NEW CONTRAINTS ON THE TECTONICS OF THE OCEAN BASIN INBOARD OF THE WRANGELLIA COMPOSITE TERRANE, SOUTH-CENTRAL ALASKA

The tectonic framework of the ocean basin between the Mesozoic margin of North America and the Wrangellia composite terrane was, until recently, largely enigmatic. New structural (at the micro-, meso-, and macroscales), lithologic, and geochemical data in south-central Alaska indicate the presence of a large subduction complex that includes an accretionary wedge (Reindeer Hills) and potential remnants of an arc (Chulitna block). The accretionary wedge of the Reindeer Hills consists of sand-shale mélange, carbonate olistostromes, radiolarian chert, and basaltic and andesitic volcanics. In the Chulitna block, serpentinite, volcaniclastic breccia with andesite, chert, and serpentinite clasts, fossiliferous limestone, pillow and columnar basalts, and sandstone have been interpreted as (1) a fault-bounded block of the Wrangellia composite terrane or (2) a sliver of an arc accreted prior to the docking of the Wrangellia composite terrane. Structures within the sand-shale mélange of the Kahiltna assemblage in Broad Pass and the deformation of the Chulitna block are consistent with northwest directed subduction. These data provide kinematic constraints on a new tectonic model for interpreting processes between the Mesozoic margin of North America and the Wrangellia composite terrane.