A Three Dimensional Analysis of the Geometry and Kinematics of a Transfer Zone

Transfer zones have been widely inferred to explain segmentation of intra-continental rift systems. Defined as discrete structures of oblique and strike-slip faulting, they facilitate the transfer of strain between offset rift systems and are oriented approximately parallel to the extension direction. Previous studies of transfer zones have primarily focussed upon onshore (outcrop) examples in areas of good exposure, such as the Basin and Range Province, USA. However, the three dimensional characteristics of the transfer zone are often difficult to infer from these field studies. This study uses interpretations of commercial 3D seismic data to assess the recognition, geometry and kinematics of transfer zones, and to compare these with previous models. The study focuses on the Voring Basin, offshore Norway - a Cretaceous basin on the NE Atlantic volcanic margin which formed after early Eocene continental break-up. The precursory rifts along the margin have a mainly NE-SW orientation, but regional potential field anomalies highlight a series of NW-SE trending lineaments, commonly referred to as transfer zones, along the length of the margin. Previous authors have inferred that some transfer zones may have originated due to reactivation of basement structures, but the three dimensional structure of these lineaments is unclear. An improved understanding of the nature and tectonic significance of these transfer zones is important, given their likely role in controlling the structural segmentation of basinal depocentres at a variety of scales. The principle aim of this study is to constrain the 3D architecture of a transfer zone and its kinematics during late Cretaceous rifting. The NW Voring Basin is characterised by two NE-SW trending structural highs, the Gjallar Ridge and Nyk High, which formed during Late Cretaceous-Paleocene rifting. These structural highs display a right step across the NW-SE trending "Rym Accommodation Zone". Previous analyses of 2D seismic data suggested that the Rym Accommodation Zone accommodated sinistral strike-slip movements during Late Maastrichtian- Paleocene rifting. New interpretations of 3D seismic data from the region have not, however, found conclusive evidence for major strike-slip activity. Rather, the Rym Accommodation Zone has characteristics that are more consistent with it being a transfer zone, with oblique-slip movements along its strike being controlled by the late Cretaceous rift movements within the basin. This Rym Transfer Zone exerted a primary control upon sedimentation during rifting and also controlled the extent of lava flows and sedimentation patterns during periods of post-rift uplift in the latest Paleocene. A further key observation is the contrast in structural styles observed across the Rym Transfer Zone. The Gjallar Ridge is characterised by half grabens formed against a series of low-angle normal faults, whilst the Nyk High comprises grabens, bounded by more steeply dipping, opposed-polarity normal faults. The structural model derived from the 3D seismic surveys, combined with regional structural interpretations, will be used to estimate the amount and distribution of crustal extension across the Nyk High and Gjallar Ridge, i.e. to either side of the Rym Transfer Zone. These data will be used to assess the impact of the differences in structural style upon the geometry and kinematics of the transfer zone and, in turn, to discuss their implications for the crustal structure in the NW Voring Basin.