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Extension orientation is a key parameter in rift tectonics. While a clockwise rotation of the extension orientation is widely accepted for the southern Rio Grande rift and the Basin and Range province in the western U.S., in which direction(s) the northern Rio Grande rift opened has been controversial. Moreover, slip re-orientation, a phenomenon caused by local stress rotation on weak faults under a uni-directional regional extension, imposes a serious impact on kinematic interpretations. We investigate the presently oblique Tusas-Abiquiu segment of the Rio Grande rift in north-central New Mexico to assess the rifting kinematics with the presence of pre-existing crustal weaknesses. Fault-slip data show an overall NE-SW extension on the NW-trending Tusas border faults, and WNW-ESE extension on the NNE-trending Abiquiu border and internal faults. The slip vectors on the border faults show a wide variety of oblique-slip component, whereas those on the internal faults are predominantly pure dip-slip. The majority of our field measurements can be reproduced by the slip re-orientation model, although this model's setting does not satisfy regional geological records. Clockwise rotation of the extension axes from NE-SW to WNW-ESE can also explain our fault-slip data, as well as other structures in the rift. Tectonic reactivation during the extension also influenced the landscape evolution of the early rift. The Rio Grande rift shared similar extension orientation with the southern Basin and Range during the late Oligocene. The extensional strain field of the southwestern North American plate shows a progressive, time-transgressive rotational pattern, implying that plate boundary processes, instead of asthenospheric upwelling, governed the initiation and early evolution of this intracontinental rift.

The accretion of the Avalon and Meguma terranes to the Laurentian margin was a major event in the development of the Appalachian orogen. The Minas Fault Zone delineates the boundary between these terranes. A strongly lineated and foliated granite intrusion (Kelly Brook pluton) exposed along the West River St. Mary's Fault intruded into Meguma Supergroup metasedimentary rocks. Well-developed C-S fabrics characterize the pluton with muscovite aligned in the S fabric, and a shallow (~10°) westerly plunging quartz stretching lineation. New U-Pb (zircon, LA-ICP-MS) geochronological data yield a crystallization age of 375.0 ± 4.6 Ma for the Kelly Brook pluton that is indistinguishable from other Late-Devonian intrusive rocks in the Meguma terrane. Petrographic data for muscovite indicate that it grew prior to deformation. A 100% plateau 40Ar/39Ar age of 369.0 ± 1.2 Ma on a single muscovite grain constrains the time of cooling of the granite to approximately 450 to 420 °C. Apatite grains also demonstrate pre- to syn-deformational fabrics and yield a U-Pb (LA-ICP-MS) age of 361.2 ± 5.6 Ma indicating the temperature of the granite was still above 350 °C at this time. Earliest Tournaisian fossils in the unconformably overlying Horton Group strata indicate that the granite was exposed by ca. 359 Ma. Exhumation rates of the Kelly Brook pluton and the Meguma terrane from approximately 9.5–13.3 km depth from ca. 375 Ma to surface exposure at ca. 359 Ma are 0.06 to 0.08 cm yr−1. Estimates for the Upper Devonian regional geothermal gradient in the northern Meguma terrane are high between 39.5 °C km−1 and 67.8 °C km−1, gradients that are common in extensional tectonic environments such as the Basin and Range province. An apatite fission-track age of 215 ± 14 Ma indicates Mesozoic reburial and reheating of the granite prior to opening of the Atlantic Ocean.

International audience; Breaking the lithosphere in extension without exceeding the driving far-field forces available on Earth is a tough quantitative modeling problem. One can tear it apart by propagation of an existing oceanic basin or weakness zone or one must assist rifting with magmatic processes, which drop the effective stress and weakens locally the lithosphere. While previous 3D models have demonstrated that non-cylindrical plumes produce almost cylindrical rift structures in a lithosphere under slight far-field loading, our contribution goes one step further by producing models of complete continental break-up. We investigate in details how the rheological stratification of the continental lithosphere interacting with active mantle plume influences the geometry and dynamics of rifting to continental break-up in 3D. We find that, irrespective of the rheological stratification, a plume-induced rifting process always occurs in two stages: an early crustal rifting stage and a late lithospheric necking (breakup) stage. In case of a rheologically decoupled lithosphere, initial brittle deformation is concentrated in the upper crust and strongly localized due to compensating ductile flow of lower-crustal material (core complex extension mode). On the contrary, rheological coupling between upper crust and lithospheric mantle results in highly distributed brittle deformation in the crust above mantle plume head (wide rift mode). Both core complex-like and wide rifting are followed by an abrupt transition to narrow rift stage when a localized ascent of mantle plume material focuses high strain along faults zones breaking through the entire lithosphere. The Main Ethiopian Rift, the Basin and Range province, and the East Shetland Basin may be natural examples of regions that have passed through these two stages of extension. Across-strike and along-strike asymmetry of break-up patterns arising spontaneously within initially symmetrical and laterally homogenous environment seems to be an intrinsic characteristic of plume-induced rifting.

The Gulf of California has a long tectonic history. Before the subduction of the Guadalupe and Magdalena plates ceased, extension of the Gulf began to the east, at the Basin and Range province. Later, it was focused west of the Sierra Madre Occidental and the opening of the Gulf started. Currently, the Gulf rifting has different characteristics to the north than to the south. In this study, we analyze the lithosphere thickness in the Gulf of California region by means of P -wave and S -wave receiver functions. We grouped our lithosphere-thickness estimates into five froups: 1) North of the Gulf, with a thin lithosphere (~ 50 km) related to the extension observed in the Salton Through region; 2) the northwestern part of Baja California, with a thicker lithosphere (~ 80 km), thinning towards the Gulf due to the extension and opening processes (~ 65 km); 3) central Baja California, with no converted phase corresponding to the lithosphere-asthenosphere boundary but evidence of the presence of a slab remnant; 4) the southern Baja California peninsula, showing a shallow lithosphere-astenosphere boundary (LAB) (

Recent national programs in Canada (Lithoprobe) and the U.S. (EarthScope) are providing vast quantities of data and many new scientific insights concerning crustal and lithospheric mantle structure and evolution. More modest but significant results are also being produced in and about Mexico. This paper primarily focuses on results published since 2005 and presents an updated Moho map of North America. This overview of crustal structure does reveal a number of interesting observations. For example, two buried distinct crustal blocks have been discovered in the past 20 years. One was caught in a Precambrian suture zone in Canada, and the other is buried under the Gulf Coast region and is still of unknown origin. The Precambrian assembly of the continent has been documented by the mapping of numerous suture zones, and shallow portions of subducted slabs have been preserved in several cases. Not surprisingly, the internal structure of these suture zones is complex and highly variable. Isostatic adjustment has been shown to be a complex process involving the lithospheric mantle not just crustal structure variations. Specifically, the correlation of crustal thickness with elevation is only approximate at best. Several crustal blocks that have been stable over very long periods of time possess unusually thick crust whose lower portion is usually fast (Vp > 7.0 km/s). Rifting has been shown to affect the crust in a variety of ways ranging from the massive magmatic modification that produced the Mid-continent rift system to the broad region of extension in the Basin and Range province. Crustal structure along the active modern western margin of North America is highly complex as would be expected as a result of transitions between subduction and transform faulting and variations in volcanism and intraplate magmatism. We clearly still have a lot to learn about the structure and evolution of this continent.

We provide the first synthesis of seismic reflection data and active present-day crustal deformation for the greater Wasatch fault zone. We analyzed a number of previously unpublished seismic reflection lines, horizontal and vertical crustal velocities from continuous GPS, and surface geology to investigate the relationships between interseismic strain accumulation, subsurface fault geometry, and geologic slip rates on seismogenic faults across the eastern third of the northern Basin and Range Province. The seismic reflection data show recent activity along high-angle normal faults that become listric with depth and appear to sole into preexisting decollements, possibly reactivating them. We interpret these listric normal faults as reactivated Sevier-age structures that are connected at depth with a regionally extensive detachment horizon. These observations of subsurface structure are consistent with the mapped geology in areas that have experienced significant extension. We modeled the crustal deformation data using a buried dislocation source in a homogeneous elastic half space. The estimated model results include a low-angle dislocation (~ 8–20°) at a locking depth of ~ 7–10 km and slipping at 3.2 ± 0.2 mm/yr. Despite the model's relative simplicity, we find that the predicted location of the dislocation is consistent with the interpreted seismic reflection data, and suggests an active regionally extensive sub-horizontal surface in the eastern Basin and Range. This result may imply that this surface represents aseismic creep across a reactivated low-angle fault plane or the onset of ductile flow in the lower crust at or beneath the brittle–ductile transition zone under the present-day Basin and Range extensional regime. This result may also have implications for crustal rheology, and suggests that geodesy might, under some circumstances, serve as an appropriate tool for inferring deeper crustal structure.

Seismic reflection profiling demonstrates that the active, significant-offset Surprise Valley Fault that marks the western boundary of the Basin-and-Range Province in northernmost California dips at a moderate angle, only ~ 30° to ~ 2 km depth. A nearby seismic refraction/wide-angle reflection profile, albeit of lower-resolution, shows the fault-plane remains approximately planar to at least 7 km depth. Variably-tilted volcanic strata of known ages within the Surprise Valley basin demonstrate that ≥ 7 km of normal slip occurred along the Surprise Valley Fault within the past 8–4 Myr at a time-averaged slip rate of ~ 1–2 mm/yr, and that the fault rotated from an initial dip of as much as 60° to its present dip of 30°. The rotation of this isolated fault was not accomplished by domino-style rotation as seen elsewhere in the Basin-and-Range province, but rather by flexural rift-shoulder uplift aided by mid-crustal flow.

Surprise Valley in northeastern California offers an ideal opportunity to examine the structural setting of a developing extensional basin due to its late Miocene to recent activity in isolation from other major normal fault-bound basins. Seismic velocity and potential field modeling help determine the nature of basin fill and identify intra-basin faults. Based on a detailed gravity and magnetic profile, we identify shallow subsurface basalt flows and several faults within the valley that may accommodate hundreds of meters of vertical offset, possibly cutting and offsetting the ~ 30° east-dipping Surprise Valley fault that rotated during footwall tilting of the adjacent Warner Mountains. Some of these intra-basin faults correspond with mapped Quaternary fault scarps, but others have no surface expression. These faults may represent the currently active fault system within the basin. If so, they would indicate that basin development is transitioning away from the main range-front normal fault to a new set of steep intra-basin faults that are more favorable for accommodating regional transtensional strain.

Approximately 25% of the differential motion between the Pacific and North American plates occurs in the Walker Lane, a zone of dextral motion within the western margin of the Basin and Range province. At the latitude of Lake Tahoe, the central Walker Lane has been considered a zone of transtension, with strain accommodated by dip-slip, strike-slip, and oblique-slip faults. Geologic data indicate that extension and strike-slip motion are partitioned across the central Walker Lane, with dip-slip motion resulting in E–W to ESE–WNW extension along the present-day western margin of the central Walker Lane since approximately 15 Ma, and dextral strike-slip motion across a zone further east since as early as 24 Ma. GPS velocity data suggest that present-day strain continues to be strongly partitioned and localized across the same regions established by geologic data. Velocity data across the central Walker Lane suggest a minimum of 2 mm/yr extensional strain focused along the western margin of the belt, with very little extension across either the central or eastern portions of the Walker Lane. These data indicate very little dextral motion across the central and western portions of the domain, with dextral motion of 3–5 mm/yr presently focused along a discrete zone of the eastern part of the central Walker Lane, coincident with existing, mapped strike-slip faults. Historic seismic data reveal little seismic activity in areas of Late Holocene dip-slip motion in the west or dextral motion in the east, suggesting a period of quiescence in the earthquake cycle and the likelihood of future activity in both areas. Based on this and previous studies, it is likely that a combination of pre-Cenozoic crustal structure, a relatively weak lithosphere beneath the Walker Lane, and long-term low stress ratios in the crust have permitted the long-term partitioning of dextral and extensional strain exhibited across the central Walker Lane. The present-day location of dextral strain in the central Walker Lane is subparallel with dextral deformation documented in the northern Walker Lane, suggesting that as strain continues to accumulate, these two discrete zones could become a continuous strike-slip system which will play a more important role in the future accommodation of relative Pacific–North American plate motion.

Records of lithospheric extension and mountain-range uplift are most continuously contained within syntectonic sedimentary rocks in basins adjacent to large structural culminations. In southeastern Arizona, metamorphic core complexes form mountain ranges with the highest elevations in the region, and supposedly much less extended terranes lie at lower elevations. Adjacent to the Santa Catalina-Rincon metamorphic core complex, within the Tucson Basin, stratigraphic-sequence geometries evident in a large suite of 2-D seismic reflection data suggest a two-phase basin-evolution model controlled by the emplacement and subsequent uplift of the core complex. In its earliest stage, Phase I of basin formation was characterized by extensive faults forming relatively small-scale proto-basins, which coalesced with the larger basin-bounding detachment fault system. Synextensional sedimentation within the enlarging basin is evidenced by sediment-growth packages, derived from adjacent footwall material, fanning into brittle hanging-wall faults. During this phase, volcanism was widespread, and growth packages contain interbedded sediments and volcanic products but, paradoxically, no mylonitic clasts from the adjacent metamorphic core complex. Phase II of basin evolution begins after a significant tectonic hiatus and consists of a symmetric deepening of the central basin with the introduction of mylonitic clasts in the basin fill. This is coupled with the activation of a series of high-angle normal faults ringing the core complex. These observations suggest a two-phase model for metamorphic core complex evolution, with an initial stage of isostatic core complex emplacement during detachment faulting that resulted in little topographic expression. This was followed, after a significant tectonic hiatus, by late-stage exhumation and flexural uplift of the Santa Catalina-Rincon metamorphic core complex through younger high-angle faulting. Moreover, the geometry of upper basin fill units suggests an extremely low effective elastic thickness in the region and that flexural uplift of the core complex induced asymmetric transfer of ductile mid-crustal rocks from beneath the subsiding Tucson Basin to the uplifting mountain range.

Following a protracted phase ( � 155–60 Ma) of crustal shortening and mountain building, widespread extension and magmatism have dominated the tectonic history of the North American Cordillera since early Tertiary. Although decades of intensive investigations have made North American Cordillera one of the best studied continental regions in the world, many fundamental issues, including the cause of metamorphic core complexes and basin-and-range extension and their relationship with the overlapping magmatism, remain controversial. Recent studies have emphasized the role of gravitational collapse in causing both extension and magmatism in the Cordillera, but the geodynamics of gravitational collapse are not well understood. Using simple thermal–rheological and thermomechanical modeling, we address the following questions. (1) Could gravitational collapse of the thickened Cordilleran crust have formed the metamorphic core complexes? (2) Did gravitational collapse induce the intensive mid-Tertiary volcanism in the Cordillera? (3) What caused basin-and-range extension and the associated basaltic volcanism? Our results show that, although a thickened crust at isostatic equilibrium is dynamically unstable and tends to collapse, major postorogenic extension happens only when the lithosphere is sufficiently weakened by thermal processes associated with orogenesis, including thermal relaxation, radioactive heating, shear heating, and mantle thermal perturbations. Ductile spreading within the lower crust plays a major role in postorogenic gravitational collapse. This mechanism can explain most metamorphic core complexes and the associated plutonism. However, it is unlikely to have induced major mantle upwelling required by the voluminous silicic eruption during the mid-Tertiary. The cause of the mid-Tertiary mantle upwelling remains speculative, but strong mantle thermal perturbations under the northern Basin and Range province may have persisted since mid-Miocene. Thermomechanical modeling shows that this mantle upwelling may have caused significant ductile deformation within the surrounding lithosphere, with lithospheric material being pushed away and downward from the upwelling asthenosphere. The loci of maximum lithospheric thinning are near the margins of the upwelling mantle and have migrated outward during basin-and-range extension. This process can explain some of the spatial–temporal evolution of extension and volcanism in the Cordillera since mid-Miocene. The resultant lithospheric structures are consistent with geophysical observations in the Basin and Range province. D 2001 Elsevier Science B.V. All rights reserved.

Middle‐late Miocene (proto-Gulf; ~12‐6 Ma) extension around the Gulf of California (Gulf Extensional Province) is commonly interpreted as resulting from partitioning of oblique Pacific‐North American plate motion into strike‐ slip displacement along the margin and east‐northeast extension perpendicular to the margin within the North American plate. We propose that this mechanism also applies to kinematically similar, predominantly east‐northeast extension that occurred at the same time throughout the southern Basin and Range province, from southern Arizona and New Mexico to the Trans-Mexican Volcanic Belt. New field and 40Ar/39Ar data in Sinaloa and Durango confirm that this episode of extension occurred on the mainland side of the Gulf and in the Basin and Range east of the Sierra Madre Occidental, which is generally considered the eastern margin of the Gulf Extensional Province. Published data indicate the middle‐late Miocene episode also occurred across the northern and southern ends of the Sierra Madre where the Gulf Extensional Province connects with the Basin and Range: (1) from central Sonora into southern Arizona and New Mexico, and (2) from Nayarit into central Mexico north of the Trans-Mexican Volcanic Belt. This episode appears to have aVected an area that continues to the eastern edge of the Basin and Range province in Texas and San Luis Potosi. Recognition that this episode of extension aVected the entire southern Basin and Range resolves the discrepancy between the amount of extension calculated based on plate reconstructions and that based on field data within the Gulf Extensional Province alone. Published plate reconstructions require 160 to 110 km of east‐northeast extension between ~12 and 6 Ma. If taken up solely within the Gulf Extensional Province, this would have generated 66 to 78% extension, which is much greater than observed. Spread across the entire southern Basin and Range it requires only ~20% total extension, which is more consistent with observations of cumulative extension between 12 and 6 Ma. Extension was partitioned into the Gulf Extensional Province because (1) it lies between two stable batholith belts (Mesozoic Peninsular Ranges on the west and mid-Tertiary Sierra Madre Occidental on the east) that resisted extension and (2) the Gulf was thermally weakened by immediately preceding arc magmatism. Extension in the main Basin and Range province in part probably avoided the relatively strong, batholithic crust of the Sierra Madre Occidental. © 2000 Elsevier Science B.V. All rights reserved.

Low-angle normal faulting is precluded by elementary rock mechanics arguments. However, the existence of many low-angle normal faults in regions such as western North America has long suggested that this familiar theory is incomplete. Initiation of a low-angle normal fault requires the stress tensor within the brittle upper crust to have inclined principal axes, which necessitates a substantial shear stress within the vertical plane. Nonetheless, no-one has previously identified any specific extensional stress field which is suitably oriented to allow shear failure of previously unfractured rock at a low-angle dip of ∼30° in preference to the steeper dip of ∼50–60° predicted by standard theory. Despite previous claims to the contrary, this study shows for the first time that appropriately oriented stress fields can exist. However, they require the shear stress to locally reach ∼100 MPa near the base of a ∼10-km-thick brittle layer. This is only possible under extreme conditions, as it requires dramatic lateral variations in the state of stress across the extending region. It is suggested that a shear stress of this order can develop due to the combined effects of lower-crustal flow (which imparts a horizontal shear traction at the base of the brittle layer) and loading (which imparts shear traction in the vertical plane) associated with the isostatic response to changes in heat flow caused by changes to the geometry of subducting slabs beneath an extending region. Regional patterns of low-angle normal faulting in western North America are thus interpreted in terms of changes to the geometry of subduction of the Farallon plate.

The fault-segmentation method is commonly used to estimate the potential earthquake size. Segment boundaries play an important role in arresting earthquake ruptures from event to event. In the Basin and Range Province, earthquake rupture terminations are commonly associated with structural discontinuities, but not all-structural discontinuities have the capability to terminate an earthquake rupture. The size of structural discontinuities with respect to the rupture length and displacement may play an important role in controlling rupture termination. Studies of the geometric pattern of seven well-documented historical earthquake ruptures in the Basin and Range Province reveal three important relationships between rupture termination and the size of structural discontinuities. Firstly, terminations of normal faulting earthquakes are often associated with structural discontinuities, at least for the five historical ruptures studied in this paper. Secondly, the sizes of structural discontinuities at the end of earthquake rupture zones are generally the largest among the structural discontinuities within the rupture zone. Thirdly, there appears to be a trend that larger earthquake ruptures are stopped by larger structural discontinuities. The size of structural discontinuities that stopped the earthquake seems to scale with the length and displacement of earthquake surface rupture.

More than 36,000 km of Deep Seismic Sounding (DSS) profiles have been collected in China since 1958. However, the results of these profiles are not well known in the West due to the language barrier. In this paper, we summarize the crustal structure of China with a new contour map of crustal thickness, nine representative crustal columns, and maps showing profile locations, average crustal velocity, and Pn velocity. The most remarkable aspect of the crustal structure of China is the well known 70+ km thickness of the crust of the Tibetan Plateau. The thick (45–70 km) crust of western China is separated from the thinner (30–45 km) crust of eastern China by the north-south trending seismic belt (105°E). The average crustal velocity of China ranges from 6.15 to 6.45 km/s, indicating a felsic-to-intermediate bulk crustal composition. Upper mantle (Pn) velocities are 8.0 ± 0.2 km/s, equal to the global continental average. We interpret these results in terms of the most recent thermo-tectonic events that have modified the crust. In much of eastern China, Cenozoic crustal extension has produced a thin crust with a low average crustal velocity, similar to western Europe and the Basin and Range Province, western USA. In western China, Mesozoic and Cenozoic arc-continent and continent-continent collisions have led to crustal growth and thickening. Inferences on the process of crustal thickening are provided by the deep crustal velocity structure as determined by DSS profiles and other seismological studies. A high velocity (7.0–7.4 km/s) lower-crustal layer has been reported in western China only beneath the southernmost Tibetan Plateau. We identify this high-velocity layer as the cold lower crust of the subducting Indian plate. As the Indian crust is injected northward into the Tibetan lower crust, it heats and assimilates by partial melting, a process that results in a reduction in the seismic velocity of the lower crust in the central and northern Tibetan Plateau.

Medicine Lake volcano (MLV) in northeastern California is the largest-volume volcano in the Cascade Range. The upper-crustal structure of this Quaternary shield volcano is well known from previous geological and geophysical investigations. In 1981, the U.S. Geological Survey conducted a teleseismic tomography experiment on MLV to explore its deeper structure. The images we present, calculated using a modern form of the ACH-inversion method, reveal that there is presently no hint of a large (> 100 km3), hot magma reservoir in the crust. The compressional-wave velocity perturbations show that directly beneath MLV's caldera there is a zone of increased seismic velocity. The perturbation amplitude is +10% in the upper crust, +5% in the lower crust, and +3% in the lithospheric mantle. This positive seismic velocity anomaly presumably is caused by mostly subsolidus gabbroic intrusive rocks in the crust. Heat and melt removal are suggested as the cause in the upper mantle beneath MLV, inferred from petro-physical modeling. The increased seismic velocity appears to be nearly continuous to 120 km depth and is a hint that the original melts come at least partly from the lower lithospheric mantle. Our second major finding is that the upper mantle southeast of MLV is characterized by relatively slow seismic velocities (−1%) compared to the northwest side. This anomaly is interpreted to result from the elevated temperatures under the northwest Basin and Range Province.

The Amargosa Desert fault system, part of a 250-km-long system of dextral strike-slip faults east of the Death Valley region, is a throughgoing, long-lived, NW-striking, dextral strike-slip shear system that lies beneath Crater Flat and Yucca Mountain. Evidence for the fault system includes gravity, seismic, structural, stratigraphic, and paleomagnetic data, the distribution of springs and basaltic volcanic centers, and patterns of late Quaternary surface faulting. Where the Amargosa Desert fault system passes beneath Miocene ash-flow tuffs of the Southwestern Nevada Volcanic Field, we infer that the fault system changes into a series of detachments near the basement/cover contact and is expressed as a system of rotational normal faults in the ash-flow tuffs. Displaced geologic features indicate that total dextral displacement on the Amargosa Desert fault system exceeds 30 km. Vertical-axis rotations in ash-flow tuffs suggest that about 25 km of dextral displacement may have occurred since about 12.75 Ma. This fault system has important implications for the proposed high-level nuclear waste repository at Yucca Mountain, since it has experienced Quaternary and possibly Holocene displacements. Both normal faults and basaltic volcanic cones near the proposed repository may be controlled by the evolution of this regional strike-slip fault system.

Late Cenozoic (mid-Miocene and younger) deformation of the Basin and Range Province in north-central Nevada is studied by an analysis of mesoscopic faults. Correlations between fault strikes, fault types and senses of displacement were found for this area. Normal faults strike N–S, and right-lateral and left-lateral faults strike NNW and NNE respectively. The existence of strike-slip faulting, mainly right-lateral, is consistent with vertical-axis counterclockwise rotation interpreted from paleomagnetic declination data. These data suggest that mid-Miocene and younger crustal deformation in north-central Nevada was accompanied by both E–W extension, and N–S compression and shortening. Moreover, the contribution of strike-slip faulting to the net extension of this area, although hard to estimate, is significant.

Deep (> 20 km) crustal earthquakes have occurred within or along the margins of at least four continental rift zones. The largest of these deep crustal earthquakes (M ⩾ 5.0) have strike-slip or oblique-slip mechanisms with T-axes oriented similarly to those associated with shallow normal faulting within the rift zones. The majority of deep crustal earthquakes occur along the rift margins in regions that have cooler, thicker crust. Several deep crustal events, however, occur in regions of high heat flow. These regions also appear to be regions of high strain, a factor that could account for the observed depths. We believe the deep crustal earthquakes represent either the relative motion of rift zones with respect to adjacent stable regions or the propagation of rifting into stable regions.

Govers R. and Wortel, M&R., 1993. Initiation of asymmetric extension in continental lithosphere. In: M.J.R. Wortel, U. Hansen and R. Sabadini (Editors), Relationships between Mantle Processes and Geological Processes at or near the Earth’s Surface. Tectonophysics, 223: X-96. The physical conditions are investigated under which the lithospheric-scale style of extension is pure shear or simple shear. We focus on the initial stages of continental extension to monitor how symmetric or asymmetric modes of extension evolve from specific tectonic conditions. Continental collision, magma intrusions and interaction between the lithosphere and the underlying mantle are investigated as sources for extension. We use a finite element method to model the thermo-mechanical evolution of continental lithosphere. Experimental flow laws are used to model the elastic, hutle. power law creep or diffusion creep rheology of lithospheric rocks. Our results indicate that if in-plane forces change from compressive to tensile immediately after a rapid mountain building phase, initiation of a lithosphere-scale detachment fault is possible. We find a strong dependence of the extensional style on the distribution with depth of residual stresses from the collision phase. This result is consistent with observations of gravitational collapse in regions, like the Aegean and the Basin and Range Province, where detachment faults have exhumed lower crustal rocks. The predicted dip direction of the fault also agrees with observations in these areas. Intrusion of magma into continental lithosphere, which is subject IO in-plane tensile forces, will cause localization of pure shear deformation. The style of deformation resulting from mantle plumes impinging to the base of the lithosphere is symmetric. Delamination of lithospheric mantle may initiate detachment faults if delamination occurs at the end of a collision phase, when in-plane forces change sign from compressive to tenstle. This result also strongly depends on the assumed residual stress distribution. If delamination occurs during the mountam huilding phase, the style of deformation will be pure shear. Another interesting outcome from our modeling is that dramatic strain weakening as a result of a deformation mechanism change from dislocation creep to diffusion creep, reduces the tendency to strain localization. A large volume of the published work on continental extension has been concentrated on quantifying the stretching factor p of the pure shear model of McKenzie (1978a). In some basins it was found that the kinematics of deformation could be more accurately described by depth-dependent stretching factors (Royden and Keen, 19801, which introduced asymmetry near basin edges. In the simple shear model of Wemicke

The principal areas of subsidence of the continental crust are wide intracontinental basins and passive continental margins. In many intracontinental basins crustal extension amounts to only a few percent and is unable to explain their subsidence, as shown for the Timan-Pechora, Pre-Caspian and West Texas basins. The deep Black Sea and South Caspian basins are underlain by highly attenuated continental crust, having velocities in the 7 km/s range, that shows no evidence for significant extension. In passive margins, upper crustal extension ranges between 10 and 40%, whereas the lower crust is attenuated by 2–3 times (e.g., Campos Basin, north Biscay margin). Large discrepancy between upper crustal stretching and lower crustal attenuation is seen in the crustal configuration of many intracontinental grabens (e.g., Baikal and Dniepr-Prypyat basins). Only in highly extended areas, such as the Basin and Range Province and the East China rift system, does crustal stretching account for the observed crustal thinning. The observed discrepancy between upper extension and crustal thinning across intracontinental rifts and passive margins, and particularly in some wide intracontinental basins suggests that crustal stretching alone cannot account for the subsidence of these basins. Phase transformations in the lower crust, probably facilitated by the presence of hydrous fluids, can cause rapid (~ 1 Ma) as well as long-term (200–1500 Ma) upward displacement of the Moho discontinuity, resulting in high amplitude subsidence of the crust. In areas of crustal extension this mechanism can overprint subsidence induced by thermal relaxation of the lithosphere. The case of the Black Sea and South Caspian basins shows that density increase of the lithosphere can also occur independently of crustal stretching.

Geological, geochemical, and geophysical data gathered within the central part of the Basin and Range and adjacent areas of the western USA suggests that considerable heterogeneity characterizes Cenozoic extension in this region. Good exposure and an abundance of pre-rifting markers indicate 250 km of extension of the upper crust over the past 16 m.y. Extension of several hundred percent has occurred in two distinct deformational domains, Death Valley and Lake Mead, separated by a relatively unextended block, the Spring Mountains. The limited topographic differences between extended and unextended regions imply that material with a crustal density has been added to the extended regions. Although igneous activity can provide some of this added material, kinematics of extension within the Death Valley region suggest that lateral flow of the middle and lower crust into the extended areas accounts for much of the needed material. Such flow is consistent with geochemical analysis of intermediate to silicic volcanic rocks in the Death Valley area. These volcanic rocks contain isotopic and geochemical trends similar to Mesozoic plutonic rocks from the western part of the Sierra Nevada, about 150–200 km to the west, thus suggesting that the upper crust has moved by that amount relative to deeper crustal levels. Geochemical analyses of basaltic magmas in the region indicate that two mantle reservoirs are present: an OIB-type asthenosphere, and an old, Precambrian continental lithosphere. The ancient lithospheric mantle is preserved beneath the Central Basin and Range, but to the west and north the basaltic rocks have a signature compatible with an asthenospheric origin. These differences indicate that the degree of thinning and removal of the mantle lithosphere varies considerably across the Central Basin and Range. These differences are compatible with the inference from geological and geophysical arguments that thinning of the mantle lithosphere at the latitude of the Central Basin and Range is localized beneath the Sierra Nevada. Geophysical measurements have shown that the thickness of the crust varies little from a mean of about 30 km over the entire Basin and Range; the crust under the high Sierra Nevada to the west might have about the same thickness. Estimates of the buoyancy of the crust and mantle based on P-wave crustal structures suggest that the most buoyant, and thus probably the warmest, mantle lies under the Sierra Nevada and not under areas of strongly thinned upper crust of the Death Valley and Lake Mead regions to the east. Similar analyses indicate that the extended upper crust of the Northern Basin and Range overlies an upper mantle more buoyant than that of the Southern and Central Basin and Range; this is in accord with geochemical and seismological inferences. Thus, the style of lithospheric extension varies considerably both along and across the strike of the Basin and Range.

Genesis of the Columbia River Basalt Group (CRBG) may be accounted for by the progressive thermal erosion of an attenuated, heterogenous subcontinental lithosphere (SCL) by back-arc convection behind the Cascade arc. Assembly of the source components of the CRBG began during the Early Mesozoic when slices of former oceanic lithosphere were isolated below terranes accreted to the continental margin. Subsequent lithospheric shearing accompanying northward translation of the terranes allowed the injection of small-degree partial melts from the asthenosphere which solidified in the SCL as pyroxenite veins. The SCL source of the CRBG underwent a further episode of shearing accompanied by attenuation during the Eocene and Oligocene when it was caught between contrasting right-lateral terrane translation and E-W extension regimes in the northern Cordillera and Basin and Range province, respectively. The attenuation resulted in the raising of the geotherm close to that of the pyroxenite and Iherzolite solidi. Melting was triggered by back-arc convection behind the Cascade arc. The pyroxenite was the first component mobilised, constituting the principal source of the Imnaha Formation. Continued back-arc convection resulted in the failure of the SCL and the generation of the voluminous Grande Ronde Formation from pyroxenite and the surrounding SCL. The change to Wanapum volcanism marks the stage of exhaustion of in situ sources and their replacement with a combination of Proterozoic SCL from the east and asthenospheric material by back-arc convection. The Saddle Mountains Formation was derived entirely from Proterozoic SCL. Small magma volumes resulted in a susceptibility to local contamination such that the distinct isotopic arrays of the Saddle Mountains Formation comprise three mixing lines and do not have any age significance.

Among convergent margins, Mexico is an excellent site for studying along-strike variation in plate tectonics at the trench and its effect on the accompanying volcanism and tectonics in the overriding plate. The progressive fragmentation and consumption of the Farallon plate and the interaction between the Pacific and North America plates during the Cenozoic produced some unique geologic features. These include the Sierra Madre Occidental, one of the largest silicic igneous provinces on Earth, and the Trans-Mexican Volcanic Belt, a complex continental volcanic arc underlain by two of the youngest slabs in the world. In addition, extension occurred concurrently in much of the region affected by volcanism, producing a significant part of the southern Basin and Range province, and the reactivation of older basement structures which controlled the emplacement of volcanism.

The region of the present Rio Grande rift and southeastern Colorado Plateau underwent lithospheric extension during middle to late Cenozoic deformation affecting the entire southwestern U.S. Lithospheric mantle was disrupted, and in many regions displaced or replaced by asthenospheric mantle at depths from which basaltic magmas were derived and erupted to the surface. Study of the igneous rocks erupted or intruded during this deformation yields insights into processes of magmatism associated with extension of continental lithosphere. Magmatic rocks associated with an early (late Oligocene-early Miocene) ductile phase of extension are dominantly basaltic andesites and related, calc-alkaline intermediate to silicic derivative rocks. Mafic magmas were probably derived from isotopically “enriched” lithospheric mantle. Igneous rocks associated with a later (middle Miocene-Holocene), more brittle phase of extension include widespread basaltic rocks and localized central volcanoes of intermediate to silicic composition. Isotopic compositions of mafic rocks, which include both tholeiitic and alkalic basalts, correlate strongly with tectonic setting and lithospheric structure. Basalts erupted in areas of greatest crustal extension, such as the central and southern rift and Basin and Range province, were derived from isotopically “depleted” (correlated with “asthenospheric”) mantle. Also, isotopic compositions of Pliocene to Holocene basalts are slightly more depleted than those of Miocene basalts, suggesting that subcrustal lithospheric mantle was thinned during late Miocene extension. Intermediate rocks of the central volcanoes formed by a complex combination of processes, probably dominated by fractional crystallization and by assimilation of upper and lower crust in isolated, small magma chambers. The petrologic, geochemical, and isotopic data are compatible with a model, derived first from geophysical data, whereby lithosphere is thinned beneath the central rift and southeastern Colorado Plateau, with greatest thinning centered beneath the axis of the rift. A lithospheric model involving uniform-sense simple shear does not appear compatible with the data as presently understood.

This paper is the first of a series which have considered the possible effects of currents in the mantle upon continents. It mentions effects of aging in oceans upon the direction of slope of coastal peneplains. When oceans are young the coasts are cliffs from which peneplains dip downward on the inland side. As the margins separate from the ridge they cool and the slopes reverse. It suggests how Jeffreys' arguments against convection currents breaking the lithosphere can be avoided and discusses the possibility that upwelling has penetrated beneath the southwestern United States.

A detailed heat flow study has been conducted at a site in the southern Wasatch Mountains, Utah, in the thermal transition between the Colorado Plateau and Basin and Range tectonic provinces of the western U.S.A. Two wells, 600 m deep and only 400 m apart, in rugged terrain provided constraints on topographic and microclimatic effects and helped demonstrate the efficacy but also some inadequacies of commonly used heat flow corrections. Microclimatic effects changed the subsurface thermal gradients by up to 6%; atmospheric temperature lapse, insolation and vegetation all contribute about equally to the subsurface effects. The topographic disturbance decreased gradients by as much as 25%. Paleoclimate effects may decrease the heat flow by 7%, but the local paleoclimate is not well constrained and this value is uncertain. The rate of erosion in the Wasatch Mountains is also very poorly known, but is an important influence on the borehole temperature measurements. For reasonable bounds on the erosion rate of 0.1–1.0 mm y −1 , acting over the past 10–20 My, the erosional history of the Wasatch Mountains contributes from 10% to 50% of the observed heat flow; lower values are more probable. The heat flow at Fifth Water is greater than 90 mW m −2 , and possibly as high as 210 mW m −2 , depending upon the paleoclimatic and erosional scenarios assumed. Our preferred value of corrected heat flow is 150 ± 10 mWm −2 . This value is significantly higher than nearby heat flow determinations in both the Colorado Plateau and Basin and Range provinces, although well within the range of all Basin and Range heat flow estimates. Cooling of magma bodies in the upper crust and upwelling groundwater are unlikely mechanisms for the elevated heat flow at this site.

The western margin of the Colorado Plateau in central Utah is similar in tectonic style to the adjacent Basin and Range Province and appears to be a transition between the two provinces. It is important to understand the isostatic compensation of the topography, the source of the associated gravity anomalies, and the regional mechanical structure since these variations can be linked to the mechanism controlling the uplift of the Colorado Plateau and adjacent regions. In this paper the isostatic response function (admittance function) technique is used to study the isostatic compensation of the Colorado Plateau-Basin and Range transition zone in central Utah and determine the applicability of this technique in the study of small geologically complex areas. The observed admittance, determined from the topography and free air gravity anomaly, had small errors for the long wavelength estimates, small (nearly zero) phase angles, and high coherence. This suggests that most of the energy in the power spectrum of the free air anomaly is correlated with the gravity fields of the topography and its compensation. The residual gravity anomaly based on the observed admittance is small except in the region of the Wasatch Front where the presence of a fairly symmetric high-low-high trend suggests compensated crustal or lithospheric structures not correlated to the topography. The observed admittance function is compared to three simple isostatic models: a surface loaded continuous elastic plate, a subsurface loaded continuous elastic plate, and a Pratt model of local compensation. It was found that the best fit elastic plate models defined similar results in spite of differing assumptions as to the form of the loading. These models both had best fit effective flexural rigidities of 10 20 Nm and Moho depths of approximately 35 km. The calculated residual gravity anomalies based on these models are nearly identical to the residual anomaly of the Pratt model with a compensation depth of 65 km. The residual anomaly of all three models reproduced the trends and magnitude of the residual anomaly based on the observed admittance. It is suggested that this local compensation is due to subsurface laterally varying thermal density anomalies. These results differ from previous isostatic rebound studies of ancient Lake Bonneville, which give larger magnitude effective flexural rigidities. These differences can possibly be attributed to differing time scales of loading between Lake Bonneville and the long term thermal loading of the Basin and Range extension and associated volcanism or the overall simplicity of the considered models. Care should be exercised in the interpretation of admittance functions determined across transition zones between adjacent tectonic provinces. Depths of compensation and flexural rigidities can represent averaged values. All additional geophysical and geologic information should be considered in the interpretations.

The Bill Williams River area of west-central Arizona includes not only the Rawhide-Buckskin metamorphic core complex, which is part of the lower Colorado River highly extended terrane (HET), but also the boundary between the extended terranes of the Basin and Range Province and the less deformed Arizona Transition Zone/Colorado Plateau. This provides important constraints on models that address the mechanisms for the mid- to late Tertiary deformation. Three phases of extension are present. The oldest is the extension associated with core-complex tectonism, which characteristically shows a lower plate composed of lineated mylonitic gneiss overlain by a detachment fault that is regionally nearly horizontal but undulates at the local scale. The fault in turn is overlain by an upper plate that includes Precambrian basement rocks, recrystallized Paleozoic sedimentary rocks, Mesozoic(?) metasedimentary and metavolcanic rocks of greenschist facies, and unaltered to hydrothermally altered syntectonic sedimentary and volcanic rocks of Miocene age. The upper plate is cut by closely spaced faults of modest structural relief that strike northwest and strongly rotate intervening blocks to face southwest. Most of these faults do not penetrate below the detachment fault. Fault spacing increases, and rotation decreases, to the northeast, away from the trace of the detachment. The second phase consists of “classic” Basin-Range high-angle normal faults that strike about north and have wide spacing, high structural relief, and modest rotation of blocks. These faults have no consistent direction of displacement and so produced horst and graben that form the ranges and basins visible today. This phase is locally superposed on Phase I, and also extends in more subdued form into the Transition Zone/Colorado Plateau. The third phase consists of tectonic quiescence and is present everywhere except parts of the Transition Zone that are still active seismically. The first phase occurred in the early and middle Miocene and was accompanied by deposition of syntectonic fluviolacustrine rocks (Suite I); the second (middle to late Miocene) was marked by interior-basin deposits (Suite II); the third (latest Miocene through Quaternary) is characterized by deposits related to through-flowing drainage. The phases grade into each other and thus are likely to be genetically related. Tectonic models must take into account not only the geographic distribution of deformation at any one time but also the time-dependent succession of deformation at any one place. A model proposed in this paper attempts to do this. The model is thermotectonic. A heating event in the lower crust, (basaltic intrusion, asthenospheric upwelling) combined with stretching, causes a sharp thermal front to rise within the crust. Embedded within the front is an “isotherm” that marks the brittle-ductile transition. As the front rises, it leaves behind a trail of shear zones, each marking a locus of preferred failure defined by mechanical or physical properties, or combinations thereof. The highest shear zone, now preserved in fossil form as the “detachment”, occurs where the front impinges on the meteoric groundwater, a few km below the topographic surface. The water steepens the thermal gradient at the front, which it stabilizes. A convective hydrothermal circulation system is established, causing alteration and mineralization above the ductile-brittle transition, as well as pore overpressure that results in hydrofracturing (producing monolithologic breccias) and the sliding of gravity-glide sheets. During these events, extension is taking place by brittle failure in the upper plate and ductile deformation below the detachment. Simultaneously, the hottest areas (core complexes) are updomed, promoting drainage reversals and the sliding of breccias and glide sheets. All this occurred only in the hottest areas or “blisters”, now marked by the core complexes. Distal areas showed less or no deformation at the surface. With time and the waning of the thermal event, the thermal front, and thus the brittle-ductile transition, smoothed out and sank, again leaving a trail of shear zones. Phase 1 deformation ceased and was replaced by Phase 2 deformation that occurred over a much wider area. Eventually, the front sank so deep that surface deformation ceased. This illustrates how the style of deformation at the surface may be a measure of the depth to the brittle-ductile transition. According to the thermotectonic model, extensional strain does not need to be constant along the detachment, in contrast to models involving simple shear through crustal-scale normal faults. On the contrary, one would expect strain to vary geographically as a function of maximum temperature attained, because of the well known relation between temperature and lithospheric strength. The thermotectonic model is also in good accord with geophysical characteristics of the Basin and Range Province, which suggests that extension was accompanied by intrusion of basalt into the lower crust, with consequent heating and anatexis. Many studies in the U.S. and elsewhere support the model by showing that continental extension commonly is accompanied by near-surface temperatures corresponding to the brittle-ductile transition, by steep thermal gradients, and by hydrothemal convective systems. A possible driving mechanism from the thermotectonic processes described by the model is the rise of asthenospheric domes or welts, which thin the lithosphere by subcrustal transfer while heating and stretching it. An asthenospheric welt that migrates northeastward while dying out might explain the encroachment of relatively subdued extension onto the Colorado Plateau, as well as the juxtaposition of compressive stress on the plateau with extensional stress in the adjacent Transition Zone and Basin and Range Province.

Several mechanisms that have been proposed to explain the high heat flow in regions of tectonic extension are examined. The transient thermal regime is computed and the results are discussed. Conductive heating of the lithosphere from below requires a time of at least 75 Ma to affect the surface heat flow. Advective heating by the injection of magmas into the lithosphere does not produce steady-state conditions before 75 to 300 Ma (depending on boundary conditions). Only mechanical stretching of at least the shallow lithosphere can explain the increase in surface heat flow in a time of the order of 30 to 50 Ma. The heat flow data are compatible with several mechanisms for the extension of the lower lithosphere (underplating or dike injection) and other geologic and geophysical data are needed to constrain the models.

A tectonic boundary should be defined by changes in tectonic elements. Tectonic elements would include such parameters as structural style, stress orientations, volcanism, heat flow, seismicity and changes in crustal thickness. Examination of these tectonic elements for the southern Colorado Plateau suggests that the southwestern part of the physiographic plateau appears to be tectonically part of the Basin and Range province. I therefore propose a boundary at 112° W longitude, the locus of seismicity and recent volcanism and also the site of crustal thickening and a change of structural style. The tectonic boundary of the southern Colorado Plateau is suggested to form part of the eastern boundary of the southern Basin and Range seismotectonic region. This seismotectonic region is bounded on the north by the Las Vegas shear zone/Nevada seismic zone and has a different stress orientation than the Northern Basin and Range from which it began to evolve separately 13 m.y. ago. The present principal stress orientation of the southern Basin and Range of Arizona appears to be the result of compound movements of the San Andreas transform and Garlock-Las Vegas shear zone resulting in a shear couple affecting the crust in Arizona and an associated NE-SW least principal stress orientation.

Two-dimensional thermal models of the lithosphere under the Basin and Range Province, the Rio Grande rift and the Colorado Plateau have been constructed using a numerical method for solution of the equation of conservation of energy for the transient temperature field. It is established that the crust in the Basin and Range Province and the Rio Grande rift is still heating on a regional scale. We suggest that intrusion of asthenospheric material in the shape of wide upwellings was a cause of lithospheric heating. The time elapsed since asthenospheric material reached the base of the crust of the Basin and Range Province and the Rio Grande rift is estimated as 2–4 m.y. About the same time a great dyke intrusion into the crust beneath the Rio Grande rift occurred. The asthenospheric surface under the Colorado Plateau reached the present depth (about 60 km) not less than 30 m.y. ago.

Seismic body-wave and surface-wave data indicate the existence of a substantial lowvelocity, low- Q zone in the upper mantle beneath western North America. Conditions in this zone are distinctively different from those that are typical of the upper mantle in shield regions. The present study, using Walsh's model for partially melted rock, suggests a common mechanism for low-velocity and low- Q zones. This parametric study also indicates that the pronounced low-velocity, low- Q zone and anomalous travel-time delays of both P and S waves in the Basin and Range Province are consistent with the combined effects of high temperature, chemical composition, phase changes, and partial melting. The observed low P n velocity in this region is consistent with high temperature, chemical composition, and the presence of a partially molten layer within the upper mantle, but the observed teleseismic delay times result principally from the thickness of the lowvelocity zone. The teleseismic delay-time variations are therefore related to the seismic Q distribution in the asthenosphere. Conditions a few kilometers beneath the Moho boundary influence the P n velocity; however, the observed correlation among the teleseismic signal amplitudes, travel-time delays, and the upper mantle Q indicates that the P n velocity is a better indicator of upper mantle Q than suggested by the P n path alone. This knowledge of the upper mantle seems to account for the anomalous effects of the Basin and Range Province and other regions of similar tectonic nature on observations of teleseismic events. This will provide a practical technique for comparing seismic observations made in unstable tectonic regions with observations made in shield areas.

The seismologically observed Pn velocity anomalies in the conterminous United States are restricted to the lithosphere, but the observed teleseismic delay-time variations are due principally to the regional variations in the physical state (i.e., thickness of lowvelocity zone and/or percent melt, etc.) of the asthenosphere. The observed low Pn velocity has been attributed to partial melting in the upper mantle, but it is shown that the partial-melting model alone cannot explain the seismologically observed Pn velocities in such an anomalous region as the Basin and Range Province. The present structure of the Basin and Range Province is possibly a result of rifting in the western conterminous United States; under it there may lie a mixed structure of old crust and mantle materials. The low-velocity zone under the Basin and Range Province would then be caused by downward chemical transition from the sub-Moho pyrolitic mantle material into a plagioclase-rich ophiolitic (old oceanic crust and upper mantle) composition and associated meltingand then into a peridotitic composition at the bottom of the lowvelocity zone. This mixed material model, with partial melting, would explain the low Pn velocity and low seismic Q in the region, as well as other geophysical observations.

The complex representation of plane stress is used for an analytic model to describe the stress regime of the Snake River Plain (SRP) region of the western United States. The area is modeled as an infinite elastic sheet cut along a circular arc with tension applied at infinity. Modeling results include the following: (1) south of the Snake River Plain, the principal direction of tension rotates from the direction of the applied stress to a direction sub-parallel to the cut, consistent with the orientation of normal faulting and nodal planes given by fault plane solutions; (2) north of the Snake River Plain, calculated stress directions are consistent with the variable nature of faulting in that region; (3) a decrease in magnitude of predicted stresses is observed within the Snake River Plain consistent with relatively low levels of seismicity on the Plain.

Very Long Baseline Interferometry experiments over the last 1-3/4 years between Owens Valley, CA and Haystack, MA Radio Observatories suggest an upper limit of east-west crustal deformation between the two sites of about 1 cm/yr. In view of the fact that the baseline between the two sites traverses most of the major geological provinces of the United States, this low rate of crustal deformation has direct relevance to intra-plate crustal tectonics. The most active region traversed by this baseline is the Basin and Range province, which was estimated by various researchers to be expanding in an east-west direction at rates of .3 to 1.5 cm/yr. The Colorado Plateau and Rocky Mountain system also appear to be expanding, but at a somewhat lower rate, while east of the Rocky Mountains, the predominant stress appears to be compressional, nearly horizontal, and east to northeast trending.

An elastic-perfectly plastic plate model has been developed to analyze the flexure associated with normal faulting. The model consists of a thin layer, which is completely cut by a normal fault, overlying a fluid substratum. For a given applied bending moment at the fault, the relationship between the amount of displacement on the fault and the extent of the failure zone can be calculated. The model is applied to the Wasatch Front region in the eastern Basin and Range Province, USA to determine the correlation of its parameters with geological and geophysical data in the vicinity of a major normal fault, the Wasatch fault, along which there has been 3–4 km of Late Cenozoic uplift. In this region, most seismic activity occurs away from the Wasatch fault in a zone 30 km wide, roughly centered 30 km east of the fault. This activity occurs at depths of 15 km or less. In order to match the observations, the lithospheric layer must have a flexural rigidity of 0.5 to 1.1 · 1022 n-m and a yield stress of 1–2 kb and must have zero applied bending moment at the fault. The effective mechanical thickness of the lithosphere in this region is 20–25 km. These results indicate that the lithosphere in long-term mechanical studies in the eastern Basin and Range is thin and weak. Evaluating these results as compared to the seismic lithospheric thickness and temperature regime of the region produces some interesting correlations with studies in oceanic regions.

This review of geological, seismological, geochronological and paleobotanical data is made to compare historic and geologic rates and styles of deformation of the Sierra Nevada and western Basin and Range Provinces. The main uplift of this region began about 17 m.y. ago, with slow uplift of the central Sierra Nevada summit region at rates estimated at about 0.012 mm/yr and of western Basin and Range Province at about 0.01 mm/yr. Many Mesozoic faults of the Foothills fault system were reactivated with normal slip in mid-Tertiary time and have continued to be active with slow slip rates. Sparse data indicate acceleration of rates of uplift and faulting during the Late Cenozoic. The Basin and Range faulting appears to have extended westward during this period with a reduction in width of the Sierra Nevada. The eastern boundary zone of the Sierra Nevada has an irregular en-echelon pattern of normal and right-oblique faults. The area between the Sierra Nevada and the Walker Lane is a complex zone of irregular patterns of horst and graben blocks and conjugate normal-to right- and left-slip faults of NW and NE trend, respectively. The Walker Lane has at least five main strands near Walker Lake, with total right-slip separation estimated at 48 km. The NE-trending left-slip faults are much shorter than the Walker Lane fault zone and have maximum separations of no more than a few kilometers. Examples include the 1948 and 1966 fault zone northeast of Truckee, California, the Olinghouse fault (Part III) and possibly the almost 200-km-long Carson Lineament. Historic geologic evidence of faulting, seismologic evidence for focal mechanisms, geodetic measurements and strain measurements confirm continued regional uplift and tilting of the Sierra Nevada, with minor internal local faulting and deformation, smaller uplift of the western Basin and Range Province, conjugate focal mechanisms for faults of diverse orientations and types, and a NS to NE—SW compression axis (σ1) and an EW to NW—SE extension axis (σ3).

(Accepted for publication May 22, 1978) ABSTRACT: Smith, R.S.U., 1979. Holocene offset and seismicity along the Panamint Valley fault zone, western Basin-and-Range province, California. In: C.A. Whitten, R. Green and B.K. Meade (Editors), Recent Crustal Movements, 1977. Tectonophysics, 52: 411-415. Holocene right-slip along the central segment of the Panamint Valley totals 20 m and dip-slip is somewhat less. The most recent offset, about 2 m right-slip, probably occurred at least several hundred years ago. If a comparable amount of slip occurred during earlier earthquakes, mean seismic recurrence intervals would have been about 700-2500 years during the Holocene.

Deep basins on continental crust can be divided into two main types: basins with normal faults and the basement both tilted in the same direction (S-type) and in opposite directions (A-type). A high intensity of stretching occurs in the basins of the A-type under a large angle of block tilting. This can also be typical of the S-type basins of a very small width L that does not exceed the thickness of the prestretched crust h c 0 ( L h c 0 ). Only in basins of these two k is extension sufficient to explain the subsidence by stretching. They can be called “rifts”. Extended basins in the Basin and Range Province are a typical example. Deep basins of A-type with a small angle of block tilting and deep basins of S-type with a width L > h c 0 cannot be produced by stretching. Under a width h c 0 L ≲ 100 km they can be call “grabens”. The Fen-Wei Graben in Central China and the Viking Graben in the North Sea belong to this structural type.

Stratigraphic studies and detailed field analyses of brittle and plastic deformations allow the reconstruction of the stress pattern and the tectonic evolution of the central sector of the Mexican Volcanic Belt. Several compressional phases affected the metamorphic basement, while the last one also involved the volcanic cover of Early Miocene age. In Middle Miocene-Early Pliocene times and ENE-WSW extensional phase developed, representing the southern protraction of the Basin and Range Province of the western United States. In Pliocene-Quaternary times there then followed the development of a complicated deformational cycle showing the superficial evidence of a sinistral lateral shear zone. At the beginning the area was affected by a NE-SW compression including reverse, dextral and sinistral strike-slip faulting. This was followed by Early Pleistocene sinistral transtension characterized by a NW-SE direction of least principal stress and sinistral normal-slip faulting. During Late Pleistocene-Holocene times the transtension was concentrated along E-W faults. Tectonic control in the distribution of volcanism has also been recognized. The Basin and Range phase controlled the deposition of the Miocene volcanic and clastic units, particularly the effusion of basaltic lavas connected with the latest extensional movements of the same phase. The Mexican Volcanic Belt sequence is strictly linked with the development of a large E-W sinistral lateral shear zone, active from the Pliocene to the Present, which shows the extensional component progressively prevailing over the shear one.

The active rift system along the postulated landward extension of the East Pacific Rise into western North America (Heezen, 1960; Menard, 1960; Cook, 1961) consists of several active rift zones which follow belts of seismicity and probable high heat flow and include: (a)the belt of grabens along the eastern margin of the Sierra Nevada, the Walker Lane, and the Churchill Arc; (b)the system of great trenches extending through western Arizona, central Utah, southeastern Idaho, western Wyoming, western Montana, and British Columbia; and (c)the Rio Grande rift belt. The present paper reviews the following additional recent geophysical evidence that defines in more detail the active rift system in the eastern part of the Basin and Range province: 1. (1) In north central Utah, gravity surveys, together with refraction and/ or reflection seismic data along some profiles, indicate that the Wasatch Trench is a northward-trending belt of grabens about 160 miles long and up to 13 miles wide. 2. (2) In south central and southwestern Utah, gravity surveys indicate grabens adjacent to the northward-trending Sevier and Hurricane fault zones, which lie along the rift belt. 3. (3) In New Mexico, several gravity profiles across the Rio Grande rift belt indicate the width and depth of the grabens. 4. (4) Sparse data tentatively indicate high heat flow in the rift belts. The possible mechanism of the Late Cenozoic vulcanism and diastrophism in the Basin and Range province is explained by a postulated mantle convection current (associated with the formation of the Rise) that rose beneath the eastern part of the Basin and Range province (or perhaps the Colorado Plateau) and flowed westward (Cook, 1961). The active rift system is in the region of tension over the upwelling convection currents. To the west, in the region of translation and compression, it is postulated that the westward-moving mantle convection current dragged along the continental crust in the California region. The crustal drag caused a large continental-type block to move westward into the Pacific Ocean about 50–100 km in the California region between the landward projections of the Mendocino and Murray fracture zones, which were probably major lines of weakness existing before the formation of the East Pacific Rise. On the northern and southern margins of the block, respectively, the movement occurred along transcurrent and/or transform faults in: 1. (1) the Cape Mendocino area (Gorda escarpment). 2. (2) the Garlock fault, Big Pine fault, Channel Islands. Transverse Ranges, and Pinto Mountain fault areas. The westward movement of the block, which occurred along transcurrent and/or transform faults associated with these two fracture zones, probably caused: (a) the folding of the Coast Ranges in California along the frontal (i.e., west) segment of the block and (b) the bending of the San Andreas fault zone in two areas along the southern margin of the block. The right-lateral movement of the San Andreas fault zone is explained by a postulated second, northwestward-trending mantle convection current with a velocity gradient increasing southwest; this movement has apparently moved into the western Basin and Range province recently to cause transcurrent faulting there (Billings, 1960), and a change in the direction of the mantle convection currents there is postulated. The concept that a westward-moving continental-type crustal block has overridden the oceanic crust in the Pacific Ocean and that the driving force probably originated in the upper-mantle spreading beneath the Basin and Range province (or perhaps the Colorado Plateau) is fundamental to the interpretation of the Late Cenozoic tectonics in the southwestern United States. If the United States, along with the North American continent, has drifted westward as a consequence of the sea-floor spreading along the Mid-Atlantic Ridge and overriden the East Pacific Rise, as postulated by Vine (1966), the westward movement of the above-mentioned crustal block would constitute about 50–100 km of horizontal displacement in addition to the “normal” westward drift of the North American continent as a whole.

The largest rift zone of Europe and Asia is located in the region of Lake Baikal. In 1968–1970 deep seismic measurements were carried out along a number of profiles with a total length of about 2000 km within the rift zone and in the adjacent parts of the Siberian platform and the region of the Baikal Mountains. These investigations were of a reconnaissance nature, and therefore the point sounding method was used. A low-velocity region for compressional waves (7.6–7.8 km/sec) has been found and could be traced over a large area in the upper parts of the mantle. The width of this anomalous zone is 200–400 km. The Baikal rift lies in its northwestern part. Within the studied part of the Siberian platform the thickness of the earth's crust is 37–39 km, while in the rift zone it is 36 km, and further to the southeast the crust-mantle boundary lies at a depth of 45–46 km. The Baikal rift proper is bounded in the northwest by a deep fracture zone and does not seem to be associated with any significant “root” or “antiroot” in the relief of the Mohorovicic discontinuity. The reduced compressional velocity in the upper parts of the mantle beneath the Baikal zone is considered to correspond to the same phenomena found under the mid-oceanic ridges and the extended rift system in the Basin and Range province of North America. The Baikal rift in the narrow sense of the word lies over the northwestern edge of the anomalous mantle region. This asymmetric position seems to be its main peculiarity.