Determination and Significance of the Bluemull Blastomylonite Zone in the Northeast——East of Yell,Shetland

There is a zone of blastomylonites containing hornblende gneisses of Lewisian Inliertype in the northeast-east of Yell,Shetland.This zone with a north—northwesterlytrend consists of blastomylonised hornblende-banded gneiss,hornblende-feldspar gneissand mica-felsie-gneiss with lenses of hornblende gneiss.It is indicated that theblastomylonite zone is actually a ductile shear zone and the sense of shear is dextral.This contributes to understanding the details of tectonic evolution in this area,and thiszone is probably a part of tectonic system of the British Caledonides.     

DEFORMATION OF THE EASTERN HIMALAYAN SYNTAXIS: EVIDENCES FROM STRUCTURES AND KINEMATICS OF ITS WESTERN BOUNDARY

DEFORMATION OF THE EASTERN HIMALAYAN SYNTAXIS: EVIDENCES FROM STRUCTURES AND KINEMATICS OF ITS WESTERN BOUNDARYtheNationalNaturalScienceFoundationofChina (grants 49732 10 0and 4980 2 0 2 0 )     

辽宁建昌养马甸子韧性剪切带基本特征

在辽宁西部建昌县查家店-养马甸子-回流水一带,发育有长度11.5 km、宽约1 km的韧性剪切带.通过多次实地调查,根据宏观的SC组构和镜下的旋转应变构造分析,判断养马甸子韧性剪切带为右行剪切机制,剪切位移1780 m.该韧性剪切带发育在新太古代麻计沟单元花岗岩中,而东侧多处可见中元古代长城纪高于庄期堆积物呈角度不整合覆于其上,从而确定该韧性剪切带形成时代的下限为新太古代晚期,上限为中元古代.     

Age and origin of magmatism along the Cenozoic Red River shear belt, China

To decipher the geodynamic significance of Cenozoic magmatism along the Red River shear belt, geochemical analyses, U-Pb and Rb-Sr dating, and Pb-Sr-Nd isotope tracing were undertaken. Zircon, monazite, titanite, and a Ti-U-oxide from foliated granitoid intrusions in the shear belt gneisses yield U-Pb emplacement ages of 33.1?±?0.2 (2σ), 31.9?±?0.3, 25.8?±?0.2 and 24.7?±?0.2?Ma, and an age of 35.0?±?0.3?Ma was obtained for the roughly 100?km long, adjacent Jinping (Phan Si Pang) alkali granite. Together with our previous data the new ages suggest that magmatism and left-lateral strike-slip movements occurred coevally during latest Eocene–Oligocene times from 33 to 22?Ma. The Rb-Sr dating of muscovite and biotite from the northernmost gneisses indicates that cooling to 500?°C occurred at 52.6?±?1.1?Ma, pre-dating the onset of magmatism, whereas further cooling to 300?°C took place at 28.9?±?0.6. This shows that unroofing in the north took place almost 9?million years earlier than in the central gneiss segments of the shear zone. Geochemical data substantiate two types of magmas: (1) amphibole-bearing intrusions of alkaline trend which are derived from sources with I_sr: 0.7065–0.7089 and _i ^Nd: ?3.7 to ?6.6; (2) leucogranitic layers and bodies having I_sr: 0.7084–0.7354 and _i ^Nd: ?3.3 to ?13.4. The former type of intrusion is found in both the gneisses and the adjacent unmetamorphosed cover rocks, whereas leucogranites are restricted to the shear belt gneisses. Source signatures of the alkaline intrusions lie adjacent to the those of OIB, plotting at the lower end of the Mantle Array. Contamination of these melts by continental material seems to be very limited. On the other hand, the leucogranitic layers are essentially crustal derived but none of the them has country rock isotope signatures, requiring melting of crust different from the actually exposed gneisses. Magma sources similar to those of ocean island basalt indicate magmatism to involve melting of light rare earth element and large ion lithophile element enriched mantle domains, most likely present in the lithosphere underneath the region. Since lithospheric thickening or subduction can be ruled out to produce both types of magmas, the presence of an important thermal anomaly is required, which is coevally active with left-lateral strike-slip shear. Adiabatic decompression and melting within the rising anomaly is the most plausible mechanism to produce the mantle magmas, which successively migrate through the crust to induce anatectic melting at 20–15?km crustal depth. Alkaline magmas largely dominate the volume of magmatism along the belt, being continuously present in the shear zone for millions of years. Such lubrication potentially explains how very large amounts of displacement can be absorbed in surprisingly narrow shear zones such as the Red River belt, possibly also playing a rôle for where and when zones of plate-scale lateral extrusion develop.     

A crustal-scale, orogen-parallel strike-slip fault in the Middle Urals: age, magnitude of displacement, and geodynamic significance

The kinematic evolution of an orogen-parallel strike-slip fault in the Middle Urals demonstrates that orogen-parallel mass transfer was an important, previously underestimated process during the syncollisional evolution of the Middle Urals. The Kyshtym strike-slip fault extends NNE, parallel and adjacent to the Main Uralian fault, which is the main suture of the Uralide orogen. The Kyshtym fault is interpreted as one of two conjugate strike-slip fault zones that have accomodated the longitudinal transfer of material along the margins of a rigid indenter belonging to the East European craton. The dextral Kyshtym shear zone was active under retrograde lower amphibolite to middle/lower greenschist facies conditions. Four metagranitic, muscovite-bearing mylonites yielded Rb-Sr internal mineral isochron ages of 247.5DŽ.9, 244.5Lj.5, 240.0ǃ.4, and 240.4DŽ.3 Ma, whereas a biotite-rich sample, without muscovite, gave a mineral isochron age of 229.1Dž.2 Ma. The results indicate almost complete Sr-isotopic reequilibration on the hand specimen scale during mylonitization. The muscovite ages are interpreted as deformation ages and demonstrate a Late Permian/Early Triassic age for the Kyshtym shear zone. The shear zone transects a pre-orogenic syenite intrusion of Ordovician age. A maximum shear strain of %=7Dž is estimated from the shape of the ductily deformed syenite body in map view and from the length/width ratios of deformed amphibolite bodies in the country rock. This shear strain suggests a maximum displacement of 28ᆠ km for the ~4-km-thick Kyshtym shear zone. A younger brittle fault, oriented subparallel to the shear zone, accomplished an additional horizontal displacement of 15Dž km; thus, the total displacement along the fault system is 43ᆣ km.     

江西武功山东区大型韧性剪切带的显微构造特征

武功山东区存在一条大型韧性剪切带。鞘褶皱倒向以及旋转变形构造(如S-C面理组构、旋转碎斑系、雪球构造和粒内显微破裂构造等)显示此剪切带为由南向北逆冲推覆性质。砾石、黄铁矿还原斑和石英斑晶的有限应变分析表明剪切带西段和东段岩石分别以收缩型椭球和压扁型椭球变形为特征。剪切带的主要变形时代是早古生代,可能与早古生代华夏陆块和扬子陆块之间的碰撞造山作用有关。     

桂北四堡韧性剪切带的发现及其构造意义

通过野外观察、室内显微构造分析和磁组构测量方法,在桂北四堡地区浅变质地层中厘定出一条NE30°走向,南东倾,倾角约40°的大型左旋斜冲韧性剪切带——四堡韧性剪切带;该韧性剪切带内发育糜棱岩系列、糜棱面理、拉伸线理、A型褶皱、S-C组构、亚颗粒、显微分层及石英条带等宏观和微观构造特征;磁各向异性度测量结果显示四堡韧性剪切带由一宽约4 km的强应变带及边缘弱带组成,全带宽达10 km,长度超30 km;在对韧性剪切带运动学、构造年代学研究的基础上,结合区域地质资料,认为四堡韧性剪切带是华南加里东晚期华夏地块由南东向北西作低角度斜冲到扬子地块的产物。这一发现揭示了扬子地块与华夏地块碰撞拼合的方式,为深化华南构造演化提供了新资料。     

河南省西峡—内乡北部元古界与古生界间的构造边界

构造制图和详细的构造解析,在东秦岭造山带核部识别出一个原先未曾注意的元古代变质岩区与古生代变质岩区间的构造边界——军马河-马蹄湾断裂带。实际上,这个构造边界是一个底部韧性变形带,带内发育构造混杂岩、糜棱岩和强直片麻岩。本文阐述了各种几何学特征,它们表明变形是发生在一个缓倾斜的剪切带之上。大量的运动学标志指出,在中生代花岗岩类岩体就位前,再造的元古代变质岩区沿着构造边界向北逆冲于古生代变质岩区之上。     

Quartz microfabric of the Laxfordian Canisp Shear Zone,NW Scotland

The Canisp Shear Zone transects layered Lewisian gneisses near Lochinver, NW Scotland. It is a vertical ductile shear zone with a dextral shear sense, formed during Laxfordian amphibolite facies metamorphism, transposing the layering to new foliation and linear structures. Minerals in the layered gneisses show little or no shape fabric, while a strong shape fabric defines the foliation. For quartz, this shape fabric is accompanied by development of a preferred crystal orientation with fabric patterns reflecting the geometry of the shear deformation. The quartz fabric shows a pole-free area around the lineation with the c-axes concentrated in an asymmetric cross-girdle or a point maximum perpendicular to the shear plane, and a monoclinic symmetry consistent with the shear sense.     

西藏阿里札达韧性剪切带特征及其X光岩组分析

文中简述了西藏阿里札达盆地的地质背景、区域地层和札达韧性剪切带的基本特征。采用X射线衍射法对札达韧性剪切带中的石英、方解石和白云母等三种矿物,进行了X光岩组分析,确定了韧性变形岩石的组构特征、韧性剪切带的属性和变形岩石的应变类型,以及韧性剪切带形成时的温压条件。研究表明,韧性变形岩石均具不对称组构,反映韧性带属于南盘(下盘)俯冲型韧性剪切带,韧性变形是在高温、高压、低应变速率条件下发生的,处于>10km的地壳深度,岩石应变类型以压扁应变为主。     

The Loch Maree Group: Palaeoproterozoic subduction–accretion complex in the Lewisian of NW Scotland

The Lewisian complex of northwest (NW) Scotland has long been correlated with intercontinental Palaeoproterozoic belts of the North Atlantic region but uncertainty about the age and origin of the supracrustal rocks of the Loch Maree Group (LMG) and the apparent lack of subduction-related intrusive rocks have precluded interpretations of a similar tectonic setting for the Lewisian. We present integrated field, geochemical and geochronological data that resolve both issues and are consistent with an intercontinental setting. The LMG is made up of two components, one oceanic (plateau basalts or primitive arcs, plus associated abyssal sediments, ferruginous hydrothermal deposits, and platform carbonates) and the other continental (deltaic flysch, greywacke shale). The metasediments have geochemical characteristics that imply a source outside the Archaean gneisses of the Lewisian, an interpretation that agrees with the detrital zircon populations (from the Flowerdale schists) that have a significant 2.2–2.0-Ga component. The Ard gneiss, formerly regarded by some as a tectonic sliver of basement, is a strongly foliated granodiorite that occurs in sheets intrusive into the LMG, and has given a UPb crystallisation age of 1903±3 Ma, consistent with its syntectonic relationship with the major D1/D2 phase of Proterozoic deformation. The gneiss has a rather primitive geochemistry, which implies that it was not generated by melting of the local metasediments but was derived by partial melting of a more mafic source. The most likely model is that the LMG evolved as an accretionary complex, modern parallels of which can be found in the Shimanto belt in Japan, Rhodope in north Greece and Colombia and the Caribbean. The various elements of the complex became tectonically intermixed and subject to extreme deformation during accretion to the overriding Lewisian continent. Eventual relaxation and exhumation of the accretionary complex may have resulted in the generation of the Ard gneiss (possibly by melting of the underplated oceanic plateau) followed by collision with the continental crust of the lower plate. The younger D3 phase of the Palaeoproterozoic deformation sequence was coincident with the emplacement of the Tollie pegmatites at 1.7 Ga, c 200 m. years after the main collisional event, and may be related to a younger accretionary event (Labradorian?).     

Structural relations and PbPb zircon ages for the Makuti gneisses: evidence for a crustal-scale Pan-African shear zone in the Zambezi Belt,northwest Zimbabwe

The Makuti Group of northwest Zimbabwe is composed of mafic and intermediate biotite-rich gneisses interlayered with quartzofeldspathic gneisses of granitic composition, and minor sedimentary units. The gneisses have experienced a multi-staged metamorphic history, including an early high temperature-high pressure event and subsequent reworking at upper- to mid-amphibolite-facies conditions. They are positioned along the strongly deformed, southern margin of the east-west trending Zambezi Belt, and have been correlated with supracrustal gneiss units along the northern margin of the Zimbabwe Craton.The Makuti Group is characterised by an intensely developed gneissic layering and complex disharmonic folds that resulted from non-coaxial deformation involving repeated stages of transposition. The basal contact of the g roup coincides with a decrease in strain intensity, but not with a directional change of characteristic structural elements (e.g. lineations, fold axes), nor with a clear change in rock types. Pink quartzofeldspathic gneisses of granitic composition are typical for the Makuti Group, but locally intrude basement gneiss as well. The quartzofeldspathic gneisses occur as porphyritic and non-porphyritic varieties that are, invariably, intensely sheared.The age and nature of the basal contact of the Makuti Group and its relationship to the quartzofeldspathic gneisses has been investigated. Samples for single zircon PbPb dating were collected from a felsic biotite gneiss just below (2704 ± 0.3 Ma) and above (2510 ± 0.4 Ma) the lower contact of the Makuti Group at an ‘unconformity’ 2 km northwest of Vuti. Further samples were collected from pink quartzofeldspathic units at the base (737 ± 0.9 Ma), central part (764 ± 0.9 Ma; 797 ± 0.9 Ma) and top (794 ± 0.5 Ma; 854 ± 0.8 Ma) of the Makuti Group. Two samples of Kariba orthogneiss (1920 ± 0.4 and 1963 ± 0.4 Ma) underlying the Makuti Group in the northwest were also collected. In all samples, long-prismatic, colourless to brown, igneous zircon grains were selected. Dates were obtained using a stepwise single-grain evaporation technique. Although this technique only allows minimum age estimates, the dates are highly reproducible, indicating that they approximate emplacement ages. The ages conform with the field observations that the basement has been reworked in the Makuti Group and that the quartzofeldspathic units may have been emplaced as granites.It is proposed that the Makuti Group represents a crustal scale shear zone that partly reworked basement gneisses and acted as a conduit for granite emplacement. Shearing took place in an extensional setting around 800 Ma ago, and may have resulted in the exhumation of lower crustal rocks.     

LONG TERM HISTORY (DEXTRAL TO SINISTRAL AND DUCTILE TO BRITTLE) OF THE DAMXUNG—JIALI SHEAR ZONE IN SOUTHERN TIBET

LONG TERM HISTORY (DEXTRAL TO SINISTRAL AND DUCTILE TO BRITTLE) OF THE DAMXUNG—JIALI SHEAR ZONE IN SOUTHERN TIBET     

西藏阿里雅鲁藏布江缝合带韧性剪切带的磁组构特征

应用磁组构测量方法,厘定、划分了札达地区印度河—雅鲁藏布江缝合带内的韧性剪切带,两条强韧性带分别位于缝合带北缘和南缘,均具有南盘(下盘)俯冲、平面右旋扭动运动特征和压扁型应变特征。韧性剪切发生于65Ma以前,它们应是印—欧两大板块俯冲-碰撞剪切应变产物,是板块缝合带的重要组成部分。     

Coupled extrusion of sub‐arc lithospheric mantle and lower crust during orogen collapse: a case study from Fiordland,New Zealand

The Anita Peridotite is a ~20 km long by 1 km wide exhumed fragment of spinel facies sub‐arc lithospheric mantle that is enclosed entirely within the ≤4 km wide ductile Anita Shear Zone, and bounded by quartzofeldspathic lower crustal gneisses in Fiordland, south‐western New Zealand. Deformation textures, grain growth calculations and thermodynamic modelling results indicate the mylonitic peridotite fabric formed during rapid cooling, and therefore likely during extrusion. However, insights into the exhumation process are gained through examination of aluminous garnet‐bearing meta‐sedimentary gneisses also enclosed within the shear zone. P–T calculations indicate that prior to mylonitization the gneisses enclosing the peridotite equilibrated at 675–746 °C in the sillimanite stability field (stage I), before being buried to near the base of thickened arc crust (stage II; ~686 ± 26 °C and 10.7 ± 0.8 kbar). From this point on, the peridotite unit and the quartzofeldspathic rocks share a deformation history involving extensive recrystallization (stage III) within the Anita Shear Zone. Coupled exhumation of these portions of lower crust and upper mantle occurred during regional thinning of over‐thickened lithosphere at c. 104 Ma (U–Pb zircon). Our favoured model for the exhumation process involves heterogeneous transpressive deformation within the translithospheric Anita Shear Zone, which provided a conduit for ductile extrusion through the crust.     

P–T conditions of decompression of the Limpopo high-grade terrane: record from shear zones

The Southern Marginal Zone of the late Archean Limpopo Belt of southern Africa is an example of a high‐grade gneiss terrane in which both upper and lower crustal deformational processes can be studied. This marginal zone consists of large thrust sheets of complexly folded low‐strain gneisses, bound by an imbricate system of kilometre‐wide deep crustal shear zones characterized by the presence of high‐strain gneisses (‘primary straight gneisses’). These shear zones developed during the decompression stage of this high‐grade terrane. Low‐ and high‐strain gneisses both contain similar reaction textures that formed under different kinematic conditions during decompression. Evidence for the early M1/D1 metamorphic phase (> 2690 Ma) is rarely preserved in low‐strain gneisses as a uniform orientation of relict Al‐rich orthopyroxene in the matrix and quartz and plagioclase inclusions in the cores of early (M1) Mg‐rich garnet porphyroblasts. This rare fabric formed at > 820 °C and > 7.5 kbar. The retrograde M2/D2 metamorphic fabric (2630–2670 Ma) is well developed in high‐strain gneisses from deep crustal shear zones and is microscopically recognized by the presence of reaction textures that formed synkinematically during shear deformation: M2 sigmoid‐shaped reaction textures with oriented cordierite–orthopyroxene symplectites formed after the early M1 Mg‐rich garnet porphyroblasts, and syn‐decompression M2 pencil‐shaped garnet with oriented inclusions of sillimanite and quartz formed after cordierite under conditions of near‐isobaric cooling at 750–630 °C and 6–5 kbar. The symplectites and pencil‐shaped garnet are oriented parallel to the shear fabric and in the stretching direction. Low‐strain gneisses from thrust sheets show similar M2 decompression cooling and near‐isobaric cooling reaction textures that formed within the same P–T range, but under low‐strain conditions, as shown by their pseudo‐idioblastic shapes that reflect the contours of completely replaced M1 garnet and randomly oriented cordierite–orthopyroxene symplectites. The presence of similar reaction textures reflecting low‐strain conditions in gneisses from thrust sheets and high‐strain conditions in primary straight gneisses suggests that most of the strain during decompression was partitioned into the bounding shear zones. A younger M3/D3 mylonitic fabric (< 2637 Ma) in unhydrated mylonites is characterized by brittle deformation of garnet porphyroclasts and ductile deformation of the quartz–plagioclase–biotite matrix developed at < 600 °C, as the result of post‐decompression shearing under epidote–amphibolite facies conditions.     

Determination of magnetic anisotropy and a ca 1.2 Ga palaeomagnetic pole from the Bremer Bay area,Albany Mobile Belt,Western Australia

Granulite facies tonalitic gneiss, mafic granulite and late metadolerite dykes from Bremer Bay in the Mesoproterozoic Albany Mobile Belt yield palaeomagnetic remanence that were acquired between ca 1.2 Ga and 1.1 Ga. A well‐constrained pole (66.6°N, 303.7°E) fits the ca 1.2 Ga part of the Precambrian Australian apparent polar wander path. This pole is in agreement with the high‐latitude position of Australia at ca 1.2–1.1 Ga shown on some Rodinia reconstructions. More data are required before any significance can be attributed to a second, poorly defined pole (41.8°S, 243.7°E) that falls at some distance from the ca 0.8 Ga part of the Australian apparent polar wander path. Magnetic anisotropy measurements from all samples except late granite dykes indicate northeast‐southwest elongation (i.e. parallel to the local trend of the orogenic belt) and northwest‐southeast contraction. This is in agreement with the orientation of principal strain axes deduced from structures formed during late stages of ductile deformation. The mean magnetic fabric lineation (long axis of the strain ellipsoid) is subparallel to a mineral elongation lineation and the axes of late upright to inclined folds. Short axes of the strain ellipsoid determined from magnetic fabric measurements are in a similar orientation to poles to the axial surfaces of these folds and to the associated cleavage. This mean shortening axis bisects late conjugate ductile shear zones that overprint the folds. This study has shown that structurally complex high‐grade gneisses and intrusive rocks with variable timing relationships may yield meaningful palaeomagnetic results for late stages of metamorphism. Magnetic anisotropy analysis is also seen to be a valuable tool in providing principal strain directions for late ductile deformation.     

Testing the model of oblique transpression with oblique extrusion in two natural cases: Steps and consequences

Kinematic models of various types of transpression have been used to explain fabric features and strain in many natural deformation studies. Here, a mathematical model that encompasses all monoclinic and triclinic transpressional deformations including triclinic deformation with inclined simple shear (ϕ) and/or inclined extrusion orientations (υ) can be tested using a step-by-step approach with available field evidence. Two cases are presented. The first case from the Wabigoon–Quetico boundary in the Archean Superior Province utilizes both fabric orientation and quantified strain data. The best fit of the field evidence to the model indicates that deformation likely took place along subvertical shear zones via transpression with subhorizontal simple shear (ϕ = 0–20°) and variable inclined extrusion direction (extrusion can be either east or west and υ typically indicates extrusion orientations between 0 and 50° from vertical). The second case of the South Iberian shear zone has fabric orientation data, but no quantifiable strain possibilities. The best fit of the field evidence to the model indicates that deformation likely took place along a moderately inclined shear zone via transpression with subhorizontal simple shear (ϕ = 0–20°) and variable inclined extrusion direction (υ values between 0° and 80° from the true dip of the shear zone). Using this protocol in other examples of natural deformation will allow further constraints to be applied to kinematic models.     

NORMAL-SLIP ALONG THE NORTHERN ALTYN TAGH FAULT, NORTH TIBET

NORMAL-SLIP ALONG THE NORTHERN ALTYN TAGH FAULT, NORTH TIBET     

Diffusion creep and partial melting in high temperature mylonitic gneisses, Hope Valley shear zone, New England Appalachians, USA

Field, petrographic, microstructural and isotopic studies of mylonitic gneisses and associated pegmatites along the Hope Valley shear zone in southern Rhode Island indicate that late Palaeozoic deformation (c. 275 Ma) in this zone occurred at very high temperatures (>650 °C). High‐energy cuspate/lobate phase boundary microstructures, a predominance of equant to sub‐equant grains with low internal lattice strain, and mixed phase distributions indicate that diffusion creep was an important and possibly predominant deformation mechanism. Field and petrographic evidence are consistent with the presence of an intergranular melt phase during deformation, some of which collected into syntectonic pegmatites. Rb/Sr isotopic analyses of tightly sampled pegmatites and wall rocks confirm that the pegmatites were derived as partial melts of the immediately adjacent, isotopically heterogeneous mylonitic gneisses. The presence of syntectonic interstitial melts is inferred to have permitted a switch from dislocation creep to melt‐enhanced diffusion creep as the dominant mechanism in these relatively coarse‐grained mylonitic gneisses (200–500 µm syn‐deformational grain size). A switch to diffusion creep would lead to significant weakening, and may explain why the Hope Valley shear zone evolved into a major regional tectonic boundary. This work identifies conditions under which diffusion creep operates in naturally deformed granitic rocks and illuminates the deformation processes involved in the development of a tectonic boundary between two distinct Late Proterozoic (Avalonian) basement terranes.