Micro-and submicrostructural evidence for high-temperature brittle-ductile transition deformation of hornblende: Case study of high-grade mylonites from Diancangshan, western Yunnan

OM (optical microscope)/TEM (transmission electron microscope) micro- and submicrostructural analysis of hornblende rocks sheared at high temperatures from the Diancangshan area, western Yunnan reveals evidence for deformation in the brittle-ductile transition of hornblende at middle crustal level (about 637℃ and 0.653 GPa) and mechanisms of deformation in the transitional regime are further discussed. Sheared hornblende rocks at middle crustal level have typical mylonitic microstructures, shown by coarse porphyroclasts and fine matrix grains. Different mineral phases in the rocks show distinct deformation characteristics. Hornblende and feldspar grains are intensely deformed with ob- vious grainsize reduction, but quartz grains are recrystallized dominantly by grain growth. Hornblende grains show typical brittle-ductile transition nature. Initial crystallographic orientations of porphyro- clasts have strong effects on the behavior of grains during deformation. There are mainly two types of porphyroclasts, type I "hard" porphyroclasts and type II "soft" porphyroclasts, with [001] perpendicular and parallel to external shear stresses respectively. "Hard" porphyroclasts generally occur as compe- tent grains that are rarely deformed or sometimes deformed by fracturing and dislocation tangling. "Soft" porphyroclasts are highly deformed primarily by dislocation tangling (as shown in the cores of the porphyroclasts), but twinning, dislocation glide and climb probably due to hydrolytic weakening also contribute to dynamic recrystallization of the porphyroclasts into fine grains in the matrix. The micro- and submicrostructures of the two types of porphyroclasts and fine-grained matrix provide powerful evidence for the behavior of brittle-ductile transition of hornblende grains. It is concluded that twinning nucleation is one of the most important processes that operate during dynamic recrystalliza- tion of hornblende crystals at the brittle-ductile transition. (100) [001] twin gliding and dislocation creep (dislocation glide and climb) are mutually enhanced during twinning nucleation. As a newly discovered mechanism of dynamic recrystallization, it may have played more important roles than ever recognized during dynamic recrystallization of crystals with twins in the brittle-ductile transition.     

Structural deformation of the Leoville chondrite

Foliations defined by alignment of elongated chondrules have been noted previously in chondrites, but none displays this effect so well as Leoville (CV3). The shapes of Leoville chondrules were produced by deformation in situ, as indicated by inclusions and clasts with similar shapes and preferred orientations to those of chondrules. Similarities in the aspect ratios of apparent strain ellipses measured for chondrules alone (1.9 and 2.0 by several methods) and for the whole meteorite (2.0) indicate either that Leoville deformed homogeneously or that it deformed as a framework of touching chondrules. This amount of strain corresponds to approximately 33% uniaxial shortening, assuming constant volume. Because the strain ellipse was measured in only one orientation, this strain value is a minimum estimate for the meteorite. Lack of correlation between foliation and either shock or thermal effects argues that impact or metamorphism are unlikely to have produced this deformation. Compaction due to overburden from progressive accretion on the chondrite parent body is suggested to have been its cause.Estimates of maximum deviatoric stresses in the interiors of asteroid-sized bodies and constraints on maximum temperatures for CV3 chondrites are consistent with diffusional flow as the deformation mechanism for olivine in these chondrules. Diffusional flow is also suggested by the scarcity of observed lattice dislocations. Deformation of Leoville olivines by this mechanism at geologically reasonable strain rates appears to require higher temperatures than those believed to have been experienced by this meteorite (< 600°C). However, differences in olivine grain size, the presence of water, or a more complex deformation history might explain this discrepancy.     

DEFORMATION MECHANISM OF GRANITIC ROCKS IN BRITTLE-PLASTIC TRANSITION ZONE

Field studies and seismic data show that semi-brittle flow of fault rocks probably is the dominant deformation mechanism at the base of the seismogenic zone at the so-called frictional-plastic transition. As the bottom of seismogenic fault, the dynamic characteristics of the frictional-plastic transition zone and plastic zone are very important for the seismogenic fault during seismic cycles. Granite is the major composition of the crust in the brittle-plastic transition zone. Compared to calcite, quartz, plagioclase, pyroxene and olivine, the rheologic data of K-feldspar is scarce. Previous deformation studies of granite performed on a quartz-plagioclase aggregate revealed that the deformation strength of granite was similar with quartz. In the brittle-plastic transition zone, the deformation characteristics of granite are very complex, temperature of brittle-plastic transition of quartz is much lower than that of feldspar under both natural deformation condition and lab deformation condition. In the mylonite deformed under the middle crust deformation condition, quartz grains are elongated or fine-grained via dislocation creep, dynamic recrystallization and superplastic flow, plagioclase grains are fine-grained by bugling recrystallization, K-feldspar are fine-grained by micro-fractures. Recently, both field and experimental studies presented that the strength of K-feldspar is much higher than that of quartz and plagioclase. The same deformation mechanism of K-feldspar and plagioclase occurred under different temperature and pressure conditions, these conditions of K-feldspar are higher than plagioclase. The strength of granite is similar to feldspar while it contains a high content of K-feldspar. High shear strain experiment studies reveal that granite is deformed by local ductile shear zones in the brittle-plastic transition zone. In the ductile shear zone, K-feldspar is brittle fractured, plagioclase are bugling and sub-grain rotation re-crystallized, and quartz grains are plastic elongated. These local shear zones are altered to local slip-zones with strain increasing. Abundances of K-feldspar, plagioclase and mica are higher in the slip-zones than that in other portions of the samples (K-feldspar is the highest), and abundance of quartz is decreased. Amorphous material is easily formed by shear strain acting on brittle fine-grained K-feldspar and re-crystallized mica and plagioclase. Ductile shear zone is the major deformation mechanism of fault zones in the brittle-plastic transition zone. There is a model of a fault failed by bearing constant shear strain in the transition zone:local shear zones are formed along the fractured K-feldspar grains; plagioclase and quartz are fine-grained by recrystallization, K-feldspar is crushed into fine grains, these small grains and mica grains partially change to amorphous material, local slip-zones are generated by these small grains and the amorphous materials; then, the fault should be failed via two ways, 1)the local slip-zones contact to a throughout slip-zone in the center of the fault zone, the fault is failed along this slip-zone, and 2)the local slip-zones lead to bigger mineral grains that are in contact with each other, stress is concentrated between these big grains, the fault is failed by these big grains that are fractured. Thus, the real deformation character of the granite can't be revealed by studies performing on a quartz-plagioclase aggregate. This paper reports the different deformation characters between K-feldspar, plagioclase and quartz under the same pressure and temperature condition based on previous studies. Then, we discuss a mode of instability of a fault zone in the brittle-plastic transition zone. It is still unclear that how many contents of weak mineral phase(or strong mineral phase)will control the strength of a three-mineral-phase granite. Rheological character of K-feldspar is very important for study of the deformation characteristic of the granitic rocks.     

Electron petrography of shock-deformed olivine in stony meteorites

We have studied olivine by high-voltage electron microscopy in meteorites showing shock effects, namely the ordinary chondrites Olivenza and Hedjaz, the ureilite Goalpara and the carbonaceous chondrite Allende. The observations are compared with published data on experimentally deformed and annealed olivine.Olivenza and Hedjaz have suffered low-temperature shock, which has produced high densities of [001] screw dislocations and cracks. Arrays of cracks are found to correspond to planar features which can be seen optically and to be responsible for mosaicism. Goalpara has suffered very heavy shock, of which the final result is extensive recrystallization of the olivine, and the new grains have distinctive low-density distributions of dislocations. Allende has not been shocked as a whole, but individual parts have deformational and heating histories dating from before the aggregation of the meteorite.     

Incipient melting in and shock classification of L-group chondrites

Thirty-three of 52 type L4 to L6 chondrites that we have examined in thin section contain closed bodies of crystal-laden glass or devitrified glass (melt pockets) that testify to in-situ melting. A close correlation between the distribution of melt pockets and shock intensity as inferred from the characteristics of olivine and plagioclase indicates that the pockets reflect shock melting. The appearance of pockets coincides with a sharp decrease of⁴⁰Ar in L-group chondrites, suggesting that shock melting was responsible for loss of argon and raising the possibility that this process redistributed other volatile elements as well. The use of three criteria for shock intensity — olivine and plagioclase characteristics, and the presence or absence of melt pockets — leads to a refined shock classification for equilibrated chondrites that is based entirely on petrographic observations.     

New kind of type 3 chondrite with a graphite-magnetite matrix

We have discovered four clasts in three ordinary-chondrite regolith breccias which are a new kind of type 3 chondrite. Like ordinary and carbonaceous type 3 chondrites, they have distinct chondrules, some of which contain glass, highly heterogeneous olivines and pyroxenes, and predominantly monoclinic low-Ca pyroxenes. But instead of the usual fine-grained, Fe-rich silicate matrix, the clasts have a matrix composed largely of aggregates of micron- and submicron-sized graphite and magnetite. The bulk compositions of the clasts as well as the types of chondrules (largely porphyritic) are typical of type 3 ordinary chondrites, although chondrules in the clasts are somewhat smaller (0.1–0.5 mm). A close relationship with ordinary chondrites is also indicated by the presence of similar graphite-magnetite aggregates in seven type 3 ordinary chondrites. This new kind of chondrite is probably the source of the abundant graphite-magnetite inclusions in ordinary-chondrite regolith breccias, and may be more common than indicated by the absence of whole meteorites made of chondrules and graphite-magnetite.     

沂沭断裂带中段基底韧性剪切带

在沂沭断裂带中段的泰山群结晶基底中,存在着一系列北东、北北东走向的左行韧性剪切带。对其中发育糜棱岩的韧性剪切带的结构构造、应变测量、变形岩石的显微构造、显微组构以及长石粒度和含量的变化规律进行了观测分析。讨论了韧性剪切带的变形条件和机制。及其递进发展的趋势。指出基底韧性剪切带是沂沭断裂带元古宙时期断裂活动时在地壳较深层次上形成的构造表象。在此基础上,讨论了地壳不同层次上的断裂变形及断裂岩石的综合分类问题     

Characteristics in naturally and experimentally shocked chondrites: A clue to P-T conditions of impacted asteroids

The aim of this study is to compare the experimentally shock-induced features with those in naturally shocked chondrites and to test the feasibility of experimentally calibrating naturally induced features in shocked H- and L-chondrites. Samples of the Jilin chondrite (H5) were experimentally shock-loaded at the following peak pressures: 12, 27, 39, 53, 78, 83, 93 and 133 GPa respectively. Chondritic melts were first obtained at P>78 GPa and more than 60% melting was achieved at P~133 GPa. No high-pressure phases were observed in any of the shocked samples, neither in the deformed nor in the molten regions. Textural relations and mineral assemblages of the shocked samples are comparable to those encountered in the heavily shocked H-chondrite Yanzhuang but differ considerably from those found in heavily shocked L6 chondrites. Shock melt veins in L6 chondrites contain high-pressure polymorphs of olivine and pyroxene and high pressure liquidus phases. Scaling from shock experiments on millimeter-sized samples to natural shock features on kilometer-sized asteroids poses considerable problems in quantifying the P-T conditions during natural shock events on asteroids.     

Fine-grained aggregates in L3 chondrites

The textures and chemical compositions of the constituent minerals of the fine-grained aggregates (FGA's) of L3 chondrites were studied by the backscattered electron image technique, electron probe microanalysis, and transmission electron microscopy. Plagioclase and glass in the interstices between fine grains of olivine and pyroxene indicate that the FGA's once partly melted. Compositional zoning and decomposition texture of pyroxenes are similar to those observed in chondrules, indicating a common cooling history of the FGA's and chondrules. Therefore, the mechanism that caused melting of the FGA's is considered to be the same as for chondrules. Bulk compositions of the FGA's are within the range of those of chondrules, so some chondrules probably were produced by complete melting of the same precursor materials as those of the FGA's. The precursor materials must have included fine olivine and other grains that probably are condensates.     

Constraints for chondrule formation from Ca–Al distribution in carbonaceous chondrites

Chondritic meteorites and their components formed in the protoplanetary disk surrounding the nascent sun. We show here that the two volumetrically dominating components of carbonaceous chondrites, chondrules and matrix did not form independently. They must have been derived from a single, common source. We analyzed Ca and Al in chondrules and matrix of the CV type carbonaceous chondrites Allende and Y-86751. The Ca/Al-ratios of chondrules and matrix of both chondrites are complementary, but in case of Allende chondrules have sub-chondritic and matrix super-chondritic Ca/Al-ratios and in case of Y-86751 chondrules have super-chondritic and matrix sub-chondritic Ca/Al-ratios. This rules out the redistribution of Ca between chondrules and matrix during parent body alteration. Tiny spinel grains in the matrix produce the high Al in the matrix of Y-86751. In Allende these spinels were most probably included in chondrules. The most plausible explanation for this Ca- and Al-distribution in the same type of chondrite is that both chondrules and matrix formed from the same chemical reservoir. Tiny differences in nebular conditions during formation of these two meteorites must have led to the observed differences. These are severe constraints for all models of chondrule formation. Any model involving separate formation of chondrules and matrix, such as the X-wind model can be excluded.     

Remarques sur la matrice des chondrites ordinaires d'apres l'observation de quelques chondrites a bronzite peu ou pas choquees

Microscopic investigations have been done on the chondrites Sena and Nadiabondi (H5, not shocked), Ste. Marguerite en Comines (H4, very slightly shocked), Allegan (H5, slightly shocked). Only in such cases can the matrix be easily observed and compared to those of type 3 chondrites. The <100 μm debris found in types 4 and 5 that we have observed are not the result of the metamorphism of type 3 fines.The abundance of tiny debris is in direct relation with the intensity of the shock though this shock was insufficient to provoke either the induration of the stones or a significant loss of rare gases. The bulk of the fines are the result of local disaggregation of the most brittle parts from chondrules and fragments.A low-temperature matrix has not been observed in these meteorites but only in H3 chondrites, as a coating around the chondrules. The accretion modelists should take into account the absence or the scarcity of fine particles in their calculations.     

Grain scale deformation in ultra-high-pressure metamorphic rocks—an indicator of rapid phase transformation

Conspicuous grain scale deformation is observed in some ultra-high-pressure (UHP) metamorphic rocks of the Dora Maira Massif, Western Alps, although no significant strain is discernible on the mesoscopic scale. In a jadeite–kyanite–quartz rock, some of the jadeite crystals reveal (100) deformation twins, indicating local differential stress levels above 0.3 GPa. Many kyanite crystals show marked kink or deformation bands, with a slip system (100)[001]. In contrast, the adjacent coarse-grained quartz matrix (grain size ca. 0.2 mm), which has formed from coesite during exhumation from >100 km depth, reveals a foam structure. The quartz grain boundary configuration is controlled by interfacial free energy, the grains are optically strain-free, and there is no crystallographic preferred orientation. Preservation of this foam microstructure, which indicates grain growth during low-stress annealing, precludes that deformation of the jadeite and kyanite crystals is a result of a late-stage low-temperature overprint. The orientation distribution of jadeite and kyanite with and without twins or deformation bands, respectively, has been investigated with a combination of universal stage and EBSD techniques. On the scale of a thin section, there is no preferred orientation of twinned jadeite and bent kyanite crystals and undeformed crystals, respectively. Thus, the orientation of the inferred local shortening direction is random. This precludes deformation driven by a homogeneous far field tectonic stress, but suggests an internally controlled stress field which is highly inhomogeneous on the scale of a few grain diameters. Laboratory experiments show that the coesite to quartz transformation proceeds within hours after decompression from 3.0 to 2.7 GPa at 800 °C. The microstructures of incompletely transformed samples indicate that the quartz growing at the expense of coesite undergoes crystal plastic deformation and recrystallizes with a very fine grain size during transformation. In this case, the deformation of quartz is attributed to the volumetric strain ΔV=+10% inherent in the coesite–quartz transformation, which causes a highly inhomogeneous stress field inside the sample related to the progress of the transformation. We propose that a similar process has taken place in the polyphase natural rock during exhumation, with the transient stresses causing mechanical twinning of jadeite and bending or kinking of kyanite. When the transformation had gone to completion, grain growth obliterated the microstructures of the quartz matrix, while the deformed jadeite and kyanite crystals preserved the record of inhomogeneous deformation at high temperatures. The peak differential stresses locally exceeded 0.3 GPa, which indicates very high strain rates and a correspondingly rapid transformation of coesite to quartz, comparable to the laboratory results.     

Small faults formed as deformation bands in sandstone

Small faults with displacements of a few millimeters or centimeters are abundant in the Entrada and Navajo Sandstones, in the San Rafael Desert, Utah, where they are important primary structures, preceding the development of large faults with displacements of several meters or tens of meters. The small faults contain no surfaces of discontinuity, rather they occur asdeformation bands about one millimeter and tens or hundreds of meters long, and across which the displacements are distributed. Two zones with different modes of deformation can be distinguished within a deformation band: an outer zone where the matrix, including pores and matrix material, deforms; and an inner zone, about 0.5 mm thick, where the sand grains fracture and further consolidation takes place. Fracturing of the grains is controlled by contact geometry; the grains tend to split into subgrains along lines connecting contact points between the grains. Then the angular subgrains, which are readily fractured, are further granulated and mixed with the matrix. The final product is the deformation band, with much smaller grain size, poorer sorting, and lower porosity than the original parent sandstone. The sandstone on either side of a deformation band is almost undisturbed-fractures are rare there — so that deformation is highly localized within the band. The material within a deformation band apparently strain hardens as a result of the deformation; perhaps this is why the shear displacement across a deformation band is at most a few centimeters.     

K–Ar age–chemistry–fabric relations of phengite from the Sanbagawa high-pressure schists, Japan

The Sanbagawa high-pressure schists from central Shikoku in Southwest Japan have experienced high-strain ductile deformation during exhumation and cooling. This study examines the effects of high-strain ductile deformation on K–Ar ages of phengites on the basis of fabric, chemistry and K–Ar ages of phengites from the pelitic, psammitic and quartzose (or albitic) schists collected from the same outcrop in the albite–biotite zone. Phengites in the pelitic and psammitic schists generally occur forming aggregates consisting of fine-grained phengite crystals and are extremely fine-grained in domains close to relatively rigid garnet and albite porphyroblasts, indicating that deformation-induced grain-size reduction had taken place in phengite during the ductile deformation accompanying the exhumation of host schists. We suggest that the grain-size reduction of phengite is due to strain-induced recrystallization or dynamic recrystallization. The matrix phengites in schists are chemically heterogeneous on the thin-section scale but the phengites from pelitic and psammitic schists from the same outcrop have similar chemical range. Phengite included in garnet has a high Si value and its Na/(Na + K) and Mg/(Mg + Fe) ratios are significantly low in comparison with those in matrix. The phengite included in garnet records the chemistry in equilibrium with other major silicate phases during the higher pressure stage of the P–T–t history of the schists. In contrast, the matrix phengites having low Si values are likely to have been formed during retrograde metamorphism in extremely restricted equilibrium domains. The two or three different types of schists from the same outcrop, which have a similar grain size of phengite, have similar K–Ar ages, suggesting that the closure temperature does not depend on chemistry. However, the hematite-rich quartzose schist with strong grain-size reduction of both phengite and quartz has a significantly younger K–Ar phengite age than the pelitic and quartzose schists in the same outcrop that do not show grain-size reduction. We suggest that the exhumation tectonics of the schists, which have experienced strong ductile deformation at temperatures less than ~350°C, played an important role resulting in the observed variation in age.     

天然麻粒岩高温流变实验研究

本实验在气体介质三轴高温流变仪上,采用怀安瓦窑口麻粒岩,在温度900～1200℃、围压300 MPa、应变速率10~(-4)～10~(-6)/s条件下,开展高温流变实验.实验样品麻粒岩由斜长石(52%)、单斜辉石和斜方辉石(40%)、石英(3%)、磁铁矿和钛铁矿(5%)组成,矿物平均粒度为:斜长石294μm、单斜辉石和斜方辉石282μm、石英97μm、磁铁矿和钛铁矿109μm.利用傅里叶变换红外光谱仪分析获得变形后样品的水含量约为0.17±0.05wt%.实验样品的强度随温度升高而降低,随应变速率降低而降低.基于力学数据,采用稳态流变方程,获得实验样品在900～1000℃时的应力指数为8.1～12.9,在1050～1150℃时的应力指数为4.8～5.8,平均值5.2.应力指数随着温度升高而降低.显微结构和成分分析表明,在900℃时麻粒岩出现矿物压扁与定向拉长特征,样品以位错滑移和微破裂变形为主;在950～1000℃时,麻粒岩样品中颗粒边界变得圆滑,表现出位错攀移特征,辉石和磁铁矿边缘出现微量熔体;在1050～1200℃时麻粒岩出现部分熔融,而且随着温度和实验时间(应变)增加,熔体含量增加,熔体结晶出微粒斜长石、辉石和橄榄石,部分辉石通过固体反应生成橄榄石.颗粒边界熔体和矿物反应促进了扩散作用,导致位错攀移和熔体引起的扩散蠕变共同控制了麻粒岩的高温流变.     

The Niger (I) carbonaceous chondrite and implications for the origin of aggregates and isolated olivine grains in C2 chondrites

The origin of olivine grains in C2 carbonaceous chondrites is a controversial topic: directly condensed material or detrital remnants of preexisting chondrules? This study shows that the Niger C2 meteorite is similar to Murchison but reveals several interesting features in relation to the origin of the olivine. Microprobe analysis of olivine (Si, Fe, Mg, Ca, Mn, Cr), glass and nickel-iron inclusions within the grains, and Fe-S-O phase as well as the relationships between the olivine grains in the aggregates, between the grains and the interstitial phyllosilicate matrix, between the inclusions and their host olivine grains, and the morphology of some aggregates all show that two populations of olivine coexist, probably crystallized from chondrule melts rather than by direct condensation from a solar nebula gas. The characteristics of the nickel-iron inclusions within the olivine suggest a magmatic chondrule-making stage from previously condensed materials.     

Fe, Ni and Co variations in the metals of some antarctic chondrites

Nearly 4,000 Fe, Ni and Co analyses have been carried out on the metal phases of 12 Antarctic chondritic meteorites by means of the electron microprobe. H-group chondrites show relatively simple patterns of variation for these elements but L- and LL-group members show much more scatter in both Ni and Co concentrations. A single member of the CO3 group investigated shows some scatter in the concentrations and also much higher Co concentrations in the high-Ni (awaruite?) phase (1.25–2%) than in the coexisting kamacite (0.2–0.5%). Thus, analysis of the metal phases can provide not only a means of identifying the group to which a meteorite belongs, but also the possibility of distinguishing between individual chondrites from the same group.

The overall concentrations of Co in the metal particles in the different groups are considered to be related inversely to the abundance of metal grains in meteorites of these groups while the scatter is interpreted as reflecting characteristics inherited at the time of accretion. The absence of homogenisation of the concentrations of Fe, Ni and Co in the metal particles, even in so-called equilibrated chondrites, provides further evidence against the widely held notion that these meteorites have been involved in a high-temperature prograde metamorphism.     

Global variability in subduction thrust zone-forearc systems

Deviations of slip vector azimuths of interplate thrust earthquakes from expected plate convergence directions at oblique subduction zones provide kinematic information about the deformation of forearcs and indirect evidence on the dynamics of the plate boundary. A global survey of slip vectors at major trenches of the world reveals a large variability in the kinematic response of forearcs to shear produced by oblique convergence. The variability in forearc deformation inferred from slip vector deflections is suggested to be caused by variations in forearc rheology rather than in the stresses acting on subduction zone thrust faults. Estimated apparent macroscopic rheologies range from elastic to perfectly plastic (or viscous). Forearc rheologies inferred from slip vectors do not correlate with age of the subducting lithosphere, but continental forearcs or old arcs appear to deform less than oceanic or young arcs. The inferred absence of forearc deformation at continental arcs from this study is counter to inferences drawn from compiled geologic information on forearc faults. Correlations of the apparent forearc rheology with backarc spreading, convergence rate, slab dip, arc curvature, and downdip length of the thrust contact are poor. However, great subduction zone earthquakes occur where forearcs are apparently more elastic (i.e., less deformed by oblique convergence), which suggests that the mechanical properties of forearcs rather than stress magnitude on thrust faults control both the kinematic behavior of forearcs and where great subduction zone earthquakes occur.     

2008汶川Ms8.0地震发生的深层过程和动力学响应

汶川M_s8.0强烈地震发生在一条现今并不活动的龙门山构造带上,造成了以汶川、映秀为中心及其周边地域的严重破坏和人员的重大伤亡.然而强烈震发生前却未见有可能的确切征兆或浅表层异常活动,即浅层过程与地震发生的深层过程并不匹配.为此对这次强烈地震“孕育”、发生和发展的深层过程进行了分析和探讨,初步研究表明：①在印度洋板块与欧亚板块陆—陆碰撞、挤压作用下,喜马拉雅造山带东构造结向NNE方向顶挤、楔入青藏高原东北缘,迫使高原深部物质向东流展,在受到以龙门山为西北边界的四川盆地阻隔下,一部分物质则转而向东南侧向运移;②龙门山地带在地形上差达3500±500 m左右,而地壳厚度在龙门山西北部为60±5 km左右,四川盆地为40±2 km左右,而龙门山地带与其东、西两侧相比则为地壳厚度变化幅度达15~20 km的突变地域,即为应力作用的耦合地带;③中、下地壳和地幔盖层物质以地壳低速层、低阻层（深20~25 km）为第一滑移面,以上地幔软流层顶面为第二滑移面,且在四川盆地深部“刚性”物质阻 隔下,深部壳、幔物质以高角度在龙门山构造带和四川盆地的耦合地带向上运移（或称逆冲）,且在龙门山地表三条断裂构成的断裂系向下延伸到20 km左右深处汇聚,二者强烈碰撞、挤压、震源介质破裂;在物质与能量的强烈交换下,应力得到释放,故形成了这次M_s.0强烈地震.为此从深部初步揭示了这次强烈地震“孕育”、发生和发展的深层动力过程.     

Small faults and kink bands in the Nankai accretionary complex: Textural observations from Site 808 of ODP Leg 131

Abstract We present backscattered scanning electron microscope and petrographic microscope observations of deformed sediments from Ocean Drilling Program (ODP) Site 808 in order to better understand the dewatering and deformation history of the Nankai accretionary complex. This synthesis of deformation textures has three implications. First, the early structures that dominate the Nankai prism, small faults and kink bands, have very different electron microscope versus optical microscopic expressions. This observation is important to investigations of fine-grained sediment in both stable and active tectonic settings, in part, because these materials have often been studied almost exclusively by electron microscope methods. In sediments of this type, investigators often forego petrographic analysis because of the relative opacity of samples at normal (i.e. 30 pm) thin section thicknesses. Second, the textural observations we have compiled suggest that these deformation structures acted as 'single-event' pathways that contributed to diffusive dewatering of the prism. Third, our observations serve as a reference frame for the early tectonic structures that are important to the dewatering history of a 'sandy' accretionary prism.