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

Abstract

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.

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

“…Assuming that fault slip rates have been constant since fault inception, our rates are slower than required by the ca. ∼1.1 Ma inception of dextral faulting proposed for the Salton Trough by Lutz et al [2006] and Kirby et al [2007]. Conversely, if we combine bedrock displacements and slip rates at Anza (22-24 km and ∼12-15 mm/yr, respectively) and Rockhouse Canyon (14.5 to 17 km and 8.9 ± 2.0 mm/yr, respectively), we would estimate the age of fault initiation at both sites as 1.8 ± 0.5 Ma.…”

“…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.…”

“…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].…”

“…[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.…”