Microstructural evidence for the deep pulverization in a lower crustal meta‐anorthosite

We report on the evidence of the pulverization in a deep‐seated meta‐anorthosite in the Eidsfjord shear zone, Vesterålen, northern Norway. Some plagioclase porphyroclasts comprise a few large relict clasts and many fine grains that preserve the outlines of the original grains. The fine‐grained plagioclase does not show any plastic‐deformation microstructures and has strong crystallographic preferred orientations, which are inherited from the twinned porphyroclast. Misorientation‐axis distributions indicate that the grains have rotated randomly, so that the misorientation axes are not aligned with either the crystallographic or kinematic axes. The observed grain‐size distribution has a fractal dimension, suggesting their fracturing/fragmentation origin. The microstructures are characterized by the fracturing/fragmentation with a very low shear strain, indicating that it may be associated with pulverization at ~20–25 km depth.

are characterized by a very low shear strain and a very high fracture density at or below the hand-specimen scale are also a candidate for the evidence of dynamic fracturing (Dor, Ben-Zion, Rockwell, & Brune, 2006;Rempe et al., 2013). Pulverized rocks have been found along various crustal-scale faults, such as the San Andreas fault (Dor et al., 2006) and the Arima-Takatsuki Tectonic Line (Mitchell, Ben-Zion, & Shimamoto, 2011). The current view on pulverized rocks is that they are formed by stress waves with high stress-or strain-rate loadings released by the rupturing fault (Aben et al., 2016;Doan & Gary, 2009). The pulverization is thought to have occurred in the top few kilometres of the crust (Dor et al., 2006). Based on the results of numerical analyses (Andrews, 2005;Ben-Zion & Shi, 2005), the spontaneous generation of damage produced by rupture propagation along the fault is suppressed by an increase in the confining stress. However, the effects of normal stress on the local behaviour of a fault tip where the intact rock is being dynamically fractured remain unclear (Ando & Yamashita, 2007).
In meta-anorthosites from an exhumed lower crustal shear zone of Vesterålen in northern Norway, some mesoscopic plagioclase porphyroclasts have been found to comprise a few large relict clasts and many fine grains that preserve the outlines of the original grains.
The microstructures indicate that fracturing and fragmentation occurred without substantial shear strain, thus suggesting the grains development of ductile shear zones, occurring in the uppermost region of the lower crust (i.e., middle crust).

| SAMPLE AND ME THODS
The Eidsfjord shear zone is a 200-m-thick mylonite zone that underwent down-to-the-west normal-slip movement along the crustal-scale detachment fault (Moecher & Steltenpohl, 2011). In this region, Caledonian metamorphism and deformation are strictly limited to shear zones (Steltenpohl et al., 2011). In the Eidsfjord shear zone, pseudotachylytes are frequently observed. Some pseudotachylytes deform plastically along with mylonites, while some pseudotachylytes cut across the mylonitic foliation and encloses angular clasts of metaanorthosite mylonite. The anorthosite mylonites are often recognized in the coastal area ( Figure 1). The lateral extent of each mylonite in the field is unclear, but the width of the mylonite shear zone is less than a few metres.
The deformed meta-anorthosite that is examined in this study constitutes the narrow shear zone (<2 m wide) occurring at the western part of the Eidsfjord shear zone (Figure 1). The outcrop is at a distance from the high-strain zone that is developed near the crustal-scale detachment fault. This meta-anorthosite sample is not overprinted by later stage deformation concentrated along the detachment fault (Steltenpohl et al., 2011). A mylonitic foliation that is defined by an alignment of mafic-mineral patches is observed (Figure 2a). The degree of development of the foliation varies within the sample and may reflect the degree of shear deformation. In a domain with high shear strains (lower part of the sample in Figure 2a), a weak stretching lineation that is defined by an alignment of the long axes of mafic-mineral patches occurs on the foliation planes. The sample comprises plagioclase, amphibole, biotite, and quartz, and minor amount of garnet, epidote, apatite, K-feldspar, and scapolite exist as secondary phases. Plagioclase occurs as fine-grained matrix grains and porphyroclasts. The matrix-forming mineral assemblage is similar to that in the meta-anorthosite mylonites that have been previously reported from the Eidsfjord shear zone the synmetamorphic deformation occurred at~600-700 MPa and~600-750°C (Leib, Moecher, Steltenpohl, & Andresen, 2016;Okudaira, Shigematsu, Harigane, & Yoshida, 2017;Steltenpohl et al., 2011), which corresponds to crustal depths of~20-25 km, assuming the rock density of 2,800 kg/m 3 . Some plagioclase porphyroclasts are partially altered to epidote and muscovite. The long axes of muscovite and epidote are randomly oriented, which indicates their postkinematic crystallization. In the matrix-forming plagioclases, An-poor cores and Anrich rims are developed; however, the crystallographic orientation between these plagioclases is continuous.
Some rectangular-shaped plagioclase porphyroclasts can be recognized at the scale of hand specimens with grain sizes of up to 4 cm and comprise a few large clasts and numerous fine microscopic grains (hereafter referred to as "pseudomorphic porphyroclasts").
We measured the crystallographic orientations of plagioclase grains within the pseudomorphic porphyroclast in a domain with low shear strains (the upper part of the sample is depicted in Figure 2a

| RESULTS
The pseudomorphic plagioclase porphyroclast for the EBSD measurements exhibits a rectangular-shaped grain outlined by the matrix-forming mafic minerals (Figure 2a  reported from the natural fault rocks (Muto, Nakatani, Nishikawa, & Nagahama, 2015) and similar to those (D = 0.9-1.1) for small grains with the particle seize of <~2 μm from the deformation experiments (Keulen, Heilbronner, Stünitz, Boullier, & Ito, 2007). The low value of the fractal dimension may be caused by metamorphic recrystallization or grain growth via absorption of small grains and growth of large grains.
Most grains are free from internal orientation differences, displaying no distinguishable subgrain boundaries. In some grains, there is a gradual increase in the internal orientation difference from the geometrical grain centre to the margin (grains 1 and 2 in Grain 1 Grain 2   (Ellis & Stöckhert, 2004;Jiang & Lapusta, 2016;Shimamoto & Noda, 2014). In rocks deformed naturally under lower crustal conditions, it has been reported the fragmentation of jadeite and garnet from the Sesia Zone, Western Alps (Trepmann & Stöckhert, 2001, 2002, and garnet and plagioclase associated with pseudotachylyte from the Bergen Arcs, SW Norway (Austrheim et al., 2017;Petley-Ragan, Dunkel, Austrheim, Ildefonse, & Jamtveit, 2018). This also implies that fragmentation or pulverization is associated with seismic faulting in the lower crust. In this study, pulverization of plagioclase   episodic fluid flow that modifies the mechanical and rheological properties of the zone and its upward extension.

ACKNOWLEDGEMENTS
We gratefully acknowledge N. Shigematsu and Y. Harigane for their assistance to use EBSD system at Geological Survey of Japan, AIST.
The original MTEX scripts used in this study are provided by D.

APPENDIX 1 A D D I T I O N A L I N F O R M A T I O N F O R T H E E B S D A N A L Y S I S
The EBSD data were recorded with a step size of 5 μm. For indexing, we used data in Aztec of andesine An38, muscovite, and epidote. The grain size (equivalent diameter) of the plagioclase was calculated using the areas of the grains having a segmentation angle of greater than 10°, and the Pericline and Albite twins are merged along the twin boundaries. We excluded the grains having sizes smaller than 10 pixels. Systematically misindexed points, which appear as a mosaic of pixels composed of two orientations in EBSD orientation maps, are removed by rotation about specific angle-andaxis pairs. These processes have been done using MTEX.
SODA AND OKUDAIRA