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_Journal of Structural Geology 33 (2011) 169-186_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com/locate/jsg_ _Pseudotachylyte in muscovite-bearing quartzite: Coseismic friction-induced melting and plastic deformation of quartz_ _Michel Bestmann a,*, Giorgio Pennacchioni b,c, Gerhard Frank d, Mathias G?ken e, Helga de Walla_ _GeoZentrum Nordbayern, University of Erlangen-Nuremberg, Germany_ _b Department of Geosciences, University of Padova, Italy_ _c Istituto Nazionale di Geo?sica e Vulcanologia, Roma, Italy_ _d Department Werkstoffwissenschaften VII, University of Erlangen-Nuremberg, Germany_ _e Department Werkstoffwissenschaften I, University of Erlangen-Nuremberg, Germany_ _article info_ _Article history: Received 12 January 2010; Received in revised form 16 September 2010; Accepted 30 October 2010; Available online 5 November 2010_ _Keywords: Pseudotachylyte Quartz Microstructures Plastic deformation EBSD TEM_ _abstract_ _Thin (0.5-2 mm thick) pseudotachylyte veins occur within muscovite-bearing (w10% volume), amphibolite-facies quartzites of the Schneeberg Normal Fault Zone (Austroalpine, Southern Tyrol, Italy). Pseudotachylytes are associated with precursor localized plastic microshear zones (50-150 mm thick) developed sub-parallel to the host-rock foliation and with conjugate sets oriented at a high angle to the foliation. Such microshear zones are characterized by recrystallization to ultra-fine-grained (1-2 mm grain size) mosaic aggregates of quartz showing a transition from a host-controlled to a random crystallographic preferred orientation towards the shear zone interior. Subsequent coseismic slip mainly exploited these microshear zones. Microstructural analysis provides evidence of extensive friction-induced melting of the muscovite-bearing quartzite, producing a bimodal melt composition. First, the host-rock muscovite was completely melted and subsequently crystallized, mainly as K-feldspar. Then, about 60% volume of the ultra-fine-grained quartz underwent melting and crystallized as spherulitic rims (mostly consisting of quartz-Ti-Fe) around melt-corroded quartz clasts. The two melts show immiscibility structures in the major injection veins exploiting microshear zones at high angles to the quartzite foliation. In contrast, they were mechanically mixed during flow along the main fault veins._ _? 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Tectonic pseudotachylytes are solidified friction-induced melts produced along a fault during seismic slip (i.e., at slip rates of 1-3 ms?1) (Sibson, 1986; Spray, 1992; Swanson, 1992; Sibson and Toy, 2006; Lin, 2008; Di Toro et al., 2009 and references within). They have been reported in a large variety of silicate-built rocks including felsic to maf-ultramaf intrusive rocks and different metamorphic rocks (Sibson and Toy, 2006; Di Toro et al., 2009). However, pseudotachylytes within quartzites have not been reported previously._ _Non-equilibrium melting is inferred to be the dominant process during friction-induced melting as indicated by the disappearance or a decrease in the percentage of host-rock clasts of low-melting point minerals (e.g., micas and amphiboles) in the pseudotachylyte. * Corresponding author. E-mail address: michel@geol.uni-erlangen.de (M. Bestmann)._ _0191-8141 $ ? 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.10.009_ _Preferential melting of mafic minerals (having relatively low single-phase melting point) results in a more basic composition of the pseudotachylyte melt (or matrix) than the host-rock, whereas the bulk pseudotachylyte composition (clasts ? matrix) is identical to that of the host-rock (Allen, 1979; Bossi?re, 1991; Camacho et al., 1995; Maddock, 1986, 1992; Magloughlin, 1989; Sibson, 1975; Spray, 1992, 1993; Di Toro and Pennacchioni, 2004). In pseudotachylytes from within granitoid rocks: (i) biotite melts completely, (ii) plagioclase (with a melting temperature under dry conditions in the range of 1100-1550 x14C) undergoes partial to complete melting in the centre of centimetre thick veins, indicative of superheating of friction-induced melts (Di Toro and Pennacchioni, 2004), and (iii) quartz, having a very high melting temperature (1720 x14C under dry conditions; Deer et al., 1992), commonly survives as clast, although embayed shapes have been reported as local evidence of quartz melting (Lin, 2008 and references within). In addition to the high melting point of quartz, the friction-induced melting of this mineral is potentially hindered by the occurrence of extreme fault weakening at high slip rates; this has been experimentally determined in quartzite and related to lubrication by silica gel (Di Toro et al., 2004)._ _170_ _M. Bestmann et al. Journal of Structural Geology 33 (2011) 169-186_ _The maximum temperature achieved during friction-induced melting is an important parameter for estimating the energy budget of an earthquake from exhumed paleoseismic faults (Di Toro et al., 2005). To estimate this temperature, minerals surviving melting have often been used (e.g., Maddock, 1983). The commonly reported range of estimated and inferred friction melt temperatures is 650-1730 x14C (Sibson and Toy, 2006). The observation of quartz melting could therefore provide evidence for unusually high thermal peaks (in a dry environment). The amount of quartz involved in melting to form a pseudotachylyte cannot be easily quantified. Spherulitic overgrowth structures around quartz clasts have been described in some pseudotachylytes (e.g., “quartz-nucleus spherulites”: Lin, 1994; Di Toro and Pennacchioni, 2004), where inclusion-rich quartz rims surround rounded quartz clasts. However, these features cannot be univocally related to the achievement of single-quartz melting point._ _In this study, we report on the occurrence of pseudotachylytes within muscovite-bearing (w10% volume) amphibolite-facies quartzites of the “Schneeberg Normal Fault Zone” (Austroalpine, Southern Tyrol, Italy), clear evidence of extensive frictional melting of quartz. The detailed microstructural and electron microscope analysis (scanning electron microscopy e SEM, electron backscatter diffraction technique e EBSD, and transmission electron microscopy e TEM) show a close spatial association between pseudotachylytes and ultra-fine-grained aggregates (grain size in the order of a few microns) delineating microshear zones in the host quartzite close to the fault vein._ _2. Methods_ _2.1 Sample preparation_ _Optical microscopy (transmitted light) and SEM analysis were carried out on oriented samples. Polished thin-sections were obtained from slabs cut parallel to the mineral lineation (X-axis) and perpendicular to the foliation (XY plane) of the quartzite hosting the pseudotachylyte. The XZ section is also orthogonal to the pseudotachylyte veins. EBSD measurements (Section 5.1.2) have shown that this reference frame also contains the main kinematic axes of the network of microshear zone precursors of seismic slip._ _For SEM analyses, the thin-sections were chemical polished using a colloidal silica suspension (SYTON) and subsequently carbon coated (coating thickness of ca. 3 nm)._ _2.2 SEM analysis_ _specific match unit (muscovite, quartz and/or orthoclase) containing 75 reflectors to index the patterns._ _EBSD orientation data are presented as processed orientation maps. Non-indexed points were replaced by the most common neighbouring orientation. The degree of processing required to fill non-indexed data points, without introducing artefacts, was tested carefully by comparing the resulting orientation map with the pattern quality map (Bestmann and Prior, 2003)._ _2.4 TEM_ _The TEM foils were examined at 300 kV in a Phillips CM 30 Twin STEM transmission electron microscope at the Central Facility for High Resolution Electron Microscopy of the University Erlangen-Nuremberg. All diffraction contrast images were produced using bright field (BF) conditions. Geochemical energy-dispersive spectroscopy (EDS) analyses (element mapping and line scans) were carried out in the scanning transmission electron microscopy (STEM) mode with an Oxford Instrument ISIS 300 EDS system, using a Si(Li) detector._ _Two different sample preparation methods were applied for the TEM analysis. Samples of 1 inch size were assembled with a 200 mm spaced copper net for conventional ion beam thinning with a BALTEC BALZER RES 010 (thinning parameter: inclination angle 11-12°, acceleration voltage 3.5-5 kV). This sample preparation was not appropriate for geochemical area analyses in the TEM because in a polyphase rock, such as the pseudotachylyte, a sample topography could not be excluded. Such small-scale irregularities on the sample surface might cause thickness-dependent artefacts especially in EDS line scans and element mappings. To guarantee plane parallel electron-transparent foils, the focused ion beam (FIB) technique was applied. This technique allows site-specific TEM foils (10-20 mm wide, 5-15 mm high and 100-200 nm thick) to be prepared through Ga-ion beam thinning on standard thin-sections. The TEM foils were prepared using a ZEISS CrossBeam 1540 EsB at the Material Science Department at the University Erlangen-Nuremberg._ _2.5 Electron microprobe analysis (EMPA)_ _Compositional data of muscovite and K-feldspar were measured on a Jeol JXA-8200 at the GeoZentrum Nordbayern (University of Erlangen-Nuremberg). Natural silicates were used as standards and a ZAF routine was applied for matrix correction. Measuring conditions using a focused electron beam were: 15 kV acceleration voltage and 15 nA beam current._ _SEM analyses were carried out with a ZEISS CrossBeam 1540 EsB equipped with a thermo-ionic field emission located at the Department of Material Sciences of the University Erlangen-Nuremberg. The cathodoluminescence (CL) images of Fig. 10d was produced with a TESCAN Vega-XM-U SEM attached with a CL-system._ _2.3 EBSD_ _Full crystallographic orientation data were_ Ключевые слова: orientation map, ebsd analysis, sem, grain size, sem-oc image, frictional melting, developed, geophysical, fault zone, spherulitic rim, tem foil, plastic, polycrystalline clast, ultrane-grained microstructure, fault-generated pseudotachylytes, terra, melt, contrast, quartz mylonites, quartz spherulites, coarse clast, image, bestmann, kfsp, melting temperature, quartz clast, muscovite, subgrain mosaic, frictional melt, pole gures, wa, magloughlin, grain boundary, journal structural, earth, aggregate, vein, ultra?ne, structure, foliation, electron, lin, dynamic recrystallization, host quartz, snfz pseudotachylytes, high, sample, latent heat, elsevier, hand sample, fault, hydrated condition, grain adjacent, angle, antithetic set, dry condition, colour, size, coseismic slip, ebsd data, showing, di, dislocation, spherulitic, sem analysis, subgrain, pattern, pseudotachylytes vein, rim, plastic deformation, deformation, melting point, plane, grain, host-rock, tectonophysics, figs, friction-induced melting, random cpo, university erlangen-nuremberg, sharp contact, ?uid, pseudotachylyte matrix, pseudotachylyte vein, grained, snfz, upper, colour coded, note, coseismic faulting, erlangen-nuremberg germany, host grain, pennacchioni, quartz melting, slip, journal structural geology, journal, deformed muscovite, evidence, s?lva, complete melting, microshear zone, small, mm, friction-induced melt, angular deviation, boundary, friction, temperature, petrology, microshear, data, k-feldspar, prior, mosaic aggregate, point, slip surface, frictional, ultra?ne-grained, zone, fault plane, earlier stage, earth sciences, quartzite, bestmann journal, misorientation analysis, area, structural geology, clast, th?ni, sciences, host quartzite, toro, k-feldspar muscovite, mineral, melting, small grain, journal geophysical, injection vein, contractional bridge, quartz aggregate, structural, polycrystalline, spherulites, faulting, white, mac mineral, main, seismic, quartz involved, di toro, rock, coseismic, discussion, deformed, microstructures, uid inclusion, friction melt, texel complex, japan, sibson, quartzite foliation, microstructure, fluid inclusion, geology, sem-bse image, inclusion, host rock, muscovite melting, stage, feldspar, maddock, transition zone, matrix, unit, quartz, high angle, feo, lloyd, condition, friction-induced, melt temperature, deer, main foliation, seismic slip, pseudotachylyte, spray, nature, adjacent, reduction, host, tem image, vorticity axis, recrystallization, grey, recrystallized grain, ebsd, bse image, pseudotachylytes, crystal, subgrains, colour banding, tem, domain, map, analysis, large, orientation, constrained, shear