The Mojave Section of the San Andreas Fault (California), 2: Pleistocene Records of Near‐Field Transpression Illuminate the Atypical Evolution of the Restraining “Big Bend”

Abstract

AbstractWith an obliquity of ∼30° relative to plate motion direction, the ∼300‐km‐long Big Bend of the San Andreas Fault is one of the world's largest restraining bends. The 5–6 Ma (∼160 km of total displacement) longevity of this mechanically inefficient structure and the lack of evidence for associated widespread uplift both challenge existing models of transpression and bend evolution. We focus on the structurally simplest section of the Big Bend (the Mojave section of the San Andreas: MSAF) to characterize the pattern of near‐field (<5 km from the San Andreas trace) uplift over two different timescales. The topography of vertically deformed alluvial surfaces is used to demonstrate that near‐field uplift along the least oblique segment of the MSAF has been significant over the last ∼40 ka (∼1–2 mm/yr), and driven by slip on two oppositely dipping blind reverse faults. Topographic and structural analyses of the MSAF near‐field are conducted at the scale of the entire fault to show that, at least on the NE side of the MSAF, these blind structures coincide with the front of a fault‐parallel bedrock ridge with clear characteristics of a young transpressive ridge. Structural, sedimentary, and geomorphic arguments converge to suggest that these blind structures were activated ∼315 ka ago and record a Mid‐Pleistocene kinematic reorganization of the MSAF fault‐zone. This reorganization is tentatively interpreted as a shift in the mode of accommodation of the transpressive component of plate motion, in turn driven by the strike‐slip advection of crustal strength gradients along the Big Bend.