CRUSTAL STRUCTURE OF THE CASCADIA SUBDUCTION ZONE, SOUTHWESTERN BRITISH-COLUMBIA, FROM POTENTIAL-FIELD AND SEISMIC STUDIES

The northern Cascadia subduction zone is a region of convergence betwe en the oceanic Explorer and northern Juan de Fuca plates and the conti nental North American plate. Potential field and new seismic reflectio n data coupled with previous seismic results and geology enable deriva tion of a series of density - magnetic susceptibility cross sections a nd a block density model from the ocean basin to the volcanic are from 2.5- and 3-dimensional interpretations. The lateral extent and thickn ess of the accreted wedge vary significantly along the zone. The narro w, metasedimentary Pacific Rim terrane lies immediately west of Wrange llia and extends the length of Vancouver Island, consistent with its e mplacement by strike-slip faulting following the accretion of Wrangell ia. The ophiolitic Crescent terrane is a narrow slice lying seaward of the Pacific Rim terrane but not extending northward beyond the Juan d e Fuca plate, In this region, the Crescent terrane was emplaced in a s trike-slip or obliquely convergent style during the latter stages of e mplacement of Pacific Rim terrane. Below the accreted terranes, the Ex plorer plate is shallower than Juan de Fuca plate, resulting in a thin ner crust. High-density lower crustal material lies beneath the wester n edge of Vancouver Island, supporting interpretations of wide-scale u nderplating of Wrangellia. The shape of the boundary region between Wr angellia and the Coast belt to the east varies along strike and may be controlled by properties of preexisting plutonic rocks. The low-densi ty Coast belt plutons extend throughout most of the crust and are unde rlain by a lowermost crustal high-density layer, which may be indicati ve of fractionation accompanying magma generation.