Crustal structure of northern Juan de Fuca Plate from multichannel reflection data

Abstract

The northern Juan de Fuca plate forms the basement of a young (<8 Ma) ocean basin off western Canada. Two subparallel multichannel seismic reflection lines, totaling 230 km in length and trending obliquely to the spreading direction, were recorded across the basin and provide high‐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, poststack 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 landward. Topography of the basement was formed at or near the spreading ridge; there is no evidence for subsequent deformation. A strong, fairly continuous 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 waveform seen in other surveys. Estimates of reflection coefficients for the Moho at several locations indicate that, in general, the crust‐mantle 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 the 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 noncontinuous 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 thickness 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.