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

With 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.

The Mojave Section of the San Andreas Fault (California): 1. Shaping the Terrace Stratigraphy of Little Rock Creek Through the Competition Between Rapid Strike‐Slip Faulting and Lateral Stream Erosion Over the Last 40 k.y.

“…The inference, based on the MSAF‐normal trend of incision, that the entire group of upstream terrace remnants belongs to that particular age interval is supported by the insignificant difference between the concentration of the amalgamated sample collected at the surface of the pristine upstream remnant (AM‐5 located in Figure 5b) and that collected at the surface of T2 NE of the MSAF (AM‐1 located in Figure 3c) (Table 2). The T2 to T1b age of the upstream remnants is also supported by the inter‐comparison of normalized uplift profiles performed in Moulin and Cowgill (2023).…”

“…Finally, we propose that the synchronicity between the lake highstand and the capture of the LR Creek by its modern outlet is not a coincidence, but instead highlights how capture events can be promoted by increased water discharge during more wet periods. Notably, 1,210 ± 110 m of strike‐slip displacement have been inferred to post‐date the capture of the Mount Emma Creek by its downstream valley (Moulin & Cowgill, 2023), implying this event was broadly coeval to the capture of the LR Creek (according to the insignificant difference between 1,210 ± 110 m and the post‐T4 FP offset of 1,140 ± 160 m). It thus seems that the drainage configuration in this area was mostly acquired during an episode of increased water discharge which terminated ∼36 k.y.…”

“…Our post‐23 ka slip‐rate estimate also indicates that the MSAF slips at a rate which is similar to the rate along the northern section of the San Andreas Fault despite the ∼30° difference in fault azimuth (Figure 1a). Moulin and Cowgill (2023) combine this rate with long‐term quantitative records of near‐field transpression to show that the MSAF accommodates the relative rotation of the San Gabriel and Western Mojave blocks about an Euler pole located as close as ∼1,000 km from the fault trace. This means that the independent rotation of these blocks decouples the MSAF from the far‐field constraints imposed by the motion of the larger plates, and thereby allows the rate of right‐lateral slip along the San Andreas Fault to be largely insensitive to the Big Bend geometry.…”

“…The G D1 complex overlies two bedrock units: the arkosic member of the late‐Miocene to early‐Pliocene Anaverde Formation, between the MSAF and Little Rock Fault, and the Holcomb Quartz Monzonite to the NE of the Little Rock Fault (Barrows et al., 1987). The underlying strata of the Anaverde Formation are tightly folded and have been incised by the LR Creek and BW stream by up to 60 m since the oldest G D1 terrace was abandoned, whereas the Holcomb Quartz Monzonite has been incised ≤15 m (Moulin & Cowgill, 2023).…”

“…(2005) that G D1 consists of two main aggradational events (their fans 0 and 1, which respectively encompass terraces T1a to T2 and T3d to T3g) and that the modern upstream floodplain provides a robust estimate of the source of these fans (piercing‐points in Figure 2c), represents an over‐simplified picture of the morpho‐tectonic evolution of that site. In addition, the strong vertical deformation in the near‐field of the MSAF (tilting, secondary flexural faulting, and reactivation of fault strands inherited from the proto‐MSAF such as the Little Rock Fault) (Moulin & Cowgill, 2023) adds a degree of complexity to the formation and evolution of the LR Creek terrace series, and clearly calls for a 4D approach. Although the identification of terrace risers represents a real advance for the determination of post‐depositional displacements at that site, the T2‐T1b timing of the upstream strain markers suggested in Section 3.6 is loosely controlled and may be better understood with additional independent data.…”