Seismic constraints on the three-dimensional geometry of low-angle intracrustal reflectors in the Southern Iberia Abyssal Plain

Abstract

Several lines of evidence suggest that simple shear rifting of the continental crust, in the form of low-angle detachment faulting, occurred during the final stages of continental breakup between West Iberia and the Grand Banks. The primary evidence for such faulting is the occurrence of low-angle, high amplitude reflectors within the basement adjacent to the ocean– continent transition zone. Here we present a series of intersecting, depth migrated seismic reflection profiles that image one such reflector, the H-reflector, located on the southern edge of Galicia Bank. ‘H’ lies beneath several boreholes drilled during ODP Legs 149 and 173, in a region where the oceanward extent of extended continental crust steps at least 150 km westward from its location in the southern Iberia Abyssal Plain to its location off the relatively shallow Galicia Bank. In our profiles ‘H’ appears to define a surface that extends over a region of at least 200 km2 and that dips down ?19? to the north, towards Galicia Bank. The profiles show that a close affinity exists between ‘H’ and the most seaward continental crust. Based on geophysical data and ODP drilling results, we infer that the basement above ‘H’ is composed of continental crust deformed by extensional faults into a series of wedge-shaped blocks and thin slivers. These basement wedges have a complex 3-D geometry. ‘H’ rises to the basement surface on a number of the seismic profiles and appears to define locally the oceanward extent of continental fault blocks.