GRAVITY AND MAGNETIC STUDIES OF THE TALKEETNA MOUNTAINS AND THEIR IMPLICATIONS FOR THE JURASSIC TO RECENT EVOLUTION OF STRUCTURES AND TECTONICS IN SOUTHERN ALASKA

New gravity and aeromagnetic data and interpretations of the Talkeetna Mountains are part of a broader study to characterize the tectonics and metallogenesis of south-central Alaska. Our goals are to: (1) understand the tectonics of south-central Alaska, especially the timing and development of the southern Alaska Orocline, (2) determine the structural relationships between tectonostratigraphic terranes, 3) understand the character of major internal and terrane bounding faults, and (5) identify the locations, size, and depth of buried sources of potential mineral targets. One of the most prominent geophysical features in the Talkeetna Mountains is a combined gravity and magnetic gradient corresponding to the Talkeetna Fault Zone (TFZ), a northeast-trending structure that forms along a first-order crustal discontinuity between dense oceanic crust of the Wrangellia terrane and the low-density Jurassic to Cretaceous flysch, that forms an overlap assemblage between Wrangellia and North America. Our data suggest that the southeast portion of the flysch basin adjacent to Wrangellia, represents a transitional zone between Wrangellia and the original North American continental margin. This zone is distinguished from northern Kahiltna basin sediments, which were deposited on continental crust, by a northeast-trending gravity gradient aligned with the Broad Pass graben. This interpretation is consistent with recent sediment provenance mapping in the Kahiltna basin (K. Ridgeway, written comm., 2001). Thus, it appears that both the Talkeetna and Broad Pass fault zones, though overlaying deep crustal discontinuities, are structures that were activated in Tertiary time, long after the initial suturing of Wrangellia to North America. The southern transition zone boundary formed a crustal weakness that has since been exploited and modified by combined thrust and dextral strike-slip motion along the TFZ and parallel structures. These structures were active during early Tertiary time and accommodated the westward escape of crust due to Tertiary oroclinal bending; activity on some segments may have continued into the Neogene or later. It appears that a dense and magnetic block of Wrangellia's margin, was stranded north of the TFZ and right-laterally offset from its southern counterpart by as much as 50km.