TECTONIC AND ISOSTATIC DEFORMATION OF LATEST PLEISTOCENE PLUVIAL LAKE SHORELINES, DIXIE VALLEY, NV

Dixie Valley marks the topographically lowest tectonic depression of the northern Great Basin and was host to the latest Pleistocene (~12 ka) pluvial Lake Dixie. High stand shoreline features (~1095 m) of Lake Dixie are locally well preserved and provide a paleohorizontal datum from which to assess post-12 ka deformation in the region. Most of the preserved shorelines on the west side of the valley formed on the steep Stillwater Range escarpment and consist of wavecut strandlines and cemented beach rock ledges. About 3 km south of Dixie Comstock Mine, high shorelines are disrupted by Holocene fault scarps (3.7-2.0 14C kyrs B.P.) that show up to 5 m of vertical offset. High shorelines mapped on both sides of the fault were surveyed by previous workers who concluded the shorelines were offset vertically by 10 m, which would require offset by multiple post-12 ka faulting events. Because these results are inconsistent with the geomorphic expression and measured offsets along the fault, we resurveyed the shorelines with a Total Station. We find that the high shorelines are vertically offset by only 5 m across the fault, identical to the offsets measured across scarps in this area. Our results indicate that the previous slip rate estimate of 1 mm/yr for the Dixie Valley fault based on the earlier shoreline survey is no longer valid. We also surveyed highstand beach barrier elevations on the east shore of the lake and found them to be 5 m lower than the highest strandlines along the Stillwater Range front to the west. We consider this difference a minimum because the beach deposits on the east side represent the maximum highstand elevation, whereas the high strandlines on the west are not required to have formed at the absolute maximum lake level. The 5 m difference in shoreline elevations across the valley is equal to the measured Holocene offset across the Dixie Valley fault. However, analogues of far field relative displacements across normal faults of similar geometry (e.g. Borah Peak) and over distances of the width of Lake Dixie (15 km) indicate that only a fraction (25%?) of the elevation difference can be accounted for by coseismic displacement. We suggest that most of the 5 m of eastward tilting of Lake Dixie shorelines is the result of differential isostatic rebound associated with the dessication of pluvial Lake Lahontan west of the Stillwater Range.