Geological evidence for shallow ductile-brittle transition zones along subduction interfaces: example from the Shimanto Belt, SW Japan
© Hashimoto and Yamano; licensee Springer. 2014
Received: 26 February 2014
Accepted: 1 October 2014
Published: 16 October 2014
Tectonic mélange zones within ancient accretionary complexes include various styles of strain accommodation along subduction interfaces from shallow to deep. The ductile-brittle transition at shallower portions of the subduction plate boundary was identified in three tectonic mélange zones (Mugi mélange, Yokonami mélange, and Miyama formation) in the Cretaceous Shimanto Belt, an on-land accretionary complex in southwest Japan. The transition is defined by a change in deformation features from extension veins only in sandstone blocks with ductile matrix deformation (possibly by diffusion-precipitation creep) to shear veins (brittle failure) from shallow to deep. Although mélange fabrics represent distributed simple to sub-simple shear deformation, localized shear veins are commonly accompanied by slickenlines and a mirror surface. Pressure-temperature (P-T) conditions for extension veins in sandstone blocks and for shear veins are distinct on the basis of fluid inclusion analysis. For extension veins, P-T conditions are approximately 125 to 220°C and 80 to 210 MPa. For shear veins, P-T conditions are approximately 185 to 270°C and 110 to 300 MPa. The P-T conditions for shear veins are, on average, higher than those for extension veins. The temperature conditions overlap in the range of approximately 175 to 210°C, which suggests that the change from more ductile to brittle processes occurs over a range of depths. The width of the shallow ductile-brittle transition zone can be explained by a heterogeneous lithification state for sandstone and mudstone or high fluid pressure caused by clay dehydration, which is controlled by the temperature conditions.
The physical properties of sediments evolve during underthrusting along subduction thrust interfaces because of mechanical (e.g., compaction) and chemical (e.g., dehydration and cementation) processes (e.g., Moore and Saffer, ). Each of these components of the lithification process can contribute to changes in deformation features and deformation mechanisms (Knipe, ). The evolution of physical properties of sediments associated with changes in deformation features and deformation mechanisms are reflected in both wedge geometry and the kinematics of earthquakes brought on by changes in the mechanical behavior of wedge materials and plate boundaries (e.g., Wang and Hu, ).
The seismogenic zone has been described as the temperature-controlled zone of plate boundary coupling, where interplate earthquakes nucleate (e.g., Hyndman and Wang, ). Recently, additional types of failure have been found along active subduction zones, such as slow slip events (e.g., Hirose et al., ; Kato et al., ; Ito et al., ), episodic tremor and slip (e.g., Rogers and Dragert, ; Obara et al., , Ishida et al., ), very low frequency earthquakes (e.g., Ito and Obara, ; Sugioka et al., ), and dynamic overshoot to the trench axis (e.g., Ide et al., ). Studies have been conducted to connect geologic deformation features from on-land accretionary complexes with seismic and aseismic deformations or with the newly found failure styles (e.g., Ikesawa et al., ; Kimura et al., ; Meneghini et al., ; Fagereng and Toy, ; Saito et al., ). Pseudotachylyte and fluidized shear zones, which indicate dynamic weakening of the fault during displacement, were found in on-land accretionary complexes; this suggests that the geologic features of the seismogenic slips along subduction interfaces are visible in on-land accretionary complexes (e.g., Ikesawa et al., ; Kitamura et al., ; Ujiie et al., ; Meneghini et al., ; Hashimoto et al., ; Saito et al., ). Further detailed studies of on-land accretionary complexes are needed to understand the nature of deformation features and how they relate to variable slip styles along subduction plate interfaces.
1.2 Tectonic mélanges
Pressure solution cleavages are composed of clay minerals approximately 0.5 μm in diameter (Figure 3B). The grain size is relatively heterogeneous compared with that of the clay coatings on shear veins that will be described later. Authigenic pyrites (up to approximately 1 μm) are also observed in the cleavages as light white patches (Figure 3B). Energy dispersive X-ray spectrometry shows that the cleavages are composed of Si, Al, K, and C, and the X-ray diffraction analysis indicates that the clay mineral present is illite, on the basis of the location of the white mica 001 peak.
1.3 Shear veins
Shear veins are commonly observed parallel or sub-parallel to the mélange foliation. In some parts, shear veins clearly cut the mélange foliation, which indicates that some of the shear veins were formed after the mélange formation (Figure 2B). The occurrence of foliations crosscutting shear veins is not observed at the outcrop scale. Displacement along the shear vein is relatively small, that is, less than 1 m. Veins consist of quartz and calcite. Matsumura et al. () and Hashimoto et al. () presented the one-dimensional distribution of shear veins in the Mugi mélange and the Yokonami mélange, which are both in the Cretaceous Shimanto Belt, Shikoku Island. The number of shear veins in a 1-m interval perpendicular to the mélange foliations is approximately 2 to 3. Much larger densities of shear veins have been reported in the Chrystalls Beach complex, New Zealand (Fagereng, [2011a]). The average thickness of shear veins ranges from 3 to 6 mm. Multiple thin clay layers, parallel to the vein, are observed within the veins (Figure 2C,D). The thickness of the clay layers varies from a few tens of micrometers to a few millimeters. In the left side of the figure, the vein surface is represented by a relatively straight surface of clay layer; whereas the clay layers within the veins show wavy shapes (Figure 2C,D). This occurrence suggests that shear vein mineralization occurred repeatedly. On the surface of the shear veins, slickenlines and slickensteps are well observed, and clay coatings (weathered mirror surfaces) are also identified on them (Figures 2E and 3C). The clay coatings are represented as a relatively shiny and thin black layer, and they are observed as an aggregate of nanoscale clay grains (Figure 3D). On the surfaces, slickenlines and slickensteps are also recognized (Figure 3D). Therefore, the deformation mechanism of shear veins is brittle failure.
1.4 Pressure-temperature conditions
Pressure-temperature conditions for the formations of the extension and the shear veins have been previously estimated on the basis of fluid inclusion analysis from three mélange zones in the Cretaceous Shimanto Belt: the Mugi mélange (Matsumura et al., ), the Miyama formation (Hashimoto et al., , ), and the Yokonami mélange (Hashimoto et al., ) (see locations in Figure 1). For the Yokonami mélange, the pressure-temperature condition only for the shear veins was obtained. The host minerals for fluid inclusions are quartz and lesser calcite. Host mineral strain by crystal-plastic processes in relatively weak minerals such as calcites can alter the fluid inclusion density in ways that render homogenization temperature data difficult to use (Roedder, ), for example, fluid-inclusion stretching during heating (Prezbindowski and Larese, ). The host mineral for fluid inclusion presented here is mostly quartz because the amount of calcite host is much less. No systematic variation in homogenization temperatures from calcite hosts has been noted suggesting that the constant density assumption for the trapped fluids is valid (e.g., Roedder, ).
The temperature gradient of up to 60°C/m (Figure 4) is much higher than that predicted by subduction zone thermal modeling. This can possibly be explained by the pressure drop, which makes the geothermal gradient apparently higher. Because the geothermal gradient for the Mugi mélange is considered to be higher than those of the Miyama formation and the Yokonami mélange, the temperature range for the extension vein and shear vein formation can be shallower than those for the Miyama formation and the Mugi mélange. Nevertheless, the temperature condition of the extension veins for each mélange zone shows a similar range, although the data are limited in the Mugi mélange, and this suggests that the extension vein development may finish in a shallower portion in the higher geothermal gradient than that in the lower geothermal gradient.
The average temperature condition for the extension veins is slightly lower (approximately 50°C) than that for shear veins. The occurrence of extension veins that are confined to sandstone blocks and do not propagate into the surrounding matrix implies that the shale matrix was ductile, and a competence contrast existed between the sandstone blocks and the shale matrix, as described above. Shear veins crosscut shale matrices in a brittle manner, which suggests that the shear veins can be formed after lithification of shale matrices in a broad sense. Therefore, this lower temperature condition for the extension veins is consistent with the observation of crosscutting relations between mélange fabrics and shear veins. However, overlapping temperature conditions for the extension and shear veins were identified in the temperature range between approximately 175 and 220°C. As described above, the deformations at the time of the extension veins and shear veins are interpreted as ductile and brittle in the shale matrices, respectively. Therefore, the overlapping area in temperature conditions can be regarded as the shallow ductile-brittle transition zone from the ductile mélange matrix to brittle shear veins. Because the slab age is comparable in the Yokonami mélange and the Miyama formation, the temperature range of the ductile-brittle transition zone is constrained by the data from the Yokonami mélange and the Miyama formation.
The temperature conditions and crosscutting relations for the extension and shear veins suggest that the deformations along the decollement change from ductile to brittle in a transition zone from shallow to deep. The change in deformation mechanisms is consistent with the outcrop observations that the shear veins crosscut the mélange foliation in some parts. The crosscutting relation also suggests a change in the deformation mechanism from pressure solution to frictional failure, from lower to higher temperature conditions.
The progressive change in deformation mechanism from ductile to brittle in the shallower subduction zone is probably related to lithification along the subduction interface in a broad sense. Mechanical compaction contributes to the lithification of sediments, especially in sandstone. The lithification of the sandstone is probably completed before mélange formation (Hashimoto et al., ). At the time the sandstone was lithified sufficiently to develop extension cracks, the shale matrix was still deforming in a ductile manner. Therefore, bulk ductile deformation was expected at the time of extension vein development. These occurrences may be associated with a relation between extensional strength of the sandstone blocks and viscosity of the shale matrix, the ratio of competent material to incompetent material, and the effect of fluid on the deformations in the ductile mélange stage. Pressure solution, a type of cementation process, can strongly contribute to lithification and strengthening of the shale matrix because of dissolution and precipitation, which reduces porosity (Angevine et al., ). Consequently, some shear veins that cut the shale matrix were developed after lithification of the shale matrices. Some extension veins can be formed in shale matrices after they are lithified (Figure 2A). The P-T conditions for the extension and shear veins indicate, however, that some of the shear veins were developed in the extension vein stage, which suggests that ductile and brittle deformation coexisted in a transition zone.
The transition from ductile to brittle behavior is suggested to occur in the temperature range of 175 to 220°C (Figure 4). The coexistence of different deformation mechanisms is not unexpected because they are controlled by competing factors such as the lithification state, strain rate, and fluid pressure. The similar occurrence of localized microfaults within a ductile mélange zone has also been reported elsewhere (e.g., Vannucchi et al., ; Fagereng and Sibson, ; Fagereng, [2011a]). Fegereng and Sibson (2010) suggested that the ratio of competent sand blocks to incompetent shale matrices may control the distribution of localized microfaulting because the shear strain is concentrated in the thin shale matrices. The higher strain rate in the thin shale matrix area enhanced the development of localized microfaults in the heterogeneous mélange zone. This model depends on a heterogeneous lithification process wherein sandstone layers strengthen more rapidly and shale matrices strengthen more slowly, and the strain rate is distributed heterogeneously. The relation between shear vein distribution and thickness of the shale matrix should be examined to test the model with quantitative evaluation of the ratio between competent and incompetent materials (Fagereng, [2011b]).
Saffer and Tobin () suggested that high pore-pressure zones tend to be distributed in the temperature range of approximately 60 to 150°C, on the basis of the fluid production rate of smectite dehydration. They suggested that the shallower ductile-brittle transition zone corresponds to the area of high pore fluid pressure because very low frequency earthquakes are observed in the area. In this study, the brittle deformation of extension veins actually exists in the ductile matrix deformation in the temperature range of 100 to 150°C, and it was possibly caused by the overpressures (due to smectite dehydration). However, the overall trend of our data indicates that the ductile-brittle transition occurs at 175 to 220°C, which is significantly higher than that suggested by Saffer and Tobin (). This compelled us to consider another mechanism for the generation of excess pore pressures at that temperature range. Kameda et al. () conducted a similar calculation (not for smectite dehydration but saponite dehydration) within the hydrated basaltic basement. They suggested that fluid production from saponite dehydration is significantly high, as much as that from smectite dehydration, and that the reaction continues from approximately 150°C to above approximately 260°C. Therefore, the higher temperature (150 to 220°C) of the ductile-brittle transition shown in this study could be explained by the high fluid pressure at least for extension veins, as Saffer and Tobin () suggested, in the case that the fluid source is from saponite dehydration. For shear veins, however, the excess pore pressure is not needed. Lithification state or strain rate as discussed above are also other controlling factors that may have contributed to the development of the ductile-brittle transition in a higher temperature range. The abnormally high fluid pressure is observed only in the ductile-brittle transition zone, as shown in Figure 4. This occurrence suggests that the high pore pressure can control the area of the ductile-brittle transition zone, from the ductile mélange matrix to brittle shear veins, in a shallower portion along the subduction interface although the relationship between the high pore fluid pressure, lithification state, and strain rate are not well constrained. The high fluid pressure could also be the result of a change in hydrologic properties by the lithification process, such as those caused by the pressure solution or clay dehydration. The resulting high fluid pressure can weaken the preexisting shear veins as feedback processes. The volume of fluid from dehydration of smectite or saponite and their relations to deformation need to be examined in detail for different thermal gradient regimes to better understand the fluid pressure effects according to the lithification state, the ratio of competent materials in deformation features, and deformation mechanisms.
We thank Dr. M. Kinoshita and two anonymous reviewers for their constructive comments and suggestions that improved this manuscript. Part of this study was supported by the Japan Society for the Promotion of Science (JSPS) via a Grant-in-Aid for Scientific Research on Innovative Areas (70359199) and a Grant-in-Aid for Scientific Research (B) (40346698).
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