Exploitation of unsuitably oriented foliation by localized mylonites and pseudotachylytes (Tauern Window, Eastern Alps)
- 1Department of Geosciences, University of Padova, Padua, Italy
- 2Structural Geology and Tectonics Group, ETH Zürich, Zürich, Switzerland
During exhumation, metamorphic rocks change their rheological behavior from dominantly ductile to brittle. Especially at the “brittle-ductile transition” at the bottom of the brittle crust, which coincides roughly with the domain where most “shallow” earthquakes nucleate, rocks exhibit a close interplay between ductile flow and fracturing.
In the Neves area (Tauern window, Eastern Alps) the exhumation across the brittle-ductile transition of amphibolite-facies meta-granitoids during the Alpine cycle is recorded by the association of pseudotachylyte veins and localized low-grade mylonites (stage-2 deformation). The stage-2 structures exploited the precursor amphibolite-facies foliation within meter-thick mylonites (stage-1 deformation) and were in turn overprinted by epidote-chlorite-bearing shear fractures and veins (stage-3 deformation). The kinematics and orientation of stage-1 and stage-3 structures indicate a slight rotation of the regional shortening direction from 345° to about 360°. This implies that stage-2 mylonites and pseudotachylytes developed at a high angle to the shortening direction.
The syn-kinematic metamorphic assemblage of stage-2 mylonites includes quartz, oligoclase (Ab75), biotite, epidote, and minor muscovite and K-feldspar; garnet was not stable. This assemblage constrains the deformation at upper greenschist facies condition and temperatures of around 400 °C. During mylonitization the coarse-grained (mm-sized) amphibolite-facies quartz recrystallized by subgrain rotation to ultra-fine (~ 3 µm average grain size determined from EBSD maps) aggregates. Such a small grain size yields differential stress > 200 MPa during stage-2 mylonitization, considering the piezometer of Cross et al., 2017 1.
Pseudotachylytes are in a close spatial association with stage-2 mylonites and share the same sense of shear. There is no evidence of a ductile overprint of pseudotachylytes. The stage-2 structures developed at a very high angle to the inferred shortening direction, which implies that the coseismic slip occurred on planes with a very low friction coefficient (estimated <0.3), contradicting the high differential stress estimated for the mylonites. We infer a genetic relationship between stage-2 mylonite and pseudotachylyte. Mylonites progressively formed the mica-rich foliation planes, continuous over large distances, that provided the weak mechanical anisotropy eventually leading to coseismic slip.
Reference:
1: Cross, et al., 2017, The recrystallized grain size piezometer for quartz: An EBSD‐based calibration. Geophys. Res. Lett., 44(13), 6667-6674.
How to cite: Toffol, G., Pennacchioni, G., Grafulha Morales, L. F., and Papa, S.: Exploitation of unsuitably oriented foliation by localized mylonites and pseudotachylytes (Tauern Window, Eastern Alps), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11145, https://doi.org/10.5194/egusphere-egu2020-11145, 2020