KINEMATIC AND GEOCHRONOLOGIC ANALYSIS OF LATE CENOZOIC FAULT SURFACES FROM THE SOUTHERN SEGMENT OF THE RIO GRANDE RIFT IN THE BIG BEND REGION OF WESTERN TEXAS AND NORTHERN CHIHUAHUA

Multiple studies have analyzed fault-kinematic data from the Rio Grande rift to investigate the history of extension in this classic continental-rift setting, but most data come from the central and northern segments of the rift. Here we present new kinematic and geochronologic data from the southern segment of the Rio Grande rift in the Big Bend region of western Texas to constrain extension directions and compare to northern studies. The study area includes faults in the Sunken Block, a north-northwest-striking basin in the Big Bend of Texas and northern Chihuahua, Mexico, and in the northwest-striking Border Corridor Transform Zone, a connector between rift segments that trends southeast from the southern end of the north-south-striking rift basins of New Mexico. Compilation of new and published fault-kinematic data from approximately twenty Cenozoic faults reveals complex and varied extension directions. On a fault-strike vs. rake plot, dip-slip data cluster within northwest strikes, shallow rakes have a wide range of strikes, and faults with northwest strikes record all rakes. Many fault surfaces preserve multiple directions of rake. Stereographic analyses grouped by location yield east-northeast and east directions of principal extension, but these calculations are weighted by the number of exposed dip-slip slickenlines on north-northwest faults; fewer but present shallow-rake lineations on many fault strikes do not influence the calculation as heavily. Grouping stereographic strain-axis calculations instead by U-Pb calcite-slickenline age may expose a hypothesized transition between crustal extension directions. While faults either offset Eocene-Miocene igneous rocks or otherwise reveal their activity during Cenozoic extension, constraints on exact timing of fault movement are limited. Further, no cross-cutting relationships among variable slickenline rakes are present to determine relative age of the different fault motions. Therefore, samples of fault-lineated calcite were collected for U-Pb geochronology to determine ages of fault activity of different orientations within this segment of the Rio Grande rift and compare these to the timing of extension in the northern segments of the rift. These samples are being analyzed at the University of Texas at El Paso.