PATTERNS OF LATE PLIOCENE TO QUATERNARY FAULTING IN THE LAKE TAHOE REGION, CA-NV

The Lake Tahoe basin lies within the Sierra Nevada-Great Basin boundary zone (SNGBBZ). Most faults in other parts of the SNGBBZ strike NW to N, dip easterly, and show east-side-down normal to dextral-oblique displacements. However, fault patterns in the Lake Tahoe region are complex, and normal fault systems root both to the east, as expected, and to the west beneath the Sierra Nevada block. Prominent Late Quaternary to Holocene faults (set A) of the Tahoe-Sierra frontal fault zone generally strike ~N30W, dip steeply NE, and show NE-side-down displacement. However, an older set of faults (set B) strikes ~N50W and shows SW-side down displacements. Faults of set B have little topographic expression and presumably are inactive, although they may have been reactivated in places. Both A and B sets of faults displace andesitic flows locally as young as ~3.6 Ma (Harwood, in Saucedo and Wagner, 1992).

Faults of set B record significant SW-NE extension after 3.6 Ma, and comprise an extensional episode not previously recognized. Set B in some cases predates 1-2 Ma basalts and latites, predates the present configuration of Lake Tahoe basin, and may have impounded earlier, pre-2 Ma lakes. Such faults appear to be widely distributed between McKinney Creek on the south and Independence Lake in the north, and between Donner Pass on the west and Reno on the east. Set B faults are probably listric and are responsible for east to northeast dips of Miocene and Pliocene volcanic rocks throughout the region. Some of the set B faults die out along strike within the Sierra Nevada block.

Normal faults of set A in many places cut and displace the east-dipping sections, resulting in anomalous tilt domains. Set A records significant WSW-ENE extension and dextral transtension. Some faults of set A were reactivated as dextral strike-slip faults in the current regime of transtensional deformation. Near the present Sierra crest, stratal dips of Miocene-Pliocene volcanic rocks locally define anticlinal folds formed by the interference of both fault sets. In contrast, domains only affected by set A (Late Quaternary east-side down normal faults) show prominent west-dipping strata in fault blocks.

The faults have important implications for evolution of the SNGBBZ and the Lake Tahoe basin, for transtensional deformation, and for Sierran crustal thicknesses.