David L. Shuster | USGS Science

USGS2023

Crustal block-controlled contrasts in deformation, uplift, and exhumation in the Santa Cruz Mountains, California, USA, imaged through apatite (U-Th)/He thermochronology and 3-D geological modeling

Deformation along strike-slip plate margins often accumulates within structurally partitioned and rheologically heterogeneous crustal blocks within the plate boundary. In these cases, contrasts in the physical properties and state of juxtaposed crustal blocks may play an important role in accommodation of deformation. Near the San Francisco Bay Area, California, USA, the Pacific−North American plate-bounding San Andreas fault bisects the Santa Cruz Mountains (SCM), which host numerous distinct, fault-bounded lithotectonic blocks that surround the San Andreas fault zone. In the SCM, a restraining bend in the San Andreas fault (the SCM bend) caused recent uplift of the mountain range since ca. 4 Ma. To understand how rheologic heterogeneity within a complex fault zone might influence deformation, we quantified plausible bounds on deformation and uplift across two adjacent SCM lithotectonic blocks on the Pacific Plate whose stratigraphic and tectonic histories differ. This was accomplished by combining 31 new apatite (U-Th)/He ages with existing thermochronological datasets to constrain exhumation of these two blocks. Additionally, surface exposures of the latest Miocene to late Pliocene Purisima Formation interpreted in 18 structural cross sections spanning the SCM allowed construction and restoration of Pliocene deformation in a three-dimensional geologic model. We found that rock uplift and deformation concentrated within individual Pacific Plate lithotectonic blocks in the SCM. Since 4 Ma, maximum principal strain computed for the more deformed block adjacent to the fault exceeded that computed for the less deformed block by at least 375%, and cumulative uplift has been more spatially extensive and higher in magnitude. We attribute the difference in uplift and deformation between the two blocks primarily to contrasts in lithotectonic structure, which resulted from diverging geologic histories along the evolving plate boundary.

California↗

USGS2020

Timing of Cenozoic extension in the southern Stillwater Range and Dixie Valley, Nevada

The Dixie Valley fault bounds the east side of the Stillwater Range in west‐central Nevada and last ruptured in 1954. Offset basalts indicate that slip began more recently than ~14 Ma, and prior work has interpreted the southern segment as an active low‐angle normal fault. Oligocene igneous rocks in the southern Stillwater Range were steeply tilted during large‐magnitude extension prior to ~14 Ma. To refine the timing of early extension and the onset of slip on the Dixie Valley fault, we collected two transects of samples for apatite fission track, apatite and zircon (U‐Th)/He (AHe and ZHe), and apatite 4 He/ 3 He thermochronometry. Apatite fission track ages from the Oligocene IXL pluton indicate rapid cooling ~18–14 Ma, and AHe and ZHe ages from the Cretaceous La Plata Canyon pluton indicate rapid cooling ~16–19 Ma. We interpret these data to record cooling during rapid extension. AHe ages from the IXL pluton are ~6–8 Ma and record cooling during slip on the Dixie Valley fault. We modeled these ages and 4 He/ 3 He spectra from one sample as the result of cooling during exhumation of a tilted fault block at a constant extension rate. The model predicts slip on the Dixie Valley fault beginning ~8 Ma. Although it does not constrain the initial fault dip, the model illustrates how a low‐angle fault requires a higher extension rate to reproduce cooling ages. Consequently, we prefer a high‐angle southern Dixie Valley fault for strain compatibility with the high‐angle northern segment.

Nevada↗