Structural segmentation and oblique extension on passive margins – the effects of pre-existing basement structures on reservoir potential

Most passive margins are segmented along strike, giving rise to discrete zones characterized by constancy in structural style, offset along segments at high angles to the margin. The boundaries between the continental margin and rift segments are generally believed to exhibit a variety of structural styles ranging from transfer faults and accommodation zones to transform faults. One possibility is that such segmentation reflects along-strike changes in the orientation of pre-existing structures in the underlying continental basement. A corollary of this model is that if such pre-existing structures undergo reactivation then they will often be significantly oblique to the direction of regional extension. Our studies show that the complexity of rifted margins can be linked to changes in the obliquity of pre-existing structures relative to the regional extension vector. Diagnostic structures recognized in such zones include: reactivation of basement fabric; non-Andersonian, polymodal fault patterns; partitioned domains of wrench- and extension-dominated transtensional deformation; complex arrays of sigmoidal and helicoidal faults; local strike-slip inversion and transfer/ accommodation zones segmenting rift basins.
The Lofoten-Vesterlen archipelago (LVA), of NW Norway, is a segmented basement high showing distinct lateral variations in trend, deformational style, and structural complexity, and is an ideal area to asses the effects of basement influenced oblique extension. We present an integrated onshoreoffshore study involving regional to outcrop scale fault analysis for the Lofoten Ridge. Regional analysis of fault lineament patterns using Geographic Information Systems (GIS) reveals that the LVA can be sub-divided into a series of distinct lineament domains. These domains are closely coincident with changes in ridge trend, and variations in structure within offshore models derived from seismic reflection studies. Digital field mapping and spatial analysis of faulting in the North Lofoten, which is highly oblique to the regional structural trend, reveal that faulting is dominated by transtensional dip-slip and oblique-slip movements associated with NWSE extension. The overall change in fault orientation, fault geometry and deformation style are consistent with models for transtension where the ridge-bounding structure becomes increasingly oblique to regional extension. Pre-existing structures that may control ridge trend in North Lofoten include Devonian detachments and Precambrian granitic plutons.

Using the LVA as an analogue, this study also explores whether segmentation can be linked more generally to differences in the obliquity of pre-existing structures relative to plate motion vectors on continental margins. Particular attention is paid to exploration areas where an investigation of complex fault system is essential to improve understanding of structural controls on reservoir architectures, trap geometries and fluid flow within reservoir intervals.