Salton Trough Research Articles

Seismic velocity structures in the southern California plate-boundary environment from double-difference tomography

SUMMARY We present tomographic images of crustal structures in the southern California plate-boundary area, with a focus on the San Jacinto fault zone (SJFZ), based on double-difference inversions of earthquake arrival times. Absolute arrival times of 247 472 P and 105 448 S wave phase picks for 5493 earthquakes recorded at 139 stations in southern California are used. Starting with a layered 1-D model, and continuing in later iterations with various updated initial models, we invert the data for Vp and Vs in a 270 km long, 105 km wide and 35 km deep volume around the SJFZ using a spatially variable grid with higher density around the SJFZ. The examined volume stretches from Cajon Pass to the northernmost Imperial Fault Zone and includes portions of the southern San Andreas Fault (SAF), the Elsinore Fault and the Brawley Seismic Zone in the Salton Trough. Because differential traveltimes used in the double-difference inversions are most sensitive to near-source structures, we obtain high resolution around the earthquake sources. After 30 iterations we improve the average traveltime misfit by a factor of 16. Though ray coverage is limited at shallow depths, we obtain detailed images of seismic velocities from 3 to 20 km throughout much of the study area. Our final velocity results show zones of low-velocity and anomalous Vp/Vs ratios associated with various fault strands and sedimentary basins, along with clear velocity contrasts across the SJFZ and the southern SAF. The velocity reductions in fault zone regions are generally highest in geometrically complex areas (up to 30–50 per cent in the top few kilometres), are higher for Vs than for Vp, and follow a flower-type pattern with depth. In the central section of the SJFZ, from the San Jacinto valley to the trifurcation area, the northeast side of the fault has generally higher seismic velocities than the southwest block. The obtained contrasts of Vp are more persistent and higher (up to 20 per cent) than the contrasts of Vs (up to 15 per cent), although the differences may stem (at least partially) from the higher resolution of Vp images. In the SJFZ sections to the northwest and southeast, there are patches with reversed velocities contrasts especially in the shallow crust near the San Jacinto Valley and other basins. Along the Banning fault there is no clear velocity contrast. For the southern SAF, the northeast side has generally lower seismic velocities in the seismogenic zone with patches of contrast reversals in the shallow crust. In the Brawley Seismic Zone, the northeast side has somewhat lower velocities in the ∼20 km section near the southern SAF and higher velocities farther to the southwest. The imaged features have important implications for various aspects of earthquake and crustal dynamics in the region.

Spatial migration of earthquakes within seismic clusters in Southern California: Evidence for fluid diffusion

Seismicity within many earthquake swarms is observed to migrate slowly with time, which may reflect event triggering due to slow fault slip or fluid flow. We search for this behavior in Southern California by applying a weighted least squares method to quantify event migration within 69 previously observed seismicity bursts. We obtain best‐fitting migration directions and velocities, and compute a statistical migration significancesm for each burst using a bootstrap resampling method. We define 37 bursts with sm ≥ 0.8 as the migration group, and 32 bursts with sm < 0.8 as the non‐migration group. To explore differences between the two groups, for each burst we compute effective stress drop (Δσquasi, the ratio between total moment and radius), the skew of the moment release time series (μ), the timing of the largest event (tmax), and the distance separation between the first half and second half of the sequence (ds). As expected, the migration group features larger ds and lower Δσquasi, consistent with higher migration significance. It also features lower μ and higher tmax, similar to observations from swarms in the Salton Trough, while the non‐migration group is more similar to main shock‐aftershock sequences. To explore possible fluid involvement, we model the migration behavior with the fluid diffusion equation, and identify 18 bursts with diffusion coefficients ranging from 0.01 to 0.8 m2/s, with the majority below 0.16 m2/s. The obtained diffusion coefficients and migration behavior are similar to the Reservoir‐induced seismicity beneath the Açu reservoir in Brazil. The majority of normal faulting events are associated with these 18 bursts, while the non‐migration group has the most reverse faulting events, indicating a possible link between sequence type and focal mechanism.

Initiation of the Southern Elsinore Fault at ∼1.2 Ma: Evidence from the Fish Creek–Vallecito Basin, southern California

Strike‐slip faults west of the San Andreas fault (SAF) accommodate more than half of the modern Pacific–North America plate motion in southern California, yet their age, kinematic evolution and long‐term slip rates are incompletely understood. The southern Elsinore fault is part of this zone and occupies the boundary between Mesozoic crystalline rocks of the Peninsular Ranges and Late Cenozoic sedimentary rocks of the Salton Trough depocenter. The Fish Creek–Vallecito Basin, a recently inverted western sub‐basin of the Salton Trough, contains a thick succession of Plio‐Pleistocene sedimentary rocks that preserve a record of crustal subsidence and uplift in response to evolving tectonic controls. Deposits older than ∼1.2 Ma accumulated during transtension and subsidence in the hanging wall of the low‐angle West Salton detachment fault. A fanning‐dip interval high in the section, dated at 1.2–1.0 Ma, accumulated during tilting caused by earliest motion on the Elsinore and Earthquake Valley faults. Total offset of 1–2 km yields a lifetime slip rate of about 1–2 mm/yr for the southern Elsinore fault, similar to a late Pleistocene rate in the Coyote Mountains but slower than published estimates for the northern Elsinore fault. This difference likely results from long‐term transfer of slip from the northern Elsinore fault to the San Felipe and San Jacinto fault zones farther east. Initiation of the Elsinore fault at 1.2 Ma coincides with a major tectonic reorganization that terminated slip on the detachment fault and formed the modern network of strike‐slip faults west of the SAF.

The Role of Fluids in Promoting Seismic Activity in Active Spreading Centers of the Salton Trough, California, USA

We interpret seismic activity in the active spreading centers of the Salton Trough to indicate 1) a magmatic intrusion in the lower crust beneath the active Brawly, Cerro Prieto, Imperial, Elsinore, and San Jacinto fault systems; and 2) fluids in the upper crust that have been released from that magmatic body. The absence of a magmatic body and fluids at the location of fossil spreading centers along the Sand Hill and Algodones faults ndicated by little or no seismic activity in those areas. We show several lines of evidence to point out that both melt and fluids related to the seismic activity. In particular, receiver function analysis, Vp/Vs ratios, and tomographic data reveal low velocity zones coincide with the location of the active spreading centers. High Vp/Vs ratios and low velocity zones in the lower crust and upper mantle attributed to melt inclusion, while low Vp/Vs ratios in the upper crust are attributed water inclusions. Frequency-mag- nitude distributions characterized by high b-values in southern California; high b-values have also been associated with crustal fluids. A crustal scale model developed from the receiver functions, gravity, and magnetic data supports the existence of a magmatic intrusion within about 20 km of the surface southwest of the Salton Sea, that intrusion extends for 70 km in a SW-NE direction.

Tomography of crustalPandStravel times across the western United States

[1] Data from the USArray combined with local, regional, and national networks provide an unprecedented opportunity for imaging the crust of the western United States. We employ a simple tomography scheme to invert Pg and Sg travel times for velocity structure. The approach is analogous to Pn tomography: we assume a great circle arc between source and receiver and solve for station and event terms along with 2-D velocity structure. Pg velocities are high in the Snake River Plain, western Washington and the Columbia Plateau, and in central California. The Colorado Plateau falls midrange while the Basin and Range is slow, as are the Geysers region north of the San Francisco Bay and the Salton Trough. For Sg, the Snake River plain is fast, as is the entire Cordillera from the Southern Sierra Nevada up through the Cascades. Both the Colorado Plateau and most of the Rocky Mountains are also fast. At the highest point of the Rocky Mountains, the Aspen Anomaly is a low-velocity region. The Basin and Range is a composite of slower to the west abutting the Sierra Nevada and faster in eastern Nevada. Coastal California is slow, with the exception of the San Francisco Bay region and far northern California. The continental crust of the North American craton appears slightly fast for both Pg and Sg. Our observations and their trends compare well with middle to upper crustal velocity estimates from previous studies. They also compare reasonably well with average crustal velocities from the same studies.

A unified analysis of crustal motion in Southern California, 1970-2004: The SCEC crustal motion map

[1] To determine crustal motions in and around southern California, we have processed and combined trilateration data collected from 1970 to 1992, VLBI data from 1979 to 1992, and GPS data from 1986 to 2004: a long temporal coverage required in part by the occurrence of several large earthquakes in this region. From a series of solutions for station positions, we have estimated interseismic velocities, coseismic displacements, and postseismic motions. Within the region from 31°N to 38°N. and east to 114°W, the final product includes estimated horizontal velocities for 1009 GPS, 190 trilateration, and 16 VLBI points, with ties between some of these used to stabilize the solution. All motions are relative to the Stable North American Reference Frame (SNARF) as realized through the velocities of 20 GPS stations. This provides a relatively dense set of horizontal velocity estimates, with well-tested errors, for the past quarter century over the plate boundary from 31°N to 36.5°N. These velocities agree well with those from the Plate Boundary Observatory, which apply to a later time period. We also estimated vertical velocities, 533 of which have errors below 2 mm/yr. Most of these velocities are less than 1 mm/yr, but they show 2–4 mm/yr subsidence in the Ventura and Los Angeles basins and in the Salton Trough. Our analysis also included estimates of coseismic and postseismic motions related to the 1992 Landers, 1994 Northridge, 1999 Hector Mine, and 2003 San Simeon earthquakes. Postseismic motions increase logarithmically over time with a time constant of about 10 days, and generally mimic the direction and relative amplitude of the coseismic offsets.

Comprehensive analysis of earthquake source spectra and swarms in the Salton Trough, California

[1] We study earthquakes within California's Salton Trough from 1981 to 2009 from a precisely relocated catalog. We process the seismic waveforms to isolate source spectra, station spectra and travel-time dependent spectra. The results suggest an average P wave Q of 340, agreeing with previous results indicating relatively high attenuation in the Salton Trough. Stress drops estimated from the source spectra using an empirical Green's function (EGF) method reveal large scatter among individual events but a low median stress drop of 0.56 MPa for the region. The distribution of stress drop after applying a spatial-median filter indicates lower stress drops near geothermal sites. We explore the relationships between seismicity, stress drops and geothermal injection activities. Seismicity within the Salton Trough shows strong spatial clustering, with 20 distinct earthquake swarms with at least 50 events. They can be separated into early-Mmax and late-Mmax groups based on the normalized occurrence time of their largest event. These swarms generally have a low skew value of moment release history, ranging from −9 to 3.0. The major temporal difference between the two groups is the excess of seismicity and an inverse power law increase of seismicity before the largest event for the late-Mmax group. All swarms exhibit spatial migration of seismicity at a statistical significance greater than 85%. A weighted L1-norm inversion of linear migration parameters yields migration velocities from 0.008 to 0.8 km/hour. To explore the influence of fluid injection in geothermal sites, we also model the migration behavior with the diffusion equation, and obtain a hydraulic diffusion coefficient of approximately 0.25 m2/s for the Salton Sea geothermal site, which is within the range of expected values for a typical geothermal reservoir. The swarms with migration velocities over 0.1 km/hour cannot be explained by the diffusion curve, rather, their velocity is consistent with the propagation velocity of creep and slow slip events. These variations in migration behavior allow us to distinguish among different driving processes.

Detecting aseismic strain transients from seismicity data

[1] Aseismic deformation transients such as fluid flow, magma migration, and slow slip can trigger changes in seismicity rate. We present a method that can detect these seismicity rate variations and utilize these anomalies to constrain the underlying variations in stressing rate. Because ordinary aftershock sequences often obscure changes in the background seismicity caused by aseismic processes, we combine the stochastic Epidemic Type Aftershock Sequence model that describes aftershock sequences well and the physically based rate- and state-dependent friction seismicity model into a single seismicity rate model that models both aftershock activity and changes in background seismicity rate. We implement this model into a data assimilation algorithm that inverts seismicity catalogs to estimate space-time variations in stressing rate. We evaluate the method using a synthetic catalog, and then apply it to a catalog of M ≥ 1.5 events that occurred in the Salton Trough from 1990 to 2009. We validate our stressing rate estimates by comparing them to estimates from a geodetically derived slip model for a large creep event on the Obsidian Buttes fault. The results demonstrate that our approach can identify large aseismic deformation transients in a multidecade long earthquake catalog and roughly constrain the absolute magnitude of the stressing rate transients. Our method can therefore provide a way to detect aseismic transients in regions where geodetic resolution in space or time is poor.

Crustal geophysics and seismicity in southern California

The geographical distribution of the (1981–2005) seismicity in southern California forms a ±150 km broad zone adjacent to the Pacific–North America plate boundary, ranging from depths of ∼1–~30 km, with the bulk of the focal depths in the range of 2–12 km. The distribution of the seismicity that includes both mainshock–aftershock sequences and background events is affected by both static and kinematic geophysical parameters of the crust. The static parameters include heat flow, topography, crustal density, V_(p)/V_(s) ratio, hypocentral fault-distance and crustal thickness from receiver functions. The tectonic loading is represented by kinematic parameters such as the crustal shear strain rate field, and the dilatational strain rate field. In our analysis, we normalize the seismicity relative to the areal density of the range of values of each of the parameters. Most of the seismicity occurs in areas of average heat flow, low to intermediate topography, average Vp/Vs and high late Quaternary fault density, and forms seismogenic zones that extend through the brittle crust. The location of late Quaternary faults, often described as zones of weakness, influences the geographical distribution of seismicity more than any other parameter. Although above or below average crustal properties such as high heat flow, thin crust or very low V_(p)/V_(s) values exist, these properties only influence the spatial distribution of seismicity in a minor way. As an example, the Salton Trough area of low topography, high heat flow, high V_(p)/V_(s), high shear strain rate and thin crust has distributed seismicity within a thin seismogenic zone. Also, somewhat surprisingly, areas of high topography, low heat flow, low V_(p)/V_(s), low shear strain rate and thick crust have low seismicity rates but a thin seismogenic zone. We determine an empirical relationship between heat flow and crustal thickness to show how the ~400 °C temperature isotherm gradually deepens with crustal thickness and forms the base of the seismogenic zone for crustal thicknesses from 22 to 37 km. For crustal thickness ranging from 37 to 43 km, the ~250 °C isotherm forms the base of the seismogenic zone, suggesting that seismic faulting in these regions is confined to the top of the upper crust (12–14 km), and thus does not accommodate plate motion.

Local Magnitude Tomography in California

Lateral variation in crustal attenuation of California is calculated by inverting 25,330 synthetic Wood–Anderson amplitudes from the California Integrated Seismic Network (CISN) for site, source, and path effects. Two-dimensional attenuation ( q or 1/ Q ) is derived from the path term, which is calculated via an iterative least-squares inversion that also solves for perturbations to the site and source terms. Source terms agree well with initial CISN M L s, and site terms agree well with a prior regression analysis; q ranges from low attenuation at 0.001 ( Q =1000) to high attenuation at 0.015 ( Q =66), with an average of 0.07 ( Q =143). The average q is consistent with an amplitude decay function (log A ) for California when q is combined with a simple geometrical spreading rate. Attenuation in California is consistent with the tectonic structure of California, with low attenuation in the Sierra batholith and high attenuation at The Geysers, at Long Valley, and in the Salton trough possibly due to geothermal effects. Also, path terms are an order of magnitude smaller than site and source terms, suggesting that they are not as important in correcting for M L .

Stratigraphic record of basin development within the San Andreas fault system: Late Cenozoic Fish Creek-Vallecito basin, southern California

The Fish Creek–Vallecito basin contains a 5.5-km-thick section of late Miocene to early Pleistocene sedimentary rocks exposed in the hanging wall of the West Salton detachment fault. These deposits preserve a high-fidelity record of late Cenozoic subsidence and basin filling that resulted from deformation in the San Andreas fault system of southern California. Existing and new paleomagnetic data, combined with new U-Pb zircon ages of two tuffs high in the section, show that the section ranges in age from ca. 8.0 ± 0.4 Ma at the base to ca. 0.95 Ma at the top. Geohistory analysis reveals: (1) moderate subsidence (0.46 mm/yr) from ca. 8.0 to 4.5 Ma; (2) rapid subsidence (2.1 mm/yr) from 4.5 to 3.1 Ma; (3) moderate subsidence (0.40 mm/yr) from 3.1 to 0.95 Ma; and (4) rapid uplift and erosion that has exhumed the section since ca. 1 Ma. Onset of sedimentation at ca. 8.0 ± 0.4 Ma records earliest extension or transtension in the area, possibly related to localization of the Pacific–North America plate boundary in the Salton Trough and Gulf of California. Alternatively, marine incursion at 6.3 Ma may be the earliest record of plate-boundary deformation in the Gulf of California–Salton Trough region. A thick interval higher in the section records progradation of the Colorado River delta into and across the basin starting ca. 4.9 Ma. Progradation continued during an abrupt increase in subsidence rate at 4.5 Ma, and fluvial-deltaic conditions persisted for 1.4 m.y. during the rapid-subsidence phase, indicating that delta progradation was driven by a large increase in rate of sediment input from the Colorado River. Uplift and inversion of the basin starting ca. 1.0 Ma record initiation of strike-slip faults that define the modern phase of dextral wrench tectonics in the western Salton Trough.

Role of sedimentation in continental rifting from comparing two narrow rift valleys the Salton Trough and Death Valley-California

To unravel the forces and better understand the processes that drive continental rifting, and to understand the role of sedimentation in promoting the rifting process, we compare; the different geological features of two narrow rifts, the Salton Trough and Death Valley, California. According to our models, the Moho is 22 km deep to the southwest of the Salton Sea on US-Mexico border and it deepens to 30 km in the region west of the Salton Trough. In Death Valley, the Moho is 24 km deep in the central part of the basin and it deepens to 32 km outside of the basin. The dome shaped Moho in both rifts is suggested to be primarily the product of magmatic activity in the lower crust and upper mantle. Death Valley is narrow rift in the initial stage of rifting with several sedimentary basins 2 - 4 km deep. In Death Valley magmatic (thermal) forces appears to drive the rifting process. The Salton Trough is wider than Death Valley and is moving toward sea floor spreading. The depth of the sedimentary basins ranges from 8 - 10 km and a combination of thermal and sedimentation appears to drive rifting processes in the Salton Trough.

Crustal Structure of the Salton Trough: Incorporation of Receiver Function, Gravity and Magnetic Data

The Salton Trough of southwestern California is inferred to be an incipient ocean basin, and is a polyphase basin with significant extension in addition to dextral shear. To further explore the origin and evolution of this basin, we have incorporated receiver function, gravity, and aeromagnetic data to construct new subsurface crustal scale models. Receiver function analysis suggests the Moho is 20 km deep to the southwest of the Salton Sea and deepens to 32 km in the region east of the Salton Trough and dome in shape. Crustal modeling shows that the density of the lower crust is 2950 kg/m3, which is an indication for gabbroic com- position, while the density of the upper crust varies from 2500 kg/m3 to 2600 kg/m3 and the depth of sedi- mentary and meta-sedimentary rocks appears to be 8 - 10 km. Most magnetic anomalies show shallow relief and are low amplitude with some exceptions in the marginal areas, suggesting the absence of shallow buried mafic intrusions and deep basement. Our models show a magmatic body to the southwest of the Salton Sea at depth of about 18 km and extend in SW-NE direction for about 90 km, We expect this magmatic body (mix- ture of lower crust and upper mantle material) is responsible for crustal thinning, stretching and rifting, ac- cording to the crustal models this body doesn’t exist in the north region of Salton Trough, thus, no further propagate of the rift is expected in the north.

Crustal shear wave velocity structure of the western United States inferred from ambient seismic noise and earthquake data

Surface wave dispersion measurements from ambient seismic noise and array‐based measurements from teleseismic earthquakes observed with the EarthScope/USArray Transportable Array (TA) are inverted using a Monte Carlo method for a 3‐D VS model of the crust and uppermost mantle beneath the western United States. The combination of data from these methods produces exceptionally broadband dispersion information from 6 to 100 s period, which constrains shear wave velocity structures in the crust and uppermost mantle to a depth of more than 100 km. The high lateral resolution produced by the TA network and the broadbandedness of the dispersion information motivate the question of the appropriate parameterization for a 3‐D model, particularly for the crustal part of the model. We show that a relatively simple model in which VS increases monotonically with depth in the crust can fit the data well across more than 90% of the study region, except in eight discrete areas where greater crustal complexity apparently exists. The regions of greater crustal complexity are the Olympic Peninsula, the MendocinoTriple Junction, the Yakima Fold Belt, the southern Cascadia back arc, the Great Central Valley of California, the Salton Trough, the Snake River Plain, and the Wasatch Mountains. We also show that a strong Rayleigh‐Love discrepancy exists across much of the western United States, which can be resolved by introducing radial anisotropy in both the mantle and notably the crust. We focus our analysis on demonstrating the existence of crustal radial anisotropy and primarily discuss the crustal part of the isotropic model that results from the radially anisotropic model by Voigt averaging. Model uncertainties from the Monte Carlo inversion are used to identify robust isotropic features in the model. The uppermost mantle beneath the western United States is principally composed of four large‐scale shear wave velocity features, but lower crustal velocity structure exhibits far greater heterogeneity. We argue that these lower crustal structures are predominantly caused by interactions with the uppermost mantle, including the intrusion and underplating of mafic mantle materials and the thermal depression of wave speeds caused by conductive heating from the mantle. Upper and middle crustal wave speeds are generally correlated, and notable anomalies are inferred to result from terrane accretion at the continental margin and volcanic intrusions.

Late Quaternary slip rate gradient defined using high‐resolution topography and 10Be dating of offset landforms on the southern San Jacinto Fault zone, California

Recent studies suggest the San Jacinto fault zone may be the dominant structure accommodating PA‐NA relative plate motion. However, because the late Quaternary slip history of the southern San Andreas fault system is insufficiently understood, it is difficult to evaluate the partitioning of deformation across the plate boundary and its evolution. Landforms displaced by the Clark fault of the southern San Jacinto fault zone were mapped using high‐resolution airborne laser‐swath topography and selected offset landforms were dated using cosmogenic 10Be. Beheaded channels at Rockhouse Canyon, displaced by 500 ± 70 m and 220 ± 70 m, have been dated to 47 ± 8 ka and 28 ± 9 ka, respectively. Farther south, near the southern Santa Rosa Mountains, an alluvial deposit displaced by 51 ± 9 m has been dated to 35 ± 7 ka. From these sites, the slip rate of the Clark fault is determined to diminish southward from 8.9 ± 2.0 to 1.5 ± 0.4 mm/yr. This implies a slip‐rate decrease along the Clark fault from Anza southeastward to its surface termination near the Salton Trough, where slip is transferred to the Coyote Creek fault, and additional deformation is compensated by folding and thrusting in the basin. These data suggest that since ∼30 to 50 ka, the slip rate along the southern San Jacinto fault zone has been lower than, or equivalent to, the rate along the southernmost San Andreas fault. Accordingly, either the slip rate of the San Jacinto fault has substantially decreased since fault initiation, or fault slip began earlier than previously suggested.

Complex subduction and small-scale convection revealed by body-wave tomography of the western United States upper mantle

New high-resolution P- and S-wave tomography of the United States upper mantle from the Pacific Coast to the Great Plains reveals strong multi-scale heterogeneity closely correlated with tectonic and magmatic activity. We invert teleseismic travel-time residuals from the EarthScope Transportable Array and more than 1700 additional temporary and permanent stations for 3-D velocity perturbations to a depth of 1000 km. The inversion uses recent advances in western U.S. crust models to better isolate the mantle component of travel-time residuals, and frequency-dependent 3-D sensitivity kernels to map travel-time residuals, measured in multiple frequency bands, into velocity structure. In addition to separate V P and V S models, we jointly invert the two datasets for V P/V S perturbations by imposing a smoothness constraint on the δ lnV S/δ lnV P field. The joint inversion helps us identify regions where partial melt is probable. The amplitude of V P, V S, and V P/V S variations is greatest in the upper 200 km of the mantle and the form of velocity anomalies suggests a provincially heterogeneous lithosphere and the occurrence of widespread small-scale convection. Partially molten mantle is inferred beneath Yellowstone and the eastern Snake River Plain (SRP), the Salton Trough, and the Clear Lake volcanic field. The inferred depth extent of partial melt is consistent with a generally hydrated upper mantle and elevated temperatures beneath the eastern SRP and Yellowstone. Despite continuous subduction since the Cretaceous, the distribution of sub-lithospheric high-velocity anomalies is dissected (similar to other recent studies). Based on our new tomography models, western U.S. geologic history, and plate–tectonic reconstructions, we infer patchy and incomplete removal of the flat-subducting Laramide slab and slab tearing associated with Eocene accretion in the northwestern U.S.

Sedimentation and crustal recycling along an active oblique-rift margin: Salton Trough and northern Gulf of California

Transtensional basins embedded in the San Andreas fault system of Southern California (United States) and northwestern Mexico are fi lled with sediment derived from the Colorado River, which drains a large area of the western U.S. interior. The sediment is rapidly buried, heated, and mingled with intrusions in the deep basins to form a new generation of recycled crust along the active plate boundary. Using a range of values for total basin depth, relative volume of mantle-derived intrusions, and composition of early rift deposits, the volume of Colorado River‐derived sediment in the basins is bracketed between 2.2 and 3.4 ◊ 10 5 km 3 , similar to the volume of rock that likely was eroded from the Colorado River catchment over the past 5‐6 m.y. The volumetric rate of crustal growth by sedimentation is ~80‐130 km 3 /m.y./km, comparable to growth rates in subductionrelated island arcs and slow seafl oor spreading centers. Sedimentary and basinal processes thus play a major role in crustal evolution and recycling in this setting, and may be important at other rifted margins where a large river system is captured following tectonic collapse of a prerift orogenic highland.

Seismic heterogeneity and small‐scale convection in the southern California upper mantle

We invert traveltime residuals of teleseismic P and S phases for 3‐D perturbations in VP, VS, and VP/VS structures in the southern California upper mantle. The tomographic inversion uses frequency‐dependent 3‐D sensitivity kernels to interpret traveltime residuals measured in multiple frequency bands and recent advances in regional crustal thickness and velocity models to better isolate the mantle component of traveltime residuals. In addition to separate VP and VS tomography, we jointly invert the P and S data sets for VP/VS perturbations by imposing a smoothness constraint on the δlnVS/δlnVP field. The regional upper mantle is very heterogeneous with the greatest amplitude of velocity and VP/VS perturbations in the upper 200 km and mantle structures that correlate spatially with the major physiographic provinces of southern California. Our imaging improves the resolution of the geometry and amplitude of these features. In addition to the major structures imaged and discussed in previous papers, we find a large high‐velocity anomaly at depths between ∼340 and 500 km beneath the Borderland, Transverse Ranges, and northern Peninsular Ranges. This anomaly is separate from the well‐known uppermost mantle Transverse Ranges anomaly, which extends no deeper than ∼175–200 km. The strongest low‐velocity, high VP/VS anomaly is found beneath and west of the Salton Trough and is attributed to relatively high partial melt in the asthenosphere as a result of lithospheric thinning. The magnitudes of VP, VS, and VP/VS perturbations and knowledge of regional 1‐D velocity and attenuation lead us to conclude that the asthenosphere contains up to ∼1% partial melt extending to depths as great as 150–200 km.

Phylogeography and systematics of the San Diego pocket mouse (Chaetodipus fallax)

The systematics and phylogeography of the San Diego pocket mouse (Chaetodipus fallax), a species restricted to the Baja California Peninsula and adjacent southwestern California, were assessed using sequences of the mitochondrial cytochrome-b gene (Cytb). Genetic relationships were evaluated among the 6 recognized subspecies of C. fallax (including the island population, C. f. anthonyi) in 3 geographic regions from individuals representing 22 populations. Analysis of molecular variance and multiple phylogenetic analyses indicated 3 main clades: northern populations in the southwestern Mojave Desert and Los Angeles Basin north of the Salton Trough; central populations from south of the Salton Trough and throughout the state of Baja California; and southern populations from west of the Vizcao ´no Desert in Baja California Sur and adjacent Isla Cedros. These clades do not correspond to the currently recognized subspecies, and each could be considered a distinct subspecies pending analysis of nuclear DNA or characters (e.g., morphology) encoded by nuclear DNA. DOI: 10.1644/09-MAMM-A-135.

Quaternary rift flank uplift of the Peninsular Ranges in Baja and southern California by removal of mantle lithosphere

Regional uplift in southern California, USA, and northern Baja California, Mexico, is interpreted to result from flexure of the elastic lithosphere driven largely by heating and thinning of the upper mantle beneath the Gulf of California and eastern Peninsular Ranges. The geometry and timing of faulting in the Salton Trough and Gulf of California, the history of recent rock uplift along the Pacific coastline, and geophysical data constrain models of lithospheric heating and thinning based on unloading of a continuous elastic plate. High topography that marks the ∼400‐km‐long rift shoulder in northern Baja California mimics the pattern of uplift observed along the Pacific coastline as defined by marine terraces. We interpret this to indicate that recent rock uplift has occurred across the entire width of northern Baja Peninsula and increases from west to east. Pliocene strata deposited at sea level along the Pacific coastline in southern California have not been uplifted significantly above Quaternary marine terrace deposits. This suggests the onset of rock uplift along the Pacific coast here is post‐Pliocene and occurs after Miocene crustal extension in the Salton Trough and Gulf of California. Strong heating of the mantle lid beneath the Peninsular Ranges in northern Baja California thus coincides with crustal extension limited to localized oceanic spreading in the Gulf of California.