EXAMINATION OF VORTICITY AT DIFFERENT CRUSTAL LEVELS: A CASE STUDY FROM THE MOINE THRUST ZONE, NW SCOTTISH CALEDONIDES

Traditionally, the Moine thrust is interpreted to mark the Caledonian hinterland-to-foreland transition zone at the base of the Scandian (~430 Ma) orogenic wedge. More recently, it has been recognized that the Moine thrust zone (MTZ) may be more appropriately subdivided into a structurally lower section (lower MTZ) dominated by brittle faulting and folding and a structurally higher section (upper MTZ) dominated by pervasive deformation and mylonitization. In the upper MTZ, significant variation in structural character and style of displacement/deformation have been recognized from north to south, and we propose that this may be due to changes in the exposure level of this crustal scale fault/shear zone. In the Loch Eriboll region to the north, the upper MTZ is composed in ascending order of three ductile thrust sheets (Arnaboll, UA-CNF, Creagan) in the footwall to the Moine thrust beneath the Moine nappe. In the Assynt region, a discontinuity interpreted to represent the Moine thrust places pervasively deformed Moine nappe rocks directly above lower MTZ rocks. Isolated lenses of penetratively deformed rocks in the footwall to the Moine thrust in Assynt are interpreted to be “orphans” of the UA-CNF thrust sheet. In this study, we applied rigid grain vorticity analysis techniques to 127 upper MTZ and Moine nappe samples collected along strike from northern Loch Eriboll to southern Assynt in order to characterize what, if any variation exists due to deformation occurring at different crustal levels. This also allowed us to examine if along strike variation in flow could be related to changes in temperature (and hence rheology), lithology, or underlying footwall structural architecture. Mean kinematic vorticity (Wm) estimates of samples ranged from 0.47-0.83 (37-68% pure shear), with averaged minimum and maximum Wm values of 0.62-0.69. Averaged minimum and maximum Wm values in the UA-CNF (0.63-0.70), Creagan (0.61-0.69), and Moine (0.62-0.69) thrust sheets are remarkably similar and indicate that, with few exceptions, limited flow partitioning occurred at all structural levels during emplacement of the Scandian nappe pile.