EVOLUTION OF CORDILLERAN CONVERGENT MARGINS—IMPLICATIONS OF OFFSHORE OBSERVATIONS THAT ARC-TRENCH WIDENING OF THE ALASKA MARGIN WAS NOT CAUSED BY ACCRETIONARY ADDITIONS TO THE OUTER FOREARC

INTRODUCTION: California’s Franciscan Fm was recognized in the early 1970s as an accretionary complex, as were similar bodies along the Pacific rim. It was conjectured that accretion of ocean floor material to the forearc would widen the distance separating the magmatic arc and trench, i.e., the arc-trench gap. The Alaska margin was recognized as an example of a widening arc-trench gap. This conceptual notion was not based on MCS records that clearly imaged upper plate structures as modern migrated profiles do.

OBSERVATIONS: Since the mid 70s, migrated MCS profiles, velocity data, dredge samples, and exploratory drilling revealed the rock fabric of offshore Alaska. These findings document that coastal basement of Late Cretaceous-earliest Tertiary age extends seaward to near the trench. Basement is the exhumed, granite-intruded, underplated accretionary body of the Kodiak and kindred Fms. A wave-base unconformity cuts across the top of basement and slopes downward and seaward beneath a 2-5-km-thick section of extensionally faulted beds of Eocene and younger age. The underlying basement thins seaward and terminates behind a ~15-25-km-wide prism of frontally accreted trench sediment.

INTERPRETATIONS AND IMPLICATIONS: Observational data attest that since the Eocene offshore Alaska has been progressively subsiding and migrating inboard toward a fix reference. We attribute subsidence and migration to crustal thinning and truncation effected by basal and frontal subduction erosion. Seismic images imply that these crust-removing processes were accompanied by sub-coastal accretionary underplating of subducted sediment and eroded debris. Similar coastal underplates are imaged beneath Japan, Cascadia, and Chile. With respect to fossil underthrust Cordilleran margins, it seems likely their rock fabric was fashioned more by offshore truncation and coastal underplating (crustal thickening) than forearc widening by accretionary processes. Fossil truncated margins would include the present Pacific margins of Baja California and northern BC, the older Jurassic margin of Alaska’s Talkeetna arc, and possibly for western and eastern California, respectively, the Franciscan Sur-Nacimiento “fault” and the Sierra-fronting sector of the Mojave-Sonora megashear.