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.

Abstract

To determine the post‐40 ka slip‐rate along the Mojave section of the San Andreas Fault (MSAF) we re‐analyze the sedimentary record preserved where Little Rock (LR) Creek flows across the fault. At this location, interaction between the northeast‐flowing stream and right‐lateral fault has resulted in the abandonment and preservation of 11 strath terraces and one paleo‐floodplain in the downstream trailing corner of the river, two of which are also preserved upstream to provide cross‐fault matches. A new model of fault‐induced river deflection, together with standard terrace riser restoration, yields strike‐slip displacements of 1,140 ± 160 m for the older terrace and 360 ± 70 m for the younger one. When combined with new 10Be dating and reinterpretation of prior measurements the displaced terraces yield right‐lateral slip‐rates of 27.7+6.9/−3.5 and 26.8+3.4/−3.0 mm/yr over the last 23 k.y. and last 40 k.y., where uncertainties are at 95% credible intervals. These new rate determinations are consistent with independent late Holocene estimates, indicating that the long‐term rate of strain accumulation along the MSAF is relatively fast and does not vary significantly when averaged over timescales of 15–20 k.y. Using our new model of stream deflection, we find that the fluvial sequence was emplaced in two distinct periods, each characterized by a temporally stable but markedly different deflected river geometry. Each period coincides with a distinct stage of erosive power along LR Creek determined from independent paleoclimate proxies. Importantly, application of the new river‐deflection model allows strike‐slip displacements to be determined in the absence of upstream piercing points.

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”

“…In our companion study (Moulin et al, 2023), we have also demonstrated that the alluvial terraces and paleo-floodplain are vertically deformed along MSAF-parallel tectonic scarps. Scarps were defined by Moulin et al (2023) as the portions of terrace which are unambiguously tilted to the NE (slope up to 18° mapped with black polygons in the inset map of Figure 3) relative to the modern gradient of the Little Rock Creek channel (0.68° from its intersection with the Mount Emma road up to at least the NW limit of Figure 2; see Text S1 in Supporting Information S1 for further details). The scarps separate terrace portions with lower slopes (less than 1°) that are hereafter referred as "benches."…”

“…(2023) for a closer view). The geomorphic setting of the Little Rock Creek catchment, as well as the description and distribution of the Little Rock Creek terraces, have been described in detail in our companion study (Moulin et al., 2023). For the sake of conciseness, that information will not be repeated in the present article, and the terrace mapping of Moulin et al.…”

“…Total slip along the MSAF has been distributed between several, now abandoned, sub-parallel fault strands (Punchbowl, Nadeau, Hitchbrook, Little Rock, and Cajon Valley Faults in addition to the modern trace) (Barrows et al, 1987;Matti & Morton, 1993;Weldon et al, 1993) (Figure 1c). Today, strike-slip motion along the MSAF appears localized along a single, structurally continuous, fault trace (Barrows et al, 1987;Moulin et al, 2023), which exhibits a sigmoidal geometry in map-view (Figure 1c) with an overall trend 20-30° more oblique than the SAF in Northern and Central California and in the Salton trough (Figure 1a) (Spotila et al, 2007). On a large scale, the MSAF broadly coincides with the sharp topographic boundary separating the rugged topography of the San Gabriel and Liebre mountains in the SW from the regular low-relief surface of the internally drained Lake Thompson basin in the NE (Moulin et al, 2023;Orme, 2008) (Figure 1c).…”

“…The procedure involves a normalization of the individual records, based on the magnitude of post-abandonment strike-slip displacement, with the underlying assumption that the rate of strike-slip faulting is temporally and spatially invariant at the scales of observation. All records were normalized to terrace T2 of the Little Rock Creek, which is the most extensive terrace spatially and for which absolute dating is available (22.7 +2.8 / −4.4 ka, see Moulin et al (2023)). T2-normalization of each record was thus performed by multiplying the uplift by (630 ± 40)/D, where D depicts the magnitude of post-depositional strike-slip displacement of the considered record, and 630 ± 40 m is the strike-slip displacement recorded by T2 (Moulin et al, 2023).…”

“…These theoretical predictions should particularly apply to the Big Bend owing to its strong obliquity relative to plate motion (20–40°: Spotila et al., 2007) (Figure 1a). Yet, available data indicate that the Late Pleistocene to Holocene contribution of the Big Bend to North‐America/Pacific plate motion (i.e., between 27 and 40 mm/yr: Moulin et al., 2023; Salyards et al., 1992; Weldon et al., 2004; Young et al., 2021) is not particularly lower than that averaged over its whole history (25–34 mm/yr, as implied by 160 ± 10 km of finite displacement over the last 5–6 Ma: Darin & Dorsey, 2013; Weldon et al., 1993). On the other hand, more optimally oriented structures or fault systems, such as the San Jacinto Fault and Eastern California Shear Zone (ECSZ; Figure 1a), contribute significantly to the transcurrent budget, but do not seem to have evolved toward increasing prevalence (see Blisniuk et al.…”