Crustal structure along the EGT profile across the Tornquist Fan interpreted from seismic, gravity and magnetic data

Abstract

The Tornquist Fan is a northwestward widening splay of late Carboniferous to early Permian fault zones in the Danish area which emanates from the Teisseyre–Tornquist Zone in the Baltic Sea. It is located over an area of deep sedimentary basins around Denmark in the southwestern corner of the former Baltica plate. This paper presents an integrated model of seismic velocity, density, and magnetic properties along the EGT profile across the Tornquist Fan between southern Sweden and northern Germany. The model shows distinct features that may be related to the main tectonic events in the area. The Caledonian crustal suture between Baltica and Eastern Avalonia is interpreted from changes in physical parameters at all crustal levels; most noticeable as an abrupt, lateral change in seismic velocity in the lowest crust at Moho level from 6.9km/s in Baltica to only 6.4km/s in Eastern Avalonia. Thick sedimentary successions beneath the Mesozoic sequence are interpreted as Palaeozoic sedimentary rocks, which may have been deposited on the southern margin of the Baltica plate, in a deep Caledonian foreland basin, and in late Carboniferous-early Permian rifts and extensional basins. Rift related magmatism is indicated by the presence of zones of high velocity, density and magnetic susceptibility as well as strong remanent magnetization. Thermal subsidence following the emplacement of these bodies may be the origin of regional subsidence of the Danish and North German basins of late Paleozoic and Mesozoic age. Late Cretaceous to early Cenozoic inversion of basin structures is detected at the Sorgenfrei–Tornquist Zone (STZ) in the transition between shield and basin areas. The STZ coincides with a pronounced change in crustal thickness and the southwestern edge of a major magmatic intra-crustal body. Significant, abrupt changes in Moho depth exist along the profile: 26–30km beneath the deep basins, ca. 35km beneath the Ringkøbing–Fyn basement High, and 48km beneath the Baltic Shield around the Sorgenfrei–Tornquist inversion Zone. The basement in the Tornquist Fan area is of Proterozoic age. The Moho topography indicates strong thinning of the crust during its Phanerozoic evolution. The model supports the hypothesis that existing tectonic structures and weakness zones are continuously being reactivated during subsequent tectonic activity.