3-D Structural Architecture and Reactivation of Faults(?) in the NE England Coalfield

P. Clegg1, R. E. Holdsworth1, K. J. W McCaffrey1 & B. Young2.

1Reactivation Research Group, Department of Geological Sciences, University of Durham. Durham DH1 3LE

2British Geological Survey, Edinburgh

p.clegg@durham.ac.uk

The periodic aseismic reactivation of geological faults and fractures is a well-known phenomenon within the coalfields of Britain. The development of mm- to m-scale surface fault scarps or fissures has caused severe structural damage to agricultural land, buildings, roads and sub-surface utilities such as pipelines. Recent localised studies undertaken in NE England by the BGS suggest that fissuring processes and associated ground movements (surface collapse, formation of crown holes, structural damage to man-made structures) are particularly prevalent in the region underlain by the Magnesian Limestone. They have demonstrated that in the Houghton-le-Spring area of County Durham, there is a close association between occurrence of surface collapse and structural damage and the position of old faults both in the Magnesian Limestone and the underlying Carboniferous rocks of the Coal Measures.

The current project aims to expand on the initial findings of the BGS by characterising the structure of the Magnesian Limestone and associated units in the concealed coalfield area of NE England. Structural data associated with Faults, fissures and associated ground deformation will be acquired using Digital Structural Mapping (DSM) via hand held computers linked to GPS, DGPS and Laser RangerTM location methods that provide dm- to cm-scale levels of accuracy (see also McCaffrey et al. and Wilson et al. this meeting). These data will be incorporated into a GIS database along with data from mine abandonment plans and known fissuring and ground deformation events. The digital format of the mapping survey permits rapid and highly accurate data collection, spatial analysis and 3-D visualization. The methodologies developed during this project can potentially be applied to all future studies of geohazards in both urban and rural settings. In addition, the data can be incorporated directly into the growing multi-dimensional database of onshore and offshore geology presently being constructed as part of the BGS-based Digital Geoscience Spatial Model (DGSM).

Preliminary field studies have identified three main fault sets: N-S, NW-SE & E-W. A number of the E-W and N-S faults contain oblique slickenlines which suggest dextral oblique movement senses. NE-SW and some E-W faults appear to have normal dip-slip movements. Locally E-W faults are post-dated by N-S faults, but it remains distinctly likely that these faults have experienced more that one phase of movement. There is some evidence that the fault patterns may be quadrimodal indicating 3-D strains. Several minor monoclinal folds, with E-W-trending fold axes are closely associated with larger E-W faults and may be associated with inversion against the E-W faults, possibly due to partitioning of wrenching deformation during regional transtension (see also Holdsworth et al. and De Paola et al. this meeting).