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

Abstract

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.

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

“…Landforms along the Clark fault strand suggests that it is the dominant strand in accommodating slip of the southern SJFZ. The right-lateral strike-slip behavior of the Clark fault strand terminates southeast of the Santa Rosa Mountains into a zone of diffuse faulting and folding in the northwestern Imperial Valley [Sharp, 1981;Kirby et al, 2007]. Quaternary features along the Clark fault strand that indicate youthful activity include folds, offset and deformed terraces, deflected channels, beheaded channels, offset surfaces, fault scarps, and linear ridges.…”

“…[7] Inception of the SJFZ as a major right-lateral strike slip fault zone has variably been inferred to have occurred as early as ∼2.4 Ma based on a slip rate of 10 mm/yr [Sharp, 1981;Rockwell et al, 1990] and a total offset of 24 km [Sharp, 1967], to 1.5 Ma [Morton and Matti, 1993], to as recently as ∼1.1 Ma [Lutz et al, 2006;Kirby et al, 2007]. For the northern SJFZ, Morton and Matti [1993] suggest an initiation age of 1.5 Ma from sedimentologic changes in the upper San Timoteo Formation deposited adjacent to the SJFZ, dated by a rodent tooth fossil identified as Microtus Californicus.…”

“…In the central SJFZ, using the best-estimated late Quaternary slip rate of 10-14 mm/yr [Rockwell et al, 1990], one can infer an inception age of 1.7-2.4 Ma based on the 24 km of bedrock displacement, although this assumes the slip rate has been fairly constant since inception. For the southern SJFZ, a 1.05-1.07 Ma initiation age has been suggested based on dramatic changes in basin dynamics inferred from sedimentary rocks [Lutz et al, 2006;Kirby et al, 2007]. Based on a magnetic reversal located between two nonconformable stratigraphic units, the Ocotillo and Borrego Formations, the initial progradation of sediment beginning at ∼1.1 Ma is interpreted as evidence for initiation of faults in the Salton Trough [Lutz et al, 2006;Kirby et al, 2007].…”

“…For the southern SJFZ, a 1.05-1.07 Ma initiation age has been suggested based on dramatic changes in basin dynamics inferred from sedimentary rocks [Lutz et al, 2006;Kirby et al, 2007]. Based on a magnetic reversal located between two nonconformable stratigraphic units, the Ocotillo and Borrego Formations, the initial progradation of sediment beginning at ∼1.1 Ma is interpreted as evidence for initiation of faults in the Salton Trough [Lutz et al, 2006;Kirby et al, 2007].…”

“…However, mechanical models of faults show a systematic relationship between fault length and displacement with displacement decreasing toward the fault tip [Cowie and Scholz, 1992]. Thus, the range in slip rate estimates for the SJFZ could suggest that 1) the slip rate of the SJFZ may have decreased since its initiation, 2) faulting may have initiated earlier than 1.1 Ma [Lutz et al, 2006;Kirby et al, 2007], 3) a slip rate gradient may exist along the SJFZ, or 4) some previously published slip rate estimates may have been compromised by insufficiently constrained ages or displacements.…”