Damage Zone Evolution in Coal Measures Strata From the Northumberland Basin, NE England
We develop kinematic models to describe damage zone evolution during fault growth and test their predictions against measurements of damage zone attributes within siliciclastic sand/shale sequences from the Carboniferous Northumberland Basin, NE England. These data, obtained from faults with throws spanning 0.1-20 m, were measured from detailed (cm-resolution) digital outcrop models captured using terrestrial laser scanning techniques. Study locations include coastal sections and areas of active open-cast mining that provide good 3D exposure of faults during progressive coal extraction.
The damage zones comprise: fault splays and oversteps; drag folds; rotated fault-bound blocks; sub-parallel fracture sets and ductile shear zones; cataclasite lenses; and intensely deformed scaly gouge. We propose two kinematic models to explain the observed structural relationships. Firstly, cataclasite lenses develop from fault-bounded blocks in contractional oversteps. In this scenario, damage zone width remains approximately constant as throw increases and is defined by the initial fault separation. The second model describes the space incompatibility that develops between discrete fault planes in coherent sandstone layers and adjacent shales where slip is distributed along ductile shear zones. In this case, damage zone width may increase with increasing fault throw. Alternatively, the width of the damage zone may be controlled by thickness of the rheologically weaker shale. These geologically-based models highlight the importance of bed thickness and host-rock rheology - in addition to fault throw - in controlling damage zone evolution and provide a basis for predicting the likely sizes of different damage zone elements associated with seismically-imaged faults in the subsurface.