The deep structure of northern England and the Iapetus Suture zone from BIRPS deep seismic reflection profiles

Abstract

The NEC deep seismic reflection profile images structures in the lower crust which represent the Iapetus Suture zone beneath southern Scotland and northern England. On the basis of reflectivity characteristics of the lower crust and reflection Moho we recognize four different crustal zones which correspond to four crustal/terrane types: Midland Valley (zone A), a sub-continental subduction complex (zone B ), Lake District (zone C ) and Midland Platform (zone D). The junctions between each of these zones are interpreted as being tectonic. A and B originate from the northern continental margin of the Iapetus Ocean and are separated from C and D (which are derived from a southern continent) by strong northerly dipping reflections between approximately 15 and 30 km deep in the crust. The structure responsible for these reflections does not displace the reflection Moho. North of the AB/CD junction, the base of zone B (and base of the reflective crust) is marked by a pair of parallel reflectors persistent for a horizontal length of about 55 km. These reflectors are interpreted as the top and bottom of a slice of remnant oceanic crust. At their northerly limit the deepest of the two parallel reflectors transects the Moho to a depth of 6-7 km into the uppermost mantle. We suggest that their termination delimits the northerly extent of a décollement or shear zone at the present continental Moho. The shear zone was active during the late stages of Caledonian collision when mantle, originally from beneath the southern continent, underthrust old and newly created crust of the collision zone.