INVESTIGATING STYLES OF CONTRACTION AND OVERPRINTING BY RIO GRANDE RIFT EXTENSION IN THE SOUTHERN INDIO MOUNTAINS, WESTERN TEXAS

Rocks exposed in the southern Indio Mountains, western Texas, provide an important geologic record of contraction and overprinting by Rio Grande rift extension. The range exposes Cretaceous sedimentary rocks that were initially deposited along the northern margin of the Jurassic to Cretaceous Chihuahua trough. Late Cretaceous to Eocene contraction inverted the Chihuahua trough and thrust these rocks towards the northeast onto the Diablo Plateau. This region has variably been referred to in the literature as being part of the Laramide orogen as well as the Mexican fold and thrust belt (MFTB), which is part of the larger Mexican orogen. The Laramide orogeny overlaps in space and time with thin-skinned Sevier deformation to the west and with the MFTB to the south, although a hallmark of Laramide contraction is the development of basement-cored faults, arches, and basins. This ongoing study aims to constrain timescales and styles of contraction in the southern Indio Mountains to compare to better-studied Laramide systems of southern New Mexico and Chihuahua trough inversion structures of the MFTB. Preliminary mapping indicates that contraction is concentrated along low-angle to almost flat NW-trending thrust faults that translated hanging wall rocks towards the northeast. Whereas some thrust faults are widely-spaced with little intervening deformation, other regions are complexly folded and faulted. Contractional structures in Cretaceous sedimentary rocks do not appear to extend into the underlying basement, which may be more similar in style to MFTB than Laramide deformation. Although Cretaceous rocks are overlain by younger Cenozoic units, contractional structures are restricted to Cretaceous rocks, are beveled by a regional unconformity, and are offset by younger Rio Grande rift normal faults. Existing geochronology suggests that contraction ceased by 38.1 Ma and shifted to a period of widespread volcanism. This ongoing research aims to utilize drone images and digital field mapping techniques to understand styles and evolution of contractional deformation in western Texas as a comparison with more-thoroughly studied regions within the Laramide orogen and MFTB.