REVISITING THE IMPORTANCE OF LOW-ANGLE NORMAL FAULTING IN THE RIO GRANDE RIFT, CENTRAL NEW MEXICO

Low-angle normal faults in the Rio Grande rift have long been noted within the literature, although their importance for explaining crustal attenuation remains unresolved. We focus on low-angle normal faulting at the base of Ladron Peak in central New Mexico, and touch on additional locations in the Albuquerque Basin: in the Lucero Uplift and the Sandia Mountains. Using a combination of detailed mapping, geo- and thermochronology, and structural relationships, we propose a model involving preservation of low-angle fault remnants in areas of rift salients, where extension has propagated toward the rift center and early-formed normal faults have been shallowed via footwall unloading. Ladron Peak is composed of Precambrian granitic and metamorphic rocks that form a conspicuous protrusion into the southwestern margin of the Albuquerque Basin. These rocks are bounded on the eastern front by the anomalously low-angle Jeter fault (dipping 15-20° east). Fault plane and associated slickenline measurements from the Jeter fault system indicate an average extension direction trending southeast. A >50-km-long movement plane cross-section was constructed utilizing available geological and geophysical datasets to understand large-scale fault geometry and evolution through time. Important features of the observed fault geometry are: 1) synthetic 60° dipping normal faults sole into the Jeter master detachment; 2) faults become progressively steeper basinward, where the Jeter fault is the lowest angle; and 3) intrabasinal synthetic normal faults are younger than the Jeter fault, which is inferred to have been active between ~20-10 Ma based on apatite fission track results. All of these observations together are consistent with a model where initially steep faults are progressively shallowed due to hanging wall unloading and footwall isostatic adjustment. Other rift margins with similar bulges into the basin and high slip include the Lucero Uplift and the Sandia Mountains of the Albuquerque Basin and Blanca Peak of the San Luis Basin. Each of these areas has mapped low-angle normal faults that may record a similar fault rotation and basinward propagation mechanism as the Jeter fault at Ladron Peak.