Constraints on rock uplift in the eastern Transverse Ranges and northern Peninsular Ranges and implications for kinematics of the San Andreas fault in the Coachella Valley, California, USA

Abstract

The nexus of plate-boundary deformation at the northern end of the Coachella Valley in southern California (USA) is complex on multiple levels, including rupture dynamics, slip transfer, and three-dimensional strain partitioning on nonvertical faults (including the San Andreas fault). We quantify uplift of mountain blocks in this region using geomorphology and low-temperature thermo chronometry to constrain the role of longterm vertical deformation in this tectonic system. New apatite (U-Th)/He (AHe) ages confirm that the rugged San Jacinto Mountains (SJM) do not exhibit a record of rapid Neogene exhumation. In contrast, in the Little San Bernardino Mountains (LSBM), rapid exhumation over the past 5 m.y. is apparent beneath a tilted AHe partial retention zone, based on new and previously published data. Both ranges tilt away from the Coachella Valley and have experienced minimal denudation from their upper surface, based on preservation of weathered granitic erosion surfaces. We interpret rapid exhumation at 5 Ma and the gentle tilt of the erosion surface and AHe isochrons in the LSBM to have resulted from rift shoulder uplift associated with extension prior to onset of transpression in the Coachella Valley. We hypothesize that the SJM have experienced similar rift shoulder uplift, but an additional mechanism must be called upon to explain the pinnacle-like form, rugged escarpment, and topographic disequilibrium of the northernmost SJM massif. We propose that this form stems from erosional resistance of the Peninsular Ranges batholith relative to more-erodible foliated metamorphic rocks that wrap around it. Our interpretations suggest that neither the LSBM nor SJM have been significantly uplifted under the present transpressive configuration of the San Andreas fault system, but instead represent relict highs due to previous tectonic and erosional forcing.GEOSPHERE GEOSPHERE, v. 16, no. 3

Insights into the geometry and evolution of the southern San Andreas fault from geophysical data, southern California

“…Rotation of the San Andreas fault from near vertical to a northeast dip predicts southwest tilts on either side of the fault, assuming the crust is rigid. Such southwest tilts on either side of Coachella Valley are not documented by available thermochronologic data or geomorphic analyses, except in the San Jacinto Mountains (Wolf et al, 1997;Spotila et al, 2020). Northeast tilting of the Little San Bernardino and Santa Rosa Mountains (Spotila et al, 2020;Dorsey and Langenheim, 2015) is opposite of what might be expected from rigid block tilting from rotating the San Andreas fault from near vertical to a northeast dip.…”

“…Such southwest tilts on either side of Coachella Valley are not documented by available thermochronologic data or geomorphic analyses, except in the San Jacinto Mountains (Wolf et al, 1997;Spotila et al, 2020). Northeast tilting of the Little San Bernardino and Santa Rosa Mountains (Spotila et al, 2020;Dorsey and Langenheim, 2015) is opposite of what might be expected from rigid block tilting from rotating the San Andreas fault from near vertical to a northeast dip. However, the crust is likely not rigid on either side of the San Andreas fault.…”

“…This uplifted block northeast of the fault would then have been translated to its current location, juxtaposed against basin fill deposited on the southwestern block developed in the hanging wall of the West Salton detachment fault. This scenario explains the juxtaposition of basin fill on the southwestern side and uplifted basement on the northeastern side of the fault in northern Coachella Valley, although thermochronologic data suggest that the timing of uplift in the Little San Bernardino Mountains was earlier at 5 Ma, when the block was interpreted to be next to the San Gabriel Mountains by Spotila et al (2020;see also Figs. 6, 7E, and 7F).…”

Late Oligocene–Early Miocene Detachment Faulting in Crystalline Basement, Mecca Hills, CA, Documented With Zircon (U–Th)/He Date‐eU‐Visual Radiation Damage Patterns

“…Previous workers have speculated ∼60 Ma exhumation and cooling may be the result of extension, surface erosion, subduction refrigeration, or some combination of the three processes, but the mechanism(s) is not fully understood (e.g., Jacobson et al, 2002;Saleeby, 2003;Wells & Hoisch, 2008;Wells et al, 2005;Yin, 2002). We suggest this sample was most likely incorporated into the current crystalline basement terrane during San Andreas fault-related deformation and exhumation at or just prior to ∼1 Ma, when its apatite (U-Th)/He dates overlap those from the other North American basement samples in the same fault-bounded block (Moser et al, 2017;Spotila et al, 2020).…”

Tectonostratigraphic record of late Miocene–early Pliocene transtensional faulting in the Eastern California shear zone, southwestern USA

“…The onset of rapid exhumation in the Little San Bernardino Mountains (Fig. 15; Spotila et al, 2020) at ca. 5 Ma records tectonic activity in the eastern Transverse Ranges that likely was related to transtensional faulting, further suggesting early development and antiquity of the western part of the modern ECSZ.…”