A seismic refraction and reflexion study of the continent-ocean transition beneath the north Biscay margin

Abstract

The structure of the northern margin of the Bay of Biscay consists of a series of tilted and rotated blocks bounded by prominent listric faults whose polarity is consistently down toward the continent-ocean boundary. These blocks formed by rifting in late Jurassic - early Cretaceous time and are now thinly covered by post-rift sediments of Aptian to Recent age. Seismic refraction profiles were occupied on the shelf, on either side of and across the continent-ocean transition to the shelf, using P ubs and O bs with explosives and a 4 x 1000 in 3 (4 x 16400 cm 3 ) airgun array. Two-ship expanding spread multichannel (48-trace) seismic reflexion profiles and 30 km fixed offset reflexion profiles were located along the seismic refraction profiles on either side of the transition. A two-ship 30 km fixed offset multichannel profile was located across the transition as well as a 5 km fixed offset multichannel profile extending from the ocean crust to the shelf. Conventional 48-trace single ship multichannel profiles were located along all the refraction and two-ship reflexion lines. Interpretation of the refraction profiles has been made by using ray tracing as well as synthetic seismograms. Conventional seismic processing techniques have been used to prepare the two-ship multichannel seismic data for interpretation. The survey is believed to be the first attempt to apply two-ship multichannel seismic data to the study of the change in crustal structure of a rifted passive margin from the shelf to the ocean crust. The results from the experiment led to the identification of a zone of transition between continental and oceanic crust about 8 km wide. The seismic refraction data show progressive thinning of the continental crust from 33 km to about 5 km close to the transition zone. However, extension values calculated in the upper crust from the rotation of fault blocks are much less (1.1—1.4) and suggest that the majority of the thinning is achieved by extensive attenuation of the lower crust.