Devonian extensional tectonics versus Carboniferous inversion in the northern Orcadian basin

Abstract

The Old Red Sandstone (Middle Devonian) Orcadian basin was formed as a consequence of extensional collapse of the Caledonian orogen. Onshore study of these collapse-basins in Orkney and Shetland provides directions of extension during basin development. The origin of folding of Old Red Sandstone sediments, that has generally been related to a Carboniferous inversion phase, is discussed: syndepositional deformation supports a Devonian age and consequently some of the folds are related to basin formation. Large-scale folding of Devonian strata results from extensional and left-lateral transcurrent faulting of the underlying basement. Spatial variation of extension direction and distribution of extensional and transcurrent tectonics fit with a model of regional releasing overstep within a left-lateral megashear in NW Europe during late-Caledonian extensional collapse. Later inversion (probably during the Upper Carboniferous) is characterized by E–W to NE–SW contraction. It induced reactivation of extensional faults as thrusts, development of small-scale folds and thrusts, and right lateral transcurrent movement of the major faults such as the Great Glen and Walls Boundary faults.

Kinematics and dynamics of Old Red Sandstone basins

“…3; Andersen et al 1991;Dewey et al 1993;Milnes et al 1997). A system of syn-extensional east-west upright folds affects the basal detachment and the ORS sedimentary basins (Serrane & Seguret 1987;Chauvet & Seranne 1994). The commencement of ORS sedimentation is not precisely constrained but is generally placed at the boundary between the Early and Mid-Devonian time .…”

Evidence from Rb–Sr mineral ages for multiple orogenic events in the Caledonides of Shetland, Scotland

“…Scandian orogenesis in the Northern Highland Terrane was complete by c. 430 Ma (Goodenough et al 2011); metamorphic grade was no higher than mid-amphibolite facies and no examples of syn-or late-convergent extensional shear zones have been identified, all features consistent with its location on the periphery of the main, longer-lived continental collision to the north. The Scandian collision was followed at c. 400 Ma by sinistrally oblique plate divergence associated with displacements along extensional and strike-slip shear zones and faults, exhumation of the ultrahighpressure rocks of SW Norway, and development of late-to postorogenic 'Old Red Sandstone' sedimentary basins in East Greenland, Svalbard, Scotland and Norway (Seranne 1992;Krabbendam & Dewey 1998;Dewey & Strachan 2003;Fossen 2010).…”

The Dalradian rocks of Scotland: an introduction

“…Syndepositional deformation of Old Red Sandstone sedimentary rocks in the Orcadian Basin has been proposed by Seranne (1992), and shear fabrics in the Upper Nappe of the Shetland Ophiolite-complex and its underlying imbricate zone might also be related to this extensional phase and hence represent the youngest deformation to affect Caledonian metamorphic rocks (Cannat, 1989).…”