THE HORN GRABEN, AND ITS RELATIONSHIP TO THE OSLO GRABEN AND THE DANISH BASIN

The tectonic development of the Horn Graben, the Danish Basin and the Oslo Graben are compared with special emphasis on the timing of their initiation. The comparison is based on seismic interpretation and is supported by analysis of tectonic subsidence. Devonian and Carboniferous sedimentary strata and Rotliegende volcanics constitutes the pre-rift succession in the Horn Graben. In the central part of the graben it rests unconformably on Precambrian (possibly Dalslandian) basement, to the south on Caledonian metamorphic basement, whereas Lower Palaeozoics occur to the north. The distribution of these pre-Devonian assemblages suggests initial early Devonian uplift of the Ringkøbing-Fyn High as the Lower Palaeozoic sediments are believed to have covered the high originally. Further uplift appears to have taken place in the Early Permian. A subdivision of the Horn Graben into a northern and a southern segment are recognized. The rifting was apparently heralded only in the southern segment in the Rotliegende, and the main rifting phase did not start before the Triassic. Subsidence modelling shows that the thermal anomaly created by rifting of the Horn Graben was less significant than in the Danish Basin and resulted in much less post-rift thermal subsidence. The reduced thermal anomaly may account for an earlier consolidation of the crust, thereby rendering the area resistant to late Cretaceous-early Tertiary inversion tectonism. The Oslo Graben was tectonically active during late Carboniferous to early Permian times, and essentially became inactive during Triassic times when the main tectonism of the Horn Graben occurred. Subsidence modelling shows that the Danish Basin, south of the Fjerritslev Trough, was tectonically active mainly during the Zechstein, whereas the rapid Triassic subsidence is modelled to be caused by phase transformations, as indicated by the minor amount of basement attached faulting. Based on the differing subsidence patterns, it is concluded that the three basins did not develop simultaneously, and that they are not genetically related. © 1990.