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Staurolite fragment orientations from the five analyzed samples (A-E). For each sample: (1) Backscattered electron images with staurolite microstructures of interest outlined and fragments identified by letters. (2) Equal-area, lower-hemisphere projections of staurolite crystallographic axes; letters refer to the fragments in (1). (3) Graphical representation of fragment orientations; shear sense indicated in upper right.
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The kinematic record and bulk viscous strength of polyphase rocks depend in part upon the relative strengths and distributions of rheologically distinct fabric elements. Here, we explore the effects of microstructural and rheological heterogeneity in porphyroblastic schists. Electron backscatter diffraction and petrographic analyses reveal asymmetr...
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... results of our EBSD analysis provided the relative crystallo- graphic orientation of staurolite fragments as well as the relative ro- tation axes and magnitudes of rotation (Figs. 5 and 6). Every staurolite fragment for which we determined a unique crystallo- graphic orientation was given a letter designation. All fragments dis- play rotation of the principal crystallographic axes relative to a parent orientation; however, the magnitude of relative rotation of any given crystallographic axis varies considerably from ...
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... Every staurolite fragment for which we determined a unique crystallo- graphic orientation was given a letter designation. All fragments dis- play rotation of the principal crystallographic axes relative to a parent orientation; however, the magnitude of relative rotation of any given crystallographic axis varies considerably from sample to sample (Figs. 5 and 6). The magnitudes of rotation (i.e. the misorientation an- gles) are calculated as the minimum amount of rotation necessary to make the fragment orientation coincident with the reference parent orientation (Fig. 6A). We report the axes necessary to achieve that ro- tation ( Fig. 6B) in a crystallographic reference ...
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... of former- ly contiguous twinned staurolite grains prior to disaggregation and relative rotation during the non-coaxial D 3 event. Relative rotation magnitudes range from 15° to 63° (Fig. 6A) with an average magni- tude across all fragments analyzed of 35°. In some cases the axial ori- entation data exhibit apparent curvilinear trends (e.g., Fig. 5A.2-B.2, and E.2a). However, the axial orientation data for the remaining sam- ples display no clear trend (e.g., Fig. 5C.2 and E.2b). Furthermore, the relative rotation axes are generally not coincident both within and between samples and individual boudin fragments consistently yield unique rotation axes out of the shear plane (Fig. ...
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... results of the EBSD analysis (Figs. 5, 6) describe the relative crys- tallographic orientation of microboudinaged staurolite grains that were formerly contiguous crystals prior to a period of dextral shearing at syn- to post-peak amphibolite facies conditions. In this section, we consider the factors that control rotation amounts and axes, including the initial grain ...
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... in the magnitude and style of rotation likely resulted in part from differences in the initial orientation of stauro- lite crystals relative to the D 3 kinematic reference frame. Based on numer- ous outcrop to thin-section scale observations and the EBSD results, we propose the conceptual relationships compiled in Fig. 7. For example, sample 10S (Fig. 5.E) preserves a near complete symmetric staurolite twin, which displays significantly different fragment morphologies and rotational trends by each of the conjugate twins (Fig. 5E.2a, b). Specifical- ly, the rotation of fragments F and G, from sample 10S, were largely accommodated by the a-and b-axes with little change c-axis ...
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... 1992;Jessell et al., 2009;Johnson et al., 2009a,b;Mandal et al., 2005). We interpret our results to be repre- sentative of these types of complexity such that staurolite fragments in the Appleton Ridge Formation display variable amounts of rota- tion out of the shear plane and yield inconsistent inter-and intra-sample rotational behavior (e.g., Figs. 5 and 7). Therefore any measurements made in the standard kinematic reference frame would underestimate the shear strain and/or mean kinematic vortic- ity number. Despite the quantitative challenges, we observe 3D rota- tional trends and microstructures qualitatively consistent with the bulk ...
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Citations
... As staurolite represents a characteristic mineral of amphibolite facies regional metamorphism, its growth is roughly restricted to a field of medium P-T conditions (Bell and Kim, 2004;Mezger, 2010;Bell and Fay, 2016). Due to their ability to form euhedral poikiloblastic porphyroblasts that can retain inclusion trails, staurolite can provide information for reconstructions of past and changing deformation environments (Frieman et al., 2013). As mentioned above, the Chonghuer area has experienced three phases of deformation, and porphyroblast-matrix relations are important tools for delineating the tectonothermal history of the area. ...
... Passchier et al., 1992;Passchier and Trouw, 2005;Passchier and Coelho, 2006;Griera et al., 2013;Stahr and Law, 2014). In the last two decades, a renewed and heated discussion has sprung out of two fundamental issues: Do rigid clasts rotate (e.g., Bell et al., 1992;Jiang and Williams, 2004;Johnson, 2008Johnson, , 2009Bell and Fay, 2016), and how do matrix properties impact the rotational behavior of rigid clasts in noncoaxial flow (e.g., Marques and Coelho, 2001;Mancktelow et al., 2002;Piazolo et al., 2002;ten Grotenhuis et al., 2002;Ceriani et al., 2003;Schmid and Podladchikov, 2005;Griera et al., 2011Griera et al., , 2013Frieman et al., 2013;Stahr and Law, 2014;Mulchrone and Meere, 2015;Marques, 2016). These issues beg the question of whether or not clast-based kinematic criteria can be used as independent shear-sense indicators. ...
... The impact of matrix anisotropy on the rotational behavior of the rigid clast in viscous flow was also explored in terms of intrinsic fabric anisotropy (e.g., Mandal et al., 2005a;Fletcher, 2009;Dabrowski and Schmid, 2011;Griera et al., 2013;Frieman et al., 2013), i.e., a viscous matrix composed of thin layers with alternating viscosities. Results of these numerical models show that the rotation of the circular clast is always synthetic relative to the applied shear stress, although it would be reduced or even fully stopped with the increase of matrix anisotropy (e.g., Dabrowski and Schmid, 2011). ...
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