Seismic and laboratory constraints on crustal formation in a former continental arc (ACCRETE, southeastern Alaska and western British Columbia)

Abstract

[1] The ACCRETE project studies continent formation by assemblage of exotic terranes and arc magmatism. A marine-land seismic profile crosses the terranes with dense sampling and remarkably strong S waves as well as P waves. Unique, high-quality S-wave seismic data provide further constraints on interpretation and are used together with new laboratory P- and S-wave velocity measurements ( corrected for high temperatures within and at the base of the crust) to make a lithologic and petrologic interpretation. V-P/V-S ratios indicate three distinct terranes that were assembled to form continental crust. These terranes are the outboard, accreted Alexander-Wrangellia terrane, the Coast Mountains Batholith (CMB) magmatic arc, and the inboard Stikinia terrane. Below the CMB, V-P and V-P/V-S to a depth of 10-15 km are appropriate for tonalite and diorite. The V-P and V-P/V-S increase with depth indicating that the rocks become more mafic, but V-P/V-S is not high enough for gabbro. The V-P and V-P/V-S of the lower crust are consistent with a mixture of mafic garnet granulite and restite, whose quartz content lowers the V-P/V-S ratio below that of gabbro. When corrected for high temperature and uplift due to exhumation, the crust under the CMB exhibits seismic properties of an average continental crust. These results suggest that gabbro could have intruded a metasedimentary pile in the deep crust to cause melting of the metasediments to form intrusions of the CMB, leaving behind a mixture of mafic garnet granulite and sillmanite-garnet-quartz restite and generating the deeper part of what becomes an average continental crustal section.