QUATERNARY SLIP HISTORY OF THE BARE MOUNTAIN FAULT (NEVADA) FROM THE MORPHOLOGY AND DISTRIBUTION OF ALLUVIAL FAN DEPOSITS

摘要

Analyses of alluvial fan sedimentation around Bare Mountain, Nevada, indicate differential slip along the Bare Mountain fault during the Pleistocene and Holocene. Analyses show that the ratios of fan area to sediment source area and the degree of Holocene sedimentation at the fan heads can be correlated with dip and displacement of the fault, The well-developed bajada and Holocene deposits near the fan toes along the southwestern flank of Bare Mountain contrast with the location of Holocene deposits near the fan heads and significantly smaller individual fan lobes on the eastern flank of the mountain (adjacent to the Bare Mountain fault), In addition, ratios of fan area to source area along the eastern flank of the mountain decrease from north to south, which we interpret to be caused by increased Quaternary slip (from north to south) along the fault, with relatively little Holocene throw at Tarantula Canyon and maximum throw near Wildcat Peak, The dip of the fault also changes from north to south, with relatively shallow dip angles (45 degrees to 50 degrees) at Tarantula Canyon and steeper (similar to 70 degrees) dips near Wildcat Peak. Geometric constraints dictate that, for any given increment of horizontal extension, vertical offsets will be smaller on those segments of the fault that have shallower dip angles. On the basis of these observations, we conclude that the recent slip rate of about 0.02 mm/yr derived from trenching studies in the Tarantula Canyon fan should be considered a minimum value that may not represent the slip rate of the Bare Mountain fault as a whole. [References: 21]