GEOPHYSICAL INVESTIGATIONS INTO THE TECTONIC AND MAGMATIC EVOLUTION OF THE KAHILTNA TERRANE, SW ALASKA

The Jura-Cretaceous Kahiltna Terrane, situated between the Peninsular Terrane to the south and the Kuskokwim Group sedimentary package to the north, is not well-defined in terms of its boundaries, its internal structure, or its extensive magmatic history. The geometry of the Mulchatna fault zone, generally believed to be the northern boundary of the Kahiltna Terrane, becomes increasingly speculative to the west, as do structures such as the Lake Clark Fault, interpreted by some as the southern Kahiltna Terrane boundary. Though mostly concealed beneath glacial and volcanic cover, K/Ar and Ar/Ar dating of rocks within the Kahiltna Terrane and beyond indicates at least three magmatic pulses (~90Ma, ~65Ma, ~45Ma).

The USGS collected magnetotelluric and gravity data over a 10,000 km² area within the central and northern Kahiltna Terrane. Regional aeromagnetic data provide the broader context for these studies. A 3D resistivity model reveals a first-order change in crustal structure across a sinuous thrust boundary south of the Mulchatna Lineament. This boundary is paralleled by a series of linear magnetic highs and lows which we interpret as upturned sediments or metasediments associated with compression during or after suturing of the Kahiltna to the Kuskokwim.

Within the central Kahiltna Terrane, large intrusives make up a significant crustal volume, with resistivity models constraining the extent of these intrusives from mid-crustal depths to at or near the surface. Different types and/or generations of intrusives can in some case be identified by their magnetic/gravity signatures and varying depth of emplacement. A particularly large intrusive cluster, parts of which date to ~90Ma, is delineated by both magnetics and magnotellurics. This cluster is associated with the Pebble Cu-Au-Mo porphyry deposit and a number of associated skarns and base-metal deposits. A strong conductive zone is imaged at mid-crustal depths beneath this cluster, and may be associated with large-scale processes that led to the shallow mineralization. Alternatively, the high conductivity may reflect a zone of crustal weakness along the Lake Clark Fault corridor, which intersects this large intrusive cluster.