Preliminary Results of a Recent Gravity Survey Across the Border Ranges Fault System, Central Kenai Peninsula, Alaska

We have collected over 580 gravity observations across the central Kenai Peninsula to better determine the structural geometry of the Border Ranges fault system (BRFS). Gravity points had a minimum spacing of 0.5 km, with denser readings taken over several moraines in an effort to determine density variations in glacial units. We also collected eight hand samples of exposed bedrock units for laboratory analysis of density and have assembled a suite of density logs from petroleum wells to estimate density variations within the Cook Inlet basin. We have tied our results to previous gravity surveys conducted in the 1960’s to 1990’s. Our preliminary results of analysis of gravity using Nettleton’s reduction method give Cretaceous accreted rocks with densities of around 2.9 g/cc whereas the densities of Quaternary alluvial and glacial deposits of the forearc basin range from 1.63 to 2.41 g/cc. Our ultimate goal is to test several plausible models of structure along the Border Ranges fault system using a 3-D inversion scheme on gravity and magnetic data constrained with other geophysical, borehole and surface geological information.