TERTIARY STRATIGRAPHY AND STRUCTURE OF THE VIRGINIA MOUNTAINS, WESTERN NEVADA: IMPLICATIONS FOR DEVELOPMENT OF THE NORTHERN WALKER LANE

The Virginia Mts (VM) in W. Nevada occupy a left-step between two NW-striking dextral faults in the northern Walker Lane, the Pyramid Lake fault (PLF) on the east and Warm Springs Valley fault (WSF) on the west. The oldest Tertiary rocks are ~30 to 23 Ma rhyolitic ash-flows and domes, associated with the southward advancing ignimbrite flareup, and rock avalanche deposits shed from the flanks of the domes. These strata accumulated in an ~E-trending paleovalley carved into Mesozoic basement. The ~14-12? Ma Pyramid sequence (PS) overlies the tuffs and consists of a nearly 1 km thick section of dominantly mafic lavas, fanglomerate, and lesser diatomite. A shield volcano in the VM was the source for most of these lavas. Some lavas, including distinct porphyritic basalts, filled N-trending paleovalleys. The PS was probably associated with the ancestral Cascade arc, which was dominated by relatively mafic volcanism and locally punctuated by large stratovolcanoes.

Kinematically linked WNW-striking normal-dextral and NNW-striking normal faults, as well as a broad N-trending anticline, characterize the structure of the VM. Many faults in the eastern VM may be splays of the PLF. The anticline was produced by the intersection of opposing rollovers developed in the hanging walls of the E-dipping Dogskin-Black Canyon and W-dipping Lake Range listric normal faults, as evidenced by increasing dips toward these faults. A paucity of both reverse faults and contractional folds indicates that the left step in the Walker Lane has not accommodated appreciable shortening. The timing of deformation is not tightly constrained. NNW-striking mafic dikes suggest some WSW extension during PS deposition. Roughly concordant tilts in the Oligocene tuffs and entire PS imply, however, that most deformation postdates the mid-Miocene. Late Miocene-Pleistocene fan deposits are cut and locally highly deformed by strike-slip and normal faults. In the mid to late Miocene, dextral motion in the central Walker Lane may have been transferred into E-W magmatic and mild mechanical extension in this region, which possibly facilitated magmatism in the ancestral arc. Since the late Miocene, the Walker Lane has propagated through this area into NE California with a corresponding NW retreat of the Cascade arc in concert with the northward track of the Mendocino triple junction.