A regional explanation for Laxfordian tectonic evolution and its implications for the Lewisian terrane model

Abstract

The later Paleoproterozoic (Laxfordian) structural and metamorphic history of the Lewisian Complex can be explained by several major kinematic changes. At c. 1.9 Ga, the neighbouring Archean cratons of Rae, North Atlantic (NAC), Kola and Karelia were isolated from each other and subduction and accretion were active at their margins. At c. 1.85 Ga, during the ‘Early Laxfordian’, the Lewisian, then part of the Nagssugtoqidian orogenic belt, experienced NW–SE-directed tectonic movements attributed to east–west convergence between the Rae Craton and the NAC, probably prompted by collision with Baltica. At c. 1.7 Ga, during the ‘Late Laxfordian’, a major change to north–south convergence produced a combination of NW–SE-trending folds and NW–SE dextral shear zones. The kinematic system changed again after c. 1.6 Ga. The latter two changes may be attributable to the docking of further large cratons as the Nuna supercontinent was assembled. The Lewisian terrane model can be interpreted in terms of the relative movements between two major cratons, Rae and NAC. Sandwiched between them was a third, composed of material partly derived from a juvenile magmatic arc or arcs situated in oceanic crust. The Assynt terrane may be an isolated remnant of the upper-plate NAC, whereas most of the remaining Lewisian outcrop may consist of modified Rae material.