Palaeozoic structural development along the Tornquist Zone, Kattegat area, Denmark |
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| Authors: | Tommy E Mogensen |
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| Institution: | Geologisk Institut, Aarhus Universitet, DK-8000, Århus C, Denmark |
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| Abstract: | The interpretation of newly released commercial 2D reflection seismic data in the Kattegat area, Denmark, has provided us with a better understanding of the Palaeozoic tectonic processes along the Tornquist Fault Zone. A Base Palaeozoic time structure map, a Lower Palaeozoic TWT isopach map, a “true” Lower Palaeozoic TWT isopach map, an Upper Carboniferous/Lower Permian syn-rift TWT isopach map, a Top pre-Zechstein time structure map and a Zechstein combined TWT isopach and Palaeogeography map have been generated. The uniform Lower Palaeozoic sequence thickness in the Kattegat, both inside and outside the Tornquist Zone indicates only minor lateral movements if any, whereas the extensive Upper Silurian sequence, increasing in thickness to the north, indicates a relatively fast regional subsidence. The Base Palaeozoic time structure map and the Late Palaeozoic syn-rift isopach map show a clear Late Palaeozoic extension in the area. The syn-rift isopach map, in combination with the time-equivalent opening of the Skagerrak graben at right angles to the Tornquist Zone in the Kattegat, indicates that this extensional tectonic event had a dextral slip component. Measurements on internal extensional faults in the Tornquist Zone, give a minimum right-lateral displacement of 10.4 km. The footwall blocks were deeply eroded during the Early Permian rifting, and at Zechstein times the area became a peneplane. The Tornquist Zone was later exposed to several tectonic phases, where dextral slip played a role, indicated by the “push up” and “pull down” structures caused by restraining and releasing bends of the Børglum Fault. The dextral displacement along the Børglum Fault since the beginning of the Permian is in the order of 5–7 km based on the displacement of a Lower Palaeozoic local depocentre. Early Permian depocentres and faults, which gives a total amount of right-lateral displacement since the Early Palaeozoic in the order of 15–20 km. The continuously repeated tectonic episodes along the Tornquist Zone throughout most of the Phanerozoic, show that the zone was easily reactivated, implying deep-seated basement faults. The Tornquist Zone can be seen as a “buffer zone”, between continental blocks, whenever changes in the regional stress field are induced. |
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