Seismic structure of oceanic crust in the western North Atlantic

Abstract

We examine the seismic structure of Cretaceous‐aged ocean crust north of the Blake Spur fracture zone in the western North Atlantic by using a combination of multichannel seismic and wide‐angle reflection/refraction data. Although the oceanic crust in this area is characterized by a relatively uniform thickness and seismic velocity structure, it displays large variations in crustal reflectivity on both ridge‐normal and ridge‐parallel profiles. The upper crust is highly reflective and contains both subhorizontal (or shallow dipping) events and more steeply dipping reflectors. Subhorizontal reflectors are typically present between 1.5 and 2 km depth and are, in some cases, laterally continuous for distances of 15 to 20 km. Generally, these events do not correlate with the depth of the seismic layer 2/layer 3 boundary determined from refraction data, and their origin is still poorly understood. The steeply dipping events are generally confined to the upper 2.5 km of the crust. We interpret these reflectors as the subsurface expression of ridge‐parallel extensional faults commonly mapped at mid‐ocean ridges. The lower crust in this area exhibits alternating regions of high and low reflectivity. The highly reflective zones are made up of packages of linear or concave‐upward dipping reflectors that flatten out in a diffuse zone of high reflectivity near the base of the crust. Some of these dipping reflectors appear to cut through the whole crustal section, although most fade out in the acoustically transparent midcrust. On ridge‐normal profiles the majority of these events dip to the east, toward the paleo‐spreading center, whereas on ridge‐parallel profiles the events typically dip south, toward the Blake Spur fracture zone. The base of the crust is generally not associated with a strong, discrete Moho reflection but is a comparatively indistinct boundary usually associated with a 1‐ to 2‐km‐thick band of diffuse high reflectivity. We interpret the lower crustal and whole crustal dipping events as the subsurface expression of major fault systems that have ruptured the entire crustal section down to depths of 8–10 km. With the available data we cannot unambiguously determine the geometrical relationship between the dipping lower crustal reflectors on the ridge‐normal and ridge‐parallel profiles. We may be imaging a single major detachment surface that dips both toward the ridge axis and away from accommodation zones linking major boundary faults. Alternatively, these events may represent two different classes of fault systems that form during different stages of the emplacement and aging of oceanic lithosphere.