GSA Connects 2022 meeting in Denver, Colorado

Paper No. 101-7
Presentation Time: 9:00 AM-1:00 PM


PETERSON, Dana1, JAMES, Stephanie1, CAINE, Jonathan S.1 and MANN, Michael Everett2, (1)Geology, Geophysics, and Geochemistry Science Center, U.S. Geological Survey, Denver, CO 80225, (2)Brown University, Providence, RI 02912

Understanding crustal-scale mineral belts requires 3D context to determine the role of seismically resolvable tectonic features in mineralization. As part of an exploratory nodal study, passive seismic methods are used to link crustal-scale seismic structure to outcrop-based tectonic fabrics. Four months of continuous seismic data were recorded in a ~1.2x1 km area of Walker Ranch in Boulder, CO along the Livingston shear zone (LSZ) adjacent to the Colorado Mineral Belt. The 20-node array spanned the LSZ, which occurs within foliated Boulder Creek Granodiorite, providing an opportunity to explore structural and seismic velocity differences across the shear zone. The nodes recorded 57 teleseismic earthquakes ≥M5.5 at distances of 30-90°. Using these events, receiver functions were calculated to illuminate near-surface velocities and depths of sharp velocity contrasts to Moho depths. Preliminary results from h-k stacking indicate the Moho beneath our seismic array is ~38-42 km. Free-surface transform analysis yields surface shear-wave velocity (Vs) estimates at each station, indicating that Vs ranges from ~2.5-3.4 km/s. Using these Vs estimates and the lag time of the first converted phase in the receiver functions, we estimate the thickness of the near-surface Vs layer to be ~1.8-2.5 km, which may represent differences in fracture density or degree of alteration. Converted phases between ~2-5 s lag time suggest the presence of additional upper- to mid-crustal interfaces. The timing of these phases is consistent across all stations, implying the LSZ is near-vertical in dip and/or lacks a sharp velocity contrast. By expanding the distance range of usable teleseismic earthquakes to 110° we may be able to identify anisotropic and dipping contrasts using receiver functions. Crustal Vs and structure results from receiver functions at Walker Ranch will be combined with Vs estimates of the upper ~1 km from ambient noise interferometry and horizontal-vertical spectral ratios, as well as structural geologic measurements of foliations and shear fabrics taken at the outcrop scale, creating a holistic picture of crustal seismic velocity and geologic structure. This work expands the utility of nodal seismic data in bridging the crustal-scale gap towards understanding mineral deposit localization processes.