Pleistocene volcanism and shifting shorelines at Lake Tahoe, California

In the northwestern Lake Tahoe Basin, Pleistocene basaltic and trachyandesitic lavas form a small volcanic field comprising ∼1 km³ of lava that erupted from seven vents. Most of these lavas erupted subaerially and produced lava flows. However, where they flowed into an early Lake Tahoe (Proto-Tahoe), they produced deltas consisting of hydrovolcanic breccias as well as pillow lavas draped downslope, pillow breccias, hyaloclastites, and mixtures of lava and wet sediments. Consequently, various former shorelines of Proto-Tahoe are marked by subaerial lava flows overlying subaqueous lava deltas. Isolated explosive interactions produced lapilli tuff cones that built upward from vents on the lake floor or grew as littoral cones where subaerial lava flows crossed the shoreline. Six new ⁴⁰Ar/³⁹Ar ages define three Pleistocene episodes when lava erupted subaerially and flowed into Proto-Tahoe. Three cycles of canyon damming by lava and down-cutting occurred at the outlet of Proto-Tahoe in the Truckee River Canyon. The canyon was dammed at 2.3 Ma by basaltic lavas at Rampart, which raised lake level from ∼1897 m above sea level to 2048 m. The canyon was again dammed at 2.1 Ma by basaltic lavas at the outlet of Proto-Tahoe near Rampart, which raised lake levels from ∼1914 m to 2073 m. And finally, the canyon was again dammed at 0.94 Ma by trachyandesitic lavas at Thunder Cliffs, which raised lake level to 2085 m. Hence, ancient shorelines that are nearly 200 m above the present lake level are documented at 0.94, 2.1, and 2.3 Ma. The present outlet of Lake Tahoe through the Truckee River canyon has been operative for at least 2.3 million years. Even though the three lava dams are now eroded away, the repeated construction (and removal by erosion) of lava dams has diminished the erosion and deepening of the Truckee River Canyon that otherwise would have occurred. Hence, the soft-sediment sill of Lake Tahoe has been protected, which has helped to maintain the great depth of the lake (500 m). The timing of this repetitive volcanic activity raises implications for future volcanic eruptions and their hazards. The lake could be dammed by lava again causing extensive shoreline flooding as its level rose, or rapid dam failure could cause extensive downstream flooding along the Truckee River on its path to Reno.