SEE SAWS AND RUPTURED HINGES: THE SEDIMENTOLOGIC AND HYDROGEOLOGIC SIGNIFICANCE OF THE UPS, DOWNS, AND TILTS OF NORTHERN RIO GRANDE RIFT BASINS

Rift-basin stratigraphic models focus on basin and facies asymmetry, with high-K axial-stream deposits displaced from the basin center toward the master fault and flanked with variable-K piedmont facies. The Santo Domingo (SDB), Española (EB), and San Luis basins (SLB) of northern New Mexico depart from this scheme. The SDB is an accommodation-zone basin where stratigraphic, structural, and geomorphic data document see-saw subsidence over the past 7 my, with alternating periods of eastward and westward tilting. Axial-stream facies, therefore, migrated back and forth across the basin causing widespread distribution of high-K gravel. The EB and SLB half grabens show evidence of rupture along the flexural hinge that raised distal hanging walls of the basin master faults to elevations close to or exceeding those of the footwall uplifts. Hinge rupture reactivated high-angle Proterozoic and Laramide faults with the same motion sense as the master fault, which accentuates basin asymmetry by increasing tilt toward the master fault. Uplift of the distal hanging wall increases relief and watershed elevations for streams constructing the alluvial-slope piedmont. The resulting larger and less variable stream discharges can produce higher-K facies than do flashy flows on alluvial fans. Faulting of Oligo-Miocene volcaniclastic rocks in the Tusas Mountains documents synrift rupture of the SLB hanging wall, but lack of basin dissection precludes study of the sedimentologic consequences of hanging-wall rupture. The ruptured hanging-wall hinge zone model for the EB is supported by: (1) variable relief interpreted to result from differential uplift of the distal hanging wall Santa Fe Range proportional to the displacement on the Pajarito master fault at the same latitude; (2) monoclinal tilting of basin fill at the north end of the range at the margin of the Peñasco embayment, a nonuplifted part of the distal hanging wall; (3) deflection of drainage routes and stream piracy related to Santa Fe Range uplift. Upward-coarsening piedmont facies may be a response to increasing slope and watershed elevations in the Santa Fe Range. Sediment textural variations related to hanging-wall drainage evolution may be hydrologically significant, and high-elevation watersheds enhance mountain-block and mountain-front recharge potential.