Scanning electron microscopy reveals more detailed deformation structures in syntectonic granite. Asymmetric folds in the SEM images of syntectonic granite are widespread (Figs. 3c, 3d).
Moreover, nanoparticles are widespread and their complexes present various structures. By analyzing and classifying the morphological characteristics, spherical nanoparticles (Cai et al., 2019; Wang et al., 2018; Sun et al., 2009), rod-like nanograins, spherical nano-aggregates (Cai et al., 2019; Shen et al., 2016; Yuan et al., 2014; Sun et al., 2008a), and rod-like nano-aggregates are identified. The spherical nanoparticles and rod-like nanograins can be further divided into three types: linear, planar and stereo nanoparticles. Linear spherical nanoparticles are very uniform and arranged in lines (Fig. 4a). They are scattered across the mineral surface and typically 50–110 nm in diameter (Fig. 4a). During the sliding, more linear spherical nanoparticles are tightly packed and planar distribution, developing the planar spherical nanoparticles (Fig. 4b). Planar spherical nanoparticles are arranged without orientation. Compared with linear spherical nanoparticles, planar spherical nanoparticles are more uniform in shape, and have better roundness and sphericity with 10–50 nm in diameter. The alienation spherical nanoparticles become changing bodies in rock shear planes, forming stereo spherical nanoparticles, which are closely aligned to each other (Fig. 4c), forming some mosaic textures (Sun et al., 2008a). The alienated, nano-sized grinding grains show irregular sizes and shapes, including round nanoparticles with tens of nano-meters in diameter, columnar nanoparticles with 100–200 nm in size (Fig. 4c). As for rod-like nanograins, they are typically linearly aligned and parallel to each other in the initial stage. The linear rod-like nanograins grow with the long axis over short axis ratio mainly between 5 and 10 (Fig. 4d). With increasing slip velocity, more linear rod-like nanoparticles are connected to each other and planar distribution, developing the planar rod-like nanoparticles (Fig. 4e). In contrast, the long axis over short axis ratio ranges from 10 to 20 and they are obvious elongated. The dense rod-like nanoparticles form the smoother surface. Based on the observation of the SEM images in the shear planes of the rocks, it is revealed that the nano-sized particle distribution in rock shear planes is characterized by layering texture (Sun et al., 2008a, b, 2005).We also found that the layering planar rod-like nano-layers are superimposed on each other to form stereo rod-like nanoparticles and aligned parallel or subparallel to the shear direction (Fig. 4f). Both the planar and stereo rod-like nanoparticles exhibit preferred orientation, which is likely subjected to the stress.
Figure 4. Scanning electron microscopy (SEM) of the spherical nanoparticles and rod-like nanograins in the samples. (a) Linear spherical nanoparticles; (b) planar spherical nanoparticles; (c) stereo spherical nanoparticles; (d) linear rod-like nanoparticles; (e) planar rod-like nanoparticles; (f) stereo rod-like nanoparticles.
During the process of the fault sliding, individual grains cluster into composed ones which are typically 200–400 nm in size (Sun et al., 2009, 2008a). Under shearing stress, most of single nanograins can concentrate into compound grains, resulting in growth and agglomeration of the spherical nanoparticles (Sun et al., 2013). The spherical nanoparticles are thus transformed into the nano-aggregates, presenting complex morphological characteristics. The spherical nano-aggregates are typically 100–500 nm in size and assume diverse shapes, including a "flower" shape (Fig. 5a), cohesive nanoparticles (Fig. 5b), "strawberry" shape and so on. These characteristics may indicate that the spherical nano-aggregates have experienced growth due to shear heating (Cai et al., 2019). As for rod-like nanograins, the monomer grains show a relatively preferred orientation (Fig. 5c). Their agglomerated nanoparticles clutter in micropores, which are predominantly 500–800 nm in diameter (Fig. 5d).
The formation of densely packed spherical nanoparticles on the sliding surface at high velocity are related to thermal decomposition (Hu et al., 2019; Smith et al., 2013; Han et al., 2007) and Fault Mirrors (FMs) (Siman-Tov et al., 2015). Some dome nanoparticles were formed when the thermal decomposition (decarbonation) developed sufficient pressure to deform the crystal lattice (Hu et al., 2019). Ultramicroscopic analyses found that FMs formed in high slip velocity (> 0.07 m/s) experiments are highly smooth and composed of densely packed, sintered, thin (~1 micron) layers of nanograins (Siman-Tov et al., 2015, 2013). The FMs surface is produced by wear and smoothing (Bifano et al., 1991), which is the super-plasticity ductile processes (Green et al., 2015). However, the nano-sized particle layer that covers slip surfaces at low slip velocity (v=0.01 m/s) experiment is porous and rough (Siman-Tov et al., 2015; Tisato et al., 2012). Below the FMs, there is a more porous nanoparticles layer (Siman-Tov et al., 2015).
On the fresh sections, it's easy to find a series of naturally polished patches (FMs) composed by numerous nanograins coating on the surface of the minerals (Figs. 6b–6d). The polished patches vary from several to dozens of micrometers in size, presenting irregular shape (Fig. 6b). These polished patches are optically smooth and their measured roughness is lower than 100 nm at scales < 550 nm (Siman-Tov et al., 2013). The upper most grains of nanograin layer form the smooth polished patches form at high slip velocities (> 0.07 m/s) (Siman-Tov et al., 2015). The outer most part of nanograin layer, 30–180 nm in thickness, consists of densely packed nanograins, having low porosity (Figs. 6c, 6d). SEM images of surfaces of polished patches reveal smooth topography and are mostly composed of tightly packed spherical nanoparticles and rod-like nanograins (Figs. 6b, 6c, 6d). The densely packed spherical nanograins developed with mean grain diameter of 40±10 nm (Fig. 6c). The rod-like nanograins formed by sintered linear chains of nanograins are densely packed and well aligned (Fig. 6d). Both densely packed spherical nanograins and rod-like nanograins forming the smooth polished patches are the products of high velocity slipping. Based on the sliding surface dotted with raised hemispherical domes (Fig. 6a), we think such domes indicate the evidence of calcite thermal decomposition (decarbonation). Damaged sites of the rough surface provide "windows" into the interior, exposing the porous structure of the nanograins inside (Figs. 6e, 6f). A series of holes, gouges with different sizes and parallel "scratches" subjected to slow slip often dissect the surfaces (Figs. 6e, 6f). SEM images show that the nanograin layer is porous with multiple discontinuities and fractures, which is mainly composed of porous rod-like and spherical nano-aggregates. The nanograin layers, 200–300 nm in thickness (Figs. 6e), cover damaged rock crystals. The rod-like nanograins randomly distributed with mean grain length of 220±30 nm are interconnected to form the rod-like nano-aggregates (Fig. 6f). The brittleness at low velocity is evidenced by microcracks and holes dissecting the surfaces (Figs. 6e, 6f), suggesting that porous nano-aggregates deform brittlely at a low velocity.
Figure 6. SEM images show that nanoparticles cover the slip surfaces. (a) Dome nanoparticles related to the calcite thermal decomposition (decarbonation); (c) flat and smooth FMs comprise densely packed, sintered, rounded spherical nanoparticles, as seen above the dashed line; (b), (d) rod-like nanograins formed by sintered linear chains of nanograins are densely packed and show a very well preferred orientation on the FMs surfaces; (e) the surface is porous and rough, showing scratches and holes (yellow dashed line) crosscutting the surface; (f) a series of holes with different sizes and parallel arrangement scratches dissect the slip surfaces.
Microscopically, the shear fracture experiments at high temperature and pressure show that the materials in the fracture zones are more fragmented than the surrounding materials (Cai et al., 2019). Spherical nanoparticles and their nano-aggregates developed only within the fracture zones, but not beyond (Cai et al., 2019). The similar microscopic nanoparticle structures are also observed in Xiaomei shear zone. The red lines represent the fractures, within which the nanoparticles are developed (Figs. 7a–7d). The fractures are thin and straight, showing a relatively preferred orientation. The width of the fractures ranges from tens to hundreds of nanometers. These parallel fractures are formed during shearing and control the alignment of nanoparticles.
Figure 7. The red dotted lines represent the fractures, within which the nanoparticles are developed. (a) Spherical nanograins are formed in straight and thin fractures; (b) spherical nano-aggregates developed in straight fracture zones show a preferred orientation; (c), (d) rod-like nanograins are distributed between the fractures.
Energy dispersive spectrometer can provide information about the mineral phases through elemental composition (Bankole et al., 2019). From the energy spectrum analyses of 20 nanoparticles with different shapes and sizes, the calculated analytical data are acquired (see Table 1). The nanoparticles are rich in SiO2, Al2O3, followed by Albite, Fe, Wollastonite, MAD-10 Feldspar, MgO and CaCO3. According to the geochemical characteristics of fracture structure, the elemental Si is easily stabilized in the slip zones (Kisters et al., 2000). The concentration of elemental Al in the nanoparticles may be related to mica (Sun et al., 2009). Chemical analyses and diffraction patterns indicate that nanoparticles are composed of calcite surrounded by a matrix mainly comprising of silicon and aluminum (Siman-Tov et al., 2013). Representative XRD analyses show that the bulk of the nanoparticles from the slide surface contain unaltered calcite, dolomite, quartz, and talc as the major minerals (Hu et al., 2018). We suggest that the nanoparticles are possibly composed of silicates. It is possible that carbonate also plays an important role in the formation of the nanoparticles, as many nanoparticles are found in carbonate rocks (de Paola et al., 2015; Smith et al., 2015). Furthermore, Fe, MgO and wollastonite are only found in the rod-like nanograins and their aggregations. The proportion of SiO2 ranges from 76.71 wt.% to 100 wt.% in the spherical nanoparticles and their aggregations except that one of the spherical nanoparticle aggregations is composed of SiO2 (23.77 wt.%) and CaCO3 (76.23 wt.%). However, the proportion of the SiO2 in the rod-like nanograins and their aggregations is lower, ranging from 41.08 wt.% to 76.84 wt.%. In general, it can be suggested that the materials rich in Fe, MgO and Wollastonite are more likely to form the rod-like nanograins, while the materials rich in SiO2 are more likely toform the spherical nanoparticles.
No. Samples Percentage SiO2 Al2O3 Albite Fe MAD-10 feldspar Wollastonite CaCO3 MgO 1 Rod-like nanograins Weight 61.28 6.66 0 22.44 6.98 0 0 2.64 2 Rod-like nanograins Weight 76.84 10.33 0 12.82 0 0 0 0 3 Rod-like nanograins Weight 59.89 12.24 0 12.92 0 11.32 0 3.62 4 Rod-like nanograins aggregations Weight 68.24 6.37 3.41 21.97 0 0 0 0 5 Rod-like nanograins Weight 62.66 2.52 0 24.1 0 5.49 0 5.23 6 Rod-like nanograins Weight 68.25 6.37 3.41 21.97 0 0 0 0 7 Rod-like nanograins Weight 74.08 16.57 0 9.35 0 0 0 0 8 Rod-like nanograins Weight 57.55 3.35 0 28.63 0 5.12 0 5.36 9 Rod-like nanograins
Weight 59.9 7.82 0 29.68 0 0 0 2.60 10 Rod-like nanograins Weight 41.08 1.14 0 57.78 0 0 0 0 11 Spherical nanoparticles
Weight 89.83 10.17 0 0 0 0 0 0 12 Spherical nanoparticles
Weight 80.64 12.95 6.41 0 0 0 0 0 13 Spherical nanoparticles Weight 77.06 13.16 1.86 0 7.92 0 0 0 14 Spherical nanoparticles
Weight 80.64 7.71 0 0 11.66 0 0 0 15 Spherical nanoparticles
Weight 76.71 15.20 2.67 0 5.42 0 0 0 16 Spherical nanoparticles Weight 82.22 8.28 0 0 9.5 0 0 0 17 Spherical nanoparticles Weight 100 0 0 0 0 0 0 0 18 Spherical nanoparticles Weight 82.72 10.93 6.36 0 0 0 0 0 19 Spherical nanoparticles
Weight 23.77 0 0 0 0 0 76.23 0 20 Spherical nanoparticles
Weight 84.6 8.92 6.48 0 0 0 0 0
Table 1. Energy dispersive spectrometer (EDS) test of the nanograins from the Xiaomei shear zone