The origin of strain patterns resulting from contemporaneous deformation and metamorphism in the Sambagawa metamorphic belt

Abstract The magnitudes of plastic strains of 104 metacherts were determined from the deformed shape of initially spherical radiolarians in the Sambagawa high- P type metamorphic belt of Western Shikoku, Japan. The strain magnitude increases with increasing metamorphic temperature from several per cent to 250%. The a₂/a₃ ratio of strain ellipsoids in the higher metamorphic grades decreases with increasing metamorphic grade while the a₁/a₂ ratio increases rapidly. The long axis of the strain ellipsoid for every grade is nearly parallel to the length of the metamorphic belt, suggesting that the flow direction of the synmetamorphic deformation was uniform along the belt. A map of strain zones within the Sambagawa high- P type metamorphic belt reveals that the metamorphic belt underwent a progressive bulk inhomogeneous shear deformation and that the high-grade zones represent a deep-seated boundary shear zone on the accretionary wedge between a subducting oceanic plate and the immobile rigid continental plate.     

Nappe structure of the Sambagawa belt

The thermal structure of the Sambagawa belt, which is reflected by minerals produced during the highest temperature of metamorphism, generally is characterized by the occurrence of the highest grade schists in the middle of the structural pile. The origin of this structure has been analysed in the upper horizon of the Sambagawa schist sequence in Central Shikoku as an example for the structure of the whole belt. The upper horizon of the Sambagawa schist sequence in Central Shikoku consists of three nappes, the Saruta nappe II, the Saruta nappe I and the Fuyunose nappe in descending order of structural level. The Saruta nappe II shows a downward increase of metamorphic grade from the garnet zone, through the albite-biotite zone, to the oligoclase-biotite zone. The Saruta nappe I consists mainly of rocks in the albite-biotite zone and partly in the oligoclase-biotite zone, but the direction of the increase of metamorphic grade is not clear. The Fuyunose nappe shows an upward increase in metamorphic grade which changes from the glaucophane zone to the barroisite zone. It is concluded that the Saruta nappe II and Saruta nappe I were overturned and then mechanically coupled with the Fuyunose nappe. The Sambagawa metamorphic field, which is of the highest temperature phase of metamorphism, appears to have had an inverted thermal gradient and a thermal structure comparable with that expected in the deeper parts of a subduction complex.     

Implications of mylonitic microstructures for the geotectonic evolution of the Median Tectonic Line,central Japan

The Median Tectonic Line (MTL), the most prominent onshore fault in Japan, demarcates the Cretaceous Hiji quartz dioritic gneiss of the Ryoke belt on the west from the high P/T type Sambagawa metamorphic rocks on the east in the Takato area. Toward the MTL, the Hiji gneiss grades into strongly mylonitized rocks characterized by grain-size reduction of quartz. In the mylonitic rocks, the development of fluxion banding (Sm) is remarkably influenced by the existence of K-feldspar, forming a myrmekitic intergrowth. Brittle microstructures indicative of truly cataclastic deformation are observed only in mylonitic rocks close to the MTL. Early deep-level ductile deformation apparently gave way to shallower, brittle deformation at a later stage. The attitude of stretching lineations (Lm) and asymmetric microstructures observed in the mylonites suggest that sinistral strike-slip shearing with a subordinate component of vertical-slip took place during mylonitization in mid-Cretaceous time. The mylonitic rocks and their protolith, supposed to have constituted the eastern limb of the shear zone, were probably eroded out and lost by upheaval of the Sambagawa belt relative to the Ryoke belt.     

Structural and metamorphic evolution of the Moornambool Metamorphic Complex,western Lachlan Fold Belt,southeastern Australia

The wedge‐shaped Moornambool Metamorphic Complex is bounded by the Coongee Fault to the east and the Moyston Fault to the west. This complex was juxtaposed between stable Delamerian crust to the west and the eastward migrating deformation that occurred in the western Lachlan Fold Belt during the Ordovician and Silurian. The complex comprises Cambrian turbidites and mafic volcanics and is subdivided into a lower greenschist eastern zone and a higher grade amphibolite facies western zone, with sub‐greenschist rocks occurring on either side of the complex. The boundary between the two zones is defined by steeply dipping L‐S tectonites of the Mt Ararat ductile high‐strain zone. Deformation reflects marked structural thickening that produced garnet‐bearing amphibolites followed by exhumation via ductile shearing and brittle faulting. Pressure‐temperature estimates on garnet‐bearing amphibolites in the western zone suggest metamorphic pressures of ～0.7–0.8 GPa and temperatures of ～540–590°C. Metamorphic grade variations suggest that between 15 and 20 km of vertical offset occurs across the east‐dipping Moyston Fault. Bounding fault structures show evidence for early ductile deformation followed by later brittle deformation/reactivation. Ductile deformation within the complex is initially marked by early bedding‐parallel cleavages. Later deformation produced tight to isoclinal D₂ folds and steeply dipping ductile high‐strain zones. The S₂ foliation is the dominant fabric in the complex and is shallowly west‐dipping to flat‐lying in the western zone and steeply west‐dipping in the eastern zone. Peak metamorphism is pre‐ to syn‐D₂. Later ductile deformation reoriented the S₂ foliation, produced S₃ crenulation cleavages across both zones and localised S₄ fabrics. The transition to brittle deformation is defined by the development of east‐ and west‐dipping reverse faults that produce a neutral vergence and not the predominant east‐vergent transport observed throughout the rest of the western Lachlan Fold Belt. Later north‐dipping thrusts overprint these fault structures. The majority of fault transport along ductile and brittle structures occurred prior to the intrusion of the Early Devonian Ararat Granodiorite. Late west‐ and east‐dipping faults represent the final stages of major brittle deformation: these are post plutonism.     

内蒙古北山小红山岩组变形特征及地质意义

为了研究红石山—百合山—蓬勃山古生代构造带(蛇绿岩带)物质组成和变形特征,利用数字填图方法,在内蒙古北山地区大红山一带,开展1:5万区域地质调查.发现大红山南出露一套变质基底,主要岩石组合为一套海相细碎屑岩夹中基性火山岩及磁铁石英岩组合,以含硅铁建造及变质基性火山岩为特征.该岩石组合无底无顶,变形变质强烈,分不出层序,故称之为小红山岩组.各类中低级变质岩说明变质作用达到了高绿片岩—低角闪岩相.该组北部被上石炭统白山组角度不整合覆盖,南部被早泥盆世、晚石炭世侵入岩侵入.区内出露面积约140 km2,厚约2890 m.明显有4次变形:前3次变形为韧性变形,形成了透入性面理S1、大型褶皱、韧性剪切带等构造形迹;后一次变形以脆性变形为特征,形成了较大规模的断层带.小红山岩组多期次变形、变质说明了红石山—百合山—蓬勃山古生代构造带(蛇绿岩带)确实存在,并经历了多期次变形和变质.     

滇西无量山地区的构造变形和变质作用

无量山构造带位于滇西兰坪—思茅盆地中段西侧北北西—南南东走向的复背斜。在复背斜中部发育北北西—南南东向的左旋走滑韧性剪切带。中部剪切带变质作用达绿帘角闪岩相的蓝晶石带高于绿片岩相黑云母带的围岩。用共生的黑云母—石榴石温压计计算得到中部剪切变质带西缘变质温度为600 ℃～650 ℃,压力为5.6 kba,东缘温度为550 ℃～600 ℃,压力为5 kba左右。结合同构造期石榴石变斑晶的环带成分和多硅白云母b0值的分析,首次在滇西提出该剪切变质带是进变质的,以不均一的、非连续的变质作用为主要特征,与区域变质作用不同。无量山中部韧性剪切变质带的进变质作用与剪切带中岩石的变形强度有关。岩石变形愈强,变形能就愈大,随之转化成的热量就愈大,该热能参与到岩石变质作用中,提高了岩石的变质程度。这一思路有可能成为研究变形与变质作用的一条有效途径。     

武夷山北缘断裂带动力学研究

华南武夷山北缘边界被绍兴－兴山－东乡断裂带所限。该断裂带到少保留了三期构造事件的形迹，第一期发生在８００Ｍａ～９００Ｍａ的晚元古代，呈ＮＷ向ＳＥ的区域推覆韧剪变形运动，以构造混杂岩和区域绿片岩相－角闪岩相变质，强烈的褶皱和韧剪变形为特征，对应于古洋盆关闭，华南复合地体与江南岛弧撞焊接过程，第二期发生在４５８Ｍａ～４２１Ｍａ的志留纪，表现为从北向南的韧剪变形运动，伴有左旋走滑韧性剪切，以糜棱岩化和进变质作用为特征．黒云母多变为硅线石。该期变形使第一期构造形迹被强烈选加置换。其动力学背景与闽东南地体朝武夷山的拼贴增生事件有关。第三期属中生代陆内变形，是一种高构造位的左旋走滑脆性剪切，以岩石的破裂和岩块的水平位移为特征．并具转换拉伸性质，导致中生代火山沉积盆地的形成。     

内蒙古色尔腾山地区花岗绿岩带糜棱岩变形变质过程探讨

采用不同研究方法分析和解读岩石和矿物的显微构造特征，能再造结晶基底岩石复杂的变形和变质演化过程和条件。通过对色尔腾山地区花岗绿岩带中韧性剪切带的糜棱岩显微构造、石英组构、矿物化学等综合分析，发现该区糜棱岩在经历了低角闪岩相区域变质及绿帘角闪岩相退变质变形之后，发生了不均匀进变质重结晶作用及中低绿片岩相退变质变形等几个演化阶段。由不同阶段温压条件所限定的p-T演化轨迹为一个在早期顺时针的基础上叠加了一个晚期逆时针环的复杂图像。     

羌塘中部高压变质带的退变质作用及其构造侵位

羌塘中部的高压变质带主要由榴辉岩、石榴石白云母片岩和蓝片岩等组成,它们在遭受高压变质作用之后折返,构造侵位于晚古生代展金组地层中,二者以韧性变形带为接触边界.本文以高压变质带中的榴辉岩和韧性变形带为研究对象,讨论了高压变质带折返过程中的退变质作用特征及折返时代.研究表明,榴辉岩在高峰期变质作用之后的折返过程中经历了由榴辉岩相→蓝片岩相→绿帘角闪岩相的退变质作用演化过程;在高压变质带构造侵位过程形成的韧性变形带中,白云母石英片岩的白云母40Ar-39Ar坪年龄为219±2Ma.高压变质带在219Ma左右构造侵位于展金组地层中,并于214Ma之前最终抬升出露地表.     

P-T-D Path of Eclogite from the Sambagawa Belt Deduced from Combination of Petrological and Microstructural Analyses

Both structural and petrological data can be used to constrainthe P–T path of an eclogitic schist unit (the Seba basicschist) in the Sambagawa belt of SW Japan. The relationshipsbetween these two sets of data are well defined by porphyroblasticand other microstructures. The derived P–T path for theSeba basic schist has an overall clockwise trajectory with thedecompression, or exhumation-related, path taking place undera lower P/T gradient than the burial, or subduction-related,path. The clockwise nature of the P–T path is qualitativelysupported by chemical zoning of amphibole coexisting with eclogiticminerals. The significant feature of the P–T path is thepresence of two temperature maxima, the first in the eclogitefacies and the second in the epidote-amphibolite facies. Theexistence of two temperature maxima gives a simple explanationfor the observation that metamorphic zonal boundaries postdatingthe eclogite facies metamorphism cross-cut the distributionof the main eclogite bodies in the Sambagawa belt. Estimatesof metamorphic pressure using the jadeite content of clinopyroxenein the Seba area demonstrate the existence of a tectonic discontinuitybetween the eclogitic schist and surrounding non-eclogitic schist.Structural studies show that although these two units have experiencedvery different peak metamorphic conditions, they became juxtaposedduring a single ductile deformation affecting both units. Thisdeformation is related to exhumation of the eclogitic schistand subduction of the non-eclogitic schist, indicating thatboth were formed during the same subduction event. The presenceof a major tectonic boundary between two units with a similarorigin as subducted and accreted material, but contrasting metamorphichistories, can be interpreted in terms of nappe tectonics, andthe existence of an ‘eclogite nappe’, the thirdnappe of the Sambagawa belt, is proposed. KEY WORDS: deformation stage; dual thermal maxima; eclogite; P–T–D path; Sambagawa belt     

K-Ar Age and Chemistry of Phengite from the Sanbagawa Schists in the Kanto Mountains, Central Japan, and their Implication for Exhumation Tectonics

EPMA analyses and K-Ar age determinations were carried out on phengite in pelitic schist from the Sanbagawa metamorphic belt of the Kanto Mountains, Central Japan.

Phengite from the Sanbagawa pelitic schist in the Kanto Mountains generally occurs as aggregates of fine-grained crystals. It is extremely fine-grained in domains adjacent to relatively rigid garnet and albite porphyroblasts. This suggests that deformation-induced grain-size reduction took place in phengite during the ductile deformation accompanying the exhumation of the host schists. EPMA analysis shows that phengite is chemically heterogeneous at the thin-section scale, suggesting that it formed during retrograde metamorphism in restricted equilibrium domains. The retrograde chemical reaction was promoted by the ductile deformation.

K-Ar ages of phengite get younger from the Southern Unit (82 Ma) to the Northern Unit (58 Ma) in the Kanto Mountains. The age range is similar to that in Central Shikoku. The older schists occur in the higher metamorphic grade zone in Central Shikoku and in the lower-grade zone in the Kanto Mountains. The thermal structures in Central Shikoku are inverted, so that the highest-grade zone occurs in the upper or middle parts of the apparent stratigraphic succession. In contrast, the Kanto Mountains have a normal thermal structure: the higher-grade zone is in the lower part of the apparent stratigraphic succession. The different tectonic features in exhumation produced the two contrasting age-temperature-structure relations at the western side of Sanbagawa belt in Central Shikoku and the eastern end of the Sanbagawa belt in the Kanto Mountains that are 800 km distant from each other. Namely, the western Sanbagawa belt in Central Shikoku underwent longer ductile deformation during the exhumation than the eastern Sanbagawa belt in the Kanto Mountains.     

豫西东秦岭造山带低压变质带的变质变形和变质反应

豫西西峡北部低压变质带受到４次变形改造。低压变质作用主要发生在Ｄ１和Ｄ２变形间期，形成黑云母－红柱石、石榴石－红柱石、十字石－红柱石和堇青石－红柱石４个递增变质带。由矿物变质反应识别出堇青石－红柱石带ｐ－Ｔ趋势具有减压增温特点，这一演化特征可能反映变质作用具有地壳拉张的地球动力学特点。     

构造动力变质作用初论

本文提出了构造动力变质作用的基本概念。即在构造动力及其引发的动力热流共同作用下,岩石、矿物在构造运动中发生的形变、相变而呈现的变质作用。按构造动力变质作用的物理化学条件,动力变质岩的结构构造、动态重结晶和混合岩化作用、应力矿物共生组合关系及其空间分布等特征,将构造动力变质作用由弱到强划分为4个发展阶段和相应的4种基本变质类型——脆性、脆-塑性、塑性和动力热流变形变质,并论述了它们各自的基本特征。      

Ductile wrench tectonics and exhumation of hercynian metamorphic basement in Sardinia: Monte Grighini Complex

Abstract

The Monte Grighini Complex (Central-Western Sardinia) is a NW-SE trending metamorphic complex of Hereynian age made up of a medium grade Lower tectonic unit with mylonitie granitoids and a low grade Upper tectonic unit exposed in the westernmost and southernmost portions of this complex. The Lower Unit shows a prograde metamor phism from garnet to sillimanite zone and the transition from MP/MT to LP/HT metamorphism. The metamorphic climax was reached at the end of the main deformative phase 1)2 (600° C. 6 kbar). After the main tectonic and metamorphic phase. the Lower Unit was affected by a wide NW-SE trending ductile dextral wrench shear zone. Intrusive rocks emplaced within the shear zone yielded radiometric ages of 305-300 Ma. Shear deformation leads to low temperature C-S mylonites and retrograde phyllonitic rocks with subhorizontal NW-SE trending stretching lineations. Kinematic analysis of the shear zone points to a dextral sense of shear with an amount of ductile displacement of about 7 km. Later low angle N-S and E-W trending normal faults are associated with cataclastic zones separating the Lower Unit from the Upper one. These faults originated during a later evolutionary stage of the shear zone. This shows a progressive change of deformation regime from duetile wrenching to brittle normal faulting. The Monte Grighini Complex is a good example of ductile wrench tectonics. followed by uplift and extension in the Paleozoic basement of Sardinia.     

西天山造山带双变质带变质变形序列分析——以新疆昭苏县察汗乌苏一带为例

西天山造山带中双变质带以新疆昭苏察汗乌苏一带较为典型,根据变质作用的不同,划分出绿片岩相绢云母-绿泥石带、绿片岩相蓝闪石带、角闪岩相黑云母-普通角闪石带、角闪岩相矽线石-铁铝榴石带等变质相带;按其变质作用以及构造变形程度分为上部层次脆性性构造层、中上部层次、深部层次3种构造层次。根据之间的相互叠加关系,确立了造山带内的岩石主要存在着3期以上构造变形特征;在主要的3期构造变形过程中见有相应的变质矿物,并根据生成的矿物组合与变形之间的相互关系,通过变质矿物先后生成关系与岩石的结构、构造建立了变质与变形序列关系,确立了变质与变形非简单一一对应,而是具有渗透叠加和循环往复的特点。     

郯庐断裂带张八岭隆起北段的左旋走滑挤压变形及其40Ar/39Ar定年

郯庐断裂带张八岭隆起北段,自西向东分别出露北北东向的韧性滑脱变形带、脆-韧性过渡带和脆性的前陆褶断带。韧性滑脱变形带内的张八岭群片岩,广泛发生了低绿片相背景下的糜棱岩化。其中呈现为平缓的糜棱面理和近南北向的矿物拉伸线理。显微构造及石英C轴组构分析显示,该韧性滑脱带一致为上盘向南的运动。该带以东依次变为上盘向南南东→南东的逆冲活动,总体上为左旋走滑挤压变形带。张八岭群所在的韧性变形带为深部陡立走滑构造与浅部脆性构造之间的滑脱变形带,其间的差异走滑变形,造成了该滑脱层在总体向北运动中出现上盘向南的剪切变形。对6处张八岭群片岩中15个不同粒级白云母的40Ar/39Ar定年指示,变形发生在(236.2±0.5)~(238.0±0.4)Ma的中三叠世晚期。这表明郯庐断裂带的左行平移发生在华北与华南板块碰撞的深俯冲阶段,起源于陆内转换断层。     

山东文登金矿成矿动力学研究

本文通过古应力估算，流体包体研究，应变测量以及矿化蚀变规律研究，探讨了山东文登金矿的动力作用与金成矿的主要关系，认为构造动力作用的金成矿的主要原因，韧性剪切作用向韧－脆性变形作用的过渡过程是金成矿的必要条件之一。     

Petrological evolution of amphibolite shear zones, Cheyenne Belt, south-eastern Wyoming, USA

Abstract Metre-scale amphibolite boudins in the Cheyenne Belt of south-eastern Wyoming are cut and deformed by shear zones which preserve a full strain transition across 7 cm, from relatively undeformed amphibolite with a relict igneous texture to mylonitic amphibolite with an L-S tectonic fabric. The strain transition is marked by the progressive rotation of amphibole + plagioclase aggregates into parallelism with the shear-zone boundary. An increase in strain magnitude is indicated by development of the tectonic fabric and progressive reduction of amphibole and plagioclase grain size as a result of cataclasis. Bulk chemistry of five samples across a single strain transition shows no significant or systematic variation in major element chemistry except for a minor loss of SiO₂, which indicates that the shear zone was a system essentially closed to non-volatile components during metamorphism and deformation. Amphibolites throughout the shear zone consist of amphibole and plagioclase with only minor amounts of quartz, chlorite, epidote, titanite and ilmenite. Within the relatively undeformed amphibolite, amphibole and plagioclase have wide compositional ranges in single thin sections. Amphibole compositions vary from actinolitic hornblende to magnesio-hornblende with increases in Al, Fe, Na and K contents and decreases in Si and Mg that can be modelled as progress along tschermakite, edenite and FeMg_-1 exchange vectors from tremolite. Plagioclase ranges from An₆₀ in cores to An₃₀ within grain-boundary domains. With increasing strain magnitude, local variation of amphibole composition decreases as amphibole becomes predominantly magnesio-hornblende. Plagioclase composition range also decreases, although grain-boundary domains still have higher albite content. These petrological data indicate that shear-zone metamorphism was controlled by the magnitude of strain during synmetamorphic deformation. SEM and microprobe imaging indicate that chemical reactions occurred by a dissolution and reprecipitation process during or after cataclastic deformation. This suggests that grain-boundary formation was an important process in the petrological evolution of the shear zone, possibly by providing zones for fluid ingress to facilitate metamorphic reactions. These results highlight the necessity for conducting detailed microstructural evaluation of rocks in order to interpret petrological, isotopic and geochronological data.     

Prograde and Retrograde Eclogites in the Sambagawa Metamorphic Belt, Besshi District, Japan

In the Besshi district of the Sambagawa metamorphic belt, thereare two types of eclogites: one occurring in the Sebadani metagabbromass and retrograded from high-temperature anhydrous ecologite,and the other in the basic schists and produced by prograde,dehydration of epidote amphibolite. The Sebadani metagabbro mass was originally layered gabbro,which was once equilibrated in the ecologite facies before emplacementinto the Sambagawa terrain as a hot eclogite mass. Basic schistssurrounding the Sebadani mass, which had suffered the Sambagawametamorphism of albite epidote amphibolite facies, were contact-metamorphosedat high pressure by the emplacement of the Sebadani mass. Asa result, the basic schists were dehydrated to form eclogiticbasic schists, i.e. garnet and omphacite porphyroblast-bearingbasic schists. Thus, two types of ecologite, retrograde andprograde, converged into the same metamorphic condition, 610–650?C, 7–17 kb, in a part of the ecologite facies duringthe Sambagawa metamorphism. Correspondingly, the values of distributioncoefficients of Fe and Mg between garnet and omphacite increasefrom core pairs to rim pairs in the retrograde eclogites anddecrease from core pairs to rim pairs in the prograde ecologites.After this stage, both the prograde and retrograde eclogitesshared a common metamorphic history; they were retrograded viathe epidote amphibolite facies to the greenschist facies. The Sebadani metagabbro mass, as a large tectonic block, hadbeen emplaced into a m?lange zone in the Sambagawa metamorphicbelt after the peak of the Sambagawa metamorphism, probablyfollowing initiation of uplift of the metamorphic rocks fromtheir deep-seated environment.     

Tectonic evolution of a crustal-scale oblique ramp,Hellenides thrust belt,Greece

This study investigates the tectonic evolution of the Omalos transverse zone, which served as a crustal-scale oblique ramp in the External Hellenides thrust belt on Crete island. The Omalos oblique ramp developed above an inherited Mesozoic fault zone that strikes NE–SW, oblique to the regional SSW-directed tectonic transport. During the Early Miocene–Pleistocene evolution of the thrust belt, the oblique ramp was repeatedly reactivated localizing deformation above the inherited structure. Geological and structural mapping combined with kinematic analysis of ductile and brittle structures suggest that the Omalos oblique ramp generated a local kinematic field, which deviated significantly from the regional kinematic pattern in the thrust belt. The most conspicuous feature in the tectonic evolution of the oblique ramp is a change from a ductile wrench-dominated to a brittle, primarily reverse faulting regime across the brittle–ductile transition, followed by brittle wrench deformation after the final exhumation of high-pressure (HP) rocks. Deflections of transport and compression orientations from the regional pattern are attributed to buttressing against basement-cover offsets produced by the pre-existing fault zone, to oblique ramping, and to transfer faulting, respectively. Our findings are potentially applicable to other examples of crustal-scale oblique thrust ramps in various tectonic settings.