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