THE EXTENT, BOUNDARIES, AND CRUSTAL CHARACTER OF WRANGELLIA IN SOUTHERN ALASKA

Prominent gravity (~3.25mGal/km), magnetic (~100nT/km) and magnetotelluric (~300ohm-m/km) gradients define the westernmost edge of oceanic crust in southern Alaska. This first-order discontinuity, which we term the Talkeetna Suture Zone (TSZ), forms the western boundary of Wrangellia. Models along three profiles across the TSZ indicate that it is a deep (2-8 km) steeply west-dipping structure, rather than the shallow, east dipping thrust fault previously interpreted (Csejtey et al 1978). Structures in the upper crust are concentrated in a 2-12 km wide zone above the TSZ. These features include the Watana Creek graben of Oligocene-Miocene terrestrial sediments, steep normal and dextral faults in the Fog Lakes Lowland, and Eocene volcanic rocks at Deadman and Prairie Creeks. Although the TSZ focused later tectonic activity, no single surficial feature directly corresponds to, or overlies the older crust break. We interpret four internally consistent regional geophysical domains as lithotectonic blocks somewhat different than previous tectonostratigraphic assignments. Mesozoic flysch west of the TSZ overlies two domains - one of continental crust northwest of Broad Pass and one of transitional crust to the southeast. This interpretation is consistent with recent provenance studies of Ridgway and others that define two flysch sub-basins that received their sediments from the North American margin and Wrangellia, respectively. We propose that the suture between the two other domains – the Wrangellia and Peninsular terranes- lies 40 kilometers southeast of its previously proposed location, a position more consistent with geologic evidence that middle Jurassic plutons intruded Wrangellia basement rocks. Within Wrangellia, basalts of the Nikolai Greenstone, and recently recognized dolerite sills that are their intrusive equivalents, have been mapped as far southwest as Sheep River. Numerous smaller (tens of km²) domains of probable ultramafic character have been subdivided into suites associated with rocks of Nikolai age or chemistry, and those of uncertain affiliation. These indicate the locations, size, and depth of buried sources that may be feeders to the Nikolai, and allows rapid identification of areas prospective for magmatic sulfide deposits of Ni, Cu, and PGE in southern Alaska.