RECONSTRUCTING THE TRANSITION FROM LARAMIDE CONTRACTION TO RIO GRANDE RIFT EXTENSION IN THE SOUTHERN INDIO MOUNTAINS, WESTERN TEXAS

Rocks exposed in the southern Indio Mountains provide an important geologic record of the transition from Laramide contraction to Rio Grande rift extension. These rocks include 1) a package of folded and faulted Mesozoic rocks deformed during Laramide compression, 2) Eocene volcanic and sedimentary rocks that are tilted but not folded, and 3) Miocene conglomerate that is deposited within a Rio Grande rift half-graben. We present a combination of geologic mapping, sedimentological and structural analysis, and geo/thermochronology to document the timing and nature of this transition in western Texas as an important comparison with the more thoroughly studied tectonic and erosional history of New Mexico and Colorado. Detailed geologic mapping and cross-section reconstruction reveal a highly irregular angular unconformity overlying Mesozoic rocks and that post-Laramide topography funneled local Eocene deposition within an EW-trending paleovalley. New and existing 40Ar/39Ar geochronology constrain the timing of paleovalley deposition to 38.1-36.6 Ma. Subsequent Rio Grande rift extension occurred primarily along several NW-striking normal faults that dissected the older rocks and led to deposition of conglomerate within a half-graben. Apatite (U-Th)/He thermochronology applied to normal fault footwall rocks indicate exhumational cooling occurred from 31-19 Ma. In addition, detrital sanidine 40Ar/39Ar geochronology of basin fill indicate a maximum depositional age of 11.9 Ma. Clast count data from the conglomerate show a prominent unroofing trend, where clasts include locally-derived Mesozoic units and Eocene volcanic rocks. Together, these data hint that Laramide deformation ceased by 38.1 Ma in western Texas, although it does not preclude translation along an underlying buried thrust. Eocene paleovalley deposition is reminiscent of the widespread Eocene erosion and fluvial deposition that occurred in Colorado and northern New Mexico and suggests that this event may have been more widespread than previously thought. The timing of Rio Grande rift extension in western Texas overlaps with the timing of extension in the central and northern segments of the rift.