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Evolution of the Hat Creek Fault System, Northern California
Abstract
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.
... Extensional reactivation of a pre-existing, basement-involved fault at depth can lead to its upward propagation and to the development of three-dimensional segmentation and formation of en échelon young faults in the sedimentary cover (Figs. 1 and 2A;Frankowicz and McClay, 2010;Giba et al., 2012;Jackson and Rotevatn, 2013;Fossen and Rotevatn, 2016;Kattenhorn et al., 2016;Camanni et al., 2019;Deng et al., 2020a;Roche et al., 2020). These en échelon fault segments are initially separated by horizontal and vertical relays, which, with increased displacement become breached such that hard linkage occurs with the long, linked faults able to accommodate greater displacements (Peacock and Sanderson, 1991;Cartwright et al., 1995;Cowie et al., 2000;Baudon and Cartwright, 2008;Bastesen and Rotevatn, 2012;Fossen and Rotevatn, 2016;Camanni et al., 2019). ...
... Fault networks consist of geometrically and kinematically linked noncolinear segments that interact to produce more complex fault surfaces (Marchal et al., 2003;Frankowicz and McClay, 2010;Jackson and Rotevatn, 2013;Nixon et al., 2014;Rotevatn and Jackson, 2014;Duffy et al., 2015;Peacock et al., 2017;Withjack et al., 2017;Collanega et al., 2019Collanega et al., , 2020Deng et al., 2020a). These structures are common in sedimentary basins that have experienced multiphase extension ( Fig. 1; Henza et al., 2010Henza et al., , 2011Nixon et al., 2014;Kattenhorn et al., 2016;Nicol et al., 2016;Peacock et al., 2017;Withjack et al., 2017). In such tectonic settings, the growth of fault networks controls the displacement distributions, strain patterns, and morphologies of extensional terranes and rift basins (Lezzar et al., 2002;Cavinato et al., 2002;Wilkins and Schultz, 2003;Bladon et al., 2015a;Duffy et al., 2015;Henstra et al., 2015;Morley, 2017;Deng et al., 2020a). ...
... In recent years, the study of fault growth by segment linkage has gained insights from detailed outcrop analyses (de Joussineau et al., 2007;Kattenhorn et al., 2016;Peacock et al., 2016Peacock et al., , 2017Peacock et al., , 2018, seismic studies (Nixon et al., 2014;Duffy et al., 2015;Morley and Nixon, 2016;Collanega et al., 2017), and analogue modeling (Bellahsen and Daniel, 2005;Bailey et al., 2005;Henza et al., 2010Henza et al., , 2011Nixon et al., 2014;Withjack et al., 2017;Collanega et al., 2020). Whereas most analyses of fault networks have been largely two-dimensional, few have shown the development of threedimensional fault topologies (Walsh et al., 1999;Nixon et al., 2014;Duffy et al., 2015;Withjack et al., 2017;Camanni et al., 2019;Delogkos et al., 2020;Roche et al., 2020), due to limited access to high-quality 3-D seismic data. ...
Basement fault reactivation, and the growth, interaction, and linkage with new fault segments are fundamentally three-dimensional and critical for understanding the evolution of fault network development in sedimentary basins. This paper analyses the evolution of a complex, basement-involved extensional fault network on the Enderby Terrace on the eastern margin of the Dampier sub-basin, NW Shelf of Australia. A high-resolution, depth-converted, 3D seismic reflection data volume is used to show that multiphase, oblique extensional reactivation of basement-involved faults controlled the development of the fault network in the overlying strata. Oblique reactivation of the pre-existing faults initially led to the formation of overlying, en échelon Late Triassic – Middle Jurassic fault segments that, as WNW–directed rifting progressed on the margin, linked by breaching of relay ramp to form two intersecting fault systems (F1 and F2-F4). Further reactivation in the Latest Jurassic – Early Cretaceous (NNW–SSE extension) produced an additional set of en échelon fault arrays in the cover strata. The final fault network consists of main or principal faults and subordinate or splay faults, together with branch lines that link the various components. Our study shows that breaching of relay ramps and/or vertical linkages produces vertical and horizontal branch lines giving complex final fault geometries. We find that repeated activity of the basement-involved faults tends to form continuous and planar fault architectures that favor displacement transfer between the main constituent segments along strike and with depth.
... The most studied fault north of Lassen Peak is the Hat Creek fault (Muffler et al., 1994;Walker, 2008;Blakeslee and Kattenhorn, 2013;Kattenhorn et al., 2016), which is well expressed geomorphically and offsets units as young as the Hat Creek Basalt, dated as 24 ± 6 ka (Turrin et al., 2007; refined to 23.8 ± 1.4 ka by Rood et al., 2015) and ca. 15 ka periglacial deposits (Muffler et al., 1994). ...
... Just north-northeast of the source vents for the Hat Creek Basalt (star in Fig. 5) is a localized high superposed on the linear high bound by the fault. This high coincides with a shift in fault activity that Kattenhorn et al. (2016) hypothesize may be related to underlying magmatic influences. A magnetic low straddles the western margin of the valley, also marked locally by faults, from Highway 44 to the latitude of Burney over a distance of 25 km, and coincides with a belt of volcanic rocks that are considerably older than those to the east and west (Fig. 2). ...
... A good fit of the steep gravity gradient is achieved if the fault dips 75° and the basin fill is as much as 2 km thick. The fill must include basalt flows older than the Hat Creek Basalt, because the Hat Creek Basalt has a maximum thickness of ~50-75 m (Walker, 2008;Kattenhorn et al., 2016). The dense basin fill, if also magnetic, can also account for the magnetic high in the eastern part of the valley, although the predicted gradient is somewhat steeper than observed. ...
Interpretation of magnetic and new gravity data provides constraints on the geometry of the Hat Creek fault, the amount of right-lateral offset in the area between Mount Shasta and Lassen Peak (northern California, USA), and confirmation of the influence of preexisting structure on Quaternary faulting. Neogene volcanic rocks coincide with short-wavelength magnetic anomalies of both normal and reversed polarity, whereas a markedly smoother magnetic field occurs over the Klamath Mountains and Paleogene cover there. Although the magnetic field over the Neogene volcanic rocks is complex, the Hat Creek fault, which is one of the most prominent normal faults in the region and forms the eastern margin of the Hat Creek Valley, is marked by the eastern edge of a north-trending magnetic and gravity high 20-30 km long. Modeling of these anomalies indicates that the fault is a steeply dipping (~75°-85°) structure. The spatial relationship of the fault as modeled by the potential-field data, the youngest strand of the fault, and relocated seismicity suggest that deformation continues to step westward across the valley, consistent with a component of right-lateral slip in an extensional environment. Filtered aeromagnetic data highlight a concealed magnetic body of Mesozoic or older age north of Hat Creek Valley. The body's northwest margin strikes northeast and is linear over a distance of ~40 km. Within the resolution of the aeromagnetic data (1-2 km), we discern no right-lateral offset of this body. Furthermore, Quaternary faults change strike or appear to end, as if to avoid this concealed magnetic body and to pass along its southeast edge, suggesting that preexisting crustal structure influenced younger faulting, as previously proposed based on gravity data.
... These techniques provide a viable alternative to traditional paleoseismologic analyses, such as trenching, which are ill-suited for the analysis of faulted lavas. The Hat Creek fault is located within a volcanic corridor between Mount Shasta and Lassen Peak, near the southern end of the Cascade Range and its associated underlying subduction system (Fig. 1) (Wills, 1991; Muffl er et al., 1994; Blakely et al. 1997; Walker, 2008). Normal faulting and recurring volcanic activity from more than 500 vents over the past 7 m.y. ...
... The oldest and largest system of scarps, referred to as the Rim, has as much as ~350 m of throw and defi nes the easternmost extent of the fault system. The 47-km-long Pleistocene Rim consists of seven right-stepping , northwest-oriented segments ranging in length from ~1–16 km (Walker, 2008). These scarps are heavily vegetated and have prominent talus piles that refl ect gradual geomorphic modifi cation of the scarps to slope angles of ~30°– 45°. ...
... Geosphere, October 2013 3 Pali (a Hawaiian term for eroded basaltic cliffs) and have accrued as much as ~175 m of throw (Walker, 2008). The Pali, also Pleistocene in age, extends for ~24 km and is made up of fi ve left-stepping segments with generally north orientations in the southern part of the fault system where the Pali intersects the Rim, but changing to north-northwest orientations in the north where the Pali segments approach the volcanic edifi ce at Cinder Butte (Fig. 2A). ...
Normal faults in basalt have distinctive surface-trace morphologies and earthquake evidence that provide information about the slip behavior and earthquake potential. The 47-km-long Hat Creek fault in northern California (USA), a useful case example of this fault style, is a segmented fault system located along the western margin of the Modoc Plateau that is a regional earthquake hazard. In response to interaction with sporadically active volcanic systems, surface ruptures have progressively migrated westward since the late Pleistocene, with older scarps being successively abandoned. The most recent earthquake activity broke the surface through predominantly ca. 24 ka basaltic lavas, forming a scarp with a maximum throw of 56 m. Past work by others identified 7–8 left-stepping scarp segments with a combined length of 23.5 km, but did not explicitly address the throw characteristics, fault evolution, slip history, or earthquake potential. We address these deficiencies in our understanding of the fault system with new field observations and mapping that suggest the active scarp contains 2 additional segments and is at least 6.5 km longer than previously mapped, thus increasing the knowledge of the regional seismic hazard. Our work details scarp geomorphic styles and slip-analysis techniques that can be applied to any normal-faulted basalt environment. Applied to the Hat Creek fault, we estimate that a surface-breaking rupture could produce an earthquake of ~Mw (moment magnitude) 6.7 and a recurrence interval of 667 ± 167 yr in response to a rapid slip rate in the range 2.2–3.6 mm/yr, creating a moderate risk given a lack of historical earthquake events.
... Vertical offsets and thickness changes in volcanic stratigraphy in the headwaters of Kosk Creek indicate oblique slip that aligns with topographic scarps. Regionally, prominent north-south-striking faults, such as the Hat Creek and Rocky Ledge faults, record primarily dip-slip normal motion (Kattenhorn et al., 2016;Martin, 2020) with limited recently recognized dextral movement on more northwest-southeast-striking fault strands (Gray et al., 2017;Martin, 2020). On the other hand, northwest-southeast-striking faults in the Walker Lane, such as the Likely and Honey Lake faults ( Fig. 2; Wills and Borchardt, 1993), generally accommodated significant dextral movement during the late Pleistocene and Holocene between regions of north-south-striking normal faulting. ...
... Moreover, derivation of an Euler pole for relative Sierra Nevada-Oregon Coast block rigid motion predicts relative west-northwest-directed dextral shear between the microplates (McCaffrey et al., 2013), which is supported by the observations in this study. The orientation of these faults may be influenced by preexisting basement structure inferred from gravity lows and Cascade volcanism (Blakely et al., 1997), where zones of primarily normal faulting, such as the Hat Creek (Muffler et al., 1994;Blakeslee and Kattenhorn, 2013;Kattenhorn et al., 2016) and McArthur (Page, 1995) fault zones and the Klamath graben (Waldien et al., 2019), are separated by zones of oblique dextral shear (Blakely et al., 1997). Recent work on the Klamath graben posited that Walker Lane-related shear extends into southern Oregon, where it dissipates into the southern Cascade arc (Waldien et al., 2019). ...
The tectonic domains of Basin and Range extension, Cascadia subduction zone contraction, and Walker Lane dextral transtension converge in the Mushroom Rock region of northeastern California, USA. We combined analysis of high-resolution topographic data, bedrock mapping, 40Ar/39Ar geochronology, low-temperature thermochronology, and existing geologic and fault mapping to characterize an extensive dextral-normal-oblique fault system called the Pondosa fault zone. This fault zone extends north-northwest from the Pit River east of Soldier Mountain, California, into moderately high-relief volcanic topography as far north as the Bartle (California) townsite with normal and dextral offset apparent in geomorphology and fault exposures. New and existing 40Ar/39Ar and radiocarbon dating of offset lava flows provides ages of 12.4 ka to 9.6 Ma for late Cenozoic stratigraphic units. Scarp morphology and geomorphic expression indicate that the fault system was active in the late Pleistocene. The Pondosa fault zone may represent a dextral-oblique accommodation zone between north-south–oriented Basin and Range extensional fault systems and/or part of the Sierra Nevada–Oregon Coast block microplate boundary.
... The Union Peak fault has strands with little till or ignimbrite cover and in which scarps offset LGM-glaciated lava surfaces. There (Fig. 6A), strands have been linked by curved lateral propagation that typically has a counterclockwise sense, forming cusps along the scarp (e.g., Ferrill et al., 2016;Kattenhorn et al., 2016). Similar curved lateral propagation is evident on a larger scale along the Bald Crater fault north of its namesake (Figs. 2 and 8A) where the fault displaces early? ...
... En echelon scarps in relatively young, glaciated lavas (e.g., Oasis Butte and Union Peak faults) may reflect early stages of fracturing above older, long-established faults (Soliva and Benedicto, 2004;Giba et al., 2012) that lava had buried rather than inception of entirely new normal faults. The predominantly left-stepping en echelon scarp patterns may be ascribed to a dextral strike-slip component of motion, such as for the Hat Creek fault (Kattenhorn et al., 2016). How-ever, theoretical modeling, and geologic studies have shown that en echelon normal fault patterns can develop in the absence of strike-slip deformation (Willemse, 1997), such as by fault growth during extension (Trudgill and Cartwright, 1994;Clifton et al., 2000). ...
Volcanoes of subduction-related magmatic arcs occur in a variety of crustal tectonic regimes, including where active faults indicate arc-normal extension. The Cascades arc volcano Mount Mazama overlaps on its west an ∼10-km-wide zone of ∼north-south–trending normal faults. A lidar (light detection and ranging) survey of Crater Lake National Park, reveals several previously unrecognized faults west of the caldera. Postglacial vertical separations measured from profiles across scarps range from ∼2 m to as much as 12 m. Scarp profiles commonly suggest two or more postglacial surface-rupturing events. Ignimbrite of the ca. 7.6 ka climactic eruption of Mount Mazama, during which Crater Lake caldera formed, appears to bury fault strands where they project into thick, valley-filling ignimbrite. Lack of lateral offset of linear features suggests principally normal displacement, although predominant left stepping of scarp strands implies a component of dextral slip. West-northwest–east-southeast and north-northwest–south-southeast linear topographic elements, such as low scarps or ridges, shallow troughs, and straight reaches of streams, suggest that erosion was influenced by distributed shear, consistent with GPS vectors and clockwise rotation of the Oregon forearc block.
Surface rupture lengths (SRL) of faults suggest earthquakes of (moment magnitude) Mw6.5 from empirical scaling relationships. If several faults slipped in one event, a combined SRL of 44 km suggests an earthquake of Mw7.0. Postglacial scarps as high as 12 m imply maximum vertical slip rates of 1.5 mm/yr for the zone west of Crater Lake, considerably higher than the ∼0.3 mm/yr long-term rate for the nearby West Klamath Lake fault zone. An unanswered question is the timing of surface-rupturing earthquakes relative to the Mazama climactic eruption. The eruption may have been preceded by a large earthquake. Alternatively, large surface-rupturing earthquakes may have occurred during the eruption, a result of decrease in east-west compressive stress during ejection of ∼50 km3 of magma and concurrent caldera collapse.
... Together these segments form the larger Kordjya fault system, which is also defined by a broad elliptical throw profile. In all, these throw patterns suggest that the Kordjya fault is defined by two soft-linked, kinematically-coherent segments (Walsh and Watterson, 1988;Dawers et al., 1993;Cartwright et al., 1995;Nicol et al., 1996;Kattenhorn et al., 2016), that are themselves a composite of multiple NNE-SSW-striking normal fault segments (Fig. 4B). The two major softlinked segments that define the Kordjya fault are also evident on fault segment and DEM maps presented in Fig. 4A and C. ...
... The WSW-ENE extension inferred from field data is expected to translate to dip-slip motion along a NNW-trending structure at basement depths (Fig. 11E). The earlier formed rift-parallel structures in the vicinity of the Kordjya fault were thus reactivated as oblique-normal faults with a component of left-lateral shear in order to accommodate dip-slip motion along this NNW-trending fault (Fig. 12), resulting in polyphase fault activity along an inherited structural fabric (e.g., Kattenhorn et al., 2016). ...
Inherited crustal weaknesses have long been recognized as important factors in strain localization and basin development in the East African Rift System (EARS). However, the timing and kinematics (e.g., sense of slip) of transverse (rift-oblique) faults that exploit these weaknesses are debated, and thus the roles of inherited weaknesses at different stages of rift basin evolution are often overlooked. The mechanics of transverse faulting were addressed through an analysis of the Kordjya fault of the Magadi basin (Kenya Rift). Fault kinematics were investigated from field and remote-sensing data collected on fault and joint systems. Our analysis indicates that the Kordjya fault consists of a complex system of predominantly NNE-striking, rift-parallel fault segments that collectively form a NNW-trending array of en echelon faults. The transverse Kordjya fault therefore reactivated existing rift-parallel faults in ∼1 Ma lavas as oblique-normal faults with a component of sinistral shear. In all, these fault motions accommodate dip-slip on an underlying transverse structure that exploits the Aswa basement shear zone. This study shows that transverse faults may be activated through a complex interplay among magma-assisted strain localization, preexisting structures, and local stress rotations. Rather than forming during rift initiation, transverse structures can develop after the establishment of pervasive rift-parallel fault systems, and may exhibit dip-slip kinematics when activated from local stress rotations. The Kordjya fault is shown here to form a kinematic linkage that transfers strain to a newly developing center of concentrated magmatism and normal faulting. It is concluded that recently activated transverse faults not only reveal the effects of inherited basement weaknesses on fault development, but also provide important clues regarding developing magmatic and tectonic systems as young continental rift basins evolve.
... The sense of step is described as left-and right-stepping. Releasing or extensional stepovers result where the sense of step is the same as the sense of the overall slip (e.g., a left-step along a left-lateral fault) Woodcock and Fischer, 1986;Sylvester, 1988;Woodcock and Schubert, 1994;Westaway, 1995;Cunningham and Mann, 2007;Mann, 2007;Crider, 2015;Cao and Neubauer, 2016;Kattenhorn et al., 2016;Peacock et al., 2016); related structures include open and dilatant cracks at cm to m-scale (e.g., Pollard, 1980, 1983;Martel et al., 1988;Kim et al., 2000Kim et al., , 2003Kim et al., , 2004Flodin and Aydin, 2004) and pull-apart basins, sag ponds and negative flower structures (e.g., Mann et al., 1983;Aydin and Nur, 1985;Mukherjee, 2015aMukherjee, , 2015b. However, all these structures need not form in every strike-slip settings (Misra et al., 2014(Misra et al., , 2016Misra and Mukherjee, 2015, in press-1, in press-2;Babar et al., in press;Kaplay et al., in press, submitted;Mukherjee et al., in press). ...
... However, all these structures need not form in every strike-slip settings (Misra et al., 2014(Misra et al., , 2016Misra and Mukherjee, 2015, in press-1, in press-2;Babar et al., in press;Kaplay et al., in press, submitted;Mukherjee et al., in press). In contrast, restraining or contractional stepovers, or contractional linking damage zone (Kim et al., 2004), result where the sense of step is opposite to the sense of the overall slip (e.g., a right-step along a left-lateral fault or vice versa) Woodcock and Fischer, 1986;Sylvester, 1988;Woodcock and Schubert, 1994;Westaway, 1995;Cunningham and Mann, 2007;Mann, 2007;Crider, 2015;Kattenhorn et al., 2016;Peacock et al., 2016); related structures include closing, anti-cracks and pressure solution seams at the mscale (e.g., Pollard, 1992, 1994;Peacock and Sanderson, 1995) and pop-up and positive flower structures at the regional scale (e.g., Aydin and Nur, 1985). Contractional fault steps, which are common features of active intra-continental strike-slip fault systems, are commonly referred to as sites of "transpression" (e.g., Curtis, 1997Curtis, , 1999McClay and Bonora, 2001;Dooley and Schreurs, 2012). ...
Two-dimensional finite-element modelling of elastic Newtonian rheology is used to compute stress distribution and strain localization patterns in a transpression zone between two pre-existing right-stepping, left-lateral strike-slip fault segments. Three representative fault segment interactions are modelled: underlapping, neutral, and overlapping. The numerical results indicate that at the onset of deformation, displacement vectors are oblique to the regional compression direction (20–90°). The orientations of the local σ1 (the maximum compressive stress) and σ3 (the minimum compressive stress) directions strongly depend on the structural position within the transpression zone. For neutral and overlapping fault steps, there is a contractional linking damage zone between the fault segments. For overlapping faults, the σ1 trajectories within the transpression zone deflects significantly forming a sigmoidal pattern, which is created by two rotational flow patterns close to the fault tips. These flow patterns are related to friction effects and different shear deformation, from pure shear outside of the fault steps toward simple shear along the fault segments. Interaction between the two fault segments perturbs the stress field and reflects the heterogeneous nature of deformation. A lozenge- (for underlapping steps), rhomboidal- (for neutral steps), and sigmoidal-shaped (for overlapping steps) transpression zone developed between the two segments. The modelled mean stress pattern shows a similar pattern to that of the contractional steps, and decrease and increase in underlapping and overlapping fault steps, respectively. Comparison of the Kuh-e-Hori transpression zone, between the Esmail-abad and West Neh left-stepping right-lateral strike-slip fault segments in SE Iran, with the modelling results shows strong similarities with the neutral step configuration.
... The resultant fault system F1a-F1m was therefore associated with reactivation of preexisting faults and regional stress changes. Cases of temporal stress change have also been reported in places such as the Hat Creek fault, northern California (Blakeslee and Kattenhorn, 2013;Kattenhorn et al., 2016) and the Gulf of Thailand (Morley et al., 2004;Morley, 2017). However, the short-lived temporal stress change remains doubtful and cannot fully satisfy the contemporaneous occurrence of intersecting faults with distinct strikes (e.g. ...
Insights of spatial and temporal development of fault network in 3D is crucial for understanding the process evolution of complex fault network and for evaluating the regional and local stresses control on structure development. We demonstrate a fault network on the eastern Dampier Sub-basin, North West Shelf of Australia, which consists of (1) a ENE-trending fault array that has a through-going segment at depth and a series of left-stepping fault splays at upper levels, and (2) a network of ENE- and NNE-trending intersecting faults decoupled from the basement structures. This research shows that the segmented ENE-trending fault array developed through three extensional phases in the Late Paleozoic, in the Early Jurassic, and in the Late Middle Jurassic. Fault analysis shows that the summed displacement of the segmented, en échelon faults behaves as a single fault and that the basement fault controlled the fault array in the upper section through vertical linkages– a typical coherent fault system. The NNE- and ENE-trending intersecting faults formed simultaneously in the Late Middle Jurassic; as such, they might have controlled by 3D strain field released from the Rosemary and Mermaid fault systems bounding the fault network. This implies that fault geometry derived from 3D seismic interpretation need to be treated with caution as the alignment of fault sets may not directly relate to regional, far-field stress but, in some cases, significantly modified by local stresses induced by reactivated larger faults. This study provides an analogue for the interpretation of other rift systems, where structures were controlled by competing forces of regional and local stresses and where reactivated and newly-formed structures coexist in polyphase of extensions.
... Our database suggests this is not always the case, however. For example, growth folds are documented on the flanks of Kilauea, Hawaii (Kattenhorn et al., 2000;Parfitt and Peacock, 2001;Peacock and Parfitt, 2002;Holland et al., 2006;Martel and Langley, 2006;Kaven and Martel, 2007;Podolsky and Roberts, 2008), the Modoc Plateau, USA (White and Crider, 2006;Blakeslee and Kattenhorn, 2013;Crider, 2015;Kattenhorn et al., 2016), and the Reykjanes Peninsula, Iceland (Bull et al., 2003;Grant and Kattenhorn, 2004;Bull et al., 2005;Trippanera et al., 2015) suggesting that cover rheology is not the principal control on growth fold occurrence. Cover lithology and rheology may affect the geometry and size of growth folds (Fig. 8D). ...
Growth folds above the upper tips of normal faults are ubiquitous in extensional settings, especially during the early phases of extension and in salt-rich basins. As slip accumulates on the underlying normal fault, the geometry and size of the fold changes. These changes reflect the dip, throw, displacement and propagation rate of the underlying normal fault, as well as the thickness and rheology of the overlying cover. These changes also have a marked impact on the architecture and distribution of synkinematic sediments, as well as the styles of secondary deformation accommodating strain within the growing fold. Here, we analyse a large dataset of natural, and physically- and numerically-modelled growth folds to: (i) characterise their diagnostic features; (ii) investigate the controls on their geometry, size and differences; and (iii) describe how they grow with increasing extensional strain. We demonstrate that larger fault throws and a thicker and weaker cover are associated with larger growth folds. In contrast, small fault throws as well as thin and strong brittle cover are associated with smaller growth folds. We show that the geometry and size of growth folds vary through time; the width (and thus, the wavelength) of the fold is established relatively early during fold growth, whereas fold amplitude increases gradually with increasing fault throw. Fold width and amplitude become increasingly similar during fold evolution, until the fold is breached by the underlying normal fault. We also derive a number of preliminary empirical relationships between readily observable structural and stratigraphic parameters in our dataset that may help estimate the geometry and size of poorly exposed (i.e. in the field) or imaged (i.e. in the subsurface) growth folds. In addition, we discuss how fault growth models (i.e. constant-length vs. propagating) may impact the three-dimensional evolution of growth folds. Finally, our work shows that growth folds are likely more common than previously thought. For example, although they are well-documented in areas characterised by weak, ductile cover strata and low strain rates, our dataset illustrates that growth folds may also occur in brittle, relatively strong rocks and in regions with high strain rates. However, the underlying controls on fold occurrence remain elusive.
... The Pliocene-Quaternary structural fabric of the Modoc Plateau is characterized by normal slip on approximately N-striking faults and oblique slip on NW-striking faults, which accommodate NW-SE dextral transtension throughout the region ( Fig. 2A; Blakely et al., 1997;McCaffrey et al., 2013). Although fault slip studies indicate primarily normal slip on faults in the Modoc Plateau (White and Crider, 2006;Kattenhorn et al., 2016), seismicity indicates that NW-SE-oriented dextral transcurrent or transtensional deformation regimes characterize a significant portion of the region (Unruh and Humphrey, 2017). Moreover, the Likely fault and Eagle Lake fault are regional NW-striking structures in the Modoc Plateau with demonstrable Pleistocene dextral displacement (Pease, 1969;Sawyer and Bryant, 1995;Colie, 2003). ...
A thorough insight into the initiation, segmentation, propagation and interaction of multitrend basin‐bounding faults is crucial to restoring the growth history of the faults and clarifying the fault growth pattern and its influence on the structures developed along the margin due to the growth of the basin‐bounding faults, but systematic studies on the individual influence of the evolution of each fault segment on the present structure are lacking. Based on 3D seismic data, the timing and growth of multitrend basin‐bounding faults were analysed using T‐z plots and throw backstripping, allowing us to determine the individual effects that each fault segment evolution exerteds on the present‐day configuration of the northern margin of the Nanpu Sag. The basin‐bounding fault is composed of the Xinanzhuang and Baigezhuang faults, and the Xinanzhuang fault comprises three linked segments with varying orientations (i.e., NE–SW, E–W, and NNE–SSW). In comparison, the Baigezhuang fault comprises only two linked NW–SE‐oriented fault segments. The evolution process can be divided into three stages. (1) During the early synrift I stage, namely, the isolated fault stage, five isolated multitrend basin‐bounding segments were active. (2) During the late synrift I stage, namely, the hard‐linkage stage, the five segments propagated laterally and linked with each other, behaving as a single fault. Meanwhile, the NE‐trending No. 5 Fault bifurcated upward from the basin‐bounding fault to accommodate local stress, and the NW‐trending Gaobei Fault deviated from the basin‐bounding fault controlled by local stresses induced by differential activities of the multitrend fault segments under the same far‐field stress. (3) During the synrift II to postrift linkage development stage, the extension orientation changed from NW–SE‐ to N–S, and additional displacement accumulated along the basin‐bounding fault without further lateral propagation. Newly formed E–W‐trending faults developed orthogonal to the extension orientation and linked with preexisting NE‐ or NW‐trending faults, forming a complex fault zone. In addition, influenced by the geometry of the basin‐bounding fault, the Laoyemiao Anticline formed by gravitational collapse under the dual action of a rollover anticline and transverse anticline. Furthermore, the evolution of the basin‐bounding faults played an important role in controlling the source‐to‐sink system, and the transition zone was the main provenance channel formed by the segmented growth of the faults. This study provides new insight into multitrend large fault evolution, and their impact on basin development provides a comprehensive explanation of the later structures developed in polyphase rifts.
Tackling structural geology problems today requires a quantitative understanding of the underlying physical principles, and the ability to apply mathematical models to deformation processes within the Earth. Accessible yet rigorous, this unique textbook demonstrates how to approach structural geology quantitatively using calculus and mechanics, and prepares students to interface with professional geophysicists and engineers who appreciate and utilize the same tools and computational methods to solve multidisciplinary problems. Clearly explained methods are used throughout the book to quantify field data, set up mathematical models for the formation of structures, and compare model results to field observations. An extensive online package of coordinated laboratory exercises enables students to consolidate their learning and put it into practice by analyzing structural data and building insightful models. Designed for single-semester undergraduate courses, this pioneering text prepares students for graduates studies and careers as professional geoscientists.
Tackling structural geology problems today requires a quantitative understanding of the underlying physical principles, and the ability to apply mathematical models to deformation processes within the Earth. Accessible yet rigorous, this unique textbook demonstrates how to approach structural geology quantitatively using calculus and mechanics, and prepares students to interface with professional geophysicists and engineers who appreciate and utilize the same tools and computational methods to solve multidisciplinary problems. Clearly explained methods are used throughout the book to quantify field data, set up mathematical models for the formation of structures, and compare model results to field observations. An extensive online package of coordinated laboratory exercises enables students to consolidate their learning and put it into practice by analyzing structural data and building insightful models. Designed for single-semester undergraduate courses, this pioneering text prepares students for graduates studies and careers as professional geoscientists.
Sketching, particularly in field settings, is a common but powerful means of communication and visualization in the geosciences. Here, we investigate the range of sketch types and annotations made by expert geoscientists and non-geoscientists during a field trip to the Hat Creek fault zone (northern California) taken during the 2013 AAPG Hedberg Research Conference. Participants (N542) included geologists and seismic interpreters employed in the oil and gas industry (n520), geologists employed in academia (n516), and non-geoscientist software developers and cognitive scientists (n56). A total of 361 sketches of the normal fault system were collected during stops at three field modules. Sketches were thematically coded by sketch type (e.g., map, perspective landscape view, cross-section, three-dimensional [3-D] block diagram) and annotation type (e.g., fault symbols, reference locations, questions, labels). Overall, two-dimensional (2-D) perspective sketches and maps were the most common representation type, whereas 3-D block diagrams were rare. Statistical analysis of code counts suggests that the choice of sketch and annotation types is largely driven by characteristics of the field trip stop and/or the particular task required. Non-geoscientists more frequently produced perspective sketches from their actual viewpoint, but were less likely to annotate diagrams. As compared to industry peers, academic geoscientists were more likely to create
The Hat Creek fault is a major, young, north-striking, normal fault along the western boundary of extensional Basin and Range deformation in the Lassen region of northeastern California. Volcanic rocks of Quaternary and late Pliocene age are displaced a total of >500 m down to the west along west-facing, en echelon scarps now retreated to ~35° slopes. Fresh, young scarps as much as 30 m high cut the Hat Creek Basalt (erupted between 15 and ~40 ka) a few tens of meters west of the retreated scarps. Prior to the late 1980s, these young scarps were interpreted as lava slump scarps formed as the Hat Creek Basalt ponded against the older fault scarps and then drained away to the northwest. Numerous pieces of geologic evidence, however, show that the young scarps formed after the Hat Creek Basalt solidified and cooled and are true fault features formed by the youngest displacements of the Hat Creek fault. Displacement of outwash gravel overlying the Hat Creek Basalt shows that vertical separation on the Hat Creek fault has averaged ~1.3 mm yr-1 for the past 15 000 yr. The Hat Creek fault thus represents a potential earthquake hazard, despite the low level and diffuse nature of modern seismicity in the region. -from Authors
Many rift zones exhibit a range of fault orientations, rather than simple colinear faults that strike orthogonal to the inferred least principal stress. The formation of non-colinear fault sets has implications for assessing rift-zone kinematics, as well as determining palaeo-stress state in extensional basins. Using 3D seismic reflection data, we deduce the likely mechanisms responsible for the formation of a population of non-colinear faults in the Måløy Slope area of the northern North Sea. Three basement-displacing fault populations exist on the Måløy Slope; (i) large (>1 km throw), N-S-striking faults, (ii) smaller (<250 m throw) N-S-striking faults and (iii) small (<250 m throw) NE-SW-striking faults. All were initiated in the Middle Jurassic. Coeval growth of these fault populations, and the apparent correlation between the NE-SW faults and a NE-SW-trending gravity and magnetic anomaly high lead us to suggest that the NE-SW faults are the result of deflection of the otherwise E-W-orientated least principal stress by NE-trending intrabasement weaknesses. Our study's results have implications for the large-scale kinematic evolution of the North Sea, arguing that major rotations in extension direction are not required to generate multiple fault sets locally or across the rift. This study also highlights the importance of using borehole-constrained 3D seismic data as a tool in understanding non-colinear fault growth, and its broader implications for regional tectonic history.
[1] The Seychelles plateau is a prime example of a microcontinent, yet mechanisms for its creation and evolution are poorly understood. Recently acquired teleseismic data from a deployment of 26 stations on 18 islands in the Seychelles are analyzed to study upper mantle seismic anisotropy using SKS splitting results. Strong microseismic noise is attenuated using a polarization filter. Results show significant variation in time delays (δt = 0.4–2.4 s) and smooth variations in orientation (ϕ = 15°–69°, where ϕ is the polarization of the fast shear wave). The splitting results cannot be explained by simple asthenospheric flow associated with absolute plate motions. Recent work has suggested that anisotropy measurements for oceanic islands surrounding Africa can be explained by mantle flow due to plate motion in combination with density-driven flow associated with the African superswell. Such a mechanism explains our results only if there are lateral variations in the viscosity of the mantle. It has been suggested that the Seychelles are underlain by a mantle plume. Predictions of flow-induced anisotropy from plume-lithosphere interaction can explain our results with a plume possibly impinging beneath the plateau. Finally, we consider lithospheric anisotropy associated with rifting processes that formed the Seychelles. The large variation in the magnitude of shear wave splitting over short distances suggests a shallow source of anisotropy. Fast directions align parallel to an area of transform faulting in the Amirantes. Farther from this area the orientation of anisotropy aligns in similar directions as plate motions. This supports suggestions of transpressive deformation during the opening of the Mascarene basin.
Normal faults in basalt have distinctive surface-trace morphologies and earthquake evidence that provide information about the slip behavior and earthquake potential. The 47-km-long Hat Creek fault in northern California (USA), a useful case example of this fault style, is a segmented fault system located along the western margin of the Modoc Plateau that is a regional earthquake hazard. In response to interaction with sporadically active volcanic systems, surface ruptures have progressively migrated westward since the late Pleistocene, with older scarps being successively abandoned. The most recent earthquake activity broke the surface through predominantly ca. 24 ka basaltic lavas, forming a scarp with a maximum throw of 56 m. Past work by others identified 7–8 left-stepping scarp segments with a combined length of 23.5 km, but did not explicitly address the throw characteristics, fault evolution, slip history, or earthquake potential. We address these deficiencies in our understanding of the fault system with new field observations and mapping that suggest the active scarp contains 2 additional segments and is at least 6.5 km longer than previously mapped, thus increasing the knowledge of the regional seismic hazard. Our work details scarp geomorphic styles and slip-analysis techniques that can be applied to any normal-faulted basalt environment. Applied to the Hat Creek fault, we estimate that a surface-breaking rupture could produce an earthquake of ~Mw (moment magnitude) 6.7 and a recurrence interval of 667 ± 167 yr in response to a rapid slip rate in the range 2.2–3.6 mm/yr, creating a moderate risk given a lack of historical earthquake events.
Geologic and paleomagnetic data from the Cascadia forearc indicate
long-term northward migration and clockwise rotation of an Oregon
coastal block with respect to North America. Paleomagnetic rotation of
coastal Oregon is linked by a Klamath Mountains pole to geodetically and
geologically determined motion of the Sierra Nevada block to derive a
new Oregon Coast-North America (OC-NA) pole of rotation and velocity
field. This long-term velocity field, which is independent of Pacific
Northwest GPS data, is interpreted to be the result of Basin-Range
extension and Pacific-North America dextral shear. The resulting Oregon
Coast pole compares favorably to those derived solely from GPS data,
although uncertainties are large. Subtracting the long-term motion from
forearc GPS velocities reveals ENE motion with respect to an OC
reference frame that is parallel to the direction of Juan de Fuca-OC
convergence and decreases inland. We interpret this to be largely the
result of subduction-related deformation. The adjusted mean GPS
velocities are generally subparallel to those predicted from elastic
dislocation models for Cascadia, but more definitive interpretations
await refinement of the present large uncertainty in the Sierra Nevada
block motion.
We have used high-resolution scanned air photos and field measurements
to analyze fracture population systematics for the Reykjanes fissure
swarm in order to determine the effect of rift obliquity on the
evolution of fracture populations on the Reykjanes Peninsula, SW
Iceland. The peninsula is oriented approximately 30° oblique to the
direction of plate motion. Data show significant differences between the
strike, length, and degree of development of fractures along the margin
of the fissure swarm and those in the center of the zone of active
volcanism. Along the margin, fractures are long, highly segmented
extension fractures, and normal faults with offset on the order of
several meters and a predominant strike 20° oblique to the trend of
the plate boundary (rift axis). In the center of the zone of active
volcanism, fractures are generally shorter, straighter, fewer in number,
and strike approximately perpendicular to the direction of maximum
horizontal extension and parallel to the trend of eruptive fissures.
Right-lateral oblique-slip faults striking approximately perpendicular
to the spreading direction are present in the center of the rift zone.
Scaled experimental models of oblique extension predict a significant
change in fracture strike at the rift margins due to the presence of a
secondary stress field; on the Reykjanes Peninsula, this corresponds to
the boundary between thicker and thinner brittle crust. The models
confirm that right-lateral oblique- or strike-slip faults accommodate
significant left-lateral shear where deformation is distributed in a
rift zone.
Geologic and geodetic studies in California indicate that about 1 cm yr-1 of right-lateral shear occurs across what has been referred to as the Eastern California Shear Zone. Northwest trending zones of dextral, sinistral, and normal faults splay eastward from the San Andreas system, continuing through the Mojave Desert, east of the Sierra Nevada, and northward along the Central Nevada and Walker Lane fault zones. Aerial photography, field investigations, and fault studies in southern and central Oregon, compiled with a comprehensive analysis of previous studies nearby, indicate that latest Pleistocene and Holocene fault activity is concentrated along four zones that stretch northward into the Cascade volcanic arc and across the northwestern edge of the Basin and Range Province. The Oregon zones appear to continue the activity in eastern California and northwestern Nevada northward and provide a connection to seismically active zones in southern and central Washington. Several techniques are applied to fault data from the Oregon zones in an attempt to estimate the overall direction and rate of motion across them. -from Authors
Obliquely spreading mid-ocean ridges, such as the Reykjanes Ridge, display two distinct fault sets distinguishable by orientation and position: on-axis faults are oriented oblique to both the trend of the axis and the normal to the relative plate separation vector, while faults on the flanks strike approximately parallel to the ridge axis. Numerical modeling techniques are used here to simulate the development of faulting on the Reykjanes Ridge. Stresses acting in a cross section through the lithosphere at a slow spreading ridge are investigated using the fast Lagrangian analysis of continua (FLAC) explicit difference modeling software. The predicted stresses from the cross-sectional models are imposed as a condition in boundary element models of fracture propagation and linkage. On-axis fault simulations run under conditions similar to the Reykjanes Ridge successfully reproduce the mapped distribution of faults and predict the observed orientation of the axial volcanic ridges. Simulations of fractures away from the axis show the development of axis-parallel faults by the interaction and linkage of fractures which have been rafted off-axis, also in accord with observations. Stresses modeled in cross section favor downdip displacement on faults dipping toward the ridge axis.
Although oceanic spreading is often perpendicular to the ridge trends, in some cases the angle between these two directions can be significantly less than 90° (40°–50°). This occurs because of either a bend of the ridge trend or a change of the spreading direction. We here describe oblique spreading in the Mohns Ridge, resulting in deformation partitioning between the valley walls, which are dominantly affected by strike-slip displacements, and the axial valley which is subject to nearly pure extension. The axial valley walls are characterized by en échelon normal faults affecting the walls, while the axial valley is affected by parallel faults grouped into oblique sets. These fault sets define different structures, horst or tilted blocks, that are regularly spaced inside the axial valley. Moreover some ridge segments mainly undergo pure extension, whereas others are affected by oblique extension. We explain this faulting pattern, including the along-strike and transverse variations, as a consequence of depth variations of the brittle-ductile transition.
Faults can severely compartmentalize pressures and fluids in producing reservoirs, and it is therefore important to take these effects into account when modelling field production characteristics. The Brent Group fields, northern North Sea, contain a complex arrangement of fault juxtapositions of a well-layered sand-shale reservoir stratigraphy, and fault zones containing a variety of fluid flow-retarding fault rock products. It has been our experience that these fault juxtapositions impact the 'plumbing' of the faulted layering system in the reservoirs and the models that are built to mimic them-and are, in fact, a first-order sensitivity on compartmentalization of pressures and fluid flow during production simulation. It is important, therefore, to capture and validate the geological feasibility of fault-horizon geometries, from the seismic interpretation through to the static geocellular model, by model building in conjunction with the interpretation. It is then equally important to preserve this geometrical information during geocellular transfer to the simulation model, where it is critical input data used for calculation of fault zone properties and fault transmissibility multipliers, used to mimic the flow-retarding effects of faults. Application of these multipliers to geometrically weak models tends to produce ambiguous or otherwise potentially misleading simulation results. We have systematically modelled transmissibility multipliers from the upscaled cellular structure and property grids of geometrically robust models-with reference to data on clay content and permeability of fault rocks present within drill core from the particular reservoir under study, or from similar nearby reservoirs within the same stratigraphy. Where these transmissibility multipliers have been incorporated into the production simulation models, the resulting history matches are far better and quicker than had been achieved previously. The results are particularly enhanced where the fault rock data are drawn from rocks that have experienced a similar burial-strain history to the reservoir under study.
Fault displacements derived from a seismic reflection survey of an offshore oilfield are projected onto a vertical plane parallel to the fault strike and the displacement values contoured. In addition to those for single faults, displacement diagrams are also constructed for fault arrays by aggregating the displacement values on selected faults. Displacement contours form regular and systematic patterns, even when there is no continuity between the fault surfaces in the array. A system in which linkage between the elements of a fault array is achieved by ductile strain of the intervening rock is referred to as soft-linked and is regarded as the general case. Geometrical coherence, with regular and systematic displacement patterns, exists at all stages in the growth of a fault array, implying a kinematic coherence requiring a high degree of synchronous movement, as opposed to sequential development, on individual elements in the array.
Conventional maps and diagrams allow representation of only two orders of magnitude of fault displacement rather than the five or more which may occur. A soft-domino model is presented in which the role of ductile strains is acknowledged both in accommodating varying displacements on fault surfaces and in extension on structures too small to be represented individually, and in which the rotation of rigid fault blocks is of reduced importance when compared with rigid-domino models.
Overlap zones between normal faults have been studied using a variety of 2D and 3D seismic reflection datasets. The overlaps are of two types, (i) relay zones in which displacement is transferred between the overlapping faults and (ii) non-relay overlaps in which displacement is not transferred.
Overlap zones are continually formed and destroyed during the growth of a fault system. Overlap zones are formed either by interference between initially isolated faults or as a result of bifurcation of a single fault. The mode of overlap formation is reflected in the 3D geometry of the overlapping faults which may be either unconnected or linked at a branch-line or branch-point.
Seismic reflection data from regions of growth faulting, and also sandbox analogue data, allow analysis of fault development through time. Reconstructions of the displacement distribution on some faults with sharp bends and associated hanging-wall splays, show that the bends originated as overlap zones which were later breached to form through-going faults.
Depending on the displacements of relay-bounding faults, the effect of relay zones on hydrocarbon reservoirs may be to (a) provide structural closure, (b) form gaps in otherwise sealing faults or (c) increase reservoir connectivity.
Neogene deformation, paleomagnetic rotations, and sparse geodetic data
suggest the Cascadia fore arc is migrating northward along the coast and
breaking up into large rotating blocks. Deformation occurs mostly around
the margins of a large, relatively aseismic Oregon coastal block
composed of thick, accreted seamount crust. This 400-km-long block is
moving slowly clockwise with respect to North America about a Euler pole
in eastern Washington, thus increasing convergence rates along its
leading edge near Cape Blanco, and creating an extensional volcanic arc
on its trailing edge. Northward movement of the block breaks western
Washington into smaller, seismically active blocks and compresses them
against the Canadian Coast Mountains restraining bend. Arc-parallel
transport of fore-arc blocks is calculated to be up to 9 mm/yr,
sufficient to produce damaging earthquakes in a broad deformation zone
along block margins.
The form of the scaling relation between the displacement and length of
faults has been a subject of considerable controversy because of
insufficient scale range and scattered data. Here we report on
displacement and length data collected from well-exposed normal faults
located on the Volcanic Tableland in northern Owens Valley, California.
These data, which exhibit little scatter, are from a fault population
that spans three orders of magnitude in fault length and were gathered
in a relatively uniform lithologic and tectonic setting. With the upper
cooling surface of the middle Quaternary Bishop Tuff used as a marker,
the displacement distribution along individual faults can be mapped in
detail. The displacement distribution profiles are consistent with a
linear relation between displacement and fault length.
The Taupo Volcanic Zone (TVZ) is a young ( 60°). Systematic fracture sets occur in all rock types in the TVZ and generally show a strong northeast‐southwest trend, indicating a component of extensional strain distributed throughout the rock mass. Extension is predominantly orthogonal to the rift axes except in “accommodation zones” where interaction between offset rifting segments perturbs the stress field, locally enhancing permeability. The varying trends of extensional structures lie perpendicular to least principal stress trajectories in a locally heterogeneous stress field with normal dip‐slip as the predominant fault mechanism.
Isostatic residual gravity anomalies in the southern Cascade Range of northern California and southern Oregon are spatially correlated with broad zones of Quaternary magmatism as reflected by the total volume of Quaternary volcanic products, the distribution of Quaternary vents, and the anomalously low teleseismic P wave velocities in the upper 30 km of crust. The orientation of Quaternary faults also appears to be related to gravity anomalies and volcanism in this area, trending generally north-south within the magmatic regions and northwest-southeast as they enter the neighboring amagmatic zones to the north and south. The relationship between gravity anomalies, vent density, and fault orientations may indicate in a broad sense the strength of the middle and upper crust. The southern Cascade Range occupies a transition zone where horizontal stress is transferred from the northwest-southeast dextral shear of the Walker Lane belt to the east-west extension characteristic of the Cascade arc in central Oregon. Faulting along north-south strikes in the volcanically active areas indicates the east-west extensional stresses in thermally weakened crust, whereas northwest faulting between the volcanically active areas reflects the northwest trending, right lateral shear strain of the Walker Lane belt. The segmentation of the arc reflected in Quaternary magmatism may be caused by differential extension behind crustal blocks of the forearc rotating clockwise with respect to North America. In this view the volcanic centers at Mount Shasta, Medicine Lake volcano, and Lassen Peak in northern California are situated along the southern parts of the trailing edges of two distinct segments of the forearc where additional extension is implied by their differential clockwise rotation.
The November 30, 1974, Mt = 5.5 and November 16, 1983, Mt = 6.6 earthquakes generated left-stepping, en echelon ground cracks within the Kaoiki seismic zone, on the southeast flank of Mauna Loa volcano, Hawaii. The general trend of the ruptures, N48-55E, parallels a nodal plane of the main shocks' focal mechanisms. The ruptures themselves consist of short, predominantly extension cracks, which are up to 20 m long and strike roughly E-W, 300-50 degrees clockwise from the overall trend of the zones. Some of the cracks are linked by secondary fractures and rubble breccia to form left-stepping crack arrays, which are themselves linked to form longer en echelon systems of ground rupture. Geologic maps and field observations indicate that these features emerge from an underlying strike-slip fault, and they form a "fracture-process zone" above its tip. The maximum displacement measured across cracks in the 1983 rupture zone is 0.5 m. Trilateration data, however, suggests that the overall shear displacementw as about 1.5 m at depth. Elastic solutions indicate that a region of significant tensile stress can exist above buried strike-slip faults. We suggest that these stresses generated the extensional ground cracks and that shear displacements were transmitted to the Earth's surface by subsequent growth and linkage of these cracks into the observed arrays. We infer
that the crack arrays accommodate increased displacement with depth and they merge downward into the "parent" strike-slip fault at an estimated 1-2 km depth, where strike-slip displacement was probably more or less continuous along the ~7 km length of the rupture. In the Kaoiki region, only three major ground ruptures traverse a series of basaltic lava flows that date back 1500 years. This suggests that the recent ~10-year periodicity of moderate-magnitude Kaoiki strike-slip events may not have extended far into the past. The tectonic significance of strike-slip faulting on Mauna Loa volcano remains enigmatic.
The authors identify 537 volcanic vents younger than 7 Ma, and they classify these into five age intervals and five compositional categories based on SiOâ content. Maps of vents by age and composition illustrate regionally representative volcanic trends. Most mafic volcanism is calcalkaline basalt and basaltic andesite. However, lesser volume of low-potassium olivine tholeiite (LKOT), a geochemically distinctive basalt type found in the northern Basin and Range province, also has erupted throughout the Lassen segment of the Cascade arc since the Pliocene. Normal faults and linear groups of vents are evidence of widespread crustal extension throughout most of the Lassen region. NNW alignments of these features indicate NNW orientation of maximum horizontal stress (ENE extension), which is similar to the stress regime in the adjacent northwestern Basin and Range and northern Sierra Nevada provinces. They interpret the western limit of the zone of NNW trending normal faults as the western boundary of the Basin and Range province where it overlaps the Lassen segment of the Cascade arc. The Lassen volcanic region occurs above the subducting Gorda North plate but also lies within a broad zone of distributed extension that occurs in the North American lithosphere east and southeast of the present Cascadia subduction zone. The scarcity of volcanic rocks older than 7 Ma suggests that a more compressive lithospheric stress regime prior to the late Miocene extensional episode may have suppressed volcanism, even though subduction probably was occurring beneath the Lassen region.
Individual segments of the Koae Fault System, Hawaii, show four patterns of structures around fault tips, and these are inferred to represent four evolutionary stages of fault growth. These are: (1) monoclinal bending occurs, probably above a steeply dipping fault at depth, (2) cracks develop in the hinges of the monocline, (3) throw starts to develop on the cracks when they reach a width of about 3m, which is probably when they link downwards to the normal fault, and (4) rollover and related cracks develops in the hanging wall as throw increases. Widths of monocline- and fault-related cracks obey a power-law distribution, with a 3m upper cut-off, beyond which the monocline- and fault-related cracks develop a throw and become faults. Relay ramps are common within the highly segmented Koae and Hilina active normal fault systems. Three distinct geometries of relay ramps can be identified at Kilauea Volcano, and these are inferred to represent the following three evolutionary stages of relay ramps. (1) Where the bounding faults understep, the relay ramps have a gentle dip, and a set of en échelon cracks may cut across the relay ramp; these cracks suggest that the two understepping faults connect into a single fault at depth. (2) The dip of the relay ramp increases as the faults overstep. Connecting faults start to cut across the relay ramp. (3) When the relay ramp is breached by the connecting fault, a single, irregular fault is produced. Cracks or small breaching faults across a relay ramp suggest the bounding faults are connected at depth, and suggest that the bounding faults may both slip during an earthquake event.
Overlap lengths, separations and throw gradients were measured on 132 relay zones recorded on coal-mine plans. Throws on the relay-bounding fault traces are usually ≤ 2 m and individual structures are recorded on only one seam. Throw gradients associated with relay zones are not always higher than on single faults, but asymmetry of throw profiles is diagnostic of relay zones. Bed geometries around larger faults in opencast mines are used to assess the displacement accommodated by shear in the vertical plane normal to the faults and displacement transfer accommodated by shear in the fault-parallel plane. Three-dimensional structure is defined for two relay zones, each recorded on five seam plans. These relay zones are effectively holes through the fault surfaces and overlap occurs between salients or lobes of the parent fault surfaces. Lobes initially terminated at tip-lines but, as the faults grew, gradually rejoined the main fault surfaces along branch lines. This type of relay zone originates by bifurcation of a single fault surface at a locally retarded tip-line and is an almost inevitable result of a tip-line irregularity.
Interpretation of magnetic and new gravity data provides constraints on the geometry of the Hat Creek fault, the amount of right-lateral offset in the area between Mount Shasta and Lassen Peak (northern California, USA), and confirmation of the influence of preexisting structure on Quaternary faulting. Neogene volcanic rocks coincide with short-wavelength magnetic anomalies of both normal and reversed polarity, whereas a markedly smoother magnetic field occurs over the Klamath Mountains and Paleogene cover there. Although the magnetic field over the Neogene volcanic rocks is complex, the Hat Creek fault, which is one of the most prominent normal faults in the region and forms the eastern margin of the Hat Creek Valley, is marked by the eastern edge of a north-trending magnetic and gravity high 20-30 km long. Modeling of these anomalies indicates that the fault is a steeply dipping (~75°-85°) structure. The spatial relationship of the fault as modeled by the potential-field data, the youngest strand of the fault, and relocated seismicity suggest that deformation continues to step westward across the valley, consistent with a component of right-lateral slip in an extensional environment. Filtered aeromagnetic data highlight a concealed magnetic body of Mesozoic or older age north of Hat Creek Valley. The body's northwest margin strikes northeast and is linear over a distance of ~40 km. Within the resolution of the aeromagnetic data (1-2 km), we discern no right-lateral offset of this body. Furthermore, Quaternary faults change strike or appear to end, as if to avoid this concealed magnetic body and to pass along its southeast edge, suggesting that preexisting crustal structure influenced younger faulting, as previously proposed based on gravity data.
Through examination of the scaling relations of faults and the use of seismic stratigraphic techniques, we demonstrate how the temporal and spatial evolution of the fault population in a half-graben basin can be accurately reconstructed. The basin bounded by the >> 62-km-long Strathspey-Brent-Statfjord fault array is located on the western flank of the Late Jurassic northern North Sea rift basin. Along-strike displacement variations, transverse fault-displacement folds and palaeo-fault tips abandoned in the hangingwall all provide evidence that the fault system comprises a hierarchy of linked palaeo-segments. The displacement variations developed while the fault was in a prelinkage, multisegment stage of its growth have not been equilibrated following fault linkage. Using the stratal architecture of synrift sediments, we date the main phase of segment linkage as latest Callovian - middle Oxfordian (10-14 Myr after rift initiation). A dense subpopulation of faults is mapped in the hangingwall to the Strathspey-Brent-Statfjord fault array. The majority of these faults are short, of low displacement and became inactive within 3-4 Myr of the beginning of the extensional event. Subsequently, only the segments of the proto-Strathspey-Brent-Statfjord fault and a conjugate array of antithetic faults located 3.5 km basinward continued to grow to define a graben-like basin geometry. Faults of the antithetic array became inactive similar to 11.5 Myr into the rift event, concentrating strain on the linked Strathspey-Brent-Statfjord fault; hence, the basin evolved into a half-graben. As the rift event progressed, strain was localized on a smaller number of active structures with increased rates of displacement. The results of this study suggest that a simple model for the linkage of 2-3 fault segments may not be applicable to a complex multisegment array.
Well exposed Central Afar (Ethiopia/Djibouti) offers direct observation of a wide range of extensional structures and rift morphologies, at regional and local scale. Large Format Camera satellite photography and aerial photography was used to investigate in stereo-models phenomena of progressive extensional faulting. Representative successive fault geometries and fault evolution patterns are presented and discussed. -Author
Examination of well-constrained three-dimensional seismic data demonstrates the role of fault interaction and linkage in controlling the nature of synrift sequences on the hanging wall of the Statfjord East fault, a typical Late Jurassic structure in the northern North Sea Brent province. Although now a single fault, the Statfjord East fault originally consisted of several en echelon segments, each of which defined individual subbasins. Structural and stratigraphic evidence, both along and across fault strike, indicates that the fault resulted from segment propagation, interaction, and linkage. Facies architecture, thickness variations, and the internal character of synrift formations are temporally and spatially related to the subbasin geometry. Variations in displacement along the fault segments exhibit characteristics of interacting en echelon faults, including anomalous displacement gradients in regions of segment overlap. We attribute the observed shifts in depocenters to local enhancement of displacement rates, resulting from the interaction of neighboring fault segments. The results have far-reaching consequences for synrift plays in the northern North Sea because they imply that only from the perspective of fault growth and linkage can the Late Jurassic structure and stratigraphy be fully understood.
Seismic reflection data across six boundary faults from east Africa have revealed information about the way boundary faults develop with respect to the synkinematic basin fill. Previous models of basin fill assume that lateral fault propagation and deposition occur simultaneously, which gives rise to along-axis onlap of the basin fill onto the prerift basement (propagating basin model). Seismic reflection data from boundary faults in east Africa show only rare evidence for along-axis broadening of the basins. Instead, the basin margins appear to be fixed from a very early stage in the basin history (nonpropagating basin model). Depocenters shift not by gradual lateral propagation, but by abrupt changes in location. Such observations indicate that boundary fault propagation to near-maximum length occurs rapidly during the very early stages of rifting, probably by linkage of numerous small faults. Displacement then builds (following the b-value earthquake model) along the fault during basin development, out of phase with the main propagation phase. Mature fault systems may have experienced several phases of jumps in fault length or the location of fault activity. Major boundary faults exhibit three basic patterns of evolution; these patterns of evolution can have many variants and include (1) simple displacement, where maximum displacement is approximately at the center of the fault and decreases gradually toward the fault tips, (2) variable along-strike displacement that incorporates linkage of in-line or en-echelon, relatively large-displacement faults of similar age leaving transverse anticlines and synclines, and (3) asymmetric propagation where one tip of a fault remains more or less fixed while the other tip propagates a relatively long distance. Asymmetric propagation can occur smoothly and gradually or, alternatively, there can be a jump in the location of the fault, and a new fault of different age can form along-strike from the older fault. Eventually the new fault propagates and links with the older fault segment, reactivating the older fault to some extent. Additionally, long-lived faults may be composites of fault types 1-3. ©Copyright 1999. The American Association of Petroleum Geologists. All rights reserved.
We propose a methodology for the analysis of normal fault geometries in three-dimensional (3-D) seismic data sets to provide insights into the evolution of segmented normal fault systems and to improve recovery efforts in fault-controlled oil fields. Limited seismic resolution can obscure subtle fault characteristics such as segmentation and gaps in fault continuity that are significant for oil migration and thus accurate reservoir characterization. Detailed seismic data analyses that incorporate principles of normal fault mechanics, however, can reveal evidence of fault segmentation. We integrate seismic attribute analyses, outcrop analog observations, and numerical models of fault slip and displacement fields to augment the utilization of 3-D seismic data for fault interpretation. We applied these techniques to the Wytch Farm oil field in southern England, resulting in the recognition of significant lateral and, to a lesser extent, vertical segmentation of reservoir-scale faults. Slip maxima on fault surfaces indicate two unambiguous segment nucleation depths, controlled by the lithologic heterogeneity of the faulted section. Faults initiated preferentially in brittle sandstone and limestone units. Subsequent growth and linkage of segments, predominantly in the lateral direction, resulted in composite fault surfaces that have long lateral dimensions and multiple slip maxima. Reservoir compartmentalization is greatest at the level of prevalent segment linkages, which corresponds at Wytch Farm with the predominant hydrocarbon-producing unit, the Sherwood Sandstone. At relatively shallower depths, fault segments are younger and less evolved, resulting in a greater degree of segmentation with intact relay zones.
Traces of many normal faults form an array of closely spaced
overstepping segments. In three dimensions, fault segments may either be
unconnected or link vertically or laterally into a single continuous
fault surface. The slip distribution along segmented faults is complex
and asymmetric, and the point of maximum slip generally is not located
at the center of a segment. In relay zones between segments, slip
gradients may be gentler or steeper, depending on the spatial fault
arrangement. Branch points are characterized by steep slip gradients.
One explanation for these observations is mechanical interaction between
neighboring faults which occurs through local perturbation of the stress
field. Three-dimensional (3-D) boundary element models show that the
degree of fault interaction and hence the degree of asymmetry in the
slip distribution increases with increasing fault overlap and downdip
fault height and with decreasing fault spacing and Poisson's ratio.
Interaction is strongest for faults with uniform shear strength and
decreases if there exists a zone of greater shear strength near the tip
line. This analysis provides a mechanical rationale for more frequent
occurrence of overlapping segments relative to underlapping segments and
for the limited range of the ratio between segment overlap and spacing
along natural faults. Echelon segment configurations promote
interaction, maximize the capacity to accommodate slip, and do not
necessarily require a strike-slip movement component. Model
idealizations of some outcropping fault arrays and of branching/merging
faults capture a wide variety of common field observations. Consistent,
mechanically based 3-D normal fault models can be obtained by combining
different types of field data such as fault slip-to-length ratios,
location of maximum slip, segment overlap-to-spacing ratios, and
footwall uplift/hanging wall subsidence. By capitalizing on these data
one can understand the mechanics of faulting, constrain the boundary
conditions that govern the formation and growth of faults, and provide a
rationale for interpreting normal faults in seismic surveys.
New data indicate that northeast-directed extensional faulting
characterizes slip across the Brothers fault zone (BFZ), which marks the
northern limit of the northwestern Basin and Range (NWBR) extensional
province in southeastern Oregon. Structural separation across individual
north-northeast striking NWBR faults decreases to zero south of the BFZ.
Field relationships and cross-sections demonstrate limited kinematic
linkage and independent evolution of the two fault systems since
˜7 Ma. West-directed extension accumulated on NWBR faults at
0.01 mm/yr and lengthened northward after 7.05 Ma. BFZ faults
accumulated northeast-directed extension at rates of 0.01 mm/yr
since 5.68 Ma. Deformation coincides with periods of heightened
basaltic magmatism in the High Lava Plains, implying that volcanism
weakened the crust and promoted extension in the BFZ. In a new model, we
reconcile the observed northward diminishing rate and clockwise motion
of the modern NWBR deformation field with regional geology. The BFZ
defines a small circle about the pole of rotation and separates a stable
block to the NE from the extending region to the south. Faults to the
south are growing northward, consistent with the northward decrease in
rate and magnitude of extension in the NWBR.
Relay zones on normal faults are unlikely to have tabular geometries as depicted in idealised models. Rotation of a relay ramp between non-parallel and non-planar relay-bounding faults will inevitably lead to strain compatibility problems causing open gaps or overlaps within the relay zone. Linkage of relay-bounding faults does not evolve from a single branch point. Rather, linkage occurs at multiple points along the fault tip lines giving rise to initially discontinuous branch lines. Where linkage occurs along a discontinuous slip-aligned branch line, displacement at different levels within the relay zone is partitioned between variable amounts of ramp rotation and slip across the branch line. The linking fault propagates when strain compatibility can no longer be maintained by continuous deformation processes, such as thickening or thinning of incompetent layers within the relay ramp. Step-like changes in vertical displacement vs. distance (d − x) profiles on horizons containing apparently intact relay ramps are probably indicative of incipient breaching and can be used predict the presence of a slip-aligned branch line in the sub-surface. Despite the complexity of the strain distribution within relay zones, the total vertical displacement across the relay remains geometrically coherent at all levels.
Exceptionally well exposed normal faults within the Solite Quarry of the Dan River rift basin range in length from a few millimetres to a few metres and are possibly the smallest visible faults studied to date. Displacement is greatest at or near the center of isolated faults and decreases toward the fault tips. Relay structures form between closely overlapping faults. The distribution of fault sizes in the study area follows a power-law (fractal) relation, and the maximum observed displacement scales linearly with fault length. The new fault data extend the global data set to more than eight orders of magnitude of fault length and indicate that there is no significant change in displacement geometry and the linear length-displacement scaling relation between small and large faults.
This paper focuses on field studies and numerical models of fracture development in the area of the Hengill Central Volcano and its northern fissure swarm containing the Thingvellir Graben, in Southwest Iceland. Apart from additional field data on normal faults, a new detailed map of the Holocene fractures in the Thingvellir Graben is presented and used as a basis for numerical models on normal fault development. Field observations and models yield four basic results. First, hyaloclastite mountains in the area (“soft mechanical inclusions”) tend to deflect or arrest propagating tension fractures and normal faults. Second, fractures with an en echelon arrangement and an overlapping configuration develop shear stress shadows and tend to prevent the linking up into larger fault segments. Third, fractures with an en echelon arrangement and underlapping configuration develop shear stress concentrations between nearby tips resulting in development of transverse shear faults (transfer fault). Fourth, colinear normal fault segments concentrate tensile stresses at their tips and encourage tip-to-tip growth into larger segments.
The lithosphere P wave velocity structure beneath northern California is determined on the basis of teleseismic P wave travel time residuals from 120 earthquakes recorded across the USGS California seismic network. Lateral P wave variations beneath the array are determined by inversion of 9383 travel time residuals. Inversion results for the crust show strong correlations to volcanic features. The active volcanic fields, Shasta-Medicine Lake, Lassen, and Clear Lake, are characterized by crustal low-velocity anomalies that average approximately -6 percent, possibly identifying partially molten magma bodies. The largest upper mantle velocity variations occur in the depth range 30-110 km, where velocities vary from -5.5 to +9.5 percent. Results provide improved constraints on Gorda plate subduction, evolution of the San Andreas fault system, and the development of the lithosphere beneath western North America.
Normal fault systems bounding extensional basins are typically adjoined by a series of subbasins separated by intrabasin highs. The strata within these basins form syndepositional anticlines and synclines whose axes are transverse to the strike of the main bounding fault. One possible explanation for these intrabasin highs is that they result from persistent along-strike deficits in fault displacement. Such deficits are incompatible with scaling relationships observed between fault displacement and length based on large populations of faults. W e present data from active normal faults within the Basin and Range province and from inactive normal faults of the Newark basin of eastern North America demonstrating a clear correlation between the along-strike position of overlapping splay faults and the location of intrabasin highs as well as syndepositional transverse folds. Summed displacements for all faults within an intrabasin high are comparable to the displacements on faults bounding flanking subbasins. Older synextensional deposits exhibit localized tilt maxima within subbasins flanking an intrabasin high whereas younger units exhibit uniform tilt patterns across the entire region. Footwall elevation profiles, used as a proxy for fault displacement, define uniform arcuate patterns independent of along-strike position of intrabasin highs. These characteristics of hanging walls and footwalls suggest that intrabasin highs do not represent locations of long-term fault-displacement deficits, but rather are the location of anastomosing fault segments, which upon linking together, rapidly compensate for initial displacement deficits by increased displacement distributed over several splay faults.
Normal faults observed in extensional clay models evolve by displacement (throw) accumulation and concomitant trace length increase, and by segment linkage. The first of these processes leads to an increase in the maximum throw to trace length (Dmax/L) ratio, whereas the second leads to a decrease in this ratio. With increasing extension individual faults evolve along stepwise tracks in Dmax/L parameter space, although at any given time a summary plot of Dmax versus L for the entire fault population will typically span one order of magnitude in Dmax/L ratio and two orders of magnitude in trace length, obscuring the stepwise nature of fault evolution and giving the impression of self-similar fault growth along a locus of constant Dmax/L ratio. In addition, the number of simple faults (faults with a single throw maximum) incorporated into a compound fault (a fault with multiple throw maxima) is strongly correlated with the fault’s trace length, indicating that trace length increase is dominated by linkage not throw accumulation. When analyzed for evidence of relict simple faults (expressed as throw maxima), normal faults observed on Mars exhibit similar characteristics to those developed in analog clay models. By analogy, we infer that Mars normal faults evolve in a similar fashion to faults in clay models.
The overlap and separation distances of relay zones follow a power-law scaling relationship over nearly 8 orders of magnitude. Approximately one order of magnitude scatter in both separation and overlap exists at all scales. The strong power-law relationship (R2 = 0.98) suggests that the primary control on relay aspect ratio (overlap/separation) is a scale-invariant process, such as the stress interaction between the overlapping fault tips as suggested by previous authors. Within the compiled global dataset host rock lithology does not appear to be a first-order control. Much of the observed scatter can be attributed to the spread of measurements recorded from individual relay zones, which relates to the evolving three-dimensional geometry of the relay zone as displacements on the bounding faults increase. Relay ramps exposed at two localities where the faults cut layered sedimentary sequences display mean aspect ratios of 8.20 and 8.64 respectively, more than twice the global mean (4.2). Such high aspect ratios can be attributed to the relay-bounding faults having initially been confined within competent layers, facilitating the development of large overlap lengths. The presence of pre-existing structures (veins) at fault tips may also enhance fault propagation, giving rise to increased overlap lengths.
In the western Great Basin of North America, a system of dextral faults accommodates 15%-25% of the Pacific-North American plate motion. The northern Walker Lane in northwest Nevada and northeast California occupies the northern terminus of this system. This young evolving part of the plate boundary offers insight into how strike-slip fault systems develop and may reflect the birth of a transform fault. A belt of overlapping, left- stepping dextral faults dominates the northern Walker Lane. Offset segments of a W- trending Oligocene paleovalley suggest ;20-30 km of cumulative dextral slip beginning ca. 9-3 Ma. The inferred long-term slip rate of ;2-10 mm/yr is compatible with global positioning system observations of the current strain field. We interpret the left-stepping faults as macroscopic Riedel shears developing above a nascent lithospheric-scale trans- form fault. The strike-slip faults end in arrays of ;N-striking normal faults, suggesting that dextral shear diffuses into extension in the Great Basin. Coeval extension and dextral shear have induced slight counterclockwise fault-block rotations, which may ultimately rotate Riedel shears toward the main shear zone at depth, thus facilitating development of a throughgoing strike-slip fault.
Segmented graben systems develop stepovers that have important implications in the exploration of oil and gas in extensional tectonic basins. We have compared and modeled a representative stepover between grabens in Canyonlands, Utah, and the North Sea Viking Graben and, despite their different structural settings, found striking similarities that pertain to other graben systems. In both cases, the stepovers represent relatively high parts within the graben systems that are likely to be among the first to be filled with hydrocarbons generated in deeper parts of the grabens. Furthermore, the relay ramps and smaller fault offsets in stepovers ease hydrocarbon migration and allow stepovers to act as preferred migration routes from deep graben kitchens to structurally higher traps in the basin. Graben stepovers and their related structures should be paid special attention during exploration because they may represent hydrocarbon accumulations complementary to larger traps along the graben flanks. These observations explain the location of the Kvitebjorn, Valemon, and Huldra fields in a stepover structure of the Viking Graben and encourage increased focus on similar graben stepovers in the Viking Graben and other graben systems.
Fieldwork within a series of mesoscale grabens in southeast Utah has revealed a particularly well-exposed system of interlinked extensional faults. A series of down-faulted grabens consist of two or more overlapping elements, which are composed of fault segments. These segments may be hard-linked (fault surfaces are joint) or soft-linked (fault surfaces are isolated, but linked by ductile strain of the rock volume between them) in map view. Relay structures are defined as zones connecting the footwalls and hanging walls of overlapping fault segments representing soft linkage of fault segments. In the Canyonlands grabens, the transfer of displacement between soft-linked fault segments is characterized by well-defined, dipping relay ramps commonly rotated and extended to accommodate the ductile strain between the overlapping fault segments. -from Authors
A dominantly basaltic late Miocene and early Pliocene (approx 5-10Ma old) lava field lies directly E of the Cascade Range on the NW edge of the Basin and Range province. >12 400km2 of Modoc County, California, and Klamath and Lake Counties, Oregon, termed the Devils Garden, is underlain by basalt with a total volume of approx 850km3. The basalt of Devils Garden is diktytaxitic olivine tholeiite, characterized by high Al content; low K, Rb, and Cs content; and high K/Rb and K/Cs ratios. In these respects, it resembles mid-oceanic-ridge basalt (MORB), although it differs in other respects, such as high concentration of Ba and Sr, low K/Ba, and higher 87Sr/86Sr (0.7936-0.7039) ratios. Chemical characteristics indicate that little or no contamination by sialic crustal material has occurred. The basalts originated in the upper mantle and were erupted through crust thinned by tectonic extension behind the Cascade Range volcanic arc. -Authors
The geomorphic characteristics of young fault scarps can be used as a key to the ages of fault displacements. The principal features of scarps younger than a few thousand years are a steep free face, a debris slope standing at about 35°, and a sharp break in slope at the crest of the scarp. The principal slope of older scarps declines with age, so that scarps of about 12,000 yr of age have maximum slope angles of 20° to 25°, and slopes as low as 8° to 9° represent ages much older than about 12,000 yr. The crestal break in slope broadens with age. The material in the scarp face, whether loose fanglomerate or indurated bedrock, controls to a large extent the rate of scarp degradation. Where more than one displacement has occurred along a fault, a composite or multiple scarp develops. Composite or multiple scarps suggest mean recurrence intervals on individual faults measured in thousands of years.
The Walker Lane and northern part of the eastern California shear zone
form a boundary zone accommodating differential motion between the
Sierra Nevada and western Great Basin. Within the boundary zone, Global
Positioning System velocities show a westward increase from 2 3 mm/yr in
the central Great Basin to ˜14 mm/yr in the Sierra Nevada and a
clockwise rotation from west-northwest to northwest. In the same region,
incremental extensional strain axes recorded by earthquake focal
mechanisms and fault-slip inversion show an east to west
counterclockwise rotation of 50°, from parallel with the velocity
field in the central Great Basin to nearly orthogonal to the velocity
field along the eastern flank of the Sierra Nevada. Unlike plane-strain
deformation within the central Great Basin, the progressive deviation
between the orientation of extension axes and the velocity field in the
boundary zone is a product of nonplane strain. The boundary zone records
active constriction formed in conditions varying from wrench-dominated
to extension-dominated transtension.
Rift-basin stratigraphy commonly records an early stage of slow
subsidence followed by an abrupt increase in subsidence rate. The
physical basis for this transition is not well understood, although an
increase in extension rate is commonly implied. Here, a numerical
fault-growth model is used to investigate the influence of segment
linkage on fault-displacement-rate patterns along an evolving normal
fault array. The linkage process we describe is controlled by a stress
feedback mechanism, which leads to enhanced growth of optimally
positioned faults. Model results indicate that, even with constant
extension rates, slow displacement rates prevail during an initial phase
of distributed extension, followed by an increase in displacement rates
as strain becomes localized on linked fault arrays. This is due to the
dynamics of fault interactions rather than mechanical weakening.
Comparison of model simulations with rift-basin subsidence and
stratigraphic patterns in the Gulf of Suez and North Sea suggests that
the occurrence and timing of rapid basin deepening can be explained by
the mechanics of fault-zone evolution, without invoking a change in
regional extension rates.
Field observations of two overlapping normal faults and associated deformation document features common to many normal-fault relay zones: a topographic ramp between the fault segments, tapering slip on the faults as they enter the overlap zone, and associated fracturing, especially at the top of the ramp. These observations motivate numerical modeling of the development of a relay zone. A three-dimensional boundary element method numerical model, using simple fault-plane geometries, material properties, and boundary conditions, reproduces the principal characteristics of the observed fault scarps. The model, with overlapping, semicircular fault segments under orthogonal extension, produces a region of high Coulomb shear stress in the relay zone that would favor fault linkage at the center to upper relay ramp. If the fault height is increased, the magnitude of the stresses in the relay zone increases, but the position of the anticipated linkage does not change. The amount of fault overlap changes the magnitude of the Coulomb stress in the relay zone: the greatest potential for fault linkage occurs with the closest underlapping fault tips. Ultimately, the mechanical interaction between segments of a developing normal-fault system promote the development of connected, zigzagging fault scarps.
Spatial, temporal, and compositional distributions of c4000 volcanic vents formed since 16 Ma in Washington, Oregon, N California, and NW Nevada illustrate the evolution of volcanism related to subduction of the Juan de Fuca plate system and extension of the Basin and Range province. Vent data were obtained from published map compilations and include monogenetic and small polygenetic volcanoes in addition to major composite centers. On the basis of the distribution of 2821 vents formed since 5 Ma, the Cascade Range is divided into 5 segments, with vents of the High Lava Plains along the northern margin of the Basin and Range province in Oregon forming a sixth segment. Some aspects of the Cascade Range segmentation can be related to gross structural features of the subducting Juan de Fuca plate.-from Authors
To determine whether the structural evolution of the Northern Cerberus Fossae (NCF) was dominated by cryospheric melting and collapse or fault-related subsidence, we used MOC, THEMIS and HiRISE images, and MOLA data to document spatial variations in vertical offset along strike. The Fossae are a series of fractures on the martian surface that cross-cut Noachian, Hesperian and, in places, very young Late Amazonian terrain. Serial cross sections across the fracture-related topography, from MOLA data, show that vertical offsets are not greater where fractures traverse older terrain, showing that offsets have accumulated since the formation of the Amazonian terrain. Vertical offsets are greater in the central portions of the fracture system with the profile resembling that for a single fault system. Topographic features that pre-date deformation are preserved on the graben floors suggesting little sediment infill, so the MOLA elevation measurements constrain total vertical offsets since the fractures formed. Deficits in vertical offset occur where fractures have not linked and remain en echelon across relay zones, or have linked, leaving palaeo-graben-tips. This indicates that the traces of the fractures propagate along strike at the surface and intersect over time periods that are likely to be in the range of 105–106 years rather than in a single collapse event. Deficits are also in places associated with collapse pits, suggesting such collapse is the early stage of graben subsidence at propagating lateral graben-tips. We use these observations to argue that the primary mechanism causing subsidence is not cryospheric melting and collapse, but faulting.
Map patterns of normal fault linkages near Summer Lake, Oregon, show a systematic relationship between échelon step-sense, oblique-slip sense, and the position of linking faults. Where the step sense is the same as the sense of oblique slip (e.g. left step and left-oblique slip), the faults are linked in the lower part of their relay ramp. Where the step-sense and slip-sense are opposite (e.g. left-step and right-oblique slip), the faults are linked in the upper part of the ramp. A boundary-element code is used to calculate the stress field around échelon normal faults during oblique slip, and the model results reveal a relationship similar to the field observations. If step sense and oblique-slip sense are the same, there is a greater potential for deformation ahead of the tip of the front fault and in the lower part of the ramp. If step sense and oblique-slip sense are opposite, there is a greater potential for deformation ahead of the tip of the rear fault and in the upper part of the ramp. The field-model comparison confirms that oblique slip modifies the mechanical interaction among fault segments and thus influences fault growth and the geometry of fault linkage.