Impact of normal faulting and pre-rift salt tectonics on the structural style of salt-influenced rifts: the Late Jurassic Norwegian Central Graben, North Sea

摘要

Studies of salt-influenced rift basins have focused on individual or basin-scale fault system and/or salt-related structure. In contrast, the large-scale rift structure, namely rift segments and rift accommodation zones and the role of pre-rift tectonics in controlling structural style and syn-rift basin evolution have received less attention. The Norwegian Central Graben, comprises a complex network of sub-salt normal faults and pre-rift salt-related structures that together influenced the structural style and evolution of the Late Jurassic rift. Beneath the halite-rich, Permian Zechstein Supergroup, the rift can be divided into two major rift segments, each comprising rift margin and rift axis domains, separated by a rift-wide accommodation zone - the Steinbit Accommodation Zone. Sub-salt normal faults in the rift segments are generally larger, in terms of fault throw, length and spacing, than those in the accommodation zone. The pre-rift structure varies laterally from sheet-like units, with limited salt tectonics, through domains characterised by isolated salt diapirs, to a network of elongate salt walls with intervening minibasins. Analysis of the interactions between the sub-salt normal fault network and the pre-rift salt-related structures reveals six types of syn-rift depocentres. Increasing the throw and spacing of sub-salt normal faults from rift segment to rift accommodation zone generally leads to simpler half-graben geometries and an increase in the size and thickness of syn-rift depocentres. In contrast, more complex pre-rift salt tectonics increases the mechanical heterogeneity of the pre-rift, leading to increased complexity of structural style. Along the rift margin, syn-rift depocentres occur as interpods above salt walls and are generally unrelated to the relatively minor sub-salt normal faults in this structural domain. Along the rift axis, deformation associated with large sub-salt normal faults created coupled and decoupled supra-salt faults. Tilting of the hanging wall associated with growth of the large normal faults along the rift axis also promoted a thin-skinned, gravity-driven deformation leading to a range of extensional and compressional structures affecting the syn-rift interval. The Steinbit Accommodation Zone contains rift-related structural styles that encompass elements seen along both the rift margin and axis. The wide variability in structural style and evolution of syn-rift depocentres recognised in this study has implications for the geomorphological evolution of rifts, sediment routing systems and stratigraphic evolution in rifts that contain pre-rift salt units.