Range-bounding Faults Research Articles

Audiomagnetotelluric characterization of range-front faults, Snake Range, Nevada

Two controlled-source audio-magnetotelluric (CSAMT) profiles were collected on the eastern flank of the Snake Range in eastern Nevada across geologically complex terrain to investigate the suspected presence of faults along the range front. The location of the range-bounding faults is not easily determined on geologic grounds because of the presence of extensive young sedimentary cover and overall geologic complexity. Characterization of the presence, location, and structural style of the range-front faults is critical to assessment of connectivity of groundwater aquifers near the mountain front and in adjacent alluvial basins. A total of 48 CSAMT soundings were recorded along two lines that were chosen to maximize subsurface geologic information. Two generations of faults were interpreted based on the CSAMT data: an older, low-angle fault that is cut by a younger, more steeply dipping fault. Lack of deep boreholes in the region required that the subsurface interpretation rely on analogy from surface outcrops within and adjacent to the study area. The success of the CSAMT method as applied in this study hinged on near-ideal collection conditions, the relatively high contrast in electrical resistivity provided by the rock types involved, and well-developed geologic conceptual models of the region.

The Vallo di Diano Fault System: New Evidence for an Active Range-Bounding Fault in Southern Italy Using Shallow, High-Resolution Seismic Profiling

Range-bounding normal faults can presentsignificant challenges for seis- mic exploration. This is the case of the fault system bounding the Vallo di Diano, the largest intermountain basin in the southern Apennines seismic belt. Industry reflection profiles define the large-scale structure of the basin but barely image the shallow fault system due to unfavorable topographic and near-surface conditions along the foothills of the eastern range. We present two high-resolution (HR) wide-aperture profiles recorded attheeastern marginof thebasin acrossunreported scarpsthat affect Middle- Late Pleistocene alluvial fans and slope debris. The survey is aimed at identifying possible recent faulting across these challenging terrains and at understanding the re- lationship between shallow structures and the master range-bounding fault at depth. Common depth point processing of wide-aperture reflection data and first-arrival tra- vel-time tomography provide detailed images of the upper 200-300 m and sounding evidence of recent activity along previously unknown splays of the fault system. These splays dissect the Mesozoic limestone bedrock and alluvial-fan sequences, affecting theirdepositionalpattern.VeryhighresolutionVPandreflectivityimagesalsogivehints ofpossiblecoseismicsurfacefaultinginHolocenecolluvia.Theseresultshaverelevant implicationsfortheevaluationoftheseismogenicpotentialoftherange-boundingfault systemandforseismichazardassessmentofthedenselyurbanizedVallodiDianobasin. Online Material: Table of acquisition and data processing parameters, and figures showing schematic stratigraphy of the Vallo di Diano basin, restoration of fault dis- placement along the HR profile, and resolution of the P-velocity tomographic images for the VHR profile.

Patterns of Quaternary deformation and rupture propagation associated with an active low-angle normal fault, Laguna Salada, Mexico: Evidence of a rolling hinge?

The Laguna Salada rift basin is within the zone of shearing between the Pacific and North American plates and is an asymmetric half-graben controlled on its eastern margin by the Laguna Salada fault and the Cañada David detachment. Both faults dip west, have accommodated >10 km of offset since the middle-late Miocene, and are associated with an extensive late Quaternary fault array. The Laguna Salada fault is a high-angle fault that strikes northwest and has an oblique normal-dextral sense of shear. The Cañada David detachment is a low-angle normal fault with a curvilinear trace that extends ~55–60 km and contains two prominent megamullion antiform-synform pairs. The late Quaternary scarp array that extends along the entire mountain front shows remarkable variations with antiformal and synformal megamullions. In antiformal domains, the scarp array is generally wider, closer to the mountain front (<100 m), and contains numerous antithetic scarps. In synformal domains, it is well removed from the mountain front (3.5–10 km) and contains more synthetic scarps. Integrated deformation across the array shows a systematic decrease in the ratio of horizontal:vertical deformation with distance from the Cañada David detachment, which reflects the mechanism of accommodation of the horizontal component of slip, and/or is the direct result of a master fault with a near-surface antilistric geometry. Patterns of sedimentation as well as gravity and seismic data are consistent with the latter and strongly suggest that the Cañada David detachment takes on a high-angle geometry within 5–10 km of the mountain front. Structural analysis of the scarps and range-bounding fault clearly demonstrates a basin-ward migration of deformation at a variety of scales along the Cañada David detachment. The largest steps (3.5–10 km) in this migration are made in synformal megamullion domains, where the strong divergence of the scarp array from the trace of the Cañada David detachment results in the abandonment of large segments of the detachment and the transfer of large lozenge-shaped tectonic blocks from the hanging wall to the footwall. The basinward migration of deformation and inferred near-surface antilistric geometry are both defining characteristics of the rolling-hinge model of normal faulting. If this model is applicable, our data indicate that the near-surface antilistric bend of the master fault is tighter and more abrupt than previously envisioned.

Quaternary alluvial-fan development, climate and morphologic dating of fault scarps in Laguna Salada, Baja California, Mexico

Late Quaternary slip across the Cañada David detachment has produced an extensive array of Quaternary scarps cutting alluvial-fans along nearly the entire length (~ 60 km) of the range-bounding detachment. Eight regional alluvial-fan surfaces (Q 1 [youngest] to Q 8 [oldest]) are defined and mapped along the entire Sierra el Mayor range-front. Terrestrial cosmogenic nuclide 10Be concentrations from individual boulders on alluvial-fan surfaces Q 4 and Q 7 yield surface exposure ages of 15.5 ± 2.2 ka and 204 ± 11 ka, respectively. Formation of the fans is probably tectonic, but their evolution is strongly moderated by climate, with surfaces developing as the hydrological conditions have changed in response to climate change on Milankovitch timescales. Systematic mapping reveals that the fault scarp array along active range-bounding faults in Sierras Cucapa and El Mayor can be divided into individual rupture zones, based on cross-cutting relationships with alluvial-fans. Quantitative morphological ages of the Laguna Salada fault-scarps, derived from linear diffusive degradation modeling, are consistent with the age of the scarps based on cross-cutting relationships. The weighted means of the maximum mass diffusivity constant for all scarps with offsets < 4 m is 0.051 and 0.066 m 2/ka for the infinite and finite-slope solutions of the diffusion equation, respectively. This estimate is approximately an order of magnitude smaller than the lowest diffusivity constants documented in other regions and it probably reflects the extreme aridity and other microclimatic conditions that characterize the eastern margin of Laguna Salada.

Analysis of Seismic Activity in the Crust from Earthquake Relocation in the Central Tien Shan

Using P - and S -wave arrivals from local earthquakes recorded by a temporary broadband seismic network (ghengis) and a Kyrgyzstan broadband seismic network (knet), we determined the source parameters of 1938 earthquakes occurring from 1997 to 1998 in the central Tien Shan and adjacent areas, based on one- dimensional and three-dimensional velocity models. The results indicate that seismic activity is not restricted to only central Tien Shan but also penetrates into the edges of the Kazakh platform and the Tarim basin to the north and south, respectively. It implies that most range-bounding faults in the central Tien Shan are active and they play a significant role in the tectonic activity of the mountain belt. However, seismic activity of the Talas-Fergana fault in the study area is different; its central segment seems more active than its eastern segment. We infer that this part of the fault may be activated by neighboring faults, whereas its eastern segment could be temporally locked. The earthquakes at the depths of 30–40 km beneath the east-central Tien Shan reveal an interaction between the upper and mid-lower crust. This suggests that brittle failure is possible to greater depth within the crust. Another possibility is that the seismic activity in the mid-lower crust is affected by a positive buoyancy force created by mantle upwelling of this area. These observations indicate that dynamic processes such as small-scale mantle convection or mantle upwelling, which resulted in lithospheric thinning or removal, still occur beneath the central Tien Shan.

Sedimentologic and tectonic origin of an incised-valley-fill sequence along an extensional marginal-lacustrine system in the Basin and Range province, United States: Implications for predictive models of the location of incised valleys

Incised valleys in extensional lacustrine systems should be common and significant petroleum targets, yet documentation and analysis of these systems are limited and, hence, so are predictive models for their location. Geologic mapping of the Miocene–Pliocene Humboldt Formation in Knoll basin, northeastern Nevada, has revealed a significant incised-valley system formed along the lacustrine margins of an extensional basin. The valley formed during a relative lake-level fall and incised into lacustrine shoreface and offshore sandstone and subsequently was filled with fluvial and eolian sediment as lake level rose. The valley's location was tectonically influenced; it is situated in the hinge zone of a syncline near the tip of the range-bounding fault system. Folding of the syncline was broadly synchronous with incision and filling, and it appears to have localized the valley along the topographically low hinge zone. Furthermore, the large relative lake-level change that produced the valley is only recorded in strata in the syncline area, suggesting that the location and cause of incision was greatly influenced by tectonics. Thus, the location of similar incised valleys in other extensional basins may be predictable if comparable tectonic features and processes are recognized. Our study suggests that the best locations to develop and preserve incised valleys are near the tips of normal faults during periods of overall high tectonic subsidence. Specific areas along basin-bounding faults where tectonically influenced incised valleys are more likely to form include fault-propagation folds, synthetic relay ramps near transfer faults, and areas that have large changes in fault slip. Although lake volume changes caused predominantly by climate change can be an important factor in producing incised valleys, tectonically influenced incised valleys are likely to be larger, better preserved, and more petroleum prone compared to climate-controlled incised valleys.

Timing of Cenozoic volcanism and Basin and Range extension in northwestern Nevada: New constraints from the northern Pine Forest Range

Eocene–middle Miocene volcanic rocks in the northern Pine Forest Range, Nevada, are ideally situated for reconstructing the timing and style of volcanism and extensional faulting in the northwesternmost part of the Basin and Range province. A conformable sequence of Cenozoic volcanic and sedimentary strata in the northern Pine Forest Range dips ~30°W, and 11 new 40 Ar/ 39 Ar ages from this sequence defi ne 3 major episodes of volcanic activity. Pre-Tertiary basement and older (ca. 38 Ma) Tertiary intrusive rocks are overlain unconformably by Oligocene (ca. 30–23 Ma) basalt fl ows and dacitic to rhyolitic ash-fl ow tuffs interbedded with fi ne-grained tuffaceous sedimentary rocks. Oligocene rocks are overlain by ~550 m of ca. 17–16 Ma basalt fl ows equivalent to the Steens Basalt in southern Oregon, and basalt fl ows are capped by a thin 16.3 Ma ignimbrite that likely is correlative with either the Idaho Canyon Tuff or the Tuff of Oregon Canyon. The northern Pine Forest Range is bounded to the east by a major down-to-the-east normal fault that dips ~40°E with well-developed fault striations indicative of dip-slip motion. This fault initiated at an angle of ~70° and was rotated ~30° during uplift of the range. A suite of 17 apatite fi ssion-track ages from the Pine Forest footwall block demonstrates that exhumation, uplift, and slip on the range-bounding fault began ca. 12–11 Ma and continued until at least 7 Ma, with moderate slip since then. The Pine Forest Range did not undergo signifi cant extension before or during peak Oligocene and Miocene volcanism, and similar geologic relationships in nearby ranges suggest that a larger region of northwestern Nevada was also little extended during this interval. Basin and Range faulting in northwestern Nevada appears to have begun no earlier than 12 Ma, making it distinctly younger than deformation in much of central and southern Nevada, where peak extension occurred in the middle Miocene or earlier.

Analysis of Seismic Activity in the Crust from Earthquake Relocation in the Central Tien Shan

Using P and S wave arrivals from local and regional earthquakes recorded by a temporary broadband seismic network, we relocated the earthquakes occurring in the central Tien Shan and adjacent areas during 1997~1998, based on one-dimensional and three-dimensional velocity models. Seismic activity within crust was analyzed according to the source parameters of these relocated earthquakes. Earthquake distribution indicates that most range-bounding faults within the mountain belt are active and they have played a significant role in tectonic activity of the crust of the Tien Shan. However, seismic activity along the Talas-Fergana fault shows different features: the activity on its southeast segment and other faults in the southwest Tien Shan is less active, while its northwest segment, probably activated by adjacent faults, is more active. In addition, seismic activity seems to penetrate from the margins of the mountain belt into foreland basins on the north and south. The earthquakes at 30~40km depths indicate that brittle failure can take place in the mid-lower crust beneath the central Tien Shan and they are probably related to thermal perturbation near the crust-mantle boundary. This means that dynamic processes such as small-scale mantle convection and asthenosphere upwelling are still ongoing beneath the central Tien Shan.

Active extension in Val d'Agri area, Southern Apennines, Italy: implications for the geometry of the seismogenic belt

SUMMARY Integration of geologic, geomorphologic and seismologic data sets is used to reconstruct the recent tectonic evolution and active deformation pattern in the Val d'Agri area, located in the seismically active axial sector of the Southern Apennines (Italy). The western portion of the Apennines thrust belt has been affected by Pliocene–Quaternary extension during easterly roll-back and crustal delamination of the Adriatic slab. The bulk of Quaternary extension has been accommodated by SW-dipping oblique and normal faults, which have attained mature morphologic and structural features and, nowadays, separate mountain ranges from intermontane basins. However, in the present seismogenic belt, coseismic faulting locally occurs on NE-dipping structures, which might cut the inherited Pleistocene landscape. In the Val d'Agri basin, in spite of the large Early–Middle Pleistocene, displacement occurred on SW-dipping faults bordering its eastern flank, our investigations show that the recent basin evolution has been controlled by a NE-dipping fault system (Monti della Maddalena fault system, MMFS). This fault system cuts across the Monti della Maddalena range, west of the Agri valley and has not yet created an evident tectonic landscape. Notwithstanding, fault motion since the Middle Pleistocene might explain geomorphologic and hydrographic anomalies of the Agri river and its valley, where fault-controlled subsidence has captured the river course and produced an aggrading plain within a regional uplift context. Recent and ongoing motion is documented by fault scarplets in loose deposits, 14C ages of palaeosols and the spatial relation with low to moderate instrumental seismicity. Results from fault kinematic analysis are compatible with fault-plane solutions of local and regional seismic events, and indicate ∼NE–SW oriented extension. Recognition of the MMFS as a potential seismogenic fault increases the longitudinal extent of the NE-dipping, morphologically immature seismic sources in the Southern Apennines and argues against the range-bounding fault model for active extension in the region. The regional size of the NE-dipping seismogenic belt may result from impingement of a mantle wedge beneath the Apenninic chain and possibly track the external front of crustal delamination.

Cenozoic tectonic evolution of the White Mountains, California and Nevada

The White Mountains represent the westernmost range of the central northern Basin and Range province. They are situated to the east of the unextended Sierra Nevada and represent a crustal block that is bounded along its western flank by the high-angle White Mountains fault zone. The fault zone accommodates up to ∼8 km of total dip-slip displacement. Investigation of the structural and thermal history of the White Mountains indicates a two-stage Cenozoic tectonic evolution. Preextensional Miocene volcanic rocks preserved along the eastern side of the range unconformably overlie Mesozoic granitic basement and currently dip up to 25° to the east, recording the total Cenozoic tilt of the crustal block. Apatite fission-track and (U-Th/He) thermochronological data indicate that the White Mountains underwent rapid exhumation and eastward tilting in the middle Miocene, starting at ca. 12 Ma. Geologic mapping (1:10,000), fault kinematic analysis, and dating of younger volcanic sequences show that following middle Miocene east-west extension, the White Mountains have been dominated by right-lateral transtensional deformation related to the Walker Lane belt. The eruption of late Miocene and Pliocene volcanic sequences in the eastern White Mountains postdates the majority of the uplift of the range, as evidenced by infilling of paleodrainages and the presence of east-directed flow fabrics. Fault kinematic indicators from the White Mountains fault zone are characterized by apparent overprinting of dip-slip fault-motion indicators by right-lateral slickenfibers and fault striations, demonstrating that the range-bounding fault system along the western side of the White Mountains was reactivated as a dextral strike-slip fault system. At the northern and southern ends of the range, Pliocene right-lateral transtension along this northwest–southeast-trending fault systems resulted in the formation of northeast-trending pull-apart basins that truncate the mountain range and transfer strike-slip displacement eastward from the Owens Valley fault zone to the Fish Lake Valley fault zone. The inception of strike-slip faulting in Fish Lake Valley occurred at ca. 6 Ma as constrained by late Miocene volcanic units. Right-lateral faulting on the western side of the White Mountains occurred at ca. 3 Ma and is distinctly younger than the faulting in the Fish Lake Valley area, indicating a westward migration of transcurrent deformation through time.

Timing and Nature of Tertiary Plutonism and Extension in the Grouse Creek Mountains, Utah

The Grouse Creek-Albion-Raft River metamorphic core complex in northwestern Utah and southern Idaho is characterized by several Tertiary plutons with a range of ages and crosscutting relations that help constrain the timing of extensional deformation. In the Grouse Creek Mountains, at least three distinct, superimposed, extension-related Tertiary deformational events are bracketed by intrusive rocks, followed by a fourth event: motion on range-bounding faults. The Emigrant Pass plutonic complex was emplaced at depths of less than 10 km into Permianage rocks. SHRIMP U-Pb zircon analysis indicates a three-stage intrusion of the complex at 41.3 ± 0.3 Ma, 36.1 ± 0.2 Ma, and 34.3 ± 0.3 Ma. The two youngest phases represent distinctly younger intrusive event(s) than the oldest phase, separated by more than 5 m.y. The oldest phase cuts several metamorphosed and deformed younger-on-older faults, providing a pre-41 Ma age bracket for oldest extension-related deformation in the region. The youngest phase(s) are interpreted to have been intruded during development of a map-scale, N-S-trending recumbent fold, the Bovine Mountain fold, formed during vertical shortening of roof rocks during intrusion. This second event folded older normal faults that are likely pre-41 Ma. Zircons from the youngest part of the pluton show inheritance from Archean basement (~2.5 Ga) and from its Proterozoic sedimentary cover (~1.65 Ga). The Red Butte pluton, emplaced at 15-20 km depth, intruded highly metamorphosed Archean orthogneiss at 25.3 ± 0.5 Ma; cores of some zircons yield latest Archean ages of 2.55 Ga. The pluton is interpreted to have been intruded during a third deformational and metamorphic event that resulted in vertical flattening fabrics formed during NW to EW stretching, ultimately leading to thinning of cover and top-to-the west motion on the Ingham Pass fault. The Ingham Pass fault represents an important structure in the Grouse Creek Mountains, as it juxtaposes two parts of the crust that apparently resided as much as 10 km apart (in depth) at times as young as the Miocene. The varied structural, metamorphic, and intrusive relations observed in the Grouse Creek Mountains reflect their formation at different levels within the crust. Data from these various levels argue that plutonism has been a key mechanism for transferring heat into the middle and upper crust, and localizing strain during regional extension. Interestingly, events documented here correlate in a broad way with cooling events documented in the Raft River Mountains, although plutons are not exposed there. Major and trace element geochemistry imply a crustal component in all of the studied plutons, indicating significant degrees of crustal melting at depth during extension, and point to mantle heat sources during the timespan of Basin and Range extension as the cause of melting. Basin and Range faulting and final uplift of the range is recorded by apatite fission track ages, averaging 13.4 Ma, and deposition of about 2 km of syn-faulting basin fill deposits along the Grouse Creek fault mapped along the western flank of the range. Similar apatite ages from the Albion Mountains to the north indicate that the western side of the Albion-Raft River-Grouse Creek core complex behaved as a single rigid crustal block at this time.

Active breaching of a geometric segment boundary in the Sawatch Range normal fault, Colorado, USA

The northwest-trending Sawatch Range normal fault system is the principal range-bounding fault of the northern Rio Grande Rift of central Colorado. It displays a geometric segment boundary in the form of a prominent en échelon step at the Chalk Cliffs, a zone of highly fractured and hydrothermally altered quartz monzonite. There, the modern fault geometry suggests that the Sawatch Range fault is presently breaching the boundary. Immediately north of the Chalk Cliffs, the range front fault is active whereas immediately south, the range front fault is inactive. Instead, it is replaced by a north-trending break that transects much of the boundary. The Chalk Cliffs also display a four-stage history of faulting and alteration/mineralization that appears to track the boundary's evolution. First-stage chlorite-filled faults and second-stage laumontite-filled faults affect the entire boundary and likely pre-date the breach. They record a change of fluid chemistry during oblique extension associated with the boundary's early history. Third stage, calcite-filled fractures are generally restricted to the eastern half of the boundary, between the breaching fault's projection into the Chalk Cliffs and the active range front fault. These fractures probably coincide with initiation of the breach. Modern (fourth stage) hydrothermal activity occurs at, and slightly beyond, the eastern edge. This coincidence of hydrothermal events with changes in large-scale fault geometry illustrates how normal fault zones, through breaching, can simplify their geometries and localize associated deformation and hydrothermal activity.

Seismic expression of Late Cretaceous to Recent structure in southwestern New Mexico

We present an interpretation of seismic-reflection data from the Playas and Hachita valleys of southwestern New Mexico which places new constraints on the geometry of structures responsible for Laramide shortening, and subsequent Basin and Range extensional structures. With seismic interpretation tied to well data and gravity modeling, we address aspects of the debate that still surrounds the question of whether Laramide shortening was accommodated by “thin-skinned” thrusting along a regional decollement or by basement-involved block uplifts. Our interpretation suggests that shortening was at least in part accommodated with low-angle, basement-involved thrusts. Subsequent extension occurred on range-bounding normal faults that become listric near 10-km depth. The relative lack of recent basin fill in the Playas Valley, and particularly the Hachita Valley, suggests either that uplift of the mountains along normal faults has been relatively recent or that extension has only just compensated for earlier shortening, and has yet to create deeply subsided basins.

Surface faulting and paleoseismic history of the 1932 Cedar Mountain earthquake area, west-central Nevada, and implications for modern tectonics of the Walker Lane

The 1932 Cedar Mountain earthquake (M s 7.2) was one of the largest historical events in the Walker Lane region of western Nevada, and it produced a complicated strike-slip rupture pattern on multiple Quaternary faults distributed through three valleys. Primary, right-lateral surface ruptures occurred on north-striking faults in Monte Cristo Valley; small-scale lateral and normal offsets occurred in Stewart Valley; and secondary, normal faulting occurred on north-northeast–striking faults in the Gabbs Valley epicentral region. A reexamination of the surface ruptures provides new displacement and fault-zone data: maximum cumulative offset is estimated to be 2.7 m, and newly recognized faults extend the maximum width and end-to-end length of the rupture zone to 17 and 75 km, respectively. A detailed Quaternary allostratigraphic chronology based on regional alluvial-geomorphic relationships, tephrochronology, and radiocarbon dating provides a framework for interpreting the paleoseismic history of the fault zone. A late Wisconsinan alluvial-fan and piedmont unit containing a 32–36 ka tephra layer is a key stratigraphic datum for paleoseismic measurements. Exploratory trenching and radiocarbon dating of tectonic stratigraphy provide the first estimates for timing of late Quaternary faulting along the Cedar Mountain fault zone. Three trenches display evidence for six faulting events, including that in 1932, during the past 32–36 ka. Radiocarbon dating of organic soils interstratified with tectonically ponded silts establishes best-fit ages of the pre-1932 events at 4, 5, 12, 15, and 18 ka, each with ±2 ka uncertainties. On the basis of an estimated cumulative net slip of 6–12 m for the six faulting events, minimum and maximum late Quaternary slip rates are 0.2 and 0.7 mm/yr, respectively, and the preferred rate is 0.4–0.5 mm/yr. The average recurrence (interseismic) interval is 3600 yr. The relatively uniform thickness of the ponded deposits suggests that similar-size, characteristic rupture events may characterize late Quaternary slip on the zone. A comparison of event timing with the average late Quaternary recurrence interval indicates that slip has been largely regular (periodic) rather than temporally clustered. To account for the spatial separation of the primary surface faulting in Monte Cristo Valley from the epicenter and for a factor-of-two-to-three disparity between the instrumentally and geologically determined seismic moments associated with the earthquake, we hypothesize two alternative tectonic models containing undetected subevents. Either model would adequately account for the observed faulting on the basis of wrench-fault kinematics that may be associated with the Walker Lane. The 1932 Cedar Mountain earthquake is considered an important modern analogue for seismotectonic modeling and estimating seismic hazard in the Walker Lane region. In contrast to most other historical events in the Basin and Range province, the 1932 event did not occur along a major range-bounding fault, and no single, throughgoing basement structure can account for the observed rupture pattern. The 1932 faulting supports the concept that major earthquakes in the Basin and Range province can exhibit complicated distributive rupture patterns and that slip rate may not be a reliable criterion for modeling seismic hazard.

Source parameters of moderate‐sized earthquakes in the Tien Shan, central Asia from regional moment tensor inversion

We estimate faulting geometry, depth, and seismic moment of 32 moderate‐sized earthquakes in the Tien Shan by matching observed and synthetic seismograms recorded at regional distances. The moment‐tensor compression axes are oriented in the direction 170°–350°, consistent with the orientation of India‐Eurasia convergence. This compression is manifested in reverse faulting on roughly east‐west trending fault planes. Faults within the mountain system have moderate‐to‐steep dips, but range‐bounding faults appear to dip more gently. Earthquake source depths are in the range 5–25 km, consistent with those determined from local earthquake studies. The distribution and mechanism of earthquakes in the Tien Shan suggests that a significant proportion of the active deformation in the Tien Shan is absorbed by shortening across intramontane basins. The absence of strike‐slip earthquake mechanisms associated with the Talas‐Ferghana Fault suggests that the fault is presently locked, with deformation accommodated on neighboring compressional structures.

Array measurements of phases used in receiver-function calculations: Importance of scattering

AbstractReceiver-function methods utilize three-component teleseismic P coda to determine structure beneath seismographs. Most methods treat coda complexity as mode conversions and reflections off subhorizontal interfaces beneath the station, such as the Moho. This assumption is evaluated beneath a broadband array in the Kopet Dagh foothills, at Geyokcha, Turkmenistan, by applying array processing techniques to P coda waves at frequencies near 1 Hz. Beneath the Geyokcha array, horizontal components show signals that deviate significantly in slowness and bearing from predictions of a plane-layered Earth. Large phases arrive at the same time as the predicted Ps conversion from the Moho yet systematically come from a single azimuth (from NNE) and travel at phase velocities of 3 to 5 km/sec. The coda waves appear to be P-to-S converted waves from a near-surface scatterer. An inversion for scatterer location is developed and applied to 29 events. Most coda energy comes from a scatterer ∼16 km NNE of the array. This location is coincident with the range-bounding faults that separate Kopet Dagh mountains from adjacent basins. Thus, array analysis shows that signals potentially interpreted as Moho conversions are instead scattered from a near-surface structure. Although these results suggest caution is necessary in interpreting single-station receiver functions, particularly at high frequencies, they also demonstrate the power of array sampling in resolving lateral structure variations.

Gold mineralization and fault evolution at the Dixie Comstock Mine, Churchill County, Nevada

Gold ore at the Dixie Comstock mine, Churchill County, Nevada, is composed of quartz breccia, quartz stockwork, incipiently crushed gabbro, and minor fault gouge. The ore zone forms a mullion within a range-bounding normal fault, the Dixie Comstock mine fault, which originated in the middle Miocene and separates Jurassic gabbro and Tertiary volcanic rocks of the Stillwater Range to the west from Quaternary colluvium of Dixie Valley to the east. Within the mullion, a potentially bulk mineable resource of several million tons grading approximately 0.06 oz of gold per ton has been defined by drilling.Within the fault zone, clasts of gabbro in quartz breccia, quartz stockwork gabbro, and incipiently crushed gabbro are altered to quartz, sericite, chlorite, montmorillonite, and sulfides. Gold occurs as electrum in quartz that cements quartz breccia, in quartz stockwork veins with quartz, pyrite, chalcopyrite, and montmorillonite in breccia clasts, and in veins in incipiently crushed gabbro. Fluid inclusion microthermometry and stable isotope analyses of mineralized quartz matrix and two later stages of quartz and calcite show that ore fluid was 180 degrees C, near-boiling meteoric water.An inverse stratigraphy of thermal spring silica detritus in hanging-wall colluvium, microthermometry and isotopic compositions of fluid inclusion water from quartz breccia, a subjacent geothermal reservoir, and radiometric ages of hydrothermal minerals in the fault cumulatively support a mid-Pleistocene age for gold mineralization. The lower average displacement rate on the Dixie Comstock mine fault that is required by the age and thickness of eroded thermal spring sinter and underlying quartz breccia and stockwork, compared to higher recent displacement rates along range-bounding faults on the western side of Dixie Valley, suggests that the current pattern of temporal clustering of fault displacements began by the mid-Pleistocene.

Relationship of Laramide and Basin and Range Structures in the Pedregosa Basin, Southwestern New Mexico: ABSTRACT

The Pedregosa basin of southwestern New Mexico has long been recognized as a frontier hydrocarbon exploration target. The Paleozoic sedimentary rocks contain petroleum source and reservoir units that correlate with important producing zones in the Permian basin of west Texas and southeastern New Mexico. Factors that are commonly considered to limit the petroleum potential of the region include multiple episodes of deformation, heating due to local igneous intrusion and volcanism, and fresh-water flushing. Laramide (late Cretaceous-early Tertiary) fold-and-thrust traps are structural targets. Subsequent fracturing by Basin and Range (late Tertiary) normal faulting may have partially destroyed the Laramide traps. Seismic reflection and gravity data from the Playas and Hatchita valleys in southwestern New Mexico constrain the structural styles of Laramide shortening and later Basin and Range extensional deformation. These data show that Precambrian basement was involved in Laramide thrusting. This observation refutes the interpretation of some previous workers who interpreted the Precambrian basement surface as a regional decollement for Laramide thrusting. An east-west seismic profile that crosses the southeast end of the Little Hatchet Mountains images a range-bounding listric normal fault and associated antithetic faults beneath the Hatchita Valley fill. Near the range, [open quotes]antithetic[close quotes] faults show a normalmore » sense of motion, but farther east, faults are reverse. The latter may be high-angle Laramide structures or unusual features of Basin and Range deformation. The major range-bounding fault is steeply dipping Near the surface, but becomes a low-angle listric fault at a depth of about 10 km.« less

A relation among geology, tectonics, and velocity structure, western to central Nevada Basin and Range

Research Article| September 01, 1992 A relation among geology, tectonics, and velocity structure, western to central Nevada Basin and Range R. D. CATCHINGS R. D. CATCHINGS 1U.S. Geological Survey, 345 Middlefield Road, M.S. 977, Menlo Park, California 94587 Search for other works by this author on: GSW Google Scholar Author and Article Information R. D. CATCHINGS 1U.S. Geological Survey, 345 Middlefield Road, M.S. 977, Menlo Park, California 94587 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1992) 104 (9): 1178–1192. https://doi.org/10.1130/0016-7606(1992)104<1178:ARAGTA>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R. D. CATCHINGS; A relation among geology, tectonics, and velocity structure, western to central Nevada Basin and Range. GSA Bulletin 1992;; 104 (9): 1178–1192. doi: https://doi.org/10.1130/0016-7606(1992)104<1178:ARAGTA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract In the northwestern to central Nevada Basin and Range, there are correlations between velocity and specific geologic structures of the crust. Mapped range-bounding faults at the surface can be traced to appreciable (10 km) depths based on velocity variations and are consistent with subsurface projections of the faults based on seismic reflection images. The limiting depth of the faults, as indicated by the velocity variations, corresponds to the maximum depth of earthquakes along the seismic profile. Correlations between velocity and the surface geology show that in the upper crust the pre-Cenozoic rocks are underlain by high-velocity (6.0 km/s) rocks, whereas the Tertiary ranges are underlain by lower-velocity (4.0-5.7 km/s) rocks to depths as great as 10 km. Although the Tertiary rocks differ in composition from the Mesozoic rocks, the lower-velocity Tertiary rocks may also be attributed to rock masses which are broken (4.0-5.7 km/s), and the higher-velocity Mesozoic rocks (6.0 km/s) may be attributed to largely unbroken rock masses. The regional seismicity pattern is consistent with this interpretation, as earthquakes are largely confined within or near the base of the low-velocity rocks. These low-velocity, highly fractured rocks are laterally distributed in discrete zones, suggesting that extension is not uniformly distributed but occurs in discrete, highly extended zones. Beneath these highly extended zones, the lower-crustal layers show structural evidence of extension, and velocity measurements suggest that the lowermost crust has been magmatically underplated. The superposition of Tertiary volcanic rocks, highly fractured upper crust, and lower-crustal magmatic underplating suggests that the Tertiary volcanic rocks originated from lower-crustal magmas that migrated to the surface via the highly extended zones. The velocity structure of one of the highly extended zones and the Lahonton Basin resembles that of many continental rifts. The velocity structure beneath central Nevada, however, is much more like normal continental crust. On the basis of isotopic studies, it is concluded that the transition between highly extended crust and more normal crust occurs in the area inferred to be the edge of the North American craton. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Late Quaternary fault segmentation from analysis of scarp morphology

A thorough knowledge of the lateral extent of individual prehistoric ruptures along a fault is necessary to assess the locations and geologic characteristics of boundaries between independent faultrupture segments. Paleoseismic studies employing only trenching may yield insufficient data to evaluate the lateral extent of individual rupture events. Systematic mapping of fault scarps and degradation-equation modeling of relative scarp ages on the Lemhi fault, a range-bounding normal fault in east-central Idaho, provide a method to assess rupture extent and boundaries between rupture segments. The youngest latest Quaternary surface ruptures occurred at distinctly different times on adjacent segments of the fault. The boundaries between rupture segments differ in characteristics, although most are at intersections of the currently active range-bounding fault and mid-Tertiary normal faults. Several major structural and tectonic discontinuities did not form segment boundaries in the most recent rupture cycle or (probably) in the previous cycle. The Lemhi fault illustrates both the importance of preexisting structures in controlling extent of ruptures and the need for detailed analysis along an entire fault zone.