The growth of faults and fracture networks in a mechanically evolving, mechanically stratified rock mass a case study from Spireslack Surface Coal Mine, Scotland

摘要

Fault architecture and fracture network evolution (and resulting bulkhydraulic properties) are highly dependent on the mechanical properties ofthe rocks at the time the structures developed. This paper investigates therole of mechanical layering and pre-existing structures on the evolution ofstrike–slip faults and fracture networks. Detailed mapping of exceptionallywell exposed fluvial–deltaic lithologies at Spireslack Surface Coal Mine,Scotland, reveals two phases of faulting with an initial sinistral andlater dextral sense of shear with ongoing pre-faulting, syn-faulting, and post-faultingjoint sets. We find fault zone internal structure depends on whether thefault is self-juxtaposing or cuts multiple lithologies, the presence ofshale layers that promote bed-rotation and fault-core lens formation, andthe orientation of joints and coal cleats at the time of faulting. Duringongoing deformation, cementation of fractures is concentrated where thefracture network is most connected. This leads to the counter-intuitiveresult that the highest-fracture-density part of the network often has thelowest open fracture connectivity. To evaluate the final bulk hydraulicproperties of a deformed rock mass, it is crucial to appreciate the relativetiming of deformation events, concurrent or subsequent cementation, and theinterlinked effects on overall network connectivity.