Practical Estimation of Near-surface Bulk Density Variations Across the Border Ranges Fault System, Central Kenai Peninsula, Alaska

Abstract

We demonstrate a near-surface density estimation approach in an area without exposed outcrop or where outcrop occurrences do not adequately represent the subsurface rock densities based on the Nettleton (1939) - Parasnis (1952) technique as extended by Rao and Murty (1973) . We applied this technique in the central Kenai Peninsula, Alaska where the region is cut by a major fault zone, the Border Ranges fault system, that juxtaposes two terranes with greatly varying geological and geophysical properties. The Kenai Peninsula region can be generally divided into two different geologic settings: recent fluvial and glacial deposits of the Cook Inlet basin to the west and accreted metamorphic terranes of the Kenai Mountains to the east. Our study region includes glacial cover, deep lakes, and large topographically-driven gravity gradients between the Kenai Mountains and Cook Inlet. We selected 11 gravity loops from ∼580 gravity points collected in 2009, which have 10–20 gravity points per loop; nine loops (A to I) are located in the Quaternary glacial and alluvial deposits and two loops (J and K) are in Cretaceous metamorphic rocks of the Kenai Mountains. Inversion of the free-air anomaly data gave estimates of near-surface rock densities and their associated uncertainties. Our inversion results are comparable to values obtained from density logs in the Cook Inlet basin ranging from 1.64 to 2.60 g/cm3 with an average uncertainty of 0.12 g/cm3. Estimated densities for the accreted metamorphic terrane in the eastern peninsula of 2.89 ± 0.02 and 2.86 ± 0.01 g/cm3 are slightly higher than measured densities from hand rock samples.