Article

Internal structure of the Median Tectonic Line fault zone, SW Japan, revealed by borehole analysis

April 2012
Tectonophysics s 532–535:103–118

April 2012
s 532–535:103–118

DOI:10.1016/j.tecto.2012.01.024

Authors:

Norio Shigematsu

National Institute of Advanced Industrial Science and Technology

Koichiro Fujimoto

Tokyo Gakugei University

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The internal structure of the Median Tectonic Line (MTL) fault zone and the processes that prevailed at depths are described based on an analysis of a borehole. The fault plane which defines the boundary between the Ryoke- and Sanbagawa-derived rocks dips at 56° to the north. Immediately beneath the boundary, approximately 40 m thick fractured rocks form the major strand of the MTL fault zone. The hanging wall above the boundary comprises variably deformed Ryoke granitoids, including several mylonite zones and cataclasite zones. The fault zone has evolved through a series of faulting events under temperatures ranging from 400 to 200 °C. The mineral assemblages of the mylonites and cataclasites immediately above the boundary indicate that these fault rocks were formed at temperatures of about 300 °C. These mylonites and cataclasites represent, therefore, fault rocks that formed immediately below and above the brittle–plastic transition, respectively. Development of dissolution seams in these cataclasites suggests that the cataclasite has low strength. The presence of pseudotachylytes in the cataclasite indicates the occurrence of seismicity immediately above the brittle–plastic transition. On the other hand, the very fine grain size of recrystallised quartz in the mylonites indicates high differential stress immediately below the brittle–plastic transition. It is therefore likely that the differential stresses immediately below the brittle–plastic transition are much higher than those immediately above the transition. Formation of laumontite in the major strand of the MTL fault zone occurred at temperatures of around 200 °C. The central slip zone of the major strand is about 30-cm thick, and is surrounded by thick gouge zones. This situation is favourable for thermal pressurisation during earthquake slips.

Quantitative logging data clustering with hidden Markov model to assist log unit classification

Article

Full-text available

Jun 2022
EARTH PLANETS SPACE

Revealing subsurface structures is a fundamental task in geophysical and geological studies. Logging data are usually acquired through drilling projects, which constrain the subsurface structure, and together with the description of drill core samples, are used to distinguish geological units. Clustering is useful for interpreting logging data and making log unit classification and is usually performed by manual inspection of the data. However, the validity of clustering results with such subjective criteria may be questionable. This study proposed the application of a statistical clustering method, the hidden Markov model, to conduct unsupervised clustering of logging data. As logging data are aligned along the drilled hole, they and the geological structure hidden behind such sequential datasets can be regarded as observables and hidden states in the hidden Markov model. When log unit classification is manually conducted, depth dependency of logging data is usually focused. Therefore, we included depth information as observables to explicitly represent depth dependency of logging data. The model was applied to the following geological settings: the accretionary prism at the Nankai Trough, the onshore fault zone at the Kii Peninsula (southwest Japan), and the forearc basin at the Japan Trench. The optimum number of clusters were searched using a quantitative index. The clustering results using the hidden Markov model were consistent with previously reported classifications or lithological descriptions; however, our method allowed a more detailed division of logging data, which is useful to interpret geological structures, such as a fault or a fault zone. Therefore, the use of the hidden Markov model enabled us to clarify assumptions quantitatively and conduct clustering consistently for the entire depth range, even for different geological sites. The proposed method is expected to have wider applicability and extensibility for other types of data, including geochemical and structural geological data.

Low-effective fault strength of a blind detachment beneath the Indo-Burmese Arc (NE-India) induced by frictional–viscous flow

Article

Mar 2022

We present evidence for frictional–viscous flow (or steady creep) behaviour of the blind detachment below the Indo-Burmese wedge (IBW), as characterised by the fault gouge exposed along the Churachandpur–Mao Fault, which is one of the splay faults to the blind detachment. The petrography of the exhumed phyllonitic gouge material (considered the wedge material from the detachment and representative of the detachment) indicates evidence of pressure solution and viscous flow. We use a microphysical model involving frictional–viscous flow, and together with microstructural observations from the exhumed gouge we interpret low shear strength (<20 MPa) within the frictional–viscous transition zone. The results of the model also suggest that the geodetically estimated strain rate in this region lies in the domain of steady creep or velocity strengthening frictional behaviour. We suggest that the presence of phyllosilicate-rich rocks may significantly weaken mature fault cores along the detachment beneath the IBW and facilitate frictional–viscous flow at shallow depth. This new evidence suggests a significantly reduced seismic hazard across the megathrust. Frictional–viscous flow along the blind detachment beneath the Indo-Burmese Arc.Exhumed fault gouge indicates evidence of pressure solution and viscous flow.Phyllosilicate rich rocks significantly weaken mature fault cores along this detachment.Microphysical model imply low shear strength of the blind detachment. Frictional–viscous flow along the blind detachment beneath the Indo-Burmese Arc. Exhumed fault gouge indicates evidence of pressure solution and viscous flow. Phyllosilicate rich rocks significantly weaken mature fault cores along this detachment. Microphysical model imply low shear strength of the blind detachment.

The architecture of long-lived fault zones: insights from microstructure and quartz lattice-preferred orientations in mylonites of the Median Tectonic Line, SW Japan

Article

Full-text available

Dec 2019

Abstract We combine field mapping with quartz microstructure and lattice preferred orientations (LPO) to constrain the mechanisms and spatio-temporal distribution of deformation surrounding the Median Tectonic Line (MTL), SW Japan. In the study area, the MTL occurs either as a narrow gouge zone or as a sharp contact between hanging-wall quartzofeldspathic mylonites to the north and footwall pelitic schists to the south. Along the northern margin of the MTL, there exists a broad zone of mylonitic rocks, overprinted by cataclastic deformation and a damage zone associated with brittle deformation. The mylonitic shear zone is dominated by coarse-grained protomylonite up to ~ 100 m from the MTL, where fine-grained ultramylonite becomes dominant. We observe a systematic variation in quartz LPO with distance from the MTL. In protomylonites, quartz LPOs are dominantly Y-maxima patterns, recording dislocation creep by prism slip at ~ 500 °C. Closer to the MTL, we observe R- and Z-maxima, and single and crossed girdles, reflecting dislocation creep accommodated by mixed rhomb and basal slip, likely under cooler conditions (~ 300 °C–400 °C). Some ultramylonite samples yield weak to random LPOs, interpreted to result from the influx of fluid into the shear zone, which promoted deformation by grainsize-sensitive creep. Following cooling and uplift, deformation became brittle, resulting in the development of a narrow cataclasite zone. The cataclasite was weakened through the development of a phyllosilicate foliation. However, healing of fractures strengthened the cataclasites, resulting in the development of anastomosing cataclasite bands within the protomylonite.

Alteration Reaction and Mass Transfer via Fluids with Progress of Fracturing along the Median Tectonic Line, Mie Prefecture, Southwest Japan

Chapter

Full-text available

Jun 2017

Toru Takeshita

We have analyzed mass transfer in the cataclasite samples collected from the Median Tectonic Line, southwest Japan, in which the degree of fracturing is well correlated with the bulk rock chemical compositions determined by the X-ray fluorescence (XRF) analysis. The results of “isocon” analysis indicate not only a large volume increase up to 110% but also the two-stage mass transfer during cataclasis. At the first stage from the very weakly to weakly fractured rocks, the weight percents of SiO2, Na 2O, and K2O increase, while those of TiO2, FeO, MnO, MgO, and CaO decrease. At the second stage from the weakly to moderately and strongly fractured rocks, the trend of mass transfer is reversed. The principal component analysis reveals that the variation of chemical compositions in the cataclasite samples can be mostly interpreted by the mass transfer via fluids and by the difference in chemical composition in the protolith rocks to lesser degree. Finally, the changes in the modal composition of minerals with increasing cataclasis analyzed by the X-ray diffraction (XRD) with the aid of “RockJock” software clearly elucidate that the mass transfer of chemical elements was caused by dissolution and precipitation of minerals via fluids in the cataclasite samples. Keywords: Median Tectonic Line, cataclasite, mass transfer, isocon analysis, dissolution and precipitation of minerals

Evolution of the Median Tectonic Line fault zone, SW Japan, during exhumation

Article

Dec 2016
TECTONOPHYSICS

Like many crustal-scale fault zones, the Median Tectonic Line (MTL) fault zone in Japan preserves fault rocks that formed across a broad range of physical conditions. We examined the architecture of the MTL at a large new outcrop in order to understand fault behaviours under different crustal levels. The MTL here strikes almost E–W, dips to the north, and juxtaposes the Sanbagawa metamorphic rocks to the south against the Izumi Group sediments to the north. The fault core consists mainly of Sanbagawa-derived fault gouges. The fault zone can be divided into several structural units, including two slip zones (upper and lower slip zones), where the lower slip zone is more conspicuous. Crosscutting relationships among structures and kinematics indicate that the fault zone records four stages of deformation. Microstructures and powder X-ray diffraction (XRD) analyses indicate that the four stages of deformation occurred under different temperature conditions. The oldest deformation (stage 1) was widely distributed, and had a top-to-the-east (dextral) sense of slip at deep levels of the seismogenic zone. Deformation with the same sense of slip, then became localised in the lower slip zone (stage 2). Subsequently, the slip direction in the lower slip zone changed to top-to-the-west (sinistral-normal) (stage 3). The final stage of deformation (stage 4) involved top-to-the-north normal faulting along the two slip zones within the shallow crust (near the surface). The widely distributed stage 1 damage zone characterises the deeper part of the seismogenic zone, while the sets of localised principal slip zones and branching faults of stage 4 characterise shallow depths. The fault zone architecture described in this paper leads us to suggest that fault zones display different behaviours at different crustal levels.

Patterns of Clay Mineral Transformations in Fault Gouges

Article

Oct 2012

Samuel Haines

Neoformed clay minerals in fault rocks in the brittle crust are increasingly recognized as being key to the sealing behaviour of faults. Academic literature has recognized the importance of neoformation of clay in fault gouge for a number of years, but the concept has not reached most industry seal analysis workflows. Clay-rich gouges that form as a consequence of new clay mineral growth are distinct from clay smears or cataclastic fault rocks that form as a result of mechanical incorporation of wall-rock phyllosilicates, in that they form by chemical and not physical processes. We report a comprehensive field study of clay mineralogy on fault rocks from sedimentary basins and low-angle normal faults in the American Cordillera. We then synthesize the field study with a literature survey to identify controlling conditions for neo-formed clay in fault gouge. Neoformed mineral in gouges are illite, illite/smectite, smectite, and chlorite/smectite phases. Chlorite and kaolinite do not form as neoformed clays in fault gouges. Controlling conditions are wallrock chemistry, temperatures of ∼60-180 C and fluid availability.

Recognition of shear heating on a long-lived major fault using Raman carbonaceous material thermometry: implications for strength and displacement history of the MTL, SW Japan

Research

Full-text available

Nov 2015

Application of Raman carbonaceous material thermometry reveals the presence of a rise in peak temperature of around 60 °C towards the MTL. The spatial association of the thermal anomaly with the fault zone implies that the cause is shear heating. Constraints on maximum slip rate, time scale of heating and width of thermal anomaly suggest a minimum coefficient of friction around 0.4.

Patterns of mineral transformations in clay gouge, with examples from low-angle normal fault rocks in the western USA

Article

Oct 2012
J STRUCT GEOL

Estimates of stress and strain rate in mylonites based on the boundary between the fields of grain-size sensitive and insensitive creep

Article

Full-text available

Mar 2012

Microstructural analyses of mylonites next to the Median Tectonic Line (MTL), SW Japan, reveal a transition in the dominant deformation mechanism of quartz from grain-size-insensitive dislocation creep to grain-size-sensitive grain-boundary sliding (GBS). The transition occurred under greenschist-facies conditions (˜300-400°C) during grain-size reduction by dynamic recrystallization. The stereologically corrected grain size for the transition is approximately 4.3 μm. At the boundary between the fields of dislocation creep and GBS, as calculated from creep constitutive relations, the differential stress and strain rate for this corrected grain size are estimated to be ˜280 MPa and 1.2 × 10-11 s-1 for 300°C, and ˜110 MPa and 1.0 × 10-10 s-1 for 400°C. The strain rates estimated for the mylonites next to the MTL are much higher than those estimated for the surrounding metamorphic rocks (˜10-14 s-1), and the displacement rates calculated based on the thickness of high-strain mylonites and their strain rates are comparable with the average slip rates of the most active intraplate faults in Japan. These inferences suggest that the high-strain mylonite zones next to the MTL are the exhumed downward extension of a seismogenic fault in the ductile region. The zones were highly localized (<10 m) and experienced very high strain rates (10-11 to 10-10 s-1).

Development of the Median Tectonic Line-related shear zone, southwest Japan: An analysis of strain localization processes

Article

Feb 2023
TECTONOPHYSICS

The development of a mylonite zone along the Median Tectonic Line (MTL), southwest Japan, which developed in the Cretaceous granitic rocks, was examined to understand strain localization during cooling. The mylonites are E-W trending, north-dipping, and represent the hanging wall of the MTL. The mylonite zone includes both protomylonite and mylonite, which were derived from two distinct protoliths: tonalite (protomylonite) and granite (mylonite) and occur at distances of c. 0–300 m and 300–700 m from the MTL, respectively. We used quartz microstructures and crystallographic preferred orientations (CPOs) and two-feldspar thermometry to infer the spatiotemporal development of deformation conditions in the MTL mylonite zone. The mylonitic granite is sub-divided into southern and northern mylonite zones, which exhibit A- and B-type quartz microstructure formed by subgrain rotation and grain boundary migration recrystallization, respectively. In both the mylonitic granite and protomylonitic tonalite, quartz c-axis CPOs primarily display a moderate-temperature Y-maximum pattern, as well as a less common low-temperature type-I crossed-girdle pattern. In the northern zone, quartz deformation initially occurred at moderate temperatures of 435 °C–490 °C, forming mylonitic granite with B-type quartz microstructure. As temperature decreased to 355 °C–435 °C, the strain localized into the southern zone to form A-type quartz microstructure. The protomylonitic tonalite formed under similar temperature conditions. At temperatures below 355 °C, ductile deformation ceased in the majority of the granite and tonalite, with only a narrow zone of ultramylonite up to 50 m wide, deforming adjacent to the MTL at conditions approximating the brittle–ductile transition in quartz (c. 300 °C). Strain localization did not occur within a single shear zone, but in several deforming strands that formed in distinct protoliths, one of which developed into the narrow ultramylonite zone reworked as the brittle MTL, likely controlled by cooling.

3D fault-zone architecture across the brittle–plastic transition along the Median Tectonic Line, SW Japan: Fault-rock characterization

Article

Sep 2021
J STRUCT GEOL

We determine the features and distribution of fault rocks along the Median Tectonic Line (MTL), SW Japan, to establish the 3D architecture of the fault zone across the brittle–plastic transition. Cataclasites exposed close to the lithological boundary fault (the MTL) can be divided into those formed by sinistral faulting at temperatures of ∼300 °C and those formed by dextral faulting at ∼250 °C. Mylonites distributed to the north of the cataclasites were formed by sinistral faulting and can be divided into lower-temperature mylonite (L-T mylonite) close to the MTL and higher-temperature mylonite (H-T mylonite) distant from the MTL, where deformation temperatures were lower and higher than 400 °C, respectively. Structures formed by sinistral faulting are oblique to those formed by dextral faulting, indicating that the former structures are older than the latter. Structures formed by sinistral faulting underwent deformation around the brittle–plastic transition. Thus, the MTL fault zone records deformation through a crustal section. Microstructural observations suggest that the differential stress just below the brittle–plastic transition (L-T mylonite) was ∼200 MPa and that this value may not change substantially in the deep crust (H-T mylonite).

Architecture of onshore fault zones

Article

Full-text available

Nov 2018

We review a wide range of research elucidating the architecture of onshore fault zones mostly by members of the Geological Society of Japan in recent decades. Drilling of the Nojima Fault was a turning point in the studies of the architecture of the shallow, brittle faults. This project also stimulated many investigations of the hydrological and frictional properties of faults. A major outcome of investigations of the deep part of the seismogenic zone on faults such as the Median Tectonic Line (MTL), was recognition that deformation mechanisms such as pressure solution creep and crystal plasticity of mica in foliated cataclasite. The cataclastic damage causes increase in intracrystalline strain in surrounding rocks. Within the brittle-plastic transition zone, several studies (the Asuke shear zone and the MTL) estimated the stress and strain rate conditions. Some studies revealed the processes involved in fracture nucleation, and documented the heterogeneity of deformation at a few 10 km scale in this regime (the Hatagawa fault zone). Within the lower crust, evidence was found for plastic deformation of plagioclase and pyroxene by dislocation creep, and that the formation of fine-grained aggregates during deformation results in the significant weakening due to switch from dislocation creep to grain boundary sliding (the Hidaka metamorphic belt). Recently evidence of fracturing has also been found in the deformed lower crustal rocks. This may be due to stress concentration at the down-dip termination of earthquake rupture, but further studies are needed to validate this hypothesis.

Trench and drilling investigation of the Median Tectonic Line in Shikoku, southwest Japan: implications for fault geometry

Article

Full-text available

Oct 2021
EARTH PLANETS SPACE

The Median Tectonic Line (MTL) is a thousand-kilometer-long fault that extends across southwest Japan. Near the Nyugawa region of Shikoku, the MTL comprises (i) a low-angle inactive terrane boundary fault (the MTLTB) that divides the Jurassic and Cretaceous geological terranes, and (ii) a subparallel high-angle active fault zone (the MTLAFZ; Kawakami Fault). To better understand the relationship between the MTLTB and MTLAFZ fault traces, we exposed a ten-meter-long trench of approximately 2-m depth across the Kawakami Fault. We also drilled and cored five boreholes with lengths 80‒330 m along a 100 m transect to understand the cross-cutting relationship between the MTL faults and to determine the fault plane geometries and their dipping values. The Kawakami Fault was found to be a high-angle (> 70°) active fault exposed at the surface; however, it represents a non-vertical or listric fault that converges to the low-angle MTLTB fault dipping to the north at 30°. The Kawakami Fault was originally formed as a reverse fault, and subsequent dextral strike–slip displacement occurred along the same fault plane. Although the MTLTB is poorly oriented with respect to the regional stress field, it is capable of rupturing owing to its significantly weak interface; the properties of local faulted rock material are expected to play an important role in determining slip behavior.

Dynamically recrystallized grains identified via the application of Gaussian mixture model to EBSD data

Article

Full-text available

Feb 2023
J STRUCT GEOL

Crystals formed during ductile deformation known as the recrystallized grains have been revealed to provide syn-deformational stress and strain information. Despite its importance, current methods of distinguishing recrystallized grains are subjective and lack consideration to the evolving microstructures. Using data obtained from electron backscattered diffraction mapping, we provide an unsupervised method to estimate the recrystallized fraction and the recrystallized grain size. The Gaussian mixture model was utilized to upon the grain orientation spread that measures intragranular lattice distortion to separate the grains into clusters. The counterpart of the recrystallized grain, the relict grains which are grains present prior to deformation are identified as grains distributed under the cluster with the largest mean grain orientation spread. Three zones could be separated from the Gaussian mixture model results representing the recrystallized zone, mixture zone and relict zone. Grains located within the mixture zone have an innate probability to be classified as either relict or recrystallized grains. The application of the Monte Carlo approximation enables the evaluation of error for the recrystallized fraction and grain size. Comparison to previously available methods shows that the current method avoids ambiguity in the selection of the cut-off threshold when identifying the recrystallized grains.

Origin of magmatic and tectonic fabrics in the Remal granite-gneiss, Singhbhum Craton, India

Article

Full-text available

Jun 2022

In the Singhbhum Craton of the Indian shield, the Remal granite-gneiss preserves felsic magmatic fabrics onto which a low-temperature segregation layering has been superposed. Planar, sub-horizontal to gently dipping layers (Sign1) comprise K-feldspar megacrysts, plagioclase and quartz, with the base of each layer defined by segregations of biotite. Sign2 consists of trough cross-bedded layers composed of K-feldspar phenocrysts, plagioclase and quartz with biotite schlieren defining the base of each layer. Microstructural features such as concentrically arranged mineral inclusions in K-feldspar phenocrysts and graphic intergrowth textures testify to the magmatic origin of these fabrics, with insignificant subsequent metamorphic reconstitution. The tectonic fabric S1 has developed sub-parallel to localized greenschist-facies mylonite bands, and is defined by weakly aligned flakes of biotite. Crystallographic preferred orientations away from the mylonitized domains show a strong alignment of K-feldspar, quartz and biotite parallel to the magmatic fabric due to efficient segregation during magmatic flow. Quartz crystallographic preferred orientations within the mylonitized domains show a strong preferred orientation and dextral asymmetry. Temperature constraints from synkinematic chlorites along with estimates of deformation temperature from quartz crystallographic preferred orientations indicate that mylonitization occurred at the lower limits of quartz crystal plasticity. The results of combined thermodynamic and multiphysics modelling studies show that felsic magmas can undergo significant convective motion for a wide range of crystallinities and water contents before solidification. Additionally, segregation layering resembling a gneissosity can develop at low temperatures owing to localized mylonitization and concomitant dissolution–precipitation of biotite.

How Fault Rocks Form and Evolve in the Shallow San Andreas Fault

Article

Full-text available

Nov 2021
GEOCHEM GEOPHY GEOSY

We document the mechanical and geochemical processes of fault rock development in the shallow San Andreas fault (Mojave segment), and quantify their importance in shaping the mineralogy, grain size, fabric, and frictional characteristics of gouge. Through a combination of field and laboratory analysis of an extensive suite of shallow (<150 m) drill cores, we show that fault rocks evolved from a granodiorite protolith via three main processes: distributed microfracturing/pulverization; cataclastic flow and incipient fabric development; and production of authigenic illite/smectite during fluid‐rock interaction. The interdependence of these mechanical and geochemical processes results in a diverse suite of fault rocks, and causes significant changes in frictional strength. Spatial variations in the effects of these mechanisms, as manifested in fault‐rock mineralogy and geochemistry, indicate marked variations in their relative contribution to fault‐rock evolution. These data reveal a complex San Andreas fault with multiple principal slip zones and damaged and altered rock hosting numerous interconnected secondary slip surfaces. The resulting picture of the San Andreas Fault zone suggests a substantial departure from the simple structures envisioned for near‐surface seismogenenic faults in numerical models is required, and may inform future efforts to forecast peak ground accelerations during southern California earthquakes.

Amalgamation of the Ryoke and Sanbagawa metamorphic belts at the subduction interface: New insights from the Kashio mylonite along the Median Tectonic Line, Nagano, Japan

Article

Sep 2021

We present a detailed petrological, structural, and geochronological study of the mylonitic Ryoke metamorphic rocks within the granitic mylonite (Kashio mylonite) and Sanbagawa metamorphic rocks along the Median Tectonic Line (MTL), Japan. Located in the Oshika area of the Chubu district, the Kashio mylonite is one of the few geologic units that can be used to determine detailed pressure–temperature–time–deformation (P–T–t–D) paths during mylonitization because it occurs as many small tectonic blocks of mylonitic metasediment. Detailed petrological analysis coupled with conventional thermobarometry and P–T pseudosection modeling give estimated peak P–T conditions (M1a) of 650–790 °C at 4.6–5.6 kbar for the Ryoke metamorphic rocks. The gneissose Ryoke granitoids were emplaced subhorizontally at around 685–710 °C and 4.6–5.8 kbar, after peak metamorphism. The Kashio shear zone developed immediately after the last igneous activity at ca. 71 Ma, and two stages of mylonitization (stages D1 and D2) can be identified from microstructural observations. The retrograde P–T conditions (M1b) recorded in the Kashio mylonite exhibit a systematic change in temperature from 710 to 450 °C at 5.2–2.6 kbar with decreasing distance from the MTL. By contrast, highly deformed mylonites with zoned garnets demonstrate a striking increase in pressure from 4.0 to 8.3 kbar with decreasing temperature from 590 to 450 °C after low-P/T type metamorphism. Such a temperature range indicating isothermal compression is consistent with deformation temperatures of stage D1 determined from quartz microstructures and quartz c-axis fabric opening-angle deformation thermometer. Moreover, the timing of the two mylonitization episodes during retrograde metamorphism are estimated to be 69–67 Ma and 66–64 Ma, respectively, with a high cooling rate of ~34 °C/Myr using the revised time–temperature relationship of the host Ryoke granitoids. The rapid change in tectonic setting with strain localization occurred during the brief period between 69 and 64 Ma. Our field and petrological observations imply that a thick D1 mylonite zone was formed by rapid subsidence (≥ 10km) with cooling of the hangingwall rocks from the middle crust to the subduction interface. It is considered that the underplating of exhumed high-P/T type metamorphic rocks led to further cooling between hangingwall and footwall rocks, and the formation of a narrow D2 mylonite zone, which served as an old plate boundary. Thus, low and high-P/T type metamorphic belts had already been amalgamated as paired metamorphic belts beneath the brittle–ductile transition of the subduction zone before exhumation. The rapid cooling of hangingwall rocks at the subduction interface is proposed to play an essential role in the thermal overprinting of exhumed high-P/T type metamorphic rocks.

Effects of geological and tectonic characteristics on the earthquake-triggered Daguangbao landslide, China

Article

Oct 2017

The Daguangbao landslide is the largest co-seismic landslide triggered by the Wenchuan earthquake (Ms 8.0) occurred on 12 May 2008. The landslide, which is 4.6 km long and 3.7 km wide, involves a volume of approximately 1.2 × 10⁹ m³. An exposed slip surface, situated at the southern flank of its source area, was observed with a length of 1.8 km along the main sliding direction and an area of 0.3 km². To study the geological and tectonic characteristics of the source area and their contributions to the landslide formation during the earthquake, detailed geological investigations were firstly conducted. And it is reached that the landslide occurred on the northwestern limb of the Dashuizha anticline with its scarp showing several geological structures, including joint sets, local faults, and folds. These tectonic-related structures potentially influenced the failure of the landslide. Secondly, further investigations were focused on the inclined planar sliding surface using 12 exploratory trenches, nine boreholes, a tunnel, borehole sonic data, and micro-images. These data reveal that the rock mass along the sliding surface was the fragmented rock of a bedding fault. A pulverized zone was observed on the sliding surface, which was the zone of shear localization during the landslide. This suggests that the shear failure of the Daguangbao landslide developed within the bedding fault. The rapid failure of the landslide was associated with the degradation of the rock mass strength of the bedding fault both before and during the 2008 Wenchuan earthquake. With this study, we propose that a pre-existing large discontinuity within a slope may be the basis for initiating a large landslide during earthquake.

Inconsistent Structure and Motion of the Eastern Median Tectonic Line, Southwest Japan, during the Quaternary

Chapter

Full-text available

Jun 2017

Pore fabric anisotropy and elastic moduli of fault rocks from the Median Tectonic Line, Shikoku, southwest Japan

Article

Apr 2022
TECTONOPHYSICS

Pores and cracks have an important role in the evolution of fault rocks because they strongly influence the behavior of the fluids that promote rock alteration and trigger the mechanical instability of faults. We used rock physics model inversion of measured elastic wave velocity and porosity to estimate the grain elastic moduli and crack aspect ratios of a range of fault rocks (intact rocks, fractured rocks, transition rocks, and fault gouge) from the Median Tectonic Line in southwest Japan. Our results show distinct gaps in the evolutionary trends of crack aspect ratios and grain elastic moduli from intact rocks to fault rocks. Crack aspect ratios show a nonlinear trend from fractured rock to fault gouge, and then these values in fault gouge were considerably higher than in fractured rock and transition rock. In contrast, grain elastic moduli decreased as fracture evolved with the development and subsequent extinction of shear planes and then increased markedly with the formation of fault gouge. Our results show that crack aspect ratios and grain elastic moduli are clearly related to the evolution of shear fabrics in faults. Therefore, they might be useful indicators of fault activity and maturity.

Recognition of broad thermal anomaly around the Median Tectonic Line in central Kii peninsula, SW Japan: Possible heat sources

Article

Jan 2022

Application of Raman spectroscopy of carbonaceous material thermometry to samples of metasedimentary rock from the low‐grade Sanbagawa belt in the central Kii peninsula reveals a progressive decrease in temperature from ~390 °C close to the northern boundary, a major continental shear zone—the Median Tectonic Line (MTL)—to ~270 °C and ~7 km to the south and roughly constant temperature distribution thereafter. Within the Sanbagawa belt, the thermal structure is not significantly modified by slip on fault boundaries between different geological units or folding. Meso‐ and micro‐structural observations combined with strain analysis using detrital grains in meta‐mudstone indicate a similar deformation history throughout the area and no correlation between ductile strain and temperature gradients. These observations suggest the observed thermal structure was developed after the main stages of ductile deformation of the Sanbagawa belt were complete and is not due to localized preferential exhumation along the MTL. The observations also require a heat source along the MTL. Order of magnitude estimates suggest the influx of warm fluid along the MTL are viable causes of the observed thermal anomaly. Although shear heating would be another possible explanation, thermal calculations require anomalous fast slip rates along the MTL and much greater frictional strength than generally considered reasonable. For these reasons fluid infiltration is our preferred model. This article is protected by copyright. All rights reserved.

In-situ permeability of fault zones estimated by hydraulic tests and continuous groundwater-pressure observations

Article

Full-text available

Dec 2018
EARTH PLANETS SPACE

In-situ permeability of the Median Tectonic Line (MTL) fault zone in Mie Prefecture, southwest Japan, was estimated using hydraulic tests and groundwater pressure observations in two boreholes. The screen depths in Holes 1 and 2 are located, respectively, in a major strand of the MTL fault zone within the Sambagawa metamorphic rocks and a branching fault developed in the hanging wall of the MTL within the Ryoke mylonite. The estimated permeability at Hole 1 ranges from 5.3 × 10−17 to 5.0 × 10−16 m2, and that at Hole 2 ranges from 4.4 × 10−16 to 1.5 × 10−15 m2. We also measured the permeability of the protolith close to the screened depth of Hole 1 (3.4 × 10−19 and 3.7 × 10−19 m2) and Hole 2 (3.1 × 10−19 and 6.2 × 10−19 m2). The permeability of the fault zone was found to be more than 100 and 700 times higher than the protolith permeability at Holes 1 and 2, respectively. The permeability data for Holes 1 and 2 are consistent with previously reported permeability data for samples from an MTL outcrop. The permeability observed in this study reflects the complex fault zone permeability structure of the MTL fault zone. Open image in new window

Toward the creation of models to predict static and dynamic fault seal potential in carbonates

Article

Oct 2016

In contrast to faults in clastic reservoirs, rules to predict the exploration and production timescale fault-seal potential in carbonates are lacking. This paper provides a summary of carbonate reservoirs with cross-fault column height differences that represent examples of apparent static fault seal, and a summary of observed examples of dynamic fault seal in carbonate reservoirs and aquifers. These include cross-fault differences in water columns across carbonate–carbonate juxtapositions, cross-fault pressure differences in carbonate aquifers separated by faults, production-induced cross-fault pressure differences in carbonate hydrocarbon reservoirs, sealing behaviour of faults in carbonate reservoirs inferred from well tests, and examples of low fault transmissibilities from history-matching exercises from carbonate reservoirs. This paper also documents the range of compositions of fault rocks in carbonates and the range of permeabilities that have been reported from low-permeability fault cores in carbonate fault zones, as well as the implications of the observed range of fault permeabilities in carbonates for sealing behaviours. The purpose of this paper is not to argue that every fault in a carbonate reservoir will seal or will even be capable of sealing. There are, however, enough examples of faults in carbonates that are sealing in a dynamic sense, and in a static sense, that the topic of carbonate fault seal should warrant much more study. Creation of predictive models will ultimately require a considerable amount of subsurface data, but these models should be created.

Recognition of shear heating on a long-lived major fault using Raman carbonaceous material thermometry: implications for strength and displacement history of the MTL, SW Japan: Shear heating on the MTL

Article

Dec 2015

The extent to which movement on major faults causes long term shear heating is a contentious issue and an important aspect in the debate about the strength of major faults in the crust. Comparing the results of experimental work on the kinetics of crystallization of carbonaceous material with results of thermal modeling show that the Raman carbonaceous material (CM) geothermometer is well suited to studying shear heating on geological time scales in suitable lithologies exposed around exhumed major fault zones. The Median Tectonic Line (MTL), SW Japan, is the largest on-land fault in Japan with a length of > 800 km. Application of Raman CM thermometry to pelitic schist adjacent to the fault reveals the presence of a rise in peak temperature of around 60 °C over a distance of around 150 m perpendicular to the MTL fault plane. The spatial association of this thermal anomaly with the fault implies it is due to shear heating. Thermal modeling shows the recorded thermal anomaly and steep temperature gradient is compatible with very high rates of displacement over time scales of a few thousand years. However, the implied displacement rates lie outside those generally observed. An alternative explanation is that an originally broader thermal anomaly that developed during strike slip faulting was shortened due to the effects of normal faulting. Constraints on displacement rate, width of the original anomaly, duration of heating and peak temperature imply a coefficient of friction, μ, greater than 0.4.

Determination of the Exact Surface Trace of the Median Tectonic Line in the Ise Area, Southwest Japan

Article

Full-text available

Sep 2015

The Miocene collision of the Izu-Bonin arc with the Honshu arc in Japan is thought to have initiated eastward bending of the Median Tectonic Line (MTL) from an approximate E-W strike to an ENE-WSW strike in the Ise area of the easternmost Kii Peninsula. However, the exact surface trace of the MTL in the Ise area remains poorly defined because of Quaternary sediment cover and the development of urban areas. Determination of the surface trace of the MTL is important to evaluate the characteristics of seismic motion upon the paired metamorphic belts divided by the MTL, which reflects the subsurface geology, and to better explain the curvature of the MTL in central Japan. This paper presents the results of petrographic observations from several localities in the Ise area, including Tamaki Town and Ise City: boring cores at (1) the factory building of Miwa Lock Co., Ltd., in Tamaki Town, which overlie the MTL, (2) Ise City Hall, (3) Ise City Tourism and Culture Hall, as well as additional boring data from Ise City. The depth of pre-Neogene basement increases eastward from the Gokatsura area, ranging from 6-12 m in the Sanbagawa belt near Ise City Hall to greater than 30m in the Ryoke Belt. A geological survey of rare exposures, including the Ryoke rocks of the Ooike pond area in Tamaki Town, Miocene conglomerate along the Miyagawa River in the Ryoke belt, and Sanbagawa schists in Ise City, have helped to constrain the exact location of the MTL in Ise City. Through this study we have also signalized the potential of investigated sites in Ise city to be used as prospect geosites in future. After proper development and promotion these sites can become important points of attention to be seen by visitors and geologists and provide various opportunities for local economy growth.

Low effective fault strength due to frictional-viscous flow in phyllonites, Karakoram Fault Zone, NW India

Article

Full-text available

May 2015
J STRUCT GEOL

Phyllosilicate-rich fault rocks are common in large-scale fault zones and can dramatically impact fault rheology. Experimental evidence suggests that multi-mechanism frictional-viscous flow (FVF) may operate in such lithologies, potentially significantly weakening mature fault cores. We report microstructures indicative of FVF in exhumed phyllonites of the Karakoram Fault Zone (KFZ), NW India. These include interconnected muscovite foliae, lack of quartz/feldspar crystal preferred orientations, and sutured grains and overgrowths indicative of fluid-assisted diffusive mass transfer. FVF microphysical modelling, using microstructural observations from the natural fault rock and experimentally-derived friction and diffusion coefficients, predicts low peak shear strengths of < 20 MPa within the frictional-viscous transition zone. Chlorite geothermometry indicates that synkinematic chlorites grew at 351 ± 34 °C (c. 10 km depth) during FVF, immediately above the transition to quartz crystal plasticity. The deformation processes and interpreted low shear strength of the exhumed KFZ fault rocks provide analogues for processes operating currently at depth in active faults of similar scale. If similar lithologies are present at depth, then the Quaternary seismic characteristics of the KFZ support faults with phyllonitic cores being able to accommodate large seismic ruptures. The results also provide rare rheological constraints for mechanical models of the India-Asia collision zone.

Multiple damage zone structure of an exhumed seismogenic megasplay fault in a subduction zone - A study from the Nobeoka Thrust Drilling Project

Article

Full-text available

Nov 2015
EARTH PLANETS SPACE

To investigate the mechanical properties and deformation patterns of megathrusts in subduction zones, we studied damage zone structures of the Nobeoka Thrust, an exhumed megasplay fault in the Kyushu Shimanto Belt, using drill cores and geophysical logging data obtained during the Nobeoka Thrust Drilling Project. The hanging wall, composed of a turbiditic sequence of phyllitic shales and sandstones, and the footwall, consisting of a mélange of a shale matrix with sandstone and basaltic blocks, exhibit damage zones that include multiple sets of `brecciated zones' intensively broken in the mudstone-rich intervals, sandwiched by `surrounding damage zones' in the sandstone-rich intervals with cohesive faults and mineral veins. The fracture zones are thinner (2.7 to 5.5 m) in the sandstone-rich intervals and thicker in the shale-dominant intervals (2.3 to 18.6 m), which indicates a preference of coseismic slip and velocity-weakening in the former, and aseismic deformation in the latter. However, the surrounding damage zones observed in the current study are associated with an increase in resistivity, P-wave velocity, and density and a decrease in porosity, inferring densification and strain-hardening in the sandstone-rich intervals and strain-weakening in the mudstone-rich intervals. These observations indicate that the sandstone-rich damage zones may weaken in the short term but may strengthen in the geologically long term, contributing to a later stage of fault activity. In contrast, the mudstone-rich damage zones may strengthen in the short term but develop weak structures through longer time periods. The observed shear zone thickness in the hanging wall is thinner (2.3 to 18.6 m) compared to the footwall damage zones (12 to 39.9 m), possibly because faults in the hanging wall were concentrated and partitioned between the preexisting turbiditic sequence of alternating shale/sandstone-dominant intervals, whereas in the footwall, faults were more sporadically distributed throughout the sandstone block-in-matrix cataclasites. A splay fault may evolve and be characterized by physical property contrasts, the lithology dependence of deformation, and the variability of damage zone thickness due to a heterogeneous lithology distribution in the hanging wall and footwall. The deformation patterns observed in the Nobeoka Thrust provide insights to the strain-hardening/weakening behaviors of sediments along megathrusts over geological timescales.

Reactivation of an old plate interface as a strike-slip fault in a slip-partitioned system: Median Tectonic Line, SW Japan

Article

Jan 2015
TECTONOPHYSICS

In models for strain-partitioning at obliquely-convergent plate boundaries, trench-parallel slip occurs on a vertical fault. Trench-parallel slip at the Nankai subduction zone, SW Japan, is mapped along the Median Tectonic Line (MTL) which dips approximately 40°N. To understand its structural context and how the MTL functions in this slip-partitioned system, we collected a set of three seismic profiles in the Kii peninsula south of Osaka, using a multi-scale acquisition strategy that provides increasingly fine resolution. To understand its fault kinematics, we analyzed microseismic activity in two locations on the fault, using source data from Japan's Hi-net monitoring network. Structural details suggest that the MTL functioned as a megathrust during subduction of the Cretaceous Sanbagawa HP metamorphic belt. Its current pattern of microseismicity shows that it behaves as a strike-slip fault with no indication of a vertical fault at or around its surface trace. Thus, trench-parallel slip at the Nankai is now accommodated on an inclined fault plane in an unusual form of partitioning. This system appears to have developed out of a two-phase tectonic history in which a thrust structure that formed under initial-phase compressive stresses has been reactivated as a strike-slip fault under subsequent-phase shear stresses. Its unusual kinematics show that shear failure can occur on an existing non-vertical fault plane at a regional scale in preference to the rupture of a new ideal (vertical) fault plane.

The mechanics of seismic faulting: Recent advances and open issues

Article

Full-text available

Apr 2014

Andrea Bizzarri

In the early '60s the fracture mechanics was mainly focused on tensile cracks (of prominent importance in engineering problems) and later on shear, antiplane cracks. Since these studies consisted in singular problems without friction, their applicability to earthquake events was extremely limited. After the seminal papers by Kostrov in the late '60s and the relevant contributions of Aki, Burridge, Andrews, Das, Ida, Madariaga and others in '70s, more elaborated, realistic and physically constrained fault models have been proposed and further intensively used and improved. Complemented by the evidence from laboratory experiments on fracture (first pioneered by Ohnaka and later by Rosakis) and friction (pioneered by Dieterich, Ruina and more recently by Shimamoto), numerical and theoretical models provide substantial improvements in the understanding of the chemical and physical, potentially competing, energy-dissipating processes occurring in the natural fault structures. In spite of these significant advances, some open issues still hover and many important ideas remain unexplored fully. Relevant challenges to relate the physics of the seismic source to the coseismic scenarios and ultimately to the seismic hazard assessment could be successfully handled in the framework of a multidisciplinary approach, which combines theory, numerical models, data analysis, geological observations and laboratory experiments.

Magnetotelluric imaging of the Median Tectonic Line in western Shikoku, southwest Japan: Implications of the fault-related low-resistivity zone

Article

Aug 2013
TECTONOPHYSICS

We carried out a magnetotelluric (MT) survey along a profile crossing the Median Tectonic Line (MTL) in western Shikoku, Japan. The MTL is a terrane boundary that formed during the Cretaceous between the Sanbagawa belt, consisting of high pressure/temperature (P/T) metamorphic rocks, and the Ryoke belt of granites and low P/T metamorphic rocks. The MT image shows a zone of remarkably low resistivity, dipping northward at 40°, at the surface coincides with the surface trace of the MTL. The low-resistivity zone probably corresponds to a fluid-filled damaged zone of porous media composed of clay minerals and cracked rocks, formed by repeated faulting of the MTL since Late Cretaceous time. The calculated maximum porosity of the damaged zone is 7.1%, which is clearly higher than that of the non-damaged crystalline rocks.

Orienting drill core using borehole-wall image correlation analysis

Article

Oct 2014
J STRUCT GEOL

A method for orienting drill core is proposed that correlates measurements of planar structures in drill core with those observed in spatially referenced images of borehole walls. The drill core orientation is expressed in terms of a transformation between the geographic coordinate system and the drill core coordinate system, using the Euler angles θ, Φ, and ϕ. The angles θ and Φ are the azimuth (trend) and the plunge of the inclined borehole, respectively. The angle ϕ is the rotation angle about the drill core axis and is determined through correlation analyses of planar structures in the drill core with those observed in the borehole wall images. Orientations of planar structures in the drill core are measured in terms of a reference line that is drawn along the length of the drill core in an arbitrary position. The proposed method is applied to drill core samples recovered from a borehole that penetrates the Median Tectonic Line (MTL) at Matsusaka-Iitaka (ITA), eastern Kii Peninsula, Japan. The results suggest that the accuracy of the determination of ϕ is about ±5°.

Late-interseismic state of a continental plate-bounding fault: Petrophysical results from DFDP-1 wireline logging and core analysis, Alpine Fault, New Zealand: Late-Interseismic State, Alpine Fault

Article

Full-text available

Sep 2013
GEOCHEM GEOPHY GEOSY

We present a geophysical characterization at 0.1–100 m scales of a major plate-bounding continental fault in a late-interseismic state. The Alpine Fault produces MW∼8 earthquakes every 200–400 years and last ruptured in 1717 AD. Wireline geophysical logs and rock cores extending from one side of the Alpine Fault to the other were acquired in two boreholes drilled in 2011 at Gaunt Creek during the first phase of the Deep Fault Drilling Project (DFDP-1). These data document ambient conditions under which the next Alpine Fault earthquake will occur. Principal component analysis of the wireline logging data reveals that >80% of the variance is accounted for by electrical, acoustic, and natural gamma properties, and preliminary multivariate classification enables the lithologies of sections of missing core to be reconstructed from geophysical measurements. The fault zone exhibits systematic variations in properties consistent with common processes of progressive alteration and comminution near the principal slip zone, superimposed on different protolith compositions. Our observations imply that the fault zone has the opposite sense of elastic asymmetry at 0.1–100 m scales to that of the crustal-scale orogen imaged by remote geophysical methods. On the basis of the fault-zone scale asymmetry, the bimaterial interface model of preferred earthquake rupture directions implies a northeastward direction of preferred Alpine Fault rupture. On-going characterization of the structural and hydraulic architecture of the Alpine Fault will improve our understanding of the relationship between in situ conditions, earthquake rupture processes, and the hazards posed by future earthquakes.

Fault zone models, heat flow, and the depth distribution of earthquakes in the continental crust of the United States

Article

Full-text available

Jan 1982

Richard H. Sibson

Formation of Clay Minerals in Hydrothermal Environments

Chapter

Full-text available

Jan 1995

Atsuyuki Inoue

Formation of clay minerals under hydrothermal influence is the result of rock alteration by circulating hot water in the Earth’s crust. A pre-existing rock-forming mineral assemblage is altered to a new set of minerals which are more stable under the hydrothermal conditions of temperature, pressure, and fluid composition. The interaction of hot water and rocks forms a spatially and temporally regular zonal pattern of new clay minerals, as the fluid with cooling temperature moves through the surrounding rock mass. This chapter discusses the formation of clay minerals in such dynamic processes of hydrothermal alteration. The approach is one of clay-mineral facies formed under conditions of massive alteration in the rocks. The chemical and mineralogical changes which occur on the scale of a rock or rock mass are considered to have been dealt with in the preceding chapter. The exact process of change via local, vein-influenced exchange processes is ignored for simplicity (see Chap. 6).

Fault zone architecture and permeability structure

Article

Full-text available

Nov 1996
GEOLOGY

Fault zone architecture and related permeability structures form primary controls on fluid flow in upper-crustal, brittle fault zones. Qualitative and quantitative schemes for evaluating fault-related permeability structures are developed by using results of field investigations, laboratory permeability measurements, and numerical models of flow within and near fault zones. The fault core and damage zone are distinct structural and hydrogeologic units that reflect the material properties and deformation conditions within a fault zone. Whether a fault zone will act as a conduit, barrier, or combined conduit-barrier system is controlled by the relative percentage of fault core and damage zone structures and the inherent variability in grain scale and fracture permeability.

Fault rocks and fault mechanics

Article

Full-text available

Mar 1977

Richard H. Sibson

Physical factors likely to affect the genesis of the various fault rocks-frictional properties, temperature, effective stress normal to the fault and differential stress-are examined in relation to the energy budget of fault zones, the main velocity modes of faulting and the type of faulting, whether thrust, wrench, or normal. In a conceptual model of a major fault zone cutting crystalline quartzo-feldspathic crust, a zone of elastico-frictional (EF) behaviour generating random-fabric fault rocks (gouge-breccia- cataclasite series-pseudotachylyte) overlies a region where quasi-plastic (QP) processes of rock deformation operate in ductile shear zones with the production of mylonite series rocks possessing strong tectonite fabrics. In some cases, fault rocks developed by transient seismic faulting can be distinguished from those generated by slow aseismic shear. Random-fabric fault rocks may form as a result of seismic faulting within the ductile shear zones from time to time, but tend to be obliterated by continued shearing. Resistance to shear within the fault zone reaches a peak value (greatest for thrusts and least for normal faults) around the EF/QP transition level, which for normal geothermal gradients and an adequate supply of water, occurs at depths of 10-15 km.

Fluid[ndash]rock interaction in faults of the San-Andreas System[mdash]Inferences from San-Gabriel fault rock geochemistry and microstructures

Article

Full-text available

Jul 1995

Optical and scanning electron microscopy and whole rock geochemical analyses are used to investigate variations in deformation mechanisms and fluid-rock interactions in rocks at three sites on the San Gabriel fault, southern California: Pacoima Canyon, Bear Creek, and North Fork. At Bear Creek, unaltered and undeformed granite, granodiorite, and diorite protolith bound a fault core several meters thick that consists of foliated cataclasite on either side of 2-20 cm thick ultracataclasite layer. The foliated cataclasite contains clays and zeolite veins which developed by alteration of protolith during slip. The ultracataclasite consists of 20-100 mum diameter feldspar and quartz fragments embedded in a clay-zeolite matrix. The matrix consists of grains

Evidence of frictional melting from disk-shaped black material, discovered within the Taiwan Chelungpu fault system

Article

Full-text available

Oct 2006

The Taiwan Chelungpu-fault Drilling Project penetrated three fault zones as the Chelungpu fault system, which slipped during the 1999 Chi-Chi earthquake, discovering disk-shaped black material (BM disk) within the middle and lower fault zones in Hole B. The microscopic features of the BM disks indicated that they were pseudotachylytes, and they showed high magnetic susceptibility, possibly the result of intense shearing or high temperature conditions. Inorganic carbon content of the BM disks was low, possibly because of thermal decomposition of carbonate minerals. The high temperatures might be related to frictional heating during the earthquake, implying that the BM disks were produced under intense shearing with frictional heating that reached melting temperature. Because the disks, which provide the only evidence of melting, pre-date the 1999 earthquake, we concluded that frictional melting did not occur during the earthquake.

Mineralogic and textural analyses of drill cuttings from the San Andreas Fault Observatory at Depth (SAFOD) boreholes: Initial interpretations of fault zone composition and constraints on geologic models

Article

Full-text available

Oct 2007
GEOSPHERE

We examine drill cuttings from the San Andreas Fault Observatory at Depth (SAFOD) boreholes to determine the lithology and deformational textures in the fault zones and host rocks. Cutting samples represent the lithologies from 1.7-km map distance and 3.2-km vertical depth adjacent to the San Andreas Fault. We analyzed two hundred and sixty-six grain-mount thin-sections at an average of 30-m-cuttings sample spacing from the vertical 2.2-km-deep Pilot Hole and the 3.99-km-long Main Hole. We identify Quaternary and Tertiary(?) sedimentary rocks in the upper 700 m of the holes; granitic rocks from 760-1920 m measured depth; arkosic and lithic arenites, interbedded with siltstone sequences, from 1920 to ~3150 m measured depth; and interbedded siltstones, mudstones, and shales from 3150 m to 3987 m measured depth. We also infer the presence of at least five fault zones, which include regions of damage zone and fault core on the basis of percent of cataclasite abundances, presence of deformed grains, and presence of alteration phases at 1050, 1600-2000, 2200-2500, 2700-3000, 3050-3350, and 3500 m measured depth in the Main Hole. These zones are correlated with borehole geophysical signatures that are consistent with the presence of faults. If the deeper zones of cataclasite and alteration intensity connect to the surface trace of the San Andreas Fault, then this fault zone dips 80-85° southwest, and consists of multiple slip surfaces in a damage zone ~250-300 m thick. This interpretation is supported by borehole geophysical studies, which show this area is a region of low seismic velocities, reduced resistivity, and variable porosity.

Roughness at the Base of the Seismogenic Zone: Contributing Factors

Article

Full-text available

Jul 1984

Richard H. Sibson

The cutout depth of microseismic activity in continental fault zones appears to correspond to the onset of greenschist metamorphic conditions at about 300°C. It can generally be modeled as the transition from frictional to quasi-plastic behavior in quartzofeldspathic crust. Shear resistance increases with depth through the frictional regime to peak at the transition, beneath which it falls off exponentially with increasing temperature. Larger earthquake ruptures (ML>5.5) nucleate around this transition depth where the highest concentrations of strain energy may accumulate. Varying depth and amplitude of the peak shear resistance along strike induce fluctuations in strain energy concentration at the base of the seismogenic zone. Factors affecting the depth of the transition include crustal composition, geometry and mode of faulting, fluid pressure levels in the frictional regime, and water content in the quasi-plastic regime, quasi-plastic strain rate, and geothermal gradient. Evaluation of their relative importance is complicated because several are interdependent. However, compositional change may cause abrupt irregularities in seismogenic depth and peak shear resistance, while regional variations in heat flow look to be particularly effective in creating long-wavelength heterogeneities in strain energy concentration affecting faulting style.

Recent advances in the understanding of fault zone internal structure: A review

Article

Full-text available

Jul 2008

It is increasingly apparent that faults are typically not discrete planes but zones of deformed rock with a complex internal structure and three-dimensional geometry. In the last decade this has led to renewed interest in the consequences of this complexity for modelling the impact of fault zones on fluid flow and mechanical behaviour of the Earth’s crust. A number of processes operate during the development of fault zones, both internally and in the surrounding host rock, which may encourage or inhibit continuing fault zone growth. The complexity of the evolution of a faulted system requires changes in the rheological properties of both the fault zone and the surrounding host rock volume, both of which impact on how the fault zone evolves with increasing displacement. Models of the permeability structure of fault zones emphasize the presence of two types of fault rock components: fractured conduits parallel to the fault and granular core zone barriers to flow. New data presented in this paper on porosity–permeability relationships of fault rocks during laboratory deformation tests support recently advancing concepts which have extended these models to show that poro-mechanical approaches (e.g., critical state soil mechanics, fracture dilatancy) may be applied to predict the fluid flow behaviour of complex fault zones during the active life of the fault. Predicting the three-dimensional heterogeneity of fault zone internal structure is important in the hydrocarbon industry for evaluating the retention capacity of faults in exploration contexts and the hydraulic behaviour in production contexts. Across-fault reservoir juxtaposition or non-juxtaposition, a key property in predicting retention or across-fault leakage, is strongly controlled by the three-dimensional complexity of the fault zone. Although algorithms such as shale gouge ratio greatly help predict capillary threshold pressures, quantification of the statistical variation in fault zone composition will allow estimations of uncertainty in fault retention capacity and hence prospect reserve estimations. Permeability structure in the fault zone is an important issue because bulk fluid flow rates through or along a fault zone are dependent on permeability variations, anisotropy and tortuosity of flow paths. A possible way forward is to compare numerical flow models using statistical variations of permeability in a complex fault zone in a given sandstone/shale context with field-scale estimates of fault zone permeability. Fault zone internal structure is equally important in understanding the seismogenic behaviour of faults. Both geometric and compositional complexities can control the nucleation, propagation and arrest of earthquakes. The presence and complex distribution of different fault zone materials of contrasting velocity-weakening and velocity-strengthening properties is an important factor in controlling earthquake nucleation and whether a fault slips seismogenically or creeps steadily, as illustrated by recent studies of the San Andreas Fault. A synthesis of laboratory experiments presented in this paper shows that fault zone materials which become stronger with increasing slip rate, typically then get weaker as slip rate continues to increase to seismogenic slip rates. Thus the probability that a nucleating rupture can propagate sufficiently to generate a large earthquake depends upon its success in propagating fast enough through these materials in order to give them the required velocity kick. This propagation success is hence controlled by the relative and absolute size distributions of velocity-weakening and velocity- strengthening rocks within the fault zone. Statistical characterisation of the distribution of such contrasting properties within complex fault zones may allow for better predictive models of rupture propagation in the future and provide an additional approach to earthquake size forecasting and early warnings.

Why large earthquakes do not nucleate at shallow depths

Article

Full-text available

Oct 1983

Although by definition tectonic earthquakes may nucleate anywhere within the seismogenic layer, in almost all cases those earthquakes that become large nucleate near the base of the seismogenic layer. Because frictional strength and stress-drop should increase with depth and in situ stress measurements suggest that the ambient stress increases with depth, we have examined spontaneous rupture models with gradients of both average stress-drop and average strength relative to ambient stress to determine whether this explains the observed phenomena. We report here that ruptures that are nucleated with in the low stress-drop (shallow) regions of the model are inhibited from propagating, but those that are nucleated within the high stress-drop regions can propagate over the entire fault plane.

The eastern Tonale fault zone: A 'natural laboratory' for crystal plastic deformation of quartz over a temperature range from 250 to 700 °C

Article

Full-text available

Dec 2002
J STRUCT GEOL

Near the eastern end of the Tonale fault zone, a segment of the Periadriatic fault system in the Italian Alps, the Adamello intrusion produced a syn-kinematic contact aureole. A temperature gradient from ∼250 to ∼700 °C was determined across the Tonale fault zone using critical syn-kinematic mineral assemblages from the metasedimentary host rocks surrounding deformed quartz veins. Deformed quartz veins sampled along this temperature gradient display a transition from cataclasites to mylonites (frictional–viscous transition) at 280±30 °C. Within the mylonites, zones characterized by different dynamic recrystallization mechanisms were defined: Bulging recrystallization (BLG) was dominant between ∼280 and ∼400 °C, subgrain rotation recrystallization (SGR) in the ∼400–500 °C interval, and the transition to dominant grain boundary migration recrystallization (GBM) occurred at ∼500 °C. The microstructures associated with the three recrystallization mechanisms and the transitions between them can be correlated with experimentally derived dislocation creep regimes. Bulk texture X-ray goniometry and computer-automated analysis of preferred [c]-axis orientations of porphyroclasts and recrystallized grains are used to quantify textural differences that correspond to the observed microstructural changes. Within the BLG- and SGR zones, porphyroclasts show predominantly single [c]-axis maxima. At the transition from the SGR- to the GBM zone, the texture of recrystallized grains indicates a change from [c]-axis girdles, diagnostic of multiple slip systems, to a single maximum in Y. Within the GBM zone, above 630±30 °C, the textures also include submaxima, which are indicative of combined basal 〈a〉- and prism [c] slip.

Neotectonics of the forearc zone and the Setouchi Province in southwest Japan

Article

Jan 1992

Y. Sugiyama

Geology of the Sanbagawa, Chichibu and Shimanto belts in the Kii Peninsula: Yoshino area in Nara prefecture and Kushidagawa area in Mie prefecture

Article

Jan 1996

M. Takeuchi

Fabrics of experimental fault zones: Their development and relationship to mechanical behavior

Article

Jan 1992

Oldest record of brittle deformation along the Median Tectonic Line: fission-track age for Pseudotachylyte in the Taki area, Mie Prefecture

Article

May 2010

Fission-track (FT) dating of pseudotachylyte (PST) associated with the Median Tectonic Line (MTL) in the Taki area, Mie Prefecture, SW Japan, provides new constraints on the timing of movement upon this fault. A PST vein with a thickness of about 5 cm yields a zircon FT age of 60.0 ± 3.5 Ma (1σ). In contrast, a weighted average of zircon FT ages obtained for protolith samples (cataclastic mylonitized Hatai Tonalite) collected 10 cm and 15 m from the PST vein boundary is 69.8 ± 1.2 Ma, which is significantly older than the age of the PST. Decomposition of feldspars in the PST suggests that the temperature exceeded 1100°C, which is sufficient to completely erase previous fission tracks in zircon within several seconds. The distribution of fission-track lengths in zircon from the PST vein also supports the interpretation that the zircon FT age was completely reset during frictional fusion of the PST vein. Considering an apatite FT age of 38.0 ± 1.5 Ma for the host rock, the age of the PST indicates that the frictional fusion was occurred during cooling of the Ryoke granite at temperatures between 250°C (closure temperature of the zircon FT system) and 100°C (closure temperature of the apatite FT system). This PST age is comparable with the oldest K-Ar age obtained for fine fractions of MTL fault gouge derived from both the Sanbagawa pelitic schist and the Izumi Group in Shikoku, indicating that the initiation of brittle fault movement associated with formation of the PST and/or fault gouges along the MTL had occurred by 60 Ma.

Aftershock Distribution of the January 17, 1995 Hyogo-ken Nanbu Earthquake Determined by the JHD Method.

Article

Jan 1996

On January 17, 1995, the Hyogo-ken Nanbu (Kobe) earthquake of magnitude 7.2 occurred in the Kansai district near Kobe, Japan. After the quake, the Net-Hyogo group set temporal seismic stations in and around Kobe and Awaji Island. Using P and S arrival data from temporal observations, the aftershock distribution of the Kobe earthquake was determined. The JHD (joint hypocenter determination) method which eliminates mislocation of hypocenters caused by errors of the simplified model of 1-dimension velocity structure and heterogeneity near stations was used. The hypocenters form a thin flat zone in the Kobe area but form a complex feature in the Awaji area. The aftershock zone strikes N51°E and the total area of the aftershock zone is 60 km wide and 15 km deep. In its northeastern half, the aftershock zone forms nearly a simple plane. In the southwestern half, it shows complicated features. These results show that surface fault traces do not necessarily connect directly with the underground fault geometry estimated by aftershock distribution if the fault dips vertical.

Origin of the Median Tectonic Line and its initial shape

Article

Jan 1980

Geohistory of the Median Tectonic Line of Southwest Japan

Article

Jan 1980

K. Ichikawa

Chapter 2 Fabrics of Experimental Fault Zones: Their Development and Relationship to Mechanical Behavior

Chapter

Dec 1992

The fracture array of simulated fault zones is shown to evolve in a predictable and reproducible manner, from a stepwise fashion to a steady-state condition. At low confining pressures and increasing shear strain the sequence is: (1) Homogeneous shearing by grain-to-grain movements. (2) R2- and R1-fractures initiate at about the same time but propagate only a few grain diameters. They are at relatively high angles to the gouge-forcing block interface and widely spaced. These first two stages are one primarily of gouge compaction characterized by strain hardening. (3) Extension of R1S and coincident reorientation to lower angles closely paralleling the interface with the forcing blocks. P-fractures initiate. These occur from the ultimate strength through a strain softening stage. (4) Y-fractures form along which most of the displacement is accommodated, with the fracture array now close to steady state. Y's initially are close to one or both interfaces with the forcing blocks, but with increasing shear strain shift to the interior of the gouge. At this stage, sliding may change from stable slip to periodic oscillations, characteristic of stick-slip sliding.

Pseudotachylyte veins associated with granitic cataclasite along the Median Tectonic Line, eastern Kii Peninsula, Southwest Japan.

Article

Jan 2001

Fault-generated pseudotachylytes showing reddish, greenish and grayish color have recently been found from the mylonitized Ryoke Granites along the Median Tectonic Line (MTL). These pseudotachylytes are suggested to have a melt origin on the basis of petrographic observations of typical melt-quenched microstructures such as microlites and amygdales. Preferential melting of low melting point minerals is the most probable melting process. The absence of lithic fragments of hornblende and biotite, and systematic decrease of bulk SiO2 contents in the pseudotachylytes compared with their host rocks support this interpretation. These pseudotachylytes were formed after mylonitization and were post-dated by weak cataclasis resulting from movement of the MTL. Similar orientation of the MTL, fault veins of pseudotachylytes, and mylonitic foliation indicate that the deformation sequence of mylonite, pseudotachylyte, and cataclasite formation probably progressed under a single tectonic stress field. These pseudotachylytes are examples of the remnants of ancient faulting with frictional melting indicating seismic activity of the MTL.

Geology of the Ryoke Belt in the eastern Kinki District, Japan:The phase-divisions and the mutual relations of the granitic rocks

Article

Jan 1982

Mylonitic rocks of the Ryoke belt in the Kayumi area, eastern part of the Kii Peninsula

Article

Sep 1985

Hideo Takagi

Isotopic ages of rocks along the Median Tectonic Line in the Kayumi area, Mie Prefecture

Article

Jan 1989

K-Ar, Rb-Sr and fission-track age determinations were carried out on rocks along the Median Tectonic Line (MTL) in the Kayumi area, Mie Prefecture. K-Ar ages on the Ryoke granitic rocks range from 44.1 to 84.3 Ma. Within about 1000m of MTL, the ages for K-feldspars and biotites decrease toward MTL, probably caused by the hydrothermal alteration associated with cataclasis, whereas the ages for homblendes increase toward MTL. The cooling rate of the granitic rocks, calculated from K-Ar ages and closure temperatures of minerals, is 31-56°C/Ma. K-Ar ages for muscovites from pelitic schists in the Sanbagawa belt are 73.6-75.6 Ma; decrease in age is not observed even near MTL. Rb-Sr ages for muscovites from the schists are 72.2-77.5 Ma, nearly the same as the K-Ar ages. These age data are assumed to indicate the time of maximum temperature for the Sanbagawa schists in this area. Fission track ages for zircons from the Ryoke granitic rocks range from 52.4 to 59.5 Ma, approximating to K-Ar ages of K-feldspars for the same rocks. Fission-track ages for zircons from the Sanbagawa schists are 59.0 and 61.2 Ma, nearly 15 Ma younger than the K-Ar ages of muscovite for the same rocks. The fine fractions (<2μm) separated from MTL fault gouges at the Nishitani River and Kayumi give K-Ar ages of 41.8 and 51.4 Ma, respectively. The former age probably represents the time of hydrothermal alteration following the major fault movement of MTL associated with cataclasis.

Geotectonic evolution in transpressional regime: Time and space relationships between mylonitization and folding in the southern Ryoke belt, eastern Kii Peninsula, southwest Japan

Article

Jan 1998

Dynamic evolution of deformation microstructures in rocks. Physical conditions for deformation. Qualitative estimation of deformation temperature and strain rate from microstructure and lattice preferred orientation in plastically deformed quartz aggregates.

Article

Jan 1996

Nobuo Sakakibara

The Role of Fault Zone Drilling

Chapter

Jan 2006

The objective of fault-zone drilling projects is to directly study the physical and chemical processes that control deformation and earthquake generation within active fault zones. An enormous amount of field, laboratory, and theoretical work has been directed toward the mechanical and hydrological behavior of faults over the past several decades. Nonetheless, it is currently impossible to differentiate between - or even adequately constrain - the numerous conceptual models of active faults proposed over the years. For this reason, the Earth science community is left in the untenable position of having no generally accepted paradigm for the mechanical behavior of faults at depth. One of the primary causes for this dilemma is the difficulty of either directly observing or inferring physical properties and deformation mechanisms along faults at depth, as well as the need to observe directly key parameters such as the state of stress acting on faults at depth, pore fluid pressure (and its possible variation in space and time), and processes associated with earthquake nucleation and rupture. Today, we know very little about the composition of active faults at depth, their constitutive properties, the state of in situ stress or pore pressure within fault zones, the origin of fault-zone pore fluids, or the nature and significance of time-dependent fault-zone processes.

Enhanced detection capability of non-volcanic tremor using a 3-level vertical seismic array network, VAnet, in southwest Japan

Article

Oct 2011

This study evaluates a method that enhances detection of non-volcanic low frequency tremor (NVT) using VA-net, a 3-level vertical seismic array network that was recently constructed in southwest Japan. The method employs semblance analysis for continuous vertical array records and calculates semblance coefficients as a function of time and apparent vertical velocity. The sign of the best apparent velocity provides information about propagation direction, either upward or downward, allowing easy discrimination between seismic signals of natural origin and unwanted noise such as man-made vibrations. We applied the method to a NVT episode that occurred beneath the Kii Peninsula during November 2008, to demonstrate that NVT signals can be detected using only the 3-level array. Furthermore, we demonstrate that the method enables us to detect minor NVT activity which is not detected by a conventional envelope cross-correlation method, resulting in a dramatic improvement in detection capability.

High magnetic susceptibility of fault gouge within Taiwan Chelungpu fault: Nondestructive continuous measurements of physical and chemical properties in fault rocks recovered from Hole B, TCDP

Article

Aug 2006

The Taiwan Chelungpu-fault Drilling Project (TCDP) was undertaken in 2002 to investigate the faulting mechanism of the 1999 Taiwan Chi-Chi earthquake. Hole B penetrated the Chelungpu fault, and recovered core samples from between 948.42 m and 1352.60 m depth. Three zones, marked 1136mFZ, 1194mFZ and 1243mFZ, were recognized in the core samples as active fault-zones within the Chelungpu fault. Multi-Sensor Core Logger measurements revealed lower densities and higher magnetic susceptibilities within the black gouge zones in all three fault zones. Even though the fault zone that slipped during the 1999 earthquake has not been identified, higher magnetic susceptibilities indicate that frictional heating has taken place in the Chelungpu fault.

Progressive strain localization in a major strike-slip fault exhumed from midseismogenic depths: Structural observations from the Salzach-Ennstal-Mariazell-Puchberg fault system, Austria

Article

Apr 2009

Analysis of a strike-slip fault exhumed from midseismogenic depths reveals that the fault experienced progressive strain localization toward a high-strain fault core. We focus on the Ennstal segment of the 400-km-long Salzach-Ennstal-Mariazell-Puchberg (SEMP) strike-slip fault system in the Eastern Alps, which accommodated ∼60 km of left lateral displacement during Oligo-Miocene time. Macroscopic and microscopic observations reveal a zoned fault featuring a high-strain core at least 10 m wide within a fault zone that is at least 150 m wide. Grain-size distribution analysis shows how the Ennstal segment of the SEMP evolved. Our data reveal a 10-m-wide high-strain fault core (characterized by a power law relationship of grain sizes, D2 ≈ 2.0) bordered by a 54-m-wide “transition zone” where the largest and smallest grains are characterized by two power law relationships (D2 ≈ 2.0 and 1.6, respectively). This zone is in turn bordered by a region with grain sizes that show a single low-strain power law relationship of D2 ≈ 1.6. We interpret these relationships to be the result of concentrated shear overprinting an initial low-strain, power law grain-size distribution before strain localized to the core. This is consistent with the theory that faults mature by smoothing geometrical complexities, forming a highly localized, high-strain fault core. The data do not support the idea that damage forms primarily in response to dynamic stresses during seismic rupture, although they do suggest that this mechanism may operate within tens of meters of the fault once it has developed its zoned structure.

Fault rocks and past to recent fluid characteristics from the borehole survey of the Nojima fault ruptured in the 1995 Kobe earthquake, southwest Japan

Article

Jul 2000

The mineralogy, fluid inclusions, and distribution of fault rocks of the Nojima fault were examined in the core recovered from a borehole drilled by the Geological Survey of Japan (GSJ) 12 months after the 1995 Kobe (Hyogo-ken Nanbu) earthquake (MJMA=7.2) in southwest Japan. The borehole was drilled across a slipped portion of the fault to a depth of 746.7 m. Nearly continuous coring between 152.2 and 746.7 m recovered granodiorite protolith, porphyry dikes, and fault-related rocks. The fault zone was intersected at 426.2 m and is characterized by a greater intensity of brittle deformation and/or hydrothermal alteration than typical host granodiorite. The fault core consists of three types of fault gouge and occurs at the depth range of 623.1 to 625.3 m. The fault-normal thicknesses of the fault core and the fault zone are 0.3 m and >46.5 m, respectively. Three types of hydrothermal alteration are recorded by mineral assemblages and fluid inclusions. The first type is characterized by chloritization of mafic minerals at >200°C and occurred prior to the fault activity during the intrusion and cooling of the granodiorite. The second type occurred during faulting and is recorded by zeolite mineralization at

Structural and chemical characterization of shear zones in the freshly activated Nojima fault, Awaji Island, southwest Japan

Article

May 2001

Behavior and role of each shear zone in a shallow fault zone of granitic origin during seismic cycles are revealed by comprehensive examinations of petrographic and chemical characterization on a fault zone in the Geological Survey of Japan (GSJ) drill core penetrating the Nojima fault which was activated during the 1995 Hyogoken-Nanbu earthquake (M=7.2). The GSJ core consists of granodiorite and porphyritic intrusive rocks including a Nojima fault zone which involves seven thin shear zones: main shear zone (MSZ, 625 m depth), upper cataclasite zone (UCZ), upper shear zone (USZ), lower shear zones (LSZ-1 and LSZ-2), and lower cataclasite zones (LCZ-1 and LCZ-2). These shear zones are generally surrounded by weakly pulverized and altered (fault-related) rocks (WPAR) which generally show volume gain. The fault zone architecture is clarified as follows: (1) Total thickness of the Nojima fault zone is ~70 m. (2) All shear zones except the older cataclasite zones (UCZ-1, LCZ-1, and LCZ-2) were evolved from WPAR, indicating that pulverization and alteration of recent activity were more diffused at the initial stage of faulting and gradually localized to each shear zone. (3) The MSZ (2 m thick) can be regarded as a high-velocity frictional zone with accompanying volume loss (compaction) and possibly with heat generation during coseismic periods. (4) The LSZ-1 (3.6 m thick), located just beneath the MSZ and typically showing explosion brecciation texture, is also regarded as a coseismic shear zone. This zone could function as a trap zone for fluid or gas during postseismic/interseismic periods. (5) The LSZ-2 (2.7 m thick), located around 710 m depth, contains foliated fault gouge enriched with clay minerals and characterized by a large degree of volume gain, possibly a result of slow velocity motion or creep during the interseismic and/or postseismic periods.

Gently north-dipping Median Tectonic Line (MTL) revealed by recent seismic reflection studies, southwest Japan

Article

Oct 1996
TECTONOPHYSICS

The Median Tectonic Line (MTL), with a length of more 1000 km, is the most significant fault in Japan. It juxtaposes the high-P/T Sambagawa metamorphic rocks against the low-P/T metamorphic rocks of the Ryoke belt. The MTL was probably formed in the Cretaceous with many subsequent reactivations. The western segment of the MTL is still active with an almost pure right-lateral sense of motion. Although a great amount of geological information on the MTL has been accumulated, information about the subsurface, especially the deep-seated structure of the MTL, is still insufficient. It has been generally assumed that the MTL is vertical or steeply dipping at depth because of its straight surface trace and its recent lateral motion. Recently, new geophysical data have suggested that the MTL dips gently northward at depth. We have acquired a complementary set of geophysical profiles (seismic reflection and refraction, gravity and MT) across the MTL in east Shikoku. Our results confirm that the MTL dips northward at about 30 to 40 degrees from the surface to about 5 km depth, where it becomes listric. This fault geometry more reasonably explains the reactivation history of the MTL: the motion has occurred on a listric-type fault in so-called oblique or lateral ramp manner.

Ultracataclasite structure and friction processes of the Punchbowl Fault

Article

Paleocene large-scale normal faulting along the Median Tectonic Line, western Shikoku, Japan

Article

Dec 2007
ISL ARC

Abstract The Median Tectonic Line (MTL) in southwest Japan, a major east–west-trending arc-parallel fault, has been defined as the boundary fault between the Cretaceous Sambagawa metamorphic rocks and Ryoke granitic and metamorphic rocks, which are unconformably covered by the Upper Cretaceous Izumi Group. The juxtaposition by faulting occurred after the deposition of the Izumi Group. Based on detailed fieldwork and previous studies, the authors reconstruct the kinematic history along the MTL during the Paleogene period, which has not been fully understood before. It is noted that although the strata of the Izumi Group along the MTL dip gently, east–west-trending north-vergent folds with the wavelength of ∼300 m commonly develop up to 2 km north from the MTL. Along the MTL, a disturbed zone of the Izumi Group up to 400 m thick, defined by the development of boudinage structures with the transverse boudin axis dipping nearly parallel to the MTL, occurs. Furthermore, east–west-trending north-vergent folds with the wavelength of 1–5 m develop within the distance up to 60 m from the MTL. The disturbed zone with the map-scale north-vergent folds along the MTL, strongly suggests that they formed due to normal faulting with a top-to-the-north sense along the MTL. Considering that the normal faulting is associated with the final exhumation of the Sambagawa metamorphic rocks, and its juxtaposition against the Izumi Group at depth, this perhaps occurred before the denudation of the Sambagawa metamorphic rocks indicated by the deposition of the Lower Eocene Hiwada-toge Formation. Dynamic equilibrium between crustal thickening at depth (underplating) and extension at shallow level is a plausible explanation for the normal faulting because the arc-normal extension suggests gravity as the driving force.

Structural evidence for large‐scale top‐to‐the‐north normal displacement along the Median Tectonic Line in southwest Japan

Article

Mar 2007
ISL ARC

Abstract The Median Tectonic Line (MTL) is a first-order tectonic boundary that separates the Sanbagawa and Ryoke Metamorphic Belts. Strike-slip movements on the MTL have been well documented by many workers. New field based structural studies in the Sanbagawa Belt close to the MTL reveal a large number of secondary faults and semibrittle shear bands indicating a top-to-the-north normal sense of displacement. The strikes of these shear zones and their spatial distributions suggest that development of these structures is related to movements on the MTL. These results imply that the MTL has a large-scale normal shear component on a regional scale that can help account for the exhumation of the Sanbagawa Belt. Our proposed history of the MTL can also account for changes in the geometry of folds in the Sanbagawa Belt.

Crustal structure of southwest Japan, revealed by the integrated seismic experiment Southwest Japan 2002

Article

Jul 2009
TECTONOPHYSICS

A multi-purpose seismic experiment named the 2002 integrated seismic experiment Southwest Japan was conducted in 2002 along a more-than-240-km-long line across southwest Japan from the Pacific coast to the Japan Sea coast. Its profile provides the first crustal-scale cross section across the Japanese island arc, which highlights a number of significant points related to the structural development of the arc. Major outstanding points are that the Japanese island arc is composed of two completely different crusts juxtaposed by the Median Tectonic Line (MTL), and that the MTL started its activity associated with lower crustal thinning and formation of an upper crustal-scale half-graben in Late Cretaceous.

Implications of mylonitic microstructures for the geotectonic evolutin of the Median Tectonic Line, central Japan

Article

Dec 1986
J STRUCT GEOL

Hideo Takagi

The Median Tectonic Line (MTL), the most prominent onshore fault in Japan, demarcates the Cretaceous Hiji quartz dioritic gneiss of the Ryoke belt on the west from the high P/T type Sambagawa metamorphic rocks on the east in the Takato area. Toward the MTL, the Hiji gneiss grades into strongly mylonitized rocks characterized by grain-size reduction of quartz. In the mylonitic rocks, the development of fluxion banding (Sm) is remarkably influenced by the existence of K-feldspar, forming a myrmekitic intergrowth. Brittle microstructures indicative of truly cataclastic deformation are observed only in mylonitic rocks close to the MTL. Early deep-level ductile deformation apparently gave way to shallower, brittle deformation at a later stage. The attitude of stretching lineations (Lm) and asymmetric microstructures observed in the mylonites suggest that sinistral strike-slip shearing with a subordinate component of vertical-slip took place during mylonitization in mid-Cretaceous time. The mylonitic rocks and their protolith, supposed to have constituted the eastern limb of the shear zone, were probably eroded out and lost by upheaval of the Sambagawa belt relative to the Ryoke belt.

Outline of the fault zone drilling project by NIED in the vicinity of the 1995 Hyogo‐ken Nanbu earthquake, Japan

Article

Sep 2001
ISL ARC

Ryuji Ikeda

Abstract Three boreholes, 1001 m, 1313 m and 1838 m deep, were drilled by the National Research Institute for Earth Science and Disaster Prevention (NIED) in the vicinity of the epicenter of the 1995 Hyogo-ken Nanbu (Kobe) earthquake to investigate tectonic and material characteristics near and in active faults. Using these boreholes, an integrated study of the in situ stress, heat flow, and material properties of drill cores and crustal resistivity was conducted. In particular, the Nojima–Hirabayashi borehole was drilled to a depth of 1838 m and directly intersected the Nojima Fault, and three possible fault strands were detected at depths of 1140 m, 1313 m and 1800 m. Major results obtained from this study include the following: (i) shear stress around the fault zone is very small, and the orientation of the maximum horizontal compression is perpendicular to the surface trace of faults; (ii) from the results of a heat flow study, the lower cut-off depth of the aftershocks was estimated to be roughly 300°C; (iii) cores were classified into five types of fault rocks, and an asymmetric distribution pattern of these fault rocks in the fracture zones was identified; (iv) country rock is characterized by a very low permeability and high strength; and (v) resistivity structure can be explained by a model of a fault extending to greater depths but with low resistivity.

Continuous Observation of Groundwater and Crustal Deformation for Forecasting Tonankai and Nankai Earthquakes in Japan

Article

Aug 2010

In 2006, we started construction of an observation network of 12 stations in and around Shikoku and the Kii Peninsula to conduct research for forecasting Tonankai and Nankai earthquakes. The purpose of the network is to clarify the mechanism of past preseismic groundwater changes and crustal deformation related to Tonankai and Nankai earthquakes. Construction of the network of 12 stations was completed in January 2009. Work on two stations, Hongu-Mikoshi (HGM) and Ichiura (ICU), was finished earlier and they began observations in 2007. These two stations detected strain changes caused by the slow-slip events on the plate boundary in June 2008, although related changes in groundwater levels were not clearly recognized. KeywordsGroundwater–strain–tremor–slow-slip event–Nankai earthquake–Tonankai earthquake

Fault structure, wear mechanisms and rupture processes in pseudotachylyte generation

Article

Apr 1992
TECTONOPHYSICS

Mark Swanson

Fault-generated pseudotachylyte is found within both cataclastic and mylonitic host rocks suggesting that rapid catastrophic displacements have occurred at a variety of depths within paleoseismogenic zones. Pseudotachylyte-bearing fault zones represent a composite of structural features associated with the process of earthquake rupture propagation and coseismic slip. The development of multiple pseudotachylyte veins in fault linkages, duplexes, sidewall ripouts, en echelon arrays and brittle zones suggests repeated rupturing during a series of characteristic earthquakes. Each earthquake, as a coseismic slip event, can be subdivided into initial rupture, acceleration, stable sliding and final deceleration stages. These evolve through distinctive sequences of wear and deformation mechanisms that vary with sliding velocity, duration of slip, total displacement and the hydrodynamics of the developing fault zone. Slip is thought to proceed toward surface refinement and possible frictional melting following the propagation of leading shear fracture process zones. The passage of the initial process zone of oblique fracturing would be followed by linkage to a throughgoing structure with asperity reduction through brecciation, comminution and refined cataclastic flow for frictional melting in an abrasive wear-dominant model. At greater depths in the presence of mylonitic anisotropy, slip would proceed through initial layer-parallel surfaces and duplex linkages with rapid surface refinement through plastic smearing and laminar flow for frictional melting in an adhesive wear-dominant model.

Deformation of granitic-rocks across the brittle ductile transition A

Article

Dec 1985
J STRUCT GEOL

Carol Simpson

A microstructural analysis has been carried out on mylonites and mylonitic gneisses of the Eastern Peninsular Ranges Mylonite Zone, which were formed over a range of metamorphic conditions from lower greenschist to amphibolite facies. Composite planar fabrics in the form of C and S planes are found at all metamorphic grades. Fractured feldspars, kinked biotites and ductile deformation of quartz characterize the lower greenschist facies mylonites. At mid-upper greenschist grade orthoclase grains show dynamic recrystallization textures whereas plagioclase exhibits low temperature plasticity with only minor recovery. Biotite ribbons form by progressive rotation and coalescence of kink band segments to produce chevron fold patterns. At epidote-amphibolite grade and above, recovery processes and annealing recrystallization predominate in all minerals. Residual orthoclase porphyroclasts show strain-related myrmekite formation along those sides of the grains that face the instantaneous shortening direction. Myrmekite formation due to replacement reactions cannot explain this geometry. It is proposed that the myrmekites formed due to a combination of exsolution, replacement and strain-enhanced diffusion.

The nature and importance of phyllonite development in crustal-scale fault cores: An example from the Median Tectonic Line, Japan

Article

Feb 2006
J STRUCT GEOL

Like many large, crustal-scale faults, the Median Tectonic Line (MTL) in SW Japan has a long history of movement, having been active predominantly as a strike-slip fault since the mid-Cretaceous. Fault rock exposures in the core of the MTL preserve a history of deformation at a range of mid- to shallow-crustal depths. Ryoke mylonites 1–4 km north of the main contact record deeper level, Cretaceous top-to-the-south sinistral movements. The remainder of the fault zone core is surprisingly narrow, exhibiting a wide variety of fault rocks that illustrate both the interaction and effects of syn-tectonic fluid influx over a range of deformation conditions. Exposures within 50 m of the central slip zone display a progressive sequence in fault rock evolution from ultramylonite→cataclasite→foliated cataclasite→phyllonite→breccia/gouge. This sequence occurs because cataclasis in the vicinity of the fault core creates permeable pathways for the ingress of chemically active fluids into the fault zone. This leads to the replacement of load-bearing phases, such as feldspar, by fine-grained, foliated aggregates of intrinsically weaker phyllosilicates such as white mica and chlorite. The grain size reduction associated with both cataclasis and mineral alteration creates conditions ideal for the operation of fluid-assisted, stress-induced diffusive mass transfer mechanisms. Comparison with the findings of recent experimental studies suggest that the fault zone processes observed in the core of the MTL will lead to long-term weakening, provided the network of phyllosilicate-rich fault rocks are able to form an interconnected thin layer of weak material on kilometre- to tens of kilometre-length scales.

Seismogenic layer, reflective lower crust, surface heat flow and large inland earthquakes

Article

Jun 1999
TECTONOPHYSICS

Kiyoshi Ito

Regional variations in the cutoff depth of seismicity in southwest Japan are derived from more than 60,000 well-determined earthquakes. The thickness of the seismogenic layer is closely related to the strength of the crust and accordingly to the occurrence of large inland earthquakes, since the seismic–aseismic boundary is thought to be related to the brittle–ductile boundary in the crust. Large inland earthquakes are likely to occur in the area where the cutoff depth of seismicity changes abruptly. It is therefore important to survey the regional variations in this depth to evaluate the potential of large inland earthquakes. The cutoff depth of seismicity roughly coincides with a temperature range of 300–400°C in the crust. In addition, distinct S-wave reflectors as well as a reflective lower crust have generally been observed in some areas in Japan. The depths of the reflector and the top of the reflective lower crust lie several kilometers below the cutoff depth of seismicity and they seem to coincide with a temperature range of about 300–450°C. Therefore, in those areas where no earthquakes occur but a reflective layer exists, the cutoff depth can be mapped from a survey of reflectivity in the crust. The heterogeneity of the crust is very important for understanding the nature of the crust where large earthquakes are frequent. This paper proposes a model of crust inhomogeneity based on observation results.

Deformation of granitoids at low metamorphic grade. I: Reactions and grain size reduction

Article

May 1993
TECTONOPHYSICS

Deformation and chemical reaction in granitoids at greenschist faciès conditions were investigated in samples from three shear zones: two from the Alps (Corvatsch Granodiorite and Aiguilles Rouges Granite, Switzerland) and one from Eastern Australia (Wyangala Granite). When examined by light and electron microscopy, all show similar features of deformation that progressed from coarse parent rock through to fine grained mylonite: quartz deformed plastically, while both K-feldspar and plagioclase fractured and recrystallized in conjunction with chemical change, which had, as its end point, an assemblage albite + quartz ± white mica ± CaAl-silicates. K-feldspar and calcic plagioclase (depending on fluid chemistry) from the parent rock were unstable at low metamorphic grades in the presence of aqueous fluid. Since Ca-bearing plagioclase was not stable in these environments, myrmekite did not replace K-feldspar in any of the rocks examined.Recrystallization of feldspar, which should perhaps be termed neocrystallization, was initiated predominantly at clast margins or along microfractures that are marked by fluid inclusions and twin offsets. In old feldspar grains, particularly in plagioclases, dislocations exist in wall-like structures that are commonly associated with voids, suggesting origins in microfractures. These dislocations had severely limited mobility and subgrain rotation does not appear to have contributed to recrystallization.We conclude that recrystallization mainly occurred via a classical nucleation mechanism, with minor contributions from twin boundary migration. We argue that the 450–550°C lower limit for recrystallization of feldspar, referenced throughout the literature, is not applicable to the rock systems under investigation and should be discarded as a universal limit. Microfracturing, microboudinage and reaction are seen as the prerequisites for mylonite formation from coarse grained granitic rocks at low metamorphic grades (see part II of this paper). Neocrystallization, fluid access and nucleation appear to have accompanied, or immediately followed, the first deformation increments and were primarily responsible for continued grain size reduction. Transgranular fracture was comparatively important only at the onset of deformation. Since the formation of shear zones in granitoids commonly takes place under conditions of low metamorphic grade, in the presence of aqueous fluids in the Earth's crust, breakdown reactions and nucleation-recrystallization in feldspars may be important influences upon localization of deformation in granitoids.

Non-volcanic deep low-frequency tremors accompanying slow slips in the southwest Japan subduction zone

Article

Apr 2006
TECTONOPHYSICS

Non-volcanic deep low-frequency tremors in southwest Japan exhibit a strong temporal and spatial correlation with slow slip detected by the dense seismic network. The tremor signal is characterized by a low-frequency vibration with a predominant frequency of 0.5–5 Hz without distinct P- or S-wave onset. The tremors are located using the coherent pattern of envelopes over many stations, and are estimated to occur near the transition zone on the plate boundary on the forearc side along the strike of the descending Philippine Sea plate. The belt-like distribution of tremors consists of many clusters. In western Shikoku, the major tremor activity has a recurrence interval of approximately six months, with each episode lasting over a week. The tremor source area migrates during each episode along the strike of the subducting plate with a migration velocity of about 10 km/day. Slow slip events occur contemporaneously with this tremor activity, with a coincident estimated source area that also migrates during each episode. The coupling of tremor and slow slip in western Shikoku is very similar to the episodic tremor and slip phenomenon reported for the Cascadia margin in northwest North America. The duration and recurrence interval of these episodes varies between tremor clusters even on the same subduction zone, attributable to regional difference in the frictional properties of the plate interface.

Characterization of slip zone associated with the 1999 Taiwan Chi-Chi earthquake: X-ray CT image analyses and microstructural observations of the Taiwan Chelungpu fault

Article

May 2008
TECTONOPHYSICS

To characterize the fault-related rocks within the Chelungpu fault, we performed X-ray computed tomography (CT) image analyses and microstructural observations of Hole B core samples from the Taiwan Chelungpu-fault Drilling Project. We identified the slip zone associated with the 1999 Chi-Chi earthquake, within the black gouge zone in the shallowest major fault zone, by comparison with previous reports. The slip zone was characterized by low CT number, cataclastic (or ultracataclastic) texture, and high possibility to have experienced a mechanically fluidized state. Taking these characteristics and previous reports of frictional heating in the slip zone into consideration, we suggested that thermal pressurization was the most likely dynamic weakening mechanism during the earthquake.

Ultracataclasite Structure and Friction Processes of the Punchbowl Fault, San Andreas System, California

Article

Sep 1998
TECTONOPHYSICS

The Punchbowl fault is an exhumed, 40+ km displacement fault of the San Andreas system. In the Devil's Punchbowl, the fault contains a continuous ultracataclasite layer along which the Punchbowl Formation sandstone and an igneous and metamorphic basement complex are juxtaposed. The fabric of the ultracataclasite layer and surrounding rock indicate that nearly all of the fault displacement occurred in the layer. By analogy with nearby active faults, we assume that the Punchbowl fault was seismogenic and that the ultracataclasite structure records the passage of numerous earthquake ruptures. We have mapped the ultracataclasite layer at 1 : 1 and 1 : 10 to determine the mode of failure and to constrain the processes of seismic slip. On the basis of color, cohesion, fracture and vein fabric, and porphyroclast lithology, two main types of ultracataclasite are distinguished in the layer: an olive-black ultracataclasite in contact with the basement, and a dark yellowish brown ultracataclasite in contact with the sandstone. The two are juxtaposed along a continuous contact that is often coincident with a single, continuous, nearly planar, prominent fracture surface (pfs) that extends the length of the ultracataclasite layer in all exposures. No significant mixing of the brown and black ultracataclasites occurred by offset on anastomosing shear surfaces that cut the contact or by mobilization and injection of one ultracataclasite into the other. The ultracataclasites are cohesive throughout except for thin accumulations of less cohesive, reworked ultracataclasite along the pfs. Structural relations suggest that: (1) the black and brown ultracataclasite were derived from the basement and sandstone, respectively; (2) the black and brown ultracataclasites were juxtaposed along the pfs; (3) the subsequent, final several kilometers of slip on the Punchbowl fault occurred along the pfs; and (4) earthquake ruptures followed the pfs without significant branching or jumping to other locations in the ultracataclasite. By comparison with rock friction experiments, the slip localization along the pfs in the ultracataclasite implies rate weakening behavior with a critical slip distance similar to laboratory values, and thus relatively small nucleation and breakdown dimensions for earthquake ruptures. Of the various mechanisms proposed to explain the low strength of the San Andreas and to produce dynamic weakening of faults, those that require or assume extreme localization of slip are most compatible with our observations.

Dislocation creep regimes in quartz

Article

Feb 1992
J STRUCT GEOL

Using optical and TEM microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates, depending on the relative rates of grain boundary migration, dislocation climb and dislocation production. Within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization. At lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion-controlled dislocation climb to be an effective recovery mechanism. In this regime recovery is accommodated by strain-induced grain boundary migration recrystallization. With an increase in temperature or decrease in strain rate, the rate of dislocation climb becomes sufficiently rapid to accommodate recovery. In this regime dynamic recrystallization occurs by progressive subgrain rotation. With a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery. However, in this regime grain boundary migration is rapid, thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation. The identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers. In addition, the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred.

Internal structure and permeability of a major strike-slip fault zones: The Median Tectonic Line in Mie Prefecture, Southwest Japan

Article

Jan 2003
J STRUCT GEOL

Fault zone structure and permeability data from the Median Tectonic Line (MTL) in Mie Prefecture, Southwest Japan suggest that fault permeability models are currently too simplistic for such large structurally complex fault zones. Ryoke Belt mylonites are cut by mineralised brittle structures up to 300 m North of the MTL that show evidence of fluid circulation. The Sambagawa schist on the south side of the MTL is deformed into foliated quartz/phyllosilicate gouge across a 15-m-wide zone. The complex fault contact area has foliated cataclasite up to 4 m wide, and is cut by a narrow central planar slip zone that probably represents the most recent seismogenic principal displacement zone. Laboratory-determined permeability data show wide variation with fault rock microstructure (e.g. gouge microclast size), controlled by structural position in the fault zone and slip zone intersections. Central slip zone gouges have the lowest permeabilities of all of the fault rocks studied. Fault permeability models should take into account asymmetry where widely contrasting protolith lithologies exist and large permeability variations within a complex central fault zone ‘core’. Pore pressure evolution during rupture propagation may vary greatly because of this complexity, but thermal pressurisation is feasible above certain pressures if the slip remains within fine-grained gouge. The different deformation behaviours of contrasting protolith lithologies control the fault zone fabrics and hence final permeability structure.

Questions about myrmekite in deformed rocks

Article

Dec 1991
J STRUCT GEOL

Ron Vernon

Several questions about the development of myrmekite in deformed felsic and metapelitic rocks remain unanswered. Although a general relationship between deformation and myrmekite appears to be well established, the fact that myrmekite grows into relatively non-deformed K-feldspar, while being simultaneously deformed and recrystallized at the rear of the growing lobes, suggests that strain energy may not be a major contributor to the advance of the growth interface of the myrmekite colony. Furthermore, because myrmekite typically nucleates on existing plagioclase, rather than in K-feldspar, strain energy may not contribute directly to nucleation of myrmekite either, at least in many instances. However, strain energy probably contributes indirectly by facilitating access of fluids to growth sites, thereby altering the local chemical environment and so promoting development of myrmekite. Because myrmekite recrystallizes readily in deformed felsic rocks, it is one of the main contributors to the development of folia.

Internal structure of the Median Tectonic Line fault zone, SW Japan, revealed by borehole analysis

Abstract

No full-text available

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