Ken-ichi Kano

Nankai Trough and Zenisu Ridge: a deep-sea submersible survey

Abstract Eight submersible dives between 3000 and 4200 m water depth were made off southern Japan in the eastern Nankai subduction zone. Benthic communities associated with chemosynthetic processes were discovered along the 800 m wide active tectonic zone, at the toe of the accretionary prism. A benthic community was also discovered along a zone of active compression, at the foot of Zenisu Ridge, 30 km south of Nankai Trough. Temperature measurements within the sediments below the benthic communities confirm that upward motion of interstitial water occurs there. Studies of water samples indicate advection of methane and light hydrocarbons. Specimens of the benthic community have been shown to have included in their shells carbonate resulting from methane consumption. Thus the benthic communities are related to overpressure-driven fluid advection along tectonic zones with active surface deformation. A 300 m high active scarp at the toe of the accretionary prism is related to relative motion in a 280° direction which is close to the 305° average direction of subduction in this area. The dives establish further that compressive deformation is presently occurring at the foot of Zenisu Ridge. The previous interpretation of the Zenisu Ridge as a zone of recent north-south intraplate shortening, 40 km south of the Nankai Trench, is confirmed. We conclude that tectonic evolution might well lead to future detachment of the Zenisu Ridge and overthrusting of this large piece of oceanic crust over the continental margin. Such a process might be an efficient one to emplace ophiolites over continents.

Quaternary folding of the eastern Tian Shan, northwest China

Abstract The Tian Shan, east–west trending more than 2000 km, is one of most active intracontinental mountain building belts that resulted from India–Eurasia collision during Cenozoic. In this study, Quaternary folding related to intracontinental mountain building of the Tian Shan orogenic belt is documented based on geologic interpretation and analyses of the satellite remote sensing images [Landsat Thematic Mapper (TM)/Enhanced Thematic Mapper (ETM) and India Remote Sensing (IRS) Pan] combined with field geologic and geomorphic observations and seismic reflection profiles. Analyses of spatial–temporal features of Quaternary folded structure indicate that the early Quaternary folds are widely distributed in both piedmont and intermontane basins, whereas the late Quaternary active folds are mainly concentrated on the northern range-fronts. Field observations indicate that Quaternary folds are mainly characterized by fault-related folding. The formation and migration of Quaternary folding are likely related to decollement surfaces beneath the fold-and-fault zone as revealed by seismic reflection profiles. Moreover, analysis of growth strata indicates that the Quaternary folding began in late stage of early Pleistocene (2.1–1.2 Ma). Finally, tectonic evolution model of the Quaternary deformation in the Tian Shan is presented. This model shows that the Quaternary folding and faulting gradually migrate toward the range-fronts due to the continuous compression related to India–Eurasia collision during Quaternary time. As a result, the high topographic relief of the Tian Shan was formed.

Interactions between andesitic magma and poorly consolidated sediments: examples in the neogene Shirahama group, south Izu, Japan

Abstract The Upper Miocene-Lower Pliocene Shirahama Group of south Izu, Japan, is a sequence of volcanielastic shallow-sea sediments and subaqueous lava flows. It is gently warped with indistinct preferred orientation and is cross-cut by many andesitic intrusive bodies which range in size from less than a meter to several hundred meters. The intrusive bodies exhibit various shapes and weak preferred orientation, and often have hyaloclastic and peperitic textures along their peripheries. Hydrothermally altered and chaotically disturbed zones of host rocks also occur along the contacts. The areas where the intrusive bodies are abundant are almost coincident with the areas where the more inclined beds and synsedimentary faults are developed. The intrusive bodies dragged the surrounding beds with or without synsedimentary faults. Some intrusions occurred along the faults. Conjugate sets of the faults indicate that they were produced under unstable stress conditions. These relationships between the beds and the intrusive bodies are assumed to have originated mostly by interaction between hot magma and poorly consolidated wet sediments at a shallow depth beneath the sea floor. When the intrusions took place, the stress conditions in the beds were irregular and unstable, judging from the shapes and orientation of the intrusive bodies, and also from the deformation characteristics of the enclosing beds. Some of the intrusive bodies are probably feeder dikes which supplied lave to the Shirahama Group.

Asymmetrical melange fabrics as possible indicators of the convergent direction of plates : a case study from the Shimanto Belt of the Akaishi Mountains, central Japan

Abstract Scaly-foliated melanges in the upper Maastrichtian-lowest Paleogene strata occurring in a zone several kilometers wide in the Shimanto Belt of the Akaishi Mountains, central Japan, often exhibit composite planar fabrics on a mesoscopic scale with distinct monoclinic symmetry. They are characterized by block-in-matrix fabrics including scaly foliations in mudstone matrices and asymmetric lensoidal and/or ellipsoidal clasts of sandstone which were generated by extension parallel to the layers. These fabrics are geometrically very similar to those of foliated fault-related rocks. Another remarkable fabric is the imbricated stacks of lensoidal sandstone clasts forming mesoscopic duplexes by layer-parallel contraction. The co-existence of these extensional and contraction planar fabrics suggests that non-coaxial deformation had considerably progressed and that strain distribution was heterogeneous during formation of the melange. Systematic analysis of attitudes of these melange fabrics in many outcrops indicates that the fabrics have a remarkably consistent preferred orientation, suggesting that a south-trending overthrust shear about 40°–70° counterclockwise with respect to the trend of strata prevailed during melange formation. This direction is fairly consistent with the relative plate motion deduced from the regional tectonic framework of this age, which indicates the oblique subduction of the Pacific plate relative to the eastern margin of the Eurasian continent. Thus, the melange fabrics with monoclinic symmetry probably reflect the relative motion of consuming plates, and may become excellent geological indicators for determining the direction of convergence of plates during melange formation, since data on melange fabrics are directly and easily available in the field.

Intra-arc deformations with vertical rotation axes: the case of the pre-Middle Miocene terranes of Southwest Japan

Abstract Various folds (bends) with vertical or subvertical rotation axes occur in the pre-Middle Miocene terranes of Southwest Japan. They are variable in shape (conical folds, accordion folds, megakink bands and so on), and some are accompanied with strike-slip faults. They range from several kilometers to more than 100 km in width, and some of their hinge surfaces are traceable more than a few tens of kilometers. Their shapes, which resemble megakink band or buckle folds, indicate that they were produced by horizontal compression with vertical or subvertical σ2; axes in a shallow level of the crust. They were superimposed on the pre-Middle Miocene fold-and-thrust structures or nappe-melange complexes, and are regarded to have been formed mostly during the Middle Miocene, contemporaneous with the clockwise rotation of Southwest Japan related to the fan-shaped opening of the Sea of Japan. Geometrical analysis clearly suggests that the drifting Southwest Japan behaved as a non-rigid body, and the formation of these structures resulted from the differential rotation of the terranes and the collision of the Izu-Bonin Arc with eastern Southwest Japan.

Average Slip Rate and Recurrence Interval of Large-Magnitude Earthquakes on the Western Segment of the Strike-Slip Kunlun Fault, Northern Tibet

Interpretations of satellite remote sensing images, field and trench excavation investigations, and radiocarbon dates constrain the Holocene slip rate and average recurrence interval of large earthquakes on the western segment of the strike- slip Kunlun fault related with the 2001 M w 7.8 Central Kunlun earthquake, northern Tibet. Streams and gullies developed on the alluvial fans having an average 14 C age of ∼7000 years are sinistrally offset by up to 115 m along the Kunlun fault. This constrains a slip rate of 16.4 mm/yr for the past ∼7000 years. Trenches and 14 C ages reveal that at least four seismic faulting events occurred in the past 6200 years and that the penultimate event prior to the 2001 M w 7.8 earthquake occurred during the past 400 years with an average left-lateral offset of 4–5 m. Coupling the slip rate of 16.4 mm/yr with the average offset of 4–5 m produced by individual large earthquakes, it is estimated that the average recurrence interval of large earthquakes is 300–400 years on the western segment of the Kunlun fault. Our results confirm that the Kunlun fault plays an important role as a major strike-slip fault in accommodating the horizontal eastward extrusion of Tibet.

Tectonic evolution along the northernmost border of the Philippine Sea plate since about 1 Ma

Abstract Recent structural, tephrochronologic and magnetostratigraphic studies conducted along the northernmost border of the Philippine Sea (PHS) plate enable us to reconstruct the precise tectonic evolution along the convergent boundary between the PHS plate and the Northeast Japan (NEJ) plate or the North American (NAM) plate since about 1 Ma. The authors of the present study split the tectonic evolution into five stages and present the characteristics of each stage. A plate tectonic interpretation is proposed, based upon the tectonic evolution, with special reference to the mode of convergent motion. In brief, our interpretations are as follows: the relative motion between the PHS and the NEJ plates was not recognized geologically within the area studied from about 1.0 to 0.9 Ma (Stage 1), suggesting either none or small influence from the coupling between the two plates during that period of time. Convergence between the PHS and the NEJ plates was possibly in N-S direction from 0.9 to 0.5 Ma (Stage II), and probably north-northwestward since 0.5 Ma (Stages III to V). The mode of the convergent motion was that of buoyant subduction in Stages II and III. The mode changed gradually from buoyant subduction during Stage IV to collision in Stage V (0.07 Ma to the present).

Late Quaternary right-lateral displacement along active faults in the Yanqi Basin, southeastern Tian Shan, northwest China

Abstract Late Quaternary right-lateral displacement and slip rates have been determined along WNW–ESE-trending active faults in the intermontane Yanqi Basin on the southeastern flank of the Chinese Tian Shan. Detailed analyses of satellite images and field investigation have revealed that the active Kaidu River fault zone on the southern margin of the basin is a strike–slip fault zone. Drainage systems incising late Pleistocene–Holocene alluvial fans record between 3 and 250 m dextral offsets and show progressive displacement along the fault zone. Fault scarps developed in the alluvial fans range in height from 1 to 25 m and alternate along the strike of the fault zone from northeast to southwest facing in a left-stepping en echelon pattern. Based on the offset of stream channels, 14C dates of alluvial deposits, and fabrics within fault rocks, we infer that (1) the average right-lateral slip rate is about 8 mm/year, with a vertical component of 1 mm/year, (2) the offset produced by individual seismic faulting event is typically 3–7 m, (3) the average recurrence interval of large seismic events (M>7) is ca. 500 years, and (4) the most recent movement occurred during the past 2.5 ky in the Kaidu River fault zone. These strike–slip faults represent partitioning of horizontal slip within an otherwise thrust dominated orogen related to the India–Eurasia collision within the Tian Shan during late Quaternary.

Application of stereoscopic satellite images for studying Quaternary tectonics in arid regions

We introduce a flexible method for creating stereoscopic pairs of images from any interesting sub-area of the same scene of Landsat Thematic Mapper (TM)/Enhanced Thematic Mapper (ETM) and Indian Remote Sensing (IRS)-1C Pan remote sensing data by setting the Z scale. As a test of this method, stereoscopic images were used to study Quaternary deformation along the Tian Shan Orogenic Belt, north-west China. The new stereoscopic images can then provide detailed information of Quaternary deformation structures, including spatial distribution and arrangement pattern of fold structures, fault scarps and displacement of alluvial fans, terraces and drainage systems along active faults, in three dimensions. The strike–slip partitioning has been revealed by interpretation of stereoscopic images within Chinese Tian Shan. Structural interpretations derived from stereoscopic analysis were confirmed to a high degree of accuracy during a subsequent field study. The satellite remote sensing stereoscopic technique is an effective method of analysing Quaternary tectonic deformation in remote arid to semi-arid regions such as the Tian Shan.

Fluidized Cataclastic Veins along the Itoigawa-Shizuoka Tectonic Line Active Fault System, Central Japan, and Its Seismotectonic Implications

Multistage veinlet cataclastic rocks, composed of aphanitic veins typical of pseudotachylyte and unconsolidated fault gouge, and sediment veins composed of alluvial deposits are widely developed within a fault shear zone (<5 m wide) as simple veins, breccias, and complex networks along the active Shimotsuburai fault, central-southern segment of the Itoigawa-Shizuoka Tectonic Line active fault system (ISTL-AFS), central Japan. Early veins are generally fractured and overprinted by younger veins, indicating that vein-forming events occurred repeatedly within the same fault shear zone. Microstructurally, both the pseudotachylyte and fault gouge veins are characterized by a superfine- to fine-grained matrix and angular-subangular fragments ranging in size from submicron scale to several centimeters. Powder X-ray diffraction patterns show that the fault veins and injection veins of fault gouge and pseudotachylyte are characterized by crystalline materials composed mainly of quartz and feldspar, similar to the granitic cataclasite host. Based on the meso- and microstructural features of veinlet cataclastic rocks and the results of powder X-ray diffraction analyses, we conclude that (i) the pseudotachylyte veins were generated mainly by crushing rather than melting, (ii) multistage veinlet fault gouge and pseudotachylyte formed repeatedly within the fault-fracture zone via the rapid fluidization and injection of superfine- to fine-grained materials derived from the host granitic rocks during seismic faulting events, and (iii) veins of alluvial deposit formed by liquefaction associated with strong ground motion during large-magnitude earthquakes that occurred along the active Shimotsuburai fault of the ISTL-AFS. Our results show that the fluidized cataclastic veins and alluvial deposit veins record paleoseismic faulting events that occurred within a seismogenic fault zone; consequently, these features are a type of earthquake fossil, as is melt-origin pseudotachylyte.