EVOLUTION OF THE CENTRAL SIERRA NEVADA FRONTAL FAULT ZONE: CLUES FROM VOLCANIC STRATIGRAPHY

Previous workers have proposed that the Walker Lane belt, situated at the western edge of the Basin and Range, became active at 10-8 Ma, but that the most prominent faults, those of the Sierra Nevada range front, did not start moving until about 3 Ma. However, our new mapping in the central Sierra shows that significant extension, coupled with strike-slip displacement, began immediately before eruption of 10-9 Ma high-K volcanic rocks of the Stanislaus formation.

We present new results from Sonora Pass that show that mid-Miocene high-potassium volcanism in the central Sierra Nevada was contemporaneous with faulting in the same area. We have mapped mid-Miocene Valley Springs ignimbrites and early Late Miocene Relief Peak debris flow deposits dipping 40° towards a large range front-fault, overlain by low dipping Table Mountain Latite lava flows. We estimate at least 2000' of vertical offset along the fault before deposition of the latites. Slide blocks, up to several kilometers long, were unleashed in the region at this time. We have mapped one slide block, dated at 10.10 ± 0.06 Ma, that directly underlies 10.25 ± 0.06 Ma flat-lying Table Mountain Latite lava flows. We infer that these widespread landslide deposits, coupled with growth faulting, are evidence of a major tectonic event immediately prior to eruption of the high-K volcanic rocks.

These faults form the western boundary of the Little Walker Volcanic Center, which many workers have proposed is the source area for the high-K volcanic rocks. Our mapping suggests that the Walker center formed at a releasing stepover on dextral transtensional faults. We propose that the eruption of the high-potassium Stanislaus group resulted from tectonism at the western edge of the Walker Lane belt at its inception.