CRUSTAL STRUCTURE OF NORTHERN JUAN-DE-FUCA PLATE FROM MULTICHANNEL REFLECTION DATA

The northern Juan de Fuca plate forms the basement of a young (<8 Ma) ocean basin off western Canada. Two subparallel multichannel seismic r eflection lines, totaling 230 km in length and trending obliquely to t he spreading direction, were recorded across the basin and provide hig h-quality images of crustal structure. The processing sequence applied to the data includes a prestack inside-trace mute of common midpoint (CMP) gathers to reduce noise levels on the deep data, CMP stack, post stack dip filtering, f-k migration, and band-pass filtering. Coherency -filtered stacks are helpful in tracing weaker reflectors. The stacked sections reveal a horizontally layered sedimentary sequence overlying a rugged and prominent basement reflector which dips slightly landwar d. Topography of the basement was formed at or near the spreading ridg e; there is no evidence for subsequent deformation. A strong, fairly c ontinuous reflection from the base of the crust (oceanic Moho) at 2.0- 2.2 s two-way travel time below the basement surface generally mimics the basement topography and shows the characteristic multicyclic wavef orm seen in other surveys. Estimates of reflection coefficients for th e Moho at several locations indicate that, in general, the crust-mantl e transition is unlikely to occur as a discrete first-order interface. Although much of the crust appears acoustically transparent, a number of discontinuous intracrustal reflectors are observed over 40 km at t he eastern end of the northern line; some of these are interpreted to arise from the oceanic layer 2/3 and layer 3A/3B boundaries. The nonco ntinuous nature of these reflectors is an indication of the complexity of the processes producing intracrustal reflectivity and the lateral variability of crustal formation. However, the interpreted crustal thi ckness varies by only about 10% away from the distal ends of spreading segments, indicating that while the process of crustal formation may be locally variable, it produces a broadly uniform result.