Quantifying fault zone thicknesses and fault rock distribution in Coal Measures sequences using Terrestrial Laser Scan data
Fault zone thickness and permeability are key parameters used to calculate fault transmissibility multipliers in sub-surface fluid flow models. Previous studies of faults in different lithological sequences suggest that fault zone thickness scales with throw, with 3-4 orders of magnitude thickness variation for any given throw. In this study, we use Terrestrial Laser Scanning (TLS) techniques to capture the detailed cm- to m-scale thicknesses and fault rock distributions of post-depositional faults from the Carboniferous Northumberland Basin, NE England. The faults have throws spanning 0.1-20 m and appear to have developed contemporaneously at broadly the same stratigraphic level within an interbedded sandstone/shale sequence exposed in an active opencast coal mine. TLS has been of critical importance in enabling us to collect quantitative data from large, unstable working faces (<100 m high) within the mine. The faults comprise complex arrays of structural elements including: splays and oversteps; drag folds; rotated fault-bound blocks; sub-parallel fracture sets and ductile shear zones; cataclastic sandstone lenses; and intensely deformed gouge zones. Observations suggest that fault zone thickness scales with throw, although there are significant dip- and strike-parallel variations in the thicknesses of individual fault traces. Regions of greatest fault zone width appear to be associated with or to have been derived from splays and oversteps. We propose that the observed scaling relationships can be explained by elastic interactions between fault segments that have increasingly wide separations as displacement on the fault zone accumulates. An observation that cataclastic sandstone lenses, or sand smears, occur along many fault traces may locally provide connectivity between sandstone beds in the footwall and hangingwall of the fault. Analysis of field and TLS data suggests that sandstone beds within fault overlaps have a fracture spacing that appears to be related to the original thicknesses of the host beds. Block rotations within the overlap zones are accommodated by progressive fracturing and cataclastic flow, which may ultimately lead to the development of sand smears and the possibility of fluid flow across otherwise sealing structures.