Salton Trough Research Articles

A silent Mw 4.7 slip event of October 2006 on the Superstition Hills fault, southern California

During October 2006, the 20‐km‐long Superstition Hills fault (SHF) in the Salton Trough, southern California, slipped aseismically, producing a maximum offset of 27 mm, as recorded by a creepmeter. We investigate this creep event as well as the spatial and temporal variations in slip history since 1992 using ERS‐1/2 and Envisat satellite data. During a 15‐year period, steady creep is punctuated by at least three events. The first two events were dynamically triggered by the 1992 Landers and 1999 Hector Mine earthquakes. In contrast, there is no obvious triggering mechanism for the October 2006 event. Field measurements of fault offset after the 1999 and 2006 events are in good agreement with the interferometric synthetic aperture radar data indicating that creep occurred along the 20‐km‐long fault above 4 km depth, with most of the slip occurring at the surface. The moment released during this event is equivalent to a Mw 4.7 earthquake. This event produced no detectable aftershocks and was not recorded by the continuous GPS stations that were 9 km away. Modeling of the long‐term creep from 1992 to 2007 creep using stacked ERS‐1/2 interferograms also shows a maximum creep depth of 2–4 km, with slip tapering with depth. Considering that the sediment thickness varies between 3 km and 5 km along the SHF, our results are consistent with previous studies suggesting that shallow creep is controlled by sediment depth, perhaps due to high pore pressures in the unconsolidated sediments.

Earthquake swarms on transform faults

SUMMARY Swarm-like earthquake sequences are commonly observed in a diverse range of geological settings including volcanic and geothermal regions as well as along transform plate boundaries. They typically lack a clear mainshock, cover an unusually large spatial area relative to their total seismic moment release, and fail to decay in time according to standard aftershock scaling laws. Swarms often result from a clear driving phenomenon, such as a magma intrusion, but most lack the necessary geophysical data to constrain their driving process. To identify the mechanisms that cause swarms on strike-slip faults, we use relative earthquake locations to quantify the spatial and temporal characteristics of swarms along Southern California and East Pacific Rise transform faults. Swarms in these regions exhibit distinctive characteristics, including a relatively narrow range of hypocentral migration velocities, on the order of a kilometre per hour. This rate corresponds to the rupture propagation velocity of shallow creep transients that are sometimes observed geodetically in conjunction with swarms, and is significantly faster than the earthquake migration rates typically associated with fluid diffusion. The uniformity of migration rates and low effective stress drops observed here suggest that shallow aseismic creep transients are the primary process driving swarms on strike-slip faults. Moreover, the migration rates are consistent with laboratory values of the rate-state friction parameter b (0.01) as long as the Salton Trough faults fail under hydrostatic conditions.

Early Pleistocene initiation of the San Felipe fault zone, SW Salton Trough, during reorganization of the San Andreas fault system

Structural and stratigraphic analyses along the western margin of the Salton Trough show that the San Andreas fault system was reorganized in early Pleistocene time from a system dominated by two fault zones (the San Andreas fault and the West Salton detachment fault) to a network of dextral faults that include the San Andreas and at least four dextral faults to the southwest. The San Felipe fault zone, one of these dextral faults, has ~5.8 ± 2.8 km of right separation and consists of three principal faults in the Peninsular Ranges. These are the San Felipe fault in the WNW, Sunset fault in the middle, and Fish Creek Mountains fault in the ESE. They form a left-stepping array and bound domains in which the Sunset Conglomerate, the older West Salton detachment fault, and Cretaceous mylonitic rocks below the detachment are folded about WNW-trending folds. A complex flower structure within the left-stepovers probably produced this fault-parallel folding. Because all the rocks within stepovers of the San Felipe fault zone, from Cretaceous to Pleistocene, are deformed about WNW-trending folds and record broadly similar shortening strains, we infer a Quaternary age of deformation. Parts of the San Felipe fault zone cut latest Pleistocene to Holocene surficial deposits, and the fault zone is likely active. Evidence for early Pleistocene initiation of the San Felipe fault zone is preserved in conglomerate NE of the Sunset fault. Poorly sorted angular boulder conglomerate and pebbly sandstone of the Sunset Conglomerate are ~600 m thick and lie in angular unconformity on the Pliocene Palm Spring Group. The conglomerate coarsens upward and toward the fault, and is dominated by plutonic clasts derived from SW of it. Conglomerate beds contain up to 10% sandstone clasts recycled from older basin fill and accumulated in proximal to medial alluvial fans that were shed to the NE from uplifted rocks along the then-active Sunset fault. Based on lithologic, stratigraphic, structural, and compositional similarities, we correlate the Sunset Conglomerate to the Pleistocene Ocotillo Formation. Clasts of recycled sandstone record erosion of detachment-related basin fill that predates the San Felipe fault and once covered the Vallecito and Fish Creek mountains. These crystalline-cored mountain ranges first emerged from beneath basin fill during early slip above the nascent San Felipe fault ca. 1.1–1.3 Ma. Later, the San Felipe fault zone cut upward, folded, cut across, and deactivated the West Salton detachment fault within a ~9-km-wide contractional bend and pair of left-steps. Areas that accumulated sediment within this step-over zone between ca. 1.1 and ca. 0.6 Ma are currently being inverted and folded. Initiation of the San Felipe fault in early Pleistocene time was a significant event in the reorganization of the southern San Andreas fault system. The Quaternary dextral faults broadened the plate boundary zone south-westward from roughly 25 km (during coeval slip on the San Andreas fault and West Salton detachment fault) to 50–70 km, and mark a change in the dominant structural style from transtension to distributed dextral faulting south of the Big Bend.

Miocene‐Pliocene exhumation along the west Salton detachment fault, southern California, from (U‐Th)/He thermochronometry of apatite and zircon

The Salton Trough is the northernmost segment of the active Gulf of California oblique rift system. The main rift‐related structure in the western Salton Trough is the low‐angle west Salton detachment fault (WSDF). Footwall and hanging wall apatite and zircon (U‐Th)/He ages record a significant contrast in thermal history across the WSDF, confirming Pliocene normal slip along the fault. Apatite (U‐Th)/He ages record rapid exhumational cooling from ∼5 to ∼2 Ma, consistent with the timing of accelerated tectonic subsidence recorded in WSDF hanging wall sedimentary strata and consistent with the initiation age of dextral plate boundary slip on the southern San Andreas fault in the Salton Trough. Our results indicate that the WSDF caused at least 2.3–4 km of exhumation and >8–10 km of approximately E directed horizontal extension since ∼5 Ma. Slip rate of the WSDF at Yaqui Ridge was likely in the range of ∼2.3–5 km/Ma from ∼7 to ∼2 Ma. The WSDF may have been active well before ∼5 Ma. Middle Miocene apatite (U‐Th)/He ages from samples at high elevations have steep age‐elevation gradients and suggest that exhumation may have initiated by ∼12 Ma, about when subduction ceased along a large segment of the Baja California margin and consistent with the age of onset of extension in northeastern Baja California.

Anthropogenic subsidence in the Mexicali Valley, Baja California, Mexico, and slip on the Saltillo fault

Deep fluid extraction in the Cerro Prieto geothermal field (CPGF) has caused subsidence and induced slip on tectonic faults in the Mexicali Valley (Baja California, Mexico). The Mexicali Valley is located in the southern part of the Salton Trough, at the boundary between the Pacific and North American plates. The Valley is characterized by being a zone of continuous tectonic deformation, geothermal activity, and seismicity. Within the Cerro Prieto pull-apart basin, seismicity is concentrated mainly in swarms, while strong earthquakes have occurred in the Imperial and Cerro Prieto transform faults, that are the eastern and western bound of the basin. Since 1973, fluid extraction at the CPGF has influenced deformation in the area, accelerating the subsidence and causing rupture (frequently as vertical slip or creep) on the surface traces of tectonic faults. Both subsidence and fault slip are causing damage to infrastructure like roads, railroad tracks, irrigation channels, and agricultural fields. Currently, accelerated extraction in the eastern part of CPGF has shifted eastwards the area of most pronounced subsidence rate; this accelerated subsidence can be observed at the Saltillo fault, a southern branch of the Imperial fault in the Mexicali Valley. Published leveling data, together with field data from geological surveys, geotechnical instruments, and new InSAR images were used to model the observed deformation in the area in terms of fluid extraction. Since the electricity production in the CPGF is an indispensable part of Baja California economy, extraction is sure to continue and may probably increase, so that the problem of damages caused by subsidence will likely increase in the future.

Buried rhyolites within the active, high-temperature Salton Sea geothermal system

Previously unrecognized pulses of rhyolite volcanism occurred in the Salton Trough between 420 ± 8 ka and 479 ± 38 ka (2 σ), based on high-spatial resolution U–Pb zircon geochronology. Presently, these rhyolite lavas, tuffs and shallow subvolcanic sills are buried to depths between ~ 1.6 and 2.7 km at ambient temperatures between 200 and 300 °C, and are overprinted by propylitic to potassic hydrothermal alteration mineral assemblages consisting of finely intergrown quartz, K-feldspar, chlorite, epidote, and minor pyrite. Alteration resistant geochemical indicators (whole-rock Nd-isotopes, zircon oxygen-isotopes) reveal that these rhyolites are derived from remelting of MORB-type crust that was chilled and hydrothermally altered by deep-circulating hydrothermal waters. U–Pb zircon dating confirms the presence of Bishop Tuff in well State 2-14 at ~ 1.7 km depth, approximately 5 km NE of the geothermal wells that penetrated the buried rhyolites. These results indicate accelerated subsidence towards the center of the Salton Trough, increasing from 2.2 mm/a to 3.8 mm/a. Based on these results, the present-day Salton Sea geothermal field is identified as a focus zone of episodic rhyolitic volcanism, intense heat flow and metamorphism that predates present-day geothermal activity and Holocene volcanism by at least ~ 400 ka.

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.

Shape and Dimensions of the Cerro Prieto Pull-Apart Basin, Mexicali, Baja California, Mexico, Based on the Regional Seismic Record and Surface Structures

The Salton Trough is a wide, actively subsiding basin where a system of an en-echelon dextral transform faults and pull-apart basins is operating. This tectonic province represents the connection between the Gulf of California fault system and the San Andreas transform fault. Within the depression, several sub-basins, such as the Cerro Prieto geothermal field in Mexicali, Baja California, Mexico, represent an incipient spreading center along the southern limits of the Salton Trough. The Cerro Prieto Basin has developed between two major, subparallel, right-stepping active transforms, the Imperial and Cerro Prieto faults. General agreement exists regarding the regional tectonic interpretation of the Cerro Prieto Basin; however, published documentation is lacking regarding its shape and dimensions. In this paper, we address this problem. We have integrated seismic data available for the whole study area and evidence from surface breaks. Our study shows that the most important factor controlling formation of the Cerro Prieto pull-apart basin is the slip on the Cerro Prieto and Imperial faults. This tectonic activity has triggered subsidence in the intervening area, creating several subsurface normal faults, oriented oblique and parallel to the major strands, that control the subsidence. All of this has resulted in the development of a NE-SW trending, immature, pull-apart basin with an areal extent of 183 km2. Thus the Cerro Prieto Basin is an important structure in terms of geology and geothermal resources.

Curie point depth from spectral analysis of aeromagnetic data from Cerro Prieto geothermal area, Baja California, México

Using aeromagnetic data acquired in the area from the Cerro Prieto geothermal field, we estimated the depth to the Curie point isotherm, interpreted as the base of the magnetic sources, following statistical spectral-based techniques. According to our results the Curie point isotherm is located at a depths ranging from 14 to 17 km. Our result is somewhat deeper than that obtained previously based only in 2-D and 3-D forward modeling of previous low-quality data. However, our results are supported by independent information comprising geothermal gradients, seismicity distribution in the crust, and gravity determined crustal thickness. Our results imply a high thermal gradient (ranging between 33 and 38 °C/km) and high heat flow (of about 100 mW/m 2) for the study area. The thermal regime for the area is inferred to be similar to that from the Salton trough.

Genealogical Concordance between Mitochondrial and Nuclear DNAs Supports Species Recognition of the Panamint Rattlesnake (Crotalus mitchellii stephensi)

The Speckled Rattlesnake (Crotalus mitchellii) is a polytypic taxon presently composed of five subspecies that range across southwestern North America, including the Baja Peninsula and islands in the Pacific Ocean and Sea of Cortés. The principles of genealogical concordance were employed to test the taxonomic status of three of the five subspecies (C. m. mitchellii, C. m. pyrrhus, and C. m. stephensi). We used two molecular marker systems: mitochondrial (mt) DNA ATPase 8 and 6 genes (675 base pairs, bp), and introns 5 and 6 of the nuclear (n) DNA ribosomal protein (RP) gene (449 bp). These markers were evaluated across 104 individuals of C. mitchellii: C. m. mitchellii (n = 3), C. m. pyrrhus (n = 83), C. m. stephensi (n = 18), with Sistrurus c. catenatus as the distant outgroup. Deep phylogenetic splits were detected in the subspecies of C. mitchellii, with 5.0–6.4% mtDNA sequence divergence (SD) separating C. m. mitchellii and C. m. pyrrhus, while C. m. mitchellii and C. m. stephensi had SD values of 6.4–7.3%. Similarly, C. m. pyrrhus and C. m. stephensi had SD values of 5.2–6.7%. In addition, C. m. mitchellii and C. m. pyrrhus were identical in all 449 intron bp, but C. m. stephensi differed from both at a single nucleotide polymorphism. Our molecular results diagnose C. m. stephensi as sister to mainland subspecies of the C. mitchellii complex, a result consistent with certain head scalation characters and its northernmost geographic distribution in this complex. Furthermore, four morphological synapomorphies (supraocular scales prominently ridged and/or creased, contact between rostral and prenasal scales, ground coloration of tail congruent with that of body, and black rings in the distal 15% of the tail) also diagnose C. m. stephensi from all other subspecies of C. mitchellii. We hypothesize that the northern distribution of C. m. stephensi likely resulted from two vicariant events: Pliocene expansion of the Sea of Cortés as the Salton Trough, and Pliocene development of the lacustrine Bouse Embayment along the Colorado River drainage. Despite earlier conclusions based on morphology, our molecular results showed no evidence of intergradation between C. m. pyrrhus and C. m. stephensi. Based on the principles of genealogical concordance, we advocate that C. m. stephensi be elevated to a full species, which renders a minimum of two species within the C. mitchellii clades we examined.

Deep structure of southern California

We used 214,210 P-wave arrival times from 7536 local earthquakes and 16,470 travel-time residuals from 332 teleseismic events recorded by the southern California Seismic Network to determine a detailed three-dimensional (3D) P-wave velocity structure of the crust and mantle down to 600 km depth beneath southern California. In this study, we have taken into account the Moho topography under this region determined by a previous receiver-function study. We found that the undulations of the Moho discontinuity affect considerably the tomographic images of the lower crust and uppermost mantle. When the Moho topography is taken into account, ray paths and travel times can be computed more accurately. The tomographic images show a very heterogeneous structure in the crust and upper mantle under southern California. The velocity structure in the shallow depth correlates well with the surface geological features. Three major anomalies in the upper mantle are revealed clearly beneath the southern Sierra Nevada, Salton Trough and Transverse Ranges. Compared with the previous result, both the shape and size of the three anomalies show some differences. Our result revealed two prominent low-velocity (low-V) anomalies above the high-velocity (high-V) anomalies in the upper mantle under southern Sierra Nevada and the Transverse Ranges. We consider that the upper-mantle anomaly under the Transverse Ranges was formed through convergence and sinking of the entire subcrustal lithosphere. Vertical motions and removal of the dense crust root caused the high-V anomaly under the southern Sierra Nevada. The prominent low-V anomalies above the two high-V anomalies were either caused by the crustal materials pulled down to the uppermost mantle or by upwelling of partial melt in asthenosphere when the sinking of lithosphere occurs. The Salton Trough low is the response to the lithospheric extension when the Pacific plate was rifted away from the North American plate.

Surface wave tomography of the western United States from ambient seismic noise: Rayleigh wave group velocity maps

We have applied ambient noise surface wave tomography to data that have emerged continuously from the EarthScope USArray Transportable Array (TA) between October 2004 and January 2007. Estimated Green's functions result by cross‐correlating noise records between every station‐pair in the network. The 340 stations yield a total of more than 55,000 interstation paths. Within the 5‐ to 50‐s period band, we measure the dispersion characteristics of Rayleigh waves using frequency‐time analysis. High‐resolution group velocity maps at 8‐, 16‐, 24‐, 30‐, and 40‐s periods are presented for the western United States. The footprint of the TA encloses a region with a resolution of about the average interstation spacing (∼70 km). Velocity anomalies in the group velocity maps correlate well with the dominant geological features of the western United States. Coherent velocity anomalies are associated with the Sierra Nevada, Peninsular, and Cascade Ranges, Great Valley, Salton Trough, and Columbia basins, the Columbia River flood basalts, the Snake River Plain and Yellowstone, and mantle wedge features associated with the subducting Juan de Fuca plate.

Modulation of the earthquake cycle at the southern San Andreas fault by lake loading

Changes in the level of ancient Lake Cahuilla over the last 1500 years in the Salton Trough alter the state of stress by bending the lithosphere in response to the applied lake load and by varying the pore pressure magnitude within the crust. The recurrence interval of the lake is similar to the recurrence interval of rupture on the southern San Andreas and San Jacinto faults, both of which are partially covered by the lake at its highstand. Furthermore, four of the last five ruptures on the southern San Andreas fault have occurred near a time of substantial lake level change. We investigate the effect of Coulomb stress perturbations on local faults due to changing level of Lake Cahuilla to determine a possible role for the lake in affecting the timing of fault rupture. Coulomb stress is calculated with a three‐dimensional model of an elastic plate overlying a viscoelastic half‐space. Plate thickness and half‐space relaxation time are adjusted to match observed vertical deformation since the last lake highstand. The lake cycle causes positive and negative Coulomb stress perturbations of 0.2–0.6 MPa on the southern San Andreas within the lake and 0.1–0.2 MPa on the southern San Andreas outside the lake. These Coulomb stress perturbations are comparable to stress magnitudes known to have triggered events at other faults along the North America‐Pacific plate boundary.

Surface Rupture of the Morelia Fault Near the Cerro Prieto Geothermal Field, Mexicali, Baja California, Mexico, during the Mw 5.4 Earthquake of 24 May 2006

The Cerro Prieto geothermal field, considered an active developing rift basin, is located in the southern limits of the Salton Trough tectonic province. The tectonic activity along the Gulf of California–Salton Trough has created spreading centers linked through a set of transform faults. Such faults are characterized by right lateral motion obliquely oriented to the gulf axis, which produces a transtensional stress regime along the Pacific–North American plate boundary. In particular, the Cerro Prieto basin has developed between the major Cerro Prieto and Imperial right-stepping transform faults, which are part of the San Andreas–Gulf of California fault system (figure 1). Both faults are seismically active and have produced several magnitude 7 earthquakes. However, in the past decades only the Imperial fault has produced superficial ground breakage during earthquakes. This is in contrast to earthquake ruptures on the Cerro Prieto fault, which have not extended up to the ground surface, so this fault trace is not yet well-constrained. The epicenter of the 24 May 2006 Mw 5.4 earthquake was located along the northern end of the Cerro Prieto fault. In a field survey performed after the earthquake, superficial fractures were clearly observed at short distances from the epicenter. Based on our field observations, we address the question of whether such fractures are evidence of rupture along the Cerro Prieto fault or are associated with a conjugate fault. ▴ Figure 1. Tectonic map of the Gulf of California region, including the peninsula of Baja California and Sonora, Mexico; California; Arizona; and the main tectonic elements: North American plate, Pacific Plate, East Pacific Rise, Gulf extensional province (GEP), Cerro Prieto fault (CPf), Imperial fault (If), Agua Blanca fault (ABf), Laguna Salada fault (LSf), Elsinore fault (Ef), San Jacinto fault (SJf), San Andreas fault (SAf), and Garlock fault (Gf). On 24 May 2006 at 04:20 (UTC) …

Regional mapping of the crustal structure in southern California from receiver functions

Lateral variations of the crustal structure in southern California are determined from receiver function (RF) studies using data from the Southern California Seismic Network broadband stations and Los Angeles Regional Seismic Experiment surveys. The results include crustal thickness estimates at the stations themselves, and where possible, cross sections are drawn. The large‐scale Moho depth variation pattern generally correlates well with the current status of the Mesozoic batholith: Deep Moho of 35–39 km is observed beneath the western Peninsula Ranges, Sierra Nevada, and San Bernardino Mountains, where the batholith is relatively intact, and shallow Moho of 26–32 km is observed in the Mojave Desert, where the batholith is highly deformed and disrupted. High‐resolution lateral variations of the crustal structure for individual geographic provinces are investigated, and distinctive features are identified. The crustal structure is strongly heterogeneous beneath the central Transverse Ranges, and deep Moho of 36–39 km is locally observed beneath several station groups in the western San Gabriel Mountains. Moho is relatively flat and smooth beneath the western Mojave Desert but gets shallower and complicated to the east. Anomalous RFs are observed at two stations in the eastern Mojave Desert, where a Moho step of ∼8–10 km is found between the NW and SE back‐azimuthal groups of station DAN in the Fenner Valley. Asymmetric extension of the Salton Trough is inferred from the Moho geometry. Depth extension of several major faults, such as the San Andreas Fault and San Gabriel Fault, to the Moho is inferred.

Earthquake swarms driven by aseismic creep in the Salton Trough, California

In late August 2005, a swarm of more than a thousand earthquakes between magnitudes 1 and 5.1 occurred at the Obsidian Buttes, near the southern San Andreas Fault. This swarm provides the best opportunity to date to assess the mechanisms driving seismic swarms along transform plate boundaries. The recorded seismicity can only explain 20% of the geodetically observed deformation, implying that shallow, aseismic fault slip was the primary process driving the Obsidian Buttes swarm. Models of earthquake triggering by aseismic creep can explain both the time history of seismic activity associated with the 2005 swarm and the ∼1 km/h migration velocity exhibited by this and several other Salton Trough earthquake swarms. A combination of earthquake triggering models and denser geodetic data should enable significant improvements in time‐dependent forecasts of seismic hazard in the key days to hours before significant earthquakes in the Salton Trough.

Chronology of Miocene–Pliocene deposits at Split Mountain Gorge, Southern California: A record of regional tectonics and Colorado River evolution

Late Miocene to early Pliocene deposits at Split Mountain Gorge, California, preserve a record of basinal response to changes in regional tectonics, paleogeography, and evolution of the Colorado River. The base of the Elephant Trees Formation, magnetostratigraphically dated as 8.1 ± 0.4 Ma, provides the earliest well-dated record of extension in the southwestern Salton Trough. The oldest marine sediments are ca. 6.3 Ma. The nearly synchronous timing of marine incursion in the Salton Trough and northern Gulf of California region supports a model for localization of Pacifi c‐North America plate motion in the Gulf ca. 6 Ma. The fi rst appearance of Colorado River sand at the Miocene-Pliocene boundary (5.33 Ma) suggests rapid propagation of the river to the Salton Trough, and supports a lake-spillover hypothesis for initiation of the lower Colorado River.

Pleistocene Brawley and Ocotillo Formations: Evidence for Initial Strike‐Slip Deformation along the San Felipe and San Jacinto Fault Zones, Southern California

We examine the Pleistocene tectonic reorganization of the Pacific–North American plate boundary in the Salton Trough of southern California with an integrated approach that includes basin analysis, magnetostratigraphy, and geologic mapping of upper Pliocene to Pleistocene sedimentary rocks in the San Felipe Hills. These deposits preserve the earliest sedimentary record of movement on the San Felipe and San Jacinto fault zones that replaced and deactivated the late Cenozoic West Salton detachment fault. Sandstone and mudstone of the Brawley Formation accumulated between ∼1.1 and ∼0.6–0.5 Ma in a delta on the margin of an arid Pleistocene lake, which received sediment from alluvial fans of the Ocotillo Formation to the west‐southwest. Our analysis indicates that the Ocotillo and Brawley formations prograded abruptly to the east‐northeast across a former mud‐dominated perennial lake (Borrego Formation) at ∼1.1 Ma in response to initiation of the dextral‐oblique San Felipe fault zone. The ∼25‐km‐long San Felipe anticline initiated at about the same time and produced an intrabasinal basement‐cored high within the San Felipe–Borrego basin that is recorded by progressive unconformities on its north and south limbs. A disconformity at the base of the Brawley Formation in the eastern San Felipe Hills probably records initiation and early blind slip at the southeast tip of the Clark strand of the San Jacinto fault zone. Our data are consistent with abrupt and nearly synchronous inception of the San Jacinto and San Felipe fault zones southwest of the southern San Andreas fault in the early Pleistocene during a pronounced southwestward broadening of the San Andreas fault zone. The current contractional geometry of the San Jacinto fault zone developed after ∼0.5–0.6 Ma during a second, less significant change in structural style.

Alteration and remelting of nascent oceanic crust during continental rupture: Evidence from zircon geochemistry of rhyolites and xenoliths from the Salton Trough, California

Rhyolite lavas and xenoliths from the Salton Sea geothermal field (Southern California) provide insights into crustal compositions and processes during continental rupture and incipient formation of oceanic crust. Salton Buttes rhyolite lavas contain xenoliths that include granophyres, fine-grained altered rhyolites (“felsite”), and amphibole-bearing basalts. Zircon is present in lavas and xenoliths, surprisingly even in the basaltic xenoliths, where it occurs in plagioclase-rich regions interpreted as pockets of crystallized partial melt. Zircons in the xenoliths are exclusively Late Pleistocene–Holocene in age and lack evidence for inheritance. U–Th isochron ages are: 20.5 − 1.2 + 1.2 ka (granophyres), 18.3 − 3.5 + 3.6 ka (felsite), 30.1 − 12.4 + 14.1 ka and 9.2 − 6.6 + 7.0 ka (basalts; all errors 1 σ). The dominant zircon population in the rhyolite lavas yielded U–Th ages between ∼ 18 and 10 ka, with few pre-Quaternary xenocrysts present. δ 18O zircon values are lower than typical crustal basement values, thus ruling out rhyolite genesis by melting of continental crust. Moreover, δ 18O zircon values are ∼ 0.5–1.0‰ lower than compositions achievable by zircon crystallization from residual melt in equilibrium with unaltered mid-ocean ridge basalt, suggesting that basaltic crust and silicic plutons in the subsurface of the Salton Sea geothermal field isotopically exchanged with meteoric waters. This is evidence for deep-reaching hydrothermal circulation and indicates rhyolite genesis by episodic remelting of altered basalts instead of fractional crystallization of unaltered basaltic magma.

Stratigraphic record of Pleistocene faulting and basin evolution in the Borrego Badlands, San Jacinto fault zone, Southern California

Sedimentary rocks in the Borrego Badlands, Southern California, contain a record of Pleistocene crustal deformation during initiation and evolution of the San Jacinto fault zone. We used detailed geologic, stratigraphic, and paleomagnetic analysis to determine the age and geometry of the deposits and reconstruct the history of fault-controlled sedimentation in this area. The base of the ~300 to 500 m thick Ocotillo Formation is a paraconformity to abrupt conformable contact that records a brief hiatus followed by rapid progradation of coarse alluvial sediment over lacustrine facies of the Borrego Formation at 1.05 ± 0.03 Ma. This coincides with regional-scale progradation of Ocotillo Formation sand and gravel, and appears to record initiation of strike-slip faults in the southwestern Salton Trough at ca. 1.1 Ma. Thickness trends, clast compositions, paleocurrents, and distribution of paleosols provide evidence for initiation of the East Coyote Mountain fault at ca. 1.05 Ma, followed by onset of NNE-ward basin tilting obliquely toward the Santa Rosa segment of the Clark fault at ca. 1.0 Ma. Stratigraphic omission of the Ocotillo Formation and progressively older units southwest of the Coyote Creek fault beneath the Fonts Point Sandstone provides evidence that tilting to the northnortheast was related in part to growth of the San Felipe anticline during deposition of the Ocotillo Formation. Map and stratigraphic data suggest that the Coyote Creek fault in the western Borrego Badlands postdates Ocotillo deposition, and thus appears to have propagated southeast into the study area at ca. 0.6 Ma. The Fonts Point Sandstone is a thin, sheetlike alluvial deposit that records the end of deposition and onset of transpressive deformation in the Borrego Badlands. The base of the Fonts Point Sandstone changes from a conformable contact in a narrow belt southeast of the Inspiration Point fault, where it is dated at 0.6 ± 0.02 Ma, to an angular unconformity on the folded Ocotillo Formation northwest of the fault. The pattern of stratal truncation records initiation of the Inspiration Point fault at ca. 0.6 Ma. This coincides with a major structural reorganization in the San Jacinto fault zone that initiated the modern phase of north-south shortening and erosion in the southwestern Salton Trough.