Seismic images of Caledonian, lithosphere-scale collision structures in the southeastern North Sea along Mona Lisa Profile 2

The unexposed suture between Baltica and Eastern Avalonia is imaged by coin cident normal-incidence reflection and wide-angle reflection/refraction sei smic data of the MONA LISA project. We present new results of the upper lit hospheric, seismic structure from the N-S-striking profile 2 across the Cal edonian Deformation Front, which represents the crustal collision suture be tween Baltica and Eastern Avalonia that formed after closure of the Tornqui st Sea during the Caledonian orogeny in Late Ordovician times. Three differ ent crustal types are identified with great similarities to the nearby prof ile 1: (1) a three-layered crust typical of shields to the north; (2) a tra nsitional crust (suture zone) in the central part; and (3) a two-layered cr ust of Caledonian origin to the south characterized by very low velocities throughout the crust. The crustal thickness varies from 38-35 km under the northern margin of the Ringkobing-Fyn High (Baltica crust) to 28-27 km bene ath the North German Basin in the Caledonian crust to the south. The suture zone is imaged by S-dipping crustal reflections from 1.9 to 10.6 s two-way travel time (twt) over a horizontal distance of similar to 70 km within th e transitional crust. The reflection suture zone terminates in a similar to 60 km wide reflective lens with velocities of 6.6-6.8 km/s in the lowermos t crust. The reflective lens may be interpreted as a remnant of oceanic or island-are crust that was accreted to the leading edge of Baltica during cl osure of the Tornquist Sea. Alternatively, the reflective lens may represen t an indentor of Baltica crust into the Avalonian terrane. The change in lo wer crustal reflectivity and the abrupt transition from a three- to a two-l ayered crust further south suggest that the Trans-European Fault and the El be Lineament continue into the North Sea. The upper mantle reflectivity is dominated by bands of strong-amplitude, S-dipping reflections from 13.5 to 21.8 s twt, and a weaker band of N-dipping reflections from 12 to 16 s twt. Reversed wide-angle refractions and reflections indicate the presence of a S-dipping high-velocity layer (8.65-8.8 km/s) in the uppermost mantle. The sub-Moho high-velocity layer partially coincides with the strong S-dipping normal-incidence mantle reflections. We propose that the S-dipping mantle structure may represent a late-Caledonian or Late Carboniferous to Early Pe rmian mantle shear zone. (C) 2000 Elsevier Science B.V. All rights reserved .