Triassic and Jurassic structural development along the Tornquist Zone, Denmark

Abstract

In the Danish area the old crustal weakness zone, the Tornquist Zone, was repeatedly reactivated during the Triassic and Jurassic/Early Cretaceous, causing minor dextral movements along the major boundary faults. These tectonic events were minor as compared to the tectonic events of the Late Carboniferous/Early Permian and the Late Cretaceous/Early Tertiary, although a dynamic structural and stratigraphical analysis indicate that the zone was highly active compared to the surrounding areas. During the Middle to Late Permian the area was exposed to erosion and became a peneplane. A regional Triassic subsidence produces seismic onlap towards the northeast, where the youngest Triassic sediment is found, supercropping the Precambrian basement. During mainly the Early Triassic, several of the major Early Permian faults became reactivated, probably with dextral strike-slip along the Børglum Fault. The Jurassic-Early Cretaceous subsidence became restricted primarily to the area between the two main faults in the Tornquist Zone, the Grenå-Helsingborg Fault and the Børglum Fault. This restricted basin development indicates a change in the regional stress field that seems to have come into existence during the transition between the Triassic and the Jurassic. The subsidence in the Middle Jurassic and the Late Jurassic-Early Cretaceous follows the Early Jurassic pattern with local subsidence in the Tornquist Zone, but even more restricted to the zone. The subsidence seems to have decreased in the Middle Jurassic; hereafter subsidence increased again during Late Jurassic/Early Cretaceous times. A set of small faults were generated during the Mesozoic internally in the Tornquist Zone. This fault pattern indicates a broad transfer of strike-slip/oblique-slip motion from the Grenå-Helsingborg Fault to the Børglum Fault.