Digital Structural Mapping Methods for Fault and Fracture Analysis: an example from the Canisp Shear Zone.

R.W. Wilson, K.J.W. McCaffrey, R.E. Holdsworth.

Reactivation Research Group, Dept of Geological Sciences, University of Durham, DH1 3LE

robert.wilson@durham.ac.uk

Field based GIS mapping tools are becoming increasingly used for capturing and visualising spatial data in many disciplines. Digital Structural Mapping (DSM) offers a number of benefits to structural geologists such as: fast data capture and analysis, real-time spatial and geometrical analysis, powerful mapping properties, 3D visualization, automated analysis of structural attributes, and the ability to view all scales of data in one interface.

The current NERC LINK-funded project at Durham seeks to quantify fault zone evolution and basement reactivation in passive margins. Reactivated basement faults are well exposed in the NE Atlantic margin. Previous onshore studies by the Reactivation Research Group suggest that there are quantifiable, scale-independent differences between fracture attributes in highly reactivated fault zones compared to little reactivated structures. In order to detect and quantify reactivation in basement faults, a digital database of fault attributes is being generated for both highly reactivated and little reactivated structures.

In this case study we present the initial results of DSM of the reactivated Canisp Shear Zone, NW Scotland. This Inverian and Laxfordian-aged ductile shear zone is spatially associated with late Laxfordian sinistral strike-slip faults. Quantitative spatial and geometrical analysis of fault patterns clearly show that the late brittle faults are lithologically controlled and reactivate the pre-existing ductile shear fabrics (Beacom et al., 2001).

In order to produce a high-resolution fault attribute database quickly and efficiently, a DSM mapping system was designed. This uses a variety of DGPS location methods to map faults at the dm- to cm-scale by collecting spatial co-ordinates on a palm-top computer whilst traversing along or across the exposed fault systems. Fault attributes observed at individual localities were collected and stored, including orientation and kinematic data, linkage features displayed, fault-rock type and overprinting relationships, in the form of ESRI shapefiles (point, line and polygon vector data). Major structures were mapped using air-photo analysis and field DGPS spatial correction. Detailed digital outcrop maps were captured using a variety of methods: i.e. digitized cairn mapping, line traverses and outcrop analysis. Using mm accuracy survey positioning systems combined with digital photographs, cm-scale features can also be mapped.

Data will be presented that illustrate mapped fault patterns at a variety of scales (from cm to km - outcrop to satellite data). Using GIS software, detailed geostatistical analysis of the data can be carried out quickly and easily. The geospatial properties of the fault population data, such as fault orientations, fault clustering and fault connectivity, are represented in a variety of ways. Individual fault properties (geometry, kinematics, etc), contained in the attribute files, are also analysed.