The structural and stratigraphic evolution of the North Sea rift system is described, from the onset of pre-rift thermal uplift in the Aalenian to the failure of the rift system caused by the transfer of extension out onto the Atlantic margin during the Neocomian.Our sequences are interpreted from a combination of lithostratigraphic, biostratigraphic and seismic stratigraphic correlation. They represent discrete phases of basin infill caused by unique combinations of subsidence and sedimentation rates. Our sequence boundaries are regional unconformities or hiatuses which develop in response to changes in relative sea-level. We recognize three processes which control relative sea-level and, therefore, the nature of the sequence boundaries; they are changes in tectonic subsidence rates, sedimentation rates and eustatic sea-level.Six sequences are described and labelled J20 to J70. Each marks a regionally correctable step in the development of the rift system. We believe the sequences are mainly controlled by changes in the rates of tectonic subsidence and sedimentation. The effects of eustatic sea-level variations are recognized, but are not considered the prime control on sediment distribution within the rift at the scale of the sequences described.Sequences J20 to J70 highlight the struggle between sedimentation rates and tectonic subsidence during the development of the rift system. The overall pattern is one of drowning of the sediment supply and starvation of the basin as faulting propagates. This trend is often interrupted by periods in which sedimentation was able to keep pace with tectonic subsidence and in local examples substantially exceed it.The first rift-related sequence (J20) records deposition during pre-rift thermal uplift over much of NW Europe. During J20 a low-angle, weakly erosive, pre-rift unconformity (the ‘Mid-Cimmerian’ unconformity) developed over much of the North Sea Basin. The domal uplift was centred on the Central Graben volcanic province in the Central North Sea.The second main phase in the development of the rift was the syn-rift period where faulting actively controlled sedimentation. J30 through J60 sequences record the diachronous onset of faulting and breakup of the pre-rift thermal dome, starting at the extremities of a three-armed rift system and propagating in towards the core of the pre-rift thermal dome in the Central Graben. Diachronous faulting is noted by dating the unconformities in fault footwalls and in the onset of fault-controlled deposition. For example, major faulting was initiated during J30 in the Brent Province in the Northern North Sea, but not until J60 in the Central Graben of the Central North Sea. Fault-controlled unconformities can be correlated across sub-provinces but no single basin-wide extension-related unconformity (i.e. no ‘Late Cimmerian’ unconformity) is identified.The third phase of the rift development records the failure of the North Sea rift and the transfer of extension out onto the Atlantic margin. This commences with the J70 sequence which in the North Sea records the onset of thermal subsidence without associated faulting. J70 and younger fault-controlled unconformities and associated depositional systems are recorded only in the Magnus Province bordering the active Atlantic margin. In the North Sea the strong seismic boundary, often known as the ‘Base Cretaceous Unconformity’, is shown to be neither an unconformity nor to coincide with the Jurassic/Cretaceous boundary.
Summary The inversion of extensional fault systems results in the reversal of slip on the faults and expulsion of the synrift fill. During inversion the beds in the cover sequence shorten before the net extension at the basement level has been cancelled. Shortening of the sedimentary cover generates folding and backthrusting in the still downthrown hanging wall block. Intracratonic inverted basins in different parts of the Alpine Foreland show similar structural geometries with the major extensional faults which controlled basin development reactivated during subsequent compression. We use examples from the Western Approaches and offshore Holland (Broad Fourteens Basin) to illustrate the structural styles developed during inversion. The fundamental control on compressional structural geometry exerted by pre-existing extensional structures is also visible in more complexly deformed orogenic belts, like the Western Alps and the Pyrenees. In these areas inversion also occurs, but more commonly extensional faults which may not have inverted act as an indirect control on the location of ramps, and/or thrust orientation. Seismic data are normally required to establish these effects with certainty. However, as the body of knowledge builds up, it is possible to recognize certain geometrical characteristics which suggest the control of extensional faults in thrust belts. These include footwall shortcuts, out of sequence structures and arcuate thrust-fold traces.
Summary The western Tauride Mountains of S.W. Turkey comprise a central relatively autochthonous carbonate platform unit, the Tauride autochthon bordered by two allochthonous units, the Lycian Nappes to the west and the Antalya Complex to the east. Sequences of Miocene clastic sediments up to 1000 m thick that were derived from both the allochthons document the timing and direction of their emplacement onto the carbonate platform. Along the western margin of the Miocene basin intial emplacement of the Lycian Nappes, from the northwest, in the Lower Miocene, was coupled with rapid subsidence of the previously stable carbonate platform. The resulting basin was ca 170 km across. Fan-deltas were derived from the leading edge of the nappes and passed basinwards into a series of small submarine fans. At the same time rapid uplift of the central parts of the carbonate-platform, along the margin of the basin opposite the nappe pile, led to a thick wedge of carbonate-derived clastics being shed northwestwards into the basin. Varying sedimentation rates and migrating facies belts in the overlying Middle and Upper Miocene basin-fill document the progressive emplacement of the Lycian Nappes ca 100 km from the northwest over the basin margin. Along the eastern margin of the basin, palaeocurrent analysis and downslope facies-transitions in the Miocene sediments show that the Antalya Complex was emplaced from the east but only advanced a short distance beyond the eastern margin of the basin. The Lycian Nappes and the Antalya Complex approached the basin from opposite directions and so their respective ophiolite units must have originated in separate ocean basins on either side of the Bey Daḡlari and the Susuz Daḡ carbonate platform.
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