Brittle-viscous deformation and the earthquake cycle in the lower crust
Abstract
The rheological behaviour of dry lower crustal rocks is commonly characterised by a cyclic interplay between viscous creep (mylonitisation) and brittle, seismic slip associated with formation of pseudotachylytes. Seismic slip may trigger fluid infiltration, grain size reduction, rheological weakening, and a transition to viscous creep along faults initially characterized by frictional melting and wall rock damage. The cyclical interplay between seismic slip and viscous creep implies transient oscillations in stress and strain rate during the activity of lower crustal faults and shear zones steered by the earthquake cycle. This study aims to identify the evidence of transient deformation in the fault rock assemblage exhumed from the granulite facies lower crust of Lofoten, northern Norway.
In Nusfjord, Lofoten, granulite facies ductile shear zones dissect an anorthosite intrusion. The shear zones localize on precursor pseudotachylyte veins, and are otherwise very rare or absent in the anorthosite host rock. Strain localisation in mylonitised pseudotachylytes occurred at ca. 700°C and 0.8 GPa and resulted from the activation of grain size sensitive creep in fine-grained recrystallized pseudotachylytes. Mylonitised pseudotachylytes locally preserve microstructures (i.e. layers of fine-grained, average 10 microns, dynamically recrystallized quartz) suggestive of transient high stress/high strain rate deformation that we interpret as episodes of accelerated post-seismic creep. The ductile shear zones occur in three main sets of parallel structures that separate undeformed blocks of anorthosite, within which pristine (non mylonitised) pseudotachylyte fault veins link adjacent or intersecting ductile shear zones. These pristine pseudotachylytes represent direct identification of earthquake nucleation as a transient consequence of ongoing, localised aseismic creep in ductile shear zones. Overall, this study indicates that (i) earthquakes are precursors of ductile shear zones in the dry and strong lower crust, and (ii) networks of coeval mylonites and pseudotachylytes can be used to estimate the variations of rheological parameters of lower crustal shear zones active throughout the earthquake cycle, and to identify mechanisms of earthquake nucleation in the deep crust.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T41A..04M
- Keywords:
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- 3902 Creep and deformation;
- MINERAL PHYSICS;
- 8159 Rheology: crust and lithosphere;
- TECTONOPHYSICS;
- 8163 Rheology and friction of fault zones;
- TECTONOPHYSICS;
- 8164 Stresses: crust and lithosphere;
- TECTONOPHYSICS