KINEMATIC AND GEOCHRONOLOGIC EVIDENCE DOCUMENTING ROTATION OF CRUSTAL EXTENSION DIRECTION AND NARROWING OF RIFT FAULTING IN THE SUNKEN BLOCK GRABEN OF THE SOUTHERN RIO GRANDE RIFT, TRANS-PECOS TEXAS

We present new fault-kinematic data (n = 308) from six locations across the Sunken Block and Black Gap grabens in the southern Rio Grande rift of Trans-Pecos Texas. Kinematic analysis indicates a prominent NE-SW extension direction preserved at each location, while locations within the Sunken Block also show evidence for multiple incompatible fault populations and changing extension directions through time. Three U-Pb ages of oriented calcite slickenlines in the Sierra del Carmen and Black Gap graben constrain a NE-SW direction of S1 to as old as 30.1 ± 3.1 Ma and as young as 13.7 ± 0.9 Ma. These data support a model in which earliest extension was oriented NE-SW and rotated clockwise to EW and NW-SE after 13.7 Ma, and in which later extension was concentrated within the Sunken Block as the active region of extension narrowed. Clockwise rotation after 13.7 Ma is in agreement with a widely cited regional clockwise rotation of tension and extension at ~10 Ma reported across the Basin and Range province. These observations are also similar to kinematic studies of the central Rio Grande rift, although the timing of rotation is significantly younger in the southern rift (<13.7 Ma), than in the central rift (early Miocene). Together, new and compiled fault kinematic data from the Rio Grande rift suggest that rotation of extension was a time-transgressive process, and that extension was driven by both lateral and underlying plate boundary forces as the transform plate margin lengthened and exerted greater influence on the interior of the North American Plate.