Reflection seismic constraints on Paleozoic crustal structure and Moho beneath the NE German Basin

Abstract

A 330-km-long, NE–SW-trending seismic transect across the NE German Basin, combined with a 75-km-long, perpendicular cross-line in the basin centre, reveals the structure of this intracontinental basin between the stable Precambrian Baltic Shield and the weaker Mid-European Variscides. The combination of high-fold vibroseis and low-fold explosive seismic profiling images both upper and lower crustal structures in detail.

Distinct faults related to a rift stage are not observed below the Permian sedimentary basin, but there is evidence of lower crustal thinning. A Moho pattern less reflective below the basin centre than at the margins may be related to magmatic underplating.

The rather flat-lying Moho dips slightly towards the basin margins. Structures further north suggest a southern extent of Baltica below the NE German Basin. The Caledonian accretionary wedge may be outlined by SW-dipping mid-crustal reflectors beneath the basin.

Introduction

The Northeast German Basin forms the centre of the North Sea–North Germany–Polish sedimentary basin system. It extends from the Tornquist Zone (an inferred Paleozoic suture) to the Harz Mountains, and from the North Sea to Poland (Fig. 1). The complex evolution of this intracontinental basin comprises several changes in tectonic regime and fault systems during the past 300 Ma, and structures generated by strike–slip and subsequent inversion tectonics.

The German Continental Reflection Seismic Program (DEKORP) investigated the deeper crust of NE Germany in 1996, to document a basin with significant crustal thinning and subsidence (DEKORP–BASIN Research Group, 1999). The focus of this study is to better resolve the reflection seismic structures in the middle-lower crust, and hence to define tectonic boundaries.

To the north, the NE German Basin onlaps on the Baltic Shield, considered as a Precambrian terrane collage (Berthelsen, 1992). During the Caledonian orogenic cycle, the Avalonia microcontinent was accreted by oblique convergence and collision with the Precambrian Fennoscandian–Baltic craton, and forms the basement towards the south (Meissner et al., 1994). Avalonia mainly comprises Gondwana-derived pre-Caledonian basement (Torsvik et al., 1993) which is overlain by an intermittent succession of Cambrian- to Carboniferous-age sediments (e.g. McCann, 1996). The Upper Carboniferous sediments were deposited in a northward-migrating foreland basin formed during the Variscan orogeny to the south.

Extensional tectonics and rift-related volcanism between Carboniferous and Permian times (Benek et al., 1996, Ziegler, 1990) resulted in faults and pull-apart basins related to the Tornquist system (Fig. 1). The surrounding basins are filled with Late Triassic to Early Cretaceous sediments, forming a structural link between the Danish Basin and the Polish Trough. Late Cretaceous to Early Tertiary inversion structures extend from Poland to the Baltic Sea, resulting in uplift of several kilometres, probably related to the Alpine/Carpathian collision (Meissner et al., 1994). A final phase of subsidence, related to North Sea thermal subsidence, occurred in the Cenozoic, accompanied by intensive salt movement.

Widespread magmatic activity accompanied the large-scale wrench faulting of the European Variscides during the late Stephanian (Lorenz and Nicholls, 1984), and thick volcanic successions were deposited (Ziegler, 1990). The composition of these volcanic rocks and their crustal contamination indicate a thermal destabilization of the lithosphere (Benek et al., 1996).

The tectonic evolution of the NE German Basin has been a subject of much debate (see Scheck and Bayer, 1999 and references therein). Bachmann and Grosse (1989) and Bachmann and Hoffmann (1995) suggested rifting and mantle diapirism, and underplating. A simple-shear mechanism has also been proposed, with a crustal-penetrating detachment rooting in the mantle either to the south (Brink et al., 1990) or to the north (Berthelsen, 1998).

In the Baltic Sea, the BABEL Working Group (1993) imaged a continuous Moho reflector and a highly reflective lower crust, while onshore seismic reflection (NIZUSE, Horst et al., 1994; EGT, Aichroth et al., 1992) and refraction profiles (Thumark II, Bormann et al., 1989) are of variable quality and do not clearly image the pre-Zechstein successions. However, it is suggested that the NE German Basin formed in late- to post-Variscan times on pre-Mesozoic consolidated continental lithosphere. Moho depth in the region has been controversially discussed including suggestions of a Moho high below the basin centre (Bachmann and Grosse, 1989), a Moho low (Hoffmann et al., 1996) or only small Moho relief (Bormann et al., 1989).