Digital fault mapping and spatial attribute analysis of basement-influenced oblique extension in passive margin settings
Oblique extension and passive margin segmentation may be attributed to the influence of basement structures. Pre-existing fabrics exert a strong control on the overall rift geometry in extensional settings, and can lead to the development of complex fault patterns, obliquely extending segments, deformation partitioning and transfer zones. In offshore settings, the nature of basement structure cannot easily be determined from seismic data, and onshore studies are increasingly used to assess basement controls. A digital mapping methodology GAVA (Geospatial Acquisition Visualisation and Analysis) has been developed to integrate regional- to outcrop-scale data. Digital field mapping methods using DGPS, Laser-Rangefinder and field-GIS are used to map faults to a dm- to mscale accuracy by collecting spatial co-ordinates on a handheld computer whilst traversing along or across the exposed fault systems. Benefits of digital mapping include: more rapid data collection and analysis; all data geospatially located and stored in a digital database; GIS-based analysis and visualisation techniques; digital data format enables direct comparison with fault arrays interpreted from seismic data. The GAVA workflow has been used to investigate three case studies on the North Atlantic Passive Margin: 1) NW Scotland; 2) Lofoten, NW Norway; and 3) Davis Strait, West Greenland. Each case study combines regional onshore and offshore mapping, using remote sensing and seismic interpretation, with detailed outcrop mapping of onshore fault exposures. Fault attributes (e.g. fault orientation, kinematics, fault linkage, fault-rock, overprinting relationships) observed at individual localities were collected in a GIS database. Kinematic fault analysis was carried out using strain inversion techniques at various scales. Spatial analysis was carried out using ArcGIS to identify relationships between various structures, while 3-D models were constructed in order to visualise these relationships over several orders of magnitude. Results show that the complexity of rifted margins may be linked to changes in the obliquity of pre-existing structures relative to the regional extension vector. However, direct reactivation of structures need not always occur. The influence of pre-existing structures may also lead to localized variations in stress/strain orientations, which if analyzed in isolation can indicate extension non-parallel to regional stresses. Therefore, spatial analysis and studies across a range of scales is essential when analyzing such zones. Digital (GIS) mapping methods are an ideal may to carry out such studies, although further development of analysis and visualisation tools for geosciences is required in the field of GIS.