Cenozoic Tectonic Evolution of the Central Sierra Nevada: Clues from Volcanic Stratigraphy

J. C. Hagan and C. J. Busby
University of California, Santa Barbara, CA, [email protected], [email protected]

We use volcanic and volcaniclastic mapping, geochemistry, and geochronology to determine the tectonic history of the central Sierra Nevada, between Carson Pass and Sonora Pass. There, Oligocene and Miocene strata fill paleochannels inherited from Cretaceous time. Through our mapping of paleochannel fills and erosional unconformity surfaces within them, we recognize three pulses of ancestral Cascades arc magmatism and four erosional episodes that may correspond to uplift events. We also use the dateable fill of these paleochannels to determine the offset history of the Sierra Nevada range-front faults.

The first erosional unconformity was cut into the granitic basement in Cretaceous time, between 93 and 32 Ma. From ~32 to ~23 Ma, the paleocanyons filled with Oligocene ignimbrites erupted in central Nevada. The second unconformity formed between ~23 and 15.5 Ma, likely due to uplift at the onset of ancestral Cascades arc activity, when most of the ignimbrite paleocanyon fill eroded away, leaving thin deposits at the base and on the walls of the paleocanyons. The paleocanyons then filled with block-and-ash-flow tuffs, which were reworked down-canyon into volcanic debris flow and streamflow deposits, between about 15 and 13 Ma. At ~11 to10 Ma a third re-incision event occurred in the region. At Sonora Pass this event was accompanied by transtensional range-front faulting and voluminous high-K volcanism, whereas ~10.5 Ma magmatism at Carson Pass is recorded by small volume andesitic lava domes, block-and-ash-flow tuffs, peperites and volcanic necks, with no clear evidence for faulting there. A fourth re-incision event preceded major volcanic activity at Carson Pass at 6.5 Ma, coupled with formation of the Hope Valley graben. We use the geometry of the paleocanyon system to show that unlike other faults in the Sierra Nevada range front, the faults in the Hope Valley graben show no dextral component of slip.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009