Canyon Caldera Research Articles

Aeromagnetic mapping of the structure of Pine Canyon caldera and Chisos Mountains intrusion, Big Bend National Park, Texas

Analysis of aeromagnetic and gravity data reveals new details of the structure, igneous geology, and temporal evolution of the prominent, enigmatic ca. 32 Ma Pine Canyon caldera and the Chisos Mountains (Big Bend National Park, Texas). The main caldera-fi lling Pine Canyon Rhyolite, the oldest member of the South Rim Formation, is reversely magnetized, allowing it to be used as a key marker bed for determining caldera fi ll thickness. Modeling of gravity and magnetic anomalies indicates that the Pine Canyon Rhyolite is probably thicker in the northeastern part of the caldera. Lineaments in the magnetic data suggest the presence of buried faults beneath the caldera that may have led to increased downdrop in the northeast versus the southwest, allowing a thicker section of caldera fi ll to accumulate there. The Pine Canyon caldera has been interpreted as a downsag caldera because it lacks surfi cial faulting, so these inferred faults are the fi rst mapped features there that could be responsible for caldera collapse. The caldera boundary correlates well with the margins of a gravity low. General features of the caldera match well with basic models of downsag calderas, meaning that the Pine Canyon caldera may be a classic example of downsagging, of which few well-described examples exist, in terms of a geophysical signature. The source of a long-wavelength magnetic high over the Chisos Mountains is interpreted as a previously unknown broad intrusion, the long axis of which trends parallel to a major crustal boundary related to the Ouachita orogeny or an even earlier Precambrian margin. This feature represents the largest intrusion (28‐ 34 km diameter, 1‐4 km thick, 700‐3000 km 3 in volume) in an area where relatively small laccoliths are ubiquitous. The intrusion most likely represents a long-lived (>1 m.y.) reservoir replenished by small batches of magma of varying composition, as refl ected in the variation of eruptive products from the Pine Canyon and Sierra Quemada calderas. The intrusion may represent the easternmost occurrence of voluminous Tertiary magmatism in the southwestern United States.

Petrogenesis and tectonic setting of the peralkaline Pine Canyon caldera, Trans-Pecos Texas, USA

The Pine Canyon caldera is a small (6–7 km diameter) ash-flow caldera that erupted peralkaline quartz trachyte, rhyolite, and high-silica rhyolite lavas and ash-flow tuffs about 33–32 Ma. The Pine Canyon caldera is located in Big Bend National Park, Texas, USA, in the southern part of the Trans-Pecos Magmatic Province (TPMP). The eruptive products of the Pine Canyon caldera are assigned to the South Rim Formation, which represents the silicic end member of a bimodal suite (with a “Daly Gap” between 57 and 62 wt.% SiO 2); the mafic end member consists primarily of alkali basalt to mugearite lavas of the 34–30 Ma Bee Mountain Basalt. Approximately 60–70% crystallization of plagioclase, clinopyroxene, olivine, magnetite, and apatite from alkali basalt coupled with assimilation of shale wall rock ( M a/ M c = 0.3–0.4) produced the quartz trachyte magma. Variation within the quartz trachyte–rhyolite suite was the result of ∼70% fractional crystallization of an assemblage dominated by alkali feldspar with subordinate clinopyroxene, fayalite, ilmenite, and apatite. High-silica rhyolite is not cogenetic with the quartz trachyte–rhyolite suite, and can be best explained as the result of ∼ 5% partial melting of a mafic granulite in the deep crust under the fluxing influence of fluorine. Variation within the high-silica rhyolite is most likely due to fractional crystallization of alkali feldspar, quartz, magnetite, biotite, and monazite. Lavas and tuffs of the South Rim Formation form A-type rhyolite suites, and are broadly similar to rock series described in anorogenic settings both in terms of petrology and petrogenesis. The Pine Canyon caldera is interpreted to have developed in a post-orogenic tectonic setting, or an early stage of continental rifting, and represents the earliest evidence for continental extension in the TPMP.

Major ignimbrites and volcanic centers of the Copper Canyon area: A view into the core of Mexico's Sierra Madre Occidental

Reconnaissance mapping along Copper Canyon highway has established ignimbrite stratigraphic relationships over a relatively large area in the central part of the Sierra Madre Occidental volcanic field in western Chihuahua, Mexico. The oldest ignimbrites are found in the central part of the area, and they include units previously mapped from north of the study area, in and around the Tomóchic volcanic complex. Copper Canyon, at the southern end of the study area, exposes younger units, including the intracaldera tuff of the Copper Canyon caldera and five overlying ignimbrites. Well-exposed calderas are found near San Juanito, in the central part of the map area, and at Sierra Manzanita, to the far north. Stratigraphic evidence for yet another caldera in the northern part of the area is found in the Sierra El Comanche. The stratigraphic and limited available isotopic age data suggest that volcanism was particularly active ∼30 m.y. ago. This reconnaissance survey also documented lava-flow lithologies consistent with previous observations from Tomóchic that intermediate lavas have erupted throughout that area's volcanic history and that basaltic andesite became particularly abundant as felsic volcanism waned. The combined Copper Canyon–Tomóchic area gives the first view into the core of the giant Sierra Madre Occidental volcanic field, expanding that offered by earlier reports, mostly from peripheral regions. The emerging picture is one of a dramatically thickened and stratigraphically complex volcanic section related to numerous overlapping caldera complexes, much like that documented for the core of the San Juan Mountains volcanic field, Colorado.

Tilted middle Tertiary ash-flow calderas and subjacent granitic plutons, southern Stillwater Range, Nevada: Cross sections of an Oligocene igneous center

Steeply tilted late Oligocene caldera systems in the Stillwater caldera complex record a number of unusual features including extreme thickness of caldera-related deposits (>4–5.5 km), lack of conclusive evidence for structural doming of the calderas despite intrusion of cogenetic plutonic rocks, and preservation of vertical compositional zoning in the plutonic rocks. The Stillwater caldera complex comprises three partly overlapping ash-flow calderas and subjacent plutonic rocks that were steeply tilted during early Miocene extension. The calderas and cogenetic plutonic rocks are exposed in cross section over an unusually large depth range of ∼10 km. The Job Canyon caldera, the oldest (ca. 29–28 Ma) caldera, consists of two structural blocks. The north block consists of 0–1500 m of precollapse intermediate composition lava flows and breccias overlain by 2000 m of intracaldera rhyolite ash-flow tuff locally interbedded with thick sequences of caldera-collapse breccia, overlain in turn by 2500 m of intermediate lava flows and minor lacustrine and fluvial sedimentary rocks. The south block consists of thinner sequences (total thickness ≤2500 m) of intermediate lava flows and ash-flow tuff with local interbedded collapse breccia. The north part of the caldera is intruded by the cogenetic IXL pluton, which is vertically zoned downward from granodiorite to quartz monzodiorite. The 25 to 23 Ma Poco Canyon and Elevenmile Canyon calderas and underlying Freeman Creek pluton overlap in time and space with each other. Caldera-related deposits in the Poco Canyon caldera comprise two cooling units of crystal-rich rhyolite and high-silica rhyolite tuff (tuff of Poco Canyon) separated by a unit of crystal-poor high-silica rhyolite tuff and caldera-collapse breccia (megabreccia of Government Trail Canyon) and by a thick unit of crystal-rich rhyolite and trachydacite ash-flow tuff related to the Elevenmile Canyon caldera (tuff of Elevenmile Canyon). Total thickness of caldera-related deposits is locally >4500 m in the Poco Canyon caldera. The Elevenmile Canyon caldera is filled by >3000 m of ash-flow tuff (tuff of Elevenmile Canyon), locally overlain by a unit of water-laid rhyolite tuff and sedimentary rocks and by a locally thick unit of rhyolite ash-flow tuff (tuff of Lee Canyon). Total thickness of caldera-related deposits in the Elevenmile Canyon caldera is >4000 m. The composite Freeman Creek pluton intrudes the central and north parts of these calderas and consists of an older granodiorite porphyry phase probably related to the Elevenmile Canyon caldera and a younger granite phase probably related to the Poco Canyon caldera. A 7-km-long, texturally zoned rhyolite-porphyry to granite-porphyry dike intruded the north edges of these calderas and is probably a ring-fracture dike related to the Poco Canyon caldera. Caldera collapse occurred mostly along subvertical ring-fracture faults that penetrated to depths of >5 km and were repeatedly active during eruption of ash-flow tuffs. Subsidiary growth faults with relatively minor displacement are present in caldera-related deposits in the interior of the Job Canyon caldera. A fault separating the two structural blocks of the Job Canyon caldera later served as the north walls of both the Poco Canyon and the Elevenmile Canyon calderas. A second, long-active fault formed the south margin of the Job Canyon caldera, separated the Poco Canyon and Elevenmile Canyon calderas into blocks with greatly different amounts of caldera-related deposits, and later was reactivated during early Miocene extension. The calderas collapsed as large piston-like blocks, and there is no evidence for chaotic collapse. Preserved parts of caldera floors are relatively flat surfaces several kilometers across.

Episodic caldera volcanism in the Miocene southwestern Nevada volcanic field: Revised stratigraphic framework, 40Ar/39Ar geochronology, and implications for magmatism and extension

The middle Miocene southwestern Nevada volcanic field (SWNVF) is a classic example of a silicic multicaldera volcanic field in the Great Basin. More than six major calderas formed between >15 and 7.5 Ma. The central SWNVF caldera cluster consists of the overlapping Silent Canyon caldera complex, the Claim Canyon caldera, and the Timber Mountain caldera complex, active from 14 to 11.5 Ma and centered on topographic Timber Mountain. Locations of calderas older than the Claim Canyon caldera source of the Tiva Canyon Tuff are uncertain except where verified by drilling. Younger peralkaline calderas (Black Mountain and Stonewall Mountain) formed northwest of the central SWNVF caldera cluster. We summarize major revisions of the SWNVF stratigraphy that provide for correlation of lava flows and small-volume tuffs with the widespread outflow sheets of the SWNVF. New laser fusion 40 Ar/ 39 Ar isotopic ages are used to refine and revise the timing of eruptive activity in the SWNVF. The use of high-sensitivity mass spectrometry allowed analysis of submilligram-sized samples with analytical uncertainties of ∼0.3% (1σ), permitting resolution of age differences as small as 0.07 Ma. These results confirm the revised stratigraphic succession and document a pattern of episodic volcanism in the SWNVF. Major caldera episodes (Belted Range, Crater Flat, Paintbrush, Timber Mountain, and Thirsty Canyon Groups) erupted widespread ash-flow sheets within 100-300 k.y. time spans, and pre- and post-caldera lavas erupted within 100-300 k.y. of the associated ash flows. Peak volcanism in the SWNVF occurred during eruption of the Paintbrush and Timber Mountain Groups, when over 4500 km 3 of metaluminous magma was erupted in two episodes within 1.35 m.y., separated by a 750 k.y. magmatic gap. Peralkaline and metaluminous magmatism in the SWNVF overlapped in time and space. The peralkaline Tub Spring and Grouse Canyon Tuffs erupted early, and the peralkaline Thirsty Canyon Group tuffs and Stonewall Flat Tuff erupted late in the history of the SWNVF, flanking the central, volumetrically dominant peak of metaluminous volcanism. Magma chemistry transitional between peralkaline and metaluminous magmas is indicated by petrographic and chemical data, particularly in the overlapping Grouse Canyon and Area 20 calderas of the Silent Canyon caldera complex. Volcanism in the SWNVF coincided with the Miocene peak of extensional deformation in adjoining parts of the Great Basin. Although regional extension was concurrent with volcanism, it was at a minimum in the central area of the SWNVF, where synvolcanic faulting was dominated by intra-caldera deformation. Significant stratal tilting and paleomagnetically determined dextral shear affected the southwestern margin of the SWNVF between the Paint-brush and Timber Mountain caldera episodes. Larger magnitude detachment faulting in the Bullfrog Hills, southwest of the central SWNVF caldera cluster, followed the climactic Timber Mountain caldera episode. Postvolcanic normal faulting was substantial to the north, east, and south of the central SWNVF caldera cluster, but the central area of peak volcanic activity remained relatively unextended in postvolcanic time. Volcanism and extension in the SWNVF area were broadly concurrent, but SWNVF area were broadly concurrent, but in detail they were episodic in time and not coincident in space

A geophysical‐geological transect of the Silent Canyon Caldera Complex, Pahute Mesa, Nevada

Revision of lithological logs for boreholes penetrating the volcanic center at Pahute Mesa, Nevada, has led to a thorough review of the volcanic stratigraphy and geologic structure. We have combined this review with a compilation of old and newly acquired gravity and seismic travel time data, producing a unified interpretation along a northwest to southeast profile. The analysis supports a new interpretation of the Silent Canyon caldera complex. The caldera is found to be more asymmetric than previously suggested, with the southeastern boundary formed by linear, high‐angle normal faults and a more gently sloping northwestern boundary. The total thickness of volcanic units within the caldera complex does not appear to exceed 5 km. The shallow structure at Pahute Mesa could have a profound effect on the seismic response for regional and teleseismic signals from this nuclear test site. The Silent Canyon caldera complex is actually a set of nested calderas first filled by thick (>1 km) postcaldera lavas and subsequently buried by outflow sheets of the Timber Mountain caldera to the south. Thick, postcaldera lavas filled a half‐graben structure formed west of the West Greeley fault, dropping the tops of the youngest caldera‐forming units to depths in excess of 2 km. Therefore the western boundary of the caldera complex is poorly defined. East of the West Greeley fault, two overlapping calderas are defined, and stratigraphic data suggest the presence of even older calderas. The youngest caldera, the calc‐alkaline Area 20 caldera, is well defined from drill hole data. The Area 20 caldera overlaps the 13.6 Ma peralkaline Grouse Canyon caldera, which is less well defined, but apparently collapsed in trap‐door style along the Almendro fault. For both these calderas, collapse continued after the main caldera‐forming eruption, concurrent with the accumulation of thick (>1 km) lavas within the peripheral collapse zones. The geophysical interpretation indicates that the major structural boundary of the caldera complex corresponds to the NNE trending Scrugham Peak and Almendro faults, which offset the pre‐Tertiary contact more than 1 km but have less than 200 m offset in rocks of 11 Ma age. Drill hole data show that offsets along these faults increase systematically within older (up to 15 Ma) units, which are commonly rotated eastward in a style similar to units at the surface. Abrupt changes in the subsurface thickness of the caldera‐forming units occur across the faults, indicating that these linear features served as caldera boundaries.

Volcanic centers of southwestern Nevada: Evolution of understanding, 1960–1988

Since about 1960, geologists of the U.S. Geological Survey and, more recently, those of Los Alamos and Lawrence Livermore national laboratories, supported largely by the U.S. Department of Energy (DOE) and its predecessors, have been unraveling a complex series of ash flow sheets, lavas, and related calderas in the southwestern Nevada volcanic field in and near the Nevada Test Site (NTS). Extensive detailed geologic mapping aided in delineation of four major calderas: Silent Canyon (∼14 Ma), Timber Mountain‐Oasis Valley (∼11.5 Ma), Black Mountain (∼7.5 Ma), and Stonewall Mountain (∼6 Ma). In the 1960s, key concepts that contributed to the understanding of volcanology were the recognition of vertical compositional zonation within ash flow sheets, the significance of caldera rim and moat lavas, the relation between caldera collapse and intracaldera breccias and ash flow facies, and the correlation of intracaldera and outflow‐sheet facies. Deep drill holes within Silent Canyon and Timber Mountain calderas provided vital information on caldera geometry and intracaldera facies. Radiometric dating has produced nearly 100 dates that define the age of the field between about 16 and 6 Ma. During the middle part of that period a major ash flow eruption occurred once in about every half million years. Continuing support by the DOE for earth science at the NTS during the 1970s and 1980s has permitted a unique longevity of studies and provided opportunities to restudy mapped areas, revise some incorrect relationships, and work out important details of caldera history and structure that otherwise would not have come to light. Petrochemical and isotopic studies contributed to the understanding of the PT environment of the magma bodies that generated the major ash flow sheets. In the last decade, specialized work has continued on stratigraphic and petrologic problems, resulting in understanding of petrochemical cycles, in wider and more accurate correlation of certain units, and in understanding the time and spatial relationship between petrochemically very similar ash flow sheets from the Black Mountain and Stonewall Mountain calderas. Drilling and detailed Earth science studies in connection with preliminary characterization of a proposed nuclear waste repository at Yucca Mountain have greatly advanced knowledge of the older and younger parts of the volcanic sequence, including the basalts. A newly defined volcano‐tectonic collapse area in Crater Flat, probably a caldera, is strikingly similar in structure, location, and geophysical expression to the Silent Canyon caldera to the north. Rhyolite lavas peripheral to the Timber Mountain‐Oasis Valley caldera complex are intercalated with the major ash flow sheets from the complex, and analyses of the lavas fit well on compositional trends determined by the ash flow sheets. Renewed studies since 1981 include eruptive dynamics and magma chemistry of the major ash flow sheets and sources of mafic and intermediate volcanism. Hydrothermal alteration and mineralization occur after major magmatic pulses. New data on the age, structure, and distribution of the volcanic rocks have resulted in revised structural models involving volcano‐tectonic and detachment faulting processes.

Petrologic evolution of divergent peralkaline magmas from the Silent Canyon Caldera Complex, Southwestern Nevada Volcanic Field

The Silent Canyon volcanic center consists of a buried Miocene peralkaline caldera complex and outlying peralkaline lava domes. Its location has been corroborated by geophysical data and more than 50 drill holes. Two widespread ash flow sheets, the Tub Spring and overlying Grouse Canyon members of the Miocene Belted Range Tuff, were erupted from the caldera complex and have volumes of 60–100 km3 and 200 km3, respectively. Eruption of the ash flows was preceded by widespread extrusion of precaldera comendite domes and was followed by extrusion of postcollapse peralkaline lavas and tuffs within and outside the caldera complex. Lava flows and tuffs were also deposited between the two major ash flow sheets. Rocks of the Silent Canyon center vary significantly in silica content and peralkalinity. The most mafic rocks are precollapse and postcollapse trachytes (65–69% SiO2). Low‐silica comendites (69–73% SiO2) were erupted as the mafic upper part of the chemically zoned Grouse Canyon Member and as postcollapse lavas. The lower part of the Grouse Canyon Member and the underlying rhyolite of Split Ridge are moderately peralkaline comendite (PI is molar ratio Na + K/Al is 1.17–1.26). These comendites have major element characteristics and trace element enrichments approaching those of pantellerites. The Tub Spring Member, by contrast, is a weakly peralkaline chemically unzoned silicic comendite (75–76% SiO2) ash flow tuff. Weakly peralkaline silicic comendites (PI 1.0–1.1) are the most abundant precaldera lavas. Postcollapse lavas range from trachyte to silicic comendite; some have anomalous light rare earth element (LREE) enrichments. Silent Canyon rocks follow a common petrologic evolution from trachyte to low‐silica comendite; above 73% SiO2, compositions of the moderately peralkaline comendites diverge from those of the weakly peralkaline silicic comendites. These contrasting differentiation paths are shown in the behavior of Fe and other transition metals, Al, Na, K; the trace elements Ba, Zr, Nb; and probably F and Cl. Weakly peralkaline silicic comendites show a LREE/heavy REE crossover in early erupted/late erupted rocks; moderately peralkaline comendites are enriched in all REE. The development of divergent peralkaline magmas, toward both pantelleritic and weakly peralkaline compositions, is unusual in a single volcanic center.

Regularized two‐dimensional Fourier gravity inversion method with application to the Silent Canyon caldera, Nevada

A modification of the 2‐D Fourier gravity inversion method includes regularization and a linear density variation with depth. Explicit downward continuation in the Fourier inversion of gravity observations from mass distributions at depth produces instability in the presence of noise and shallow mass distributions. A data‐adaptive regularization filter tapers growth of the exponential continuation function. An empirical relationship between the regularization filter parameter and a parametric model of potential field spectra results in automatic selection of the filter parameter for a given continuation depth. Inversion of synthetic data from a random noise‐contaminated basin type model produces a depth model that agrees with the synthetic structure with an rms error commensurate with the data noise. A model of the Silent Canyon caldera, buried beneath Pahute Mesa at the Nevada Test Site, results in a gravity field that agrees with the observations to within a 4 percent rms error. The caldera gravity model supports the hypothesis of a high‐density half‐space (precaldera lithology) beneath a lower density caldera infill (postcaldera volcanic activity).

Analysis of near-source contributions to early P-wave coda for underground explosions. III. Inversion for isotropic scatterers

A systematic array processing analysis is applied to 916 teleseismic shortperiod P-wave signals from 25 underground explosions at Pahute Mesa in order to establish whether coherent arrivals scattered from near-source velocity heterogeneity can be identified. The signal coherence is measured using semblance, which can be statistically evaluated using the noncentral F-distribution if the noise properties are suitable. The effects of unsuitable correlated noise due to common path and receiver contributions are suppressed by applying station weighting functions obtained by stacking all observations at a given receiver. Similar corrections for individual events are applied to reduce biases due to variations in surface reflection times and spall effects. The characteristics of the spatial and phase velocity resolution of the semblance analysis are established by a series of synthetic calculations, including cases with random arrivals in the P-wave coda. The procedure is applied to the actual data under the assumption that any scattering structures present radiate isotropically, although this constraint could be relaxed given independent knowledge of a particular scatterer's nature. The Pahute Mesa data show some evidence for enhanced scattering contributions to the first 15 sec of the P-wave signals from the western edge of the Silent Canyon Caldera. The overall confidence level in this interpretation is fairly low because simulations with random scattering in the P coda successfully match many features observed in the stacked data.

Velocity structure of Silent Canyon Caldera, Nevada Test Site

Abstract A laterally averaged, one-dimensional P- and S-wave velocity structure is obtained for Silent Canyon Caldera, located beneath Pahute Mesa at the Nevada Test Site. The velocity models are derived from a linearized damped least-squares travel-time inversion for slowness gradient perturbations. A total of 72 P- and 20 S-wave travel times generated from 13 different explosions, obtained at locations above the caldera, are used in the inversion. The structure is modeled using linear velocity gradients in a series of horizontal layers. The inversion produces a model in which velocity for P waves increases from 1.5 km/sec at the surface to 4.5 km/sec at 2.3 km depth and from 1.0 km/sec at the surface to 2.3 km/sec at 2.0 km depth for S waves. The P- and S-wave models produce travel-time residuals which are typically less than 0.1 and 0.2 sec, respectively, which is within observational error. In an attempt to estimate the degree of lateral velocity variations over which the one-dimensional model averages, waveform data collected above the caldera are examined for indications of a laterally heterogeneous structure.

Eruptive history and structural development of the Toquima caldera complex, central Nevada

The Toquima caldera complex, located in the Toquima Range of central Nevada, consists of three overlapping to nested calderas. The Moores Creek caldera is the largest (∼30 by 20 km); it formed ∼27.2 Ma in response to eruption of the high-silica rhyolite tuff of Moores Creek. Because of recurrent volcanic activity and subsequent basin-range faulting, only the northern segment of the Moores Creek caldera is preserved; its eastern and western margins are downfaulted below valley fill, and its southern part was obscured by collapse of the Mount Jefferson caldera. Eruption of the tuff of Mount Jefferson resulted in collapse of the 18- by 20-km Mount Jefferson caldera ∼26.5 Ma. The ash-flow tuff exposed at Round Mountain is a silicic outflow-facies equivalent of the compositionally zoned (76–67 wt % SiO 2 ) intracaldera tuff of Mount Jefferson. Pyroclastic eruptive activity in the complex concluded ∼23.6 Ma with formation of the comparatively small 8- by 10-km Trail Canyon caldera. With time, caldera size diminished, and the focus of volcanic activity shifted progressively southeastward. The southeastward migration of volcanism in the complex approximately parallels regional northwest-striking faults, suggesting fundamental structural control in the rise and eruption of magma. Arcuate faults and small, aphyric to porphyritic plugs outline the structural margins of the calderas. Some plugs, on the basis of gradational textural changes from vitroclastic in the tuff to flow-layered and porphyro-aphanitic in the plug, appear to represent lava-choked, ash-flow-tuff, feeder vents. Caldera collapse breccias are locally well exposed in all three calderas. Caldera resurgence is not strongly developed in the Toquima caldera complex. The Mount Jefferson massif, which dominates the complex at an elevation of 3,640 m, is an artifact of young basin-range block faulting. The limited development of resurgence in the Mount Jefferson caldera is attributed to ponding of the bulk of erupted material within the caldera. Intermediate to mafic volcanic rocks, premonitory or coeval to silicic ash-flow tuff, are volumetrically minor over central Nevada and are lacking in the Toquima caldera complex. The less silicic magma apparently became lodged in the middle to lower crust below an extensive zone of more silicic, less dense magma.

Magnitude bias in the Great Basin and its implications for explosion magnitude versus yield estimates

Comparison of teleseismic P‐wave delays in the vicinity of the Central Nevada Test Site in Hot Creek Valley, Nevada, with P‐delay data over a wider region in the Great Basin, shows that upper mantle speeds under Hot Creek Valley stations are higher than the average for the Great Basin as a whole, but lower than those beneath Pahute Mesa. These observations indicate that the caldera complex in Hot Creek Valley may have a high speed root similar to that proposed to exist beneath the Silent Canyon caldera at Pahute Mesa, however the Hot Creek Valley anomaly is not as strong as the Pahute Mesa anomaly. These high‐speed upper mantle structures cause defocusing for outgoing rays resulting in a shadow zone at the distance range corresponding to many European stations. The shadow zone caused by the Pahute Mesa structure is much more pronounced and consequently magnitudes of Pahute Mesa explosions can be underestimated relative to the magnitude of Hot Creek Valley explosions of similar yield. When magnitude versus yield relationships developed using Pahute Mesa data are applied to other areas, such as Hot Creek Valley, they can result in a biased estimate of yield.

Interpretation of gravity data in a complex volcano‐tectonic setting, southwestern Nevada

This regional gravity study, based on an irregular 2‐km data grid, was conducted during the past few years at Yucca Mountain, southern Nye County, Nevada, as part of a program to locate a suitable repository for high‐level nuclear waste. About 100 surface rock samples, three borehole gamma‐gamma logs, and one borehole gravity study provide excellent density control. A nearly linear increase in density of 0.26 g/cm3 per kilometer of depth is indicated in the thick tuff sequences that underlie the mountain. Isostatic and 2.0‐g/cm3 Bouguer corrections were applied to the observed gravity values to remove regional gradients and topographic effects, respectively. The Bare Mountain gravity high, with an isostatic anomaly maximum of 48 mGal, is connected with a greater gravity high over the Funeral Mountains, to the southwest; together, these highs result from a continuous block of dense, metamorphosed Precambrian and Paleozoic rocks that stretches across much of the Walker Lane from the east edge of Death Valley to Bare Mountain. The Calico Hills gravity high appears more likely to originate from a northeast trending buried ridge of Paleozoic rocks that extends southwestward beneath Busted Butte, 5 km southeast of the proposed repository, where two‐ and three‐dimensional modeling indicates that the pre‐Cenozoic rocks lie less than 1000 m beneath the surface. Tuff, at least 4000 m thick, fills a large steep‐sided depression in the pretuff rocks beneath Yucca Mountain and Crater Flat. The gravity low and the thick tuff section lie within a large collapse area that includes the Crater Flat‐Timber Mountain‐Silent Canyon caldera complexes. Gravity lows in Crater Flat itself are interpreted to coincide with the source areas of the Prow Pass Member, the Bullfrog Member, and the Tram Member of the Crater Flat Tuff; these source areas add nearly 350 km2 to the previously recognized extent of the local caldera complexes. Southward extension of the broad gravity low associated with Crater Flat into the Amargosa Desert is evidence for sector graben‐type collapse segments related to the formation of the Timber Mountain caldera and superimposed on the other volcanic and extensional structures within Crater Flat.

Zeolite Diagenesis Below Pahute Mesa, Nevada Test Site

AbstractThe Tertiary silicic volcanic rocks in the Silent Canyon Caldera beneath Pahute Mesa, of the Department of Energy's Nevada Test Site have been divided into three vertical mineralogical zones that vary in thickness and transgress stratigraphic boundaries. Zonal contacts are generally sharp. Zone 1, the uppermost zone, includes unaltered or incipiently altered rhyolitic glass. Zone 2 is characterized by a predominance of clinoptilolite and subordinate amounts of smectite, cristobalite, and mordenite. Zone 3 is a complex mineral assemblage that includes analcime, quartz, calcite, authigenic K-feldspar and albite, kaolinite, chlorite, and mixed-layer illite/smectite. The mixed-layer clay shows an increase in ordering and a decrease in expandability with depth.Shortly after deposition and after shallow burial, hydration of relatively impermeable, highly porous vitric rocks resulted in the rapid formation of the Zone 2 assemblage (except mordenite). This stage of alteration resulted in a net porosity loss and negligible mass transfer. Continued burial and rise in temperature led to a dehydration stage in which the Zone 2 assemblage was replaced by the Zone 3 minerals. The dehydration stage resulted in a porosity increase and an increase in permeability of several orders of magnitude. This process, like the earlier reactions, also conserved mass. Precipitation of mordenite followed the formation of this zonal configuration. The diagenetic zones below Pahute Mesa were caused by: (1) changing pore-water chemistry in an essentially closed hydrologic system; (2) disequilibrium or kinetic precipitation of metastable phases; and (3) a higher thermal gradient than that now present.

P-Wave Residual Differences and Inferences on an Upper Mantle Source for the Silent Canyon Volcanic Centre, Southern Great Basin, Nevada

Summary Teleseismic P-wave travel-time residuals from six large nuclear detonations in the caldera of the Silent Canyon volcanic centre of the Nevada Test Site have been reduced by corresponding P-wave residuals of a nearby reference event (Dumont) in an attempt to map the deep structure of this volcanic centre. Each set of residual difference data is plotted on a projection of the lower focal hemisphere of Dumont. After corrections were applied for known low velocity rocks in the Silent Canyon caldera, data clusters of at least -1.0 s were found to dominate observations in the distance range 60–90 degrees. These large negative delays indicate the existence of a major high velocity anomaly that is directly beneath the Silent Canyon volcanic centre. Teleseismic P-wave residuals of two additional nuclear explosions, outside both the Silent Canyon caldera and the region of the reference event, were also reduced by Dumont residuals. The resulting data are distinct from the Silent Canyon residual difference fields and serve as a control on the primary experiment. The observed differential residuals, independent data on trace and major element chemistry of Silent Canyon lavas, and direct observations of effective Q argue that the high velocity anomaly is in the upper mantle. A model that satisfies the observed residual differences and which is consistent with additional seismic data possesses an anomalous zone which begins beneath the Mohorovicic discontinuity, at a depth of 40–50 km, extends to a depth of 170–190 km, and has a volume of 350000–600000 km3. This zone is interpreted as representing upper mantle material that has been largely depleted of its partial melt component by various differentiation processes and by associated eruption of the partial melt material through the Silent Canyon volcanic centre.