Figure 2 - uploaded by Donald Sweetkind
Content may be subject to copyright.
Source publication
The U.S. Geological Survey (USGS), the Desert Research Institute (DRI), and a designee from the State of Utah are currently conducting a water-resources study of aquifers in White Pine County, Nevada, and adjacent areas in Nevada and Utah, in response to concerns about water availability and limited geohydrologic information relevant to ground-wate...
Contexts in source publication
Context 1
... related to large-volume ash-flow eruptions and associated caldera collapses are enumerated by Smith and Bailey (1968) and by Lipman (1984) and are summa- rized below. Calderas can be as much as 120 km in diameter, are structurally complex features, and most are bounded by a pair of geologic discontinuities, a structural margin and a topographic margin ( fig. 2), both of which may be obscured by subsequent volcanism and erosion. These discontinuities are generally concentric and related to the structural collapse that is the hallmark of caldera-forming eruptions. Calderas form when large volumes of magma are nearly instanta- neously erupted from shallowly emplaced magma reservoirs. As an eruption of this type ensues, the associated magma reservoir is partially evacuated by the eruption of frothy magma, and the central block of roof rock that lay above the reservoir collapses downward along a series of arcu- ate faults. The resulting system of faults forms a generally circular system of normal faults that constitute the caldera's structural margin. The lithologic discontinuity across the steeply inclined structural margin can be profound and can extend to depths of several kilometers . The resulting caldera wall begins to retreat outward as landslides calve off the oversteepened walls and contribute material to the deepening depression caused by the eruption and concomitant central collapse of the volcanic edifice ( fig. 2). Outward retreat of the caldera boundary by subsequent landsliding forms a sec- ond, more gently inclined concentric discontinuity known as the topographic margin. Simultaneous with central collapse and landslide formation, the evolving central depression begins to be filled by the volcanic products derived from the ongoing eruption. This rapidly evolving intracaldera environment is usually filled by a kilometers-thick accumu- lation of ash-flow tuff and interleaved landslide materials ( fig. 2). The discontinuity across the caldera's topographic margin, between intracaldera tuff and the country rock that host the caldera, can be at least as profound as that across the structural margin. Following caldera-forming eruptions, some of these igneous systems experience a central upward resurgence of unerupted magma from the underlying magma reservoir. Resurgence further complicates and disrupts the geology within the caldera ( fig. ...
Context 2
... related to large-volume ash-flow eruptions and associated caldera collapses are enumerated by Smith and Bailey (1968) and by Lipman (1984) and are summa- rized below. Calderas can be as much as 120 km in diameter, are structurally complex features, and most are bounded by a pair of geologic discontinuities, a structural margin and a topographic margin ( fig. 2), both of which may be obscured by subsequent volcanism and erosion. These discontinuities are generally concentric and related to the structural collapse that is the hallmark of caldera-forming eruptions. Calderas form when large volumes of magma are nearly instanta- neously erupted from shallowly emplaced magma reservoirs. As an eruption of this type ensues, the associated magma reservoir is partially evacuated by the eruption of frothy magma, and the central block of roof rock that lay above the reservoir collapses downward along a series of arcu- ate faults. The resulting system of faults forms a generally circular system of normal faults that constitute the caldera's structural margin. The lithologic discontinuity across the steeply inclined structural margin can be profound and can extend to depths of several kilometers . The resulting caldera wall begins to retreat outward as landslides calve off the oversteepened walls and contribute material to the deepening depression caused by the eruption and concomitant central collapse of the volcanic edifice ( fig. 2). Outward retreat of the caldera boundary by subsequent landsliding forms a sec- ond, more gently inclined concentric discontinuity known as the topographic margin. Simultaneous with central collapse and landslide formation, the evolving central depression begins to be filled by the volcanic products derived from the ongoing eruption. This rapidly evolving intracaldera environment is usually filled by a kilometers-thick accumu- lation of ash-flow tuff and interleaved landslide materials ( fig. 2). The discontinuity across the caldera's topographic margin, between intracaldera tuff and the country rock that host the caldera, can be at least as profound as that across the structural margin. Following caldera-forming eruptions, some of these igneous systems experience a central upward resurgence of unerupted magma from the underlying magma reservoir. Resurgence further complicates and disrupts the geology within the caldera ( fig. ...
Context 3
... related to large-volume ash-flow eruptions and associated caldera collapses are enumerated by Smith and Bailey (1968) and by Lipman (1984) and are summa- rized below. Calderas can be as much as 120 km in diameter, are structurally complex features, and most are bounded by a pair of geologic discontinuities, a structural margin and a topographic margin ( fig. 2), both of which may be obscured by subsequent volcanism and erosion. These discontinuities are generally concentric and related to the structural collapse that is the hallmark of caldera-forming eruptions. Calderas form when large volumes of magma are nearly instanta- neously erupted from shallowly emplaced magma reservoirs. As an eruption of this type ensues, the associated magma reservoir is partially evacuated by the eruption of frothy magma, and the central block of roof rock that lay above the reservoir collapses downward along a series of arcu- ate faults. The resulting system of faults forms a generally circular system of normal faults that constitute the caldera's structural margin. The lithologic discontinuity across the steeply inclined structural margin can be profound and can extend to depths of several kilometers . The resulting caldera wall begins to retreat outward as landslides calve off the oversteepened walls and contribute material to the deepening depression caused by the eruption and concomitant central collapse of the volcanic edifice ( fig. 2). Outward retreat of the caldera boundary by subsequent landsliding forms a sec- ond, more gently inclined concentric discontinuity known as the topographic margin. Simultaneous with central collapse and landslide formation, the evolving central depression begins to be filled by the volcanic products derived from the ongoing eruption. This rapidly evolving intracaldera environment is usually filled by a kilometers-thick accumu- lation of ash-flow tuff and interleaved landslide materials ( fig. 2). The discontinuity across the caldera's topographic margin, between intracaldera tuff and the country rock that host the caldera, can be at least as profound as that across the structural margin. Following caldera-forming eruptions, some of these igneous systems experience a central upward resurgence of unerupted magma from the underlying magma reservoir. Resurgence further complicates and disrupts the geology within the caldera ( fig. ...
Context 4
... related to large-volume ash-flow eruptions and associated caldera collapses are enumerated by Smith and Bailey (1968) and by Lipman (1984) and are summa- rized below. Calderas can be as much as 120 km in diameter, are structurally complex features, and most are bounded by a pair of geologic discontinuities, a structural margin and a topographic margin ( fig. 2), both of which may be obscured by subsequent volcanism and erosion. These discontinuities are generally concentric and related to the structural collapse that is the hallmark of caldera-forming eruptions. Calderas form when large volumes of magma are nearly instanta- neously erupted from shallowly emplaced magma reservoirs. As an eruption of this type ensues, the associated magma reservoir is partially evacuated by the eruption of frothy magma, and the central block of roof rock that lay above the reservoir collapses downward along a series of arcu- ate faults. The resulting system of faults forms a generally circular system of normal faults that constitute the caldera's structural margin. The lithologic discontinuity across the steeply inclined structural margin can be profound and can extend to depths of several kilometers . The resulting caldera wall begins to retreat outward as landslides calve off the oversteepened walls and contribute material to the deepening depression caused by the eruption and concomitant central collapse of the volcanic edifice ( fig. 2). Outward retreat of the caldera boundary by subsequent landsliding forms a sec- ond, more gently inclined concentric discontinuity known as the topographic margin. Simultaneous with central collapse and landslide formation, the evolving central depression begins to be filled by the volcanic products derived from the ongoing eruption. This rapidly evolving intracaldera environment is usually filled by a kilometers-thick accumu- lation of ash-flow tuff and interleaved landslide materials ( fig. 2). The discontinuity across the caldera's topographic margin, between intracaldera tuff and the country rock that host the caldera, can be at least as profound as that across the structural margin. Following caldera-forming eruptions, some of these igneous systems experience a central upward resurgence of unerupted magma from the underlying magma reservoir. Resurgence further complicates and disrupts the geology within the caldera ( fig. ...
Context 5
... published isopach maps were scanned and geo- referenced in a Geographic Information System (GIS) and the location and thickness values as measured at outcrops of individual tuffs were digitized from these published maps. In order to preserve the original author's interpreted contour patterns, additional thickness data were created by digitiz- ing a regular series of points along each contour line of the isopach maps. For each tuff, these thickness data were gridded as 2,500-m square cells within the GIS to produce a digital thickness grid as raster data sets. Thickness was gridded using either inverse distance or simple kriging algo- rithms; the gridding methodology was chosen on the basis of how closely the digital grid resembled the original published contour map. Grids were locally hand-edited in order to recre- ate abrupt thickness changes at known or inferred caldera boundaries. Using this approach, digital thickness grids were created for the following ash-flow tuffs: the Kalamazoo Tuff ( fig. 5), the Windous Butte Formation ( fig. 6), the Monotony Tuff ( fig. 7), the Shingle Pass Tuff ( fig. 8), the Cottonwood Wash Tuff ( fig. 9), the Wah Wah Springs Formation ( fig. 10), the Lund Formation ( fig. 11), the Isom Formation ( fig. 12), the Leach Canyon Formation ( fig. 13), the Condor Canyon Formation ( fig. 14), and the Harmony Hills Tuff ( fig. 15). In each of these figures two maps are shown. The larger map portrays the thickness of the individual ash-flow tuff con- toured using intervals that display most clearly the thickness variations of the unit. The smaller map in each figure shows the same thickness data, but contoured at intervals consistent with those used for the thickest tuffs in the region. Thus the reader may see the details of the thickness variations within each tuff and also gain an understanding of the thickness of the ash-flow tuff as compared to the largest eruptions in the study area. In many cases, the thickest intervals do not exactly correspond to mapped caldera boundaries. This disparity is usually the result of limited outcrop data; in most cases the thickness and distribution of the intracaldera fill on published isopach maps may be represented by a single data point. In a few cases, the caldera boundaries themselves are only generally located, or the thickness of the intracaldera volcanic rocks is poorly known due to disruption by younger structural events, or burial by younger rocks. Thickness data portrayed on the individual isopach maps were added together to produce a composite isopach map that combines the gridded thickness for all of the previously named units ( fig. 16). The composite isopach map is dominated by the thick intracaldera accumulations within the Indian Peak caldera complex and the central Nevada caldera complex ( fig. 3). The thickness of intracaldera rocks within the Caliente caldera complex ( fig. 3) may be underrepresented in this compilation due to the rela- tive lack of published thicknesses of intracaldera rocks; how- ever, these eruptions were generally much smaller in volume than the other two main caldera centers. Using the composite isopach map ( fig. 16), one can predict the relative thickness of outflow tuffs between the caldera ...
Similar publications
Geologic mapping and structural analysis in the Bannock Range, SE Idaho, indicate that the bulk of Cenozoic extension in southeast Idaho was accommodated by slip on the low-angle Bannock detachment system. The Bannock detachment system consists of low-angle normal faults with a likely N-S extent of >130 km. It was active from ca. 10.3 Ma to ca. 3-4...
Citations
... and corrected for an assumed uniform 50% east-west crustal extension post-dating volcanism (Appendix). From published geologic maps, supplemented by fi eld measurements, more than 800 determinations of the thickness (in meters) of individual ignimbrite units were made in the Indian Peak-Caliente fi eld (Supplemental File 6 6 ; see also Sweetkind and du Bray, 2008). Most thicknesses (in meters) of outfl ow sheets were measured at sites where older and younger deposits are exposed to constrain the entire cooling unit; erosion of any of the upper part of the sheet prior to deposition of the younger unit is assumed to be nil. ...
The Indian Peak-Caliente caldera complex and its surrounding ignimbrite field were a major focus of explosive silicic activity in the eastern sector of the subduction-related southern Great Basin ignimbrite province during the middle Cenozoic (36-18 Ma) ignimbrite flareup. Caldera-forming activity migrated southward through time in response to rollback of the subducting lithosphere. Nine partly exposed, separate to partly overlapping source calderas and an equal number of concealed sources compose the Indian Peak-Caliente caldera complex. Calderas have diameters to as much as 60 km and are filled with as much as 5000 m of intracaldera tuff and wall-collapse breccias. More than 50 ignimbrite cooling units, including 22 of regional (> 100 km(3)) extent, are distinguished on the basis of stratigraphic position, chemical and modal composition, Ar-40/Ar-39 age, and paleomagnetic direction. The most voluminous ash flows spread as far as 150 km from the caldera complex across a high plateau of limited relief-the Great Basin altiplano, which was created by late Paleozoic through Mesozoic orogenic deformation and crustal thickening. The resulting ignimbrite field covers a present area of similar to 60,000 km(2) in east-central Nevada and southwestern Utah. Before post-volcanic extension, ignimbrites had an estimated aggregate volume of similar to 33,000 km(3). At least seven of the largest cooling units were produced by super-eruptions of more than 1000 km(3). The largest, at 5900 km(3), originally covered an area of 32,000 km(2) to outflow depths of hundreds of meters. Outflow ignimbrite sequences comprise as many as several cooling units from different sources with an aggregate thickness locally reaching a kilometer; sequences are almost everywhere conformable and lack substantial intervening erosional debris and angular discordances, thus manifesting a lack of synvolcanic crustal extension. Fallout ash in the mid-continent is associated with two of the super-eruptions. Ignimbrites are mostly calc-alkalic and high-K, a reflection of the unusually thick crust in which the magmas were created. They have a typical arc chemical signature and define a spectrum of compositions that ranges from high-silica (78 wt%) rhyolite to andesite (61 wt% silica). Rhyolite magmas were erupted in relatively small volumes more or less throughout the history of activity, but in a much larger volume after 24 Ma in the southern part of the caldera complex, creating 10,000 km3 of ignimbrite. The field has some rhyolite ignimbrites, the largest of which are in the south and were emplaced after 24 Ma. But the most distinctive attributes of the Indian Peak-Caliente field are two distinct classes of ignimbrite: 1. Super-eruptive monotonous intermediates. More or less uniform and unzoned deposits of dacitic ignimbrite that are phenocryst rich (to as much as similar to 50%) with plagioclase > biotite approximate to quartz approximate to hornblende > Fe-Ti oxides +/- sanidine, pyroxene, and titanite; apatite and zircon are ubiquitous accessory phases. These tuffs were deposited at 31.13, 30.06, and 29.20 Ma in volumes of 2000, 5900, and 4400 km(3), respectively, from overlapping, multicyclic calderas. A unique, and pos-sibly kindred, phenocryst-rich latiteandesite ignimbrite with an outflow volume of 1100 km(3) was erupted at 22.56 Ma from a concealed source caldera to the south. 2. Trachydacitic Isom-type tuffs. Also relatively uniform but phenocryst poor (< 20%) with plagioclase >> clinopyroxene approximate to orthopyroxene approximate to Fe-Ti oxides >> apatite. These alkali-calcic tuffs are enriched in TiO2, K2O, P2O5, Ba, Nb, and Zr and depleted in CaO, MgO, Ni, and Cr, and have an arc chemical signature. Magmas were erupted from a concealed source immediately after and just to the southeast of the multicyclic monotonous intermediates. Most of their aggregate outflow volume of 1800 km3 was erupted from 27.90 to 27.25 Ma. Nothing like this couplet of distinct ignimbrites, in such volumes, have been documented in other middle Cenozoic volcanic fields in the southwestern U. S. where the ignimbrite flareup is manifest. Magmas were created in unusually thick crust (as thick as 70 km) where large-scale inputs of mantle-derived basaltic magma powered partial melting, assimilation, mixing, and differentiation processes. Dacite and some rhyolite ignimbrites were derived from relatively low-temperature (700-800 C), water-rich magmas that were a couple of log units more oxidized than the quartz-fayalite-magnetite (QFM) oxygen buffer at depths of similar to 8-12 km. In contrast to these "main-trend" magmas, trachydacitic Isom-type magmas were derived from drier and hotter (similar to 950 degrees C) magmas originating deeper in the crust (to as deep as 30 km) by fractionation processes in andesitic differentiates of the mantle magma. "Off-trend" rhyolitic magmas that are both younger and older than the Isom type but possessed some of their same chemical characteristics possibly reflect an ancestry involving Isom-type magmas as well as main-trend rhyolitic magmas. Andesitic lavas extruded during the flareup but mostly after 25 Ma constitute a roughly estimated 12% of the volume of silicic ignimbrite, in contrast to major volcanic fields to the east, e. g., the Southern Rocky Mountain field, where the volume of intermediate-composition lavas exceeds that of silicic ignimbrites.
Uncertainty surrounds the fate of the orogenic plateau in what is now the Great Basin in western Utah and Nevada, which resulted from the Mesozoic and earliest Cenozoic contractile deformations and crustal thickening. Although there is some consensus regarding the gravitational collapse of the plateau by extensional faulting and consequent crustal thinning, whether or not the plateau existed during the middle Cenozoic Great Basin ignimbrite flareup – one of the grandest expressions of continental volcanism in the geologic record – had remained in doubt. We use compositions of contemporaneous calc-alkaline lava flows as well as configurations of the ignimbrite sheets to show that the Great Basin area during the middle Cenozoic was a relatively smooth plateau underlain by unusually thick crust. We compare analyses of 376 intermediate-composition lava flows in the Great Basin that were extruded at 42–17 Ma with compositions of >6000 analyses of the late Cenozoic lava flows in continental volcanic arcs that correlate roughly with known crustal thickness. This comparison indicates that the middle Cenozoic Great Basin crust was much thicker than the present ca. 30 km thickness, likely as much as 60–70 km. If isostatic equilibrium prevailed, this unusually thick continental crust must have supported high topography. This high terrain in SE Nevada and SW Utah was progressively smoothed as successive ignimbrite outflow sheets were emplaced over areas currently as much as tens of thousands of square kilometres to aggregate thicknesses of as much as hundreds of metres. The generally small between-site variations in the palaeomagnetic directions of individual sheets lend further support for a relatively smooth landscape over which the sheets were draped. We conclude that during the middle Cenozoic, especially towards the close of the ignimbrite flareup, this Great Basin area was a relatively flat plateau, and because it was also high in elevation, we refer to it as an Altiplano. It was not unlike the present-day Altiplano-Puna in the tectonically similar central Andes, where an ignimbrite flareup comparable to that in the Great Basin occurred at ca. 9–3 Ma. Outflow ignimbrite sheets that were deposited from 35 to 23 Ma on the progressively smoothed Altiplano in south-eastern Nevada were derived from source calderas to the west. Of the 12 major sheets from seven sources, nine are distributed unevenly east of their sources while the remaining three sheets are spread about as far east as west of their sources. This eccentricity of sources near the western margin of 75% of the sheets indicates the existence of a NS-trending topographic barrier in central Nevada that restricted westward dispersal of ash flows. In a symmetric manner, eastward dispersal of ash flows from sources farther west seemed to have been impeded by this same topographic barrier. The westward dispersal was controlled in part by westward-draining stream valleys incised in the sloping flank of the Great Basin Altiplano in western Nevada and adjacent California; at least one of these ash flows travelled as far west as the western foothills of the Sierra Nevada. The nature and origin of the implied topographic barrier are uncertain. It is possible that heavy orographic precipitation on the western slope of the Altiplano and consequent focused denudation and isostatic uplift created a NS-trending topographic high at the crest of the western slope and facing the smoothed Altiplano to the east. The barrier also lies near and essentially parallel to the buried western edge of the Precambrian basement and to a zone of thermal-diapiric domes that were spawned in thickened crust as the basement edge was overrun by late Palaeozoic–Mesozoic thrust sheets.
