New Insights Into Atlantic Opening From the Bay of Biscay

Abstract

The formation and evolution of the Bay of Biscay is the result of multiple, complex geodynamic processes, including regional rifting and subsequent compression and subsidence of the Atlantic rift system. Its evolution provides invaluable insights into the crustal processes acting in the region, as well as the Newfoundland conjugate margin, but also non-magmatic margins more generally as it provides a unique example of an unseparated conjugate margin.

Using newly reprocessed reflection data from Spectrum, the northern margin is best characterised by a series of distinct fault blocks to the west in the Western Approaches, and a transition into the Armorican Margin, to the east, which is dominated by regional necking. The fault blocks are dominated by syn-rift sediments and are formed during the major extension in the Mesozoic. Towards the centre of the Bay is the abyssal plain; here the sediments are relatively flat and undeformed, and are largely made up of syn-orogenic sediments from the Alpine Orogeny. The south of the Bay of Biscay is interpreted as being dominated by north verging thrust faults and sediment thinning indicative of an accretionary wedge, and is some of the best evidence of the Tethys plate subduction. Further east is a distinct deformation pattern showing an early sag basin, pre-rift, and later backarc extension. The combination of an accretionary wedge and distinct backarc extension is good evidence for Tethys Ocean subducting into the northern European plate. There is little to no evidence of volcanic sedimentation, and inversion in the Bay is rare and localised. The subduction combined with limited basaltic rock is best explained by changes in rock mechanics, and is supported by crustal thinning and chaotic basement reflectivity. Evidence of later compression through inversion is limited past the Celtic Sea; inversion in the Bay of Biscay is the result of later Pyrenean deformation, and the reactivation of earlier normal faulting. The lack of inversion in the area is attribute to buffering from the Landes High in the southeast corner of the Bay.

A crustal model was created using a combination of re-processed seismic data, gravity and magnetic data. This resulted in a large area of continental crust in the north and east, including thinned continental crust; a triangle zone of oceanic crust to the west, and a thinner zone of thinned continental crust in the south.

By placing this new crustal insight into a restored paleogeography we consider the implications on the hydrocarbon potential in the area, specifically in the far northwest where fault blocks are more prominent and interest in the equivalent position offshore Newfoundland margin is high.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019