A NEW TECTONO-METAMORPHIC MODEL FOR THE EMPLACEMENT OF THE ICEBERG LAKE AND LIBERTY CREEK BLUESCHIST BODIES, SOUTH CENTRAL ALASKA

Detrital zircon ages established that there are two distinct blueschist-facies assemblages in southern Alaska: widespread older rocks with Early Jurassic cooling ages, and the much younger Liberty Creek assemblage with Early Cretaceous maximum depositional ages. In the central Chugach Mountains, blueschists representing both groups are exposed near upper mantle/lower crustal rocks of the Talkeetna arc. The Klanelneechena klippe is adjacent to the Early Jurassic Iceberg Lake schist and the Tonsina ultramafic assemblage is juxtaposed against the Liberty Creek schist. Both blueschists have similar post-peak metamorphic ductile overprints, suggesting similar strain paths despite their very different depositional and cooling histories. We present two models for the structural and geographic relationships between these assemblages. The first model invokes Early Jurassic and Early Cretaceous periods of accretion and subduction erosion for the formation of these high pressure assemblages. These assemblages passed through closure, as a result of subduction erosion but remain at depth until a Cretaceous period of transpressional accretion. This transpression-related exhumation creates the observed reorganization of these assemblages and produces the younger overprints in the blueschists. An alternative model for the organization of these assemblages is that the blueschist terranes were subducted and exhumed through their respective cooling temperatures and lower-grade metamorphic facies independent of one another. Similarities in strain history are a result of deformation along a similarly oriented subduction margin with coincidental strain paths. In this scenario their geographic locations are a result of younger, unrelated, strike-slip reorganizations along the Border Ranges fault. Future research could distinguish these hypotheses through clarification of the cooling histories of both assemblages.