Abstract
Abstract. Fault architecture and fracture network evolution (and resulting bulk hydraulic properties) are highly dependent on the mechanical properties of the rocks at the time the structures developed. This paper investigates the role of mechanical layering and pre-existing structures on the evolution of strike–slip faults and fracture networks. Detailed mapping of exceptionally well exposed fluvial–deltaic lithologies at Spireslack Surface Coal Mine, Scotland, reveals two phases of faulting with an initial sinistral and later dextral sense of shear with ongoing pre-faulting, syn-faulting, and post-faulting joint sets. We find fault zone internal structure depends on whether the fault is self-juxtaposing or cuts multiple lithologies, the presence of shale layers that promote bed-rotation and fault-core lens formation, and the orientation of joints and coal cleats at the time of faulting. During ongoing deformation, cementation of fractures is concentrated where the fracture network is most connected. This leads to the counter-intuitive result that the highest-fracture-density part of the network often has the lowest open fracture connectivity. To evaluate the final bulk hydraulic properties of a deformed rock mass, it is crucial to appreciate the relative timing of deformation events, concurrent or subsequent cementation, and the interlinked effects on overall network connectivity.
Highlights
Differences in the mechanical properties of rock layers have long been recognised as influencing the style and evolution of faults (Anderson, 1951; Donath, 1961; Ranalli and Yin, 1990; Ferrill et al, 2017)
We investigate how the internal structure of strike–slip faults at Spireslack Surface Coal Mine depends on the lithology, presence of pre-existing weaknesses, and synchronous cementation
The exceptional exposures of the Limestone Coal Formation at Spireslack Surface Coal Mine (SCM) provides an excellent opportunity to examine the role of lithology and pre-existing structures on fault evolution, internal structure, and connectivity
Summary
Differences in the mechanical properties (mechanical stratigraphy) of rock layers have long been recognised as influencing the style and evolution of faults (Anderson, 1951; Donath, 1961; Ranalli and Yin, 1990; Ferrill et al, 2017). The lithology being cut by the fault influences fault dip, e.g. strands in competent layers have steeper dips than those in incompetent layers (Ferrill and Morris, 2008), with important consequences for vein geometry and mineralisation potential (Dunham, 1948). The ratio of competent to incompetent lithologies affects fault style and displacement profiles (Ferrill et al, 2017; Ferrill and Morris, 2008). Fault-related folding of thin competent layers (e.g. limestones) is common in successions otherwise dominated by incompetent lithologies (e.g. shale) (Ferrill and Morris, 2008; Lapadat et al, 2017). The presence of incompetent lithologies restricts fault growth with strands terminating at incompetent beds and leads to formation of faults with high length to height ratios orientated parallel to the strike of bedding (e.g. Nicol et al, 1996; Soliva and Benedicto, 2005; Roche et al, 2013)
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