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We integrated high-resolution bare-earth airborne light detection and ranging (LiDAR) imagery with field observations and modern geochronology to characterize the Tahoe-Sierra frontal fault zone, which forms the neotectonic boundary between the Sierra Nevada and the Basin and Range Province west of Lake Tahoe. The LiDAR imagery clearly delineates active normal faults that have displaced late Pleistocene glacial moraines and Holocene alluvium along 30 km of linear, right-stepping range front of the Tahoe-Sierra frontal fault zone. Herein, we illustrate and describe the tectonic geomorphology of faulted lateral moraines. We have developed new, three-dimensional modeling techniques that utilize the high-resolution LiDAR data to determine tectonic displacements of moraine crests and alluvium. The statistically robust displacement models combined with new ages of the displaced Tioga (20.8 ± 1.4 ka) and Tahoe (69.2 ± 4.8 ka; 73.2 ± 8.7 ka) moraines are used to estimate the minimum vertical separation rate at 17 sites along the Tahoe-Sierra frontal fault zone. Near the northern end of the study area, the minimum vertical separation rate is 1.5 ± 0.4 mm/yr, which represents a two- to threefold increase in estimates of seismic moment for the Lake Tahoe basin. From this study, we conclude that potential earthquake moment magnitudes (M w ) range from 6.3 ± 0.25 to 6.9 ± 0.25. A close spatial association of landslides and active faults suggests that landslides have been seismically triggered. Our study underscores that the Tahoe-Sierra frontal fault zone poses substantial seismic and landslide hazards.

The seismically active eastern and western margins of the northern Basin and Range have been extensively studied, yet the northwestern margin of the province remains incompletely understood. The Black Rock Range of northwestern Nevada straddles the transition from the Basin and Range province to the south and east, and flat-lying volcanic plateaus to the west. This poorly understood range preserves a remarkably complete record of Cenozoic magmatism and provides an important window into the pre-Miocene history of the unextended volcanic plateaus of northeastern California and southern Oregon. Geologic mapping and 40Ar/39Ar geochronology from the northern Black Rock Range document three significant episodes of Eocene to middle Miocene volcanism. Eocene (35 Ma) basalts directly overlie Mesozoic granites and arc-related volcanic and sedimentary rocks. Locally erupted Oligocene to early Miocene (27–21 Ma) bimodal volcanic rocks comprise the bulk of the Cenozoic section and conformably overlie the Eocene basalt flows. These bimodal units include rhyolitic lavas, variably welded rhyolitic ash flows, unwelded ash-fall deposits, and thin basalt flows. In the neighboring Pine Forest Range ∼20 km to the north, similar Oligocene to early Miocene units are overlain by more than 500 m of ca. 16.4 Ma Steens-equivalent basalt flows and are capped by ca. 16 Ma rhyolitic ash-flow tuffs. In the northern Black Rock Range, the ca. 16.4 Ma middle Miocene basalts are absent from the section, and a 16.2 Ma rhyolitic ash-flow tuff directly overlies the early Miocene flows. Basaltic and rhyolitic volcanic products in the northern Black Rock Range span 35–16 Ma, with many of the Oligocene volcanic units derived from local vents and dikes. Despite the map-scale complexities of locally derived lava flows, the Cenozoic section is broadly conformable and dips gently (∼5°–10°) to the northwest. The region experi enced no significant tilting between 35 and 16 Ma, with moderate tilting (∼5°–10°) and concomitant uplift occurring after 16 Ma. This tectonic history is consistent with that of the nearby Pine Forest and Santa Rosa Ranges, where low-temperature thermochronology documents footwall exhumation along the range-bounding normal faults after 12 Ma. The velocity structure of the crust beneath the northern Black Rock Range is constrained by a recent geophysical survey (seismic reflection, refraction, and gravity) and contains gradients that correspond to basin depths predicted by our geologic mapping. Together with recently completed geological and geophysical studies from the surrounding region, our results suggest that the evolution of the northwestern margin of the Basin and Range was characterized by long-lived and voluminous volcanism without significant tectonism, followed by low-magnitude (≤20%) extension along high-angle normal faults.

New geologic mapping, tectonic geomorphologic, and cosmogenic radionuclide geochronologic data provide the fi rst numerical constraints on late Quaternary vertical slip and horizontal extension rates across the southern Sierra Nevada frontal fault zone, California. This fault zone exposes numerous NNW-striking, east-facing normal fault scarps that offset seven distinct Quaternary alluvial surfaces and a rockslide deposit. Beryllium-10 cosmogenic radionuclide surface-exposure dating of these surfaces provides surface abandonment model ages of 123.7 ± 16.6 ka, 60.9 ± 6.6 ka, 25.8 ± 7.5 ka, 4.4 ± 1.1 ka, and 4.1 ± 1.0 ka on alluvial fan surfaces, and 18.7 ± 3.9 ka on the rockslide deposit. These age constraints, combined with measurements of vertical surface offset across fault scarps, yield preferred late Pleistocene to Holocene vertical and horizontal extensional slip rates of 0.2‐0.3 mm/yr and 0.1‐0.2 mm/yr. Vertical slip-rate estimates in this study are comparable to late Pleistocene vertical slip-rate estimates across other prominent range-front normal faults within the Basin and Range Province. These geologic and geochronologic results indicate that the eastern escarpment of the southern Sierra Nevada has remained tectonically active throughout the late Quaternary. Combining our data with slip data from the Owens Valley and Lone Pine faults implies that slip along the eastern escarpment of the Sierra Nevada block is partitioned into three components: dominantly dextral slip along the Owens Valley fault, intermediate oblique slip along the Lone Pine fault, and subordinate normal slip along the Sierra Nevada frontal fault zone. These observations are consistent with global positioning system (GPS) data, which indicate that dextral shear-strain accumulation dominates today along the western boundary of the Eastern California Shear Zone and Basin and Range Province.

We report new U-Pb detrital zircon sensitive high-resolution ion microprobe (SHRIMP) age data (702 grains) from 13 samples collected from Miocene to Holocene sedimentary deposits in the western Snake River Plain region. These samples effectively show that modern stream sediments of the Snake River system reliably and repeatedly record the detrital zircon age populations that are present as sources in their drainage basins across the Cordilleran thrust belt and Basin and Range Province. We use this framework and the provenance of Neogene sedimentary rocks in the region to test the effect of the migrating Yellowstone hotspot on regional drainage patterns in southern Idaho since the middle Miocene. Our results indicate that Neogene paleodrainages were fi rst directed radially away from the tumescent Yellowstone highland, then subsequently reversed their fl ow toward the subsiding Snake River Plain basin. This occurred in east-progressing time-constrained intervals starting at 16 Ma. In northern Nevada, the drainage divide is represented by a northeast-trending, southeast-migrating crest of high topography. Specifi cally, middle to late Miocene (16‐ 10 Ma) sedimentary deposits of the western Snake River Plain and Oregon-Idaho graben contain early to middle Eocene (52‐42 Ma) detrital zircon populations sourced in Challis magmatic rocks north of the Snake River Plain. Middle Jurassic (160 Ma) and middle to late Eocene (42‐35 Ma) detrital zircons, sourced from rocks in northern Nevada, are not present. Late Eocene detrital zircons from Nevada are present in two younger than 7 Ma sedimentary units of the Idaho Group along the Oregon-Idaho border. This indicates that by the late Miocene, southeastward headward erosion of the paleo‐Owyhee River into the Owyhee Plateau had captured drainage from north-central Nevada and directed it northwestward toward the subsiding western Snake River Plain. The modern Owyhee Plateau is still a topographic high, in contrast to the modern Snake River Plain, suggesting that lowering of the regional Snake River Plain base level, rather than crustal subsidence, drove stream capture. By the late Pliocene (3 Ma), Middle Jurassic detrital zircons are recorded in the Glenns Ferry Formation and Tuana Gravel of the central Snake River Plain, suggesting that surface subsidence reversed the fl ow direction of paleo‐Salmon Falls Creek from southward into Nevada to northward toward Idaho. Miocene strata of the western Snake River Plain lack recycled Proterozoic detrital zircons that are ubiquitous in sedimentary rocks of the central and southeast Idaho thrust belts. Such detrital zircons appear on the central and western Snake River Plain in early Pliocene to Holocene (4‐0 Ma) deposits. This records capture of drainage from the eastern Snake River Plain. The Yellowstone hotspot controlled the east-migrating continental divide, in the wake of which formed the western-draining, and progressively eastward-collecting, Snake River system.

The unique low-temperature sensitivity of the apatite (U-Th)/He (AHe) technique provides valuable new information relating to the late-stage thermal evolution of metamorphic core complexes in the southern Basin and Range Province. In all the ranges studied, the data show greater complexity than previously seen from other thermochronometers. In the Chemehuevi and northern Sacramento Mountains, a trend of decreasing ages toward the northeast applies only for the structurally shallowest parts of the footwall. This is followed by essentially age-invariant segments of 15 ± 1 Ma, reflecting an increase in the rate of detachment fault movement. The timing of the change suggested by the data agrees with recently published AHe results from the nearby Harcuvar Mountains, located ∼125 km to the southeast, suggesting that an underlying regional mechanism triggered this change. Plate reconstructions indicate that the Pioneer-Mendocino fracture zone migrated northward through the area at this time, leaving a slab window in its wake. We suggest that the additional heat applied to the base of the lithosphere from the slab window resulted in mechanical weakening of the area and that this, combined with the extensional strain regime present, resulted in an increased rate of slip within the actively extending metamorphic core complexes. This agrees well with timing constraints for extension onset in the central Basin and Range Province of southern Nevada, and suggests that weakening of the southern Basin and Range Province at 15 ± 1 Ma was partially responsible for the onset of extension in the central Basin and Range. These findings demonstrate that improved constraints on changes in extension rate within the Basin and Range Province should be possible through the application of the AHe technique to other metamorphic core complex footwalls.

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.

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.

The Camargo volcanic field is the largest mafic alkalic volcanic field in the Mexican Basin and Range province, and the relationship between volcanism and normal faulting is especially strong. The Camargo volcanic field lies in the northern part of the province, midway between the Sierra Madre Occidental and Trans-Pecos Texas. It is formed by Pliocene–Pleistocene (4.7–0.09 Ma) intraplate mafic alkalic volcanic rocks, some of which contain peridotite, pyroxenite, and granulite xenoliths. The volcanic field covers ∼3000 km2 and has an estimated volume of ∼120 km3 erupted from >300 recognized vents. Twenty-six new 40Ar/39Ar age determinations for the Camargo volcanic field and its environs show that volcanic activity began in the southwest part of the field and shifted toward the northeast at ∼15 mm/yr. The average magmatic eruption rate during growth of the field was ∼0.026 km3/k.y. The Camargo volcanic field lies within an accommodation zone with west-dipping faults and east-tilted blocks to the north and east-dipping faults and west-tilted blocks to the south. These faults are expressed in the volcanic field by a N30°W-trending graben with scarps up to ∼100 m high through its central part. Volcanism and faulting were at least in part coeval, and younger volcanic products commonly drape fault scarps that cut earlier lavas. Normal faulting is bracketed between 4.7 and 2.1 Ma and may have also migrated northeastward. Estimated vertical slip rates on four Pliocene faults range from 0.03 mm/yr, a likely long-term rate, to 1.67 mm/yr, interpreted as a short-term rate operative during periods of active faulting. Northwest-striking normal faults that cut alluvial-fan deposits and Pleistocene lavas in the northern Camargo volcanic field and geomorphic evidence for recent uplift to the south of the volcanic field suggest that the region is still extending.

Mono Basin, on the eastern flank of the central Sierra Nevada, is the highest of the large hydrographically closed basins in the Basin and Range province. We use geomorphic features, shoreline deposits, and basalt-filled paleochannels to reconstruct an early to middle Pleistocene record of shorelines and changing spillways of Lake Russell in Mono Basin. During this period of time, Lake Russell repeatedly attained altitudes between 2205 and 2280 m—levels far above the present surface of Mono Lake (∼1950 m) and above its last overflow level (2188 m). The spill point of Lake Russell shifted through time owing to late Tertiary and Quaternary faulting and volcanism. During the early Pleistocene, the lake periodically discharged through the Mount Hicks spillway on the northeastern rim of Mono Basin and flowed northward into the Walker Lake drainage basin via the East Walker River. Paleochannels recording such discharge were incised prior to 1.6 Ma, possibly between 1.6 and 1.3 Ma, and again after 1.3 Ma (ages of basaltic flows that plugged the paleochannels). Faulting in the Adobe Hills on the southeastern margin of the basin eventually lowered the rim in this area to below the altitude of the Mount Hicks spillway. Twice after 0.76 Ma, and possibly as late as after 0.1 Ma, Lake Russell discharged southward through the Adobe Hills spillway into the Owens–Death Valley system of lakes. This study supports a pre- Pleistocene aquatic connection through Mono Basin between the hydrologically distinct Lahontan and Owens–Death Valley systems, as long postulated by biologists, and also confirms a probable link during the Pleistocene for species adapted to travel upstream in fast-flowing water.

East-northeast extension ca. 24 Ma at Rodeo and Nazas, Mexico, was accompanied by eruption of hawaiites, marking some of the earliest intraplate-type mafic alkalic volcanism associated with development of the southern Basin and Range Province. An earlier extensional pulse, 32.3–30.6 Ma, concurrent with subduction- related rhyolitic volcanism of the Sierra Madre Occidental, is the earliest established extension in the southern Basin and Range Province. The Rodeo hawaiites are mostly along or just west of the north- northwest−striking, west-dipping Rodeo fault, a major breakaway fault that separates moderately extended terrane to the west from less extended terrane to the east. Hawaiites and interbedded conglomerates in the Rodeo half graben are flat lying to gently tilted and cut by the Rodeo fault. Underlying Eocene−Oligocene ash-flow tuffs to the west are cut by numerous west- dipping faults and tilted as much as 40°. Nazas hawaiites are also along north-northwest−striking faults but are generally unfaulted. Like other Miocene mafic alkalic volcanic rocks from the southern Basin and Range Province, those from Rodeo and Nazas are characterized by (1) moderately evolved hawaiitic compositions, (2) abundant megacrysts, including sodic plagioclase (An26–51), olivine (∼Fo55), Al-augite (7– 9 wt% Al2O3), and a wide variety of spinels, (3) lack of granulitic or peridotitic xenoliths, (4) variable Cs enrichments, and (5) isotopic compositions that indicate interaction with crust (87Sr/86Sri = 0.7037−0.7041; ϵNd = 4.8−2.8; 206Pb/204Pbi = 18.91−18.77; 207Pb/204Pbi = 15.57−15.60). These observations are consistent with a model whereby Miocene intraplate-type magmas rose slowly through the lithosphere, differentiating and interacting with the crust. The megacrysts are interpreted as disrupted gabbroic bodies, formed by slow cooling of mafic alkalic magmas that stagnated in the lower crust, possibly during the earlier extensional episode. Many of the elemental and isotopic parameters used to probe mantle sources of mafic volcanic rocks have been seriously obscured by crystallization, crustal interaction, and megacryst incorporation, complicating efforts to identify temporal changes in mantle source regions during development of the southern Basin and Range Province.

In areas with prolonged extensional histories or multiple episodes of extension, temporally distinct extensional fault systems, including dip-slip-normal, oblique-slip, and/or strike-slip faults, may form. These fault systems may be spatially distinct, overlapped, or completely superimposed, and different fault systems may have different fault strikes and extension directions. Although fault strike is not a very sensitive indicator of principal stress directions, changes through time in fault strike of as much as 90° exceed the uncertainty. Overprinted extensional fault systems of different ages with significantly different (to 90°) fault strikes and extension directions provide strong evidence of changes in the regional stress field with time. Tens of millions of years of extension occurred in the Basin and Range Province of the United States and many temporally distinct fault sets formed through that time. We document at least four extensional episodes and the fault kinematics of each, where possible, in the central Hiko Range, eastern Nevada. The four Cenozoic extensional events are prevolcanic, synvolcanic, postvolcanic Miocene–Pliocene (?), and postvolcanic Pliocene (?)–Quaternary. The prevolcanic faults strike north-south and are interpreted as footwall faults to the regional Eocene–Oligocene Snake-Stampede extensional system, which caused approximately east-west extension. The synvolcanic faults are late Oligocene–Miocene age and strike east-west. Although they are rare and have minor offset, they accommodated north-south–directed extension. Postvolcanic faults, part of the Timpahute lineament, are approximately east-west– striking normal and northeast-striking oblique-slip faults. They also accommodated approximately north-south extension during Miocene time. The northeast-striking left-normal oblique-slip fault set formed as new faults and records a change between north-south and east-west extension in Miocene time. Later east-west extension occurred along the Pliocene (?) to Quaternary Hiko fault zone. These four overprinted extensional systems show two separate changes in normal fault strike of ∼90°, from north-south to east- west and from east-west back to north-south. Consequently, the extension direction similarly changed through time. We suggest that causes for the changes in direction of upper crustal extension include a change from stresses created by (1) plate interactions, which generated approximately east-west extension, to (2) mantle motions, which generated approximately north-south extension, and (3) back again. The extension related to mantle motions is associated with a band of volcanism that passed across the northern Basin and Range Province, moving generally from north to south, during the protracted period of extension. Therefore, a change from plate boundary– or interaction-derived stresses to mantle- or volcanism-related stresses and back again is possible.

Conglomeratic strata from middle Miocene sections in the central Resting Spring Range and nearby Eagle Mountain, California, contain a clast assemblage including marble, orthoquartzite, fusulinid grainstone, and coarse (∼1 m) monzogabbro, interstratified with tephras yielding laser-fusion ^(40)Ar/^(39)Ar ages of 11.6, 13.4, and 15.0 Ma. Petrographic and geochronologic evidence ties the clast assemblage to a source area in the southern Cottonwood Mountains, California, >100 km west-northwest of their present location. In the upper 100 m of the Resting Spring Range section, conglomerates are derived almost exclusively from the southern Cottonwoods source, and sandstone modes are as much as 50% angular plagioclase derived from the monzogabbro. The lack of dilution of this detritus by other sources and sedimentary features in both sections indicate (1) that deposition occurred on an alluvial fan with a north- northeast paleoslope and (2) that transport of the gravels by sedimentary processes was probably

The South Virgin Mountains and Grand Wash trough comprise a mid-Miocene normal fault system that defines the boundary between the unextended Colorado Plateau to the east and highly extended crust of the central Basin and Range province to the west. In the upper 3 km of the crust, the system developed in subhorizontal cratonic strata in the foreland of the Cordilleran fold and thrust system. The rugged topography and lack of vegetation of the area afford exceptional three-dimensional exposures. Compact stratigraphy and well-defined prefaulting configuration of the rocks permitted a detailed reconstruction of the system. Reconstruction of cross sections based on more than 300 km2 of detailed mapping at a scale of 1:12 000 shows that the fault system accommodated more than 15 km of roughly east-west–directed Miocene extension. Extension was initially accommodated on moderately to steeply dipping listric normal faults. As the early faults and fault blocks tilted, steeply to moderately dipping faults initiated within the fault blocks, soling into the early faults. Some of the early faults were active at dips of

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.

The latest Cenozoic (

Late Cenozoic (

Seismic reflection profiles, drill-hole data, and geologic maps delimit the form of normal faults and Tertiary sedimentary rocks in the southern half of the Cache Valley basin in northern Utah. Dips of faults and sedimentary rocks were estimated from time-migrated reflection profiles by using the stacking velocities for the data. At the southern end of the basin, the East Cache fault zone is listric; it shows 50°W dips near the surface and ≈20°W dips at depth. The fault zone has been the site of ≈5.6 km of net dip slip. Tertiary rocks dip 18°E–25°E and exhibit a rollover geometry above the fault zone. No faults are interpreted on the western side of the basin. In the central part of the basin in Utah, the East Cache fault is a single fault that dips at least 45°W near the surface and is the site of 4.5–6.4 km of net dip slip. Here, the basin is bounded to the west by the West Cache fault, which is the site of at least 1 km of net slip that increases northward to 2 km of net slip. Slip on the East Cache fault resulted in planar, east-dipping, older Tertiary rocks near the bottom of the basin. Younger Tertiary strata, with southwest, west, and northwest dips, reflect complex tilting due to slip on the West and East Cache faults. Anticlines in the Tertiary basin-fill deposits are present in the central part of the basin and may reflect changes in normal-fault geometry at depth. Northward, dip slip on the East Cache fault zone decreases to 2.5 km. The basin is broad, shallow, and filled with nearly flat lying Tertiary rocks. This area, near the north-south midpoint of the basin, is bounded by the West and East Cache faults, but slip on the West Cache fault appears to diminish northward. A north-trending reflection profile tied to both drill-hole data and the east-trending seismic profiles indicates that the basin is deeper in the southern end. The along-strike changes in fault geometry, the amount of fault-slip, the subsurface form of the basin-filling sedimentary rocks, and the form of the basin indicate a complex history of faulting and deposition during its formation. This study and other recent ones from the Basin and Range province indicate that such complexities may be typical of many Tertiary basins in the region.

Crater Flat is an alluvium-filled structural basin on the west side of Yucca Mountain, Nevada, which is under consideration for a high-level nuclear waste repository. North-trending, late Quaternary faults offset alluvium in Crater Flat both along the canyons of the western flanks of Yucca Mountain and out on the piedmont slope. We believe the initial lack of young offsets at Yucca Mountain was in part due to unrecognized late Quaternary stratigraphy. We hypothesize that alluviation in the Yucca Mountain region was more active during the late Quaternary than previously thought. Several techniques were tried to test this hypothesis. Results are compared with previous soils and surface-exposure dating studies, and correlated to stratigraphy of other late Quaternary units in the southern Nevada, Death Valley, and Mojave Desert areas, and provide new stratigraphic data relevant to understanding climatic-alluvial processes in the Basin and Range Province during the late Quaternary. 76 refs., 7 figs., 6 tabs.

Two east-west seismic reflection profiles crossing Antelope Valley, Smokey Valley, Railroad Valley, and Big Sand Springs Valley in east-central Nevada demonstrate the evolution of Tertiary basin extension in the Basin and Range Province from broad sags to narrow, fault-bounded basins. Reprocessing of a 480 channel, 60 fold, dynamite-source experiment enabled good imaging of basin stratigraphy. These data suggest two distinct phases of basin development occurred, separated by a regional unconformity. The early phase is characterized by development of a broad symmetric basin riddled with many small offset normal faults. The later phase shows a narrowing of the basin and subsidence along one dominant structure, an apparent planar normal fault. The unconformity separating the two phases of extension marks a transition from broad subsidence to local asymmetric tilting that took place over a short period of time relative to sedimentation rates. Antelope Valley, Smokey Valley, and Railroad Valley clearly show evidence for two-phase development. Big Sand Springs Valley represents only the later phase of extension. The absence of dating of stratigraphic units within the basins precludes us from determining if the abrupt tectonic transition within the basins resulted from differences in local strain rates or amounts, or was due to changes in regional stress fields.

Three groups (Miocene, Pliocene, and Pleistocene) off oceanic-type basalts associated with normal faulting have been identified in an area surrounding the city of Guadalajara, Mexico. Although most basalts within these groups have compositional characteristics of an asthenospheric source, each group is also associated with lavas that have subduction-related traits. The first group, the San Cristobal plateau basalts, has a minimum volume of 1,800 km 3 of predominantly alkali olivine basalt and lesser basaltic andesite and fills a pre-existing extensional basin of unknown configuration ∼10 m.y. ago. The relatively rapid eruption of these basalts along with their volume, structural association, and chemistry suggest an upwelling mantle plume beneath the Guadalajara region. This region may be the earliest indication of the separation of the Jalisco block from North America. The second basalt group, the Guadalajara basalts, consists of small volumes of porphyritic basalts and basaltic andesites, 3.3-5.0 Ma in age, that outcrop near the northern outskirts of Guadalajara and near the town of Hostotip-aquillo to the northwest. The youngest basalts in the Guadalajara area are the 0.4-1.4 Ma Santa Rosa suite of basalts-hawaiites-mugearites close to the contemporary (>0.2 Ma) andesitic central volcano, V. Tequila; these either flowed from nearby cinder cones toward the Rio Santiago canyon or erupted in the canyon to dam the river. The flat-lying Tertiary (23-27 Ma) ash flows of the Sierra Madre volcanic province that lie to the north of Guadalajara are faulted (north-northeast) into horsts and grabens that in one case can be dated as being older than 21.8 Ma. This fault trend has been reactivated closer to Guadalajara, as normal faults (200- to 300-m displacement) cut the Miocene plateau basalts but are hidden by a cover of younger volcanic rocks close to the city. Also hidden by the young volcanic succession there is the north-west-southeast fault zone that extends to the Gulf of California as the Tepic-Zacoalco graben system. Faulting of this zone is well displayed in the region of the Santa Rosa dam in the Santiago canyon, northwest of Guadalajara. Two styles of faulting are found there: an older (>1 Ma, 3 , has been approximately north-northeast for at least 4 m.y., characteristic of the Mexican Basin and Range province.

The Nazas area, in the central-eastern portion of the state of Durango, is within the poorly studied southern half of the Basin and Range province in Mexico. At Nazas, high-angle normal faults that cut part of the mid-Tertiary volcanic sequence strike between N 20° and 70° W, with 40% of them between N 40° and 50° W. Tilting of faulted blocks varies from 5° to 35° to the northeast, most commonly around 25° NE. A few blocks are tilted to the southwest as much as 25°. Fault offsets range from 40 m to nearly 300 m. The earliest faulting occurred between 31 and 29 Ma, before the emplacement of mid-Tertiary ash-flow tuffs had ended. The Santa Ines Formation, a widespread fanglomerate, was deposited after the earliest faulting episode and is overlain by 24 to 20 Ma alkalic basalt flows. Although not cut by faults, the flows are adjacent to or cover normal faults. Some of the mapped faults could have been coeval with basalt eruption, as is the case in Trans-Pecos Texas, where alkalic basalt having similar age and composition to that in the Nazas area erupted contemporaneously with normal faulting. The Nazas alkalic basalt also has similar age and stratigraphic position as, but is compositionally distinct from, the Southern Cordilleran Basaltic Andesite (SCORBA), a widespread mafic suite in southwestern North America that has been linked to regional extension.

Outcrops, shallow well control, and coincident geophysical surveys are used to interpret a seismic reflection profile in the Amargosa Desert, within the Basin and Range province, of southern Nevada. The east-west-trending, 27-km-long seismic line crosses all or parts of three Tertiary subbasins, revealing that basin growth occurred by progressive shifts of basin-bounding faults. The reflection line images Tertiary strata that is rotated by steeply dipping listric faults and that noses into normal faults. A shallow (less than 100 to 200 m deep), laterally continuous, flat-lying, low-frequency reflector, interpreted as a Tertiary basalt flow, suggests that little vertical deformation has occurred within the easternmost of the small Tertiary basins since the eruption of the flow about 10 million years ago. Moderately dipping reflections within the pre-Tertiary bedrock may image Mesozoic thrust faults. The reflection data indicate that, whereas the top of the reflective lower crust shallows to the west, possibly in the direction of increasing crustal extension, the Moho is relatively flat between 30 and 33 km deep. Apparent bright-spot reflections from the lower crust are interpreted as evidence for ductile shearing of the lower crust, not for active magma chambers. Doming of the lower crust resembles that observed elsewhere in the Basin and Range province and is consistent with ductile flow in the lower crust.

At Crater Island, northern Silver Island Mountains, northwestern Utah, an unbroken Tertiary fault block within the Basin and Range province exhibits Jurassic or older structures that are virtually unmodified by subsequent tectonism, providing an opportunity to examine the tectonics of the Jurassic back arc of the Cordillera. Widespread high-angle faults, mainly striking north and northeast, offset the moderately west-dipping strata down to the west, thereby extending the strata parallel to bedding by 10% to 20%. These faults apparently belong to a single kinematic event of west-northwest oriented extension subparallel to bedding. The normal faults merge with a north-northwest dextral strike-slip fault system. The two fault systems are kinematically compatible, suggesting that they may have been contemporaneous. A low-angle (thrust?) fault nearly parallel to bedding within Ordovician strata is cut by the high-angle faults. Intrusive relations with about 160 Ma granitoid rocks show that all of these faults are Late Jurassic or older, with a reasonable lower age limit of Early Jurassic established on the basis of paleogeographic reconstructions using sedimentary deposits in the region. Jurassic dikes indicate a minimum horizontal compression direction that is parallel to the extension direction indicated by the normal faults, thus supporting the inference that normal and strike-slip faulting took place during the late Middle or early Late Jurassic. We interpret these data to indicate that minor thrusting, probably during the Jurassic, was followed by extensional faulting within a strike-slip fault system, probably close in time to intrusion. These relations are similar to those reported previously in the nearby Newfoundland Mountains. Other mountains of northern Utah and adjacent Nevada also have examples of Jurassic normal faults that contrast with the more generally observed thrust faults and tectonite fabrics. These data point to a possible regional extensional tectonic event affecting the crust far inland of the Jurassic magmatic arc, an event probably associated with back-arc magmatism.

Pre-caldera and caldera-related igneous rocks near Questa, New Mexico, provide a well-constrained example of upper-crustal extension over a magma chamber. Extensional strain exceeding 200% occurred over many square kilometers in a region now underlain by a batholith. The most intense deformation followed emplacement of intrusions related to formation of Questa caldera at 25.7 Ma and preceded the cooling of 24.6 Ma plutons. Detailed mapping of the southern part of Questa caldera indicates the sequence (1) extension concurrent with plutonism, resulting in an overlying carapace of highly tilted blocks bounded by low-angle faults; (2) renewed intrusion into overlying units, with the orientation of pluton tops, dikes, and molybdenum-bearing alteration zones controlled by previously formed low-angle faults; (3) reactivation or continued motion of low-angle normal faults, possibly aided by hydrothermal fluids; and (4) high-angle normal faults that formed after the magmatic system waned. Early high-angle faults remained active, again possibly aided by fluids, during their rotation to low angles. At upper-crustal levels, the magma chamber is believed to have provided a zone of ductility into which overlying brittle normal structures were rooted. At mid-levels in the crust, regional magmatism thermally softened the crust and allowed it to deform in a ductile manner. Regional extension appears to have been focused in a thermally and mechanically weakened area during the period of maximum plutonic activity. The features at Questa provide insights into processes that may have occurred in similar systems in the Basin and Range province and elsewhere.

Subsidence of Summer Lake basin, situated in the northwestern Basin and Range province in south-central Oregon, has exposed a kilometer-thick Neogene sequence of dense volcanic flow rocks overlying very weak tuffaceous sedimentary rocks in the bounding Winter Ridge–Slide Mountain escarpment. Subsidence occurs along the active, 40-km-long, Winter Ridge–Slide Mountain fault, which is capable of producing Mw ≈ 7 earthquakes with near-field, maximum horizontal acceleration approaching 1 g. Gigantic landslides cubic kilometers in volume scallop the southwestern part of the escarpment, and the deposits run out several kilometers, characteristic of rock avalanches. Geotechnical rock-mass characterization and slope- stability analyses confirm observations that these landslides were initiated within the weak tuffaceous sedimentary rocks along shallow, east-dipping, planar failure surfaces, are insensitive to groundwater fluctuations, and are stable under static conditions. Pseudostatic analyses reveal that strong shaking is required to trigger landsliding. The Bennett Flat landslide failed as a single event between 900–270 ka and 16.8 ka; the landslide has a depletion-zone volume of 1.1 km3, but only 0.3 km3 of debris is discernible in the accumulation zone. The Foster Creek landslide also appears to have failed as a single event between 180 and 16.8 ka; the landslide has a depletion-zone volume of 2.9 km3, but only 0.3 km3 of debris is discernible in the accumulation zone. The Punchbowl landslide evidences multiple events, the most recent of which failed in the Holocene and had a volume of 0.4 km3; 0.5 km3 of debris is found in the accumulation zone.

Extensional collapse after crustal shortening is a common process in mountain belts. The resulting overprinting of structures can make structural reconstruction quite challenging, especially where both extensional and shortening structures are geometrically complex. We address this challenge in a segment of the Sevier thrust belt located in the Basin and Range province of southwestern Utah. The Wah Wah Mountains are unusual because all known thrusts in this part of the Sevier belt are exposed in a single range, and therefore it is unnecessary to correlate thrusts between ranges in order to assemble a complete cross section across the thrust belt. However, the thrusts are overprinted by a three-dimensional array of extensional faults that make conventional two-dimensional cross-section restoration methods inadequate. On the basis of new mapping in the Blawn Mountain–Rose Spring Canyon area and published maps of adjacent areas, we re construct the geometry and kinematics for part of the Sevier thrust stack, which we here refer to as the “Wah Wah thrust system,” by iterative restoration and balancing of a grid of three-dimensional cross sections. We removed the effects of Tertiary extensional faulting and then restored the thrust geometry. The results of the final restoration indicate that the structure of the Wah Wah thrust system is more complex than previously recognized and comprises a roof thrust above a stack of two major and four minor footwall imbricates. Minimum total shortening across the thrust system is 38 km. The internal evolution of the thrust system appears to have been mainly backward-breaking, in contrast to the overall regional forward-breaking sequence of the Sevier thrust belt along strike. Cross-section restorations indicate that the roof thrust accumulated at least 15 km of slip after emplacement of all footwall imbricates. We speculate that bleeding off of pore pressure from the decollement by fluid discharge along ramp thrusts caused incremental strengthening and abandonment of the frontal area of the decollement and therefore led to development of the backward-breaking thrust sequence.

A combination of three independent lines of evidence (hydrogeological, geochemical, and isotopic) suggests that as much as 3.7 × 10 7 m 3 per year of interbasin flow may occur through faulted and fractured bedrock in the southern Sierra Nevada. The proposed interbasin flow path is through fractured Sierran bedrock from the Kern River drainage basin (which supplies recharge to the San Joaquin Valley to the west) into the southwestern Indian Wells Valley, a desert basin that is east of the Sierra. The interbasin flow produces an amount of recharge that significantly increases the ground-water budget for the desert valley. The interbasin flow is identified by aquifer flux values much higher than local recharge and by ground water that has geochemical and isotopic signatures that differ from the local recharge. Interbasin flow that crosses topographic watershed divides has been identified in fractured carbonate aquifers in the southwestern United States. However, igneous and metamorphic rocks in the Basin and Range province are generally considered to be flow barriers, and watershed boundaries are usually delineated by topography. In this study we present an example in which recharge appears to be transported between adjacent watersheds through the fractured bedrock of an extensional area in the southern Sierra. If confirmed, this example shows that igneous and metamorphic rocks may not always act as barriers to ground-water flow in extensional tectonic regimes.

An analysis of the Miocene Horse Camp basin in the central Basin and Range province demonstrates that individual components of an extensional basin system may undergo initial subsidence and subsequent uplift and exhumation during extension. The Horse Camp basin developed during Miocene time above a west-dipping detachment fault and between two east-striking accommodation faults. Growth strata indicate that the west-dipping detachment was active throughout basin filling. Lateral facies variations in alluvial-fan, fan-delta, and lacustrine deposits indicate that the east-striking accommodation faults along the northern and southern basin margins bounded primary sediment source areas. During late Miocene time, the hanging-wall block of the detachment system was broken by the west-dipping Railroad Valley normal fault, thereby inducing development of an extensional basin, Railroad Valley, in the newly formed hanging wall and uplift and exhumation of the older Horse Camp basin in the footwall. The total amount of exhumation during Neogene extension is calculated for a single range block, the northern Grant Range. Early exhumation is recorded by the detrital compositions of middle-late Miocene strata in the southern part of the Horse Camp basin. Provenance modeling indicates that approximately 1600 m of Mississippian to Silurian section was exhumed in the range during middle-late Miocene Horse Camp deposition. From late Miocene to Holocene time, both the northern Grant Range and Horse Camp basin were exhumed in the footwall of the Railroad Valley fault. A comparison between the currently exposed bedrock geology and calculated late Miocene exposure levels from the provenance model indicates that 1000–2000 m of Cambrian–Ordovician section must have been exposed in the range after late Miocene deposition in the Horse Camp basin. Modern alluvial fans in eastern Railroad Valley have drainage networks that contain exposures of the fill of the Horse Camp basin, indicating that older basin fill is being recycled into the modern extensional basin. Extensional basin systems having similar basinward shifts of the major fault system are particularly susceptible to uplift, exhumation, and erosional recycling, not unlike the structurally dismembered margins of foreland basin systems.

Several well-dated stratigraphic markers permit detailed assessment of the temporal and spatial variation in slip rates along the interconnected faults of the Fish Lake Valley, Emigrant Peak, and Deep Springs fault zones in west-central Nevada and east-central California. Right-lateral motion on the Fish Lake Valley fault zone apparently began ca. 10 Ma (11.9–8.2 Ma). Associated extensional faulting probably began ca. 5 Ma (6.9–4 Ma) and resulted in the opening of Fish Lake Valley and Deep Springs Valley. The long-term lateral-slip rate for the Fish Lake Valley fault zone since about 10 Ma is 5 mm/yr (3–12 mm/yr). Our preferred lateral-slip rate for the central, most active part of the Fish Lake Valley fault zone decreased from about 6 to 3 mm/yr from the late Miocene to the early Pleistocene, increased to about 11 mm/yr during the middle Pleistocene, and decreased to about 4 mm/yr during the late Pleistocene. Extension may account for some of the change in lateral-slip rate during the Pliocene. The large increase in lateral-slip rate during the middle Pleistocene is circumstantially linked to an increase in vertical-slip rates on the Fish Lake Valley and Deep Springs fault zones at about the time of the eruption of the Bishop ash (0.76 Ma). Vertical-slip rates along the three fault zones are also related to fault strike; vertical rates are highest on north-striking faults and approach zero on northwest-striking faults. The long-lived slip history of the Fish Lake Valley fault zone fits a tectonic model in which the Death Valley–Furnace Creek–Fish Lake Valley fault system is integrated with right-lateral shear on faults of the central Walker Lane and the Eastern California shear zone to accommodate part of the Pacific–North American relative plate motion. Our research demonstrates that the Fish Lake Valley fault zone accounts for about half the rate of 10–12 mm/yr of Pacific-North American plate-boundary shear accommodated within the Basin and Range province between about lat 37° and 38°N.

The Aljibes half-graben is located 140 km west-northwest of Mexico City and is characterized by several parallel, ≥10-km-long, south-dipping, and east-striking normal faults. Late Miocene basalt flows with 40 Ar/ 39 Ar ages of 7.7 ± 0.1 Ma and 7.1 ± 0.5 Ma are displaced along these faults. With reference to this basalt, the composite throw and heave of the half-graben are 480 m and 270 m, respectively, and the relative extension is 5%, whereas the vertical slip rate along the faults of the Aljibes half-graben after the deposition of the basalt is ≥0.07 mm/yr. The Aljibes half-graben is located at the northern margin of the trans-Mexican volcanic belt and perpendicularly intersects northstriking normal faults of the Basin and Range Province. The relative sequence of activity of the two stress fields and fault systems cannot be resolved in the study area, and the two systems may have been active simultaneously. We explain this structural configuration by horizontal migration of the boundary between the two stress provinces with time, which requires intermittent permutations between the intermediate and least principal stresses σ 2 and σ 3 . This is supported by the low-stress ratio Φ = (σ 2 − σ 3 /(σ 1 − σ 3 ) obtained from modeling the stress tensor associated with the slip along the faults of the Aljibes half-graben. The orientations of the fault planes and slip vectors of the Aljibes half-graben are practically identical to those of the nearby Acambay graben and the Venta de Bravo fault, which caused the Acambay earthquake of November 19, 1912 (M S = 6.9), and the Maravatio earthquake of February 22, 1979 (m b = 5.3). Furthermore, the Aljibes half-graben forms the extrapolated western continuation of the northern master fault of the Mezquital graben, which ruptured partly in the Cardonal earthquake of March 26, 1976 (m b = 5.3). This suggests the Aljibes half-graben may also be seismically active, although no historical earthquake is known to have occurred along this structure. Further evidence for the likely seismic activity of the Aljibes half-graben are two east- to eastnortheast–trending epicenter alignments between the Aljibes halfgraben and the Mezquital graben, and structural indication for stickslip behavior. Based on the structure of the Aljibes half-graben, we believe the maximum credible earthquake that may occur along the faults of the Aljibes half-graben is on the order of 2 × 10 27 dyne cm ≥ M 0 ≥ 6 × 10 25 dyne cm and 7.5 ≥ M w ≥ 6.5.

High-precision 40 Ar/ 39 Ar isotopic ages obtained from Cenozoic volcanic rocks and subvolcanic intrusions document the age of initiation and the temporal evolution of extensional and strike-slip faulting in the western Basin and Range Province. In the northern Wassuk Range, faulting began between ca. 26 and 24.7 Ma; both normal and strike-slip faults are bracketed between 23.1 and 22.2 Ma, and between 15 and 14 Ma. These ages document inception of the Ancestral Walker Lane, a northwest-trending zone of right-transtensional faulting in western Nevada that separated extending crust on the east from the unextended Sierra Nevada block on the west at lat 39°N. We speculate that the southwesterly migrating, episodic Oligocene–early Miocene, east-west extensional faulting in the Basin and Range thinned and weakened the crust, allowing right-slip faults to develop in the Walker Lane in response to San Andreas right-shear in California. Southwest of the Walker Lane there was no faulting prior to 15 Ma. Here, in the Yerington district, andesitic magmatism began at ca. 15 Ma and was followed by >150% east-west extension along closely spaced (1–2 km) normal faults with up to 4 km of offset each (Proffett, 1977). These faults tilted older Cenozoic rocks 35°–40°W. Our new 40 Ar/ 39 Ar ages substantially revise earlier K-Ar ages of the timing of extension and establish that andesite lava flows cut by normal faults are 13.8–15 Ma, and that these faults are intruded by 12.6–13.0 Ma dacites. Rapid extension is thus bracketed to a 0.7–1.7 m.y. interval at 95% confidence, indicating local, east-west strain rates of 2–4 × 10 −14 /s (5–10 mm/yr). Following this period, lower rates of extension prevailed near Yerington along more widely spaced normal oblique-slip faults that localized clastic sedimentation of the Wassuk Group between 11 and 8 Ma. These faults and sedimentary rocks are more abundant southwest of Yerington in a belt parallel to the Walker Lane in previously little-extended crust. From 7 Ma to present, normal right-oblique slip faults with a lower rate of extension than the previous two periods produced the modern ranges near Yerington and extend 100 km southwest of the Walker Lane, which continues to be the locus of strike-slip faulting. Thus, since 15 Ma the margin of the Basin and Range has moved progressively 100 km west creating the broad Walker Lane belt and lower strain rates near Yerington.

The area that surrounds the Gulf of California was intensely faulted during the late Cenozoic prior to opening of the gulf. In a representative 10,000-km 2 area of southern Sinaloa, the faults consist of a predominant north-northwest-striking set having normal displacement and a subsidiary east-northeast set having largely strike-slip displacement. Two domains are recognized: one in which both fault trends are abundant and one in which east-northeast faults are minor. In the former, north-northwest faults form a series of mostly southwest-tilted half-grabens filled with upper Tertiary sediments. Displacement on individual faults is commonly as much as several kilometers. East-northeast faults probably represent accommodation zones between areas of differential extension. In the second domain, north-northwest faults form an extensive graben system. Major faults, dipping 40° to 70° into the graben and having several kilometers of cumulative displacement, are spaced every 5 to 10 km. Bedding attitudes indicate that fault blocks are rotated as much as 65°. Dependent upon assumptions about subsurface geometry of the faults, total extension may range from 20% to 50%. Fault geometry and stress orientations calculated from fault-slip data indicate that the least principal stress was east-northeast. K-Ar ages of three north-northwest-striking dikes indicate that east-northeast extension began as early as 32 Ma. Ages of tilted volcanic rocks, however, indicate that measurable tilting, and therefore most faulting, began after about 17 Ma. Faulting having similar timing, style, and orientation occurred throughout the area surrounding the Gulf of California, as far south as Nayarit on the Mexican mainland and as far north as areas in Sonora that are unequivocally within the Basin and Range province. This continuity and similarity in characteristics of faulting around the gulf to those of early Basin and Range faulting in the United States indicate that the area around the gulf is part of the Basin and Range province. This Basin and Range faulting probably created the widely recognized but debated proto-Gulf of California. The present Gulf of California opened by ocean-floor spreading and transform faulting in a zone already weakened by Basin and Range extension. The lack of correspondence between areas of extension and likely areas of crustal thickening related either to Laramide contraction or magmatism argues against a model of Basin and Range extension related solely to spreading of overthickened crust.

Large Cenozoic clockwise rotations defined by paleomagnetic data are an established fact in the Pacific Northwest, and many tectonic models have been proposed to explain them, including (1) rotation of accreted oceanic microplates during docking, (2) dextral shear between North America and northward-moving oceanic plates to the west, and (3) microplate rotation in front of an expanding Basin and Range province. Stratigraphic onlap relations and local structure indicate that microplate rotation during docking was not a major contributor to the observed rotations. Coast Range structures, Basin and Range extension, and paleomagnetic data from middle Miocene (15 Ma) Coast Range rocks indicate that dextral shear is responsible for at least 40% of the post-15 Ma rotation of the Coast Range and that Basin and Range extension is responsible for the remainder. Reconstructions based on extrapolation of this ratio back to 37 and 50 Ma are consistent with reconstructions based on paleomagnetic and stratigraphic relations in older rocks and suggest that dextral shear has, been a significant contributor to rotation during most of Tertiary time. Changes in the dextral-shear rotation rate over the past 50 m.y. correlate directly with changes in the velocity of the Farallon plate parallel to the coast and provide a strong argument for oblique subduction as the driving mechanism. Continental reconstructions incorporating shear may provide constraints on the rate of extension in the northernmost Basin and Range region and suggest 17% extension since 15 Ma, 39% since 37 Ma, and 72% since 50 Ma near latitude 42°N.

We have determined remanent magnetization directions of the lower Miocene Peach Springs Tuff at 41 localities in western Arizona and southeastern California. An unusual northeast and shallow magnetization direction confirms the proposed geologic correlation of isolated outcrops of the tuff from the Colorado Plateau to Barstow, California, a distance of 350 km. The Peach Springs Tuff was apparently emplaced as a single cooling unit about 18 or 19 Ma and is now exposed in 4 tectonic provinces west of the Plateau, including the Transition Zone, Basin and Range, Colorado River extensional corridor, and central Mojave Desert strike-slip zone. As such, the tuff is an ideal stratigraphic and structural marker for paleomagnetic assessment of regional variations in tectonic rotations about vertical axes. From 4 sites on the stable Colorado Plateau, we have determined a reference direction of remanent magnetization (I = 36.4°, D = 33.0°, α 95 = 3.4°) that we interpret as a representation of the ambient magnetic field at the time of eruption. A steeper direction of magnetization (I = 54.8°, D = 22.5°, α 95 = 2.3°) was observed at Kingman where the tuff is more than 100 m thick, and similar directions were determined at 7 other thick exposures of the Peach Springs Tuff. The steeper component is presumably a later-stage magnetization acquired after prolonged cooling of the ignimbrite. When compared to the Plateau reference direction, tilt-corrected directions from 3 of 6 sites in the central Mojave strike-slip zone show localized rotations up to 13° in the vicinity of strike-slip faults. The other three sites show no significant rotations with respect to the Colorado Plateau. Both clockwise and counterclockwise rotations were measured, and no systematic regional pattern is evident. Our results do not support kinematic models which require consistent rotation of large regions to accommodate the cumulative displacement of major post-middle Miocene strike-slip faults in the central Mojave Desert. Most of our sites in the Transition Zone and Basin and Range province have had no significant rotation, although small counterclockwise rotation in the McCullough and New York Mountains may be related to sinistral shear along en echelon faults southwest of the Lake Mead shear zone. The larger rotations occur in the Colorado River extensional corridor, where 8 of 14 sites show rotations ranging from 37° clockwise to 51° counterclockwise. These rotations occur in allochthonous tilt blocks which have been transported northeastward above the Chemehuevi-Whipple Mountains detachment fault. Upper-plate blocks within 1 km of the exposed detachment unexpectedly show no significant rotation. From this relation, we infer that rotations are accommodated along numerous low-angle faults at higher structural levels above the detachment surface.

Understanding of the crust has improved dramatically following the application of seismic reflection and refraction techniques to studies of the deep crust. This is particularly true in areas where the last tectonic event was extensional, such as the Basin and Range province of the western United States and much of western Europe. In these regions, a characteristic reflective pattern has emerged, whereby the lower crust is highly reflective and the upper crust and upper mantle are either poorly reflective or strikingly nonreflective. In the metamorphic-core-complex belt in the western United States, where extension can be as much as an order of magnitude greater than in the more classic continental rift zones, the lower crustal reflectivity thickens and rises, yielding a picture of a crust that is reflective throughout. Synthetic seismic studies have documented that the reflectivity in these regions can be modeled by numerous laminae tens of meters thick and hundreds of meters across, characterized by inter-layered high and low velocities. Two geologic factors are interpreted as contributing to this layered character: ductile strain, responding to stress in the thermally weakened middle and lower crust, and intrusive layering, corresponding to injection of subhorizontal sheets of mantle-derived magmas. These two processes yield a variety of geologic structures, including transposed compositional layering, mylonitic ductile shear zones, and intrusive mafic sheets, all of which occur at the proper scales to cause the prominent reflectivity observed. If metamorphic core complexes are representative of extended continental crust world-wide, then these results suggest that magmatism and ductile flow have also contributed to the evolution of the middle and lower crust in many other areas around the world.

Introduction The Basin-and-Range physiographic province, as outlined by a committee of the Association of American Geographers and the Physiographic Committee of the U. S. Geological Survey,[2][1] extends in the United States from southern Oregon to trans-Pecos Texas, and contains not less than 125 closed drainage basins—that is, basins which have no surface outlets for the water that falls upon them. The mountainous rims of these basins, with their great relief and remarkable exposures of geologic formations and structures, have, from the beginning of explorations in the region, compelled the attention of geologists. In contrast, the interior depressions of the basins, mantled by recent sediments and with surface features that are hardly discernible except by an experienced observer, have received relatively little attention and are commonly regarded as lacking in geologic interest. However, the serious work that has been done in these interior depressions, or basin valleys, has shown that they . . . [1]: #fn-2

INTRODUCTION Ajo lies in the Papago country of southwestern Arizona, in the Sonoran desert section1 of the Basin and Range province (Fig. 1). The region, which displays a wide variety of topographic forms, offers unusual opportunity for study to those interested in the physiography of arid regions because the United States Geological Survey has completed topographic maps, one of the Ajo quadrangle on the scale of 1: 48,000, and one of a few square miles in the mining area near Ajo on the scale of 1: 12,000. The Ajo quadrangle includes the small mountain masses of the Little Ajo Mountains, Childs Mountain, and part of the Batamote Mountains. Much of the area is occupied by desert plains, of which the largest is the Valley of the Ajo. Altitudes range from 1,150 to 3,200 feet. Local relief is moderate and only exceptionally reaches 1,000 feet in a square mile. The climate . . .

For many years the terms orogeny and mountain building have been used so loosely that they no longer convey definite meanings. Because of this lack of precision, hypotheses of diastrophism are often based on false premises. Folding and thrusting, often referred to as tectogenesis or orogeny, is best displayed by stratified rocks. Some geologists believe that folds and thrusts are due to vertical movements accompanied by lateral spreading, that the strata are stretched an amount commensurate with the intensity of the folding, and that the opposite sides of the sedimentary packet do not move toward each other. Most geologists, however, agree that during folding and thrusting the strata are shortened and that the two opposite sides of the sedimentary packet move toward each other by an amount commensurate with the intensity of the folding. But it is not clear to what extent the entire crust in the deformed belt is shortened. In one extreme view, the two opposite ends of the deformed belt do not move toward each other, and the folds and thrusts result from gravity sliding. At the other extreme, the entire crust is believed to be shortened to the same extent as the sedimentary strata. An excellent example of broad vertical movements (epeirogenesis) that has never been sufficiently emphasized is in eastern North America, involving the Appalachians, Coastal Plain, and Continental Shelf. During the Mesozoic and Cenozoic the Fall-Line surface was depressed as much as 20,000 feet beneath the Continental Shelf, whereas it was uplifted at least 8000 feet over the Appalachians. The northeastern two-thirds of the Appalachian-Ouachita belt has been going up since the Jurassic, whereas the southwestern third has been going down. Thus the present Appalachians are unrelated to the late Paleozoic folding. Certain metamorphic minerals—such as kyanite—or certain metamorphic assemblages—such as jadeite and quartz—form at confining pressures found only at depths of tens of miles, implying tremendous erosion wherever such minerals or assemblages are exposed. Broad vertical movements, accompanied by high-angle faulting, are illustrated by the fault-block mountains of the Basin and Range Province and by such features as the Rhine graben and the Rift Valleys of Africa. Although the nature of the displacement along the San Andreas fault has been known for more than 50 years, only in recent decades have other large strike-slip faults been recognized. But it is now the fad to assign all kinds of faults and even nonexistent faults to the strike-slip category. In recent years seismologists have emphasized the importance of strike-slip faults. The author suggests that in the Fairview Peak-Dixie Valley, Nevada, earthquakes there is evidence that strike-slip movements are invading an area previously characterized by Basin-Range structure. The geologic record indicates that large strike-slip faults have been distinctly subordinate to folding, broad vertical movements, and broad vertical movements accompanied by high-angle faulting. Possible displacement of the crust or the entire earth relative to the axis of rotation has been emphasized in recent years. Continental drift, if it occurs, is one such type of movement. Slipping of the entire crust over the mantle is another. Much research has been accomplished in paleomagnetism in recent years, but the data are too scanty and conflicting to permit any definite conclusions. Among the possible causes of diastrophism are (1) contraction of the earth, (2) convection currents, (3) formation of large pockets of magma, (4) sialic material leaking out of the mantle, (5) conversion of sial to mantle by change of low-pressure minerals to high-pressure minerals, or the reverse, and (6) serpentinization or deserpentinization of the upper part of the mantle. Mountain building is merely one manifestation of vertical movements of the crust. In the past mountain building has been erroneously considered by many to be primarily the result of folding and thrusting. Certainly many modern ranges are the result of vertical uplift unrelated to folding. Many such uplifts are accompanied by high-angle faulting to produce fault-block mountains. Blocks caught between strike-slip faults may be squeezed upward if the blocks move toward each other. Mountainous uplifts have also resulted from folding and thrusting. Conversely, in some folded belts erosion appears to have kept pace with the rise of the folds so that no mountains developed.

INTRODUCTION During a study of the geology and ore deposits of the Magdalena mining district in the central part of New Mexico for the United States Geological Survey in cooperation with the New Mexico Bureau of Mines and Mineral Resources, the writers obtained the data here presented. The manuscript was submitted for criticism to Kirk Bryan, who has done much work in the surrounding country, and the authors are indebted to him for valuable suggestions and for hitherto unpublished data on the area east of the Magdalena Range. The district covers the northern part of the Magdalena Range and a small area just north of it; its geographic and geologic relations to the surrounding region are shown in Figure 1. The area studied in detail is shown in Figure 2. The district is in the southeastern part of the Basin and Range Province and possesses features typical of the maturely . . .

INTRODUCTION THE AREA The area with which this paper is concerned is in the southwestern part of the state of Coahuila, Mexico (Fig. 1). It lies in the Mexican Plateau Division of the Basin and Range Province, and is characterized by dissected broad upland areas separated by plains of aggradation. The semi-arid climate has discouraged settlement and scientific investigation, and only a small part has been more than casually visited, although the area presents several highly interesting geological problems. Within its borders are the vestiges of the Jurassic-Neocomian landmass Llanoria, the position of which controlled sedimentation throughout the Cretaceous, and affected the structural trends of the mountains formed at the close of that period. The Paleozoic stratigraphy of the area records marine sedimentation, and a possible glacial period, followed by deformation and intrusion. It throws additional light on the position of the ancient landmass and its bordering geosyncline. PREVIOUS REPORTS . . .

The Rio Grande rift, which is marked by a positive heat-flow anomaly in southern New Mexico, has been the subject of a gravity study based on 4,500 stations which cover a strip across New Mexico. The area consists of a series of basins and intervening ranges formed during Miocene time. This basin-and-range structure is strongly reflected in the Bouguer gravity anomalies, which range from -125 mgal over unlifts to -190 mgal over basins. Lineaments in trends of gravity anomalies are oblique to the predominant north-south trend of the rift and suggest that, in detail, the crust broke upon fractures oblique to the large-scale north-south trend. Thicknesses of Cenozoic sediments, determined from gravity measurements, range from 2 to 3 km in basins. The gravity effect of sediments is removed by stripping, and a broad +30-mgal gravity anomaly is located over the rift. Regional and residual Bouguer gravity anomaly maps were constructed. The source of the 30-mgal gravity high is interpreted to ba a shallow slab of basalt or a deep upwarp of the mantle that results in crustal attenuation. The low-velocity zone may project up toward the base of the crust under the Basin and Range province. Experiments and the observedmore » fault pattern suggest an extensional origin for the Rio Grande rift fracture system.« less

The geology and petrology of the Cenozoic volcanic rocks of the region of California between 38° and 35°459N latitude and 117°309 and 120°W longitude, including the rocks of a major potassic magmatic province on the west slope of the Sierra Nevada, have been described in Part I (Moore and Dodge, 1980) of this study. The geochemical features of these rocks may provide clues to their origin and aid in comparing and contrasting the Sierra Nevada potassic province with other localized potassic provinces elsewhere in the world. Basaltic rocks occur in numerous small lava-flow remnants, dikes, and plugs that intrude the predominantly granitic terrain of the western slope of the southern Sierra Nevada. Leucite-bearing rocks are present at several localities in this western region. More voluminous basalt, commonly associated with rhyolite, is present east of the range in the Basin and Range province. However, leucite-bearing rocks have been found-at only one locality in this region. Most of the late Cenozoic volcanic rocks in the southern Sierra Nevada eastward through Owens Valley and in the extreme western Basin-Range occur in five rather distinctive areas—the San Joaquin-Kings, Kern, Big Pine, and Coso volcanic fields, and the Mono-Long Valley volcanic center (Fig. 1).

Among the many granitic plutons within the Basin and Range province of western North America, those in the Kern Mountains of eastern Nevada and western Utah are unusual structurally, mineralogically, and chemically. The two largest intrusions, which together are exposed over an area of 130 sq km (51 sq mi), have well-defined border facies of leucocratic and generally aplitic rocks, or of protoclastic rock developed from coarser core-facies material. Within the core facies are abundant aplitic, leucocratic dikes. The core facies of the larger intrusion (Tungstonia Granite) is a two-mica granite with conspicuous phenocrystic books of muscovite up to 5 cm across. The smaller intrusion (Skinner Canyon Granite) lacks muscovite, and Fe-Ti oxides and sphene are constant accessory phases in it. Rb-Sr and K-Ar isotopic analyses suggest a wide range of initial Sr 87 /Sr 86 ratios, which for the most part are unusually high—to nearly 0.7246 for the Tungstonia Granite—and a complex, protracted period of magmatism and isotopic adjustment that extended from possibly Mesozoic into mid-Cenozoic time. The simplest interpretation is that the final emplacement of the Tungstonia body occurred 60 m.y. ago and that the Skinner Canyon intrusion occurred between 30 and 45 m.y. ago. The data prompt the conclusion that the magmas represent remobilized sialic basement whose model dates (assuming an initial ratio typical for mantle-derived magmas) are in excess of 1.5 b.y. The occurrence of such anatectic intrusions within the relatively intensely metamorphosed parts of the infrastructure of the Sevier orogenic belt, rather than elsewhere in the Basin and Range province, is not unexpected, and it possibly represents a more advanced evolutionary stage than do the gneiss domes previously documented within this same belt.

In the southern Rio Grande rift, two extensional regimes of different origin (but transitional with each other through the Miocene) can be interpreted from structures and rocks formed within the past 28 to 29 m.y. The earlier regime, which began about 28 to 29 m.y. B.P., is characterized by emplacement of “basaltic andesite” flows with relatively high strontium isotope ratios; formation of broad, relatively deep, northwest-trending basins; and incipient uplift of some of the region9s fault-block mountains. This regime appears to have developed in a back-arc setting, perhaps behind a rapidly steepening slab and a westward-sweeping arc system. The younger episode seemingly represents a renewal or acceleration of block faulting and volcanism during the latest Miocene and Pliocene, 9 to 3 m.y. B.P., after a long transitional period during the early and mid-Miocene when volcanism was absent and tectonism was less vigorous. The latest Miocene-Pliocene episode produced the modern northerly-trending rift basins and uplifts, regional uplift of the rift 1 to 2 km above sea level, and renewal of volcanism, this time dominated by relatively primitive alkali-olivine basalt. New basalt dates reveal that in the southern rift, modern ranges and basins were almost fully developed and that near-modern drainage ways were established across uplifts into bolsons by about 5.0 m.y. B.P. An ancestral Rio Grande had extended itself southward into the southern rift by 3 to 4 m.y. B.P., and the river entrenched itself into its modern valley between 0.7 and 0.5 m.y. B.P. Horst-graben development of the southern Basin and Range province, as well as associated basaltic volcanism, swept progressively eastward from southeastern California in the past 20 m.y., culminating in formation of the Rio Grande rift and other fault-block terrane in west Texas, New Mexico, and northern Chihuahua in the latest Miocene and Pliocene. Late Quaternary Basin and Range fault scarps increase in density eastward, which also suggests that more easterly parts of the province are youngest. These relationships support a previous model of an eastward-expanding, slab-free triangle (related to growth of the San Andreas transform), through which mantle upwelling triggers eastward-younging patterns of tectonism, volcanism, and uplift and promotes lithospheric thinning and increased heat flow. Across most of the southern Basin and Range and Rio Grande rift, the horst-graben structures related to growth of this triangle are superimposed on somewhat older (late Oligocene-middle Miocene) extensional terrane that appears to have formed in a back-arc or arc setting.

Heat-flow measurements made in Lake Tahoe, California-Nevada, demonstrate that the transition from subnormal heat flow in the Sierra Nevada to above-normal heat flow in the Basin and Range province occurs west of the assumed physiographic boundary between these two areas, contrary to earlier belief. In addition, these data, together with data of other workers, clearly reveal the sharpness of this transition, which suggests that the causative thermal sources and (or) sinks must be restricted to depths not greater than the uppermost mantle. The way in which heat-flow data constrain the current hypotheses of crustal structure and evolution of the Sierra Nevada-Basin and Range provinces is illustrated with a tectonic model that employs a post-Cretaceous shallow-dipping subduction zone beneath the Sierra Nevada and an active upper-mantle diapir under the Basin and Range province during late Cenozoic time.

The region comprising the Candelaria Hills and Excelsior Mountains, Nevada, is structurally anomalous with respect to surrounding tracts of the Basin and Range Province. Its major faults strike east, and some, if not all of them, have undergone significant left slip. Ratios of lateral to vertical displacement components on strands of such faults vary from 2 to 6.5. The region of left-oblique slip faulting seems to constitute the intermediate leg of a gigantic Z pattern of Basin and Range faults. One of these faults, the Candelaria fault system, has been studied closely in pursuit of kinematic details that might characterize the regional faulting. The Candelaria system consists of three en echelon faults of 5 to 10 km trace length joined by connector zones that possess compressional features. The amount of lateral slip transferred from one en echelon fault to another is uncertain; analysis of a complex of faults in one connector zone indicates at least some degree of integrated motion. The same connector zone is the site of a bulge that has an uplift estimated to have been 275 m since 2.8 m.y. B.P. On the basis of fault-plane striae, the extension direction is ∼ N82°W. We propose that the Candelaria fault system and a major asymmetric trough in its northern wall were created in Oligocene time by a regional left-lateral shear system oriented in the extension direction. The connector zones may be kinematic impediments which, at shallow levels at least, have prevented the system from maturing into a single plane fault. The existence of the bulge and the estimated rate of slip on part of the Candelaria fault system suggest that the system is still active.