Seismicity recorded in hematite fault mirrors in the Rio Grande rift

Exhumed fault rocks provide a textural and chemical record of how fault zone composition and architecture control coseismic temperature rise and earthquake mechanics. We integrated field, microstructural, and hematite (U-Th)/He (He) thermochronometry analyses of exhumed minor (square-centimeter- scale surface area) hematite fault mirrors that crosscut the ca. 1400 Ma Sandia granite in two localities along the eastern flank of the central Rio Grande rift, New Mexico. We used these data to characterize fault slip tex-tures; evaluate relationships among fault zone composition, thickness, and inferred magnitude of friction-generated heat; and document the timing of fault slip. Hematite fault mirrors are collocated with and crosscut specular hematite veins and hematite-cemented cataclasite. Observed fault mirror microstructures reflect fault reactivation and strain localization within the comparatively weaker hematite relative to the granite. The fault mirror volume of some slip surfaces exhibits poly gonal, sintered hematite nanoparticles likely created during coseismic temperature rise. Indi-vidual fault mirror hematite He dates range from ca. 97 to 5 Ma, and similar to 80% of dates from fault mir -ror volume aliquots with high-temperature crystal morphologies are ca. 25-10 Ma. These aliquots have grain-size-dependent closure temperatures of similar to 75-108 degrees C. A new mean apatite He date of 13.6 +/- 2.6 Ma from the Sandia granite is consistent with prior low-temperature thermochronometry data and reflects rapid, Miocene rift flank exhuma-tion. Comparisons of thermal history models and hematite He data patterns, together with field and micro structural observations, indicate that seismi-city along the fault mirrors at similar to 2-4 km depth was coeval with rift flank exhumation. The prevalence and distribution of high-temperature hematite grain morphologies on different slip surfaces correspond with thinner deforming zones and higher propor-tions of quartz and feldspar derived from the granite that impacted the bulk strength of the deforming zone. Thus, these exhumed fault mirrors illustrate how evolving fault material properties reflect but also govern coseismic temperature rise and asso-ciated dynamic weakening mechanisms on minor faults at the upper end of the seismogenic zone.