LATE CRETACEOUS-EARLY EOCENE RECORD OF OBLIQUE CONVERGENCE ALONG THE NORTHEAST PACIFIC RIM: CONSTRAINTS ON RELATIVE MOTION OF THE NORTH AMERICA-FARALLON PLATES AND LOCATION OF THE KULA-FARALLON TRIPLE JUNCTION

Plate tectonic models for the northern Cordillera in the Late Cretaceous-early Cenozoic show two oceanic plates converging on western North America: a more northerly Kula plate and a southerly Farallon plate. Between 70 and 50 Ma plate reconstructions show the Kula plate had strongly oblique convergence relative to North America and the Farallon plate had only slightly oblique convergence.

Our structural and Ar/Ar geochronologic data from southern Alaska documents dextral strike-slip along a fault that formed at the inner edge of the accretionary prism during subduction of an oceanic plate under North America. Strike-slip on this fault, the Hanagita fault system, began no later than 70 Ma and continued until ~50 Ma. During this time span a ridge-trench-trench triple junction, thought to have been Kula-Farallon-North America, also migrated along the convergent margin, as seen in the record of diachronous near-trench plutonism sweeping from west to east (in present-day coordinates). The ridge migrated east of the zone of strike-slip between 56-50 Ma. Thus prior to 56 Ma dextral slip recorded along the Hanagita fault formed in response to oblique convergence between the Farallon and North American plates, based on current plate models. The Hanagita fault system is probably continuous with dextral slip systems described in Glacier Bay and Baranof Island in southeast Alaska, which would extend its length to ~700 km. The total amount of displacement along the Hanagita fault is not well constrained. One diorite in the fault zone with an age of 170 Ma lacks correlative ages for a minimum of 700 km along strike to the southeast. Total displacement quite probably is >1200 km, based on correlating the Middle Jurassic pluton and related rocks to a similar suite on the west side of Vancouver Island. This amount of displacement is also consistent with regional geology elsewhere in the accretionary prism. To achieve high enough rates of strike-slip motion to account for this total displacement, Farallon-North America relative convergence may have been more oblique than previously suggested. The geologic data also indicate that the Kula-Farallon triple junction must have been not much further south than southern Vancouver Island in the Late Cretaceous.