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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-fi delity record of late Cenozoic subsidence and basin fi lling 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 Pacifi c-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 fl uvial-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.

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.

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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.