CONSTRAINTS ON THE STRUCTURE OF THE BORDER RANGES FAULT SYSTEM, SOUTH-CENTRAL ALASKA FROM INTEGRATED 3-D INVERSION OF GRAVITY DATA

The Border Ranges fault system (BRFS) bounds the forearc and accretionary terranes within the Aleutian subduction zone in central-southern Alaska. The location and geometry of the BRFS are unclear because of thick Quaternary glacial cover and lack of high-quality geophysical data across the BRFS. We have collected ~1,000 new gravity points within the Anchorage and Kenai Peninsula regions between 2009-2011 and merged these data with existing data collected prior to 1997 to better image the BRFS. The goal is to test several plausible models for structure across the BRFS using a novel 3-D inversion scheme. This approach utilizes available geophysical and geological information such as borehole data, seismic tomography and the DEM topography as constraints in the inversion problem. The contributions of body proportions and densities of each a single gridded point are computed in order to compare the theoretical gravity calculations from forward models to the observed gravity (Free air anomaly). Calculated a priori information is used to address the issue of numerous uncertainties among alternative model solutions.

As a first step, we will test the 3-D inversion solutions on the three smaller sub-regions of Skilak Lake, Anchorage, and Palmer, where the densest spacing of gravity points is located. All subregions extend between the Cook Inlet lowlands and the topographically higher Kenai and Chugach Mountains, and cross the BRFS. We will produce several alternative starting models by varying the body proportions and densities. Plausible models resulting from these 3-D inversions will be compared. The most reasonable models will be used as starting models for the structure of the entire study area.