Mojave Block Research Articles

Kinematics and Evolution of the Southern Eastern California Shear Zone, Based on Analysis of Fault Strike, Distribution, Activity, Roughness, and Secondary Deformation

AbstractThe Eastern California shear zone is a complex set of dextral faults that accommodates significant plate motion and has produced large earthquakes. The evolution of this system and why it consists of closely spaced, irregular faults that fail in multi‐fault ruptures are not well understood. Here we analyze the geometry, spatial distribution, and Quaternary slip activity of right‐lateral faults in the southern Mojave block. We find these faults are oriented favorably for accommodating regional dextral plate motion and do not show evidence of replacement following counterclockwise rotation to unfavorable positions, although activity may be migrating westward as previously proposed. We also confirm that the shear zone is transpressive, with widespread restraining bends, distributed convergent deformation, and significant impact on near‐fault topography. Observations also show that faults are geometrically complex, as represented by along‐strike variability in fault strike. We document a correlation between strike variability and fault activity (slip rate or net slip), which is evident within the shear zone as well as for a control group of other faults. We suggest that strike variability represents a form of geometric roughness, which may inhibit fault slip and result in complex ruptures, slip‐strengthening behavior, and a prevalence of off‐fault deformation. Other factors, including preexisting crustal fabric, edge effects, and changes in the stress field, may further complicate kinematics. These results suggest that faults of the shear zone are still juvenile and somewhat unique, yet offer an important window into how broadly distributed shear may evolve into a through‐going continental transform system.

Western pond turtles in the Mojave Desert? A review of their past, present, and possible future

The western pond turtle (WPT) was formerly considered a single species (ActinemysorEmys marmorata) that ranged from southern British Columbia, Canada to Baja California, México. More recently it was divided into a northern and a southern species. WPTs are found primarily in streams that drain into the Pacific Ocean, although scattered populations exist in endorheic drainages of the Great Basin and Mojave deserts. Populations in the Mojave Desert were long thought to be restricted to the Mojave River, but recently another population was documented in Piute Ponds, a terminal wetland complex associated with Amargosa Creek on Edwards Air Force Base. WPT fossils in the Mojave Desert are known from the Miocene to the Pleistocene. Recently, Pleistocene fossils have been found as far into the desert as Salt Springs, just south of Death Valley. The oldest fossil records suggest that WPTs were present in wetlands and drainages of the geological feature known as the Mojave block prior to the uplift of the Sierra Nevada Range about 8 Ma and prior to the ~ 3 Ma uplift of the Transverse Ranges. Archaeological records document use of turtles by Native Americans for food and cultural purposes 1,000 or more years ago at the Cronese Lakes on the lower Mojave River and Oro Grande on the upper river. The first modern publication documenting their presence in the Mojave River was 1861. Museum specimens were collected as early as 1937. These fossil and early literature records support the indigenous status of WPTs to the Mojave River. However, mtDNA-based genetic evidence shows that Mojave River turtles share an identical haplotype with turtles on the California coast. Limited nuclear data show some minor differences. Overdraft of water from the Mojave River for municipal and agricultural uses, urban development, and saltcedar expansion are threats to the continued survival of WPTs in the Mojave River.

Tools for uranium characterization in carbonate samples: case studies of natural U–Pb geochronology reference materials

Abstract. Laser ablation U–Pb analyses of carbonate (LAcarb) samples has greatly expanded the potential for U–Pb dating to a variety of carbonate-producing settings. Carbonates that were previously considered impossible to date using isotope dilution methods may preserve radiogenic domains that can be dated using spatially resolved laser ablation geochronology techniques. Work is ongoing to identify reference materials and to consider best practices for LAcarb. In this study we apply standard and emerging characterization tool sets on three natural samples with the dual goal of enhancing the study of carbonates and establishing a new set of well-characterized natural reference materials for LAcarb studies. We start with the existing carbonate reference material WC-1 from the Permian Reef Complex of Texas, building on the published description to offer a deeper look at U and associated trace elements. We consider a tufa sample from the Miocene Barstow Formation of the Mojave Block, California, as a possible secondary calcite reference material due to its well-behaved U–Pb systematics. There are currently no natural dolomite standards. We present an unusual dolomite sample with very well-behaved U–Pb systematics from the Miocene of the Turkana Basin of Kenya as a possible dolomite reference material for LAcarb dating. In addition to using X-ray fluorescence (XRF) mapping and spectroscopy to better understand U in these natural samples, we have analyzed multiple aliquots of each of them for 87Sr/86Sr by thermal ionization mass spectrometry (TIMS). The Sr isotope compositions are analytically homogeneous within petrographically homogeneous regions of all three samples, and thus these materials could be used as Sr isotope standards as well. While not part of the current contribution, this combination could streamline simultaneous LA analyses of 87Sr/86Sr and U–Pb geochronology.

Mantle flow distribution beneath the California margin

Although the surface deformation of tectonic plate boundaries is well determined by geological and geodetic measurements, the pattern of flow below the lithosphere remains poorly constrained. We use the crustal velocity field of the Plate Boundary Observatory to illuminate the distribution of horizontal flow beneath the California margin. At lower-crustal and upper-mantle depths, the boundary between the Pacific and North American plates is off-centered from the San Andreas fault, concentrated in a region that encompasses the trace of nearby active faults. A major step is associated with return flow below the Eastern California Shear Zone, leading to the extrusion of the Mojave block and a re-distribution of fault activity since the Pleistocene. Major earthquakes in California have occurred above the regions of current plastic strain accumulation. Deformation is mechanically coupled from the crust to the asthenosphere, with mantle flow overlaid by a kinematically consistent network of faults in the brittle crust.

A kinematic model for the evolution of the Eastern California Shear Zone and Garlock Fault, Mojave Desert, California

The Eastern California shear zone in the Mojave Desert, California, accommodates nearly a quarter of Pacific–North America plate motion. In south-central Mojave, the shear zone consists of six active faults, with the central Calico fault having the fastest slip rate. However, faults to the east of the Calico fault have larger total offsets. We explain this pattern of slip rate and total offset with a model involving a crustal block (the Mojave Block) that migrates eastward relative to a shear zone at depth whose position and orientation is fixed by the Coachella segment of the San Andreas fault (SAF), southwest of the transpressive “big bend” in the SAF. Both the shear zone and the Garlock fault are assumed to be a direct result of this restraining bend, and consequent strain redistribution. The model explains several aspects of local and regional tectonics, may apply to other transpressive continental plate boundary zones, and may improve seismic hazard estimates in these zones.

Constraining the kinematics of metropolitan Los Angeles faults with a slip-partitioning model.

Due to the limited resolution at depth of geodetic and other geophysical data, the geometry and the loading rate of the ramp‐décollement faults below the metropolitan Los Angeles are poorly understood. Here we complement these data by assuming conservation of motion across the Big Bend of the San Andreas Fault. Using a Bayesian approach, we constrain the geometry of the ramp‐décollement system from the Mojave block to Los Angeles and propose a partitioning of the convergence with 25.5 ± 0.5 mm/yr and 3.1 ± 0.6 mm/yr of strike‐slip motion along the San Andreas Fault and the Whittier Fault, with 2.7 ± 0.9 mm/yr and 2.5 ± 1.0 mm/yr of updip movement along the Sierra Madre and the Puente Hills thrusts. Incorporating conservation of motion in geodetic models of strain accumulation reduces the number of free parameters and constitutes a useful methodology to estimate the tectonic loading and seismic potential of buried fault networks.

Tectonostratigraphic evolution of the c. 780–730 Ma Beck Spring Dolomite: Basin Formation in the core of Rodinia

Abstract The Beck Spring Dolomite is a mixed carbonate–siliciclastic succession exposed in Death Valley, California, that was deposited between 780 and 717 Ma. Along with its bounding units, the Horse Thief Springs Formation below and unit KP1 of the Kingston Peak Formation above, the Beck Spring Dolomite were deposited in one of the ChUMP (Chuar–Uinta Mountains–Pahrump) basins with subsidence commonly attributed to the nascent rifting of Rodinia. These pre-Sturtian successions preserve eukaryotic microfossil assemblages, diverse microbialites, and large carbon isotope anomalies directly below Sturtian-age glacial deposits. Here we present new geological mapping, measured stratigraphic sections, carbon isotope chemostratigraphy and detrital zircon geochronology from the Beck Spring Dolomite and its bounding units. The carbon isotope excursion at the top of the Beck Spring Dolomite has previously been attributed to meteoric diagenesis associated with karst breccias, but here we demonstrate that these breccias are instead mass flow deposits that formed during deposition of the Kingston Peak Formation and that the carbon isotope excursion is not only reproducible throughout the basin, but is associated with transgression rather than regression and exposure. In addition, we refine local correlations and discuss the use of chemostratigraphic curves from these units for regional and global correlations. The Beck Spring Dolomite was deposited during the second of three distinct basin-forming events recorded in the Pahrump Group with basin inversion occurring between each event. The presence of syn-sedimentary faults, the character of the lateral facies change and detrital zircon provenance analyses indicate that the Beck Spring Dolomite fringed a coeval palaeo-high to the south in a tectonically active basin. Detrital zircon age distributions in the Beck Spring Dolomite show sharp probability peaks at c. 1200, 1400 and 1800 Ma, consistent with local sources to the SW in the Mojave block rather than transcontinental rivers. The c. 1800 Ma probability peak is less prominent in the KP1 samples. In addition, KP1 also records slump folding and is overlain by an unconformity. We suggest that these features are consistent with the emergence of a local fault to the NE. Deposition of the Beck Spring Dolomite and bounding units do not record evidence of incipient rifting of the western margin of Laurentia but instead reflect a distinct and separate tectonothermal event. Supplementary material: Carbon (δ 13 C) and oxygen (δ 18 O) isotopic measurements, detrital zircon laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) data, detrital zircon sample information and data from reference materials used for LA-ICPMS analyses are available at http://www.geolsoc.org.uk/SUP18823 .

Seismic evidence for lithospheric modification beneath the Mojave Neovolcanic Province, Southern California

The Mojave Neovolcanic Province (MNVP), located in the Mojave block of southern California, comprises late Miocene to Quaternary small‐volume basaltic centers. Geochemistry indicates an asthenospheric source for the MNVP beginning in the late Miocene, but no physical evidence of missing mantle lithosphere has been presented. We utilize receiver functions and ambient noise tomography to image the lithosphere beneath the Mojave block. Regionally, we find thin crust that thickens distal to sites of MNVP volcanism. Shear wave velocities between 40 and 75 km depth are consistent with the presence of mantle lithosphere in the southern Mojave block and very thin or missing mantle lithosphere to the north. With one exception, MNVP volcanoes lie along this sharp boundary. Our observations, together with the established geologic history and geochemistry of the MNVP, can be explained by small‐scale edge‐driven convection producing ongoing lithospheric removal within the Mojave block. Our results provide another example of lithospheric instability that occurs in response to rapid changes in mantle dynamics induced by major changes in tectonic plate geometry.

Clustering of GPS velocities in the Mojave Block, southeastern California

AbstractWe find subdivisions within the Mojave Block using cluster analysis to identify groupings in the velocities observed at GPS stations there. The clusters are represented on a fault map by symbols located at the positions of the GPS stations, each symbol representing the cluster to which the velocity of that GPS station belongs. Fault systems that separate the clusters are readily identified on such a map. The most significant representation as judged by the gap test involves 4 clusters within the Mojave Block. The fault systems bounding the clusters from east to west are 1) the faults defining the eastern boundary of the Northeast Mojave Domain extended southward to connect to the Hector Mine rupture, 2) the Calico‐Paradise fault system, 3) the Landers‐Blackwater fault system, and 4) the Helendale‐Lockhart fault system. This division of the Mojave Block is very similar to that proposed by Meade and Hager []. However, no cluster boundary coincides with the Garlock Fault, the northern boundary of the Mojave Block. Rather, the clusters appear to continue without interruption from the Mojave Block north into the southern Walker Lane Belt, similar to the continuity across the Garlock Fault of the shear zone along the Blackwater‐Little Lake fault system observed by Peltzer et al. []. Mapped traces of individual faults in the Mojave Block terminate within the block and do not continue across the Garlock Fault [Dokka and Travis, ].

Clustering of GPS Velocities in the Mojave Block southeastern California

Clustering of GPS Velocities in the Mojave Block southeastern California

The Sandstone-Derived Provenance Record of the Gualala Basin, Northern California, U.S.A

Abstract Detrital zircon U/Pb ages and Pb isotope compositions of detrital feldspars from the Late Cretaceous–Eocene Gualala Basin in northern California indicate that the basin received sediments from the southern Sierra Nevada, the Mojave block, and the Salinian block. Mesozoic detrital-zircon age components and Pb isotope compositions of detrital feldspars from the basin are similar to U/Pb zircon ages and Pb isotope compositions from previously studied coeval conglomerate clasts. The detrital zircons and feldspars, however, also include additional age and Pb isotope components that are unobserved or poorly represented in the conglomerate-based provenance record. These new data suggest a more complex provenance history than that inferred from previously studied conglomerate clasts, and strongly imply a connection to the Mojave Desert. Provenance evolution of the sediments in the basin indicates that significant contribution of sediment from the Mojave Desert began during the Paleocene, when it is inferred that the drainage network expanded eastward from its previous extent. The presence of Mojave-derived Late Permian zircons, as well as feldspars that have high 208Pb/204Pb ratios, in the Eocene-age German Rancho Formation indicates that the basin remained in the vicinity of the Mojave Desert until the Eocene. This result demonstrates that the Gualala Basin had undergone very little northward motion at that time, and consequently requires that no significant northward translation took place prior to ~ 50 Ma. The results of this study highlight the value of additional provenance constraints combined with age distributions when identifying sediment source terranes.

Phylogeography of the ground squirrel subgenusXerospermophilusand assembly of the Mojave Desert biota

AbstractAim The Mohave ground squirrel (Xerospermophilus mohavensis) is one of a few endemic species of the Mojave Desert of south‐western North America. We describe phylogeographic patterns within this species and its sister taxon (Xerospermophilus tereticaudus) and test hypotheses concerning their biogeographical history using genetic signatures of stable versus expanding populations. We compare these patterns with those of other Mojave species to evaluate the role of vicariance in producing phylogeographic structure during the assembly of the Mojave Desert biota.Location The Mojave Desert and adjacent desert regions of south‐western North America.Methods Complete cytochromebgene sequences ofX. mohavensis(46 individuals representing 11 localities) andX. tereticaudus(38 individuals representing 14 localities) were analysed using Bayesian methods to infer phylogenetic relationships. Genetic signals of stable or expanding populations were examined based on the distribution of recent mutations and pairwise differences, as well as with a coalescent‐based approach.Results The two species are reciprocally monophyletic and may have diverged in response to the late Pliocene–early Pleistocene uplift of the Transverse Ranges and Mojave block. Little phylogeographic structure is evident withinX. mohavensis, but there is a signature of northern expansion from a presumably full‐pluvial refugium in the Mojave River basin. Four geographic subgroups are evident withinX. tereticaudus, and there is a signature of northern expansion from a presumably full‐pluvial refugium in the Sonoran coastal plains. Roughly congruent phylogeographic patterns are found within five arid‐adapted taxa, indicating a strong element of vicariance during the assembly of the generally transitional Mojave Desert biota.Main conclusions We present a preliminary model for the historical assembly of the Mojave Desert biota that indicates a strong vicariant element producing autochthonous lineages (includingX. mohavensis) that diverged during the major geological and climatic events of the last 5 Myr. Phylogeographic partitioning within the Mojave Desert underscores the necessity of immediate conservation measures for this unique and fragile arid ecosystem that is locked between two large metropolitan population centres and is the target of continued adverse environmental impact.

Development of the Eastern California Shear Zone — Walker Lane belt: The effects of microplate motion and pre-existing weakness in the Basin and Range

The role that the motion of microplates has played in the development of the Eastern California Shear Zone in western North America is unclear. It was previously proposed that shear related to Baja California motion has propagated from the Gulf of California northward into Nevada. However, there is evidence that the formation of the Eastern California Shear Zone predates the inception of Baja California motion. Thus, either Baja California began to move northwestward prior to 6 Ma, or the Eastern California Shear Zone formed by shear strain resulting from different microplate motion than Baja California. The role of Sierra Nevada motion on the development of an adjacent shear zone remains untested. We present a numerical modeling study that examines the deformational response of western North America to Baja California and/or Sierra Nevada microplate motion. In particular we study if, and under what condition of microplate motion, shear strain can localize in the northern and southern part of the Eastern California Shear Zone (separated by the Garlock fault). We find that given the pre-existing weakness from Miocene Basin and Range extension (a lithospheric strength contrast or normal faults) shear strain from either the Baja California or Sierra Nevada microplates can lead to the formation of the Eastern California Shear Zone. Furthermore, the presence of a pre-existing weakness explains the earlier initiation of strike-slip faulting north of the Garlock fault. Our models suggest that today, Baja California microplate motion is the major player for present-day deformation rates along the Eastern California Shear Zone and was in the past 6 Myrs also a major driver of strain localization in southern Eastern California Shear Zone within the Mojave block.

Quantifying Dextral Shear on the Bristol-Granite Mountains Fault Zone: Successful Geologic Prediction from Kinematic Compatibility of the Eastern California Shear Zone

AbstractFor regional kinematic compatibility to be a valid boundary condition for continental tectonic reconstructions, there must be tests that validate or invalidate kinematic model predictions. In several reconstructions of western North America, the displacement history of the Mojave block continues to be unresolved. The magnitude of displacement along the Bristol-Granite mountains fault zone (BGMFZ), which is the eastern margin of the Eastern California Shear Zone (ECSZ) in the Mojave block, is a key example of a long-standing kinematic prediction that has defied a positive field test until now. The ECSZ is a network of late Neogene and Quaternary right-lateral strike-slip faults that extend from the Gulf of California north through the Mojave Desert, linking Pacific–North America plate motion with Basin and Range extension. This network of faults accounts for ∼15% of post-16-Ma plate transform motion. Geologic estimates of net dextral offset along the Mojave portion of the ECSZ (53 ± 6 km) are appro...

Small‐scale upper mantle convection and crustal dynamics in southern California

We present numerical modeling of the forces acting on the base of the crust caused by small‐scale convection of the upper mantle in southern California. Three‐dimensional upper mantle shear wave velocity structure is mapped to three‐dimensional density structure that is used to load a finite element model of instantaneous upper mantle flow with respect to a rigid crust, providing an estimate of the tractions acting on the base of the crust. Upwelling beneath the southern Walker Lane Belt and Salton Trough region and downwelling beneath the southern Great Valley and eastern and western Transverse Ranges dominate the upper mantle flow and resulting crustal tractions. Divergent horizontal and upward directed vertical tractions create a tensional to transtensional crustal stress state in the Walker Lane Belt and Salton Trough, consistent with transtensional tectonics in these areas. Convergent horizontal and downward directed vertical tractions in the Transverse Ranges cause approximately N–S crustal compression, consistent with active shortening and transpressional deformation near the “Big Bend” of the San Andreas fault. Model predictions of crustal dilatation and the forces acting on the Mojave block compare favorably with observations suggesting that small‐scale upper mantle convection provides an important contribution to the sum of forces driving transpressional crustal deformation in southern California. Accordingly, the obliquity of the San Andreas fault with respect to plate motions may be considered a consequence, rather than a cause, of contractional deformation in the Transverse Ranges, itself driven by downwelling in the upper mantle superimposed on shear deformation caused by relative Pacific–North American plate motion.

Diffuse interseismic deformation across the Pacific–North America plate boundary

Research Article| April 01, 2007 Diffuse interseismic deformation across the Pacific–North America plate boundary Shimon Wdowinski; Shimon Wdowinski 1Division of Marine Geology and Geophysics, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149-1098, USA Search for other works by this author on: GSW Google Scholar Bridget Smith-Konter; Bridget Smith-Konter 2Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, La Jolla, California 92093-0210, USA Search for other works by this author on: GSW Google Scholar Yehuda Bock; Yehuda Bock 2Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, La Jolla, California 92093-0210, USA Search for other works by this author on: GSW Google Scholar David Sandwell David Sandwell 2Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, La Jolla, California 92093-0210, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Shimon Wdowinski 1Division of Marine Geology and Geophysics, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149-1098, USA Bridget Smith-Konter 2Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, La Jolla, California 92093-0210, USA Yehuda Bock 2Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, La Jolla, California 92093-0210, USA David Sandwell 2Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, La Jolla, California 92093-0210, USA Publisher: Geological Society of America Received: 04 May 2006 Revision Received: 15 Nov 2006 Accepted: 20 Nov 2006 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2007) 35 (4): 311–314. https://doi.org/10.1130/G22938A.1 Article history Received: 04 May 2006 Revision Received: 15 Nov 2006 Accepted: 20 Nov 2006 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Shimon Wdowinski, Bridget Smith-Konter, Yehuda Bock, David Sandwell; Diffuse interseismic deformation across the Pacific–North America plate boundary. Geology 2007;; 35 (4): 311–314. doi: https://doi.org/10.1130/G22938A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Crustal movements and deformation within the diffuse Pacific–North America (Pa-NA) plate boundary are dominated by the right-lateral motion between the two plates. By using the Pa-NA pole of rotation (PoR) spherical coordinate system, we decompose observed crustal movements into parallel and normal components to the Pa-NA plate motion. We transformed the 840 velocity vectors of the Southern California Earthquake Center (SCEC) 3.0 velocity field into the Pa-NA PoR system in order to characterize the interseismic velocity across the plate boundary. Our results show that despite the very different deformation styles occurring across the San Andreas fault, the fault trace follows the half plate motion contour. Deviation occurs in the southern section, where the half motion contour correlates with the San Jacinto and Imperial fault segments. Our analysis yields interesting asymmetric patterns in both parallel and normal components. The parallel component shows asymmetrical velocity gradients across the San Andreas fault, and the normal component indicates compression southwest of the Big Bend, but not northeastward. The observations are compared with visco-elastic modeling results, which show a similar velocity field. The main disagreements between the observations and the model are in a narrow band along the San Andreas fault and in the Mojave block, suggesting that crustal heterogeneities and additional unmodeled fault segments should be considered in future models. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Two lithospheric profiles across southern California derived from gravity and seismic data

Abstract We present two detailed 2-D density transects for the crust and uppermost mantle across southern California using a linear gravity inversion technique. This technique parameterizes the crust and upper mantle as a set of blocks that are based on published geologic and seismic models. Each block can have a range of densities that are constrained where possible by borehole measurements, seismic velocities, and petrologic data. To further constrain the models, it is assumed that the lithosphere is close to isostatic equilibrium at both ends of the profiles, in the deep ocean and east of the Mojave Desert. We calculate the lithostatic pressure variations field for the whole cross section to rule out the geophysically insignificant solutions. In the linear equation, ρ = a + bV ( V , seismic P-wave velocity; ρ , density), which approximates the mantle density–velocity ( ρ – V ) relationship, different coefficients for b were evaluated. Lower coefficients ( b b > 0.3) imply that other effects, such as composition and/or metamorphic changes, play an important role in the mantle. Density models were constructed with the coefficient b ranging from 0 to 0.6. The results indicate that a high b value in the mantle ρ – V relationship is associated with less dense crust in the Mojave block and more dense crust in the Catalina schist block. In the less dense Mojave block, the average density of the whole crust is ∼2.75 g/cm 3 , while that of the lower crust is ∼2.72 g/cm 3 . These densities imply a high silica content in the crust, and a minor fraction of basic rock in the lower crust, or perhaps the absence of a basaltic layer altogether. By comparison, the average density of a typical continental stable platform is ∼2.85 g/cm 3 . Models with higher b coefficients (0.5–0.6) are characterized by a large isostatic imbalance. On the other hand, lower b values (0–0.2) require a consolidated whole crust density in the Mojave Desert of ∼2.78 g/cm 3 , and a lower crust density of ∼2.89 g/cm 3 with mostly basaltic composition. This contradicts the observed, lower V p / V s -ratio in the Mojave Desert associated with mostly felsic and low-density crust. Models with lower b coefficients (0.1–0.2) are characterized by an absence of local Airy compensation beneath the San Gabriel Mountains at the LARSE-1 profile. These, and other non-gravity arguments, suggest optimal solutions to the mantle ρ – V relation of b ∼ 0.2–0.4. This, in turn, means that both thermal and petrological effects occur inside the downwelling of the uppermost mantle high velocity body located beneath the Transverse Ranges. During the development of this mantle downwelling, the basaltic layer of the Mojave block was likely eroded and pulled down into the high velocity body. Those basaltic fragments may have been transformed into eclogites, and this metamorphic change implies a higher b -coefficient density–velocity relationship than would be expected for a purely thermal process.

Rayleigh wave phase velocities, small‐scale convection, and azimuthal anisotropy beneath southern California

We use Rayleigh waves to invert for shear velocities in the upper mantle beneath southern California. A one‐dimensional shear velocity model reveals a pronounced low‐velocity zone (LVZ) from 90 to 210 km. The pattern of velocity anomalies indicates that there is active small‐scale convection in the asthenosphere and that the dominant form of convection is three‐dimensional (3‐D) lithospheric drips and asthenospheric upwellings, rather than 2‐D sheets or slabs. Several of the features that we observe have been previously detected by body wave tomography: these anomalies have been interpreted as delaminated lithosphere and consequent upwelling of the asthenosphere beneath the eastern edge of the southern Sierra Nevada and Walker Lane region; sinking lithosphere beneath the southern Central Valley; upwelling beneath the Salton Trough; and downwelling beneath the Transverse Ranges. Our new observations provide better constraints on the lateral and vertical extent of these anomalies. In addition, we detect two previously undetected features: a high‐velocity anomaly beneath the northern Peninsular Range and a low‐velocity anomaly beneath the northeastern Mojave block. We also estimate the azimuthal anisotropy from Rayleigh wave data. The strength is ∼1.7% at periods shorter than 100 s and decreases to below 1% at longer periods. The fast direction is nearly E‐W. The anisotropic layer is more than 300 km thick. The E‐W fast directions in the lithosphere and sublithosphere mantle may be caused by distinct deformation mechanisms: pure shear in the lithosphere due to N‐S tectonic shortening and simple shear in sublithosphere mantle due to mantle flow.

Interseismic strain and rotation rates in the northeast Mojave domain, eastern California

The northeast Mojave domain, a type locality for bookshelf faulting, is a region of east striking, left‐lateral faults in the northeast corner of the Mojave block, a block otherwise dominated by ∼N40°W striking, right‐lateral faults. Paleomagnetic evidence suggests that blocks within the domain have rotated clockwise about a vertical axis as much as 60° since 12.8 Ma [Schermer et al., 1996]. In 1994, and again in 2002, the U.S. Geological Survey surveyed an array of 14 geodetic monuments distributed across the northeast Mojave domain. The 2002 survey results were adjusted to remove the coseismic offsets imposed by the nearby Hector Mine earthquake (16 October 1999, Mw = 7.1). The adjusted deformation across the array appears to be uniform and can be approximated by the principal strain rates ε1 = 28.9 ± 9.1 N77.2°W ± 4.8° and ε2 = −48.2 ± 8.9 N12.8°E ± 4.8° nstrain yr−1; extension reckoned positive, and quoted uncertainties are standard deviations. That strain accumulation could be released by slip on faults striking N32°W but not by bookshelf faulting on the east striking faults alone. The vertical axis rotation rate of the northeast Mojave domain as a whole relative to fixed North America is 71.0 ± 6.4 nrad yr−1 (4.07° ± 0.37° Myr−1) clockwise, about twice the maximum tensor shear strain rate. The observed rotation rate acting over 12.8 Myr would produce a clockwise rotation of 52.1° ± 4.7°, exclusive of possible coseismic rotations. That rotation is in rough agreement with the paleomagnetic rotation accumulated in the individual fault blocks within the northeast Mojave domain since 12.8 Ma.

Thermal Constraints on Earthquake Depths in California

The high-quality databases for California seismicity (from the Southern and Northern California Earthquake Centers) and an extensive compilation of thermal measurements in California are used to quantify the concept of temperature as a fundamental parameter for determining the thickness of the seismogenic zone. The base of this zone, below which only a small percentage of crustal earthquakes occur, is termed the “cutout depth,” and it is at or near the brittle-ductile transition in the crust. Based on laboratory deformation studies, this transition should be temperature, strain rate, lithology, and stress-state dependent. In this study, qualitative comparisons between the heat flow in California and earthquake hypocentral distributions show first that, as expected, earthquake cutout depths are inversely related to heat flow. Second, the epicentral distributions tend to parallel thermal transitions. This correlation is probably related to stress concentrations in these locations. An ancillary observation is that the cooler western Mojave block in southern California appears to be behaving similarly to the Tibetan indenter block in Asia, as faults tend to go around it into areas of higher heat flow where the seismogenic zone is thinner and the crust may be weaker, and it is pushed toward (and under) the southern cold Sierra Nevada block. Third, to quantitatively compare the data sets, temperatures for the seismicity cutout depth in California were calculated using the steady-state, 1D heat conduction equation, with the variables based on published values for heat flow, heat generation, and thermal conductivity. The analysis was restricted to profiles along which the heat flow and earthquake databases were constrained, which allowed the error in the temperature calculation to be determined using Monte Carlo simulations. The results show that two distinct ranges of temperatures (dependent upon location) describe the cutout depth ( D 99%) of seismicity for California: 450 ± 50°C and 260 ± 40°C. The 450 ± 50°C cutout depth temperature is most widespread geographically, occurs within many provinces, and is higher than the frequently referenced temperature of 300 ± 50°C for the brittle-ductile transition. The lower temperature (260°C) seems to be restricted to provinces where the heat flow is near or below 50 mW m-2, such as the Mojave block and Sierra Nevada. The differences in these cutout temperatures suggest that additional factors, such as strain rate, stress regime, and/or lithology, may contribute to the seismicity cutout depth. Fourth, along a profile with significant seismic activity and a 450°C cutout temperature, the envelope for the maximum energy released by earthquakes falls at or below the 300°C isotherm. Detailed characterization of the heat flow and earthquake synergy in this manner furthers the understanding of the earthquake process and can aid in the estimation of the maximum depth of rupture for great earthquakes, particularly in areas of low seismicity, thus reducing uncertainties in hazard calculations. Manuscript received 26 February 2003.