Crustal thinning, mantle exhumation and serpentinization in the Porcupine Basin, offshore Ireland: evidence from wide-angle seismic data

Abstract

New wide-angle seismic data were gathered along a 230 km long profile that runs east-west across a deep structural feature in the Porcupine Basin, offshore Ireland, known as the Porcupine Arch. Ocean bottom seismometers were deployed at 3-4 km intervals and seismic sources fired every 120 m along it. Prominent primary and secondary arrivals indicate that the continental crust is extremely thin (locally less than 2 km) across the basin centre. The sedimentary succession is up to 12 km thick and comprises three distinctive seismic layers. The two uppermost layers are interpreted as mostly a post-rift succession of Cretaceous and Cenozoic strata. The lowest layer thins rapidly towards the basin centre and is interpreted as a succession of predominantly Jurassic synrift sediments. A strong asymmetry in both the geometry of the crust and the sedimentary layers is probably related to a simple shear mode of extension and the subsidence that it induced. Crustal thinning is far greater than in the adjacent Rockall Basin and local exhumation of continental mantle lithosphere may have occurred in parts of the Porcupine Basin. Low P n velocities beneath the Porcupine Arch are compatible with larger amounts of mantle serpentinization than in the Rockall Basin.

A review of the NE Atlantic conjugate margins based on seismic refraction data

“…A HVLC is also often observed, which is commonly explained by magmatic underplating or as a result of sill intrusions into the lower crust at the distal parts of the margins. However, high velocities are also observed below more proximal parts of the margins, such as beneath the southern Rockall Basin and the Porcupine Basin, where they are interpreted as partially serpentinized mantle (Shannon et al 1999;O'Reilly et al 2006). At the mid-Norwegian margin, interpretations vary from magmatic underplating to eclogite bodies and partially serpentinized mantle (cf.…”

“…The crustal thinning to either side of the basin is asymmetrical, with a narrower necking zone in the west. O'Reilly et al (2006) interpreted this to be the result of simple shear along low-angle westwards-dipping detachment surfaces formed during later stages of extension.…”

“…Along RAPIDS line 4 in the Porcupine Basin (Fig. 3d), an up to 12 km-thick sedimentary sequence is underlain by continental crust that is extremely thin, in some places less than 2 km (O'Reilly et al 2006 (2006) argued against this alternative for several reasons, such as the lack of a double reflection from the top and base of an underplated body. Beneath the adjacent Porcupine Bank and the Irish Shelf, the Moho deepens to 30 km, which is similar to full-thickness continental crust beneath Ireland and the UK .…”

Three-dimensional gravity and magnetic modelling of the Irish sector of the NE Atlantic margin

“…The results of wide-angle seismic and potential field modelling indicate that its sedimentary infill is underlain by either very thin continental crust (e.g. Conroy and Brock, 1989;Johnson et al, 2001) or partially serpentinised upper mantle (Reston et al, 2004;O'Reilly et al, 2006). The basin has been drilled by approximately 30 exploration wells (see Croker and Shannon, 1987), mainly along its northern flanks and is considered to contain in excess of 8 km of Permo-Triassic to Recent strata, overlying a Carboniferous and Devonian succession of uncertain distribution and thickness Readman et al, 2005).…”

Controls on the location of compressional deformation on the NW European margin

“…If stretching increases with depth, the cooling effect will be smaller and may not be sufficient for embrittlement to occur. Such a contrast in stretching style may explain the presence of serpentinized upper mantle beneath the southern Rockall and Porcupine basins (Hauser et al 1995;O'Reilly et al 1996O'Reilly et al , 2006, and its absence beneath the Orphan Basin on the Eastern Canadian conjugate margin, where stretching factors are similar (Welford et al 2012). In the Cenozoic strength profiles, the lower temperatures in the former case reduces the likelihood of ductile lower crust developing above the serpentinized zone (although this may be offset by the thermal blanketing effect of a subsequently deposited, thick sedimentary sequence).…”