DEFORMATION IN CRETACEOUS STRATA NEAR THE FRONTAL LIMIT OF LARAMIDE TECTONISM, DEVILS RIVER UPLIFT REGION OF SOUTHWEST TEXAS, USA

Regionally extensive mesoscale deformation features in the Cretaceous Boquillas Formation and laterally equivalent Eagle Ford Formation in south and southwest Texas record evidence of extensional tectonism, dominated by normal faulting and vertical extension fractures formed in a normal faulting stress regime. Deformation in the Devils River uplift region, however, includes contractional folds, thrust faults, and bedding parallel veins indicative of contractional deformation in a thrust faulting stress regime. The contractional folds have wavelengths of meters to tens of meters, and generally NW-SE (WNW-ESE to NNW-SSE) trending, gently plunging fold axes. A 50-m-wavelength open anticline east of Langtry, TX contains bed-parallel fibrous veins that record up to 14% bed-perpendicular thickening that is consistent with a thrust faulting stress regime (vertical minimum principal stress). Mescoscale deformation in other exposures west of Del Rio, TX consists of small-wavelength folds that are open to tight and have upright to inclined axial surfaces with variable vergence. These structures tend to have detachment fold geometries with detachments in volcanic ash or mudrock beds. Thickening of strata by ductile flowage, small-scale folding, and thrust faulting in cores of anticlines is common. Veins associated with these folds include vertical calcite veins, rare bed-parallel calcite veins, and bed-perpendicular calcite veins that are in some cases offset by flexural slip on fold limbs. Analysis of fluid inclusions from vein cements in extension fractures and faults associated with the folds in this study area yielded homogenization temperatures of 67-89 and 90-101 °C, and estimated trapping depths of 1.75-3 km and 3-3.5 km, respectively. From these structural and fluid inclusion analyses, we interpret that the deformation structures observed in the Devils River uplift region are associated with regional Laramide contractional tectonism rather than related to syndepositional fluid escape or relatively recent weathering and caliche formation as previously interpreted.