INSIGHTS FROM THE CHUGACH ACCRETIONARY COMPLEX IN SOUTHERN ALASKA INTO UNDERPLATING PROCESSES AND MEGATHRUST RHEOLOGY AT SHALLOW SUBDUCTION CONDITIONS

The Chugach-Kodiak accretionary complex of southern Alaska extends 1200+ km along-strike from Kodiak Island through and around the Alaskan orocline. Although some sections of the Chugach are locally to regionally overprinted by later magmatic and tectonic events, along most of the southern Kenai Peninsula the Chugach preserves structures representative of shallow- to intermediate-depth (prehnite-pumpellyite to greenschist-blueschist facies) underplating during late Triassic to mid-Tertiary oceanic subduction. Furthermore, the complex shows distinctive changes in sediment input from northwest to southeast with the older, northwestern-most Seldovia Schist and McHugh Complex dominated by thinly sedimented oceanic crustal sequences, passing structurally downward to the younger turbidite-dominated Valdez Group. The Chugach thus provides an ideal natural laboratory to investigate timescales and mechanisms of basal underplating, and variations in interface rheology, as a function of changing trench lithological inputs and metamorphic grade. We used high resolution coastal mapping, geo- and thermochronology, graphite crystallinity-derived temperature estimates, and [micro]structural analysis to: a) identify several paleo-subduction megathrust segments in the Chugach complex formed at different depths, b) compare and contrast their outcrop-scale deformation patterns, and c) probe rheological and fluid flow properties. We present examples and interpretations of megathrust shear zones, including: 1) documentation of a 1-2 km-thick deformation zone juxtaposing low-grade McHugh Complex chert-shale-basalt with epidote blueschist facies Seldovia Schist that is interpreted to exhibit several kilometers of displacement, and 2) a comparative analysis of smaller-displacement basalt-hosted vs. sediment-dominated megathrust shear zones within the McHugh Complex.