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

Abstract

The unexposed suture between Baltica and Eastern Avalonia is imaged by coincident normal-incidence reflection and wide-angle reflection/refraction seismic data of the MONA LISA project. We present new results of the upper lithospheric, seismic structure from the N–S-striking profile 2 across the Caledonian Deformation Front, which represents the crustal collision suture between Baltica and Eastern Avalonia that formed after closure of the Tornquist Sea during the Caledonian orogeny in Late Ordovician times. Three different crustal types are identified with great similarities to the nearby profile 1: (1) a three-layered crust typical of shields to the north; (2) a transitional crust (suture zone) in the central part; and (3) a two-layered crust 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 Ringkøbing-Fyn High (Baltica crust) to 28–27 km beneath 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 ∼70 km within the transitional crust. The reflection suture zone terminates in a ∼60 km wide reflective lens with velocities of 6.6–6.8 km/s in the lowermost crust. The reflective lens may be interpreted as a remnant of oceanic or island-arc crust that was accreted to the leading edge of Baltica during closure of the Tornquist Sea. Alternatively, the reflective lens may represent an indentor of Baltica crust into the Avalonian terrane. The change in lower crustal reflectivity and the abrupt transition from a three- to a two-layered crust further south suggest that the Trans-European Fault and the Elbe 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 Permian mantle shear zone.