Modeling Gravity Data From a Recent (2009-2010) Survey Across the Border Ranges Fault System, Alaska

We have collected ~1,000 gravity observations within the Anchorage and Kenai Peninsula regions to better determine the structural geometry of the Border Ranges fault system (BRFS). The BRFS is characterized by a strong gradient between the deep low (~-130 mGal) of the Cook Inlet Basin and the gravity high of -10 mGal associated with the western range front of the Chugach Mountains. On the Kenai Peninsula the gravity field remains high across the Chugach Mountains, but in the Anchorage region it decreases, possibly due to the presence of the Eagle River thrust sheet. We have begun 2.5-D forward modeling of the combined new and existing gravity data using densities constrained by density logs, hand samples, seismic velocities and Nettleton’s density inversion method. Our preliminary results suggest the main fault of the BRFS dips steeply (60 to 70 degrees) toward the west. Many subsidiary buried faults are also apparent. Our ultimate goal is to test several plausible models of structure along the BRFS by implementing a novel 3-D inversion scheme that directly models known geology, and revises a priori uncertainties on the geologic model to let us compare alternative interpretations.