EXPLORING THE SPATIAL AND TEMPORAL VARIABILITY OF FAULT ACTIVITY ALONG THE SANGRE DE CRISTO RANGE WITH LOW-TEMPERATURE THERMOCHRONOLOGY, RIVER PROFILE INVERSIONS, AND ANALYSIS OF FAULT SCARP MORPHOLOGY

Regional low-temperature thermochronology compilations from throughout the Rio Grande Rift suggest a synchronous initiation of extension along the nearly 1,000 km length of the rift between 25 and 10 ma (Ricketts et al., 2014 GSAB), with local variations in timing potentially related to the formation and propagation of individual structures (Abbey and Niemi, 2018 Geology). The Sangre De Cristo Mtns define one of the principle boundaries of the Rio Grande Rift and the San Luis Basin. Here we focus on west-dipping normal faults that define the western margin of the Sangre De Cristo Range between Blanca Peak and Poncha Pass, a generally north-west trending ~100 km range front that contains multiple bends. We characterize the spatial and temporal variability of activity along this fault segment with three methods that integrate over orders-of-magnitude different timescales. First, Airborne light detection and ranging (lidar) from the San Luis Basin highlights a network of fault scarps that define the range-bounding fault. In an automated routine, we systematically characterize the amplitude and smoothness of fault scarps by fitting profile elevations to a model of vertical offset and diffusive smoothing of an initial sloping landform. Second, in non-glaciated catchments we assume time-invariant catchment boundaries, uniform rock uplift, and a power-law bedrock incision rule to enable inversion of river profile geometries in terms of temporal variations in rock uplift rate. Finally, and to calibrate stream profile inversions, we conduct bedrock Apatite (U-Th)/He (22 samples) and Zircon (U-Th)/He (6 samples) thermochronology in two vertical transects that each span >1 km of relief. We use inverse methods to infer the timing of exhumation rate changes consistent with thermo-kinematic models and based on analysis of entire thermochronology transects rather than analysis of individual samples. Taken together, these observations provide a record of spatial and temporal variability in normal fault activity spanning 100 km and over timescales ranging from millions of years to individual seismic events.