POST-3MA DEFORMATION ALONG THE SIERRA NEVADA - BASIN AND RANGE TRANSITION ZONE NEAR RENO, NV

Exhumation and faulting of Neogene sedimentary rocks records the deformation history of the Sierra Nevada - Basin and Range transition zone near Reno, NV in the last 3 Ma. Tephras and mammal fossils high in the Neogene Verdi basin section constrain the timing of the deformation. Deposition in the Verdi basin was terminated by tectonism that produced dramatic down-cutting, an abrupt shift in paleocurrent direction, and unroofing of crystalline source rocks. This uplift/arroyo-cutting event was followed by eastward tilting and broad synclinal folding of both the Neogene section and the coarse clastic arroyo fill. Some faulting within the section accompanied tilting, but most appears to have post-dated the tilting event.

Several fault sets cut the tilted Neogene section. The dominant ones are: NW-striking dextral faults, NE-striking sinistral faults, and N-striking normal or normal-dextral oblique faults. Strike-slip faults with m- to 10m-scale offset are broadly distributed; those with the largest slip are most commonly dextral faults. Mutually cross-cutting relationships between the strike-slip fault sets are observed locally. Several NNE-striking faults with 10s to 100s of m of down-to-the-east offset repeat the Neogene section and appear to be some of the most recent structures in the area.

The distributed strike-slip faulting in the Verdi basin is anomalous for post-depositional deformation of Neogene basins along the eastern boundary of the Sierra in northern Nevada. In the Gardnerville basin to the south and Long Valley basin to the north, the sedimentary section is preserved in a west-tilted half-graben and internal deformation is normal faulting. The pre-3Ma record is also unlike its neighbors -- the Verdi basin records a relatively stable depocenter with little active tectonism from 11 to 3 Ma.