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1.
H. Tabata  S. Maruyama  & Z. Shi 《Island Arc》1998,7(1-2):142-158
The ultrahigh- and high-pressure (UHP–HP) metamorphic belt of the Dabie Mountains, central China, formed by the Triassic continental subduction and collision, is divided into four metamorphic zones; from south to north, the greenschist facies zone, epidote amphibolite to amphibolite facies zone, quartz eclogite zone, and coesite eclogite zone, based on metabasite mineral assemblages. Most of the coesite-bearing eclogites consist mainly of garnet and omphacite with homogeneous compositions and have partially undergone hydration reactions to form clinopyroxene + plagioclase + calcic amphibole symplectites during amphibolite facies overprinting. However, the least altered eclogites sometimes contain garnet and omphacite that preserve compositional zoning patterns which may have originated during their growth at peak temperature conditions of ∼ 750 °C, suggesting a short duration of UHP metamorphic conditions and/or consequent rapid cooling during exhumation. Systematic investigation on peak metamorphic temperatures of coesite eclogite have revealed that, contrary to the general trend of metamorphic grade in the southern Dabie unit, the coesite eclogite zone shows rather flat thermal structure (T = 600 ± 50 °C) with the highest temperature reaching up to 850 °C and no northward increase in metamorphic temperature, which is opposed to the previous interpretations. This feature, along with the preservation of compositional zonation, implies complicated differential movement of each eclogite mass during UHP metamorphism and the return from the deeper subduction zone at mantle depths to the surface.  相似文献   

2.
Within the Tethyan realm, data for the subduction history of the Permo–Triassic Tethys in the form of accretionary complexes are scarce, coming mainly from northwest Turkey and Tibet. Herein we present field geological, petrological and geochronological data on a Triassic accretionary complex, the A?vanis metamorphic rocks, from northeast Turkey. The A?vanis metamorphic rocks form a SSE–NNW trending lozenge‐shaped horst, ～20 km long and ～6 km across, bounded by the strands of the active North Anatolian Fault close to the collision zone between the Eastern Pontides and the Menderes–Taurus Block. The rocks consist mainly of greenschist‐ to epidote‐amphibolite‐facies metabasite, phyllite, marble and minor metachert and serpentinite, interpreted as a metamorphic accretionary complex based on the oceanic rock types and ocean island basaltic, mid‐ocean ridge basaltic and island‐arc tholeiitic affinities of the metabasites. This rock assemblage was intruded by stocks and dikes of Early Eocene quartz diorite, leucogranodiorite and dacite porphyry. Metamorphic conditions are estimated to be 470–540°C and ～0.60–0.90 GPa. Stepwise 40Ar/39Ar dating of phengite–muscovite separates sampled outside the contact metamorphic aureoles yielded steadily increasing age spectra with the highest incremental stage corresponding to age values ranging from ～180 to 209 Ma, suggesting that the metamorphism occurred at ≥ 209 Ma. Thus, the A?vanis metamorphic rocks represent the vestiges of the Late Triassic or slightly older subduction in northeast Turkey. Estimated P–T conditions indicate higher temperatures than those predicted by steady state thermal models for average subduction zones, and can best be accounted for by a hot subduction zone, similar to the present‐day Cascadia. Contact metamorphic mineral assemblages around an Early Eocene quartz diorite stock, on the other hand, suggest that the present‐day erosion level was at depths of ～14 km during the Early Eocene, indicative of reburial of the metamorphic rocks. Partial disturbance of white‐mica Ar–Ar age spectra was probably caused by the reburial coupled with heat input by igneous activity, which is probably related to thrusting due to the continental collision between Eastern Pontides and the Menderes–Taurus Block.  相似文献   

3.
We have estimated the timescale of material circulation in the Sanbagawa subduction zone based on U–Pb zircon and K–Ar phengite dating in the Ikeda district, central Shikoku. The Minawa and Koboke units are major constituents of the high‐P Sanbagawa metamorphic complex in Shikoku, southwest Japan. For the Minawa unit, ages of 92–81 Ma for the trench‐fill sediments, are indicated, whereas the age of ductile deformation and metamorphism of garnet and chlorite zones are 74–72 Ma and 65 Ma, respectively. Our results and occurrence of c. 150 Ma Besshi‐type deposits formed at mid‐ocean ridge suggest that the 60‐Myr‐old Izanagi Plate was subducted beneath the Eurasian Plate at c. 90 Ma, and this observation is consistent with recent plate reconstructions. For the Koboke unit, the depositional ages of the trench‐fill sediments and the dates for the termination of ductile deformation and metamorphism are estimated at c. 76–74 and 64–62 Ma, respectively. In the Ikeda district, the depositional ages generally become younger towards lower structural levels in the Sanbagawa metamorphic complex. Our results of U–Pb and K–Ar dating show that the circulation of material from the deposition of the Minawa and Koboke units at the trench through an active high‐P metamorphic domain to the final exhumation from the domain occurred continuously throughout c. 30 Myr (from c. 90 to 60 Ma).  相似文献   

4.
Abstract Deformation in the Sanbagawa Belt is characterized by ductile flow in an east-west direction sub-parallel to its length. The east-west flow (D1) caused large-scale recumbent folding of the metamorphic sequence in central Shikoku, which can explain the inverted thermal structure of this region. Chemical zoning of metamorphic minerals associated with D1 microstructures also suggest that the east-west flow developed under retrograde conditions. D1 is therefore related to exhumation rather than subduction processes. A variety of kinematic indicators show that during the east-west flow, deformation was partitioned into structurally continuous domains with opposed senses of shear. This suggests that bulk deformation was not simple shear but included a component of flattening.  相似文献   

5.
The main tectono‐stratigraphic unit (Shirataki unit) of the Sanbagawa metamorphic complex in central Shikoku is characterized by abundant mafic schist layers that show the mid‐ocean ridge basalt (MORB) affinity. These MORB‐derived schist layers are absent in a southern (structurally lower) domain within the unit. Instead, sporadic occurrences of small metabasite lenses that contain relict igneous minerals (Ti‐rich augite and kaersutite) indicative of alkali basalt magmatism are newly recognized in the southern domain. Compositions of relict clinopyroxene in metabasalt are useful to identify the tectonic setting and origin of the protolith basalt, and those in each unit of the Sanbagawa metamorphic complex are presented. The metamorphic grade of the Shirataki unit generally increases structurally upwards in the southern side of the highest‐grade zone, and metamorphic zonation is subparallel to lithostratigraphic succession. The protolith assemblage of the Shirataki unit shows a distinct change from the southern low‐grade domain (lower Shirataki subunit) composed of terrigenous sedimentary rocks (mudstone and sandstone) with minor alkali basalt to the northern higher‐grade domain (upper Shirataki subunit) consisting of terrigenous and pelagic sedimentary rocks with abundant MORB. The youngest detrital zircon U–Pb ages (ca 95–90 Ma) suggest that both domains have Late Cretaceous depositional ages at the trench. Progressive peeling of oceanic plate stratigraphy during subduction can account for the observed change of lithological association in the Shirataki unit.  相似文献   

6.
Blueschist-bearing Osayama serpentinite melange develops beneath a peridotite body of the Oeyama ophiolite which occupies the highest position structurally in the central Chugoku Mountains. The blueschist-facies tectonic blocks within the serpentinite melange are divided into the lawsonite–pumpellyite grade, lower epidote grade and higher epidote grade by the mineral assemblages of basic schists. The higher epidote-grade block is a garnet–glaucophane schist including eclogite-facies relic minerals and retrogressive lawsonite–pumpellyite-grade minerals. Gabbroic blocks derived from the Oeyama ophiolite are also enclosed as tectonic blocks in the serpentinite matrix and have experienced a blueschist metamorphism together with the other blueschist blocks. The mineralogic and paragenetic features of the Osayama blueschists are compatible with a hypothesis that they were derived from a coherent blueschist-facies metamorphic sequence, formed in a subduction zone with a low geothermal gradient (~ 10°C/km). Phengite K–Ar ages of 16 pelitic and one basic schists yield 289–327 Ma and concentrate around 320 Ma regardless of protolith and metamorphic grade, suggesting quick exhumation of the schists at ca 320 Ma. These petrologic and geochronologic features suggest that the Osayama blueschists comprise a low-grade portion of the Carboniferous Renge metamorphic belt. The Osayama blueschists indicate that the 'cold' subduction type (Franciscan type) metamorphism to reach eclogite-facies and subsequent quick exhumation took place in the northwestern Pacific margin in Carboniferous time, like some other circum-Pacific orogenic belts (western USA and eastern Australia), where such subduction metamorphism already started as early as the Ordovician.  相似文献   

7.
Kazuo Kiminami 《Island Arc》2010,19(3):530-545
This study examines the geology of low‐grade (chlorite zone) metamorphic rocks in the Sanbagawa belt and of a Jurassic accretionary complex in the Northern Chichibu belt, eastern Shikoku, Japan. The bulk chemistries of metasandstones and metapelites in the Sanbagawa belt of eastern Shikoku are examined in order to determine their parentage. The Sanbagawa belt can be divided into northern and southern parts based on lithology and geologic structure. Geochemical data indicate that metasediments in the northern and southern parts are the metamorphic equivalents of the KS‐II (Coniacian–Campanian) and KS‐I (late Albian–early Coniacian) units of the Shimanto belt, respectively. The depositional ages of the parent sediments of low‐grade metamorphic rocks found in the Sanbagawa belt and the Jurassic Northern Chichibu belt, indicate a north‐younging polarity. In contrast, sedimentological evidence indicates younging to the south. These observations suggest that a tectonic event has resulted in a change from a northerly to southerly dip direction for schistosity and bedding in the Sanbagawa and Northern Chichibu belts of eastern Shikoku. The younging polarity observed in the Sanbagawa and Northern Chichibu belts, together with previously reported data on vitrinite reflectance and geological structure, indicate that the Northern Chichibu belt was part of the overburden formerly lying on top of the Sanbagawa low‐grade metamorphic rocks.  相似文献   

8.
Tetsumaru  Itaya  Hironobu  Hyodo  Tatsuki  Tsujimori  Simon  Wallis  Mutsuki  Aoya  Tetsuo  Kawakami  Chitaro  Gouzu 《Island Arc》2009,18(2):293-305
Laser step heating 40Ar/39Ar analysis of biotite and muscovite single crystals from a Barrovian type metamorphic belt in the eastern Tibetan plateau yielded consistent cooling ages of ca. 40 Ma in the sillimanite zone with peak metamorphic temperatures higher than 600°C and discordant ages from 46 to 197 Ma in the zones with lower peak temperatures. Chemical Th‐U‐Total Pb Isochron Method (CHIME) monazite (65 Ma) and sensitive high mass‐resolution ion microprobe (SHRIMP) apatite (67 Ma) dating give the age of peak metamorphism in the sillimanite zone. Moderate amounts of excess Ar shown by biotite grains with ages of 46 to 94 Ma at metamorphic grades up to the high‐grade part of the kyanite zone probably represent incomplete degassing during metamorphism. In contrast, the high‐grade part of the kyanite zone yields biotite ages of 130 to 197 Ma. The spatial distribution of these older ages in the kyanite zone along the sillimanite zone boundary suggests they reflect trapped excess argon that migrated from higher‐grade regions. The most likely source is muscovite that decomposed to form sillimanite. The zone with extreme amounts of excess argon preserves trapped remnants of an ‘excess argon wave’. We suggest this corresponds to the area where biotite cooled below its closure temperature in the presence of an elevated Ar wave. Extreme excess Ar is not recognized in muscovite suggesting that the entrapment of the argon wave by biotite took place when the rocks had cooled down to temperatures lower than the closure temperature of muscovite. The breakdown of phengite during ultrahigh‐pressure (UHP) metamorphism may be a key factor in accounting for the very old apparent ages seen in many UHP metamorphic regions. This is the first documentation of a regional Ar‐wave spatially associated with regional metamorphism. This study also implies that resetting of the Ar isotopic systems in micas can require temperatures up to 600°C; much higher than generally thought.  相似文献   

9.
A high‐temperature (T) metamorphic complex occurs in the Omuta district, northern Kyushu, Japan. Three metamorphic zones are defined based on pelitic mineral assemblage, i.e. chlorite–biotite zone, muscovite–andalusite zone and sillimanite–K‐feldspar zone with ascending metamorphic grade from north to south. Two isograds trend approximately east–west, which is oblique to the boundary between the metamorphic complex and the Tamana Granodiorite located on the southeast. The metamorphic condition of two pelitic rocks that occur in the muscovite–andalusite zone and sillimanite–K‐feldspar zone are estimated as 510 ±30 °C, 300 ±60 MPa and 720 ±30 °C, 620 ±60 MPa, respectively. Thermodynamic consideration reveals that use of the same geothermobarometer enables precise determination of the difference in pressure between the samples as 320 ±10 MPa. This indicates that the pelitic samples were metamorphosed at different depth by 11–12 km that is significantly larger than the geographic distance of 6.8 km between the sample localities. This also suggests that crustal thinning took place after the high‐T metamorphism. The high‐T metamorphic complex is, therefore, not of static contact metamorphism but of dynamic regional metamorphism. The present result combined with petrological and chronological similarities implies that this complex suffered the regional Ryoke metamorphism.  相似文献   

10.
The Sanbagawa metamorphic terrain of the study area is divided into two units, the Shirakura and Sejiri units. The metamorphic thermal structure is interpreted on the basis of the degree of graphitization (GD) of carbonaceous material in pelitic schists. The areal variations of the metamorphic grade are presented by the distribution of GD calculated using the Lc and d002 of carbonaceous material. As a result, the two units are classified into four metamorphic zones, respectively: A1, A2, B1 and B2 for the Shirakura Unit; and I1, I2, II1 and II2 for the Sejiri Unit. The metamorphic grades of A1, A2, I1 and I2 are included in the chlorite zone, and that of B1, B2, II1 and II2 in the garnet zone of the Sanbagawa metamorphism. The degree of graphitization at the boundary between A2 and B1 zones is the same as that between I2 and II1 zones. Detailed study on the variation of GD suggests that the present‐day structure of the study area is best interpreted as a model of shuffled‐cards structure. An estimated minimum thickness of a stack of continuous cards is about 25 m. The compositions of garnet in pelitic schists and of amphibole in basic schists are different from those in the identical metamorphic range of the Shirakura and Sejiri units. It is suggested that rocks of the Shirakura Unit were metamorphosed under higher P/T conditions than those of the Sejiri Unit.  相似文献   

11.
Masumi  Sakaguchi  Hideo  Ishizuka 《Island Arc》2008,17(3):305-321
Abstract   The mineral assemblages of the pumpellyite–actinolite facies such as pumpellyite + actinolite + epidote + chlorite or actinolite + epidote + hematite + chlorite occur in the Sanbagawa low-grade metamorphic region, central Shikoku, southwest Japan. Chemical compositions of these minerals from the eight newly studied areas were analyzed in order to evaluate the areal extent and thermal structure of the region. In the buffered assemblage of pumpellyite + actinolite + epidote + chlorite, the Fe3+/(Fe3+ + Al) values of epidote decrease slightly with decreasing Fe2+/(Fe2+ + Mg) values for chlorite. The changes in these values show a general correlation with temperature. The presence of this relationship implies that the Fe3+/(Fe3+ + Al) values of epidote can be used to divide the Sanbagawa low-grade metamorphic region into low-, medium- and high-grade subzones. The areal distribution of these subzones indicates that: (i) the temperature seems to decrease in the same sense as envisaged by the zonal mapping of the higher-grade pelitic schists; and (ii) there is no significant gap of metamorphic conditions through the boundary between the two structural units (Besshi and Oboke units). It follows that the Sanbagawa low-grade metamorphic region decreases in temperature going up the structural section, and tectonic discontinuities have not affected the thermal structure.  相似文献   

12.
马尼拉俯冲带热结构数值模拟与地震意义   总被引：1，自引：0，他引：1  
研究马尼拉俯冲带地震分布的成因机制,根据马尼拉俯冲带最新的莫霍面深度和地壳厚度等地质与地球物理资料,选取3条典型剖面,模拟俯冲带热结构。结果表明:1俯冲带热结构主要受俯冲角度、俯冲速度和俯冲板块本身地质条件等因素影响;2 BB′剖面和CC′剖面属于热俯冲;3当洋壳俯冲至软流圈边界时,俯冲板块温度迅速升高,容易形成地震活动。BB′剖面的俯冲角度和俯冲速度比CC′剖面小,使得BB′剖面发生地震的深度更浅。俯冲洋壳底部温度比顶部低,地震活动也持续到更大的深度。  相似文献   

13.
New U–Pb ages of zircons from migmatitic pelitic gneisses in the Omuta district, northern Kyushu, southwest Japan are presented. Metamorphic zonation from the Suo metamorphic complex to the gneisses suggests that the protolith of the gneisses was the Suo metamorphic complex. The zircon ages reveal the following: (i) a transformation took place from the high‐P Suo metamorphic complex to a high‐T metamorphic complex that includes the migmatitic pelitic gneisses; (ii) the detrital zircon cores in the Suo pelitic rocks have two main age components (ca 1900–1800 Ma and 250 Ma), with some of the detrital zircon cores being supplied (being reworked) from a high‐grade metamorphic source; and (iii) one metamorphic zircon rim yields 105.1 ±5.3 Ma concordant age that represents the age of the high‐T metamorphism. The high‐P to high‐T transformation of metamorphic complexes implies the seaward shift of a volcanic arc or a landward shift of the metamorphic complex from a trench to the sides of a volcanic arc in an arc–trench system during the Early Cretaceous. The Omuta district is located on the same geographical trend as the Ryoke plutono‐metamorphic complex, and our estimated age of the high‐T metamorphism is similar to that of the Ryoke plutono‐metamorphism in the Yanai district of western Chugoku. Therefore, the high‐T metamorphic complex possibly represents the western extension of the Ryoke plutono‐metamorphic complex. The protolith of the metamorphic rocks of the Ryoke plutono‐metamorphic complex was the Jurassic accretionary complex of the inner zone of southwest Japan. The high‐P to high‐T transformation in the Omuta district also suggests that the geographic trend of the Jurassic accretionary complex was oblique to that of the mid‐Cretaceous high‐T metamorphic field.  相似文献   

14.
Abstract Fifty calc-schists have been systematically collected from the Piemonte zone of the western Italian Alps and examined in terms of petrology, X-ray powder diffractometer (XRD) analysis of carbonaceous materials, and K-Ar ages of white mica separates. The petrological study and XRD analysis of carbonaceous materials have shown that calc-schists have suffered blueschist-facies metamorphism in the subduction zone of the convergent margin between the Apulian (African) continental and Tethyan oceanic plates. The metamorphic sequence is divided into three mineral zones based on increasing metamorphic temperature: chlorite (lower than 300°C), chloritoid, and rutile (higher than 450°C). The chlorite zone has dispersed ages of white mica separates, ranging from 115 to 44 Ma, whereas the rutile zone has a comparatively uniform age distribution from 60 to 40 Ma. The chloritoid zone has an intermediate age variation. The large variation in the chlorite zone is attributed to mixing of variable amounts of detrital mica derived from older high temperature metamorphic rocks in the separates, which have not been completely reset during Alpine metamorphism. The uniform age (average ca 50 Ma) in the rutile zone is the cooling age of blueschist-facies calc-schists, which have been episodically exhumed at the collision event of the European and Apulian continents in the Paleocene-Eocene.  相似文献   

15.
日本俯冲带与IBM俯冲带位于太平洋板块、菲律宾海板块和欧亚板块三者的交汇地带,是典型的"俯冲工厂"地区,具有重要的研究意义.本文利用震源分布资料与卫星重力数据对日本俯冲带与IBM俯冲带进行了研究.通过空间重力异常反映了俯冲带地区的区域构造形态,在此基础上基于艾利模式计算了均衡异常以反映地壳均衡特征.利用震源分布资料,分别从垂直俯冲带走向与沿俯冲带走向划定了横截剖面(cross-sections)进行了地震提取,讨论了俯冲带地区的Wadati-Benioff带形态特征,并借助于俯冲带地震等深线图直观描述了俯冲带的俯冲形态.在日本俯冲带与伊豆—小笠原俯冲带各选取了一条典型剖面进行了重力2.5D反演,研究了俯冲带地区的壳幔结构特征.研究结果表明,九州—帕劳海脊与IBM岛弧在均衡异常上存在差异,前者已逐渐趋向于地壳均衡.IBM的Wadati-Benioff带存在明显的南北差异,反映出伊豆—小笠原俯冲板片停留在了660km转换带中,而马里亚纳俯冲板片很可能垂直穿过了这一转换带,造成这种南北差异的原因与板块相对运动、岩石圈黏性和年龄差异以及俯冲板片的重力效应等因素有关.在IBM的中部和南部存在板片撕裂现象.日本俯冲带的俯冲洋壳密度随俯冲深度变化较小,洋幔存在一定程度的蛇纹岩化,地幔楔蛇纹岩化作用不典型,海沟处有一范围较小的含水畸变带;伊豆—小笠原俯冲带俯冲洋壳密度随深度增大而明显增大,洋幔蛇纹岩化程度较日本俯冲带低,地幔楔蛇纹岩化作用强烈,板块交汇处存在明显的蛇纹岩底辟.日本俯冲带与IBM俯冲带一线自北向南板片俯冲变陡,两侧板块耦合度降低,与俯冲带两侧的板块运动速率差异有关.  相似文献   

16.
During the Late Paleozoic Alleghanian orogeny, the mid-Atlantic Piedmont experienced transpressional deformation dominated by dextral strke-slip shear zones. The dextral displacement on these shear zones greatly influenced the geographic distribution of lithotectonic units. Transpressional deformation is evident in the Piedmont with the cogenetic development of domes and en-echelon antiforms between many of the shear zones. In the core of the Pennsylvania reentrant, major Alleghanian structures include the dextral Pleasant Grove shear zone and Tucquan-Mine Ridge antiform. Recent field mapping coupled with detailed metamorphic and deformation fabric studies have revealed that a major thrust, the Martic thrust, was also active during this time. Shear bands were identified during petrofabric analysis of the hanging wall rocks to the Martic thrust. The direction of displacement on these shear bands was parallel to the orogen, a direction contrary to earlier studies. Metamorphic mineral assemblages and ceased reaction textures, associated with ductile shear fabrics in the hangingwall rocks, are consistent with lower greenshist facies deformation. This low grade metamorphism, which is generally confined to sheared rocks, overprints the regional upper greenshist- to lower amphibolite-facies assemblages. Structural and magnetic modeling of the hangingwall block has revealed a complex geometry. A model of orogen parallel structural escape, or orogenic float, related to late Paleozoic dextral transpression is employed to explain the late reactivation on this important central Appalachian structure.  相似文献   

17.
The Bloody Bluff fault zone, which divides the New England Avalon zone and Nashoba zone, contains at least two shear zones that are within Avalonian rocks. The Rice Road shear zone (sinistral, strike-slip) affects the Westboro Formation and is intruded by the 630 Ma Dedham Granite. The Rice Road shear zone, and equivalent pre-granite mylonites appearing in drill cores, parallel the terrane boundary, and may have controlled the later mylonitization. The Nobscot shear zone (dextral, strike-slip) is a prograde shear zone cutting a granite assumed to be related to the surrounding 630 Ma plutons. Similar shear zones have been seen cutting Late Proterozoic plutons in the New England Avalon zone, and represent a series of en echelon strike-slip shears. The Burlington mylonite zone (shear sense equivocal) is part of the terrane boundary. This is a retrograde shear zone that forms the southeastern border of the Wolfpen lens, a lenticular body of sheared and altered metamorphic and intrusive rock that has been assumed to be part of the New England Avalon zone. Microstructural characteristics indicate that the Burlington mylonite zone was active after the Nobscot shear zone. In particular, quartz in the Nobscot shear zone was dynamically recrystallized by a combination of grain boundary migration and rotation recrystallization processes, thought to occur during shearing at upper-greenschist conditions. In contrast, quartz in the Burlington mylonite zone was recrystallized predominantly by rotation recrystallization, indicating lower-greenschist, retrograde, deformation. The two shear zones are too close for these differences to be a result of a simple thermal field gradient.While mineral assemblages in most of the study area indicate no metamorphic grade higher than upper-greenschist temperatures, the Wolfpen lens contains amphibolites with assemblages formed at temperatures above the oligoclase isograd, indicating mid-amphibolite facies metamorphism. As metamorphic contrast is one of the key features differentiating the Nashoba zone from the New England Avalon zone, the Wolfpen lens cannot be assumed to be part of Avalon. It may be a small block of rocks of intermediate grade between the two terranes.  相似文献   

18.
Melt‐origin pseudotachylyte is the most reliable seismogenic fault rock. It is commonly believed that pseudotachylyte generation is rare in the plate subduction zone where interstitial fluids are abundant and can trigger dynamic fault‐weakening mechanisms such as thermal pressurization. Some recent studies, however, have discovered pseudotachylyte‐bearing faults in exhumed ancient accretionary complexes, indicating that frictional melting also occurrs during earthquakes in subduction zones. To clarify the pseudotachylyte generation mechanism and the variation of slip behavior in the plate subduction zone, a pseudotachylyte found in the exhumed fossil accretionary complex (the Shimanto Belt, Nobeoka, Japan) was re‐focused and microscopic and three‐dimensional observations of the pseudotachylyte‐bearing fault were performed based on optical, electron, and X‐ray microscope images. Based on the patterns contained in the fragment, the pseudotachylyte is divided into four domains, although no clear domain boundaries or layering structures are not found. Three‐dimensional observation also suggests that the pseudotachylyte were fragmented or isolated by cataclasite or carbonate breccia. The pseudotachylyte was rather injected into the surrounding carbonate breccia, which is composed of angular fragments of the host rock and a matrix of tiny crystalline carbonate. The pseudotachylyte volume was extracted from the X‐ray microscope image and the heat abundance consumed by the pseudotachylyte generation was estimated at 2.18 MJ/m2, which can be supplied during a slip of approximately 0.5 m. These observations and calculations, together with the results of the previous investigations, suggest hydrofracturing and rapid carbonate precipitation that preceded or accompanied the frictional melting. Dynamic hydrofracturing during a slip can be caused by rapid fluid pressurization, and can induce abrupt decrease in fluid pressure while drastically enhancing the shear strength of the shear zone. Consequently, frictional heating would be reactivated and generate the pseudotachylyte. These deformation processes can explain pseudotachylyte generation in hydrous faults with the impermeable wall rock.  相似文献   

19.
用板块构造学说对中国部分地区构造发展的初步分析   总被引：16，自引：2，他引：16       下载免费PDF全文
为便于在大陆上研究板块构造,作者提出了几项原则,作为参考:1.板块接触带时常表现为一条大断层或断裂带,延伸至少数百公里,经常超过一千公里;2.在两个相邻的板块上,沉积岩相和古生物群有显著的划分;3.有混杂堆积的出现;4.有蛇绿岩带的出现;5.有蓝片岩的出现;6.侵入岩与喷出岩具有规律性的分布;7.地震震中的分布;8.两个相邻板块所指极向的不同移动轨迹. 根据上述原则,结合中国地质情况进行分析,作者认为我国有几条山脉和地区可能是板块构造接触带.这些是:1.秦岭东西构造带,2.台湾省东岸,3.西藏的雅鲁藏布江,4.金沙江上游,5.龙门山及“康滇地轴”,6.祁连山北部边缘. 在工作中也遇到了以下几个有待进一步研究的问题:1.大陆板块对大陆板块互相碰撞的说法,似应作一定的修改.因为几乎所有大陆上的俯冲带都是沿着地槽的一边或两边发生的,而不是两个大陆板块直接相互接触.它和大陆边缘的海洋板块对大陆板块的移动,极为相似。2.超基性岩是否能侵入到大陆地壳,还是只能生成于地幔?3.如果说超基性岩只出现于地槽,则古老地块中有超基性岩出现时,是否可以说,这是以前地槽沉积的变质岩,而不是古老的岩浆岩体,例如秦岭的大华群和大别山的淮阳地盾等。  相似文献   

20.
Kazuaki Okamoto 《Island Arc》1998,7(1-2):283-294
The orientation of straight inclusion trails within albite porphyroblasts from basic schists has been measured around a north-closure fold, in the Besshi district of the Sambagawa Belt, central Shikoku, Japan. The porphyroblasts are aligned with their longest dimension parallel to both the subhorizontal, east–west-directed mineral lineation and to the fold axis. There is a systematic variation in inclusion-trail geometry between the upper (northern) and lower (southern) fold limbs. The shear sense deduced from quartz c-axis fabrics is top-to-the-west in the upper limb and top-to-the-east in the lower limb. Based on observed variations in porphyroblast inclusion trails, the structural history can be modelled as follows: (i) shear flow caused east–west stretching and folding of the metamorphic zonation; (ii) east–west ductile shear resulted in opposing senses of shear in the upper and lower limbs as the eclogite body situated in the core of the fold was extruded to the east.  相似文献   

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