Age, chemical, and isotopic data from late Cenozoic volcanic rocks in the northern Sierra Nevada, California (USA), from Lake Tahoe north to the southern end of the modern Cascades volcanic arc, were obtained to investigate the evolution of the upper mantle beneath this continental margin during the transition from active subduction to the opening of a slabless window, and to test the possibility that the foundering of mantle lithosphere proposed for the southern Sierra Nevada extended to the northern reaches of the mountain range. Our data are consistent with previous work in the region and illustrate that volcanism shifted from widespread intermediate composition magmatism to small volume, localized trachybasalts to trachyandesites ca. 3 Ma. Similar to southern Cascades volcanism, 87 Sr/ 86 Sr and 206 Pb/ 204 Pb decrease, and ε Nd increase in the older (older than 3 Ma) volcanic rocks with increasing proportions of a slab component, as measured by increasing (Sr/P) N , where N is primitive-mantle normalized. We interpret these observations as evidence that the older volcanic rocks are subduction related and represent the products of basaltic melts derived from fl ux melting of mantle wedge that interacted to varying degrees during ascent with lower ε Nd and higher 87 Sr/ 86 Sr sub-Sierra Nevada continental mantle lithosphere. The younger volcanic rocks lack evidence for the involvement of a slab compo- GEOLOGIC SETTINGThe Sierra Nevada mountain range in eastern California (Fig. 1) has been the subject of intense investigation into the processes involved For permission to copy, contact editing@geosociety.org

In the northwestern Lake Tahoe Basin, Pleistocene basaltic and trachy andesitic lavas form a small volcanic field comprising ~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 ( ProtoTahoe), they produced deltas consisting of hydrovolcanic breccias as well as pillow lavas draped downslope, pillow breccias, hyaloclastites, and mixtures of lava and wet sediments.

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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.

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