FAR-FIELD EFFECTS OF EARLY TERTIARY RIDGE SUBDUCTION IN ALASKA

Early Tertiary ridge subduction in Alaska was responsible for a suite of near-trench plutons that extends 2100 km along the Pacific rim. The plutons young from west to east (61 to 50 Ma), tracking a migrating trench-ridge-trench triple junction and corresponding slab window between the Kula, Resurrection, and N. American plates. Near-trench effects include high-T, low-P metamorphism; strike-slip and normal faulting; and gold mineralization. Distal effects extend 100s of km inboard. Detailed links between far-field and near-trench events are uncertain owing to possibly large strike-slip offsets between the near-trench plutonic belt and its backstop. Along the arc, 250 km inboard of ~56-Ma near-trench plutons, the chain of events is: ~70-58 Ma—voluminous arc magmatism, including emplacement of Sn- and Mo-rich systems, correlated with subduction of young seafloor; 58-55 Ma—emplacement of mafic dikes, interpreted as marking a transition from a hot, thin slab to a slab window; ~55-45 Ma—magmatic lull, corresponding to passage of the slab window; and 45-0 Ma—renewed arc magmatism. The lull is diachronous along strike, in concert with the age trend of near-trench plutons, and is analogous to the present volcanic gap east of the Chile triple junction. Paleocene to Eocene basins in the forearc (Matanuska), intra-arc (Cantwell), and backarc (Rampart) underwent extensional subsidence and bimodal volcanism, which correlate with or slightly predate predicted arrival of the slab window. Isolated metamorphic complexes (Hatcher, Minook) were unroofed along strike-slip faults in the forearc and backarc regions; these yield cooling ages of 58-61 Ma and slightly predate arrival of the slab window. Apatite fission-track results show that as far as 900 km inboard of the paleotrench, the ancestral Brooks Range and its foreland basin experienced a pulse of uplift at 60-55 Ma, perhaps due to rebound of the upper plate upon cessation of corner flow in the asthenospheric wedge.