Interpretation of gravity and magnetic data and development of two-dimensional cross-sectional models for the Border Ranges fault system, south-central Alaska

Abstract

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 Alaska-Aleutian forearc region, difficult. 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 ultramafic and mafic 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.