Extensive Quaternary glacial cover and a lack of dense geophysical data within the Cook Inlet basin (CIB) of south-central Alaska make locating and determining the geometry of the Border Ranges fault system (BRFS), a major feature of the AlaskaAleutian forearc region, diffi cult. We use recently collected gravity data, available aeromagnetic data, and other geophysical information as constraints to develop plausible two-dimensional cross-section models that better image the BRFS and related geologic structures of the CIB. Our integrated models show a thick sequence of late Mesozoic sedimentary rocks and the Peninsular terrane basement (6-20 km depth) overlying a serpentinized body at a depth of 16-34 km. The late Mesozoic rocks and serpentinite are interpreted as possible sources of the south Alaska magnetic high over the CIB. The eastern boundaries of the CIB are characterized by gravity and magnetic highs of the emplaced Border Range ultramafi c and mafi c assemblages (BRUMA). Formation of the BRUMA may be related to the serpentinized rocks that composed a Jurassic oceanic arc. Our models constrain the BRFS as a structural boundary between the overthrusted BRUMA and the Chugach terrane to the east. The BRFS dips 50°-70° toward the west-northwest and extends to at least 15 km. The BRFS may penetrate steeply or shallowly to a form a décollement at greater depths. A model that includes underplated sediments at the base of the accretionary complex (12-40 km) is consistent with the observed gravity low over the Chugach Mountains (Chugach terrane). The underplating may be associated with the subducting and shortening of the Yakutat microplate in south-central Alaska.

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.

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