Implications of Pleistocene Volcanic Rocks in the Northwest Part of the Lake Tahoe Basin for Evolution of Proto-Tahoe and Crust-Mantle Structure

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

Implications of Pleistocene Volcanic Rocks in the Northwest Part of the Lake Tahoe Basin for Evolution of Proto-Tahoe and Crust-Mantle Structure

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

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 similar to 1 km(3) 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 Ar-40/Ar-39 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 similar to 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 similar to 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

The Tahoe-Sierra frontal fault zone, Emerald Bay area, Lake Tahoe, California: History, displacements, and rates

The location and geometry of the boundary between the Sierra Nevada microplate and the transtensional Walker Lane belt of the Basin and Range Province in the Lake Tahoe area have been debated. Two options are that the active structural boundary is (1) a few km west of Lake Tahoe, along the northwest-trending Tahoe-Sierra frontal fault zone (TSFFZ) or (2) within Lake Tahoe, along the largely submerged, north-trending West Tahoe-Dollar Point fault zone (WTDPFZ). Emerald Bay, a famous scenic locality at the southwest end of Lake Tahoe, is at the juncture between the TSFFZ and the WTDPFZ. There, utilizing high-resolution, multibeam-echosounder maps and derived bathymetric profiles, detailed field studies on land are integrated with bathymetric data and remotely operated vehicle observations to clarify the existence and activity of faults and sedimentology of the bay. Results include the most detailed structural maps of glacial moraines and the bottom of Lake Tahoe ever produced. Glacial moraines on both sides of Emerald Bay clearly have been deformed by normal displacements on faults within the TSFFZ and the WTDPFZ. Tectonic geomorphic features include scarps along moraine crests, locally back-tilted crests, and tectonic reversal of moraine crests, where older, higher moraines locally lie at lower elevations than younger, lower moraines. The alignment of crests of lateral moraines shows that dextral slip has not occurred during or since late Pleistocene glaciations. On the floor of Emerald Bay, submerged youthful faults that correspond to onshore faults that displace glacial moraines have numerous distinct, well-preserved, postglacial fault scarps, for which the vertical component of slip (vertical separation) is estimated. This study clearly demonstrates that the TSFFZ is the active structural boundary of the Sierra Nevada microplate and that the TSFFZ has a higher rate of slip than the WTDPFZ. It also provides evidence for complex range-front evolution, with both zones of normal faults active concurrently at various times