LATE OLIGOCENE AND MIOCENE FAULTING AND SEDIMENTATION, AND EVOLUTION OF THE SOUTHERN RIO-GRANDE RIFT, NEW-MEXICO, USA

The distribution of nonmarine lithofacies, paleocurrents, and provenan ce data are used to define the evolution of late Oligocene and Miocene basins and complementary uplifts in the southern Rio Grande rift in t he vicinity of Hatch, New Mexico, USA. The late Oligocene-middle Mioce ne Hayner Ranch Formation, which consists of a maximum of 1000 m of al luvial-fan, alluvial-flat, and lacustrine-carbonate lithofacies, was d eposited in a narrow (12 km), northwest-trending, northeast-tilted hal f graben, whose footwall was the Caballo Mountains block. Stratigraphi c separation on the border faults of the Caballo Mountains block was a pproximately 1615 m. An additional 854 m of stratigraphic separation a long the Caballo Mountains border faults occurred during deposition of the middle-late Miocene Rincon Valley Formation, which is composed of up to 610 m of alluvial-fan, alluvial-flat, braided-fluvial, and gyps iferous playa lithofacies. Two new, north-trending fault blocks (Sierr a de las Uvas and Dona Ana Mountains) and complementary west-northwest -tilted half graben also developed during Rincon Valley time, with app roximately 549 m of stratigraphic separation along the border fault of the Sierra de las Uvas block. In latest Miocene and early Pliocene ti me, following deposition of the Rincon Valley Formation, movement cont inued along the border faults of the Caballo Mountains, Dona Ana Mount ains, and Sierra de las Uvas blocks, and large parts of the Hayner Ran ch and Rincon Valley basins were segmented into smaller fault blocks a nd basins by movement along new, largely north-trending faults. Analys is of the Hayner Ranch and Rincon Valley Formations, along with previo us studies of the early Oligocene Bell Top Formation and late Pliocene -early Pleistocene Camp Rice Formation, indicate that the traditional two-stage model for development of the southern Rio Grande rift should be abandoned in favor of at least four episodes of block faulting beg inning 35 Ma ago. With the exception of two northwest-trending border faults of the Caballo Mountains block that may be reactivated along Eo cene compressional structures, the majority of border faults and compl ementary basins throughout the history of the southern Rio Grande rift were north-trending, which challenges the conventional idea of a cloc kwise change in stress through time.