LARAMIDE DETACHMENT FOLDING IN THE DEVILS RIVER UPLIFT, WEST TEXAS, USA

Structures in Cretaceous strata in the Devils River Uplift of west Texas include folds, thrust faults, and bedding-parallel veins indicative of contractional deformation in a thrust faulting stress regime. In this study we analyze 120 small-wavelength (meters to tens of meters) anticlines and associated calcite veins in a series of roadcut exposures of the Boquillas Formation along Highway 90 northwest of Del Rio, Texas. The folds have WNW-ESE dominant trends, gently plunging axes, are gentle to tight with upright to inclined axial surfaces of variable vergence, and formed above detachments within volcanic ash or mudrock beds. Observed thickening in anticline cores was by ductile flow, centimeter-wavelength folding, and thrust or reverse faulting. Bed-perpendicular calcite veins are common and in some cases are offset by flexural slip on fold limbs. Subhorizontal bed-parallel opening-mode calcite veins in the exposure are rare and indicative of a thrust faulting stress regime (i.e., minimum principal stress vertical). Fluid inclusion analysis of vein cements from extension fractures and faults yield homogenization temperatures of 71 to 101 °C. These fluid inclusion analysis results indicate that veins and associated folding formed at depths of 2.2 to 3.5 km, rather than at the surface via processes penecontemporaneous with Late Cretaceous deposition or near-surface caliche formation during the late Cenozoic as previously interpreted. Dominant WNW-ESE fold trends are consistent with larger-scale folds in the surrounding Devils River Uplift and to the south in the Sabinas Basin in Mexico. The detachment beds were likely severely overpressured at the time of folding, as indicated by the local presence of sub-horizontal bed-parallel blocky calcite veins. Tectonic compaction without draining of these inherently low-permeability strata would have strongly contributed to development and maintenance of the overpressure that allowed bedding-plane slip, ductile behavior of volcanic ash and mudrock beds, and detachment folding.