Exploitation of unsuitably oriented foliation by localized mylonites and pseudotachylytes (Tauern Window, Eastern Alps)
Authors: Toffol Giovanni , Pennacchioni Giorgio , Grafulha Morales Luiz Fernando , Papa Simone 
Published: March 2020

&lt;p&gt;During exhumation, metamorphic rocks change their rheological behavior from dominantly ductile to brittle. Especially at the &amp;#8220;brittle-ductile transition&amp;#8221; at the bottom of the brittle crust, which coincides roughly with the domain where most &amp;#8220;shallow&amp;#8221; earthquakes nucleate, rocks exhibit a close interplay between ductile flow and fracturing.&lt;/p&gt;&lt;p&gt;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 (&lt;em&gt;stage-2&lt;/em&gt; deformation). The &lt;em&gt;stage-2&lt;/em&gt; structures exploited the precursor amphibolite-facies foliation within meter-thick mylonites (&lt;em&gt;stage-1&lt;/em&gt; deformation) and were in turn overprinted by epidote-chlorite-bearing shear fractures and veins (&lt;em&gt;stage-3&lt;/em&gt; deformation). The kinematics and orientation of &lt;em&gt;stage-1&lt;/em&gt; and &lt;em&gt;stage-3&lt;/em&gt; structures indicate a slight rotation of the regional shortening direction from 345&amp;#176; to about 360&amp;#176;. This implies that &lt;em&gt;stage-2&lt;/em&gt; mylonites and pseudotachylytes developed at a high angle to the shortening direction.&lt;/p&gt;&lt;p&gt;The syn-kinematic metamorphic assemblage of &lt;em&gt;stage-2&lt;/em&gt; mylonites includes quartz, oligoclase (Ab&lt;sub&gt;75&lt;/sub&gt;), 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 &amp;#176;C. During mylonitization the coarse-grained (mm-sized) amphibolite-facies quartz recrystallized by subgrain rotation to ultra-fine (~ 3 &amp;#181;m average grain size determined from EBSD maps) aggregates. Such a small grain size yields differential stress &gt; 200 MPa during &lt;em&gt;stage-2&lt;/em&gt; mylonitization, considering the piezometer of &lt;em&gt;Cross et al., 2017&lt;/em&gt; &lt;sup&gt;1&lt;/sup&gt;.&lt;/p&gt;&lt;p&gt;Pseudotachylytes are in a close spatial association with &lt;em&gt;stage-2&lt;/em&gt; mylonites and share the same sense of shear. There is no evidence of a ductile overprint of pseudotachylytes. The &lt;em&gt;stage-2&lt;/em&gt; 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 &lt;0.3), contradicting the high differential stress estimated for the mylonites. We infer a genetic relationship between &lt;em&gt;stage-2&lt;/em&gt; 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.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Reference:&lt;/p&gt;&lt;p&gt;1: Cross, et al., 2017, The recrystallized grain size piezometer for quartz: An EBSD&amp;#8208;based calibration.&amp;#160;Geophys. Res. Lett.,&amp;#160;44(13), 6667-6674.&lt;/p&gt;