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Volume 31 Issue 5
Oct.  2020
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Yang Zhou, Baoyun Shen, Yi Yan, Hailing Liu, Yan Yan. Nanoparticles Study on the Indosinian Xiaomei Shear Zone in the Hainan Island, China: Implication to Developmental Stage and Formation Mechanism of Nanoparticles in a Fault Zone. Journal of Earth Science, 2020, 31(5): 957-967. doi: 10.1007/s12583-020-1286-x
Citation: Yang Zhou, Baoyun Shen, Yi Yan, Hailing Liu, Yan Yan. Nanoparticles Study on the Indosinian Xiaomei Shear Zone in the Hainan Island, China: Implication to Developmental Stage and Formation Mechanism of Nanoparticles in a Fault Zone. Journal of Earth Science, 2020, 31(5): 957-967. doi: 10.1007/s12583-020-1286-x

Nanoparticles Study on the Indosinian Xiaomei Shear Zone in the Hainan Island, China: Implication to Developmental Stage and Formation Mechanism of Nanoparticles in a Fault Zone

doi: 10.1007/s12583-020-1286-x
More Information
  • Nanoparticles are widely observed in the natural shear zone and experimental slip faults, which can lubricate the fault and significantly reduce the friction coefficient during seismic slip. But it is still not clear how the nanoparticles develop during the process of sliding. Clarifying the development stage of nanoparticles in a fault zone is critical to understanding the formation mechanisms of nanoparticles and the mechanism of fault weakening from a nanoperspective. In this study, four types of nanoparticles were found in the Indosinian Xiaomei shear zone, including spherical nanoparticles, rod-like nanograins and their aggregations. Ultramicroscopic analyses indicate that polished patches are highly smooth and composed of tightly packed spherical nanoparticles and well orientated rod-like nanograins during slip at high velocities. Meanwhile, the dome nanoparticles were formed by the calcite thermal decomposition due to frictional heat during high-speed sliding. The polygonal grooves are possibly related to high temperature (>900¦) grain boundary sliding deformation mechanisms. However, the porous and rough surfaces are accompanied by a series of holes and parallel "scratches" during a subsequent low-velocity stage. To ascertain the chemical composition of these nanoparticles, the energy dispersive spectrometer (EDS) test were conducted. The results suggest that materials rich in Fe, MgO and wollastonite are likely to form the rod-like nanograins, while materials rich in SiO2 are likely to form the spherical nanoparticles.
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Nanoparticles Study on the Indosinian Xiaomei Shear Zone in the Hainan Island, China: Implication to Developmental Stage and Formation Mechanism of Nanoparticles in a Fault Zone

doi: 10.1007/s12583-020-1286-x
    Corresponding author: Hailing Liu, ORCID:0000-0002-8279-7441, liuh82@126.com

Abstract: Nanoparticles are widely observed in the natural shear zone and experimental slip faults, which can lubricate the fault and significantly reduce the friction coefficient during seismic slip. But it is still not clear how the nanoparticles develop during the process of sliding. Clarifying the development stage of nanoparticles in a fault zone is critical to understanding the formation mechanisms of nanoparticles and the mechanism of fault weakening from a nanoperspective. In this study, four types of nanoparticles were found in the Indosinian Xiaomei shear zone, including spherical nanoparticles, rod-like nanograins and their aggregations. Ultramicroscopic analyses indicate that polished patches are highly smooth and composed of tightly packed spherical nanoparticles and well orientated rod-like nanograins during slip at high velocities. Meanwhile, the dome nanoparticles were formed by the calcite thermal decomposition due to frictional heat during high-speed sliding. The polygonal grooves are possibly related to high temperature (>900¦) grain boundary sliding deformation mechanisms. However, the porous and rough surfaces are accompanied by a series of holes and parallel "scratches" during a subsequent low-velocity stage. To ascertain the chemical composition of these nanoparticles, the energy dispersive spectrometer (EDS) test were conducted. The results suggest that materials rich in Fe, MgO and wollastonite are likely to form the rod-like nanograins, while materials rich in SiO2 are likely to form the spherical nanoparticles.

Yang Zhou, Baoyun Shen, Yi Yan, Hailing Liu, Yan Yan. Nanoparticles Study on the Indosinian Xiaomei Shear Zone in the Hainan Island, China: Implication to Developmental Stage and Formation Mechanism of Nanoparticles in a Fault Zone. Journal of Earth Science, 2020, 31(5): 957-967. doi: 10.1007/s12583-020-1286-x
Citation: Yang Zhou, Baoyun Shen, Yi Yan, Hailing Liu, Yan Yan. Nanoparticles Study on the Indosinian Xiaomei Shear Zone in the Hainan Island, China: Implication to Developmental Stage and Formation Mechanism of Nanoparticles in a Fault Zone. Journal of Earth Science, 2020, 31(5): 957-967. doi: 10.1007/s12583-020-1286-x
  • Earthquake instability requires dynamic and relatively small weakening of the fault during slip. The mechanism of fault weakening is the key to understanding earthquake sliding. Microstructural examination of experimental sliding surfaces shows that nanoparticles (microscopic particles with at least one dimension < 100 nm) have been sufficient to trigger fault lubrication in the slip zones (Green et al., 2015; di Toro et al., 2011; Han et al., 2011). The formation condition of nanoparticles varies quite differently based on observations of natural and experimental faults in the slipping zones. As temperature rises due to frictional heating, dynamically recrystallized calcite accompanied by thermal decomposition (Hu et al., 2019, 2018; Smith et al., 2013) plays a key role in producing nanoscale subgrains in the slip zone. Nanoscale grains are thermally unstable and show a trend of grain coarsening (Koch et al., 2008; Koch, 2007). The grain-coarsening temperature decreases for smaller grains, in particular those below 100 nm (Lu, 2016). Temperature experiments in granitic migmatites and dolomites show that nanoparticles can also be developed at room temperature (Cai et al., 2019). At low temperatures (room temperature), nanoparticles produced are small, about 10 to 50 nm in size. At high temperatures (> 500 ℃), they are more compacted to form the nano-aggregates, with a diameter ranging from 100 to 400 nm (Cai et al., 2019). Nanograins can be formed during both low and high velocity experiments, but after slip at low velocities, the nanograin surface is porous and rough, while it is flat and smooth after slip at high velocity (Siman-Tov et al., 2015). According to confining pressure experiments, a high enough stress and strain are necessary for the formation of nanoparticles and the higher the differential stresses are, the more abundant the nanoparticles developed in the fracture zones are (Cai et al., 2018). Overall, two types of nanoparticles have been observed in the fault zones (Siman-Tov et al., 2015, 2013; Fondriest et al., 2013; Sun et al., 2008a; Chester et al., 1993), including the spherical nanoparticles and their nano-aggregates (Cai et al., 2018; Sun et al., 2013), representing different formation environments.

    Complicated temperature, slipping velocity and stress conditions make a way to form nanoparticles with different morphological characteristics in the experiments, which provide a potentially promising indicator for studying the structure and development process of the nanoparticles. In this study, we focus on the nanoparticles characteristics of the natural fault zones. Based on scanning electron microscope (SEM) observations and energy dispersive spectrometer (EDS) test of rock samples collected in the Xiaomei shear zone of Hainan Island, this paper presents an in-depth analysis of the formation process and spatial distribution patterns regarding nanoparticles, shedding light on possible formation mechanisms of nanoparticles in a fault zone.

  • Hainan Island in the south of China is derived from the northern margin of Gondwanaland and had undergone multiphased structural overprinting and complex tectonic evolution (Gou et al., 2019; Metcalfe, 2017). Based on aeromagnetic data (Liu et al., 2006; Guangdong BGMR, 1988), two major fault systems divide the island into several tectonic blocks. One is an E-W-trending belt, and the other is a NNE-trending fault (Xu et al., 2008; Li et al., 2002; Metcalfe et al., 1994). Four large E-W-trending faults, the Wangwu-Wenjiao, Changjiang-Qionghai, Jianfeng-Diaoluo and Jiusuo-Lingshui faults, distributed from north to south (Fig. 1). The study area, NNE trending Xiaomei shear zone, located in the southeastern part of Hainan Island (Fig. 1), which is adjacent to the Jiusuo-Lingshui fault zone and considered to be part of the Jiusuo-Lingshui suture zone (Liu et al., 2017, 2006). U-Pb zircon dating yields an age of Early Triassic (252–251 Ma) for Xiaomei syntectonic granites (unpublished data), which are used to constrain the Indosinian shearing event (Schärer et al., 1994). So the nanoparticles developed in syntectonic granites are mainly affected by the Indosinian shearing movement.

    Figure 1.  Simplified geological map of Hainan Island (modified after Guangdong BGMR, 1988) showing sampling locations. The Ar-Ar ages of the high-strain shear zones are from Zhang et al. (2011).

    The NNE-trending Xiaomei shear zone is a strike-slip fault zone that extends more than 40 km with a width of 100–120 m. It comprises a series of individual shear zones (50–80 m wide and hundreds of meters long) and crops out discontinuously as tectonic slices. The shear zone is characterized by the development of syntectonic granites with feldspar and quartz elongated to form lineations (Fig. 2a). It suggests that the syntectonic granites are subjected to strong stress and experienced plastic deformation during magma crystallization (Xie, 2002). The kinematic indicators observed in the Xiaomei shear zone is characterized by outcrop-scale quartz veins, presenting S-shaped asymmetrical folds, echelon joint (Fig. 2d) and structural lenses (Fig. 2b). The asymmetric porphyroblasts developed with attenuated tails (Fig. 2a) and asymmetric fold (Fig. 2c) suggest predominant sinistral shearing (Xu et al., 2018). Polarized microscopic observations reveal that elongated feldspar and quartz porphyroclasts present undulose and inhomogeneous extinction. Fine mica grains occur in the tails of feldspar and quartz porphyroclasts (Fig. 3a), presenting gneissic texture (Fig. 3b). In general, the different scales of deformation features reflect sinistral shearing along the Xiaomei shear zone. In this study, we collected the syntectonic granites from the Xiaomei shear zone. The syntectonic granites are typically pale gray to white in color and contain plagioclase (~50%), potassium-feldspar (~25%), quartz (~20%), and minor biotite (3%) and hornblende (~2%).

    Figure 2.  Field photographs of the Xiaomei shear zone. (a) Asymmetric porphyroblasts with attenuated tails in syntectonic granites, the red dashed lines indicate the linear structure of the outcrops, which are caused by the orientation of the minerals experienced sinistral shearing; (b) felsic structural lenses in the syntectonic granites indicate sinistral shearing; (c) S-shaped asymmetrical folds of the quartz veins; (d) echelon joint.

    Figure 3.  (a) Photomicrographs of the syntectonic granites showing asymmetric porphyroblasts with recrystallized tails; (b) gneissic texture. The red dashed lines indicate the linear structure under the microscope; (c), (d) scanning electron microscopy (SEM) images of asymmetrical folds showed sinistral shearing deformation of the syntectonic granite. Qz. Quartz; Bt. biotite; Pl. plagioclase.

  • In order to study the formation process and spatial distribution patterns of nanoparticles in the shear zones, we used scanning electron microscopy (SEM) (Model of NOVA NANOSEM 450) coupled with the energy dispersive X-ray spectrometer (EDS) (Model OXFORD Inca Energy X-Max20) to depict the accurate characteristics of nanoparticles. The syntectonic granite samples were analyzed with a NOVA NANOSEM 450 scanning electron microscope (SEM) and dispersive X-ray spectrometer (EDS) (Model OXFORD Inca Energy X-Max20) at the State Key Laboratory of Nuclear Resources and Environment, East China University of Technology. Before measurements, the fresh rocks were cut into cubes of approximately 1 cm3. Then, the samples were wrapped with conductive glue, exposing the observed surface, which should be parallel to the lineation and perpendicular to the foliation. Subsequently, the gold-plating instrument of model Q150RS was used to spray gold onto the surface of the samples with a duration 100 or 150 s.

  • 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).

    Figure 5.  The characteristics of the nano-aggregates in the samples. (a) Flower-like nanoparticles; (b) cohesive nanoparticles; (c) rod-like nanograins show a relatively preferred orientation; (d) rod-like nanograins agglomerations clutter in micropores.

  • 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
    aggregations
    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
    aggregations
    Weight 89.83 10.17 0 0 0 0 0 0
    12 Spherical nanoparticles
    aggregations
    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
    aggregations
    Weight 80.64 7.71 0 0 11.66 0 0 0
    15 Spherical nanoparticles
    aggregations
    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
    aggregations
    Weight 23.77 0 0 0 0 0 76.23 0
    20 Spherical nanoparticles
    aggregations
    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

  • Nanoparticles are widely discovered in a fault zone, and their development are closely related to the fault sliding (de Paola et al., 2015; Siman-Tov et al., 2015; Sun et al., 2008b). Different mechanisms have been proposed to explain the formation of nanoparticles in a fault zone. The fragmentation process of the felsic granules is expected to form ~10 nm nanograins from the microparticles (Wibberley et al., 2005; Yund et al., 1990). Fracturing, abrasion and milling presumably generated the observed nanograins from the starting "gouge" (Verberne et al., 2014; Tisato et al., 2012; Shen et al., 1995). There is a paragenetic relationship between the development of shear frictional-viscous nano-particles and that of the layered ones in rock shear planes, likely due to fracturing, attrition, comminution and grinding (Sun et al., 2008a). The grains in the fracture zones are more fragmented than the surrounding materials, indicating the nanoparticles have experienced mechanical milling (Siman-Tov et al., 2013; Koch, 1997). The ultramicroscopy imaging of the FMs suggests that the FMs are formed from sintering nanograins during shearing process (Siman-Tov et al., 2015). FMs surfaces comprise of densely packed, sintered, rounded nanograins at high slip-velocity (Siman-Tov et al., 2015). In the displacement-controlled experiments performed on carbonate, intracrystalline plasticity mechanisms start to accommodate intragranular strain in the slip zone due to frictional heating, and play a key role in producing nanoscale subgrains (≤100 nm) (de Paola et al., 2015). The formation of nanograins in carbonate-bearing rocks is the result of a shock-like stress release associated with the migration of fast moving dislocations (Spagnuolo et al., 2015). According to low confining pressure experiments, nanoparticles are not observed in the fracture zone where the differential stress is lower than 300 MPa in granitic migmatites and dolomites (Cai et al., 2019). Experiments on simulated faults in Carrara marble demonstrated that particles of tens of nanometers in size can be produced by thermal decomposition (Han et al., 2007). Due to high-speed sliding, nanoparticles are created from calcite thermal decomposition accompanied by dynamic recrystallization (Hu et al., 2019, 2018; Smith et al., 2012).

    The SEM images in our study suggest that the formation of nanograins may be associated with collective effect of mechanical milling, grain boundary sliding deformation and thermal decomposition. Mechanical milling is often used in industrial processes to produce nanograins (Koch, 1997), producing numerous crystal defects that form nanoscale grain boundaries. Density of defects increases up to a point where the crystal lattice disintegrates or fracturing is facilitated (Siman-Tov et al., 2015, 2013; Collettini et al., 2014). Grain size reduction in fault zones is enabled by displacement of fragments causing further communition by abrasion and attrition (Keulen et al., 2007). We find that crystal defects are damaged regions accompanied by parallel arrangement grinding lines on the slip surface (Figs. 8a, 8b), consisting of polygonal grooves (Figs. 8a, 8d), crystallographic-preferred orientations domes (Fig. 8b), and densely packed granulated particles (Fig. 8c) with size between 50 and 300 nm. The grooves and granulated particles are likely integral and separated by the slip surfaces during the sliding process. SEM analyses of polygonal equiaxial grooves exhibit nearly 120° internal cusp angles (Fig. 8d). The granulated particles and polygonal grooves display no preferred elongation, no crystal preferred orientation, which are possibly related to high temperature (> 900 ℃) grain boundary sliding deformation mechanisms (de Paola et al., 2015).

    Figure 8.  The formation mechanism of nanoparticles. (a) Grinding lines are caused by wear and attrition; (b) the domes are elongated and developed with crystallographic-preferred orientations; (c) the ultrafine granulated particles occur in the surface of the rock, which integrated with rocks or minerals; (d) the granulated particles and polygonal grooves are possibly related to grain boundary sliding deformation mechanisms.

    The domes are weakly elongated and developed with well-defined shape and crystallographic-preferred orientations (Fig. 8b). Such microstructures are commonly observed in the principal slip surfaces during seismic slip rates ≥0.1 m/s (Hu et al., 2019; Smith et al., 2015, 2013). Thus, intense frictional heating along the principal slip surfaces caused the dynamic recrystallization accompanied by fault weakening and thermal decomposition of calcite. The dynamically recrystallized domes with strong shape-preferred orientation are formed by thermal decomposition of calcite.

  • Nanograins formation accompanied by fault sliding is a complex process. In the initial stage, mechanical milling with grinding lines plays an important role in producing nanoparticles. During the next fault sliding process, the intense frictional heating along the slip surfaces caused the thermal decomposition of calcite thermal decomposition of calcite, producing elongated and crystallographic-preferred orientations domes. When the frictional heating temperature is over 900 ℃, grain boundary sliding deformation mechanisms trigger polygonal equiaxial grooves and dynamic weakening of faults.

    Considering the morphological characteristics of nanoparticles in three-dimensional space, the formation of nanoparticles at high slip rates is believed to have experienced three stages: (1) the linear nanoparticles, (2) planar nanoparticles, (3) stereo nanoparticles. In the first stage, the linear nanoparticles are very uniform and arranged in a line under a high differential stress and strain. In the second stage, more nanoparticles are developed into tightly packed planar nanoparticles which are arranged in disorder, without obvious orientation. In the third stage, planar nanoparticles become changing bodies in rock shear planes, forming stereo nanoparticles with mosaic textures.

    At the subsequent low slipping velocity, friction coefficient remains high, fault surfaces are covered by brittle fracturing and cataclasis during frictional sliding. The brittleness at low velocities is evidenced by microcracks and holes dissecting the surfaces. The porous nanograins are randomly distributed in the fracture zones. The strain weakening stage produces the nano-sized grain rheology (Sun et al., 2013).

  • (1) SEM images show that spherical nanoparticles, rod-like nanograins and their nanograin aggregations are developed in the Xiaomei shear zone. The formation of nanoparticles is believed to have experienced three stages: (1) the linear nanoparticles, (2) planar nanoparticles, (3) stereo nanoparticles.

    (2) Based on energy dispersive spectrometer analyses, we found that the nanoparticles are rich in SiO2, Al2O3, followed by Albite, Fe, Wollastonite, MAD-10 Feldspar, MgO and CaCO3. Materials rich in Fe, MgO and Wollastonite are more likely to form the rod-like nanograins and their aggregations, while materials rich in SiO2 are more likely to form the spherical nanoparticles and their aggregations.

    (3) Nanograins formation is a complex process. The formation of nanograins is associated with mechanical milling, grain boundary sliding deformation and accompanied by calcite thermal decomposition.

  • This work was financially cosupported by the National Natural Science Foundation of China (Nos. 41776072, 41676048, U1701641, 91328205). The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1286-x.

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