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Geochemical and mineralogical characteristics of fault gouge in the Median Tectonic Line, Japan: evidence for earthquake slip
© Ishikawa et al.; licensee Springer. 2014
- Received: 13 November 2013
- Accepted: 30 April 2014
- Published: 20 May 2014
We carried out geochemical and mineralogical analyses on fault-zone rocks from the Anko section of the Median Tectonic Line in Nagano Prefecture, Japan, to investigate coseismic physicochemical processes in the fault zone. The latest fault zone in the Anko section contains cataclasite, fault breccia, and fault gouge of granitic composition, and brecciated basic schist. Protoliths of the granitic composition are from the Ryoke metamorphic belt and those of the basic schist from the Sambagawa metamorphic belt. X-ray diffraction analyses show a selective decrease of clay minerals coupled with an increase of amorphous phase in an intensely deformed layer of black gouge (5- to 10-cm thick). SEM observation reveals that the black gouge is characterized by a drastic reduction of grain size and abundant ultrafine particles of submicrometer to several tens of nanometers with well-rounded spheroidal shapes. These observations for the black gouge are indicative of strong mineral lattice distortion and granulation associated with earthquake slip. Geochemically, the black gouge is characterized by distinctly higher Li content and 87Sr/86Sr isotope ratio than surrounding cataclasites, breccias, and gouges, which have similar major element compositions. Model analysis reveals that the trace element composition of the black gouge is consistent with high-temperature (up to 250°C) coseismic fluid-rock interactions. Thermal and kinetic constraints indicate that there have been repeated slips on the fault at moderate depths (e.g., 600 m), although the tectonic process by which the fault zone has been uplifted and exposed in this area is not well understood.
- Active fault
- Frictional heat
- Fault gouge
- Trace elements
- Isotope ratios
Transient frictional heating in a fault zone during earthquake slip affects the slip behavior itself. Increased temperature on a fault can induce dynamic fault weakening by processes such as pressurization of interstitial fluid by thermal expansion, known as thermal pressurization (Sibson 1973; Lachenbruch 1980; Andrews 2002), and melt lubrication (Sibson 1975; Fialko and Khazan 2005). Geochemical and mineralogical analyses of fault rocks combined with geological, structural, and geophysical observations can provide useful means for elucidating such slip-weakening processes. In the case of the Chelungpu fault in Taiwan, which slipped during the 1999 Mw 7.6 Chi-Chi earthquake (Ma et al. 1999), the slip-zone rocks show marked anomalies in both fluid-mobile trace element concentrations (Sr, Cs, Rb, and Li) and Sr isotope ratios that are consistent with fluid-rock interactions at >350°C (Ishikawa et al. 2008). Thermal signals recorded in the slip-zone minerals (Mishima et al. 2006; Hirono et al. 2007) and the low permeability of the fault zone (Doan et al. 2006; Tanikawa et al. 2009) strongly suggest that thermal pressurization took place during the earthquake (Ishikawa et al. 2008). Geochemical analyses of the slip-zone rocks accompanied with generation of pseudotachylite in an ancient megasplay fault in the Shimanto accretionary complex (Mukoyoshi et al. 2006) are also indicative of fluid-rock interactions at >350°C (Honda et al. 2011). These observations provide a key for gaining a detailed understanding of slip processes in the presence of pore fluids.
However, there have been few studies of coseismic geochemical and mineralogical processes in fault zones that have investigated both the geochemistry of high-temperature fluid-rock interactions and structural observations. Three of the previously studied faults are the Chelungpu fault (Ishikawa et al. 2008), the major thrust of the Boso accretionary complex, Japan (Hamada et al. 2011), and the ancient megasplay fault of the Shimanto accretionary complex, Japan (Honda et al. 2011). Lithology, stress, temperature conditions, maturation (repeated activity and slip accumulation) and tectonic history may influence which coseismic physicochemical processes occur in slip zones, and how they occur. The aforementioned three faults are within sedimentary rocks (mudstone and sandstone) and are considered to be branch faults from plate-subduction megathrusts or major thrusts within the accretionary prism near a megathrust (Ishikawa et al. 2008; Hamada et al. 2011; Honda et al. 2011).
In this study, we focused on a fault from the Anko section of the Median Tectonic Line (MTL) in Nagano Prefecture, Japan, an inland fault in rocks of different lithology from the previous studies. We examined the structural, geochemical, and mineralogical characteristics of the fault zone and surrounding rocks to assess the relationship of coseismic fluid-rock interactions and other physicochemical processes in the slip zone with earthquake slip.
Geological background and outcrop of the MTL at the Anko section
For mineralogical and geochemical investigation, we collected 12 samples from the fault zone: two from the orange-stained cataclasite, three from the gray breccia, one from the black gouge, two from the gray-green gouge, and four from the brecciated greenschist (samples FZ01 to FZ12 in Figure 3). Additional three samples were collected from the black gouge zone (samples FZ06b to FZ06d; up to 2 m from FZ06 but beyond the extent of Figure 3) for trace element and isotope analyses. We also collected some oriented samples from the fault zone for microscopic observations.
To observe the microscopic structures and fabrics in the fault zone, we used polarization microscope and scanning electron microscope (SEM: JSM-6500 F, JEOL, Tokyo, Japan) operated at an acceleration voltage of 10 kV. Thin sections were prepared to be the plane perpendicular to the mesoscopic foliation and/or structural zone boundaries. The samples for the SEM observation were prepared by being dispersed in ethanol and then dropped on the observation stage.
X-ray diffraction analysis
X-ray diffraction (XRD) with monochromatized CuKα radiation (Spectris PANalytical, X'Pert PRO MPD, Almelo, the Netherlands) was used to analyze the mineralogy of the samples. Each sample was gently powdered in an agate mortar. Part of each original sample was mixed with α-alumina (25 wt.%) for use as an internal standard. These samples were then mounted on XRD glass holders, side-loaded to minimize preferred alignment of phyllosilicates. XRD patterns were obtained with the spectrometer operated at 45 kV, 40 mA, 0.004° (Δ2θ) step width with 0.25° divergence, anti-scattering slits, and a high-speed semiconductor array detector. For quantitative determination of the mineral composition of the samples containing α-alumina, we used the RockJock program of Eberl (2003). The XRD patterns were compared with integrated XRD patterns of standard minerals stored in the RockJock program, and the weight percent of the component minerals in the samples was determined.
The XRD patterns of some samples on a zero diffraction plate, made from a single silicon crystal, were obtained under the same operating conditions at around 20° to 30° 2θ for evaluation of a broad bump caused by amorphous state.
The clay fraction (<2 μm) was separated by centrifugation from each of the bulk-powdered samples (without α-alumina). Oriented samples of the clay fraction were prepared by sedimentation onto glass slides in dry air. XRD patterns were obtained using the spectrometer operated under the same conditions as for the internal standard determinations. XRD analysis was also undertaken for the clay fraction saturated with ethylene glycol vapor. The relative abundances of kaolinite, smectite, illite, and chlorite were calculated by the method of Biscaye (1965).
Major and trace elements and Sr isotope analyses
Major and trace element concentrations and Sr isotope ratio of the fault gouges and surrounding rocks
Microscopic structures and fabrics
The microscopic structures and fabrics of oriented samples from the fault zone were examined in thin section.
In contrast, the black gouge exhibits highly intense foliation, indicated by the preferred orientation of grains (submicron to several tens of microns) of fragmented quartz, K-feldspar, plagioclase, carbonate minerals (veins), and clay minerals (Figure 4c,d). Anastomosing black seams, probably chlorite, are developed parallel to the foliation.
The gray-green gouge is composed of abundant fragmented grains of quartz, K-feldspar, and plagioclase accompanied by carbonate veins and a matrix of very fine-grained clay minerals (Figure 4e).
The brecciated greenschist shows mylonitic foliation, which is composed of amphibole porphyroclasts with tails and fish of clay minerals (Figure 4f). Very fine grains of serpentinite are locally observed in the brecciated greenschist.
Mineral compositions from X-ray diffraction data
RockJock's bulk mineral composition
Biscaye's relative clay mineral composition
The orange-stained cataclasite (FZ01 and FZ02) consists mainly of quartz, illite, and kaolinite, with small amounts of carbonate minerals (siderite and ankerite). Clay minerals are predominantly illite. The gray breccia (FZ03 to FZ05) also consists of mainly of quartz, illite, and kaolinite with small amounts of siderite and ankerite. Illite is the dominant clay mineral.
The black gouge (FZ06) consists of quartz, plagioclase, K-feldspar, chlorite, illite, and ankerite. XRD intensities for the clay fraction are relatively weak. The total mineral abundances (71.8%) estimated by the RockJock program are markedly lower in the black gouge than in the neighboring fault zone material.
The green-gray gouge (FZ07 and FZ08) also consists of quartz, plagioclase, chlorite, and illite with small amounts of ankerite and siderite. K-feldspar, kaolinite, and smectite were detected in only sample FZ08.
The brecciated greenschist (FZ09 to FZ12) consists mainly of amphibole and chlorite. Antigorite and lizardite were identified only in clasts incorporated in the schist (FZ09). Clay minerals in samples FZ09 to FZ12 are rich in chlorite.
Geochemical characteristics and identification of protoliths
Major and trace element compositions and Sr isotope data obtained in this study are summarized in Table 1. Samples FZ01 to FZ08 (orange-stained cataclasite, gray breccia, black gouge, and gray-green gouge) show relatively high SiO2 and Al2O3 concentrations (about 63 to 71 and 15 to 18 wt.%, respectively), whereas samples FZ09 to FZ12 (brecciated greenschist) have low SiO2 concentrations and high Fe2O3 and MgO concentrations (about 42 to 47, 13 to 15, and 15 to 34 wt.%, respectively). Similar compositional contrasts between samples FZ01 to FZ08 and samples FZ09 to FZ12 are also apparent in other major and trace element concentrations (K2O, Cr, Ni, Rb, Cs, Ba, Pb, and Th) and in Sr isotope ratios (Table 1).
In contrast, samples FZ09 to FZ12 are only moderately enriched in LREE. For these samples, REE patterns from La to Sm are flat or slightly depleted in La and Ce. REE concentrations of sample FZ09 are markedly lower than all other samples. These REE patterns (apart from sample FZ09) are consistent with those of normal or enriched mid-ocean ridge basalts (MORB) and MORB-type greenschists from the Sambagawa belt (Honda et al. 1986; Figure 8b). The low SiO2 and high MgO, Fe2O3, Cr, and Ni concentrations of these samples are also in accord with a basaltic origin. These observations suggest that samples FZ09 to FZ12 originated from accreted oceanic basalts. The extremely low REE concentrations of sample FZ09 can be attributed to entrainment of highly REE-depleted ultramafic clasts, which is supported by the high serpentinite content (Table 2) and very high MgO, Cr, and Ni concentrations in this sample.
The 87Sr/86Sr ratios of samples FZ01 to FZ08 (0.7070 to 0.7084) are consistent with those of Ryoke granites and gabbros reported previously (Kagami et al. 1985; Morioka et al. 2000; Okano et al. 2000; Kawamoto et al. 2013a). In contrast, samples FZ10 to FZ12 show distinctly low 87Sr/86Sr ratios (0.7032 to 0.7046), which accord with those of MORB (0.7021 to 0.7054; Hoffman 2003). The relatively high 87Sr/86Sr ratio (0.7066) obtained from sample FZ09 may reflect the involvement of an external source of Sr during serpentinization, or might be an aging effect. The high Rb/Sr ratio of this sample (0.0050, cf. 0.0015 to 0.0039 for samples FZ10 to FZ12) reflects a rapid increase with time of the 87Sr/86Sr ratio of the sample because of decay of 87Rb to 87Sr.
Thus, we concluded that the protoliths of samples FZ01 to FZ08 were the granitic rocks from the Ryoke belt and those of samples FZ09 to FZ12 were the greenschists of the Sambagawa belt.
Anomalous mineral assemblage of the black gouge
The total mineral abundance (72%) estimated by RockJock analysis of XRD data for black gouge sample FZ06 was much lower than those of the surrounding deformed rocks (83% to 109%; Table 2). This reflects the markedly lower total clay mineral abundance (22%) of the black gouge than that of the surrounding Ryoke belt rocks (35% to 43%; Table 2). The XRD pattern of the black gouge, obtained by using the zero diffraction plate, confirmed the presence of broad low bumps at around 20° to 30° 2θ indicating the existence of amorphous phase (Figure 7). Furthermore, large amount of well-rounded spheroidal ultrafine particles ranging in size from submicrometer to several tens of nanometers, were observed only in the black gouge (Figure 5). Because weak X-ray diffraction intensity of minerals could be attributed not only to lattice distortions but also to reductions of particle size to the submicrometer to nanometer range (e.g., Lönnberg and Lundström 1994), the ‘missing’ mineral content in the XRD analysis of black gouge sample FZ06 could be attributed to the observed ultrafine particles.
Such features resemble the characteristics of the latest slip zone within the Taiwan Chelungpu fault that slipped during the 1999 Chi-Chi earthquake (Hirono et al. 2014). The transformation of clay minerals (kaolinite and chlorite-ripidolite) into fully amorphous particles of several tens of nanometers in size has been confirmed by milling experiments (Vdovic et al. 2010), and such amorphous ultrafine particles have also been produced by friction experiments using quartzite, granite, and diorite (Yund et al. 1990; Hirose et al. 2012; Hirono et al. 2013). The Chelungpu fault comprises three dominant fault zones (Hirono et al. 20062008), and the shallowest fault zone is most likely the one that slipped during the Chi-Chi earthquake because recent heating and a major stress-orientation anomaly were observed in that zone (Kano et al. 2006; Wu et al. 2007). Although aforementioned low total mineral abundance and ultrafine particles were observed in the latest slip zone of the Chelungpu fault, almost 100 wt.% total weight percentages of mineral abundance and no ultrafine particles were observed in all samples from two deeper fault zones. Hirono et al. (2014) suggested that amount of time elapsed after the 1999 Chi-Chi earthquake at the time that the samples were collected may have been insufficient for recrystallization to occur. Therefore, large amounts of ultrafine particles with weak X-ray diffraction intensity could be characteristics of the slip zone associated with the recent earthquake.
Taken together, the observations of this study and the results of previous studies suggest that anomalously low total mineral and total clay mineral abundances observed in the black gouge could be arisen from comminution during the recent earthquake.
Anomalous elemental and isotope compositions of the black gouge
The black gouge samples (FZ06, FZ06b, FZ06c, and FZ06d) are indistinguishable from other Ryoke belt samples in terms of concentrations of Ti, Nb, Nd, Cs, Ba, La, Sm, and Pb (Figure 9g,h,i,j,k). However, the black gouge samples are enriched in Li, MgO, Cr, and Ni, have a higher 87Sr/86Sr ratio, and are depleted in Rb and Sr (Figure 9a,b,c,d,e,f,l). Mg, Cr, and Ni are relatively fluid-immobile elements that are generally abundant in mafic rocks. As described in the previous section, the Sambagawa belt samples (FZ09 to FZ12) originated from basalt (with ultramafic clasts) and are highly enriched in these elements (Table 1). Thus, it is possible that the higher MgO, Cr, and Ni concentrations in the black gouge samples are the result of entrainment of small amounts of Sambagawa belt rocks. Indeed, the concentrations of MgO, Cr, and Ni (and of Sr, Cs, Ba, La, Sm, Pb, Ti, Nb, and Nd) in the black gouge samples can be explained by a source comprising about 70% Ryoke (FZ04) and 30% Sambagawa (FZ10) rocks (Figure 9). The SiO2, Al2O3, and Fe2O3 contents of the mixed product (63.1, 14.6, and 7.2 wt.%, respectively) are also comparable to those of sample FZ06 (63.0, 14.5, and 7.7 wt.%, respectively). We concluded that the chemical composition of the black gouge from this outcrop has been modified by entrainment of Sambagawa belt rocks during deformation associated with MTL activity.
Even if the effect of source-rock mixing is taken into account, the concentrations of Li and Rb and the Sr isotope ratio of the black gouge samples remain anomalous (Figure 7). Li, Rb, and Sr are known to be fluid-mobile elements, and mobilization and isotope exchange of these elements by coseismic high-temperature fluid-rock interactions has been recognized in the slip zones of the faults of Chelungpu, Boso, and Shimanto (Ishikawa et al. 2008; Hamada et al. 2011; Honda et al. 2011).
The above chemical characteristics of the black gouges could be produced by preferential uptake or extraction of elements associated with fluid-rock interactions. The enrichments of Li and 87Sr/86Sr observed here for MTL black gouge samples contrast strikingly with depletions of Li and 87Sr/86Sr observed in the Chelungpu fault black gouges (Ishikawa et al. 2008). These differences imply that fluid-rock interactions occurred under considerably different conditions in the two regions.
Element distribution modeling for high-temperature fluid-rock interactions
from which we can calculate the trace element concentration of the solid after interaction with the fluid.
where (87Sr/86Sr)s0 and (87Sr/86Sr)f0 are the Sr isotope ratios in the solid and fluid, respectively, prior to solid-fluid interaction.
Parameters used for modeling of trace-element compositions and Sr isotope ratio of the black fault gouge
Our XRD analysis also shows that black gouge sample FZ06 contained K-feldspar and chlorite (Figure 6 and Table 1), which are typical secondary minerals formed by hydrothermal alteration. The development of black chlorite-bearing seams within the intense foliation in the black gouge indicate that chlorite was precipitated during or after shearing. Jefferies et al. (2006) also reported precipitation of fibrous chlorite within MTL cataclasites at the onset of grain-scale brittle deformation. These results and observations support the scenario for occurrence of high-temperature fluids in the black gouge inferred from geochemical data.
The temperatures of up to 250°C during fluid-rock interactions in the black gouge are unlikely to have been attained without earthquake-associated frictional heating or influx of a hot transient pulse of fluid, which could result from post-seismic discharge from deep rupture zones. The occurrence of fluid-induced geochemical anomaly is restricted within the black gouge zone that is characterized by a large amount of ultrafine particles and weak X-ray diffraction intensity. This strongly suggests that the high temperature resulted from earthquake slip. Although the fluid-rock interactions require some local flow or circulation of fluids, lack of extensive veins developed along the black gouge zone (Figure 3) is indicative of fluid-rock interactions induced by frictional heating, not by the hot transient pulse of fluid or hydrothermal alteration at the depths.
Kinetic evaluation of the geochemical signals recorded in the black gouge
where Cs/Cf is the ratio of a trace element concentration in the solid to that in the fluid, A/M is the ratio of solid surface area to fluid mass, k is the reaction rate, and t is time. Only limited data are available for k values associated with trace element equilibration. The reported k values for Sr isotope exchange during calcite recrystallization at 300°C, 350°C, and 400°C are 4.68 × 10−9, 2.82 × 10−8, and 1.32 × 10−7 mol m−2 s−1, respectively (Beck et al. 1992), and the extrapolated k value at 150°C is 1.62 × 10−12 mol m−2 s−1. We derived a value of 19 for Cs/Cf from the Cs0 and Cf0 values for Sr in Table 3, and estimated A/M by assuming A = 3.23 × 107 m2 m−3, the value determined by Ma et al. (2006) for fault gouge from the Chelungpu fault, and 10 MPa fluid pressure and 20% porosity.
Element partitioning within the fault zone may be controlled by complicated processes that reflect the interaction of fluids with multiple mineral phases, and it is not clear what F values we require to detect the geochemical signal derived from fluid-rock interactions. However, if the parameters we assumed here are valid, then 99%, 90%, and 50% equilibrium can be achieved at 250°C in 2.1 × 103, 1.1 × 103, and 3.3 × 102 s, respectively; at 200°C in 3.0 × 104, 1.6 × 104, and 4.9 × 103 s, respectively; and at 150°C in 7.8 × 105, 4.2 × 105, and 1.3 × 105 s, respectively. Thus, near- or quasi-equilibrium requires heat for a longer duration than can be provided by a single earthquake event, especially the durations required at temperatures of 200°C and 150°C. Although the temperature in a fault during a single earthquake event varies with time in response to frictional heating and heat conduction, because k decreases rapidly as cooling proceeds, it is likely that only the geochemical signals that represent the peak temperatures reached are recorded in the fault rocks.
It should be noted that the achievement of fluid-rock near-equilibrium in a short time period requires very large solid surface area. For instance, if we assume smaller A of 1.0 × 106 m2 m−3, which represents the value for very fine-grained sediments, it takes much longer time, 1.4 × 104 and 2.0 × 105 s at 250°C and 200°C, respectively, to achieve 90% equilibrium with the same parameters described above. This implies that fluid-rock interactions coupled with dynamic production of ultrafine solid grains as well as frictional heating during the earthquake slip are essential to satisfy the physicochemical condition under which the high-temperature geochemical signals as observed in the black gouge become detectable.
Earthquake slip along the MTL at the Anko section
The tectonic conditions under which temperatures of 150°C to 250°C are reached during earthquake slip can be simply constrained by the relationships among depth, slip displacement, and frictional coefficient in the fault gouge. Here, we assumed 2.70 g cm−3 bulk density, 1.20 J g−1 K−1 specific heat capacity, no thermal diffusion, 30°C km−1 geothermal gradient, 15°C ground-surface temperature, and hydrostatic confining stress (normal stress on the slip zone equal to overburden stress). The combinations of depth and slip displacement required to attain temperatures of 150°C, 200°C, and 250°C for friction coefficients 0.30 and 0.15 are shown in Figure 13d,e,f. For frictional heating of 200°C to 250°C, slip displacement of more than 15 m is required at shallow depths (e.g., 200 m), whereas a shorter displacement of <10 m would attain higher temperatures at greater depth (e.g., 600 m). Considering that other recent inland earthquakes have not had displacements as large as 15 m (e.g., 2008 Wenchuan earthquake, approximately 8 m, Yagi et al. 2012;; 1999 Chi-Chi earthquake, approximately 8 m, Shin and Teng 1999 Chi-Chi earthquake, approximately 8 m, Shin and Teng ; 1999 Chi-Chi earthquake, approximately 8 m, Shin and Teng 2001), past earthquakes in the Anko section of the MTL have probably been caused by a small number of relatively short displacements (<10 m) at moderate depths (e.g., 600 m). However, there is as yet no geological evidence to explain how fault zones viewed today in outcrop have been uplifted from such depths to the surface. It can be speculated that uplift at approximately 4 mm year−1 during the Quaternary in and around the Akaishi Mountain Range (Chichibu Belt in Figure 1) caused by the Izu-Honshu collision (e.g., Dambara 1971) might have affected the Anko section of the MTL.
On the other hand, it is possible that high-temperature fluids might have triggered thermal pressurization during earthquakes in the Anko section of the MTL. An experimental evaluation by Wibberley and Shimamoto (2005) of hydraulic properties of fault-zone rocks from the Tsukide section of the MTL (Mie Prefecture) suggested that thermal pressurization likely occurred in the fault gouge there. This mechanism might also have affected the most recently active fault zone in the Anko section, but there are insufficient data on the hydraulic properties for the black gouge and surrounding rocks there to evaluate this possibility.
Our field and microscopic observations, mineralogical and geochemical analyses, modeling of fluid-rock interactions, and kinetic evaluation of geochemical data have shown that the black gouge within the most recently active fault in the Anko section of the MTL has been subjected to selective distortion of crystalline structure and granulation in clay minerals and high-temperature (up to 250°C) fluid-rock interactions associated with earthquake slip.
Our results demonstrate that the analytical approach and modeling we used are useful for investigations of earthquake slip, not only for sediment-hosted thrusts at subduction boundaries (Ishikawa et al. 2008; Hamada et al. 2011; Honda et al. 2011), but also for active inland granite-hosted faults. We have shown that in faults that define geological boundaries, entrainment of different types of rocks (granite versus basalt in this study) in slip zones must be taken into account during analysis and modeling. Our modeling also indicates that Arima-type fluids from depth might have been involved in earthquake slip on the MTL.
However, to more accurately evaluate the geochemistry of coseismic fluid-rock interactions on the MTL, experimental data are needed on the hydrothermal properties of black gouge from the MTL. The remarkably low abundances of total minerals and total clay minerals from XRD analyses of the black gouge probably indicate that insufficient time has elapsed since the most recent earthquake event to allow neocrystallization of clay minerals to occur. Further studies are needed on the kinetics of neocrystallization of minerals after distortion and amorphization by frictional forces.
We thank J. Matsuoka and K. Nagaishi for their work on chemical analyses. We also thank two anonymous reviewers and Editor C. Rowe for their constructive comments. This work was supported by Grants-in Aid for Science Research from MEXT and JSPS.
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