Cenozoic Inversion of a Regional Scale Mesozoic Normal Fault: Structural Analysis of the Dowsing Fault, Southern North Sea United Kingdom

Abstract

An effective decoupling of pre- and post-salt sequences has hindered understanding of the initiation, reactivation and nature of the Dowsing Fault: a regional scale fault system in the Southern North Sea basin. Seismic reflection data indicates that the NNW-SSE trending Dowsing Fault accommodates minor (<50 m) present-day vertical offsets within sub-salt Permian and Carboniferous sequences. In contrast, a series of supra-salt grabens, the Dowsing Graben System (DOGS), located directly above the Dowsing Fault are bounded by normal faults which accommodated large displacements (>300 m) during the Mesozoic. In this paper, established kinematic forward modelling techniques are used to constrain the development of the Dowsing Fault and an adjacent ca. 5 km wide sub-salt anticline. The results support a model where the Dowsing Fault accommodated ca. 1 km of Jurassic normal movement with displacement soft-linking through the Permian salt to the coeval DOGS structures. Subsequent Cenozoic shortening, attributed to far-field Alpine collision, resulted in ca. 1km of reverse movement on the Dowsing Fault thereby reducing net displacement to the metre-scale offsets observed at present day. The magnitude of shortening accommodated on the sub-salt Dowsing Fault is comparable to the amplitude of salt-cored anticlines within the overlying supra-salt units. Moreover, kilometre scale displacements are consistent with expected displacement/length relationships (ca. 1:50) derived from established fault scaling data.

Significantly, this work rejects the notion that the Dowsing Fault was chiefly active as a strike-slip structure during the Cenozoic. Whilst this sense of slip is not in conflict with displacement/length relationships, here it is demonstrated that the principal horizontal shortening direction (SHmax) is likely to have been oriented NE-SW during the Cenozoic: an orientation that would have been mechanically unfavourable for major strike-slip movement on the Dowsing Fault.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018