The architecture, growth and tectono-stratigraphic significance of rift-oblique lineaments on the NE Atlantic Margin

Abstract

Fault domain boundaries are characteristic features of segmented rift systems and have been recreated in analogue models. Two end member conceptual models of fault domain boundaries currently exist. 1) Accommodation zones, which are broad regions of overlapping normal faults and which trend oblique to the rift axis. 2) Transfer zones, which are discrete sub-vertical fault systems that directly link en-echelon normal fault domains. These structures are commonly believed to segment natural rift systems on a variety of scales and impact directly upon the stratigraphic and magmatic evolution of a basin.

The NE Atlantic Margin is a volcanic passive margin which has undergone a series of rift events culminating with continental breakup in the Early Cenozoic. From potential field, seismic reflection, seismic refraction and ocean bottom seismometer datasets, a series of rift-oblique lineaments (loosely referred to as ‘transfer zones’) have been identified which are commonly inferred to compartmentalise and laterally offset structural highs and depocentres developed within the Mesozoic – Cenozoic rift basins. A range of hypotheses are proposed to explain the origin of these lineaments, including fault domain boundaries, basin-wide strike-slip faults and other, non-tectonic origins. Using well-calibrated 2D and 3D seismic data, this study critically assesses the structural, stratigraphic and magmatic evidence for the rift-oblique lineaments in the Faroe-Shetland Basin and Vøring Basin, both located upon the NE Atlantic Margin.

Results from the Faroe-Shetland Basin show structures previously attributed to basin-wide strike-slip deformation can be more simply explained as igneous intrusions, hydrothermal vent complexes, gas chimneys and/or faults that transfer extensional strain between en-echelon rift segments (i.e. fault domain boundaries). There is little evidence to suggest that activity along a series of discrete, basin-wide lineaments controlled Paleocene sedimentation in the basin.

In the northern Vøring Basin, a previously identified fault domain boundary (the Rym Accommodation Zone) is analysed to understand if, and how strain is transferred between two adjacent fault domains. The results of this study highlight major differences between the offset rift segments in view of the style of rifting, timing, the loci of faulting, the relative uplift and subsidence histories as well as the impact of variations in the deep crustal structure. Analyses reveal that strain is not fully transferred across the fault domain boundary, with significant variation in beta factors calculated for each rift segment. The structural style within the Rym Accommodation Zone is complex, with the rotation of normal fault orientations, major relay ramp formation and rift perpendicular normal oblique faulting observed, elements that are not present in most existing conceptual models of accommodation zones. The results also imply that transfer zones may be an integral part of a larger accommodation zone rather than an opposite end member as previously believed.

In the final aspect of the study, a second rift-oblique lineament is analysed in the northern Vøring Basin: the Gleipne Lineament. Results highlight the close structural relationship between the Gleipne Lineament and underlying basement structure, with the lineament acting as a conduit for sediment to enter the Vøring Basin during phases of rifting. Under periods of minimal upper crustal deformation, the lineament exerted a lesser control upon basinal sedimentation. The Rym Accommodation Zone in contrast did not source sediment into the Vøring Basin, instead, it compartmentalised the basin during rifting which increased the complexity of the predicted basin fill. Increased Late Paleocene intrusive and extrusive igneous deposits are observed along the strike of both lineaments but are not directly linked to active tectonic deformation.

In conclusion, rift-oblique lineaments are unlikely to be basin-wide features and each appears to be unique in its structural style and geological origin. In turn, this means that different lineaments are likely to have different impacts upon the stratigraphical and magmatic development of a basin. Previous inferences that basin-wide lineaments have controlled sediment entry and transport within rift basins on the NE Atlantic Margin need to be substantiated on a case-by-case basis. The results of this study are further considered and discussed to predict the nature of rift-segmenting structures in the sub-basalt region of the Faroe-Shetland Basin, which is poorly resolved by current 2D and 3D seismic imaging.