Insights from low-temperature thermochronometry into transpressional deformation and crustal exhumation along the San Andreas fault in the western Transverse Ranges, California

Abstract

The San Emigdio Mountains are an example of an archetypical, transpressional structural system, bounded to the south by the San Andreas strike‐slip fault, and to the north by the active Wheeler Ridge thrust. Apatite (U‐Th)/He and apatite and zircon fission track ages were obtained along transects across the range and from wells in and to the north of the range. Apatite (U‐Th)/He ages are 4–6 Ma adjacent to the San Andreas fault, and both (U‐Th)/He and fission track ages grow older with distance to the north from the San Andreas. The young ages north of the San Andreas fault contrast with early Miocene (U‐Th)/He ages from Mount Pinos on the south side of the fault. Restoration of sample paleodepths in the San Emigdio Mountains using a regional unconformity at the base of the Eocene Tejon Formation indicates that the San Emigdio Mountains represent a crustal fragment that has been exhumed more than 5 km along the San Andreas fault since late Miocene time. Marked differences in the timing and rate of exhumation between the northern and southern sides of the San Andreas fault are difficult to reconcile with existing structural models of the western Transverse Ranges as a thin‐skinned thrust system. Instead, these results suggest that rheologic heterogeneities may play a role in localizing deformation along the Big Bend of the San Andreas fault as the San Emigdio Mountains are compressed between the crystalline basement of Mount Pinos and oceanic crust that underlies the southern San Joaquin Valley.

A Model for the Initiation, Evolution, and Controls on Seismic Behavior of the Garlock Fault, California

“…But if the early phases of slip along the newly established, mechanically immature western Garlock occurred at slower rates than the current 7.5 mm/year, then the ~25 km of SAF bending could have taken much longer than 3 Myr, and the Garlock could have reached the SAF at an earlier date. Indeed, earlier contractional deformation manifest in early Pliocene (4–6 Ma) exhumation documented by Apatite (U‐Th)/He thermochronology within the Big Bend region (Niemi et al, ) is consistent with an earlier initiation of the Big Bend. This onset of accelerated exhumation rates in the Big Bend area circa 4–6 Ma (Niemi et al, ) suggests that the current efficient conjugate pairing of the SAF and the Garlock fault had been established prior to the onset of the earliest well‐developed NNW‐trending faults in the currently active ECSZ circa 3–4 Ma, when the Death Valley, Panamint Valley, and Owens Valley fault systems all began to accommodate rapid dextral shear (Burchfiel et al, ; Lee et al, ; Monastero et al, ; Norton, ) and dextral slip began in the southern ECSZ on the Blackwater fault (Andrew et al, ; Andrew & Walker, ; Oskin & Iriondo, ).…”

Thermochronologic Evidence for Late Eocene Andean Mountain Building at 30°S

“…Apatite fission track (AFT) thermochronology and apatite (U‐Th‐Sm)/He (AHe) dating provide a powerful means of identifying and constraining the timing and rate of exhumation of the crust in response to mountain building (e.g., Farley, ; Fitzgerald et al, ; Gallagher, Brown, & Johnson, ; Niemi et al, ; Reiners & Brandon, ). The combination of AFT thermochronology and AHe dating provide complementary but independent constraints on thermal (and hence exhumation) history of the uppermost crust and lead to more robust interpretations than that of a single thermochronometer (e.g., Fitzgerald et al, ; Reiners et al, ).…”

Talas–Fergana Fault Cenozoic timing of deformation and its relation to Pamir indentation

“…In the last decade, bedrock exhumation has been used as a powerful tool to quantify transpressive deformation (e.g. Spotila et al 2007;Niemi et al 2013). In strikeslip settings, restraining bends and horsetail splays accommodate local contraction and focus transpressional deformation (e.g.…”