A. N. Steely

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.

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.

Pleistocene reorganization of the southern 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.