UPDATED MODELS ON THE EVOLUTION OF THE RIO GRANDE RIFT: EMBRYONIC CORE COMPLEXES, FLUID-ENHANCED QUATERNARY EXTENSION, AND APATITE (U-TH)/HE CONSTRAINTS FOR LARGE- MAGNITUDE 18-15 MA FAULT SLIP

Updated models for the evolution of the Rio Grande rift need to address the presence of low-angle normal fault networks, a large-magnitude slip event at 18-15 Ma, and continued Quaternary extension. The low-angle (~ 25°) Jeter and low-angle (~30°) Knife Edge faults are counterparts on SW and NE margins of the Albuquerque basin. In both cases, synthetic faults in the hanging wall become progressively steeper and younger basinward, the footwall block is the highest elevation along the rift flanks, and apatite fission track (AFT) and (U-Th)/He (AHe) data suggest large magnitude slip at 18-15 Ma, similar to the timing of rapid exhumation in other regions along the rift based on AFT data. These observations are consistent with a model where initially high-angle faults are shallowed in regions of maximum extension, and new intrabasinal faults emerge to facilitate continued rifting. We view these uplifted rift flank segments as embryonic core complexes in the sense that, if slip continued, the process could eventually place mid-crustal ductily deformed rocks in the footwall against surficial deposits in the hanging wall across faults that have been isostatically rotated to shallow dips.

Continued minor extension in the rift is likely and, for example, is indicated by Quaternary travertine deposits that are cut by extensional vein sets along the western margin of the Albuquerque basin in the Lucero uplift. Calculated strain rates at this location (15-105 nstr/yr) are higher than both the long-term strain rates (3-14 nstr/yr) and GPS-constrained modern strain rates (1.23-1.39 nstr/yr). We suggest a model where elevated fluid pressures in low differential stress environments result in anomalously-high strain rates, which are active over the timescales of the hydraulic system. Such regions need not be widespread, nor do they last long enough to shallow fault planes, but they are nevertheless an underappreciated mechanism of progressive extension in the rift.

Additional AHe analyses are underway to further bridge the gap between long-term evolution of fault networks in individual basins and continued Quaternary extension in the rift. These samples will be combined with apatite fission track ages from the same rocks to provide time-temperature profiles of various rift flanks as they cooled from >110°C to near- surface temperatures.