Deformation mechanisms in the Gwna mélange on Anglesey (UK) and the implications for aseismic subduction zones

Abstract

The Gwna m ́elange, exposed on Anglesey (north Wales) allows for the study of pre-Cambrian (∼610-540 Ma) subduction-related rocks. Knowledge about how, where and why deformation has occurred in these rocks give insights about seismicity in the unaccessible rocks in currently active subduction zones. This type of field-based studies is needed to evaluate the applicability of conceptual models and laboratory experiments to natural rocks. Microstructural observations are used to study the deformation mechanisms in these rocks, which differ between the m ́elange blocks and the matrix. Before any other research question can be considered, the first thing is to decipher are the main deformation phases , to see which deformation mechanisms have been active during the main subduction event. These deformation mechanisms, together with chlorite geothermometry are then used to put temperature constraints on the three deformation phases observed in these rocks. The first deformation phase D1 is characterized by pressure solution seams parallel to the bedding, due to low temperature pressure solution as the result of vertical compaction due to the accumulation of sediments on the seafloor. The main subduction event D2 occurred mainly by deformation in the pelagic sediments, in which oblique solution seams formed, and in the basalts. Both the basalts and the sediments consist of large amounts of very fine grained muscovite, which allows easy deformation along the basal phyllosilicate planes. Within the matrix-supported microstructure there are blocks of quartz and carbonates, which show some brittle veins, but have mostly deformed by recrystallization and ductile creep processes. The occurrence of these ductile creep processes, in spite of relatively low deformation temperatures of ∼280◦C, might be explained by hydrolitic weakening or high inherited dislocation densities. The last deformation event D3 developed a spaced foliation in the weak pelagic sediments and muscovite-rich basalts, and is possibly linked to continental collision late in the deformation history of these rocks. To quantify the deformation during the main subduction event, conceptual models are used for deformation of a phyllosilicate matrix with single quartz grains and for strong quartz blocks in a weak matrix. This allowed for estimation of the differential stress and shear stress, deformation depth and strain rates in the blocks and the matrix. There was a clear distinction between strong slowly deforming blocks and high strain rates in the m ́elange matrix, which can reach values high enough to allow aseismic creep without the need for seismic events.