New Mexico Geological Society Annual Spring Meeting — Abstracts

Our goal is to understand the extent, timing, and mechanisms of formation of low-angle normal faults in the Rio Grande rift, with initial focus on the Jeter fault system. Fault plane lineations from the Jeter fault system indicate a slip direction oriented toward 150°. We constructed a movement-plane cross-section from the summit of Ladron Peak along the western margin of the rift, through the Joyita Hills mid-rift horst, to the Los Piños eastern rift flank. The cross section shows the low-angle Jeter fault (<35°) and < 20° normal faults in Joyita Hills. Estimated percent extension is about 35%.

Apatite fission-track cooling data from age-elevation transects of the east ridge of Ladron Peak and in Joyita Hills were projected onto the cross-section. AFT data are interpreted here to record a polyphase extensional unroofing of Ladron Peak, with the eastern ridge unroofing from 18-10 Ma. Additional apatite helium dating is underway to refine the lower temperature cooling history.

Four mechanisms for formation of the low-angle faults are considered and may have worked in concert. 1) Initiation and slip along flat normal faults is possible in very weak horizons, which may explain low-angle faults in nearby regions with detachments in salt horizons, but the Jeter and Joyita Hills low-angle faults cut basement and other strong lithologies. 2) Domino-style passive rotation of both faults and bedding from initial ~60° fault dip explains normal fault networks in the Lemitar Mountains to the south, but existing mapping does not show the Jeter system to have the necessary fault system geometry of tilted crustal blocks and overprinting of faults. 3) The rolling hinge model implies that as the hinge migrates into the hanging wall, isostatic uplift rotates older steep faults to lower angle and causes them to become mechanically unfavorable for slip. 4) Observed surface uplift of 1.75 mm/yr over the last few decades could accumulate significant fault rotation if emplacement of numerous mid-crustal magma chambers took place over the last several million years, as suggested by heat flow data. These models will be tested by additional structural studies of low-angle faults on the west side of the Rio Grande rift.