LATE MIOCENE TILTING OF THE RESTING SPRING RANGE, CALIFORNIA

Models of crustal extension for the central Basin and Range Province depend critically upon the timing of faulting implied by the tilt of Cenozoic volcanic and sedimentary rocks. The Resting Spring Range, located 50 km east of Death Valley, is of particular interest because a systematic upward decrease in dip within a Miocene succession of breccias, tuffaceous sandstones, dacite flows, ashes and welded ashflow tuffs can be interpreted to constrain fault timing on both sides of the present-day range. The inclination of Miocene strata varies from ~30° NE at the base, to 18-23° ENE for at least several hundred meters of dacite, to in the order of 5-10° for the youngest ashflow tuffs. The orientations of paleovalleys with at least several tens of meters of erosional relief and of paleocurrents low in the succession are consistent with fluvial transport to between north and west, approximately parallel to the present range in the vicinity of Resting Spring Pass. This implies that much if not all of the ~16° NE excess tilt of unconformably underlying Cambrian carbonate rocks occurred before the onset of Miocene extension. We therefore infer that 20-40% of the extensional tilt was acquired prior to dacitic volcanism, provisionally dated as 9.8 Ma (K/Ar age; E. Heydari Laibidi, 1981, Masters thesis), on one or more faults within the Chicago Valley to the east. The remaining 60-80% of the tilt occurred later on the range-bounding fault on the west side of the Resting Spring Range and on a mostly buried fault on the NE side of Resting Spring Pass. How this younger deformation was partitioned in time with respect to the welded ashflow tuff, for which Wright et al. (1991) report a K/Ar age of 9.6 Ma, is not yet resolved because the welded tuff overlies erosional topography and lacks consistent indicators of paleohorizontal. Nevertheless, if much of the tilting occurred after ~9.8 Ma, the timing is currently indistinguishable from our ⁴⁰Ar/³⁹Ar geochronology of upper Miocene volcanic rocks at Sheephead Mountain, 20 km to the WSW, where most of the tilting took place after 9.5 Ma. ⁴⁰Ar/³⁹Ar geochronology of the volcanic succession in the vicinity of Resting Spring Pass is under way to sharpen constraints on the timing of tilting.