Devonian basins of Northern Scotland: extension and inversion related to Late Caledonian — Variscan tectonics

Abstract

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.

Devonian extensional tectonics versus Carboniferous inversion in the northern Orcadian basin

“…Orkney and Caithness Enfield & Coward (1987) and Enfield (1988) have presented evidence for Middle Devonian extensional tectonics in the offshore West Orkney Basin (Fig. 1).…”

“…These slickensided faults do not allow the reconstruction of the regional palaeostress field as they respond to local conditions. Consequently, the Middle Devonian regional extension direction in Caithness and Orkney is best approximated by the geometry of the normal faults and associated lateral ramps in the West Orkney Basin: a NW-SE direction of extension is suggested by the 'bow and arrow' rule (Enfield & Coward 1987;Enfield 1988) (Fig. 1).…”

“…Extensional tectonics in the West Orkney Basin is accommodated mainly by normal faults and subordinate transfer faults, that correspond to WNW-ESE extension (Enfield & Coward 1987). In Shetland, trancurrent tectonics becomes dominant over extension.…”

“…Later inversion of these Scottish basins resulted in the formation of folds within the Old Red Sandstone and reactivation of basement faults (e.g. Coward et al 1989). A consensus relates the Late Palaeozoic tectonic evolution of that area to the kinematics of the Great Glen fault which is a major tectonic boundary (Flinn 1977).…”

“…A consensus relates the Late Palaeozoic tectonic evolution of that area to the kinematics of the Great Glen fault which is a major tectonic boundary (Flinn 1977). Attempts to decipher directions of extension and subsequent compression have been based on fold geometry (Mykura 1976), orientation of faults and associated lateral ramps (Coward & Enfield 1987), or by the kinematics of the Great Glen fault (Norton & Way 1991). However, the history of movement and amount of offset of this trancurrent fault remain controversial (see review in Rogers et al 1989).…”