NEOGENE-QUATERNARY EVOLUTION AND DECREASES IN THROW RATES ON MAJOR FAULTS IN THE NORTHERN PALOMAS BASIN AND ITS STRUCTURAL BOUNDARY WITH THE ENGLE BASIN, SOUTHERN RIO GRANDE RIFT, NM

Results of recent geologic mapping and geochronology are used to decipher fault evolution near the boundary of the overlapping Palomas and Engle basins of the Rio Grande rift. The northern Palomas basin is separated from the central Palomas basin by a gravity saddle that is due-west of the Mud Spring Mountains (MSM). The asymmetric, central Palomas basin tilts eastward against the west-down, northward-branching Caballo fault. The east branch, the W-down Hot Springs fault, continues NNE across a structural high into the Engle basin. The west branch projects NW to the southern Mud Springs fault (MSF). The W-down MSF wraps around the west side of the structural high and has produced a footwall uplift (the MSM) along its central and southern segments. Four km west of the Mud Springs fault lies the N-striking, mostly W-down, Cuchillo Negro fault zone (CNFZ). Since ~5 Ma, slip has shifted from the MSF to the CNFZ along an E-W transverse fault, as evidenced by >2 km of 5-20 Ma throw but only <3 m of post-5 Ma throw along the northern MSF.

Dip changes, apatite-fission track ages, and stratigraphy across faults are used to infer a progressive decrease in throw rates since the early late Miocene. Post-6 Ma basin fill dips 0-2° whereas exposed Miocene strata dip 20-40° (as does Paleozoic bedrock of the MSM). These tilt changes, and apatite-fission track ages in the MSM of 8.3±3.8 and 9.9±2.2 Ma (2 sigma), indicate a notable decrease in tilt rates and footwall uplift, and presumably activity along major faults, between the late Miocene and early Pliocene. Using basin fill thickness changes and elevations of an offset contact, throw rates on the southern MSF decrease from 0.08 m/k.y. in the late Pliocene to 0.02-0.03 m/k.y. in the Quaternary. Offset of two geomorphic datums by the CNFZ indicate throw rates of 5 mm/k.y. for the past 800 k.y. and 0.1 to 2 mm/k.y. for the past ~600 k.y. The northern Hot Springs fault produced notable drag folding of latest(?) Miocene strata, yet does not offset a maar vent with an Ar/Ar age of 2.468±0.014 Ma, indicating no motion along the fault since this eruption. We propose that subsurface dike injection feeding numerous 2-2.5 Ma basaltic vents counteracted extensional strain, leading to the shut-down of the Hot Springs fault and abruptly decreasing rates along nearby faults amidst a long term slowing of regional extension.