MULTI-METHOD THERMOCHRONOMETRIC RECORD OF EXHUMATION AND FAULT SLIP IN THE CENTRAL RIO GRANDE RIFT, SANDIA MOUNTAINS

Low-temperature thermochronometry of exhumed fault rocks captures related fault slip and exhumation processes. The Sandia Mountains, NM, form the eastern flank of the central Rio Grande rift (RGR) and faults cutting Mesoproterozoic Sandia granite record a potentially protracted history of deformation from the Mesoproterozoic to Quaternary. Minor striated, high gloss, hematite-coated faults, or “fault mirrors,” occurring within mapped traces of the La Cueva and Tijeras Canyon faults, were targeted for microtextural analysis and hematite (U-Th)/He (He) thermochronometry. Hematite He dates reflect formation, cooling, or thermal resetting depending on the conditions of hematite formation, the post-formation thermal history, and aliquot specific closure temperatures controlled by the hematite grain size distribution.

Mean hematite He dates from four fault mirrors co-located with the La Cueva fault are 23.4 ± 4.1 Ma (1σ std dev), 18.2 ± 0.7 Ma, 20.1 ± 1.4 Ma, 12.8 ± 1.4 Ma, and one sample has dispersed individual aliquot dates of ~14.1-4.9 Ma. Two Tijeras Canyon samples yield individual dates between ~97-30 Ma and ~26-13 Ma. A new apatite He date from the Sandia granite of 13.6 ± 2.6 Ma (1σ std dev) is consistent with previously published work and records rift flank exhumation. Hematite He dates may also record RGR extension. However, scanning electron microscopy reveals hematite fault mirrors comprise sintered, polygonal grain morphologies characteristic of high temperature recrystallization during seismic slip. Scattered intrasample hematite He dates over similar Th/U imply polycrystalline aliquots capture variable He loss from variable grain (domain) size distributions during exhumation and/or frictional heating. Some individual hematite He dates older than apatite He data imply an early-RGR or even pre-RGR exhumation history. Ongoing textural characterization, including grain size and associated closure temperature analysis, and numerical modeling will evaluate if hematite He dates record non-monotonic thermal processes operative during Miocene to present deformation or are inherited from earlier tectonic phases (e.g., Ancestral Rocky Mountain or Laramide orogeny). Although the timing is unresolved, hematite textures support fault mirrors accommodated past seismicity.