Shearing and Stretching the Porcupine Basin – A Tectonic Model Based on Integrated Geophysical Analysis

Abstract

The Porcupine Basin is located 150 kilometers offshore western Ireland. The basin bathymetry varies from less than 500 meters at its deepest in the north to nearly 3,000 meters along its southwestern fringe where a prominent bathymetric gap separates the Porcupine Bank from the Goban Spur. Along the basin’s eastern and western flanks the bathymetry deepens rapidly from the shallow waters of the Celtic Shelf and Porcupine Bank, respectively. The variation in bathymetry and the steep bathymetric gradients attest to the dramatic changes in crustal thickness that underpins this region. The basin was formed throughout the Early to Middle Mesozoic as the European and North American plates diverged. The crustal structure of the Porcupine Basin is interpreted through analysis of 2D reflection seismic, aeromagnetic and gravity data and published refraction lines. Collectively, the geophysical data confirm the results of previous studies which have suggested that the region experienced extreme rifting and thus the basin is floored with a combination of highly attenuated crust and exhumed mantle. The basin is defined by a series of north-south striking normal faults which have undergone various degrees of rotation. The rifting was guided by a series of pre-existing NE-SW striking, regional scale crustal sutures which outcrop onshore in Ireland & Scotland as well as on the conjugate margin of onshore Newfoundland. Our analysis proposes that these preexisting orogenic sutures facilitated the basins formation by providing surfaces which sheared during an extended period of Mesozoic rifting and thus strongly influenced the present day geometry of the basin. These ancient suture zones controlled the configuration of the southern limit of the basin as well as limiting the migration of rifting northward. The integrated analysis of all available data leads to a variation in the plate reconstruction of the Porcupine Basin relative to previous studies. The new reconstruction restores the crustal blocks of the Porcupine Bank to their early Jurassic prerift position against the Celtic Shelf by translating the blocks eastward. This eastward translation closes the Porcupine Basin with no significant rotation of the Porcupine Bank. The far southern regions of the basin are underlain by highly attenuated crust where it is postulated that Triassic rifting thinned the crust prior to Jurassic and early Cretaceous rifting.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018