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

Abstract

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 SanAndreas 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 fl ower structure within the left-stepovers probably produced this faultparallel 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 surfi cial 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 fi ll 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 detachmentrelated basin fi ll that predates the San Felipe fault and once covered the Vallecito and Fish Creek mountains. These crystalline-cored mountain ranges fi rst emerged from beneath basin fi ll 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 stepover 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 signifi cant event in the reorganization of the southern San Andreas fault system. The Quaternary dextral faults broadened the plate boundary zone southwestward from roughly 25 km (during coeval slip on the San Andreas fault and West Salton detachment fault) to 50-70 ...

Stratigraphic record of basin development within the San Andreas fault system: Late Cenozoic Fish Creek-Vallecito basin, southern California

“…1.1-1.3 Ma terminated slip on the West Salton detachment fault along most of its length and initiated the modern San Jacinto, San Felipe, and Elsinore strike-slip faults (Figs. 1 and 2; Matti and Morton, 1993;Kirby, 2005;Lutz, 2005;Lutz et al, 2006;Steely, 2006;Kirby et al, 2007;Steely et al, 2009;Janecke et al, 2010). This event initiated present-day uplift and erosion of the southwestern parts of the former supradetachment basin in the western Salton Tough.…”

“…4), and it consists of pebble-to cobble-boulder conglomerate that accumulated in alluvial fans around the fault-bounded margins of the basin. North of the Vallecito Mountains, Canebrake Conglomerate in the lower Palm Spring interval is faulted against the West Salton detachment fault (Steely et al, 2009;Belgarde, 2007;Matti et al, 2002).…”

“…1.1-1.3 Ma (Figs. 1 and 2; Steely et al, 2009;Janecke et al, 2010). These sedimentary rocks contain a rich record of basin subsidence, sediment accumulation, delta progradation, and changing patterns of crustal deformation since late Miocene time.…”

Sedimentation and crustal recycling along an active oblique-rift margin: Salton Trough and northern Gulf of California

“…Colorado River sand fi rst arrived in the basin at 5.3 Ma and quickly overwhelmed the basin fi ll (Dorsey et al, 2007). A tectonic reorganization at 1.1-1.3 Ma terminated slip on the western basin-bounding low-angle detachment fault, initiated the modern San Jacinto and Elsinore strike-slip fault zones, and initiated uplift and erosion of late Cenozoic basins in the western Salton Trough (Steely et al, 2009). Subsidence continued beneath the axial Salton Trough and produced a sedimentary basin as deep as 10-12 km, where Colorado River sediment is intruded by young mafi c and silicic sills (Elders et al, 1972;Fuis et al, 1984;Schmitt and Vazquez, 2006).…”

Particle-size distributions of low-angle normal fault breccias: Implications for slip mechanisms on weak faults

“…All structures related to WSDF slip are brittle; cataclastic footwall rocks consist of a 0e5 m thick breccia zone overlain by 0e2 m of ultracataclasite. Folding of the WSDF occurred synchronous with late slip (the last w200 ka of WSDF activity) and after detachment slip; some folding during earlier WSDF slip cannot be ruled out (Axen and Fletcher, 1998;Steely et al, 2009).…”