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A new interpretation of deformation rates in the Snake River Plain and adjacent Basin and Range regions based on GPS measurements
Abstract
We evaluate horizontal Global Positioning System (GPS) velocities together with geologic, volcanic, and seismic data to interpret extension, shear, and contraction within the Snake River Plain and the Northern Basin and Range Province, U.S.A. We estimate horizontal surface velocities using GPS data collected at 385 sites from 1994 to 2009 and present an updated velocity field within the Stable North American Reference Frame (SNARF). Our results show an ENE-oriented extensional strain rate of 5.9 {+-} 0.7 x 10â»â¹ yrâ»Â¹ in the Centennial Tectonic belt and an E-oriented extensional strain rate of 6.2 {+-} 0.3 x 10â»â¹ yrâ»Â¹ in the Intermountain Seismic belt combined with the northern Great Basin. These extensional strain rates contrast with the regional north-south contraction of -2.6 {+-} 1.1 x 10â»â¹ yrâ»Â¹ calculated in the Snake River Plain and Owyhee-Oregon Plateau over a 125 x 650 km region. Tests that include dike-opening reveal that rapid extension by dike intrusion in volcanic rift zones does not occur in the Snake River Plain at present. This slow internal deformation in the Snake River Plain is in contrast to the rapidly-extending adjacent Basin and Range provinces and implies shear along boundaries of the Snake River Plain. We estimate right-lateral shear with slip rates of 0.5-1.5 mm/yr along the northwestern boundary adjacent to the Centennial Tectonic belt and left-lateral oblique extension with slip rates of 10ⶠyrs, the low-strain field in the Snake River Plain and Owyhee-Oregon Plateau would extend through the Quaternary.
... Both earthquakes occurred in the CTB-a northern extension of the Basin and Range province north of the eastern Snake River Plain (ESRP). Deformation in the CTB is characterized by a northeast-southwest-directed extension (e.g., Payne et al., 2012;Schmeelk et al., 2017) focused on four prominent northwest-trending normal faults in Idaho. The westernmost fault is the ∼65 km long, east-dipping Sawtooth fault along which Thackray et al. (2013) inferred 2-9 m offsets in the latest Pleistocene-to-Holocene glacial deposits from light detection and ranging (LiDAR) data. ...
... The T axes show no spatial or event-type-related differences and have an average azimuth of ∼26° (Fig. 10). This direction is in rough agreement with the extensional strain direction of 57°for the CTB estimated from GPS data by Payne et al. (2012), but we observe that their model lacks observations in the Stanley region. ...
... The northern termination of the morphologically prominent normal faults in the CTB is close to the TCFS. Strong earthquakes north of the TCFS are expected to be rare due to low strain rates (Payne et al., 2012;Schmeelk et al., 2017). The Stanley earthquake highlights the potential for such earthquakes at and beyond the northern end of the CTB. ...
The magnitude 6.5 Stanley, Idaho, earthquake occurred on 31 March 2020 in a sparsely populated region north of the Sawtooth normal fault. We used seismic data from temporary and permanent stations to derive a 1D velocity model and relocate 1401 M ≥ 2.4 earthquakes with hypoDD, including a foreshock, the mainshock, and 3 yr of aftershocks. We used broadband data to determine seismic moment tensors for 173 Mw≥3.1 earthquakes. Combining locations and mechanisms shows the mainshock ruptured an unmapped north-trending, steeply west-dipping, left-lateral strike-slip fault. Rupture initiated near the bottom of the seismogenic zone and propagated upward and bilaterally for ∼20 km north and ∼3–5 km south, where the fault likely ends and deformation changes to extension. There, the rupture may have jumped west to another unmapped blind fault accommodating oblique extension. Support for a late mainshock rupture on a northwest-trending, likely east-dipping fault comes from normal faulting aftershocks. The total rupture length is ∼25–30 km, because oblique fault activity ends at the latitude of the northern terminus of the Sawtooth fault, but its trace, if it reached the surface, would be ∼6 km to the west. The Sawtooth fault was not active, even though aftershock clusters indicate that short strike-slip and normal faults are abundant in its footwall and hanging wall. Extension in the northern Basin and Range seems to terminate where major normal faults reach the inactive Eocene Trans-Challis fault system (TCFS), suggesting the TCFS exerts structural control. The deformation north of the TCFS is low, and the strike-slip character was unknown before the Stanley earthquake. Faults rupturing in the Stanley earthquake lack surface expression and are immature with low cumulative displacement. Complex transitions between tectonic regimes are common and may result in blind ruptures on unknown, immature faults, posing an underrated hazard.
... These general extension directions would persist to the present (Figs. 14B-14I) with the exception of the northern Nevada rift, which was only active from 17 to 15 Ma and is thought to be related to the initiation of the hotspot (e.g., Rodgers et al., 2002;Payne et al., 2012;Colgan, 2013). ...
... 14H, 14I). Today, global positioning system data from Payne et al. (2012) suggest that the eastern part of the Snake River Plain is behaving as a relatively coherent, unextending block with right-lateral shear along its margins that transitions to west-northwest-to east-northeast-directed extension away from the plain (Fig. 14I). Furthermore, modern extension directions mimic those spanning the development of the Basin and Range (Figs. 14B-14I), suggesting the persistence Camilleri et al. | Tectonics, basin evolution, and paleogeography, Knoll Mountain-Ruby-East Humboldt Range region GEOSPHERE | Volume 13 | Number 6 of extension directions from the onset of Basin and Range extension to the present (e.g., Colgan et al., 2004;Puskas et al., 2007;Payne et al., 2012). ...
... Today, global positioning system data from Payne et al. (2012) suggest that the eastern part of the Snake River Plain is behaving as a relatively coherent, unextending block with right-lateral shear along its margins that transitions to west-northwest-to east-northeast-directed extension away from the plain (Fig. 14I). Furthermore, modern extension directions mimic those spanning the development of the Basin and Range (Figs. 14B-14I), suggesting the persistence Camilleri et al. | Tectonics, basin evolution, and paleogeography, Knoll Mountain-Ruby-East Humboldt Range region GEOSPHERE | Volume 13 | Number 6 of extension directions from the onset of Basin and Range extension to the present (e.g., Colgan et al., 2004;Puskas et al., 2007;Payne et al., 2012). ...
New geologic mapping and tephrochronologic assessment of strata in extensional basins surrounding Knoll Mountain (Nevada, USA) reveal a geologic history linked to tectonic development of the Yellowstone hotspot and Snake River Plain to the north, and to the Ruby-East Humboldt-Wood Hills metamorphic core complex to the south. Data from these areas are utilized to present a paleogeographic reconstruction of northeastern Nevada-south-central Idaho depicting the architecture of extensional faulting and basin development during collapse of the Nevadaplano over the past 17 m.y. Knoll Mountain is a northeast-trending horst along the southern margin of the Snake River Plain and track of the Yellowstone hotspot. The horst is bounded on the east by the Thousand Springs fault system and basin, and on the west by the Knoll Mountain fault and basin, where streams currently drain north into the Snake River Plain. The Knoll and Thousand Springs basins form half-grabens that are filled with the ca. 16 Ma to ca. 8-5 Ma Humboldt Formation, which was deposited in alluvial, eolian, and lacustrine environments during slip along range-bounding faults and a series of late-stage synthetic intrabasin faults. Structural, chronologic, and sedimentologic assessment of the Humboldt Formation in the Knoll basin indicates that it records overall southward fluvial drainage with slip along the Knoll Mountain fault beginning ca. 16 Ma and continuing to at least 8 Ma, and that between 8 and ca. 5 Ma, a west-dipping intrabasin fault system had developed. Between ca. 8-5 Ma to ca. 3 Ma, several fundamental changes took place, beginning with the cessation of faulting followed by widespread erosion that in turn was followed by deposition of older alluvium. The reversal of drainage direction from south to north flowing in the Knoll basin also took place during this time period, but its age relative to the widespread erosion or older alluvium is unknown. An integration of our work with previous studies north of Knoll Mountain reveal that the Knoll Mountain and intrabasin faults terminate to the north in the vicinity of the Jurassic Contact pluton, and that this area forms an accommodation zone separating broadly coeval and colinear faults bounding the ca. 10-8 Ma north-trending Rogerson graben, the northern end of which merges with the Snake River Plain. Furthermore, an integration of our work with previous work south of Knoll Mountain reveals that the Knoll Mountain fault formed part of a > 190-km long, west-dipping fault zone that included the Ruby-East Humboldt detachment. This fault zone, which we refer to as the Knoll-Ruby fault system, had an extensive hanging-wall basin, the Knoll- Ruby basin. The Knoll-Ruby fault system was a prominent structure facilitating collapse of the Nevadaplano in northeastern Nevada between ca. 16 and ca. 8-5 Ma, and its central part produced partial exhumation of high-grade, mid-crustal metamorphic rocks in the Ruby-East Humboldt-Wood Hills metamorphic core complex. By 8-5 Ma, during the waning stages of extension along the Knoll-Ruby fault system, a series of intrabasin faults developed at about the same time as the integration of streams to form the incipient eastern reaches of the Humboldt River system. Profound changes in tectonics and paleogeography took place between ca. 8-5 Ma and ca. 3 Ma, that included the extinction of the Knoll-Ruby and intrabasin basin fault systems followed by southward migration of significant tectonism away from the Snake River Plain, resulting in development of a set of modern normal faults responsible for uplift of the southern Snake Mountains, Ruby Mountains, East Humboldt Range, and Pequop Mountains. These new faults cut and dismembered the central and southern part of the Knoll-Ruby fault system and basin, effectively ending any fluvial connection between the northern and southern parts of the Knoll-Ruby basin. Since ca. 8-5 Ma to the present, the Knoll Mountain region has remained relatively tectonically quiescent, and continued subsidence in the Snake River Plain to the north inducecapture of the drainage system in the Knoll basin and reversed the drainage direction from south to north flowing. Our new findings indicate that (1) the Knoll-Ruby fault system and associated intrabasin faults were active until ca. 8-5 Ma, which is younger than the 12-10 Ma age generally recognized for cessation of major extension elsewhere in the northern Nevada region; (2) although this fault system was responsible for partial exhumation of core-complex metamorphic rocks, it extended well beyond the confines of the core complex proper; and (3) slip along faults in the Knoll Mountain region occurred before, during, and after passage of the hotspot at the longitude of Knoll Mountain. With the exception of significant faulting postdating passage of the hotspot, the timing of faulting in the Knoll Mountain area is consistent in a general way with the space-time pattern of extension recognized elsewhere along the southern margin of the Snake River Plain. However, it is unknown if the rate of fault slip increased during passage of the hotspot as it did in other areas.
... Most of Oregon and southern Idaho are seismically quiet at the magnitude-3 level ( Figure 3b). Geodetic results in this area Payne et al., 2012) show westward motion and clockwise rotation, with regions of several hundred kilometers in scale showing negligible departures from rigidity ( Figure 3). The Yakima fold and thrust belt of south-central Washington, recognized as having accumulated many kilometers of ∼N-S shortening (e.g., Reidel, 1984), is now contracting along longitude 120°W by 1.6 mm/yr (McCaffrey et al., 2016). ...
... Though some of the smoothness reflects the youth of the lava cover, Anders et al. (1989) and Anders (1994) argue that passage of the Yellowstone hotspot since ∼15 Ma accelerated extensional deformation directly over the hotspot, creating a "collapse shadow" along its former positions, with minimal faulting after the hotspot passed under the site. This interpretation is strongly supported by recent precision geodesy, showing the Snake River Plain moving WSW as a rigid body at 3-4 mm/yr, faster than the extending areas to the north and south (Payne et al., 2012). The contrast between the rigid Snake River Plain and the extending areas to the north and southeast is especially evident in the seismicity (Figure 3b). ...
Widely accepted tectonic reconstructions indicate at least 100 km of coast‐parallel northwestward translation of the Sierra Nevada block of California and 15–20° clockwise rotation of most of Oregon since the current phase of Basin and Range extension began ∼17 Ma. These reconstructions require at least 100 km of convergence between the central Coast Range of Oregon and rigid North America in mainland British Columbia, yet there is little discussion of how such convergence might be distributed. This study offers a kinematic model of the distribution of such deformation, constrained by geodesy, paleomagnetism, and fault offsets in Nevada, California and Oregon. The model includes differential rotation across the thrust faults of the Yakima fold and thrust belt (YFTB), compressive right‐lateral faulting in the Washington Cascade Range, substantial thrust faulting within the Puget Lowland, and oroclinal bending and doming in the Olympic Mountains. Shortening across YTFB along 120°W longitude is modeled as 47 km, across Puget Lowland at 123°W (Olympia‐Bellingham) is 94 km, and total shortening between the central Oregon Coast Range and northern Washington (Corvallis‐Bellingham) is 125 km. Current motion of the coastal regions above the Cascadia subduction zone results from both permanent deformation of the continent and elastic coupling to the subducting plate. Permanent deformation in the model is based on extrapolating geodesy from east of 120°W or south of 40°N, indicating a very uniform convergence velocity with the Juan de Fuca plate for northernmost California and Oregon near 31 mm/yr at N61°E.
... Continuous Global Positioning System (GPS) measures an extensional ENE-oriented strain rate of 7.3 ± 0.4 × 10 −9 yr −1 across the Centennial Tectonic Belt (CTB) of the Northern Basin and Range province (McCaffrey et al., 2013;Payne et al., 2012), which is mainly accommodated by four major normal faults (the Sawtooth, Lost River, Lemhi and Beaverhead faults). Fault morphology and gravity studies show that recent slip of the Sawtooth fault has concentrated along its northern trace, and the fault long-term activity YANG ET AL. ...
... Therefore, similar to the 1983 M w 6.9 Borah Peak earthquake, the extensional strain in the CTB is one of the most important factors for the 2020 M w 6.5 Stanley earthquake. In addition, surface deformation studies based on GPS measurements (Payne et al., 2012) reveal that the southern Idaho Batholith (IB) moves to the west with a different velocity from the ENE-directed extension in the adjacent CTB. The different deformation orientations and rates may contribute to the strike-slip faulting during the Stanley earthquake, for which the epicenter is located at the boundary between the IB and the CTB blocks (Figure 1). ...
Plain Language Summary
The northwest of Stanley, Idaho, is struck by an Mw6.5 earthquake on March 31, 2020, which is the largest event in Idaho since the 1983 Borah Peak earthquake. This event has several intriguing aspects: (1) the epicenter located by USGS is not on a mapped fault and the source region has little historical seismicity over the past 50 years; (2) it predominantly involves strike‐slip faulting, which is inconsistent with the extensional strain accommodated by the closest Sawtooth fault; (3) long‐period point‐source solutions have more than 35% non‐double‐couple components, indicating a more complex source than slips on a planar fault. Based upon an integrative analysis for both seismological and remote sensing data, we prescribe an opposing‐dip two‐fault model to reconcile all observations. The rupture initiated near the USGS epicenter, and then propagated to the southeast about 20 km along the northerner subfault trajectory. When reaching the northwestern terminus of the Sawtooth fault, the rupture changed its original trajectory and moved southwestward, traversing a 10‐km‐wide step‐over. After passing the mapped Sawtooth fault terminus, it propagated to the southeast about 25 km along the southern subfault, which is subparallel to the Sawtooth fault scarp.
... Using the methodology of Minson and Dreger (2008), and assuming a deviatoric source mechanism, we obtained an M w 4.4 oblique-normal solution ( Figure S4) with one nodal plane dipping 62°to the northeast and striking to the northwest at 327°, which is subparallel to the 1959 M w 7.2 Hebgen Lake fault scarp ( Figure 2). The northeast-southwest oriented T axis is consistent with the GPS-determined regional strain field as well as historic earthquakes (Payne et al., 2012;Puskas et al., 2007;Schmeelk et al., 2017). The best fitting moment centroid depth is 15 km,~6 km deeper than the focal depth derived from arrival times. ...
... The aftershock duration time in years, t a , can be estimated by applying t a = 314/v from Stein and Liu (2009), where v is the loading rate in millimeters per year. Considering the regional extensional strain rate determined from GPS measurements (Payne et al., 2012), we estimate a loading rate of~3 mm/year, corresponding to an aftershock duration of~100 years for the 1959 M w 7.2 Hebgen Lake earthquake. ...
We explore the detailed spatiotemporal evolution of 3,345 earthquakes that occurred near Maple Creek, Yellowstone, for the time period of 12 June 2017 to 13 March 2018. We generate high-accuracy relocations and near source V P /V S ratios using 4.4 million P wave and S wave differential travel times derived from waveform cross correlation. The hypocenters can be subdivided geographically into two major subpopulations: a northern cluster with planar structures striking mainly NW-SE and a southern cluster with planar structures striking mainly E-W. We observe V P /V S ratios of 1.39–1.66 in the northern cluster and a steady ratio of 1.50 in the southern cluster, suggesting the presence of CO 2 -filled cracks. We interpret the northern earthquake cluster primarily as long-lived aftershocks of the 1959 M w 7.2 Hebgen Lake earthquake but with some influence of magmatic fluids. We interpret the southern earthquake cluster as a more classic, swarm-like sequence induced primarily by the migration of magmatic fluids.
... Waning subduction was accompanied by eruption of the Challis volcanics and Eocene extension in central and eastern Idaho (Lewis & Kiilsgaard, 1991). The most recent tectonism to affect the study area includes, apparently, passage over the distal portions of the Yellowstone hotspot swell, Miocene-present Basin and Range extension (Payne et al., 2012;Vogl et al., 2014), and-west of the WISZ-the Blue Mountains terranes are extensively covered by Miocene Columbia River Flood Basalts (Reidel et al., 2013). ...
... The BMP units and areas of southern Idaho have been strongly affected by Miocene-present Basin and Range extensional deformation (e.g., Payne et al., 2012). The BMP is also the locus of prolific eruptions of the Columbia River flood basalts, beginning ∼17 Ma. ...
We developed 3‐D isotropic crustal seismic velocity models of central Idaho and eastern Oregon from the IDOR (western IDaho and eastern ORegon) Passive seismic data. Ambient noise tomography yielded crustal velocity structure from vertical component Rayleigh wave group and phase velocity measurements. Results include a strong shear wave velocity contrast—faster in accreted Blue Mountains terranes west of the western Idaho shear zone (WISZ), slower in the Idaho batholith, emplaced within the Archean Grouse Creek block east of the WISZ—restricted to the upper‐to‐middle crust. In deeper crust not affected by mafic underplating during Columbia River Flood Basalt magmatism, the shear wave velocity of the Mesozoic Olds Ferry continental arc terrane is indistinguishable from that of the Archean Grouse Creek block basement. Crustal columns of the Olds Ferry terrane and the Permian‐Jurassic Wallowa intraoceanic arc terrane are characterized by low seismic velocities, consistent with felsic lithologies down to ∼20 km. West of the WISZ, the Bourne and Greenhorn subterranes of the Baker terrane, an accretionary complex between the arc terranes, have distinct shallow crustal seismic velocities. The Greenhorn subterrane to midcrustal depths is in an overthrust geometry relative to the Bourne subterrane. Lack of mafic lower crust in our results of the Wallowa or Olds Ferry arcs may be due to imbrication of upper crustal felsic plutonic complexes of these arcs. Shortening and thickening of the Blue Mountains arc terranes crust to >30 km, and subduction or delamination of their mafic lower crustal sections is a viable mechanism for growth of a felsic continental crust.
... Quaternary-age fault scarps are observed along the three sections of the EBLF, and on the southern section, there is paleoseismic evidence for 5-7 surface-rupturing (M6.8-7.2) earthquakes in the last 40,000 years, including two in the Holocene (McCalpin, 2003). Recent geodetic observations confirm significant present-day deformation near the EBLF with an extensional strain rate of 6.4 ± 0.5 × 10 À9 yr À1 (Payne et al., 2012). ...
... Although no GPS or strain meter data are available to test this idea directly, the 2017 Sulphur Peak aftershock migration rates are similar to those of other seismic sequences with confirmed afterslip (e.g., Canitano et al., 2018), as well as creep events in California (e.g., Linde et al., 1996;Lohman & McGuire, 2007). The combination of afterslip in the 2017 Sulphur Peak sequence and the cyclic/repeating nature of seismicity in this area-as indicated by the previous energetic, co-located sequences in 1960 and 1982-suggests that southeastern Idaho might be a region with slow-slip or creep (Peng & Gomberg, 2010), a style of deformation that is consistent with the relatively high strain rates (Payne et al., 2012;Schmeelk et al., 2017) and high heat flow (Blackwell et al., 2011) in the region. ...
An energetic earthquake sequence occurred during September to October 2017 near Sulphur Peak, Idaho. The normal-faulting Mw 5.3 mainshock of 2 September 2017 was widely felt in Idaho, Utah, and Wyoming. Over 1,000 aftershocks were located within the first 2 months, 29 of which had magnitudes ≥4.0 ML. High-accuracy locations derived with data from a temporary seismic array show that the sequence occurred in the upper (<10 km) crust of the Aspen Range, east of the northern section of the range-bounding, west-dipping East Bear Lake Fault. Moment tensors for 77 of the largest events show normal and strike-slip faulting with a summed aftershock moment that is 1.8–2.4 times larger than the mainshock moment. We propose that the unusually high productivity of the 2017 Sulphur Peak sequence can be explained by aseismic afterslip, which triggered a secondary swarm south of the coseismic rupture zone beginning ~1 day after the mainshock.
... We first calculated the total values of differential velocity (Vd) and direction between these stations (3.8 ± 0.1 mm/yr at 331°, NAM08 reference frame), and then calculated the extension-only component (2.4 ± 0.1 mm/yr at 280°) using an estimate of the regional extension direction (100°/280°) based on the strikes of the seven Holocene-active faults in the transect area (Tables 1 and 2). Our estimated regional extension direction is very similar to those derived from analysis of regional GPS (e.g., 104°/284°; Payne et al., 2012) and fault strike data and is also consistent with the few published earthquake focal mechanisms in the region (Patton and Zandt, 1991;Pezzopane and Weldon, 1993). ...
... The ~50° more northward orientation of the total Vd vectors with respect to the regional extension direction (Table 2) indicates a substantial component (~3 mm/yr) of dextral shear across the transect, but the shear component likely decreases eastward away from active strike-slip faulting in the Walker Lane Thatcher, 2005, 2007;Payne et al., 2012). So how might this shear be expressed on faults in the transect? ...
We use new and existing data to compile a record of ~18 latest Quaternary large-magnitude surface-rupturing earthquakes on 7 fault zones in the northwestern Basin and Range Province of northwestern Nevada and northeastern California. The most recent earthquake on all faults postdates the ca. 18-15 ka last glacial highstand of pluvial Lake Lahontan and other pluvial lakes in the region. These lacustrine data provide a window in which we calculate latest Quaternary vertical slip rates and compare them with rates of modern deformation in a global positioning system (GPS) transect spanning the region. Average vertical slip rates on these fault zones range from 0.1 to 0.8 mm/yr and total ~2 mm/yr across a 265-km-wide transect from near Paradise Valley, Nevada, to the Warner Mountains in California. We converted vertical slip rates to horizontal extension rates using fault dips of 30°-60°, and then compared the extension rates to GPS-derived rates of modern (last 7-9 yr) deformation. Our preferred fault dip values (45°-55°) yield estimated longterm extension rates (1.3-1.9 mm/yr) that underestimate our modern rate (2.4 mm/yr) by ~21%-46%. The most likely sources of this underestimate are geologically unrecognizable deformation from moderate-sized earthquakes and unaccounted-for coseismic off-fault deformation from large surface-rupturing earthquakes. However, fault dip values of ≤40° yield long-term rates comparable to or greater than modern rates, so an alternative explanation is that fault dips are closer to 40° than our preferred values. We speculate that the large component of right-lateral shear apparent in the GPS signal is partitioned on faults with primary strike-slip displacement, such as the Long Valley fault zone, and as not easily detected oblique slip on favorably oriented normal faults in the region.
... Multiple historical earthquakes of M6.5+ have occurred in the region on Quaternary active normal faults (Figure 1a; Haller et al., 2015;Stickney et al., 2022). Regional extension and rotation are also demonstrated by geodetic strain (Payne et al., 2012). However, most known active faults remain undercharacterized given earthquake timing uncertainties and infrequent ground-rupturing earthquakes (Bartholomew et al., 2002;Ruleman et al., 2014;Stickney & Bartholomew, 1987;Zreda & Noller, 1998). ...
We investigate a shallow lake basin for evidence of a large historic intraplate earthquake in western North America. Henrys Lake, Idaho is an atypical candidate for lacustrine paleoseismic study given its shallow depth (∼7 m) and low relief (≤2° slopes). Here, we test the earthquake‐recording capacity of this basin type by showing sedimentological evidence of the 1959 M7.3 Hebgen Lake earthquake within sediment cores, using anthropogenically produced ¹³⁷Cs activity to constrain timing. In addition to expanding the morphologic range of basins targeted for lacustrine paleoseismic studies, this work has implications for sediment response in dam‐enhanced basins. Lack of sedimentological evidence for other earthquakes coupled with radiocarbon chronology reveals that the 1959 event is the only clearly recorded earthquake within Henrys Lake since the mid‐Holocene. Henrys Lake offers a proxy for paleo‐earthquake signatures within similar lacustrine environments and underscores the importance of further paleoseismic studies in the region.
... The inversion was performed using the TDEFNODE software package which is an enhanced version of the DEFNODE program developed by McCaffrey (2005). The software package has been widely adopted to image the interseismic fault kinematic and locking pattern in many active fault zones, such as the Cascadia subduction zone (McCaffrey et al. 2000), the North Island of New Zealand (Wallace et al. 2007), the Northern Basin and Range Province of America (Payne et al. 2012), and the Southeastern Tibet (Li et al. 2021(Li et al. , 2023. In the step of parameter estimation, a nonlinear simulated annealing algorithm was adopted to obtain the preferred value of parameters to be estimated. ...
The Tuolaishan–Lenglongling fault (TLSF–LLLF) is located in the middle-western segment of the Qilian–Haiyuan fault zone. The 2022 Menyuan Mw 6.7 earthquake that occurred in the TLSF–LLLF highlights the urgent need for understanding the mechanical property and seismicity over this fault segment. In this study, Persistent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR) technique was used to process Sentinel-1 acquisitions covering the TLSF–LLLF fault from 2016 to 2022 to determine the interseismic velocity field along the satellite line-of-sight. The interseismic deformation field confirmed the absence of surface creep behavior across the whole TLSF–LLLF segment. Then, we utilized both the screw dislocation and block modeling strategies to invert the comprehensive spatial distribution of fault slip rate and locking depth across the TLSF–LLLF fault. The new fault locking model, constrained by all GNSS and InSAR measurements, suggests comparable fault slip rates between 4.7 and 5.6 mm/yr in the TLSF–LLLF segment, which is generally consistent with long-term geological slip rates. The locking depth increases gradually from 8 km in the western segment of the TLSF to 18 km in the eastern segment, while the locking depth for most sections of the LLLF is relatively deep (15–18 km), indicating existence of asperities on the locking along the TLSF–LLLF fault zone. In particular, a fault segment with obvious shallow locking depth was identified in the stepover region where the TLSF and LLLF intersect. The shallow locking section shows a good spatial correlation with the coseismic rupture of the 2022 Menyuan earthquake. The calculated moment rate deficit suggests that the TLSF is capable of producing an Mw 7.3 earthquake given the high seismic moment accumulation rate and a lack of small-to-moderate earthquakes.
... It contains multiple tectonic features including the northern portion of the Intermountain Seismic Belt and the Centennial Tectonic Zone (Fig. 1A). The intraplate region is characterized by basin and range style extension and orogenic collapse (Faulds and Varga 1998;Payne et al. 2012Payne et al. , 2013Schmeelk et al. 2017), resulting in seismically active normal and strike-slip faults. We use hypocenters from the United States Geological Survey (USGS) (2022) Advanced National Seismic System Comprehensive Earthquake Catalog (ANSS comcat) from the region spanning all of Idaho north of the Snake River Plain and Montana west of longitude -109.5 (Fig. 1A). ...
Declustering of earthquake catalogs, that is determining dependent and independent events in an earthquake sequence, is a common feature of many seismological studies. While many different declustering algorithms exist, each has different performance and sensitivity characteristics. Here, we conduct a comparative analysis of the five most commonly used declustering algorithms: Garnder and Knopoff (1974), Uhrhammer (1986), Reasenberg (J Geophys Res: Solid Earth 90(B7):5479–5495, 1985), Zhuang et al. (J Am Stat Assoc 97(458):369–380, 2002), and Zaliapin et al. (Phys Rev Lett 101(1):4–7, 2008) in four different tectonic settings. Overall, we find that the Zaliapin et al. (Phys Rev Lett 101(1):4–7, 2008) algorithm effectively removes aftershock sequences, while simultaneously retaining the most information (i.e. the most events) in the output catalog and only slightly modifying statistical characteristics (i.e. the Gutenberg Richter b-value). Both Gardner and Knopoff (1974) and Zhuang et al. (J Am Stat Assoc 97(458):369–380, 2002) also effectively remove aftershock sequences, though they remove significantly more events than the other algorithms. Uhrhammer (1986) also effectively removes aftershock sequences and removes fewer events than Gardner and Knopoff (1974) or Zhuang et al. (J Am Stat Assoc 97(458):369–380, 2002), except when large magnitude events are present. By contrast, Reasenberg (J Geophys Res: Solid Earth 90(B7):5479–5495, 1985) only effectively removed aftershocks in one of the test regions.
... One such complex fault region is the Centennial Tectonic Belt (CTB) within the northernmost Basin and Range Province ( Fig. 1), characterized by moderate-to-large-magnitude seismicity (Stickney and Bartholomew, 1987;Pang et al., 2018), low geodetic strain, and northwest-trending normal faults that accommodate regional extension and crustal rotation (Payne et al., 2012;McCaffrey et al., 2013). The Eocene Trans-Challis fault system (TCFS)-a 270 km long extensional zone of northeast-trending normal faults, grabens, and eruptive centers-forms the northern terminus of the CTB (Bennett, 1986). ...
The 2020 moment magnitude (Mw) 6.5 Stanley, Idaho, earthquake raised questions about the history and extent of complex faulting in the northwestern Centennial Tectonic Belt (CTB) and its relation to the Sawtooth normal fault and Eocene Trans-Challis fault system (TCFS). To explore faulting in this area, we excavated a paleoseismic trench across the Sawtooth fault along the western margin of the CTB, and compared an early Holocene (9.1 ± 2.1 ka, 1σ) rupture at the site with lacustrine paleoseismic data and fault mapping in the 2020 epicentral region. We find: (1) a history of partial to full rupture of the Sawtooth fault (Mw 6.8–7.4), (2) that shorter ruptures (Mw≤6.9) are likely along distributed and discontinuous faults in the epicentral region, (3) that this complex system that hosted the 2020 earthquake is not directly linked to the Sawtooth fault, (4) that the northeast-trending TCFS likely plays a role in controlling fault length and rupture continuity for adjacent faults, and (5) that parts of the TCFS may facilitate displacement transfer between normal faults that accommodate crustal extension and rotation. Our results help unravel complex faulting in the CTB and imply that relict structures can help inform regional seismic hazard assessments.
... Considering the time intervals 15-8 ka and 8-0 ka, the slip rate increased by about a factor of ~3 at the northern fault end (Steamboat Mountain) and in the central section (Jenny Lake) and by factor of ~2 in the southern section (Buffalo Bowl). Note that the decrease in the slip rate after the postglacial slip acceleration does not imply that the Teton fault is presently inactive because GPS data clearly show ongoing horizontal extension across the Teton region (Payne et al., 2012). ...
Along the eastern front of the Teton Range, northeastern Basin and Range province (Wyoming, USA), well-preserved fault scarps that formed across moraines, river terraces, and other geomorphological features indicate that multiple earthquakes ruptured the range-bounding Teton normal fault after the Last Glacial Maximum (LGM). Here we use high-resolution digital elevation models derived from lidar data to determine the vertical slip distribution along strike of the Teton fault from 54 topographic profiles across tectonically offset geomorphological features along the entire Teton Range front. We find that offset LGM moraines and glacially striated surfaces show higher vertical displacements than younger fluvial terraces, which formed at valley exits upstream of LGM terminal moraines. Our results reveal that the tectonic offsets preserved in the fault scarps are post-LGM in age and that the postglacial slip distribution along strike of the Teton fault is asymmetric with respect to the Teton Range center, with the maximum vertical displacements (27–23 m) being located north of Jenny Lake and along the southwestern shore of Jackson Lake. As indicated by earlier three-dimensional numerical models, this asymmetric slip distribution results from postglacial unloading of the Teton fault, which experienced loading by the Yellowstone ice cap and valley glaciers in the Teton Range during the last glaciation.
... Meanwhile, as for the velocity component perpendicular to the profile lines, positive slopes of blue lines mean left-lateral. On the contrary, negative slopes are rightlateral [28,30,39]. ...
The Tanlu fault zone, extending over 2400 km from South China to Russia, is one of the most conspicuous tectonic elements in eastern Asia. In this study, we processed the Global Positioning System (GPS) measurements of Anhui Continuously Operating Reference System (AHCORS) between January 2013 and June 2018 to derive a high-precision velocity field in the central and southern segments of the Tanlu fault zone. We integrated the AHCORS data with those publicly available for geodetic imaging of the interseismic coupling and slip rate deficit distribution in the central and southern segments of the Tanlu fault zone. This work aims at a better understanding of strain accumulation and future seismic hazard in the Tanlu fault zone. The result indicates lateral variation of coupling distribution along the strike of the Tanlu fault zone. The northern segment of the Tanlu fault zone has a larger slip rate deficit and a deeper locking depth than the southern segment. Then, we analyzed three velocity profiles across the fault. The result suggests that the central and southern segments of the Tanlu fault zone are characterized by right-lateral strike-slip (0.29-0.44 mm/y) with compression components (0.35-0.76 mm/y). Finally, we estimated strain rates using the least-squares collocation method. The result shows that the dilatation rates concentrate in the region where the principal strain rates are very large. The interface of extension and compression is always accompanied by sudden change of direction of principal strain rates. Especially, in the north of Anhui, the dilatation rate is largest, reaching 3.780 × 10 −8 /α. Our study suggests that the seismic risk in the northern segment of the Tanlu fault zone remains very high for its strong strain accumulation and the lack of historical large earthquakes.
... The landscape includes many deeply carved canyons and tracts of land with scoured basalt and stream-lined loess deposits, indicating floods traversed and eroded several different pathways at different times (16)(17)(18)(19). GIA caused crustal deformation in the Channeled Scabland with rates up to 10 mm/y (2) orders of magnitude above regional tectonic uplift rates (20)(21)(22) and, therefore, may have influenced flood routing. ...
Significance
The glacial Lake Missoula outburst floods are among the largest known floods on Earth. Dozens of these floods scoured the landscapes of eastern Washington during the last Ice Age, from 18 to 15.5 thousand years ago, forming what is known as the Channeled Scabland. We explored how changes in topography due to the solid Earth’s response to ice sheet loading and unloading influenced the history of megaflood routing over the Channeled Scabland. We found that deformation of Earth’s crust played an important role in directing the erosion of the Channeled Scabland.
... Most Quaternary fault traces are confined to older Plio-Pleistocene deposits (Wood and Anderson, 1981;Othberg and Stanford, 1992). Recent GPS measurements of strain rates in the northern Basin and Range Province, together with geologic, volcanic, and earthquake data, reveal that the region of the Snake River Plain and Owyhee-Oregon plateau is experiencing a very low strain rate (near zero), distinct from the actively extending adjacent Basin and Range regions (Payne et al., 2012). ...
The western Snake River Plain (WSRP) in southwest Idaho has been characterized as an intracontinental rift basin but differs markedly in topography and style from other Cordilleran extensional structures and structurally from the down-warped lava plain of the eastern Snake River Plain. To investigate mechanisms driving extension and topographic evolution, we sampled granitoid bedrock from Cretaceous and Eocene-aged plutons from the mountainous flanks of the WSRP to detail their exhumation history with apatite (U-Th)/He (AHe) thermochronometry. AHe cooling dates from seventeen samples range from 7.9 ± 1.4 Ma to 55 ± 10 Ma. Most cooling dates from Cretaceous plutons adjacent to the WSRP are Eocene, while Eocene intrusions from within the Middle Fork Boise River canyon ~35 km NE of the WSRP yield Miocene cooling dates. The AHe dates provide evidence of exhumation of the Idaho batholith during the Eocene, supporting a high relief landscape at that time, followed by decreasing relief. The Miocene AHe dates show rapid cooling along the Middle Fork Boise River that we take to indicate focused river incision due to base level fall in the WSRP. Eocene AHe dates limit magnitudes of exhumation and extension on the flanks of the WSRP during Miocene rift formation. This suggests extension was accommodated by magmatic intrusions and intrabasin faults rather than basin-bounding faults. We favor a model where WSRP extension was related to Columbia River Flood Basalt eruption and enhanced by later eruption of the Bruneau-Jarbidge and Twin Falls volcanic fields, explaining the apparent difference with other Cordilleran extensional structures.
... Geodetic data indicate <2 mm/yr of extension rate across the Bitterroot valley (Payne et al., 2012;Schmeelk et al., 2017). A comparison of geodetic and geologic extension rates reveals the percentage of the geodetic rate accounted for by slip on the fault system, and the degree of slip partitioning. ...
The Bitterroot fault is a 100-km-long Quaternary active normal fault that bounds the eastern margin of the north-south trending Bitterroot Mountains and accommodates extension within the Intermountain Seismic Belt. Earthquake and fault history are unknown for the Bitterroot fault, although the seismic risk is potentially high given the proximity of the rapidly growing towns in the Missoula-Bitterroot valleys. New detailed mapping using LiDAR along the southern Bitterroot Range documents multiple generations of fault scarps in Holocene-Pleistocene deposits with vertical offsets that increase in magnitude with age. Fault mapping indicates a complex fault geometry characterized by an en echelon pattern of discontinuous segments of 45-70° east-dipping normal faults that appear to cut the older Eocene detachment fault, and locally 70-80° west-dipping antithetic normal faults. 10 Be cosmogenic radionuclides exposure dating technique provides in situ age control for 32 surface boulders (>1m) sampled in glacial deposits. Near Como Dam, two Pinedale aged glacial moraine sequences yield peak age distributions of 15.0 ± 0.4 ka and 16.4 ± 0.6 ka assuming zero erosion rate. Maximum ages on the same two Pinedale moraines yield peak ages distribution of 15.4 ± 0.4 ka and 16.8 ± 0.6 ka for 2 mm/ka erosion rate. Vertical separation of 3.5 ± 0.1 m across the dated ~16-17 ka glacial moraine offset by the Bitterroot fault scarp, yields a fault slip rate of 0.2-0.3 mm/yr. Glacial Lake Missoula high stand shorelines inset into the dated ~15 ka glacial moraine and vertically offset 4.6 ± 1.5 m by an antithetic strand of the Bitterroot fault, yield fault slip rates of 0.2-0.4 mm/yr. In the Ward Creek Fan located ~15 km to the north of Lake Como, two dated glacial debris fan sequences yield peak age distributions of 16.6 ± 0.4 ka and 62.8 ± 1.7 ka for zero erosion, and 17.0 ± 0.4 ka and 69.9 ± 2.2 ka for 2 mm/ka erosion rate. Vertical separations of 2.4 ± 0.2 m and 4.5 ± 0.1 m on the dated ~17 ka and ~63-70 ka fan surfaces offset by the Bitterroot fault, yield fault slip rates of 0.1-0.2 mm/yr and 0.1 mm/yr, respectively. Reported exposure ages are calculated using LSDn nuclide-dependent scaling age model. Our results indicate broadly consistent fault slip rates for fault segments at Lake Como (0.2-0.4 mm/yr) and Ward Creek fan (0.1-0.2 mm/yr) with an along-strike preferred average of 0.2-0.3 mm/yr for the southern Bitterroot fault. Fault scaling relations and evidence of multiple late Quaternary fault surface ruptures suggest the Bitterroot fault could produce a Mw ~7.2 earthquake. Structural model constraints and our slip rate results indicate both high angle and low angle fault geometry are possible at depth. A seismogenic low angle fault model could generate a larger earthquake of Mw > 7.2. Data from this study suggest seismic hazards from the Bitterroot fault potentially pose a significant risk to the Missoula metropolitan area, the State's second most populous region, and major infrastructures across the Missoula-Bitterroot valleys.
... The surficial geology of the RCCZO consists of Cenozoic volcanic rocks underlain by Cretaceous granite of the Idaho Batholith (McIntyre, 1972). There are no regional stress measurements in the Owyhee region, however Payne et al. (2012) showed that the Snake River Plain and Owyhee-Oregon Plateau experience very low rates of deformation. ...
In the northern hemisphere within snow-dominated mountainous watersheds north-facing slopes are commonly more deeply weathered than south-facing slopes. This has been attributed to a more persistent snowpack on the north facing aspects. A persistent snowpack releases its water into the subsurface in a single large pulse, which propagates the water deeper into the subsurface than the series of small pulses characteristic of the intermittent snowpack on south-facing slopes. Johnston Draw is an east-draining catchment in the Reynolds Creek Critical Zone Observatory, Idaho that spans a 300 m elevation gradient. The north-facing slope hosts a persistent snowpack that increases in volume up drainage, while the south-facing slope has intermittent snowpack throughout the drainage. We hypothesize that the largest difference in weathering depth between the two aspects will occur where the difference in snow accumulation between the aspects is also greatest. To test this hypothesis, we conducted four seismic refraction tomography surveys within Johnston Draw from inlet to outlet and perpendicular to drainage direction. From these measurements, we calculate the weathering zone thickness from the P-wave velocity profiles. We conclude that the maximum difference in weathering between aspects occurs ¾ of the way up the drainage from the outlet, where the difference in snow accumulation is highest. Above and below this point, the subsurface is more equally weathered and the snow accumulations are more similar. We also observed that the thickness of the weathering zone increased with decreasing elevation and interpret this to be related to the observed increase soil moisture at lower elevations. Our observations support the hypothesis that deeper snow accumulation leads to deeper weathering when all other variables are held equal. One caveat is the possibility that the denser vegetation contributes to deeper weathering on north-facing slopes via soil retention or higher rates of biological weathering.
... Prior to breaking through to the surface in the Holocene, the BRF underwent an indeterminate period of development, presumably governed by slow regional extension. The present-day regional velocity field is poorly constrained (the nearest GPS station at the longitude of the BRF is 65 km to the north), but appears to constitute generally westward motion of <1 mm/yr, equivalent to <2 nanostrain/yr of extension, relative to a stable North America (Kreemer et al., 2012;Payne et al., 2012;McCaffrey et al., 2013;Schmeelk et al., 2017). We surmise that, over time, concentration of stress and resulting localization of strain within the incipient BRF lengthened and linked a complex array of preexisting and precursory faults and sets of fractures that included the Hogsback thrust ramp, in a manner consistent with field observations of early-stage faulting (Crider and Peacock, 2004). ...
Earthquake clustering (grouping in space and time) is a widely observed mode of strain release in the upper crust, although this behavior on individual faults is a departure from classic elastic rebound theory. In this study, we consider factors responsible for a cluster of earthquakes on the Bear River fault zone (BRF), a recently activated, 44-km-long normal fault on the eastern margin of Basin and Range extension in the Rocky Mountains. The entire surface-rupturing history of the BRF, as gleaned from paleoseismic and geomorphic observations, began only 4500 years ago and consists of at least three large events. Rupture of the BRF is spatially complex and is clearly conditioned by preexisting structure. In particular, where the south end of the fault intersects older thrust faults and upturned strata along the south-dipping flank of the Precambrian basement-cored Uinta arch, the main trace ends abruptly in a set of orthogonal splays that accommodate down-dropping of a large hanging-wall graben against the arch. We hypothesize that the geomechanically strong Uinta arch crustal block impeded the development of the BRF and, over time, enabled a significant accumulation of elastic strain energy, eventually giving rise to a pulse of strain release in the mid- to late Holocene. We surmise that variations in fault strength, both in space and time, is a cause of earthquake clustering on the BRF and on other faults that are structurally and tectonically immature. The first two earthquakes on the BRF occurred during the same period of time as a regional cluster of earthquakes in the Middle Rocky Mountains, suggesting that isolated faults in this slowly extending region interact through widespread changes in stress conditions.
... Although Montana lies more than 500 km away from a tectonic plate boundary, the region is undergoing complex and active deformation, as evidenced by recent geodetic and seismic observations (Payne et al., 2012;Stickney, 2015;Schmeelk et al., 2017). The Lincoln earthquake occurred within the ISB, which is a ∼100 km wide band of seismicity that extends from southwestern Utah through northwestern Montana (e.g., Stickney and Bartholomew, 1987;Smith and Arabasz, 1991). ...
One of the most seismically active regions in the United States, located hundreds of kilometers inland from the nearest plate boundary, is the Intermountain Seismic Belt (ISB). The 6 July 2017 M 5.8 earthquake occurred 11 km southeast of Lincoln, Montana, within the ISB. This was the largest earthquake to rupture in the state of Montana since the 1959 M 7.3 Hebgen Lake earthquake. We use continuous seismic data from the University of Montana Seismic Network, the Montana Regional Seismic Network, and the U.S. Geological Survey to investigate the Lincoln aftershock sequence and to evaluate crustal stress conditions. We manually picked P- and S-wave arrival times, computed 4110 hypocenter locations and 2336 double-difference relocations, and generated focal mechanisms for 414 aftershocks (12+ polarities) in the 2 yr following the mainshock. Based on the alignment of aftershocks, we infer that the mainshock occurred on a north–northeast-trending left-lateral strike-slip fault. The orientation of the fault is unexpected, given that it strikes nearly perpendicular to the prominent Lewis and Clark line (LCL) faults in the area. Although most aftershocks concentrate near the mainshock, several distinct clusters of microseismic activity emerge along subparallel faults located primarily to the west of the mainshock. The subparallel faults also exhibit left-lateral strike-slip motion oblique to the LCL. We postulate that the aftershocks reveal the clockwise rotation of local-scale crustal blocks about vertical axes within a larger, right-lateral shear zone. The inferred block rotations are consistent with a bookshelf-faulting mechanism, which likely accommodates differential crustal motion to the north and south of the LCL region. The tension axes of well-constrained focal mechanisms indicate local northeast–southwest extension with a mean direction of N60°E.
... The northern mapped extent of the Sawtooth fault terminates near its intersection with the northeast-striking TCFS, a 24 km wide zone of normal faults that accommodated Eocene northwest-southeast extension (Fig. 1). Early extension was directly related to the emplacement of Challis volcanic rocks that lie to the north and east (Bennett, 1986;Payne et al., 2012). The TCFS extends across Montana as the Great Falls lineament to form a regionally extensive structure (O'Neill and Lopez, 1985). ...
We report on the tectonic framework, seismicity, and aftershock monitoring efforts related to the 31 March 2020 Mw 6.5 Stanley, Idaho, earthquake. The earthquake sequence has produced both strike-slip and dip-slip motion, with minimal surface displacement or damage. The earthquake occurred at the northern limits of the Sawtooth normal fault. This fault separates the Centennial tectonic belt, a zone of active seismicity within the Basin and Range Province, from the Idaho batholith to the west and Challis volcanic belt to the north and east. We show evidence for a potential kinematic link between the northeast-dipping Sawtooth fault and the southwest-dipping Lost River fault. These opposing faults have recorded four of the five M≥6 Idaho earthquakes from the past 76 yr, including 1983 Mw 6.9 Borah Peak and the 1944 M 6.1 and 1945 M 6.0 Seafoam earthquakes. Geological and geophysical data point to possible fault boundary segments driven by pre-existing geologic structures. We suggest that the limits of both the Sawtooth and Lost River faults extend north beyond their mapped extent, are influenced by the relic trans-Challis fault system, and that seismicity within this region will likely continue for the coming years. Ongoing seismic monitoring efforts will lead to an improved understanding of ground shaking potential and active fault characteristics.
... From heat flux measurements and S-wave speeds, Prieto et al. (2017) inferred a temperature of 750°C at that depth. Strain rates in the region, if not tightly constrained, are~6 × 10 −9 yr −1 , or~2 × 10 −16 s −1 (Payne et al., 2012;Schmeelk et al., 2017), which is roughly 3 orders faster than intraplate settings. Thus, a temperature~700-750°C seems reasonable (Figures 4 and 5). ...
Maximum depths of earthquakes in different settings are commonly thought to lie within a transition, many kilometers in width, from brittle deformation at shallow depths to plastic deformation by high‐temperature creep at greater depth. A review of temperatures and strain rates, both of which are low in intraplate settings, shows faster deformation in warmer tectonically active regions and highest strain rates in regions where the movement of magma affects strain rates and temperatures are especially high. Although intraplate earthquakes appear to occur in oceanic lithosphere only where colder than ~600°C, in tectonically active regions, where strain rates are several orders of magnitude higher than intraplate settings, earthquakes occur where temperatures exceed 600°C and perhaps 800°C. The temperature cutoff increases by ~100°C as strain rates increase by ~2 to 2.5 orders of magnitude. Nowhere, however, do any two of average strain rates, background stresses, and temperatures seem to be determined well enough that, when combined with laboratory‐based flow laws for high‐temperature creep, they can place a tight constraint on the third of these quantities. At the same time, the pattern of higher temperatures at higher strain rates follows the general forms of power law and Peierls creep that are appropriate for lithospheric conditions.
... More than a decade of continuous and campaign GPS geodesy of the greater Yellowstone region (Fig. 1) has generated a surface velocity field used to test geodynamic models of Northern Basin and Range (NBR) extension. These models include a 40-45 km-wide, province-scale, right-lateral shear zone called the Centennial shear zone (CSZ) that separates the NBR, Snake River Plain (SRP), and Southern Basin and Range (SBR) in the wake of the Yellowstone hotspot [1][2][3][4][5] . Seismicity across the CSZ reveals distributed deformation due to strike-slip faulting, distributed simple shear, regional-scale rotation, or some combination thereof. ...
We acquired a ~9-km long, high-resolution reflection seismic profile in the Centennial Valley, Montana, to better understand the kinematics of basin bounding faults and their role in accommodating proposed right-lateral shear in the Northern Basin and Range adjacent to the Yellowstone hotspot. In pursuing these goals, our findings have also shed light on the development of hanging wall stratigraphy and seismic hazards for this part of the SW Montana seismic belt. Here we present the profile and a working interpretation that identifies fault inversion, and an oblique, anticlinal accommodation zone linking the Centennial and Lima Reservoir faults in the Centennial Valley. These interpretations are consistent with seismicity and GPS-geodetically observed right-lateral shear aligned with the Centennial Valley north of the Yellowstone hotspot. Data were acquired using dense, wide-aperture arrays and illuminate the subsurface stratigraphy and faults down to ~1200 m, showing that the basin is a half-graben with a southern depocenter driven by the listric geometry of the north-dipping Centennial fault. Reflectors onlap basement highs with growth geometry against these faults. Our interpretation of a bright basal reflection as the Timber Hill Basalt (~6 Ma) or related flow, is consistent with a late Miocene – Pliocene inception of the basin proposed by other research. We also note a small inversion structure that we interpret as local evidence of transpression in the shear zone. This transpression is part of the accommodation zone and seismogenic faults including the Lima Reservoir fault that has well-expressed Holocene surface ruptures a few kilometres west of the seismic line along the northern edge of the Centennial basin.scient
... Hebgen Lake is situated within the Centennial Tectonic Belt (Figure 1a), an area of diffuse, mostly NW-SE-trending normal faulting separated from the larger Basin and Range extensional province by the aseismic Snake River Plain (Stickney & Bartholomew, 1987). GPS velocities show NE-SW-oriented extension of ∼3 mm/year over the whole Centennial Tectonic Belt (e.g., Payne et al., 2012;Schmeelk et al., 2017). Derived block models assign the most important individual fault slip rates of ∼1 mm/year, but GPS velocities around Hebgen Lake are still dominated by postseismic deformation from the 1959 earthquake, preventing firm constraints on strain accumulation rates in this vicinity (Schmeelk et al., 2017). ...
The 1959 Mw ∼7.2 Hebgen Lake earthquake is among the largest continental normal faulting events recorded, as well as one of the earliest associated with a multifault rupture. Multimeter vertical slip was observed on three main, morphologically distinct strands: the Hebgen fault and southeastern section of the Red Canyon fault, which both follow sharp topographic rangefronts, and the Red Canyon fault Kirkwood Ridge section, which cuts steep topography in the footwall of the Hebgen fault. We augment early field, seismological, and geodetic studies by investigating the modern surface rupture using newly acquired airborne lidar topography. By estimating throw from scarp profiling of the ∼36.5 km primary surface rupture, we show both that peak 1959 slip occurred at a structurally mature part of the fault and that many 1959 slip minima are associated with clear structural complexities. Vertical slip often substantially exceeds throw measured at the fault free face immediately after the earthquake; the scarps do not conclusively express beveled forms characteristic of repeated slip and degredation, yet must in places capture both the 1959 earthquake (for which we estimate an average throw of 2.64 m) and one or two preceding latest Pleistocene–Holocene events known from trenching. This has wider, cautionary implications for interpreting paleo-earthquake chronologies and deriving magnitudes from morphologically simple scarps. By comparing 1959-only and multievent vertical displacement populations, and considering preliminary paleoseismic data, we suggest that large surface-rupturing earthquakes on the Hebgen and Red Canyon faults involve highly variable slip distributions.
... The stress data are used to weight fault and lineament slip and dilation tendencies, both of which are proxies for permeability on these structures (e.g., Jolie et al, 2015;Siler et al., 2017). These data were integrated with regional stress and strain estimates from previous studies (e.g., Payne et al., 2012;Kessler et al., 2017). Our data suggest that rightstepping geometries between the two WNW-and NNW-striking, right-oblique fault systems (releasing steps) are more conducive to permeability development and geothermal circulation than left-stepping (restraining steps) geometries ( Figure 6). ...
Camas Prairie, Idaho, is an EW-trending structural graben that lies north of the Snake River Plain hotspot track. It is bounded on the north by the Idaho Batholith, and on the south by the Mount Bennett Hills. This region was investigated by the SRP Geothermal Play Fairway Analysis team, which included focused geologic, geochemical, and geophysical studies. The Camas Prairie geothermal system resource is indicated by warm springs and wells, geophysical analysis of buried faults and basins, mapped faults, structural analysis of all faults and lineaments, high 3 He/ 4 He, cation and multicomponent geothermometry, and the occurrence of young basalt vents and lava flows along the range front. Our provisional conceptual model includes the following components: (1) High permeability is indicated by the confluence of intersecting faults, including the major range front system and The Pothole fault system, with releasing bends as deduced from detailed field studies, and the presence of springs along mapped structural features; (2) High heat is inferred to result from mid-to shallow crustal sills, as evidenced by the location of vents along the range front; the youngest dated vent is 692 ka, but 2 ka vents lie to NW; (3) an effective seal is indicated by magnetotelluric studies, which suggest a clay seal over the prospective target area that is likely a result of hydrothermal alteration. This model is similar to that proposed for the western SRP but is less energetic due to the smaller volume of magma inferred. It is also similar to Basin-and-Range geothermal systems, but differs by including a distinct magmatic heat component.
... Mid-Miocene to ongoing extensional processes also affected southwestern Montana and northeastern Idaho, some of which have been linked to the Yellowstone hotspot (Fritz and Sears, 1993;Sears et al., 2009). This extensional tectonic regime continues today along the western side of Wyoming and extends southward past Salt Lake City, Utah, as part of the Inter mountain seismic belt and eastern edge of the Basin and Range province (Payne et al., 2012). ...
The upper Wind River Basin in northwest Wyoming (USA) is located ~80- 100 km southeast of the Yellowstone Plateau volcanic field. While the upper Wind River Basin is a manifestation of primarily Cretaceous to Eocene Laramide tectonics, younger events have played a role in its formation, including Eocene Absaroka volcanism, Cenozoic lithospheric extension, and the migration of the North American plate over the Yellowstone hotspot tail. New 40Ar/39Ar ages coupled with existing K-Ar results from intrusives and lavas in the upper Wind River Basin show that igneous activity younger than ca. 5 Ma occurred locally. Field and geochemical data show that these < ca. 5 Ma upper Wind River Basin magmas were either erupted or emplaced along normal fault zones at different locations and range in composition from tholeiitic basalt (Spring Mountain) to calc-alkaline basaltic andesite through dacite (Lava Mountain, Crescent Mountain, and Wildcat Hill), and include a lamprophyre intrusion (Pilot Knob). Together, these igneous rocks define the Upper Wind River Basin volcanic field (UWRB). All UWRB rocks have large ion lithophile element enrichments, high field strength element depletions, and other geochemical characteristics associated with subduction and that are identical to those of the Miocene Jackson Hole volcanics, even though the former erupted in an intraplate setting. Our results suggest that UWRB magmatism, as well as the Jackson Hole volcanics and other small-volume, similarly aged intermediate to felsic magmatism in eastern Idaho, are the result of the interaction between the North American plate and the progression of the tectonic parabola associated with the Yellowstone hotspot tail.
... The stress data are used to weight fault and lineament slip and dilation tendencies, both of which are proxies for permeability on these structures. These data can be compared with regional stress and strain estimates from previous studies (e.g., Payne et al., 2012;Kessler et al., 2017). ...
... The stress data are used to weight fault and lineament slip and dilation tendencies, both of which are proxies for permeability on these structures. These data can be compared with regional stress and strain estimates from previous studies (e.g., Payne et al., 2012;Kessler et al., 2017). ...
Play Fairway Analysis (PFA) is a methodology adapted from the petroleum industry that
integrates data at the regional or basin scale to define favorable plays for exploration in a
systematic fashion. Phase 2 of our Play Fairway Analysis of the Western Snake River Plain
(WSRP) province in southern Idaho had three primary goals: first, to fill data gaps in critical
areas in order to better define potential prospects, second, to integrate these data into new
thermal and structural models, and finally, to infer the location of potential resources and drilling
targets that could be validated during Phase 3. Prospects in the WSRP identified as potential
target resources for Phase 3 validation include the Mountain Home region close to the Air Force
Base, and the Camas Prairie. The Mountain Home region represents a blind geothermal resource
in an area of high heat flow and young volcanism. The Camas Prairie is a, structurally controlled
resource in an area with indicators of magmatic heat. New geophysical data acquired at these
sites includes reflection seismic, gravity and magnetic surveys, and a magnetotelluric field
survey. New geochemical data collection focused on the Camas Prairie, and included the
aqueous and isotope geochemistry of hot springs, cold springs, and wells (geothermal,
groundwater, and irrigation). New field mapping, sampling, and basalt flow chronology was also
conducted at Camas Prairie. Integrated results from Phase 1 and 2 studies suggest that the system
near the Mountain Home Air Force Base is located at ~1.5–2.3 km depth, and the structurallycontrolled
system at Camas Prairie is shallower, with upper reservoir depths perhaps only ~0.5–
0.7 km.
... The persistent and successive magmatic interaction of the plume with the overriding North American lithosphere has left a trail of volcanic calderas in southern Idaho that stretches along the Snake River Plain (SRP) to the YNP (Smith and Braile 1993;Link and Phoenix 1996;Link et al. 2005;Beranek et al. 2006;Wegmann et al. 2007). The SRP, which marks the track of the sustained migration of the YHS, is an elongate, ENE-trending swath of prominent, mainly silicic volcanic calderas which progressively become younger toward the present location of the YHS (Smith 2000;Fritz and Thomas 2011;Sears et al. 2009;Payne et al. 2012). The eruptive patterns (clustered, dispersed, random) and spatial and temporal relationships among the Neogene-Quaternary lavas along the SRP are statistically not well known. ...
The sequence of eruption, spatial pattern, and spatio-temporal relationships among the Neogene-Quaternary rhyolitic and basaltic lava along the Snake River Plain (SRP) in Idaho are analyzed applying the spatial methods of global and local Moran’s I, standard deviational ellipse, and Ripley’s K-function. The results of the analyses by the Moran’s I and K-function methods indicate a higher spatial autocorrelation, hence clustering, of rhyolitic lava compared to the more dispersed basaltic lava in each center of eruption along the SRP. The clustered nature of rhyolitic lava around each caldera either reflects the original spread and large thickness of the rhyolitic lava, or the absence of younger cover strata or lava like the distribution of rhyolite in the present caldera at the Yellowstone National Park. The standard deviational ellipses (SDEs) of the lavas indicate that younger basaltic lava that erupted from newer calderas overlapped older rhyolitic and basaltic lava as the position of the Yellowstone hotspot progressively migrated to the northeast along the SRP. The less eccentric SDEs of rhyolitic lava in each caldera probably reflect the original caldera-scale spread of viscous felsic lava, compared to the more eccentric and larger SDEs of basaltic lava which represent basalt’s wider and more directed spread due to its higher fluidity and ability to flow longer distances along the trend of the SRP. The alignment of the long axes of the lava SDEs with the trend of the Eastern SRP and the trend of systematic spatial overlap of older lava by successively younger basaltic lava corroborate the previously reported migration of the centers of eruption along the ESRP as the Yellowstone hotspot migrated to the northeast.
... By comparison, at the Snake River Plain volcanic rift in the Basin and Range Province (Idaho, United States), dike intrusions accommodate 5%-11% (0.16-0.33 mm/yr) of the total extension of 3 mm/yr (Payne et al., 2012). With over double the extension rate of the Snake River Plain, the Tongariro graben shows a globally intermediate rate of extension, and it is also low to intermediate in the context of the overall Taupo volcanic zone continental rift. ...
In volcanic rift zones, surface faulting from tectonic faults or from dike intrusions can be difficult to discriminate because they have similar geomorphic expression. At the Ton-gariro graben, near the southern end of the Taupo Rift, New Zealand, crustal extension over the last 350 k.y. has been accommodated by a combination of magma intrusion and tectonic faulting. Normal faults prevail along this 30-km-wide, NNE-oriented graben, with vents of the Tongariro volcanic complex lying parallel to and overlapping the graben axis. This study quantifies the geological extension at the Tongariro graben (7 ± 1.2 mm/yr since 20 ka) and the relative contributions from tectonic faulting and dike intrusion. Field observations were used to interpret fault geometry and activity. To discriminate between tectonic faulting and that associated with dike intrusion (volcano-tectonic), theoretical fault dislocations were modeled from dike intrusion for likely fault-dike spatial relationships and compared to measured displacements. Most of the mapped faults are tectonic in origin. The calculations indicate that the rift-bounding normal faults (National Park and Upper Waikato Stream faults) and the intrarift inward-dipping faults (Waihi and Poutu faults) accommodate 78%–95% (5.8– 7.0 mm/yr) of the total extension across the graben (tectonic extension), while dike intrusions could accommodate only 5%–22% (0.4–1.6 mm/yr; magmatic extension), from which 4%–5% is associated with volcanic eruptions and the remainder with deep arrested dikes. These results help to refine the seismic and volcanic hazards of the region and raise questions on the spectrum of volcano -tectonic interactions possible in similar continental rifts worldwide.
... Historical seismicity in the region (Fig. 1) helps to define the western extent of the Intermountain seismic belt (Smith and Sbar, 1974), a north-south-oriented zone of seis micity characterized by mostly 15-20 km focal depths. Horizontal velocities using global positioning system (GPS) data (e.g., Bennett et al., 2003;Hammond and Thatcher 2004) indicate that the Intermountain seismic belt crustal block moves as a coherent block to the west-southwest relative to stable North America, with slip rates between 0.5 and 1.5 mm/yr (Payne et al., 2012). ...
The 1934 Ms 6.6 Hansel Valley, Utah earthquake produced an 8-km-long by 3-km-wide zone of north-south trending surface deformation in an extensional basin within the easternmost Basin and Range province. Less than 0.5 m of only vertical displacement was measured at the surface, although seismologic data suggest mostly strike-slip faulting at depth. Resolving the origin and kinematics of faulting in the Hansel Valley earthquake is important to understand how complex fault ruptures accommodate regions of continental extension and transtension. Here, we address three questions: (1) how does the 1934 surface rupture compare with faults in the subsurface, (2) are the 1934 fault scarps tectonic or secondary features, and (3) did the 1934 earthquake have components of both strike-slip and dip-slip motion? To address these questions, we acquired a 6.6-km-long high-resolution seismic profile across Hansel Valley, including the 1934 ruptures. We observe numerous, east- and west-dipping normal faults that dip 40°-70° and offset late Quaternary strata from within a few tens of meters of the surface down to a depth of about 1 km. Spatial correspondence between the 1934 surface ruptures and subsurface faults suggests that ruptures associated with the earthquake are of tectonic origin. Our data clearly show complex basin faulting that is most consistent with transtensional tectonics. Although the kinematics of the 1934 earthquake remain underconstrained, we interpret the disagreement between surface (normal) and subsurface (oblique-slip) kinematics as due to slip partitioning during fault propagation and to the effect of preexisting structural complexities. We infer that the 1934 earthquake occurred along a ~3-km wide, off-fault damage zone characterized by distributed deformation along small displacement faults that may be alternatively activated during different earthquake episodes.
... In eastern Idaho and western Montana nearly all of the NW trending, normal faults in the Centennial shear zone and the Centennial Tectonic Belt northwest of Yellowstone [Stickney and Bartholomew, 1987;Payne et al., 2008Payne et al., , 2012 trend within 5°of being radial to the pole of rotation ( Figure 17). From SW to NE these 40 to 120 km long faults include the Sawtooth, Lost River, Lemhi, Beaverhead, Deadman, Red River, Timber Butte section, Blacktail, and Sweetwater faults. ...
Paleomagnetic and GPS data indicate that Washington and Oregon have rotated clockwise for the past 16 Myr. Late Cenozoic and Quaternary fault geometries, seismicity lineaments, and focal mechanisms provide evidence that this rotation is accommodated by north-directed thrusting and strike-slip faulting in Washington, and SW- to W-directed normal faulting and strike-slip faulting to the east. Several curvilinear NW- to NNW-trending high-angle strike-slip faults and seismicity lineaments in Washington and NW Oregon define a geologic pole (117.7°W, 47.9°N) of rotation relative to North America. Many faults and focal mechanisms throughout northwestern US and southwestern British Columbia have orientations consistent with this geologic pole as do GPS surface velocities corrected for elastic Cascadia subduction zone coupling. Large Quaternary normal faults radial to the geologic pole, that appear to accommodate crustal rotation via crustal extension, are widespread, and can be found along the Lewis and Clark Zone in Montana, within the Centennial fault system north of the Snake River Plain in Idaho and Montana, to the west of the Wasatch Front in Utah, and within the northern Basin and Range in Oregon and Nevada. Distributed strike slip faults are most prominent in western Washington and Oregon but may help transfer slip between faults throughout the northwestern US.
... Geophysicists aspiring to image continental architecture must therefore be prepared to interpret resolved subsurface structures in terms of the totality of tectonic history of any particular study region with the view that current geological provinces may not reflect dominant structures at depth. The tectonic history of northern U.S. Rocky Mountains region began in the Mesoarchean and comprises events from the Proterozoic accretion of Laurentia (cratonic North America) [e.g., Hoffman, 1988;Ross et al., 1991;Karlstrom et al., 2002;Sims et al., 2005;Foster et al., 2006] to the Miocene-Recent Basin and Range extension, vertical axis rotation, and Yellowstone-Snake River Plain magma systems [e.g., Payne et al., 2012]. ...
We present new images of lithospheric structure obtained from P-to-S conversions defined by receiver functions at the 85 broadband seismic stations of the EarthScope IDaho-ORegon experiment. We resolve the crustal thickness beneath the Blue Mountains province and the former western margin of cratonic North America, the geometry of the western Idaho shear zone (WISZ), and the boundary between the Grouse Creek and Farmington provinces. We calculated P-to-S receiver functions using the iterative time domain deconvolution method, and we used the H-k grid search method and common conversion point stacking to image the lithospheric structure. Moho depths beneath the Blue Mountains terranes range from 24 to 34km, whereas the crust is 32-40km thick beneath the Idaho batholith and the regions of extended crust of east-central Idaho. The Blue Mountains group Olds Ferry terrane is characterized by the thinnest crust in the study area, ~24km thick. There is a clear break in the continuity of the Moho across the WISZ, with depths increasing from 28km west of the shear zone to 36km just east of its surface expression. The presence of a strong midcrustal converting interface at ~18km depth beneath the Idaho batholith extending ~20km east of the WISZ indicates tectonic wedging in this region. A north striking ~7km offset in Moho depth, thinning to the east, is present beneath the Lost River Range and Pahsimeroi Valley; we identify this sharp offset as the boundary that juxtaposes the Archean Grouse Creek block with the Paleoproterozoic Farmington zone.
... For example, there is evidence for Late Cretaceous-early Cenozoic right-lateral strike-slip faulting in the region north of the SRP in the Southwest Montana Transverse Zone~125 km north of Yellowstone [Schmidt and O'Neill, 1982;Whisner et al., 2014]. The right-lateral Centennial Shear Zone is adjacent to the SRP, and GPS-based data indicate present-day deformation along this structure, that is attributed to differential extension between the SRP itself and the area north of the SRP [Payne et al., 2008[Payne et al., , 2012[Payne et al., , 2013. Indirect evidence that the region of the SRP was an area characterized by ESE-WNW trending crustal structures and associated landforms is the inference that the zone could have hosted a major river system that transported sediment to the west in the Eocene, delivering it to the Tyee forearc basin and to the Franciscan complex trench [Dumitru et al., 2013]. ...
Geochronologic data compiled from 12 metamorphic core complexes and their flanking regions outline important differences in tectonic and magmatic histories north and south of the Snake River Plain-Yellowstone Province (SRP-Y). Magmatism, crustal flow, metamorphism, and extensional exhumation of core complexes north of the SRP occurred mostly between 55 and 42Ma as compared to 42-25Ma south of the SRP, with final exhumation of the southern complexes occurring only during younger Miocene (20-0Ma) Basin and Range faulting. These significant differences in the timing of events suggest that the now lava-covered SRP, which is at a high angle to Cordilleran trends, may have at times operated as a steep shear or transfer zone accommodating difference in strain to the north and south. Following previous suggestions, we infer that this proposed accommodation or transfer zone developed above an important lithospheric boundary localized above a tear in the subducting slab (shallower slab angle to the south) used to explain both the locus of Late Cretaceous-Paleocene magmatism and the different ages and mechanisms of slab reconfiguration and removal north and south of the SRP during the Cenozoic. The details of these different histories help outline the complex evolution of this zone and also suggest that this zone of lithospheric weakness may have subsequently focused Miocene SRP-Y hot spot magmatism.
... ∼1.5 × 10 −16 s −1 ) up to 25 nstrain yr −1 (i.e. ∼8 × 10 −16 s −1 ) at the northeast termination of the parabola, whereas no deformation is detected in the ESRP Payne et al. 2012;McCaffrey et al. 2013). Moreover, GPS observations combined with levelling and InSAR observations reveal alternative uplift and subsidence up to 2 cm yr −1 , commonly interpreted as recharge and discharge episodes of the crustal magma reservoirs and are furthermore directly linked to the presence of a mantle plume (Saunders et al. 2007;Pickering White et al. 2009;Pierce & Morgan 2009). ...
S U M M A R Y The Yellowstone–East Snake River Plain hotspot track has been intensely studied since several decades and is widely considered to result from the interaction of a mantle plume with the North American plate. An integrated conclusive geodynamic interpretation of this extensive data set is however presently still lacking, and our knowledge of the dynamical processes beneath Yellowstone is patchy. It has been argued that the Yellowstone plume has delaminated the lower part of the thick Wyoming cratonic lithosphere. We derive an original dynamic model to quantify delamination processes related to mantle plume–lithosphere interactions. We show that fast (∼300 ka) lithospheric delamination is consistent with the observed timing of formation of successive volcanic centres along the Yellowstone hotspot track and requires (i) a tensile stress regime within the whole lithosphere exceeding its failure threshold, (ii) a purely plastic rheology in the lithosphere when stresses reach this yield limit, (iii) a dense lower part of the 200 km thick Wyoming lithosphere and (iv) a decoupling melt horizon inside the median part of the lithosphere. We demonstrate that all these conditions are verified and that ∼150 km large and ∼100 km thick lithospheric blocks delaminate within 300 ka when the Yellowstone plume ponded below the 200 km thick Wyoming cratonic lithosphere. Furthermore, we take advantage of the available extensive regional geophysical and geological observation data sets to design a numerical 3-D upper-mantle convective model. We propose a map of the ascending convective sheets contouring the Yellowstone plume. The model further evidences the development of a counter-flow within the lower part of the lithosphere centred just above the Yellowstone mantle plume axis. This counter-flow controls the local lithospheric stress field, and as a result the trajectories of feeder dykes linking the partial melting source within the core of the mantle plume with the crust by crosscutting the lithospheric mantle. This counter-flow further explains the 50 km NE shift observed between the mantle plume axis and the present-day Yellowstone Caldera as well as the peculiar shaped crustal magma chambers.
The Inland Pacific Northwest documents geologic processes from Proterozoic time to the Present. This volume presents field trips from the 2024 GSA Cordilleran and Rocky Mountain Joint Sections Meeting, exploring the genesis of bedrock in Idaho, Neoproterozoic development of supercontinents in Washington, Cambrian tectonic and biostratigraphic history of Washington, and paleoecology of Miocene woodlands in Idaho. The Spokane guide illustrates the critical connection between Anthropocene activity and the past. An overview of Pleistocene megaflood effects is presented through outcrops and drone images, as are advances in understanding of the Columbia River Basalt Group presented with a strong conviction to volcanology and flood basalt evolution. The story of the Sevier orogeny accretionary margin is examined, as is a dedication to the landmark studies of the Mesoproterozoic Belt Basin through a Missoula to Spokane transect.
Declustering of earthquake catalogs, that is determining dependent and independent events in an earthquake sequence, is a common feature of many seismological studies. While many different declustering algorithms exist, each has different performance and sensitivity characteristics. Here, we conduct a comparative analysis of the four most commonly used declustering algorithms: Garnder and Knopoff (1975), Reasenberg (1985), Zhuang et al. (2002), and Zaliapin and Ben-Zion (2008) in four different tectonic settings. Overall, we find that the Zaliapin and Ben-Zion (2008) algorithm effectively removes aftershock sequences, while simultaneously retaining the most information (i.e. the most events) in the output catalog and not significantly modifying statistical characteristics (i.e. the Gutenberg Richter b-value). Both Gardner and Knopoff (1975) and Zhuang et al. (2002) also effectively remove aftershock sequences, though they remove significantly more events than the other algorithms. By contrast, Reasenberg (1985) only effectively removed aftershocks in one of the test regions.
The North American continent has a rich record of the tectonic environments and processes that occur throughout much of Earth history. This Memoir focuses on seven “turning points” that had specific and lasting impacts on the evolution of Laurentia: (1) The Neoarchean, characterized by cratonization; (2) the Paleoproterozoic and the initial assembly of Laurentia; (3) the Mesoproterozoic southern margin of Laurentia; (4) the Midcontinent rift and the Grenville orogeny; (5) the Neoproterozoic breakup of Rodinia; (6) the mid-Paleozoic phases of the Appalachian-Caledonian orogen; and (7) the Jurassic–Paleogene assembly of the North American Cordillera. The chapters in this Memoir provide syntheses of current understanding of the geologic evolution of Laurentia and North America, as well as new hypotheses for testing.
Hot spots are the surface expression of plumes of hotter and lighter material upwelling from the Earth’s mantle. The current number of hot spots is estimated to range between 45 and 70: these are mostly in intraplate settings, especially on oceanic lithosphere, and along divergent plate boundaries. Neglecting hot spots along divergent boundaries and shaped by the related far-field tectonics (as with Afar and Iceland, described in Chap. 11), oceanic hot spot volcanoes show a considerable variability in distribution, evolution and activity, as at Hawaii, Galapagos, Easter Island, Reunion, Canary Islands and Azores. This results from the different mantle plume properties (plume configuration and productivity) and local tectonic context (age of the intruded oceanic lithosphere, rate of plate motion, pre-existing structures), which make each hot spot distinctive. Despite this variability, most oceanic hot spot volcanoes also display recurrent structural features, which include overlapping mafic edifices with summit calderas, radial volcanic rift zones and flank instability. Then, there is the less common and more evolved volcanism derived from continental hot spots, of which Yellowstone is the most dramatic example and, at the same time, quite distinct from other less productive continental cases, as for instance Tibesti. All these characteristics make hot spots widely different, stimulating from structural and magmatic perspectives and complicating established models.
Hydrologic processes have been shown to influence seismic productivity in many regions around the world, especially on active plate boundaries. To examine the influence of hydrologic loading cycles on seismicity in intraplate regions, we investigate temporal patterns of seismic productivity in the northern Rocky Mountains of Montana and Idaho. Seasonal variations in seismicity are present, with enhanced productivity in December and January, and reduced productivity in June and July. Using Snowpack Telemetry and GPS data, we find that seismicity is temporally correlated with the highest hydrologic loading rates rather than peak load, consistent with rate and state models of fault behavior for faults in critically stressed domains. However, we cannot distinguish between high hydrologic stress rates and pore pressure increases at seismogenic depths lagging ∼6 months after peak snowmelt.
Pulau Jawa terletak tepat di utara zona subduksi jawa yang merupakan zona pertemuan Lempeng Indo-Australia dengan Lempeng Sunda. Beberapa Sesar terbentuk di Pulau Jawa mengakomodasi stress yang dihasilkan oleh subduksi jawa yang berada di selatan Pulau Jawa. Studi deformasi dengan menggunakan data GNSS telah dilakukan untuk mengestimasi laju geser dari sesar-sesar utama di Pulau Jawa. Koulali dkk (2016) mengestimasi laju geser untuk Sesar Baribis dan Sesar Kendeng sebesar 2.3 – 5.6 mm/tahun dan dinyatakan sebagai sesar-sesar aktif. Pada studi ini, 15 data GNSS kontinyu dari tahun 2010 hingga 2016 di bagian timur Pulau Jawa digunakan untuk mengidentifikasi mekanisme sesar yang berada di wilayah ini meliputi Sesar Kendeng dan ekstensinya. Data fase GPS dari setiap stasiun GNSS diolah dengan menggunakan GAMIT/GLOBK 10.6 untuk mendapatkan koordinat di dalam sistem koordinat kartesian 3D di dalam kerangka referensi International Terrestrial Reference Frame 2008 (ITRF2008). Sebanyak 15 vektor kecepatan GNSS digunakan untuk menghitung strain rate dan laju geser untuk setiap segmen sesar yang dilalui oleh 3 profil. Ketiga profil tersebut menunjukkan adanya kompresi sebagai akomodasi stress dari subduksi Jawa dan laju geser untuk segmen barat Sesar Kendeng, segmen timur Sesar Kendeng, dan ekstensinya sebesar 1.93 mm/tahun, 0.90 mm/tahun, dan 0.60 mm/tahun secara berurutan dengan mekanisme sesar mengiri. Mekanisme yang sama yang terjadi pada ekstensi Sesar Kendeng menunjukkan adanya potensi sumber gempa yang baru di sekitar Selat Madura. Hal ini merupakan informasi penting untuk mengidentifikasi potensi sumber gempa dari Sesar Kendeng dan ekstensinya mengingat zona dari sesar aktif ini merupakan zona yang berpenduduk cukup padat.
In April 2014, after about 20 yrs of relatively low seismicity, an energetic earthquake sequence (maximum ML 4.8) began 25-30 km northwest of the 1983 Ms 7.3 Borah Peak earthquake rupture area near the town of Challis, Idaho. This sequence ended in the fall of 2014, but in January 2015, a second energetic sequence (maximum ML 5.0) began about 20 km to the southeast. Modest seismicity has continued in both regions with 1000 earthquakes detected and located through May 2017. To better characterize the seismicity in the area, we deployed a seven-station local seismometer network during April-October 2014; one of the stations remained active until July 2015. Here, we report updated locations for earthquakes in the Challis area for 1 January 2014-31 May 2017. Using a combination of absolute and differential arrival times, we generated a catalog of high-accuracy relocations. The earthquakes clustered into four primary groups, three of them with strikes similar to the Lost River fault-the fault responsible for the 1983 Borah Peak event. We used a modified cut-and-paste method to determine moment tensors for 15 of the largest events. All of the moment tensors showed normal faulting with nodal plane strikes consistent with the trend of the relocated seismicity and the regional stress field. We suggest that the recent seismicity near Challis is best interpreted as a continuation of the 1983 Ms 7.3 Borah Peak aftershock sequence, which is unusually long compared to plate boundary aftershock sequences because of the lower regional strain rate.
The Columbia River Basalt Group covers an area of more than 210,000 km2 with an estimated volume of 210,000 km3. As the youngest continental flood-basalt province on Earth (16.7–5.5 Ma), it is well preserved, with a coherent and detailed stratigraphy exposed in the deep canyonlands of eastern Oregon and southeastern Washington. The Columbia River flood-basalt province is often cited as a model for the study of similar provinces worldwide. This field-trip guide explores the main source region of the Columbia River Basalt Group and is written for trip participants attending the 2017 International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI) Scientific Assembly in Portland, Oregon, USA. The first part of the guide provides an overview of the geologic features common in the Columbia River flood-basalt province and the stratigraphic terminology used in the Columbia River Basalt Group. The accompanying road log examines the stratigraphic evolution, eruption history, and structure of the province through a field examination of the lavas, dikes, and pyroclastic rocks of the Columbia River Basalt Group.
We derive surface velocities from GPS sites in the interior Northwest U.S. relative to a fixed North American reference frame to investigate surface tectonic kinematics from the Snake River Plain (SRP) to the Canadian border. The Centennial Tectonic Belt (CTB) on the northern margin of the SRP exhibits west directed extensional velocity gradients and strain distributions similar to the main Basin and Range Province (BRP) suggesting that the CTB is part of the BRP. North of the CTB, however, the vergence of velocities relative to North America switches from westward to eastward along with a concomitant rotation of the principal stress axes based on available seismic focal mechanisms, revealing paired extension in the northern Rockies and shortening across the Rocky Mountain Front. This change in orientation of surface velocities suggests that the change in the boundary conditions on the western margin of North America influences the direction of gravitational collapse of Laramide thickened crust. Throughout the study region, fault slip rate estimates calculated from the new geodetic velocity field are consistently larger than previously reported fault slip rates determined from limited geomorphic and paleoseismic studies.
Project HOTSPOT, the Snake River Scientific Drilling Project (International Continental Scientific Drilling Program), tested for deep geothermal resources and examined the petrology of volcanic rocks with three drillholes in the central and western Snake River Plain (western USA). The MH-2 drillhole targeted fractured crystalline and hydrothermally altered basalt in the area of the Mountain Home Air Force Base (Idaho) to a total depth of 1821 m. At 1745 m depth the drillhole encountered flowing artesian hydrothermal fluids of at least 150 °C. We integrate geological analyses of core, image log, and borehole geophysical data, and in situ stress analyses to describe the structural environment that produces permeability for artesian flow. The rocks in the lower 540 m of the drillhole consist of basalt flows as much as 30 m thick, altered basalt, and thin sedimentary horizons. The mechanical stratigraphy is defined by nine mechanical horizons that are in three ranges of rock strength on the basis of experimentally determined strength data, core logging, and geophysical log signatures. Hydrothermal alteration products and mineralization in the core are associated with three highly faulted sections; the lowermost section is associated with the zone of flowing thermal water. Shear slip indicators on faults observed in core indicate slip ranging from pure strike slip to normal failure mechanisms in the stronger horizons. The borehole breakouts indicate that the maximum horizontal stress, SH, is oriented 047° ± 7°, and drilling-induced tensile fractures indicate that SH is oriented at 67° ± 21°.The in situ stress orientations exhibit little variation over the depth of the measured interval, but the SH magnitude varies with depth, and is best explained by an oblique normal fault stress regime.The geomechanical model indicates that if pore pressures at depth are elevated above the normal hydrostatic gradient, as observed here, the system has the potential to deform by mixed normal and strike-slip failure. Our observations and interpretations suggest that the MH-2 borehole was drilled into oblique normal faults that intersect a buried 300°-trending fault block masked by the basaltic volcanic complex. These data indicate that the transition from the central to western Snake River Plain is characterized by complex structures developed in response to a transitional stress state related to Snake River Plain and western Basin and Range stress regimes. The western Basin and Range stress and tectonic regime may extend from northern Nevada into western Idaho and may enhance the potential for geothermal resources by creating interconnected fracture and fault-related permeability at depth.
The 50 km (31 mi) long Hat Creek fault, located along the western margin of the Modoc Plateau in northern California, is a geometrically complex segmented normal fault that offsets Pleistocene lavas by at least 570 m (1870 ft) of cumulative throw. Three subparallel, ~NNW-trending sets of scarps (Rim, Intermediate, and Recent) reflect a progressive westward migration of surface rupture locations that offset progressively younger Pleistocene volcanic deposits during a ~1 Myr fault history. The 50 km (31 mi) long Rim scarp comprises predominantly right-stepping segments with a maximum throw of ~370 m (1214 ft) in ~925 ka lavas. The 17.5 km (10.9 mi) long Intermediate scarp occurs 0.4 to 3.5 km (0.2–2.2 mi) west of the Rim, comprising left-stepping segments with a maximum throw of ~177 m (581 ft). The 30.5 km (19 mi) long Recent scarp occurs several tens of meters west of the bases of older scarps, and is composed of left-stepping segments with a maximum throw of 56 m (184 ft). The northernmost segment of the Recent scarp offsets 53.5 6 2 ka basaltic lavas, whereas the remaining segments offset 24 6 6 ka basalt flows that erupted into Hat Creek Valley, indicating a youthful scarp system. Vertical propagation of the fault through young lavas produced fault-trace monoclines with amplitudes of up to 30 m (98 ft). The monoclines are commonly breached along their upper hinges by a vertical, dilational fault scarp. Shaking associated with repeated earthquakes progressively broke down these monoclines, causing disaggregation or partial to complete collapse. Fracture patterns and fault segment geometries and linkages were used to deduce the kinematic and stress history. The oldest segments of the Rim and Intermediate systems suggest initial NE-SW to ENE-WSW extension. Later Rim, Intermediate, and Recent segments responded to E-W extension, consistent with the previously documented stress state of the Cascades backarc. Complexity in Intermediate and Recent fault segments near a small shield volcano (Cinder Butte) suggests spatial variability in the stress field caused by a currently dormant magmatic system. Evidence for recent dextral-oblique kinematics along the Recent scarp, implying a slightly WNW-ESE extension, may reflect the transfer of dextral shear into the system from the Walker Lane Belt in western Nevada. Our interpretations require ~45o of clockwise rotation of the horizontal principal stresses in the vicinity of the Hat Creek fault over the past ~1 Myr, implying that significant complexity can develop in segmented normal fault systems over relatively short periods of geologic time.
The Middle Miocene Columbia River Basalt Group (CRBG) is the youngest and
smallest continental flood basalt province on Earth, covering over 210,000 km2 of
Oregon, Washington, and Idaho and having a volume of 210,000 km3. A well-established
regional stratigraphic framework built upon seven formations, and using physical and
compositional characteristics of the flows, has allowed the areal extent and volume
of the individual flows and groups of flows to be calculated and correlated with their
respective dikes and vents. CRBG flows can be subdivided into either compound flows
or sheet flows, and are marked by a set of well-defined physical features that originated
during their emplacement and solidification. This field trip focuses on the Lewiston
Basin, in southeastern Washington, western Idaho, and northeastern Oregon, which
contains the Chief Joseph dike swarm, where classic features of both flows and dikes
can be easily observed, as well as tectonic features typical of those found elsewhere in
the flood basalt province.
The geologic characteristics and evolution of the Eastern Snake River Plain are consistent with transfer of large amounts of magma advected heat at mid- to upper-crustal levels. We speculate that some of this heat is stored below surficial basalt lavas, within underlying dense, hydrothermally altered rhyolites that infill large nested calderas. Cryptic hydrothermal systems may occur within caldera-related fracture systems, or megabreccias near caldera collapse scarps. Blind hot zones are also possible below young chemically evolved volcanoes on the ESRP. Other factors in addition to high temperatures and high heat flow that support EGS development are also prevalent on the ESRP. A number of deep exploration wells exist, and hundreds groundwater wells provide additional information on the character and makeup of the subsurface. The abundant cold groundwater resource can provide a significant heat sink for power generation, and also provide working fluid and makeup water as necessary. Regional stress and earthquake data suggest that the Plain is extensional to isotropic and "quiet". The seismic risks of the eastern Snake River Plain have been extensively investigated because of the nuclear facilities at Idaho National Laboratory. The epicenter of the 1983 (magnitude 7.3) Borah earthquake was located about 100km (70 miles) from INL's nuclear facilities, but no significant damage occurred. This is attributed to the alternating layers of hard basalt and soft sediment that lie beneath the Snake River Plain. This attenuation reduces the seismic risk from natural and induced seismic occurrences. Additional studies are currently underway to better understand the EGS potential for the eastern SRP. These include GIS and VSRP analysis of existing data, new geotechnical and geomechanical analysis of core samples, geochemical studies of water-rock interaction, and numerical modeling studies of fracturing, fluid flow, and heat recovery.
The Oregon-Idaho graben is a newly identified north-south–trending synvolcanic graben in southeastern Oregon and southwestern Idaho within the middle Miocene backarc rift system that extends 1100 km from southern Nevada to southeastern Washington. The graben formed along the western margin of the North American craton shortly after the largest volumes of tholeiitic flood basalt erupted (Columbia River Basalt Group, Steens-Pueblo Basalt, basalt of Malheur Gorge, basalt and latite unit of Ekren et al., 1981). Rhyolite flows and ash-flow tuffs (16.1–14.0 Ma) erupted from northeastern Oregon (Dooley Volcanics) to northern Nevada (McDermitt volcanic field) shortly after the flood basalt was emplaced. Subsidence of the Oregon-Idaho graben (15.5–15.3 Ma) coincides with eruption of rhyolite flows and caldera-related ash-flow tuffs from vents along the margins and within the graben. Mafic and silicic intragraben volcanism accompanied sedimentation from about 15.3 to 10.5 Ma. Sedimentary and volcanic rocks from extrabasinal sources, especially southwestern Idaho, were introduced periodically.
After initial subsidence, the evolution of the Oregon-Idaho graben is divided into three stages. Stage 1 (15.3–14.3 Ma) followed intragraben caldera collapse and was marked by deposition of fluvial and lacustrine sediment across the graben. Stage 2 (14.3–12.6 Ma) movement on intragraben fault zones divided the graben into distinct subbasins and marked the onset of calc-alkalic volcanism. Fine-grained tuffaceous sediment derived from glassy rhyolite and pyroclastic deposits and basalt tuff cones interbedded with rhyolite ash and lapilli-fall deposits and locally erupted basalt hydrovolcanic deposits predominated during synvolcanic subsidence. Synsedimentary hot-spring alteration and precious-metals mineralization of graben fill were controlled by the same intragraben fault zones that served as magmatic conduits. During stage 3 (12.6–10.5 Ma) the subbasins were filled, and graben-wide fluviatile and lacustrine sedimentation resumed. At about the same time, renewed rhyolitic volcanism occurred on both flanks, and tholeiitic volcanism resumed within the Oregon-Idaho graben. Subsidence in the Oregon-Idaho graben ceased as west-northwest–striking faults related to the formation of the western Snake River plain became active.
The International GNSS Service (IGS) is an international activity involving more than 200 participating organisations in over 80 countries with a track record of one and a half decades of successful operations.
Syndepositional features, interpreted as paleoseismites indicative of six late Quaternary surface ruptures, were exposed in a trench across the Lima Reservoir fault in the Centennial Valley of southwestern Montana. Younger events progressively displaced four loess deposits capped by paleosol, and overstepping unconformities. Older events are marked by paleo-sandblow vents, ejected sand, and a deformed stream-channel deposit. Fossils of horse (Equus) and northern pocket gopher (Thomomys talpoides), place a 120-10 ka range on the oldest event. Stratigraphy and soils suggest that it occurred during the last Pinedale interstade (ca. 20 ka). Line-length balancing and progressive retro-deformation of faults show that > 8.8 m of cumulative fault displacement with ∼2 m of horizontal extension occurred since then. Orientation of faults, clastic dikes, and slip vectors, along with earthquake focal mechanism solutions, were used to determine stress-field orientations for the six events. Normal faulting associated with northeast-southwest Basin and Range extension produced three events with 4.7 m offset, a displacement rate of ∼23.5 cm/k.y., and a recurrence interval of∼6.7 k.y. Oblique-reverse faulting associated with east-northeast-west-southwest compression produced two events with 3.7 m offset, clastic dike injection along joints or joint-reactivated faults in arched areas above the oblique-reverse faults, a displacement rate of ∼18.5 cm/k.y., and a recurrence interval of ∼10 k.y. Reverse faulting, associated with northsouth compression, produced clastic dike injection during one event with 0.4 m offset, a displacement rate of <2 cm/k.y., and a recurrence interval of >20 k.y.
The Oregon-Idaho graben is a newly identified north-south-trending synvolcanic graben in southeastern Oregon and southwestern Idaho within the middle Miocene backarc rift system that extends 1100 km from southern Nevada to southeastern Washington. The graben formed along the western margin of the North American craton shortly after the largest volumes of tholeiitic flood basalt erupted (Columbia River Basalt Group, Steens-Pueblo Basalt, basalt of Malheur Gorge, basalt and latite unit of Ekren et al., 1981). Rhyolite flows and ash-flow tuffs (16.1-14.0 Ma) erupted from northeastern Oregon (Dooley Volcanics) to northern Nevada (McDermitt volcanic field) shortly after the flood basalt was emplaced. Subsidence of the Oregon-Idaho graben (15.5-15.3 Ma) coincides with eruption of rhyolite flows and caldera-related ash-flow tuffs from vents along the margins and within the graben. Mafic and silicic intragraben volcanism accompanied sedimentation from about 15.3 to 10.5 Ma. Sedimentary and volcanic rocks from extrabasinal sources, especially southwestern Idaho, were introduced periodically. After initial subsidence, the evolution of the Oregon-Idaho graben is divided into three stages. Stage 1 (15.3-14.3 Ma) followed intragraben caldera collapse and was marked by deposition of fluvial and lacustrine sediment across the graben. Stage 2 (14.3-12.6 Ma) movement on intragraben fault zones divided the graben into distinct subbasins and marked the onset of calc-alkalic volcanism. Fine-grained tuffaceous sediment derived from glassy rhyolite and pyroclastic deposits and basalt tuff cones interbedded with rhyolite ash and lapilli-fall deposits and locally erupted basalt hydrovolcanic deposits predominated during synvolcanic subsidence. Synsedimentary hot-spring alteration and precious-metals mineralization of graben fill were controlled by the same intragraben fault zones that served as magmatic conduits. During stage 3 (12.6-10.5 Ma) the subbasins were filled, and graben-wide fluviatile and lacustrine sedimentation resumed. At about the same time, renewed rhyolitic volcanism occurred on both flanks, and tholeiitic volcanism resumed within the Oregon-Idaho graben. Subsidence in the Oregon-Idaho graben ceased as west-northwest-striking faults related to the formation of the western Snake River plain became active.
Partitioning of slip at oblique subduction zones is common and fre-quently results in lithospheric blocks being detached from the overriding plate. A method is described to solve simultaneously for both block rotations on a sphere and locking on block-bounding faults using GPS vectors and other geo-physical data. Example cases from Sumatra, Oregon, and Costa Rica, where mobile blocks are suspected, show that considering both block rotations and fault locking significantly improves the fit to the data over models that consider only one of them.
The deformational history of the eastern Snake River Plain (SRP) is interpreted from rocks, structures, and land-forms within and adjacent to it. Crustal extension is mani-fested by west-dipping normal faults that define a half-graben fault style along the north and south margins of the plain. Cumulative extension averages 15-20 percent and diminishes only slightly from the south margin to the north, evidence of no lateral shear beneath the plain. Miocene-Quaternary volcanic and sedimentary rocks fill the half grabens and provide evidence of two pulses of extension that migrated northeast through time. One pulse of minor extension began 16-11 Ma and ended 11-9 Ma, and a second pulse of major extension began 11-9 Ma and lasted a few million years in any one location as it migrated eastward to the eastern edge of the Basin and Range, where it continues today. Because many Miocene faults project into the eastern SRP with no evidence of diminishing offset, normal faults are interpreted to have characterized the eastern SRP in middle and late Miocene time. Since then, upper crustal extension has been ac-commodated primarily by mafic dike injection. Subsidence of eastern SRP rocks relative to Basin and Range rocks is manifested by the accumulation of a thick volcanic succession, crustal flexure of its margins, and marginal normal faults. Northeast-striking marginal nor-mal faults only accommodated 2-5 percent extension as-sociated with crustal flexure, not downfaulting of east-ern SRP crust. Crustal flexure along the northern margin of the eastern SRP is evidenced by Cretaceous fold hinges whose southward plunges increase regularly toward the plain. Isoplunge contours define a zone of flexure about 20 km wide that accommodated 4.5-8.5 km of subsid-ence of eastern SRP rocks. Age-depth relations of east-ern SRP volcanic rocks indicate 1.7 km of rock subsid-ence since 8.0 Ma, 3.1 km since 8.5 Ma, and the initia-tion of subsidence before 10 Ma. Subsidence of the eastern SRP surface relative to the Basin and Range surface is manifested by axial drainage that extends 50 km beyond the SRP margins. Streams that are tributary to the Snake River have deeply incised Basin and Range valleys along the southern margin of the eastern SRP, a process attributed to SRP surface sub-sidence and consequent base level lowering of the Snake River. The age and amount of surface subsidence are re-corded by a strath terrace near Pocatello that cuts across tilted 7.3-Ma basin fill and is perched 900 m above the modern plain. Drainage reversals at tributary stream head-waters provide evidence of ongoing Pleistocene subsid-ence. Space-time patterns of extension and subsidence are used to support a three-stage model of deformation. Stage 1 involved minor half-graben normal faulting and possi-bly regional subsidence of the proto-eastern SRP. Stage 2 involved regional rock subsidence and major extension via normal faulting. Extension was focused slightly east of coeval silicic magmatism, and both migrated north-east after 11 Ma. Deformation and coeval magmatism were widely dispersed across the plain at about 10 Ma. Stage 3 has involved ongoing rock subsidence, surface 122 Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province subsidence after about 7 Ma, and extension via dike in-jection after about 4 Ma. Stage 3 deformation has been located west of coeval silicic magmatism.
Version 3.1 of the Generic Mapping Tools (GMT) has been released. More than 6000 scientists worldwide are currently using this free, public domain collection of UNIX tools that contains programs serving a variety of research functions. GMT allows users to manipulate (x,y) and (x,y,z) data, and generate PostScript illustrations, including simple x-y diagrams, contour maps, color images, and artificially illuminated, perspective, and/or shaded-relief plots using a variety of map projections (see Wessel and Smith [1991] and Wessel and Smith [1995], for details.). GMT has been installed under UNIX on most types of workstations and both IBM-compatible and Macintosh personal computers.
Geophysical (potential-field) data can provide an efficient means of delineating subsurface geology and structure over areas at varying scales. We are using regional aeromagnetic and gravity surveys, and their various derivative maps (e.g., Blakely, 1995; Hildenbrand and others, 2000), to investigate the geologic and tectonic framework of the Columbia–Missouri Rivers headwaters region (herein referred to as the study area), which encompasses the Northern High Plains and the Northern Rocky Mountains. The major focus of the project is to develop regional digital earth science data and interpretations that can be used by Federal land managers in regional planning and special assessments such as road-less area withdrawals. The study area encompasses all lands managed by the U.S. Forest Service (USFS) in Idaho north of the Snake River Plain and in western Montana. Geophysical techniques can delineate major crustal boundaries and deep crustal structures that may be localizing mineral deposits and controlling regional fluid flow. These techniques are particularly useful for investigating areas that are covered by thick, surficial deposits. Our purpose here is to describe, briefly, techniques to enhance particular anomaly characteristics (e.g., trend or wavelength), present the resulting maps, and suggest ways in which they may prove most useful. In-depth interpretations are deferred to subsequent publications.
Microearthquake monitoring suggests that infrequent, small-magnitude earthquakes are charactersitic of eastern Snake River Plain (ESRP) seismicity. Although a total of only 19 earthquakes have been observed to date, their relatively shallow occurrence at depths of 8km is consistent with the hypothesis that elevated crustal temperatures in the ESRP confine the brittle portion of the crust to the upper 6 to 10km. A composite focal mechanism of two microearthquakes located near the axis of the ESRP indicates normal faulting with a minor component of strike-slip motion. -from Authors
The Eastern Snake River Plain (ESRP) is a linear volcanic basin
interpreted by many workers to reflect late Cenozoic migration of North
America over the Yellowstone hotspot. Thermal subsidence of this
volcanic province with respect to Yellowstone has been documented by
several workers, but no one has characterized subsidence with respect to
the adjacent Basin and Range Province. This paper documents crustal
flexure along the northwest edge of the ESRP, uses flexure to model the
dimensions of a dense load beneath the basin, and presents evidence in
support of density-driven subsidence and lower crustal flow away from
the basin. Crustal flexure adjacent to the ESRP is reflected by the
attitudes of Mesozoic fold hinges and Neogene volcanic rocks. Fold
hinges formed with a subhorizontal plunge and a trend perpendicular to
the ESRP but now show a southward plunge near the ESRP of as much as
20°-25°. We present a contour map of equal fold plunges proximal
to the ESRP that shows flexure is roughly parallel to and extends 10-20
km north of the average edge of the ESRP. Flexural profiles indicate the
minimum amount of ESRP subsidence, with respect to the Basin and Range;
subsidence ranges from 4.5 to 8.5 km. The structural contour map and
published seismic and gravity data were used to develop and constrain
flexural subsidence models. These models indicate the flexed crust is
very weak (flexural parameter of 4-10 km), interpreted to be a result of
the high heat flow of the ESRP. Assuming subsidence was induced by
emplacement of a dense crustal layer beneath the ESRP, a midcrustal
"sill" identified in previous seismic surveys is too wide and probably
too thin to produce the measured flexure. New dimensions include a
thickness of 17-25 km and a half width of 40-50 km, which place the edge
of the sill beneath the edge of the ESRP. The dimensions of the ESRP
sill are based on isostatic compensation in the lower crust because
compensation in the asthenosphere requires an unreasonable sill
thickness of 30+ km and because ESRP seismic, gravity, and heat flow
data support lower crustal compensation. Density-driven lower crustal
flow away from the ESRP is proposed to accommodate subsidence and
maintain isostatic equilibrium. Timing of subsidence is constrained by
ESRP exploratory wells, where 6.6 Ma rhyolites at a depth of 1.5 km
indicate most subsidence occurred prior to their emplacement, and by
strong spatial correlations between plunge contours and Quaternary
volcanic rift zones. Two processes interpreted to contribute to the load
include an extensive midcrustal mafic load emplaced at ˜10 Ma,
which provided the heat source for the initial rhyolitic volcanism on
the ESRP, and continuing, localized loads from dikes and sills
associated with Quaternary basalts. Widespread ˜10 Ma magmatism
and subsidence conflicts with simple time-transgressive migration of the
Yellowstone hotspot, indicating a need for revision of the hotspot
paradigm.
The spatial and temporal distributions of Cenozoic extension and magmatism in the Cordillera suggest that the onset of major crustal extension at a particular latitude was confined to a relatively narrow belt (<100 km, pre-extension) and followed the onset of intermediate and silicic magmatism by no more than a few million years. Extension began in early Eocene time in southern British Columbia, northern Washington, Idaho and Montana. Farther S, extension began at about the Eocene-Oligocene boundary in the Great Basin and slightly later in the Mojave-Sonora Desert region. The intervening area, at the latitude of Las Vegas, remained quiescent until mid-Miocene time. Compositional and isotopic characteristics of most pre-Miocene magmas are consistent with their containing major components of melted continental crust.
In mid-Miocene time, two major changes occurred: widening of the area of extension and the widespread appearance of basaltic magmas. The area affected by extension, from southwestern Montana to the Lake Mead region, widened to several hundred kilometres. By this time extension in southern British Columbia, northern Washington and northern Montana had ceased (probably before the end of the Eocene), and extension S of Lake Mead (except in the Gulf of California) had waned. Regions affected by the broader belt of extension during late Miocene, Pliocene, and Quaternary time experienced basaltic magmatism, which began along a central rift zone in the northern part of the region, and which had within a few million years spread to include most of the region; later basaltic activity has tended to concentrate in restricted zones, especially near the margins of the extended area.
We recognize a correlation between the amount of earlier crustal thickening and Cenozoic extension, and between the length of time after shortening but before extension and the degree to which a given region was intruded by Late Cretaceous plutons. The localization of extension in areas of previous crustal thickening and the dependence of the timing of extension on the thermal state of the overthickened crust is consistent with a simple thermal-mechanical model developed in a companion paper (Sonder et al. ). This raises the possibility that stresses inherent in the North American Plate dominated over plate-interaction forces as controls of the Cenozoic tectonomagmatic evolution of the North American Cordillera, especially in its earlier stages.
The Boise River Group of late Cenozoic age consists of basaltic lavas and intercalated fluvial and lacustrine sediments. Basalts of the Boise River Group can be divided into two groups based on major and trace element chemistry. Boise River Group 1 basalts (BRG 1), are silica-saturated olivine tholeiites charcterized by low alkalis, Mg/Fe, and Ni, high concentrations of high field strength elements (HFSE), and enrichment in the heavy isotopes of Sr, Nd, and Pb. Boise River Group 2 basalts (BRG 2) are all younger than BRG 1(<0.7 my) and are transitional between olivine tholeiites and alkali olivine basalts. BRG 2 basalts are characterized by high alkalis, Mg/Fe, and Ni, lower HFSE concentrations, and isotopic compositions of Sr and Nd near bulk earth. These lavas have distinct geochemical characteristics and are not cogenitic with the older, BRG 1 flows or with coeval lavas of the Snake River Plain. The high Mg numbers and Ni of the younger BRG 2 basalts are not consistent with enrichment of the light field strength elements by crustal assimilation; this enrichment must reflect a mantle source region characteristic. -from Authors
Interest and controversy exist on the origin of forces that move and tectonically deform plates, especially regarding the relative importance of loads applied to the plate margins and base and those created internally (e.g., by elevated potential energy in uplifted regions). To quantify these loads, we evaluate predicted interplate stress through two-dimensional finite element analysis of the North American plate, finding that boundary loads are most important, followed by internal and basal loads. Craton root basal drag of ∼4 MPa opposes absolute plate motion, compared to basal tractions elsewhere that average ∼0.4 MPa, suggesting that North America is separated from a relatively static deep Earth mantle by a weak asthenosphere. San Andreas shear (∼1.5 TN/m), gravitational collapse, and southern Cascadia pull all contribute importantly to western U.S. deformation; the region also is relatively weak. Important future work includes incorporating three-dimensional plate structure onto global flow calculations and including the global set of plates.
GPS measurements in the northwestern US and adjacent parts of Canada describe the relative motions of crustal blocks, the interseismic friction on faults, and the permanent deformation associated with convergence between the Juan de Fuca and North American plates. To estimate angular velocities of the oceanic Juan de Fuca and Explorer plates and several continental crustal blocks, we invert the GPS velocities with seafloor spreading rates, earthquake slip vector azimuths, fault slip azimuths and rates. We also determine the distribution of frictional properties on the block-bounding faults. The Cascadia megathrust is locked offshore, except in southern Oregon, where significant locking extends inland. Most of Oregon and southwest Washington rotate clockwise relative to North America at rates of 0.4 to 1.0°/Myr. No shear or extension along the Cascades volcanic arc is occurring at the mm/yr level during the past decade. The agreement of spin rates derived from GPS velocities with those estimated from paleomagnetic declination anomalies suggests that the rotations have been steady for several millions of years. Rotations in the PNW do not result in net westward flux of crustal material. Rotation of Oregon and western Washington indicates that the rate of permanent shortening, the type that causes upper plate earthquakes, across the Puget Sound region is 4.4±0.3 mm/yr.
The development of the Snake River Plain volcanic province has been attributed to the passage of the North American plate over the Yellowstone mantle plume. As this province was being formed by an eastward progres- sion of younger silicic volcanic centers, basaltic volcan- ism has continued through the present day in the western regions of the "plume track." Of particular interest is the Owyhee Plateau of southeastern Oregon, located at the westernmost end of the SRP volcanic province. The Owyhee Plateau preserves the best documented continu- ous record of mid-Miocene to Recent basaltic volcanism in the northwestern United States, allowing an investiga- tion of variations in magmatic sources and processes through time. In addition, this region is underlain by litho- sphere transitional between that of the Proterozoic- Archean Wyoming Craton to the east and accreted ter- ranes younger than 200 Ma to the west. Basaltic prod- ucts that erupted from a variety of vents on the Owyhee Plateau illustrate chemical and Sr isotopic diversity that cannot be solely attributed to lateral lithospheric hetero- geneities. Rather, this diversity appears to be a function of eruptive age. For example, Sr isotope data show a sys- tematic increase in 87 Sr/ 86 Sr from 0.704 to 0.707 with decreasing age from 17.5-11 Ma, but decreasing
The western Idaho suture zone juxtaposes continental and oceanic terranes with distinctive Sr and Nd isotopic signatures across a sharp isotopic transition coincident with the Mesozoic 87Sr/86Sr = 0.706 line (MSL). Late Cretaceous to Eocene granitic plutons that define the MSL are allochthonous and were transported eastward by ˜150 km during late Mesozoic to Tertiary (ca. 100-55 Ma) thrusting. Sr and Nd isotopic compositions of Neogene rhyolites and basalts from southwestern Idaho, southeastern Oregon, and northern Nevada define isotopic transitions coincident with the MSL; there are continental affinities east of long 116°30'W and oceanic affinities west of 119°W. Between these longitudes, intermediate isotopic compositions imply derivation of magmas from both continental and oceanic mantle (basalts) and crustal (rhyolites) sources. It is inferred that the basal decollement of the Mesozoic and Tertiary contractional orogen dips westward below depths of segregation of basaltic magmas, and that oceanic lithospheric mantle was thrust eastward over a shelf of old, isotopically evolved continental lithospheric mantle that now protrudes ˜150 km west of the MSL.
We have estimated patterns and rates of crustal movement across 800 km of the Basin and Range at ~39° north latitude with Global Positioning System surveys in 1992, 1996, 1998, and 2002. The total rate of motion tangent to the small circle around the Pacific-North America pole of rotation is 10.4 +/- 1.0 mm/yr, and motion normal to this small circle is 3.9 +/- 0.9 mm/yr compared to the east end of our network. On the Colorado Plateau the east end of our network moves by ~1-2 mm/yr westerly with respect to North America. Transitions in strain rates delimit six major tectonic domains within the province. These deformation zones coincide with areas of modern seismicity and are, from east to west, (1) east-west extension in the Wasatch Fault zone, (2) low rate east-west extension centered near the Nevada-Utah border, (3) low rate east-west contraction between 114.7°W and 117.9°W, (4) extension normal to and strike-slip motion across the N10°E striking Central Nevada Seismic Zone, (5) right lateral simple shear oriented N13°W inside the Walker Lane Belt, and (6) shear plus extension near the Sierra Nevada frontal faults. Concentration of shear and dilatational deformation across the three westernmost zones suggests that the Walker Lane Belt lithosphere is rheologically weak. However, we show that linear gradients in viscosity and gravitational potential energy can also effectively concentrate deformation. In the Basin and Range, gradients in gravitational potential are spatially anticorrelated with dilatational strain rates, consistent with the presence of horizontal variations in viscosity of the lithosphere.
The active deformation of the southwestern United States (30°–41°N) is represented by a finite number of rotating, elastic-plastic spherical caps. GPS-derived horizontal velocities, geologic fault slip rates, transform fault azimuths, and earthquake-derived fault slip vector azimuths are inverted for block angular velocities, creep on block-bounding faults, permanent strain rates within the blocks, and the rotations of 11 published GPS velocity fields into to a common North American reference frame. GPS velocities are considered to be a combination of rigid block rotations, recoverable elastic strain rates resulting from friction on block-bounding faults, and nonrecoverable strain rates resulting from slip on faults within the blocks. The resulting Pacific–North America angular velocity is similar to some published estimates and satisfies transform azimuths and one spreading rate in the Gulf of California, earthquake slip vectors in the Gulf of California and Alaska, and GPS velocities along coastal California and within the Pacific Basin. Published fault slip rates are satisfied except in the southern Mojave Desert where the motion of the Mohave block relative to North America is faster than can be explained by mapped faults. The largest blocks, the Sierra Nevada–Great Valley and the eastern Basin and Range, show permanent strain rates, after removing elastic strain, of only a few nanostrain per year, demonstrating approximately rigid behavior. Observed horizontal strain rates correlate strongly with predicted strain rates from known faults suggesting that the short-term strains evident in GPS velocities are largely elastic. In only about 20% of the region is distributed deformation needed to match the data, indicating that a plate tectonic style description of the deformation of the western United States is plausible. Most blocks rotate about vertical axes at approximately the same rate as the Pacific (relative to North America), suggesting that locally, spin rates are communicated from block to block, arguing against both floating block and ball-bearing mechanisms of block rotation. The similarities of the blocks' spin rates to that of the Pacific suggests that the Pacific strongly influences their motions through edge tractions. However, it is shown that the blocks cannot rotate about the Pacific–North America pole without spinning counter to the sense of Pacific–North America shear. Unlike some other broad plate boundaries, in the western United States, vertical axis rotations take up very little of the slip rate budget across the region.
We infer rates of crustal deformation in the northern Walker Lane (NWL) and western Basin and Range using data from the Mobile Array of GPS for Nevada transtension, and other continuous GPS networks including the EarthScope Plate Boundary Observatory. We present 224 new GPS velocities, correct them for the effects of viscoelastic postseismic relaxation, and use them to constrain a block model to estimate fault slip rates. The data segregate the NWL into domains based on differences in deformation rate, pattern, and style. Deformation is transtensional, with highest rates near the western and eastern edges of the NWL. Some basins, e.g., Tahoe, experience shear deformation and extension. Normal slip is distributed throughout the NWL and Basin and Range, where 11 subparallel range-bounding normal fault systems have an average horizontal extension rate of 0.1 mm/yr. Comparison between geologic and geodetic slip rates indicates that out of 12 published geologic rates, 10 agree with geodetic rates to within uncertainties. This suggests that smaller crustal blocks move steadily, similar to larger lithospheric plates, and that geodetic measurements of slip rates are reliable in zones of complex crustal deformation. For the two slip rates that disagree, geologic rates are greater. The vertical axis rotation rate of the Carson domain is -1.3 0.1/My clockwise, lower than the 3 to 6/My obtained in paleomagnetic measurements. This suggests that vertical axis rotation rates may have decreased over the last 9-13 My as the role of faulting has increased at the expense of rigid rotations.
The eastern Snake River Plain of southern Idaho poses a paradoxical problem because it is nearly aseismic and unfaulted although it appears to be actively extending in a SW-NE direction continuously with the adjacent block-faulted Basin and Range Province. The plain represents the 100-km-wide track of the Yellowstone hotspot during the last ∼16–17 m.y., and its crust has been heavily intruded by mafic magma, some of which has erupted to the surface as extensive basalt flows. Outside the plain's distinct topographic boundaries is a transition zone 30–100 km wide that has variable expression of normal faulting and magmatic activity as compared with the surrounding Basin and Range Province. Many models for the evolution of the Snake River Plain have as an integral component the suggestion that the crust of the plain became strong enough through basaltic intrusion to resist extensional deformation. However, both the boundaries of the plain and its transition zone lack any evidence of zones of strike slip or other accommodation that would allow the plain to remain intact while the Basin and Range Province extended around it; instead, the plain is coupled to its surroundings and extending with them. We estimate strain rates for the northern Basin and Range Province from various lines of evidence and show that these strains would far exceed the elastic limit of any rocks coupled to the Basin and Range; thus, if the plain is extending along with its surroundings, as the geologic evidence indicates, it must be doing so by a nearly aseismic process. Evidence of the process is provided by volcanic rift zones, indicators of subsurface dikes, which trend across the plain perpendicular to its axis. We suggest that variable magmatic strain accommodation, by emplacement and inflation of dikes perpendicular to the least principal stress in the elastic crust, allows the crust of the plain to extend nearly aseismically. Dike injection releases accumulated elastic strain but generates only the small earthquakes associated with dike propagation. The rate of dike emplacement required to accommodate the estimated longitudinal strain rate of the plain is roughly a composite width of 10 m every 1000 years for the geologically youngest and most active part of the plain. The locus of most rapid intrusion and strain has migrated toward Yellowstone and is now in the northeastern 100–150 km of the plain. Reduced magmatic input in the transition zone of the plain causes the transitional expression of seismicity and faulting there.
Receiver functions from seismic stations about the Yellowstone hot spot track are migrated to depth using a Vp/Vs map constructed from stacking of the direct and free surface Moho reverberations (i.e., H-K analysis) and a shear velocity tomogram constructed from surface wave measurements. The thickest crust (48–54 km) resides in the Wyoming province beneath the sampled Laramide age blocks, and the thinnest crust (32–37 km) resides in the Montana Basin and Range province. The eastern Snake River Plain (ESRP) crust is thickest (47 km) at its NE end beneath the young calderas and thinnest (40 km) at its SW end beneath the older Twin Falls caldera. Two ESRP crustal thickness domains are found: (1) at the older Twin Falls and Picabo calderas, the mean ESRP crust is 4 km thicker with respect to its margins and (2) adjacent to the Heise caldera field, the mean ESRP crust is 4 km thicker with respect to its SE margin crust but no thicker with respect to its NW margin crust. This lobe of anomalously thick crust is explained as resulting from lower crustal outflow from beneath the Heise caldera field. Confirmation of these crustal thickness variations is provided by inspection of common conversion point (CCP) stacks that delineate several secondary features: the top of a thick high-velocity (3.9 km/s) lower crust layer within the Wyoming province up to 17 km thick and a paired negative and positive amplitude arrival at 12 km depth and 18 km depth beneath the Yellowstone Caldera. This paired arrival would be consistent with a low-velocity zone perhaps associated with magma staging beneath the caldera. Our most important finding is that the magmatic loads injected into the ESRP crust over the last 4–12 Myr, in tandem with the ESRP crustal viscosity structure, have been sufficient to drive significant outflow of the ESRP lower crust.
Crustal thicknesses in the Intermontane system-Colorado Plateau, Basin and Range province, and High Lava Plains of the Cordilleran region-have been reliably determined by an extensive network of seismic-refraction profiles as about 30 km in the Basin and Range and 42 km in both the Snake River Plain and the Colorado Plateau. The upper crust (velocity, 5.2 to 6.3 km/sec) averages about 20 km in thickness in the Basin and Range, 10 km in the Snake River Plain, and 28 km in the Colorado Plateau. The velocity in the lower crust ranges from 6.4 to possibly 7.5 km/sec. Upper-mantle velocities of 7.4 to 8.0 km/sec have been reported, but the lower P, velocities are probably apparent downdip velocities. The true upper-mantle velocity throughout the Intermontane system is probably 7.8 to 7.9 km/sec. Moho depths from seismic-reflection studies are about the same as those from refraction studies in the Basin and Range, but they are 5 to 10 km deeper in the Colorado Plateau. The patterns of crustal reflections are distinctly different in the western and eastern parts of the Basin and Range. Geophysical studies indicate that the lithosphere is 50 to 65 km thick in the Basin and Range, 50 km in the Snake River Plain, and 90 to 100 km in the Colorado Plateau. Bouguer gravity values range from -50 mGal in southwestern Arizona to -250 mGal in the Uinta basin of northeastern Utah. Gravity correlates inversely with topography in the Basin and Range, indicating isostatic equilibrium, but the Snake River Plain is slightly undercompensated. Long-wavelength gravity anomalies indicate that much of the isostatic compensation results from a mass deficiency in the mantle, probably related to variations in the thickness of the lithosphere. Prominent magnetic-high anomalies in the Snake River Plain and north-central Nevada result from mafic rock formed in Miocene rifts. Heat flow is high throughout the Intermontane system, ranging from 1.5 heat-flow units (HFU) in central Nevada and the Colorado Plateau to more than 2.5 HFU in northwestern Nevada and the Snake River Plain. Electrical conductivity is anomalously high in the lower crust and upper mantle of the system. Seismicity is distributed in broad bands in the Nevada and intermountain seismic zones, and epicenters commonly do not coincide with mapped faults; the larger earthquakes (magnitude greater than 6.0), however, occur only on planar, normal faults with dips of 50 to 60°. The stress field is compatible with lateral extension in the Basin and Range. Examination of evidence for a "double Moho" in Utah suggests that the apparent velocities associated with the Moho propagation paths on which this model is based vary widely, indicating severe relief on the refractor surface; the evidence favors a single Moho, below which the velocity is 7.8 to 7.9 km/sec. Ray tracing and amplitude studies leave unresolved the question of whether a transitional layer of velocity 7.5 km/sec exists in the lower crust of the Basin and Range. Many geologic and geophysical characteristics are similar throughout the Intermontane system, but lithospheric models are distinctly different in the separate provinces, owing to the different geologic processes that have prevailed. The crust throughout the system prior to Cenozoic extension and volcanism probably consisted of an upper layer of silicic gneisses and schists overlying a lower crust of silicic to intermediate composition in the granulite facies. During extension, the crust of the Basin and Range was thinned and invaded by gabbroic magma from the mantle, resulting in a lower crust consisting of as much as two-thirds mafic material. During crustal rifting or extension in the Snake River Plain, the crust was extensively invaded by mafic magma from the mantle, and a silicic fraction was fused and erupted as rhyolite. The Snake River Plain crust is now predominantly mafic, and the 10-km-thick upper crust consists primarily of sedimentary and volcanic rocks. Resolution of remaining unsolved problems must await thorough reanalysis of available data and new field experiments based on coincident application of different geophysical methods, combined with geologic studies and deep drilling.
The rhyolite plateau of Yellowstone National Park and nearby areas is part of the latest Pliocene and Quaternary Yellowstone Plateau volcanic field, which originally covered nearly 17,000 km2 but has since been disrupted by erosion and by continued volcanism and tectonism. Igneous rocks of the field consist predominantly of rhyolites and subordinately of basalts; there are virtually none of intermediate compositions. The rhyolites comprise numerous lava flows and three major sheets of welded ash-flow tuff separated by unconformities. Stratigraphically these sheets - from oldest to youngest, the Huckleberry Ridge, Mesa Falls, and Lava Creek Tuffs - constitute the Yellowstone Group. The geologic history of the field defines three cycles, in each of which the sequence of volcanic events was similar, climaxed by the eruption of a voluminous sheet of rhyolitic ash-flow tuff and the formation of a large caldera. Preceding and succeeding events included eruptions of rhyolitic lavas and tuffs in and near the source areas of the ash-flow sheets and the eruption of basalts around the margins of major rhyolitic volcanism. Before the first eruptions, the area was a mountainous terrain built by regional uplift and normal faulting; there was no extensive basin or plateau before about 2 Ma. Volcanism much like that soon to begin in the Yellowstone region was active 50 to 150 km west, in the eastern Snake River Plain. The first cycle of the Yellowstone Plateau volcanic field began just before 2 Ma. The oldest recognized rocks, erupted between about 2.2 and 2.1 Ma, are basalts in northern and eastern Yellowstone National Park and a rhyolitic lava flow at the south end of Island Park, Idaho. The oldest ash-flow sheet of the Yellowstone Group, the Huckleberry Ridge Tuff, was erupted at 2.1 Ma and was emplaced as a single cooling unit of more than 2,450 km3 over an area of 15,500 km2. Collapse of the roof of the Huckleberry Ridge magma chamber formed a caldera more than 75 km long, extending from Island Park into central Yellowstone National Park and probably consisting of three overlapping but distinct collapse zones over separate high-level parts of the magma chamber. Three subsheets of the Huckleberry Ridge Tuff can be related to these three caldera segments. Rhyolitic lava flows west of Island Park are, in part, postcollapse lavas of the first cycle that overflowed the caldera rim; others may be buried within the caldera. Basalts again erupted in northern Yellowstone Park later in the first volcanic cycle. The second cycle was simpler than the other two. Rocks of this cycle are present just west of Yellowstone National Park and probably are buried beneath the Yellowstone Plateau. Early second-cycle rhyolite flows crop out west of Island Park. The Mesa Falls Tuff, exposed near Island Park, is a cooling unit of more than 280 km3, erupted at 1.3 Ma within the northwestern part of the first-cycle caldera. This locus of eruption largely restricted the Mesa Falls within the south and east walls of the older caldera. Collapse of the Mesa Falls magma-chamber roof formed another caldera about 16 km in diameter, nested against the northwest wall of the first-cycle caldera. Postcollapse rhyolite domes erupted within and adjacent to this second-cycle caldera, and basaltic lavas erupted intermittently around the margins of the volcanic plateau, particularly southeast of Island Park. The third cycle perhaps overlapped the second, beginning about 1.2 Ma with eruption of rhyolitic lavas and related tuffs around a growing annular fissure system encircling central Yellowstone National Park. Flows vented periodically along this fissure system for about 600,000 years until ring-fracture development was terminated by rapid and voluminous ash-flow eruptions of the Lava Creek Tuff at 640 ka, probably through the same ring-fracture zone. These ash flows buried more than 7,500 km2. Collapse occurred along the ring-fracture zone immediately after, and perhaps during, the eruption of the more than 1,000 km3 of Lava Creek Tuff to form the Yellowstone caldera. Both the Lava Creek Tuff and the Yellowstone caldera display clear evidence for ash-flow eruptions from two separate high-level culminations of the Lava Creek magma chamber, analogous to those related to the three first-cycle caldera segments. The Lava Creek Tuff is a single cooling unit with two distinct subsheets centered around different parts of the caldera. The Yellowstone caldera consists of two ring-fracture zones, each enclosing a cauldron block; these two segments overlap in a single topographic basin 85 by 45 km. Postcollapse resurgence of the two cauldron blocks formed a pair of structural domes broken by axial grabens. Postcollapse rhyolitic volcanism, which began soon after resurgent doming, has continued to at least 70 ka. Renewed doming of the western cauldron block at 160 ka initiated volcanism of increased intensity between 160 and 70 ka. Basaltic lavas have erupted intermittently throughout the third volcanic cycle on the northeast, north, west, and south margins of the rhyolite plateau. Although the highly active hydrothermal system of Yellowstone National Park is the only current manifestation, volcanism probably has not yet ended. The erupted magmatic volume of the Yellowstone Group, about 3,700 km3, accounts for more than half the material erupted in the Yellowstone Plateau volcanic field. The magma bodies that initiated and sustained each cycle must have totaled many times more than even that enormous volume. Although eruptions of the second cycle were less voluminous than the other two, all three produced major volcanic sequences. Each cycle was more or less complete and separate; each lasted about a half-million to a million years. At least the first and third cycles clearly reflect emplacement of large crustal rhyolitic magma bodies; e