DOMAIN ANALYSIS TO ASSESS REGIONAL-SCALE STRUCTURES IN THE COLORADO FRONT RANGE

The Front Range of central Colorado hosts a complex suite of multi-aged Proterozoic and Cretaceous-Cenozoic plutonic rocks hosted in poly-deformed Proterozoic metamorphic units of varying protolith and grade. The region contains NE-trending plastic shear zones of 10’s of km strike length, including the Idaho Springs-Ralston Shear Zone (IRSZ), that overprint the overall, orogen-scale NW- to N-trending tectonic grain. The NE-trending shear zones have been proposed to have formed with the growth and amalgamation of Laurentia in the early and middle Proterozoic. Interpretations hold that such shear zones extend through the lithosphere and potentially act as zones of persistent weakness in the crust thus creating conduits for deep-seated fluids and/or creating structures which allow for preferential orientation of fabrics developed during reactivation. In order to test such interpretations, we analyze legacy structural data (Horton and Caine, 2019, USGS Data Release) over an area of the basement of 40 km by 40 km using the software Orient (Vollmer, 2015, GSA Abstracts) to iteratively search for domains in which fabric orientations have the highest coherence, and hence identify regional-scale tectonic fabric patterns and discern variance within the domains. The IRSZ is identified as a discrete strike-limited domain that separates regional-scale domains to the northwest and southeast in which the dominant fabrics strike respectively approximately N-S to NW-SE and E-W. The analytical results indicate that the IRSZ is a localized structure associated with dip-slip shear along the hinge-zone of a steeply-plunging km-scale fold in the metamorphic rock suite. These attributes are of sufficient scale and strength of fabric alignment to be discernable in high resolution geophysical data and the domains hence be potentially expandable and mappable into the subsurface.