Abstract

Large rock-avalanche deposits are extremely useful tools in paleogeographic and tectonic reconstructions. These deposits are large (>0.1 km3), semicoherent "snapshots" of source area that were emplaced instantaneously in a sedimentary basin. A rock-avalanche deposit with a known age in a sedimentary basin requires the restoration of that basin to a position adjacent to the rock- avalanche source area at the time of emplacement. The Miocene sedimentary section of the Amargosa Chaos and Sperry Hills basins in the southern Death Valley region contains deposits of large rock-avalanches from both spatially and temporally separated source terranes.

The rock-avalanche deposits in the Amargosa Chaos 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 Amargosa Chaos basin is equivalent to the pre-7.8 Ma stratigraphy of the Miocene sedimentary section in the Sperry Hills. Prior to ∼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 was a half-graben opening along a west-dipping normal fault with substantial vertical relief in the uplifted footwall. Evidence for this topography exists in the form of multiple large rock-avalanche deposits derived from footwall bedrock.

The axis of this basin trends northwest with the active basin-bounding normal fault on the northeast margin. At ∼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 is required between 7.8 Ma and 4.9 Ma between the Sperry Hills basin and the location of Amargosa Chaos basin in the southern Black Mountains. Continued, post-4.9 Ma, northwest- directed extension is required between 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 Range.

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