The role of shear zones in Northwest Scotland for Precambrian crustal assembly and evolution

The geological mechanisms of continent formation and growth on Earth have changed significantly through time, with a global subduction-driven (mobile lid) regime having replaced a stagnant-lid tectonic regime sometime during the Mesoarchean. This critical geodynamic transition, marking the onset of plate tectonics, paved the way for the rearrangement and juxtaposition of discrete continental fragments and island arc systems that became caught up in the supercontinent cycle.

The Lewisian Complex, northwest Scotland, is dominated by Archean tonalite–trondhjemite– granodiorite (TTG) orthogneiss, with subordinate metasedimentary and meta-volcanic supracrustal sequences (Mason, 2016). These units were variably reworked and retrogressed during Paleoproterozoic assembly of the supercontinent Nuna. These Scourian (c. 2.75–2.5 Ga) and Laxfordian (c. 1.9–1.65 Ga) metamorphic and deformational events have been documented throughout the Assynt terrane and Outer Hebrides, and can be directly correlated with equivalent tectonometamorphic episodes in Greenland and northern Canada (Whitehouse and Bridgwater, 2001). Yet, despite decades of study, fundamental questions remain within the community concerning the assembly and geological evolution of the Lewisian Complex. In particular, major tectonic boundaries provide a structural framework for correlating crustal fragments between the mainland and Outer Hebrides, with prominent crustal-scale ductile shear zones thought to represent the loci of amalgamation of separate crustal blocks with distinct geological histories (e.g. Goodenough et al., 2010). However, it is unclear whether these deformation zones record large-scale horizontal motion and welding of continental masses and/or island arc complexes previously separated by an ocean of indeterminate size, such as occurs at the terminal stages of the Wilson Cycle (e.g. Park and Tarney, 1987), or alternatively they record the effects of intracontinental deformation that post-date major episodes of assembly. Understanding the sequence of events that led to the current configuration of the Lewisian Complex has wider implications for palaeotectonic reconstructions of supercontinent formation during Earth’s middle age. 

This project will involve fieldwork, structural mapping, and sample collection from several localities within the Lewisian Complex, both on the mainland and on the outer islands, including deformed units belonging to Paleoproterozoic shear zones. Strategic sampling will allow competing hypotheses of terrane accretion in the region to be tested. Detailed petrological and microstructural observations will be obtained at a range of scales across the Complex in order to construct a model for pre-, syn-, and post-shear terrain growth. This research will complement ongoing research by the supervisory team focused on the geological processes responsible for the growth and long-term stabilization of Archean terrains, and provide new insight into secular changes in metamorphism and tectonics throughout geological time. 

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