LATE NEOGENE DEPOSITION AND GROWTH OF A WEST-NORTHWEST TRENDING FAULT-BOUNDED BASIN, NORTHERN PALMETTO MOUNTAINS, SOUTHWEST NEVADA

The northern flank of the Palmetto Mountains in southwest Nevada exposes a 2 km section of Neogene volcanic and sedimentary rocks deposited during and after subsidence of a basin bounded by WNW-striking faults. The basin is at least 11 km long and up to 4 km wide and contains a synextensional succession of rocks consisting of a 1.2 km sequence of mid- to late Miocene andesite flows and lahar overlain conformably by about 350 m of tuffaceous sedimentary rocks and ashflow tuff. The basin is bounded to the south by the central segment of the Palmetto Mountain fault that separates the synorogenic rocks from Paleozoic strata and Jurassic granitoid plutons. To the north, the basin is bounded by south-dipping WNW-striking faults that separate the synorogenic rocks from Paleozoic strata. Internally, the basin is segmented by NNE-striking faults that juxtapose different stratigraphic successions and show evidence of differential displacement during and following deposition of the Miocene andesite and tuff-tuffaceous sequence. In the eastern segment of the basin, the dip of synextensional rocks decrease in southwest dip upsection from 45° to about 20°. In the western part of the basin, the andesite rests on Paleozoic strata and dips shallowly to the northeast and thickens from 70 m to over 260 m. Paleozoic basement and the andesite are intruded by a series of east-west striking mafic dikes dated elsewhere as 10 to 12 Ma that may be related to development of the basin. The synextensional rocks are unconformably overlain with angular unconformity by a 15 to 25 m thick succession of welded tuff and tuffaceous sedimentary rocks. The tuff and tuffaceous sedimentary rocks locally are folded in gentle WNW trending folds and unconformably overlie and seal WNW-trending syndepositional faults. The synorogenic rocks and overlying welded tuff and tuffaceous sedimentary rocks are both overlain with angular unconformity by a regionally extensive ashflow tuff and tuffaceous sedimentary sequence dated elsewhere as 6.0 to 4.0 Ma. The uppermost succession of tuff and tuffaceous sedimentary rocks is homoclinal with NNW strikes and 10° to 20° NE dips. The upper succession is cut by the Palmetto Mountain fault, which has contemporary displacement.