Initiation of the Southern Elsinore Fault at ∼1.2 Ma: Evidence from the Fish Creek-Vallecito Basin, southern California

Strike-slip faults west of the San Andreas fault (SAF) accommodate more than half of the modern Pacific-North America plate motion in southern California, yet their age, kinematic evolution and long-term slip rates are incompletely understood. The southern Elsinore fault is part of this zone and occupies the boundary between Mesozoic crystalline rocks of the Peninsular Ranges and Late Cenozoic sedimentary rocks of the Salton Trough depocenter. The Fish Creek-Vallecito Basin, a recently inverted western sub-basin of the Salton Trough, contains a thick succession of Plio-Pleistocene sedimentary rocks that preserve a record of crustal subsidence and uplift in response to evolving tectonic controls. Deposits older than similar to 1.2 Ma accumulated during transtension and subsidence in the hanging wall of the low-angle West Salton detachment fault. A fanning-dip interval high in the section, dated at 1.2-1.0 Ma, accumulated during tilting caused by earliest motion on the Elsinore and Earthquake Valley faults. Total offset of 1-2 km yields a lifetime slip rate of about 1-2 mm/yr for the southern Elsinore fault, similar to a late Pleistocene rate in the Coyote Mountains but slower than published estimates for the northern Elsinore fault. This difference likely results from long-term transfer of slip from the northern Elsinore fault to the San Felipe and San Jacinto fault zones farther east. Initiation of the Elsinore fault at 1.2 Ma coincides with a major tectonic reorganization that terminated slip on the detachment fault and formed the modern network of strike-slip faults west of the SAF.