PALEOGEOGRAPHIC RECONSTRUCTION OF THE DEATH-VALLEY EXTENDED REGION - EVIDENCE FROM MIOCENE LARGE ROCK-AVALANCHE DEPOSITS IN THE AMARGOSA CHAOS BASIN, CALIFORNIA

Large rock-avalanche deposits are extremely useful tools in paleogeogr aphic and tectonic reconstructions. These deposits are large (>0.1 km3 ), semicoherent ''snapshots'' of a source area that were emplaced inst antaneously in a sedimentary basin. A rock-avalanche deposit with a kn own age in a sedimentary basin requires the restoration of that basin to a position adjacent to the rock-avalanche source area at the time o f emplacement. The Miocene sedimentary section of the Amargosa Chaos a nd Sperry Hills basins in the southern Death Valley region contains de posits of large rock-avalanches from both spatially and temporally sep arated source terranes. The rock-avalanche deposits in the Amargosa Ch aos basin require the restoration of the basin to within 10 km of the Kingston Range prior to 7.8 Ma. West-derived alluvial fan sediments in this basin can be confidently tied to the southern Panamint Mountains , with early east-derived alluvial fan sediments (pre-7.8 Ma) tied to the Kingston Range. Moreover, the pre-7.8 Ma stratigraphy of the Amarg osa Chaos basin is equivalent to the pre-7.8 Ma stratigraphy of the Mi ocene sedimentary section in the Sperry Hills. Prior to approximately 7.8 Ma, the Amargosa Chaos and Sperry Hills basins were both parts of a larger sedimentary basin. Sedimentologic constraints on the geometry of the Amargosa Chaos basin indicate that throughout its history it w as a half-graben opening along a west-dipping normal fault with substa ntial vertical relief in the uplifted foot-wall. Evidence for this top ography exists in the form of multiple large rock-avalanche deposits d erived from footwall bedrock. The axis of this basin trends northwest with the active basin-bounding normal fault on the northeast margin. A t approximately 7.8 Ma, the extension direction in the southern Death Valley region changed from southwest to northwest. Younger sediments, including three younger rock-avalanche deposits, record the motion of the Amargosa Chaos basin away from the Sperry Hills basin. Top-to-the- northwest displacement of 30 km along the Amargosa detachment system i s required between 7.8 Ma and 4.9 Ma between the Sperry Hills basin an d the location of Amargosa Chaos basin in the southern Black Mountains . Continued, post- 4.9 Ma, northwest-directed extension is required be tween the Amargosa Chaos basin and the southern Panamint Mountains. In addition, 15 km of right-lateral slip on the southern portion of the Grand View fault is required from 7.8 to 3 Ma to move the Sperry Hills basin away from its pre-7.8 Ma position adjacent to the Kingston Rang e.