Seismic induced anisotropy and kinking in quartz

Abstract

Recognition of seismically induced microstructures is important to unravel the different deformation processes during seismic cycles, especially at the base of the upper crust where many earthquakes nucleate. Deformed quartz veins related to a strike-slip shear zone within the Schobergruppe (Austroalpine Crystalline Complex, Eastern Alps) contain intense kinking in elongated quartz grains. The kink band boundaries are inclined into the general dextral sense of shear. Cathodoluminescence (CL) images reveal that the entire thin section contains a very high density of intragranular, sub-planar microstructures developed as thin dark CL lamellae accompanied with nanometre-scale fluid inclusions. Based on the oscillating orientation variation across low angle boundaries (misorientation angle 1-9°) these lamellar microstructures are referred as short-wavelength undulatory extinction microstructures - SWUE (Trepmann and Stöckhert, 2013, Solid Earth, 4). Only grains with SWUE, orientated parallel to the foliation, are kinked. In general, kinked microstructures mainly develop in strongly anisotropic material or within lamellar minerals, i.e. micas. Deformation at high stresses (e.g. at greenschist conditions or during coseismic loading) can produce a strong anisotropic microstructure in quartz by the development of deformation lamellae. Trepmann and Stöckhert (2013) showed in deformation experiments of quartz that SWUE preserve evidence of an earlier coseismic stress peak, even when overprinted during subsequent crystal plastic creep deformation at lower stress. The SWUE in the deformed Schober quartz veins are interpreted in a similar way. These microstructures were primary deformation lamellae developed during coseismic loading. Pseudotachylyte veins within the analysed shear zone next to the sample give evident for a seismic event. Subsequent overprint by ongoing creep at lower stresses is recorded by the vein quartz mylonites. The densely spaced sub-planar microstructures cause a high anisotropy of the quartz grains, which finally were kinked. Electron Backscatter diffraction data give evidence of different slip system active in the formation of the deformation lamellae/SWUE and the subsequent kinking. The opposite direction of the Burges vectors (based on Weighted Burges Vector analysis, Wheeler et al., 2009, Journal of Microscopy, 233) in the kink band domain is consistent with sinistral shearing along the anisotropic deformation lamellae/SWUE in the dextral sheared kink band. Kinked micas (muscovite and biotite) in the mica-rich host rock, next to the kinked quartz vein sample, point to seismic induced kinking. However, the question arise if the kinking of micas and quartz was caused instantaneously during the same seismic event or if the quartz kinking is related to post-seismic creep at transient high stresses during ongoing deformation at lower greenschist facies conditions under dextral sense of shear (Bestmann et al., JGPR – Solid Earth, 126).