EVIDENCE FOR EARLY TERTIARY RIDGE SUBDUCTION ALONG THE SOUTHERN ALAKSAN MARGIN

Abundant evidence exists marking a ridge subduction event during the Paleogene southern Alaksan margin. Plutonism and metamorphism associated with the Sanak-Baranof volcanic belt and Chugach metamorphic complex (CMC), respectively, reveal a high T, low P upper lithospheric environment during the Paleocene to Eocene. West to east age progression of volcanism along the Sanak-Baranof belt suggests migration of the ridge subduction event. Similarly, kinematic variation along the Hanagita fault and shear zones adjacent to the CMC indicate changes in strain patterns during the Paleogene. Recent studies of crustal stratification and kinematic deformation style at the western termination of the CMC support the accumulating evidence for early Tertiary ridge subduction. Modification of a crustal detachment model provides a good first order analog to structural mapping and finite strain measurements conducted near the Copper River east of Valdez, AK. In this area, shallowly plunging thermal stratification adjacent to the high-grade gneissic core of the CMC provides an interesting case study of an important, well-exposed complete crustal section. Present day ridge subduction along the southernmost Cordilleran margin provides an ideal analog for the dynamics of the Sanak-Baranof triple junction. Consideration of the link between current upper crustal dynamics in the southern Cordillera and middle to lower crustal dynamics of the Paleogene along the northern Cordillera could provide important insight into lower crustal flow beneath the Patagonian margin.