Tectonophysics, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 140 (1987) 297-305 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 297 Deformation mechanisms in a high-temperature quartz-feldspar mylonite: evidence for superplastic flow in the lower continental crust J.H. BEHRMANN ’ and D. MAINPRICE 2 ’ Institut fti Geowissenschaften und Lithosphiirenforschung, Universitiit Giessen, Senckenbergstr. 3, D-6300 Giessen (West Germany) 2 Luboratoire de Tectonophysique, Universite des Sciences et Techniques du Lmguedoc, Place Eug&te Bqtaillon, F-34060 Montpellier cedex (France) (Received March 24,1986; revised version accepted January 13,1987) Abstract Behnnann, J.H. and Mainprice, D., 1987. Deformation mechanisms in a high-temperature quartz-feldspar mylonite: evidence for superplastic flow in the lower continental crust. Tectonophysics, 140: 297-305. Microstructures and crystallographic preferred orientations in a fme-grained banded quartz-feldspar mylonite were studied by optical microscopy, SEM , and TEM. Mylonite formation occurred in retrograde amphibohte facies metamorphism. Interpretation of the microstructures in terms of deformation mechanisms provides evidence for millimetre scale partitioning of crystal plasticity and superplasticity. Strain incompatibilities during grain sliding in the superplastic quartz-feldspar bands are mainly accommodated by boundary diffusion of potassic feldspar, the rate of which probably controls the rate of superplastic deformation. There is evidence for equal flow stress levels in the superplastic and crystal-plastic domains. In this case mechanism partitioning results in strain-rate partitioning. Fast deformation in the superplastic bands therefore dominates flow, and deformation is probably best modelled by a superplastic law. If this deformational behaviour is typical, shearing in mylonite zones of the lower continental crust may proceed at exceptionally high rates for a given differential stress, or at low differential stresses in case of fixed strain rates. Introduction elongate quartz-ribbons anastomosing around un- deformed or only slightly deformed feldspars. The deformation mechanics of monomineralic Crystal plasticity has become known as a com- quartzite has become reasonably well understood paratively “hard” deformation mechanism. This is in both, experimental (e.g., Tullis et al., 1973; underlined for quartz by the deformation mecha- Koch et al., 1980) and natural creep (e.g., Mitra, nism map of Rutter (1976, figs. 7, 9) as well as by 1976; White, 1976; Bouchez, 1977; Behrmann, palaeostress indicators and their calibrations (e.g., 1985). Crystal plasticity has been identified as an Mercier et al., 1977; Christie and Ord, 1980; Ord important mechanism, and there is evidence in the and Christie, 1984). If crystal plasticity of quartz literature (Bossiere and Vauchez, 1978; Berth6 et is a dominant mechanism in granitoid rocks at al., 1979; Watts and Williams, 1979) that crystal high temperatures, a considerable flow strength plasticity of quartz is one of the main factors must be assigned io the quartzo-feldspathic lower controlling the deformation of quartzo-feldspathic continental crust. For geologically reasonable granitoid rocks. Deformation leads to the familiar strain rates (lo-l3 to lo-l4 s-i) this may be in mesoscopic augen-gneiss structure formed by the order of 1 kbar (e.g., Parrish et al., 1976). In 0040-1951/87/$03.50 0 1987 Elsevier Science Publishers B.V.