Carbo-Permian pteridophyll leaf fragments from an amphibolite facies basement, Tauern Window, Austria

AbstractAmphibolites in the Mesozoic part of the parautochthonous Lower Schieferhülle (LSH), the allochthonous Upper Schieferhülle (USH) and the overlying Austroalpine basement (AA) in and around the western Tauern Window (Eastern Alps) suffered a progressive Alpine deformation. Lineations and foliations L1-S1, L2-S2 defined by preferentially oriented (Na-Ca) amphiboles as well as F3 folds and further foliations Smyl and S4 in the metabasites are structures of successive deformational stages with a constant W-E main extension axis of strain. The (Na-Ca) amphiboles in assemblages with epidote, chlorite, albite/oligoclase and quartz are zoned with similar continuous zonation trends from early actinolite in the cores to magnesio-hornblende and tschermakitic hornblende, and from magnesio-hornblende to late actinolite in the rims in the three lithostratigraphic units. Geothermobarometry involving tremolite-edenite and (pargasite-hastingsite)-tremolite end-member equilibria in amphiboles allowed us to reconstruct prograde-retrograde P-T paths for the Alpine greenschist-amphibolite facies event. The paths passed P/Tmax at 6–7 kbar/600°C. Similar shapes of the paths in AA, USH and Mesozoic LSH indicate a common metamorphic history and a stacking of these units prior to or during the pre-Pmax evolution. Moderate P-T ratios are characteristic for the temperature-dominated compression paths and indicate continental collisional rather than subduction zone metamorphism. The middle to late Alpine greenschist-amphibolite facies event appears as an independent metamorphism along a complete P-T loop which may have followed an earlier and poorly documented high-pressure/low-temperature event.

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A Clinopyroxene Paragenesis of Albite--Epidote--Amphibolite Facies in Meta-Syenites from the South-East Tauern Window, Austria

Journal of Petrology ◽

10.1093/petrology/23.2.163 ◽

1982 ◽

Vol 23 (2) ◽

pp. 163-185 ◽

Cited By ~ 10

Author(s):

G. T. R. DROOP

Keyword(s):

Amphibolite Facies ◽

The South ◽

Tauern Window

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Exploitation of unsuitably oriented foliation by localized mylonites and pseudotachylytes (Tauern Window, Eastern Alps)

10.5194/egusphere-egu2020-11145 ◽

2020 ◽

Author(s):

Giovanni Toffol ◽

Giorgio Pennacchioni ◽

Luiz Fernando Grafulha Morales ◽

Simone Papa

Keyword(s):

Grain Size ◽

Eastern Alps ◽

Amphibolite Facies ◽

Mechanical Anisotropy ◽

High Angle ◽

Coseismic Slip ◽

Differential Stress ◽

Tauern Window ◽

Brittle Ductile Transition ◽

Stage 1

During exhumation, metamorphic rocks change their rheological behavior from dominantly ductile to brittle. Especially at the &#8220;brittle-ductile transition&#8221; at the bottom of the brittle crust, which coincides roughly with the domain where most &#8220;shallow&#8221; 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&#176; to about 360&#176;. 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 &#176;C. During mylonitization the coarse-grained (mm-sized) amphibolite-facies quartz recrystallized by subgrain rotation to ultra-fine (~ 3 &#181;m 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.&#160;Reference:1: Cross, et al., 2017, The recrystallized grain size piezometer for quartz: An EBSD&#8208;based calibration.&#160;Geophys. Res. Lett.,&#160;44(13), 6667-6674.

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