RIFTING MECHANISMS AND GEOMETRIES FROM THE PERSPECTIVE OF SOUTH-CENTRAL TIBET AND WEST TEXAS

Rifts are complex systems formed in extensional settings. Their structural evolution may be contested on rock mechanics and geologic history (e.g. inherited structures). Their map-view geometries offer a glimpse of their evolution which we can assess via field mapping. We will revisit the classic rifting models and discuss locations where we can test ideas on rift evolution. One place is in the Tibetan Plateau where rifting is associated with the on-going collision between India and Asia. It is not entirely clear how extension is accommodated across the orogen, although the consensus is that it is synorogenic. Questions abound on the kinematic evolution of rift systems, the timing of development, and their role in accommodating extension. In the Lopukangri Rift (LKR) in south-central Tibet, geochronology data indicate footwall rocks were exhumed from the mid-crust at high rates (~1 km/My) in the middle Miocene. Tibet may be divided into extension domains based on the kinematics of structures; these are regions that are bounded by kilometer-scale conjugate strike-slip faults along the ~E-W trending suture zones. Another location is in west Texas where field mapping on the southern Tascotal Mesa transfer zone in the vicinity of the Christmas Mountains shows similar kinematics to what we predict from oblique rifting. Our data corroborates previous hypotheses of stress transfer from west to east in the southernmost Rio Grande Rift (RGR). The RGR trends N-NW through northern Mexico, west Texas, New Mexico and central Colorado for ~1000 km. It consists of various structural segments associated with transfer and accommodation zones. In west Texas, the rift geometry is offset to the east along the E-W Tascotal Mesa transfer zone. It is debated how the southernmost segment fits into the overall kinematic evolution because the magnitude of extension is relatively low. Both rift systems are asymmetric suggesting variations in the stress field during rifting or due to pre-existing weak zones (i.e. inherited structures). We shouldn’t expect rifts to behave similarly because so many factors contribute to their evolution. Even though the LKR and RGR are in distinct tectonic settings, the interaction of inherited structures, magmatism, exhumation, neotectonics, and tectonic geomorphology can elucidate rifting mechanisms.