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1.
We describe in detail the deformation structures and textures of a large-scale landslide body that developed in the Betto-dani
Valley in northern central Japan. We studied the shape-preferred orientation of clasts and clay flakes and the development
of internal shear planes within the slip zone of the landslide. The slip has an average rate of 5–10 cm/year under the overburden
pressure of approximately 1.6 MPa; these values are similar to those of the proto-decollement zone of the Nankai accretionary
prism in SW Japan. The anisotropy of magnetic susceptibility of samples obtained from the slip zone reveals that the long
axes of clay flakes define an imbricate structure. The slip was due to a long-term periodical creep, which occurs during the
thaw seasons with an average slip rate of 0.16–0.32 μm/min. During the creep, the long axes of grains including clay flakes
in the slip zone are developed from parallel to perpendicular to the slip direction. The observed textures provide a clue
to elucidate the deformation textures and process in the decollement zone of the Nankai prism. 相似文献
2.
Chemical and chronologic complexity in the convecting upper mantle: Evidence from the Taitao ophiolite, southern Chile 总被引：1，自引：0，他引：1
Ruth F. Schulte Manuel Schilling Ryo Anma James Farquhar Mary F. Horan Tsuyoshi Komiya Philip M. Piccoli Lynnette Pitcher Richard J. Walker 《Geochimica et cosmochimica acta》2009,73(19):5793-5819
Exposure of the ca. 6 Ma Taitao ophiolite, Chile, located 50 km south of the Chile Triple Junction, allows detailed chemical and isotopic study of rocks that were recently extracted from the depleted mantle source of mid-ocean ridge basalts (DMM). Ultramafic and mafic rocks are examined for isotopic (Os, Sr, Nd, and O), and major and trace element compositions, including the highly siderophile elements (HSE). Taitao peridotites have compositions indicative of variable extents of partial melting and melt extraction. Low δ18O values for most whole rock samples suggest some open-system, high-temperature water–rock interaction, most likely during serpentinization, but relict olivine grains have δ18O values consistent with primary mantle values. Most of the peridotites analyzed for Nd–Sr isotopes have compositions consistent with estimates for the modern DMM, although several samples are characterized by 87Sr/86Sr and 143Nd/144Nd indicative of crustal contamination, most likely via interactions with seawater. The peridotites have initial 187Os/188Os ratios that range widely from 0.1168 to 0.1288 (γOs = −8.0 to +1.1), averaging 0.1239 (γOs = −2.4), which is comparable to the average for modern abyssal peridotites. A negative correlation between the Mg# of relict olivine grains and Os isotopic compositions of whole rock peridotites suggests that the Os isotopic compositions reflect primary mantle Re/Os fractionation produced by variable extents of partial melting at approximately 1.6 Ga. Recent re-melting at or near the spatially associated Chile Ridge further modified these rocks, and Re, and minor Pt and Pd were subsequently added back into some rocks by late-stage melt–rock or fluid–rock interactions.In contrast to the peridotites, approximately half of the mafic rocks examined have whole rock δ18O values within the range of mantle compositions, and their Nd and Sr isotopic compositions are all generally within the range of modern DMM. These rocks have initial 187Os/188Os ratios, calculated for 6 Ma, that range from 0.126 (γOs = −1) to as high as 0.561 (γOs = +342). The Os isotopic systematics of each of these rocks may reflect derivation from mixed lithologies that include the peridotites, but may also include pyroxenites with considerably more radiogenic Os than the peridotites. This observation supports the view that suprachondritic Os present in MORB derives from mixed mantle source lithologies, accounting for some of the worldwide dichotomy in 187Os/188Os between MORB and abyssal peridotites.The collective results of this study suggest that this >500 km3 block of the mantle underwent at least two stages of melting. The first stage occurred at 1.6 Ga, after which the block remained isolated and unmixed within the DMM. A final stage of melting recently occurred at or near the Chile Ridge, resulting in the production of at least some of the mafic rocks. Convective stirring of this mantle domain during a >1 Ga period was remarkably inefficient, at least with regard to Os isotopes. 相似文献
3.
The Izu-Ohshima-Kinkai earthquake (M = 7.0 on the Richter scale) occurred at 12h24 Japan Standard Time on 14 January 1978 in Sagami Bay, and caused hazards in the eastern to central Izu Peninsula. The meizoseismal zone extended 15 km from east to west by 20 km from north to south.
The aftershock zone shifted westward to the interior of the peninsula. The largest aftershock (M = 5.8) struck the central to western part of the peninsula on 15 January, causing severe damage in its mountainous terrain.
Most of the hazards related to slope failures of sea cliffs, river cliffs and road cuttings of convex vertical section with the upper parts sloping gently and the lower parts steeply.
Large slope failures occurred close to the activated faults, indicating that the shocks and displacement were locally amplified along the activated faults.
In the meizoseismal zone, high embankments of roads and housing sites on natural slopes were damaged by slides and tension cracks between the embankment and natural ground.
Subsidence and tilting of retaining structures were generally observed. Damage to high concrete retaining walls of gravity type was due to the insufficient bearing capacity of the foundation soil. Any type of retaining structure, even if seismic-resistant, would be considered ineffective in case of failure of the back slope.
Buildings in the meizoseismal zone were damaged due to sliding of the underlying embankments and ground displacement due to the activated faults.
Falling rock blocks and sliding soil masses struck cars and houses close to the foot of cliffs and steep slopes, causing many deaths and injuries.
A number of tension cracks in the middle to upper parts of steep slopes and on the shoulders of road embankments threatened to cause secondary failures, rockfalls and avalanches during heavy rain.
At a gold and silver mine, the tailings dam of a concentration plant failed due to liquefaction of the tailings.
The above experience may throw some light on the possibility of seismic failures in mountainous countries in seismically active regions of the world. 相似文献
4.
Tectonic rotations during the Chile Ridge collision and obduction of the Taitao ophiolite (southern Chile) 总被引：2，自引：0，他引：2
Abstract The < 6 Ma young Taitao ophiolite, exposed at the westernmost promontory of the Taitao Peninsula, is located approximately 40 km southeast of the Chile triple junction and consists of a complete sequence of oceanic lithosphere. Systematic sampling for paleomagnetic study was performed to understand the complex obduction processes of the ophiolite onto the forearc of the South American Plate. Two representative demagnetization paths of remanent magnetization vectors were observed. One is characterized by stable univectorial demagnetization paths and was observed in volcaniclastic rocks and dyke complexes. Orientations of their remanent magnetization vectors indicate various degrees of counterclockwise rotations. The other is characterized by multivectorial demagnetization paths and was observed in the plutonic units (gabbros and ultramafic rocks). From these, two distinct stable remanent magnetization vectors were isolated; one has high coercivity and the other has low coercivity along the demagnetization paths with little influence of viscous magnetizations. This suggests that the complex deformation history involved at least two rotational events. The clockwise rotation, inferred from high coercivity remanent magnetization vectors, was attributed to a ridge collision event and the counterclockwise rotation, inferred from the low coercivity remanent magnetization vectors, was attributed to an accommodation phase into the South American forearc during obduction and final emplacement of the ophiolite. Folds developed during this period. Paleomagnetic restorations of the internal structures of the plutonic units and dyke complexes suggest that they probably originated in a mid‐oceanic ridge environment near a transform fault. The counterclockwise rotation of the plutonic and dyke complex units during the obduction generated tectonic gaps between these and the basement. The volcaniclastic rocks must have been deposited at nearly their present location, filling the tectonic gaps, as less effect of tectonic rotation was identified on these rocks. 相似文献
5.
Takazo Shibuya Tsuyoshi Komiya Ryo Anma Tsutomu Ota Soichi Omori Yoshiaki Kon Shinji Yamamoto Shigenori Maruyama 《Lithos》2007,98(1-4):233-260
We estimated metamorphic conditions for the 6 Ma Taitao ophiolite, associated with the Chile triple junction. The metamorphic grade of the ophiolite, estimated from secondary matrix minerals, changes stratigraphically downwards from the zeolite facies, through the prehnite–actinolite facies, greenschist facies and the greenschist–amphibolite transition, to the amphibolite facies. The metamorphic facies series corresponds to the low-pressure type. The metamorphic zone boundaries are subparallel to the internal lithological boundaries of the ophiolite, indicating that the metamorphism was due to axial hydrothermal alteration at a mid-ocean ridge.
Mineral assemblages and the compositions of veins systematically change from quartz-dominated, through epidote-dominated, to prehnite-dominated with increasing depth. Temperatures estimated from the vein assemblages range from 230 °C in the volcanic unit to 380 °C at the bottom of the gabbro unit, systematically 200 °C lower than estimates from the adjoining matrix minerals. The late development of veins and the systematically lower temperatures suggest that the vein-forming alteration was due to off-axis hydrothermal alteration.
Comparison between the Taitao ophiolite with its mid-ocean ridge (MOR) affinity, and other ophiolites and MOR crusts, suggests that the Taitao ophiolite has many hydrothermal alteration features similar to those of MOR crusts. This is consistent with the tectonic history that the Taitao ophiolite was formed at the South Chile ridge system near the South American continent (Anma, R., Armstrong, R., Danhara, T., Orihashi, Y. and Iwano, H., 2006. Zircon sensitive high mass-resolution ion microprobe U–Pb and fission-track ages for gabbros and sheeted dykes of the Taitao ophiolite, Southern Chile, and their tectonic implications. The Island Arc, 15(1): 130–142). 相似文献
6.
Abstract The Kokchetav Massif of Kazakhstan includes high to ultrahigh-pressure (HP–UHP) metamorphic rocks (some of which were recrystallized at depths in excess of 150 km), juxtaposed against much lower pressure metamorphic components. We investigated the relationship between the HP–UHP metamorphic unit and the low pressure (LP) unit (Daulet Suite) in the Sulu–Tjube area, where the metamorphic rocks have previously been interpreted as constituting a megamelange with subvertical structural attitudes. Analyses of fold structures suggest that the HP–UHP metamorphic unit overlies the LP unit across a west-dipping subhorizontal boundary. In addition, kinematic indicators display top-to-the-north senses of shear along the tectonic contact between the two units, indicating that the HP–UHP unit has been extruded northward onto the LP unit. Following the juxtaposition of the two units, upright folds developed in both units, and these are associated with the previously reported steeply dipping metamorphic foliations. These data have important implications for the mode of exhumation of the UHP rocks from upper mantle to shallow crustal depths. 相似文献
7.
The Middle Miocene Tsushima granite pluton is composed of leucocratic granites, gray granites and numerous mafic microgranular enclaves (MME). The granites have a metaluminous to slightly peraluminous composition and belong to the calc‐alkaline series, as do many other coeval granites of southwestern Japan, all of which formed in relation to the opening of the Sea of Japan. The Tsushima granites are unique in that they occur in the back‐arc area of the innermost Inner Zone of Southwest Japan, contain numerous miarolitic cavities, and show shallow crystallization (2–6 km deep), based on hornblende geobarometry. The leucocratic granite has higher initial 87Sr/86Sr ratios (0.7065–0.7085) and lower εNd(t) (?7.70 to ?4.35) than the MME of basaltic–dacitic composition (0.7044–0.7061 and ?0.53 to ?5.24), whereas most gray granites have intermediate chemical and Sr–Nd isotopic compositions (0.7061–0.7072 and ?3.75 to ?6.17). Field, petrological, and geochemical data demonstrate that the Tsushima granites formed by the mingling and mixing of mafic and felsic magmas. The Sr–Nd–Pb isotope data strongly suggest that the mafic magma was derived from two mantle components with depleted mantle material and enriched mantle I (EMI) compositions, whereas the felsic magma formed by mixing of upper mantle magma of EMI composition with metabasic rocks in the overlying lower crust. Element data points deviating from the simple mixing line of the two magmas may indicate fractional crystallization of the felsic magma or chemical modification by hydrothermal fluid. The miarolitic cavities and enrichment of alkali elements in the MME suggest rapid cooling of the mingled magma accompanied by elemental transport by hydrothermal fluid. The inferred genesis of this magma–fluid system is as follows: (i) the mafic and felsic magmas were generated in the mantle and lower crust, respectively, by a large heat supply and pressure decrease under back‐arc conditions induced by mantle upwelling and crustal thinning; (ii) they mingled and crystallized rapidly at shallow depths in the upper crust without interaction during the ascent of the magmas from the middle to the upper crust, which (iii) led to fluid generation in the shallow crust. The upper mantle in southwest Japan thus has an EMI‐like composition, which plays an important role in the genesis of igneous rocks there. 相似文献
8.
Abstract The Late Miocene–Pliocene Taitao ophiolite is composed of a complete sequence of classic oceanic lithosphere and is exposed approximately 50 km southeast of the Chile triple junction, where the Chile Ridge subducts beneath the South American Plate. Gabbros and ultramafic rocks are folded into a complex pattern, but only evidence for block rotation has been reported in the overriding sheeted dyke complex. In the present study, sensitive high mass-resolution ion microprobe U–Pb and fission-track dating methods were applied to zircon crystals separated from gabbros and sheeted dykes. Two sets of radiometric ages of gabbros range between 5.9 ± 0.4 and 5.6 ± 0.1 Ma. These ages coincide within their error ranges and imply rapid intrusion and cooling of gabbros. The U–Pb age of a dacite dyke intruded into the sheeted dyke complex was determined to be 5.2 ± 0.2 Ma. These data indicate that the magmas of the Taitao ophiolite were formed during the 6 Ma Chile Ridge collision event and emplaced in a shorter period than previously thought. A short segment of the Chile Mid-oceanic Ridge must have been emplaced during the 6 Ma event. 相似文献
9.
Y. Kaneko S. Maruyama M. Terabayashi H. Yamamoto M. Ishikawa R. Anma † C. D. Parkinson T. Ota Y. Nakajima I. Katayama J. Yamamoto and K. Yamauchi 《Island Arc》2000,9(3):264-283
Abstract Ultrahigh-pressure metamorphic (UHPM) rocks of the Kokchetav Massif of Kazakhstan contain metamorphic microdiamond and coesite inclusions inside rigid capsules such as garnet and zircon. Precambrian protoliths of the UHPM rocks were metamorphosed at around 530 Ma, at pressures of about 7 GPa, which suggests that crustal protoliths were subducted to depths of over 200 km. Primary UHPM minerals are poorly preserved due to partial obliteration by subsequent Barrovian overprint during exhumation and later collision events in Caledonian times. We report the results of detailed mapping of the Kokchetav Massif and use structural data to propose intrusion and exhumation mechanisms for the UHPM rocks. Detailed mapping revealed that many subvertical structures in the ultrahigh-pressure–high-pressure (UHP–HP) units were formed due to later folding. The primary structure appears to be subhorizontal and the total thickness of the UHP rocks is estimated at around 2 km. The first order structure is sandwich-like; that is, the UHP–HP units are separated from underlying low-P metamorphic rocks of the Daulet Series and from feebly metamorphosed to unmetamorphosed sedimentary strata on the top by subhorizontal faults. Kinematic indicators show top-to-the-south sense of shear along the top, and top-to-the-north displacement along the bottom boundaries. These shear senses, together with the observed metamorphic gradients, suggest that the thin UHPM sheet was extruded toward the north. We consider wedge extrusion to have been the most effective mechanism for the exhumation of the UHPM rocks. 相似文献
10.
Abstract The central part of the Kokchetav Massif is exposed in the Chaglinka–Kulet area, northern Kazakhstan. The ultrahigh-pressure–high-pressure (UHP–HP) metamorphic belt in this area is composed of four subhorizontal lithological units (Unit I–IV) metamorphosed under different pressure–temperature (P–T) conditions. The coesite- and diamond-bearing Unit II, which consists mainly of whiteschist and eclogite blocks, is tectonically sandwiched between the amphibolite-dominant Unit I on the bottom and the orthogneiss-dominant Unit III on the top. Total combined thickness of these units is less than 2 km. The rocks of the UHP–HP metamorphic belt are affected by at least four deformational events post-dating peak metamorphism: (i) The earliest penetrative deformation is characterized by non-coaxial ductile flow in a NW–SE direction. The shear sense indicators in oriented samples from Unit I provide consistent top-to-the-northwest motions and those from Unit III provide top-to-the-southeast, south or south-west motions; (ii) Upright folds with subhorizontal enveloping surface refold earlier foliations including shear-indicators throughout the metamorphic belt; (iii) The third stage of deformation is denoted by large-scale bending around a subvertical axis; and (iv) Late localized fault (or shear) zones cut all earlier structures. The fault zones have subvertical shear planes and their displacements are essentially strike-slip in manner. The subhorizontal structure and opposite shear directions between Unit I and Unit III during the earlier deformation stage suggest north-westward extrusion of UHP Unit II. 相似文献