Reconstructing the erosional and tectonic record of Laramide contraction to Rio Grande rift extension, southern Indio Mountains, western Texas, USA

Abstract

Rocks exposed in the southern Indio Mountains provide an important geologic record of the transi-tion 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 fill a paleovalley, and (3) Miocene conglomer-ate 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 through an EW-trending paleovalley. New and existing 40Ar/39Ar geochronol-ogy constrains the timing of paleovalley deposition to 38.1-36.6 Ma. The presence of megabreccia units, coupled with paleoflow analysis, argue that major Laramide topographic relief in western Texas per-sisted into the Middle Eocene and that detritus was shed toward the Tornillo basin to the east. These data, when viewed within the context of regional tectonic patterns, suggest that Laramide defor-mation in western Texas had ceased by 38.1 Ma, although they do not preclude translation along an underlying buried thrust. Eocene paleovalley cutting and filling are reminiscent of widespread Eocene erosion and fluvial deposition that occurred in Colorado and northern New Mexico and suggest that this event may have been more widespread than previously thought, extending into Chihuahua and western Texas. Subsequent Rio Grande rift extension occurred primarily along several NW-striking normal faults that dissected the older structures and the paleovalley and led to deposition of conglomerate within a half-graben. Apatite (U-Th)/He thermo-chronology applied to normal fault footwall rocks indicates exhumational cooling was occurring by 27 Ma, and detrital sanidine 40Ar/39Ar geochronol-ogy of basin fill indicates a maximum depositional age of 11.9 Ma. Clast count data from the conglom-erate show a prominent unroofing trend, wherein clasts include locally derived Mesozoic units and Eocene volcanic rocks.