Implications of Pleistocene Volcanic Rocks in the Northwest Part of the Lake Tahoe Basin for Evolution of Proto-Tahoe and Crust-Mantle Structure
AuthorKortemeier, Winifred T.
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Basaltic and trachyandesitic volcanic rocks in the northwest part of the Lake Tahoe basin, northern Sierra Nevada, California (USA), have been studied to obtain new insights regarding the interactions of lavas with the waters and sediments of ancient Lake Tahoe (Proto-Tahoe), and the characteristics and evolution of the source for the magmas, and crust and mantle structure. Field relationships, volcanologic characteristics, ages, and geochemistry of these rocks have been examined. The Pleistocene volcanic rocks were erupted in a tectonic setting in which normal fault activity associated with the encroachment of Basin and Range extension was synchronous with cessation of subduction and development of a slab window as the southern edge of the Gorda plate migrated north of this region. From 2.3-2.0 Ma, basaltic lavas were erupted, followed by eruption at 0.92 Ma of lavas with an adakitic trachyandesite composition. This study documents a spectacular set of hydrovolcanic features formed when Pleistocene volcanic rocks erupted and/or flowed into an ancestral lake called Proto-Tahoe and, in some cases, interacted with wet sediments. Some of the lava flows crossed Proto-Tahoe shorelines and quenched beneath water. This study demonstrates that lava interacted with water and wet sediments during three intervals: basaltic lava flowed into Proto-Tahoe and over wet sediments at 2.3 Ma, and from 2.1-2.0 Ma; and trachyandesitic lava flowed into the lake and invaded wet sediments at 0.92 Ma. This study shows that the interactions of lava with lake water and/or sediments produced in situ-fragmented breccias, peperites, littoral cones, tuff cones, lava deltas, pillow lavas, and pillow breccias. This study confirms earlier suggestions that Pliocene-Pleistocene lava flows may have dammed the Truckee River, and establishes that the Pleistocene volcanic rocks record three cycles of damming and down-cutting within the Truckee River canyon. At 2.3 Ma, basaltic lavas dammed the outlet of Proto-Tahoe and raised lake level from ~1896 m to 2048 m. At 2.1-2.0 Ma, another series of basaltic lava flows dammed the outlet of Proto-Tahoe and raised lake levels from 1914 m to 2073 m. And finally, trachyandesitic lavas dammed the outlet of Proto-Tahoe and raised lake level to 2085 m at 0.92 Ma. The lava flows document ancient lake levels up to 186 m above the present lake level during the interval 0.924 to 2.27 Ma. The new data show that the outlet for Proto-Tahoe/Lake Tahoe through the Truckee River canyon was established as early as 2.3 million years ago. The results of this study agree with previous studies that indicated that post-subduction magmatism in the Tahoe region formed from partial melting of sub-continental mantle lithosphere that was enriched by subduction-derived fluids. New geochemical and isotopic data reveal that the source for the Pleistocene volcanic rocks was also enriched by slab melts. Melting occurred in the presence of garnet and amphibole and in the absence of plagioclase and formed magmas with a subducted slab-melt signature, referred to as adakites, even though subduction had ended in this region at approximately 4 Ma. Newly published data indicates that the lower crust and upper mantle are not present in the northern Sierra, yet the volcanic rocks described here paradoxically show evidence of melting of such rocks. To resolve the paradox, it is proposed that post -subduction magmatism at Lake Tahoe was triggered by piecemeal removal (delamination) of lowermost crust and underlying heterogeneous mantle lithosphere and melting of this foundered material. As proposed by other authors, the initiation of Basin and Range extension in the Tahoe area within the last 3 Ma, coupled with magmatism associated with the ancestral Cascade arc, may have destabilized the dense batholithic root at the base of the crust and initiated the delamination event.