UNDERSTANDING THE STRUCTURAL ATTRIBUTES AND UNDERLYING CONTROLS OF CONTINENTAL MARGIN ARCHITECTURE: CROSSING THE ONSHORE-OFFSHORE DIVIDE

In common with many rifted margins, several offshore basin-bounding faults in the UK and Norwegian sectors of the NW European continental shelf can be traced onshore where their geometry, spatial characteristics, kinematic history and textural evolution can be directly studied in exposed basement rocks. Our work in the NE Atlantic region demonstrates that a number of major, crustal-scale faults have been reactivated, some during Mesozoic rifting episodes. Fault rock assemblages associated with these structures consistently preserve evidence for the action of tectonic and metamorphic processes leading to long-term weakening following syn-tectonic fluid influx. It is also clear that pre-existing heterogeneities in basement rocks can markedly influence the spatial attributes and geometric evolution of fault systems. However, basement control of later, basin-related faulting is not always as widespread as is often assumed. Many faults are discordant with exposed basement structures and, in some areas (e.g. NW Britain) the distribution of basement outcrop is largely controlled by vertical displacements and erosion related to Mesozoic faults.

The recognition of spatial attributes that may be used to quantify the degree of basement involvement and reactivation presents an exciting new tool for use in exploration of offshore and onshore regions in both the petroleum and mining industries. Onshore, it is now possible to compile 3-D digital images of onshore fault zones to an accuracy of centimetres using GPS and Laser Ranger™ equipment. If necessary, these can then be compared, using state-of-the-art GIS-based visualization software, to images of equivalent offshore fault zones derived from interpretation of 2-D and 3-D seismic reflection profiles. Scale-independent analytical software tools can be used to recognize spatial attributes that are diagnostic of high degrees of reactivation. These techniques can be potentially applied to ancient fault zones in almost any tectonic setting. They can be combined with fluid flow data, measurements of in-situ stress and fault rock mechanical properties to investigate neotectonic fault systems, seismic hazard and active crustal kinematics and dynamics.