| dc.description.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. | |
