ASSESSING RUPTURED HANGING-WALL HINGE-ZONE UPLIFTS IN THE NORTHERN RIO GRANDE RIFT, NEW MEXICO

Distal hanging wall regions of half grabens in the northern Rio Grande rift stand topographically as high or higher than corresponding footwall uplifts. In areas of broad extension (e.g., Basin and Range) distal hanging walls of one basin are commonly the footwall uplifts to an adjacent basin but this does not explain high-standing hanging walls in the discrete, one-basin-wide, rift in northern New Mexico. Distal-hanging-wall ranges in New Mexico rose on reactivated faults of Proterozoic, Ancestral Rocky Mountain, and Laramide ancestry. Faults exhibit the same displacement sense as half-graben master faults but do not border another basin. The Tusas Mountains rise to 3400 m in the distal hanging wall of the east-tilted San Luis basin. Principal NE-side-up motion on NW-striking faults occurred after burial of the older Laramide uplift by ~30-20 Ma volcaniclastic deposits, capping the highest points of the range, and at least as recently as the Pliocene. The Santa Fe Range rises to 3600 m in the distal hanging wall of the west-tilted Española basin. An explicit record of Neogene uplift of the Proterozoic rocks comprising the range remains to be demonstrated but existing data suggest post-25 Ma, west-side-up motion on the Picuris-Pecos fault. Stratigraphic and geomorphic data imply that the range and adjacent basin rose as a doubly-plunging, west-tilted uplift throughout the late Cenozoic, leaving the Peñasco and Santa Fe embayments as nonuplifted parts of the distal hanging wall to the north and south. We hypothesize distal hanging-wall uplift by rupture of the bending half-graben hinge zone along preexisting, steep, deeply penetrating faults with the same displacement sense as the master fault to enhance half-graben tilt rather than increasing graben symmetry. We will present preliminary geodynamic models evaluating this hypothesis. This process, if rigorously demonstrated, has implications for the dynamics of extensional basin formation and the stratigraphic and geomorphic evolution of basin-fill where tilting toward the footwall escarpment is enhanced by uplift of the distal hanging wall.