Seismic structure across the Caledonian Deformation Front along MONA LISA profile 1 in the southeastern North Sea

Abstract

Seismic data from the MONA LISA (Marine and Onshore North Sea Acquisition for Lithospheric Seismic Analysis) project in the southeastern North Sea image the Caledonian Deformation Front (CDF), which is the collisional suture between Baltica to the north and east and Avalonia to the south and west. The NS-trending MONA LISA normal-incidence reflection profile 1 was recorded to 26 s twt. Coincident wide-angle data were acquired on nine ocean bottom hydrophones and several onshore mobile seismographs along and off the profile. The model of compressional seismic velocity shows three different crustal types: (a) a typical three-layered shield-type crust below the Ringkøbing Fyn High to the north: (b) a highly complex transitional crust in the central part; and (c) a two-layered crust of Caledonian origin to the south. Sharp and strong normal-incidence and wide-angle reflections from Moho were recorded south of the Caledonian Deformation Front in contrast to less distinct reflections further north. S-dipping crustal reflections from 4 to 11 s twt over ∼70 km horizontal distance terminate at Moho and coincide with a change in the seismic velocity structure. This indicates northward obduction of Avalonian crust. Non-migrated normal-incidence seismic sections show crossing weak N-dipping and stronger S-dipping reflections to 20 s twt from the uppermost mantle. We propose a tectonic model where the closure of the Tornquist Sea took place along a N-dipping subduction zone which was later overprinted by a late-or post-Caledonian S-dipping shear zone. Sub-Moho velocities are 7.8–7.9 km/s under 34–35-km-thick Baltica crust and 8.1–8.3 km/s under 25–26-km-thick Caledonian crust. The sub-horizontal Moho across the Caledonian collision zone implies late- or post-Caledonian re-equilibration of the seismological Moho. We interpret the low-velocity upper mantle (7.8–8.1 km/s) to the north as former Baltica lower crust in eclogite facies after pressure-induced metamorphism as a result of lithospheric flexure during the Caledonian orogeny. These rocks today appear as upper mantle that was uplifted to their present position during the Middle Devonian collapse of the North German-Polish Caledonides.