EXPLORING NEAR-SURFACE TECTONIC FABRICS AND MINERALIZATION IN THE COLORADO FRONT RANGE WITH HIGH-FREQUENCY PASSIVE SEISMIC METHODS

The Rocky Mountain Front Range in east-central Colorado has a long history of crustal deformation, magmatism, and mineralization. However, discrepancies between spatial footprints of mineral deposits and mapped crustal structures indicate the controls on mineral deposit formation and localization are not fully understood. Advancement in passive seismic methodologies and nodal sensor technology provide a unique opportunity to measure rock properties at intermediate scales (10s to 1000s of meters) using multiple complementary techniques. We collected continuous seismic data with a nodal pool of 15-20 sensors at Hall Ranch and Walker Ranch Open Space parks in Boulder County in small-aperture arrays (0.25-1 km²) for 2-4 months. Resulting data are combined with surface geologic mapping to explore quantification of radial and azimuthal anisotropy pertaining to rock fabrics, shear zones, and mineralization associations through high-frequency (> 1 Hz) ambient seismic noise tomography. Hall Ranch is dominated by Mesoproterozoic Longs Peak granite with relatively homogeneous geology and fabric. Outcrop measurements of magmatic foliations reveal distinct SW-NE striking, and moderately NW and SE dipping clusters. Conversely, Walker Ranch is underlain by Paleoproterozoic Boulder Creek batholith, composed of variably foliated granodiorite, and the site is crosscut by the Livingston shear zone. Thereby each location provides a unique endmember for this study. Cross-correlations from each dataset produced measurements of Rayleigh and Love wave phase velocity for all station pairs. Initial dispersion measurements reveal multiple higher-order modes present in both datasets. Mode ambiguity has necessitated careful selection of the fundamental mode, exhibiting decreasing velocities from 2.5 to ~0.75 km/s over the 2-30 Hz range. However, a multimode inversion scheme, in which pre-assignment of modes is not required, may be necessary to reduce uncertainties in mode selection and inversion, and potentially increase model resolution. The outcomes of this research will push the boundaries of high-frequency ambient noise tomography and anisotropy studies, with the potential to fill a critical scale-gap in understanding near-surface rock fabrics and shear zones in relation to mineral deposit formation.