PROPAGATING RIFT PSEUDOFAULTS - ZONES OF CRUSTAL UNDERPLATING IMAGED BY MULTICHANNEL SEISMIC-REFLECTION DATA

Two multichannel seismic reflection lines recorded over the northern Juan de Fuca plate off western Canada provide images of the crustal structure of three pseudofaults, the traces of propagating rifts. Anomalous reflectivity, including both subhorizontal and dipping events at times below the normal crust-mantle transition, is prevalent in the region of the pseudofaults. Although the crustal thickness elsewhere on the lines varies by only about 10%, crust associated with the pseudofaults is as much as 30% thicker than average. This may be due to an increase in thickness of the interpreted oceanic crustal layer 3B or crustal underplating. The results indicate that magma supply at propagating rifts may at times be large, perhaps due to the effects of nearby hotspots or convective partial melting associated with the initiation of rifting.