LOCALIZED SHEAR ZONES VERSUS DISTRIBUTED TECTONIC FABRICS: AN EXAMPLE FROM GEOLOGIC AND SEISMIC OBSERVATIONS IN PROTEROZOIC COLORADO BASEMENT

Ductile shear zones play a major role in continental assembly and evolution, and they are important indicators of lithospheric rheology. While exposed shear zones are relatively easily recognized, they are more difficult to detect at depth, particularly when they are seismically inactive. Detection methods for seismic anisotropy are promising tools for imaging deep crustal deformation but also present challenges, especially when reconciling observations with surface geology. For example, a distributed network of exposed NE-striking shear zones in Colorado is one of the most prominent features portrayed on Proterozoic tectonic maps of the SW U.S. This region experienced significant 1.8-1.6 Ga crustal growth, widely considered to have involved successive NW-directed terrane accretion. The resulting tectonic boundaries are commonly inferred to have NE trends, extending from well-studied exposures in central Arizona, and supported by the shear zones in Colorado. However, receiver functions from EarthScope’s Transportable Array and CREST stations across Colorado show seismic anisotropy suggesting that significant volumes of present-day deep crust contain fabrics in a distinctly different NW-striking geometry. Possible causes for the discrepancy fall into two categories: those that involve a) bias in seismic sampling and/or b) deformation processes that lead to either weaker anisotropy in the shear zones compared to adjacent domains or to a symmetry that is different from that conventionally assumed. Most of these explanations imply that the seismically sampled domains contain important structural information that is distinct from the shear zones. Previous work shows that these structures typically closely post-date and overprint earlier, more broadly distributed, and higher grade fabric domains. Thus, the seismic anisotropy patterns appear to reflect structures that developed at an orogenic stage when the crust was hotter and rheologically weaker, whereas the exposed shear zones reflect cooler and stronger episodes of strain localization. An additional implication is that NW-striking structures are more important for the assembly of Colorado lithosphere than commonly considered, as supported by a published study in SW Colorado and ongoing work in the northern Colorado Front Range.