NEW DIGITAL GEOLOGIC MAP OF THE NORTHERN LAKE TAHOE AND DONNER PASS REGION, NORTHERN SIERRA NEVADA, CALIFORNIA

Nearly 1300 sq km of bedrock and surficial geologic mapping have been completed at the crest of the northern Sierra Nevada by 138 geology UCSB students in nine intermittent summer field camp seasons between 1988 to 2006. The only previous comprehensive mapping in the area was published by Waldemar Lindgren in his 1895 USGS Folio of the Truckee Quadrangle. The UCSB map is compiled in GIS format using ArcMap9. It straddles the Tahoe-Truckee graben from the Pacific Crest Trail east to the California-Nevada state line, and from Homewood on the west shore of Lake Tahoe, northward a few kilometers north of I-80. It comprises the Norden, Truckee, Martis Peak, Granite Chief, Tahoe City, and Kings Beach quadrangles, and includes the towns of Truckee and Tahoe City, as well as the Upper Truckee River drainage. The west side of the graben is marked through the map area by the northern extension of the Eastern Sierra Fault Zone of Schweickert et al (2000).

The rocks and alluvial deposits have been subdivided into 100 map units, ranging from late Paleozoic and Jurassic metamorphic rocks; several Cretaceous granitic plutons on the shoulders of the graben; a thick section of nine Oligocene rhyolitic ash flow tuffs of the Valley Springs Formation in the Onion Creek paleocanyon; three volcanic members of the Neogene Mehrten Formation that include pyroxene andesite, hornblende andesite, andesitic basalt, debris avalanche deposits, and corresponding intrusive and pyroclastic units; diverse basaltic and trachybasaltic lava flows and intervening alluvial units assigned to the newly designated Truckee River Formation (2.5-1 my); Pleistocene till and outwash of the Donner, Tahoe, and Tioga glaciations; to recent landslides, talus, lake deposits, and stream alluvium.

Mehrten Formation members are: Mt. Lincoln (13-7 my), Martis Peak (7-5 my), and Squaw Peak (5-3 my). Eruptive centers at Squaw Peak and Martis Peak contributed major proportions of rock to each member. A swarm of mafic dikes emanates from Squaw Peak. The mapping suggests that Lake Tahoe formed 3.7-3.4 my ago when voluminous biotite-hornblende andesite and dacite domes erupted and blocked the ancestral Truckee River that once drained the floor of the graben. We find no compelling field evidence for recent surface displacement on any of the faults in the region.