SEISMIC-REFLECTION CONSTRAINTS ON IMBRICATION AND UNDERPLATING OF THENORTHERN CASCADIA CONVERGENT MARGIN

An interpretation of the deep structure of the continental shelf offsh ore southern Vancouver Island, subject to constraints from other geoph ysical data, is derived by combining seismic reflection profiles shot in 1989 with those from an earlier 1985 survey, Accretionary wedge sed iments, which extend landward beneath the volcanic Crescent terrane, c omprise two primary units, both of which have shortened through duplex formation. The maximum thickness of the Crescent terrane, 6-8 km, occ urs just seaward of its contact with the inboard, largely metasediment ary Pacific Rim terrane. The E region of reflectivity, first detected dipping landward beneath Vancouver Island, is regionally extensive, be ing observed on all the seismic profiles, The E reflectivity thins sea ward and splits into two or more strands that probably link into major faults within the accreted sedimentary wedge. Reflections from the in terplate decollement beneath the outer continental shelf separate from the downgoing plate, continue into the deepest level of the E reflect ivity, and are interpreted to represent a single decollement surface a bove which imbrication of accreted units occurred, It is proposed that at the southern end of Vancouver Island the E reflections represent m ainly underthrust sediments above a former subduction decollement, bot h of which were incorporated into the overlying continent when the sub duction thrust stepped down into the descending oceanic plate. This ch ange in depth of the subduction thrust underplated one or more mafic u nits to the continent. The reflection from the top of the subducting J uan de Fuca plate appears to be around 5 km shallower farther north al ong the margin, indicating that the underplated region could be confin ed to the embayment in the Cascadia subduction zone.