Occurrence of mylonite zones and pseudotachylyte veins around the base of the upper crust: An example from the southern Hidaka metamorphic belt, Samani area, Hokkaido, Japan

Shimada, Kōji; Tanaka, Hidemi; Toyoshima, Tsuyoshi; Obara, Tomohiro; Niizato, Tadafumi

doi:10.1186/bf03353343

Cited by 7 publications

(3 citation statements)

References 8 publications

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“…Similar occurrence of localized mylonite zones and pseudotachylyte veins are also observed along Simono-sawa stream (B in Fig. 1, Shimada et al, 2004).…”

Section: Descriptions Of Mylonite and Pseudotachylyte In Localized Shsupporting

confidence: 76%

Heterogeneous material distribution, an important reason for generation of strain-localized mylonite and frictional slip zones in the Hidaka metamorphic belt, Hokkaido, Japan

et al. 2014

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Lithological heterogeneity of low P/T metamorphic rocks in southern area of Hidaka metamorphic belt (HMB) was formed through historical development of HMB while these rocks had been laid in ductile lower crust. Many strain-localized mylonite zones (<100 m in thickness) are preferentially developed within S-type tonalite and pelitic gneiss, which are characterized by a large modal amount of phyllosilicates (biotite+muscovite+chlorite) and quartz, compared to other lithofacies in HMB. Mylonitic foliations are more conspicuous with close to the center of the shear zone associated with increase in amounts of phyllosilicate minerals, indicating fluidenhanced weakening mechanisms were operated in plastic shear zones. Pseudotachylyte veins are observed exclusively in these mylonite zones, which were generated during exhumation stage of HMB. We conclude the seismic slip zones in southern HMB had been initiated in the ductile lower crust by concentration of localized plastic shear zones within the phyllosilicate-and quartz-rich lithofacies, which were heterogeneously formed by old metamorphic and magmatic events. Then these zones were further weakened by fluid-enhanced plastic deformation, and finally seismic slips occurred at the bottom of seismogenic upper crust, during exhumation of HMB.

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“…Similar occurrence of localized mylonite zones and pseudotachylyte veins are also observed along Simono-sawa stream (B in Fig. 1, Shimada et al, 2004).…”

Section: Descriptions Of Mylonite and Pseudotachylyte In Localized Shsupporting

confidence: 76%

Heterogeneous material distribution, an important reason for generation of strain-localized mylonite and frictional slip zones in the Hidaka metamorphic belt, Hokkaido, Japan

et al. 2014

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“…Melting and fluidization seems to be the main mechanisms of dynamic weakening of shear zones at this depth [ Tanaka et al , 2001c; Ohtsuki et al , 2003]. Similar occurrences of deep shear zones are also recognized in Hidaka metamorphic zones, northern Japan [ Toyoshima et al , 2004; Tanaka et al , 2004; Shimada et al , 2004]. Fracture zones became greatly widened at the middle depth of the crust, associated with the precipitation of zeolite (stage II shear zones).…”

Section: Discussionmentioning

confidence: 78%

Architectural evolution of the Nojima fault and identification of the activated slip layer by Kobe earthquake

Tanaka¹,

Omura²,

Matsuda³

et al. 2007

J. Geophys. Res.

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Evolutionary history of Nojima Fault zone is clarified by comprehensive examinations of petrological, geophysical, and geochemical characterizations on a fault zone in deep‐drilled core penetrating the Nojima Fault. On the basis of the results, we reconstruct a whole depth profile of the architecture of the Nojima Fault and identify the primal slip layer activated by 1995 Kobe earthquake. The deepest part (8‐ to 12‐km depth) of the fault zone is composed of thin slip layers of pseudotachylite (5 to 10 mm thick each, 10 cm in total). Middle depth (4‐ to 8‐km depth) of the fault zone is composed of fault core (6 to 10 m thick), surrounded by thick (100 m thick) damage zone, characterized by zeolite precipitation. The shallow part of the fault zone (1‐ to 4‐km depth) is composed of distributed narrow shear zones, which are characterized by combination of thin (0.5 cm thick each, 10 cm in total) ultracataclasite layers at the core of shear zones, surrounded by thicker (1 to 3 m thick) damage zones associated with carbonate precipitation. An extremely thin ultracataclasite layer (7 mm thick), activated by the 1995 Kobe earthquake, is clearly identified from numerous past slip layers, overprinting one of the shear zones, as evidenced by conspicuous geological and geophysical anomalies. The Nojima Fault zone was 10 to 100 times thicker at middle depth than that of shallower and deeper depths. The thickening would be explained as a combination of physical and chemical effects as follows. (1) Thickening of “fault core” at middle depth would be attributed to normal stress dependence on thickness of the shear zone and (2) an extreme thickening of “damage zone” in middle depth of the crust would result from the weakening of the fault zone due to super hydrostatic fluid pressure at middle depths. The high fluid pressure would result from faster sealing with low‐temperature carbonate at the shallower fault zone.

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“…A large temperature anomaly can result in a weak zone with low seismic velocity that can be observed as a heterogeneous velocity structure in the seismic tomography data (Wittlinger et al 1998). Furthermore, mylonite outcrops of exhumed faults (White et al 1980) provide direct evidence for the existence of ductile shear zones in the lower crust under interplate (e.g., Rutter 1999;Little et al 2002) and intraplate faults (e.g., Shimada et al 2004;Fusseis et al 2006;Takahashi 2015).…”

Section: Introductionmentioning

confidence: 99%

Shear strain concentration mechanism in the lower crust below an intraplate strike-slip fault based on rheological laws of rocks

Zhang

Sagiya

2017

Earth Planets Space

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We conduct a two-dimensional numerical experiment on the lower crust under an intraplate strike-slip fault based on laboratory-derived power-law rheologies considering the effects of grain size and water. To understand the effects of far-field loading and material properties on the deformation of the lower crust on a geological time scale, we assume steady fault sliding on the fault in the upper crust and ductile flow for the lower crust. To avoid the stress singularity, we introduce a yield threshold in the brittle-ductile transition near the down-dip edge of the fault. Regarding the physical mechanisms for shear strain concentration in the lower crust, we consider frictional and shear heating, grain size, and power-law creep. We evaluate the significance of these mechanisms in the formation of the shear zone under an intraplate strike-slip fault with slow deformation. The results show that in the lower crust, plastic deformation is possible only when the stress or temperature is sufficiently high. At a similar stress level, ∼100 MPa, dry anorthite begins to undergo plastic deformation at a depth around 28-29 km, which is about 8 km deeper than wet anorthite. As a result of dynamic recrystallization and grain growth, the grain size in the lower crust may vary laterally and as a function of depth. A comparison of the results with constant and nonconstant grain sizes reveals that the shear zone in the lower crust is created by power-law creep and is maintained by dynamically recrystallized material in the shear zone because grain growth occurs in a timescale much longer than the recurrence interval of intraplate earthquakes. Owing to the slow slip rate, shear and frictional heating have negligible effects on the deformation of the shear zone. The heat production rate depends weakly on the rock rheology; the maximum temperature increase over 3 Myr is only about several tens of degrees.

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Occurrence of mylonite zones and pseudotachylyte veins around the base of the upper crust: An example from the southern Hidaka metamorphic belt, Samani area, Hokkaido, Japan

Cited by 7 publications

References 8 publications

Heterogeneous material distribution, an important reason for generation of strain-localized mylonite and frictional slip zones in the Hidaka metamorphic belt, Hokkaido, Japan

Heterogeneous material distribution, an important reason for generation of strain-localized mylonite and frictional slip zones in the Hidaka metamorphic belt, Hokkaido, Japan

Architectural evolution of the Nojima fault and identification of the activated slip layer by Kobe earthquake

Shear strain concentration mechanism in the lower crust below an intraplate strike-slip fault based on rheological laws of rocks

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