Summary The Devonian sedimentary basins of Northern Scotland illustrate the different processes of inversion along reactivated frontal and lateral thrust ramps, and the importance of strike-slip fault movements to late and post-Caledonian tectonics. The basins of the West Orkneys and West Shetland, west of the Great Glen Fault developed by collapse of thickened Caledonian crust during the Devonian. The basins extended in a NW-SE direction on dominantly SE-dipping faults, which curve and become parallel to the earlier Caledonian fabric at depth. The Caledonian structures developed a crustal anisotropy which was subsequently reworked during the later Palaeozoic and Mesozoic by extensional and compressional movements. Hence in the West Orkney and Minches basins, Caledonian structures influenced the kinematics of Early to Middle Old Red Sandstone extension, later (Devonian and Permo-Carboniferous) compression, Mesozoic extension and later Mesozoic-Tertiary compression. During basin extension, minor inversion of early listric normal faults occurred within the basin sediments, driven by gravitational gliding down tilted fault blocks. SE of the Great Glen Fault in eastern Shetland and possibly also the Inner Moray Firth, basins were affected by NE-SW extension during Middle to Upper Devonian times. This is possibly a result of sinistral strike-slip displacements on the Great Glen-Walls Boundary Fault system. This strike-slip episode may have been responsible for inversion of the earlier and more regionally widespread Lower and Middle Devonian basins formed by NW-SE extension. Localized zones of folding in Caithness and the Orkney Islands and wide zones of more intense folding in western Shetland are thought to be related to this deformation. Late Palaeozoic Variscan dextral strike-slip displacements juxtaposed basins along the Great Glen-Walls Boundary fault system which have distinct stratigraphies and structural evolutions and formed separately to the east and west of these faults. This strike-slip deformation inverted basins both to the east and west of the Great Glen Fault. In the East Shetland basins it produced local zones of overturned beds above oblique and frontal ramps. Further subsequent dextral movements in the Mesozoic produced pull-apart basins in the Moray Firth and East Orkneys.
The West Orkney Basin developed in Devonian times, as the western part of the Orcadian intermontane basin. It has been studied using commercial speculative seismic reflection data and the MOIST deep seismic data. The NW edge of the West Orkney Basin is formed by listric faults which are also strongly arcuate in plan, while the SE part is composed of straight domino-type faults which formed parallel to earlier (Caledonian) layering in the basement. Fault restoration and balancing suggest that initial extension in the basin occurred on low-angle reactivated Caledonian thrust faults. Steeper breaching faults cut the low-angle set, forming planar (domino-type) faults in the centre of the basin but listric faults at the NW margin. The maximum extension is about 45% in the basin centre, most of this being taken up on the later breaching fault system. This extension decreases to the SW, where fault tips occur on-shore, but some may transfer to fault systems in the Minches. The faults apparently decouple at a depth of 18-20 km and the extension suggests an initial post-Caledonian crustal thickness of up to 40 km. However, the sedimentary thickness is an average of only 3 km in the basin centre, much less than would be expected had the lithosphere thinned homogeneously, and there is no evidence of a thermal subsidence phase to the basin. This suggests that the extension shown by the West Orkney Basin was transferred to lower lithospheric levels to the east along the deep decoupling zone. The Devonian sediments on-land show facies changes and periods of uplift and erosion which may be related to extension during basin development. They also show a phase of pre-Late-Permian tectonic inversion where the beds are locally folded and thrusted, probably related to the Hercynian events further south. The West Orkney Basin is capped by Mesozoic sediments and was probably reactivated during Mesozoic basin development in the Minches and Moray Firth. The shape of the faults, their orientation and decoupling levels are strongly controlled by the earlier Caledonian structure, in particular by the layering and crustal anisotropy developed along and above the Moine thrust. The West Orkney Basin with its 20 km deep decoupling level formed by extension of Caledonian thickened crust. It is notable that the major basin-bounding faults to the NW, the Outer Isles and Flannan faults, which developed where the crust was thinner and hence less ductile at depth, decoupled at much deeper structural levels, at the Moho or below.
Unconventional hydrocarbon resources found across the world are driving a renewed interest in mudrock hydraulic fracturing methods. However, given the difficulty in safely measuring the various controlling factors in a natural environment, considerable challenges remain in understanding the fracture process. To investigate, we report a new laboratory study that simulates hydraulic fracturing using a conventional triaxial apparatus. We show that fracture orientation is primarily controlled by external stress conditions and the inherent rock anisotropy and fabric are critical in governing fracture initiation, propagation, and geometry. We use anisotropic Nash Point Shale (NPS) from the early Jurassic with high elastic P wave anisotropy (56%) and mechanical tensile anisotropy (60%), and highly anisotropic (cemented) Crab Orchard Sandstone with P wave/tensile anisotropies of 12% and 14%, respectively. Initiation of tensile fracture requires 36 MPa for NPS at 1‐km simulated depth and 32 MPa for Crab Orchard Sandstone, in both cases with cross‐bedding favorable orientated. When unfavorably orientated, this increases to 58 MPa for NPS at 800‐m simulated depth, far higher as fractures must now traverse cross‐bedding. We record a swarm of acoustic emission activity, which exhibits spectral power peaks at 600 and 100 kHz suggesting primary fracture and fluid‐rock resonance, respectively. The onset of the acoustic emission data precedes the dynamic instability of the fracture by 0.02 s, which scales to ~20 s for ~100‐m size fractures. We conclude that a monitoring system could become not only a forecasting tool but also a means to control the fracking process to prevent avoidable seismic events.
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