Tectonites in the Shear Zone Array of the Tongbai-Dabie Orogenic Belt

A series of tectonites were formed in the shear zone array of the Tongbai--Dabie Orogenic Belt, including mylonites, blastomylonites, semi--plastic mylonites and foliated cataclasitesas a result of multiple strain localization, strain softening and deformation partitioning.     

Shear Wave Velocity in the Top of the Earth’s Inner Core

Doklady Earth Sciences - Analysis of PKIIKP waves reflected off the inner surface of the solid core boundary and recorded close to the antipode indicates that the shear wave velocity at its top can...     

Structure of Masuleh Shear Zone: Evidence for Early–Middle Jurassic Dextral Shear Along Paleo-Tethys Suture Zone in the Western Alborz,NW Iran

The Paleo-Tethys suture zone in northern Iran was formed when the Paleo-Tethys Ocean, (between Gonwana-derived Alborz Microcontinent and the Turan Plate), closed during the Eocimmerian orogeny and after they collided together in the Mid-Late Triassic. The NW-striking Boghrov-Dagh basement Fault Zone that lies in the vicinity of Masuleh village and the southern boundary of Gasht Metamorphic Complex is a part of the Eocimmerian suture zone in the Western Alborz. Along this part of the suture zone, tourmaline leucogranites intruded in metamorphic rocks. We recognize three distinct deformation stages (D1 to D3) in the study area especially in the Masuleh Shear Zone. D1 which was synchronous with formation of the main metamorphic minerals, such as sillimanite and staurolite under medium- to high-grade metamorphic conditions probably during the Hercynian event and a NE-directed shortening. The slaty cleavage in metamorphosed Upper Paleozoic rocks and crenulation cleavage and folds in the older rocks were produced due to D2 deformation during the Eocimmerian event under greenschist facies conditions. The Masuleh Shear Zone formed as a result of a ductile strike-slip shear during the Early-Middle Jurassic Mid-Cimmerian D3 event with a pure dextral to transtension shear sense at low to locally medium-grade conditions. All of the D3 structural features agree with a NNW-directed compression and an ENE-directed extension caused by overall dextral shear parallel to the Masuleh shear zone and the Boghrov-Dagh Fault Zone. Based on the available evidence, especially cross-cutting relationships between structural fabrics and rock units, emplacement of the Gasht-Masuleh leucogranites occurred after the D2 collisional event coeval to the possible slab break-off and before the D3 event, between Eocimmerian and Mid-Cimmerian movements.     

The Palomares brittle—ductile Shear Zone of southern Spain

The Palomares Shear Zone is a major Neogene-Quaternary strike—slip zone which transects the crust of the Betic Cordillera in SE Spain. The shear zone and the mechanisms that led to its formation are discussed and illustrated on the basis of detailed compilations of both the local and regional geology. It is emphasized that the formation of the Palomares Shear Zone was not an isolated tectonic event, but part of a complex Neogene tectonic history. The Neogene evolution of the Betic-Rif orogen and its central Alboran Basin is characterised by the following events: (1) emplacement of the Alboran Diapir with resulting nappe-shedding from the overlying crust between 25 and 20 Ma ago; (2) onset of the subsidence of the Alboran Basin between 20 and 15 Ma ago due to cooling of the Alboran Diapir and the overlying crust; (3) formation of the Cabo de Gata Volcanic Chain between 15 and 8 Ma ago; and (4) refolding of the nappe sheets in the Betic-Rif orogen into a basin and range structure about 7 Ma ago. Continuous activity of the Crevillente Fault of southern Spain may have occured over a period from 20 Ma ago up to the present. The interrelated Palomare Fault in SE Spain was probably formed between 15 and 8 Ma ago and seem to be active still. The Palomares Shear Zone affects a rock volume 44 km wide, at least 80 km long and 30 km deep. A shear strain—distance diagram constructed across the Palomares Shear Zone and its axial Palomares Fault involves a new method to estimate or constrain the shear strain magnitude along brittle-ductile shears. The typical tensor shear strain rates in the approximately 20 km thick ductilely deformed walls of the Palomares Fault are of the order 10⁻¹³–10⁻¹⁴ s⁻¹. The tensor shear strain rate along the Palomares Fault itself is of the order 10⁻¹²s⁻¹ and the time averaged relative displacement rate of its walls is about 2 mm a⁻¹. The range of strain rates within the Palomares Shear Zone are interpreted to be due to a combination of various flow-softening mechanisms: geometric, structural, thermal and strain-rate softening. These softening mechanisms might explain the difference in vertically averaged viscosities of 10²⁰ Pa s and 10²⁵ Pa s or lower suggested for the crustal rocks in the Palomares Fault proper and that of the relatively rigid boundaries of the Palomares Shear Zone, respectively.     

Some Effects of Shearing Velocity on the Shear Stress-Deformation Behaviour of Hard–Soft Artificial Material Interfaces

Shear behaviour of the joints formed by the interface of two different material types, such as rock and cemented paste backfill, rock and concrete or two different rock types, have practical importance in many rock engineering activities. This paper presents the results of an experimental investigation into the shear behaviour of these special joints under pseudo-static shear velocity. Direct shear tests on concrete–plaster interfaces were carried out under boundary conditions of constant normal load and constant normal stiffness. Shearing velocities of the performed tests were in the range of 0.3–30 mm/min. The results of the shear tests conducted on the planar and rough artificial prepared joints showed that the shearing velocity has a significant influence on the shear strength, friction angle and shear stiffness of the hard–soft material interface. So that, these parameters were decreased when shear velocity was increased. Also, comparison of the tests results that performed on the concrete–plaster joints with those from tests on the plaster–plaster and concrete–concrete interfaces showed that the shear behaviour of concrete–plaster interface is governed mainly by the shear parameters of the plaster block (namely softer material).     

Tectonic Features and Stages of Evolution of the Baltic–Mezen Shear Zone in the Phanerozoic,Northwestern Russia

Geotectonics - The general tectonic features of the Baltic-Mezen zone developed along the border of the Fennoscandian shield and the Russian Plate in the north of the East European platform, are...     

Introduction of Simulating the Motion of Rigid Ellipsoidal Objects in Ductile Shear Zone Based on the Jeffery’s Theory

<正>Rigid ellipsoidal objects(gravels and porphyroclasts)in ductile zone is an important factor to indicate the kinematics and dynamics.Jeffery’s theory(Jeffery G,1922),a quantitative research method,for the rotation ofthe rigid objects(no deformation)in the Newtonian fluid of the simple deformation field has been widely applied by geologists to the study of fabrics in rocks.The theory     

Tectonic Evolution of the Himalayan Collision Belt

This paper discusses the tectonic divisions of the Himalayan collision belt anddeals with the tectonic evolution of the collision belt in the context of crustal accretion in thefront of the collision belt, deep diapirism and thermal-uplift extension and deep material flow-ing of the lithosphere-backflowing. Finally it proposes a model of the tectonic evolution-progressive intracontinental deformation model-of the Himalayan belt.     

Isotope Geochemistry of Gold Ore Deposits in the Gezhen Shear Zone, Qiongxi, Hainan Island

Gold deposits hosted in the Gezhen shear zone at Qingxi, Hainan Island occur in the Preeambrian metamorphic rock series and are regionally developed in the N-E direction along the tectonic zone. From northeast to southwest are distributed the Tuwaishan-Baoban gold mining district, the Erjia gold mining district and the Bumo gold mining district, making up the most industrially important gold metallogenesis zone on the Hainan Island. Isotope geochemical studies of the typical gold deposits in this metallogenesis zone indicate that their ore-forming materials stemmed largely from the Baoban Group migmatite series, though the involvement of some plutonic materials could not be ruled out. The ore fluids are the mixture of migrnatitized hydrothermal solutions and meteoric waters in addition to the involvement of local magmatic hydrothermal solutions. The superimposition of plutonie materials and magmatic hydrothermal solutions is controlled by the deformation environment of the shear zone and later magrnatic activities. Obvious variations are noticed in isotopic composition in the region studied, probably related to tectonic deformation, metamorphism and other evolutionary characteristics. This study is of great significance in understanding the relationship between the shear zone and gold metallogenesis,the rules of gold metallogenesis and gold ore prognosis.     

Fluid Inclusion Studies of Gold Deposits in the Gezhen Shear Zone,Hainan Province,China

The Tuwaishan, Baoban, Erjia, Bumo and other gold deposits in western Hainan occur in Precambrian metamorphic clastic rocks and are structurally controlled by the Gezhen shear zone. Fluid inclusion studies have been carried out of the gold deposits mentioned above. The homogenization temperatures of the whole fluid inclusion population range from 140°C to 370°C, indicating that gold was precipitated mainly at 240–250°C. The salinities are within the range of 2.0–9.2 wt% NaCl equiv. and the pressure of formation of the deposits was estimated at about 270×10⁵−500×10⁵Pa, corresponding to a depth of about 1.1–2.0 km under lithostatic confinement. Chemical studies show that the ore fluid is of the Na⁺(K⁺)-Ca²⁺-Cl⁻(F⁻) type. Theδ ¹⁸O andδD values of the fluid vary from −2.7‰- +4.4‰ and −50‰–−87‰ Evidence developed from fluid inclusions and geological setting indicates that the ore fluid was a mixture of magmatic and meteoric-hydrothermal waters. Changes in chemical composition andδ ¹⁸O andδD of fluid inclusions from one ore field to another seem to be related with regional tectonism, metamorphism and magmatism.     

Geodynamic Causes of the Emergence and Termination of Cenozoic Shear Deformations in the Khatanga–Lomonosov Fault Zone (Arctic)

Doklady Earth Sciences - In the context of models of mantle circulation beneath the continent, coupled with the subduction of the oceanic lithosphere, a direct link between the kinematics of plate...     

Palghat Gap — A Dextral Shear Zone from the South Indian Granulite Terrain

The origin of Palghat Gap, the most prominent discontinuity in the Western Ghats, is controversial. The rocks of the Gap — hornblende-biotite gneiss, mylonitic augen gneiss, charnockite and amphibolite are characterised by three groups of folds formed during two deformations. Shear indicators like augens, pull-aparts, rootless folds, sheath folds, minor shears and quartz fabric strongly suggest ductile dextral shearing. Kinematic analysis of structural elements supports shearing in a N-S compressional field with XY being subvertical. Features on both mesoscopic and microscopic scales strongly favour Palghat Gap being treated as a ductile shear zone. The present Gap topography is the product of shearing and erosion.     

The Geological Significance of the Deformation and Geochronology of the Xiaotian–Mozitan Shear Zone in the Dabie Orogenic Belt (East-Central China)

The Xiaotian–Mozitan Shear Zone(XMSZ) is the boundary of the Dabie High-grade Metamorphic Complex(DHMC) and the North Huaiyang Tectonic Belt. It was deformed in ductile conditions with a top-to-NW/WNW movement.Geothermometers applied to mineral parageneses in mylonites of the shear zone give a temperature range of 623–691°C for the predeformation and 515–568°C for the syndeformation, respectively, which indicates a retrograde process of evolution.A few groups of zircon U-Pb ages were obtained from undeformed granitic veins and different types of deformed rocks in the zone. Zircons from the felsic ultramylonites are all magmatic, producing a weighted mean 206 Pb/238 U age of 754 ± 8.1 Ma, which indicates the time of magmatic activities caused by rifting in the Neoproterozoic. Zircons from the granitic veins, cutting into the mylonites, are also of magmatic origin, producing a weighted mean 206 Pb/238 U age of 130 ± 2.5 Ma,which represents the time of regional magmatic activity in the Cretaceous. Zircons from the mylonitic gneisses are of anatectic-metamorphic origins and are characterized by a core-mantle interior texture, which yielded several populations of ages including the Neoproterozoic ages with a weighted mean 206 Pb/238 U age of 762 ± 18 Ma, similar to that of the felsic ultramylonites and the Early Cretaceous ages with a weighted mean 206 Pb/238 U age of 143 ± 1.8 Ma, indicating the anatectic metamorphism in the Dabie Orogenic Belt(DOB). Based on integrated analysis of the structure, thermal conditions of ductile deformation and the contact relations of the dated rocks, the activation time of the Xiaotian–Mozitan Shear Zone is constrained between ～143 Ma and 130 Ma, during which the DOB was undergoing a transition in tectonic regime from compression to extension. Therefore, the deformation and evolution of this shear zone plays an instrumental role in fully understanding this process. This research also inclines us to the interpretation of it as an extensional detachment, with regard to the tectonic properties of the shear zone. It may also be part of a continental scale extension in the background of the North China Block's cratonic destruction, dominated by the subduction and roll-back of the Paleo-Pacific plate, but more detailed work is needed in order to unravel its complicated development.     

Structure and Evolution of the Belomorian–Severodvinsk Shear Zone in the Late Proterozoic and Phanerozoic,East European Platform

Geotectonics - The tectonics, morphological features, and development stages of the Belomorian‒Severodvinsk shear zone (northwestern part) found in the East European Platform are considered....     

Quantifying 3D post-accretionary tectonic strain in the Arabian–Nubian Shield: Superimposition of the Oko Shear Zone on the Nakasib Suture,Red Sea Hills,Sudan

Deformed conglomeratic clasts exposed along the Neoproterozoic Nakasib Suture and the Oko Shear Zone are used to calculate three-dimensional (3D) tectonic strain associated with the latter to quantify strain associated with post-accretionary deformational belts in the Arabian–Nubian Shield. The Nakasib Suture is a NE-trending fold and thrust belt that is sinistrally offset (∼10 km) by the cross-cutting NNW- to NW-trending strike-slip faults of the Oko Shear Zone. The Nakasib Suture was formed as a result of collision between the Haya terrane and the Gebeit terrane at ∼750 Ma ago. The Oko Shear Zone was subsequently formed as a result of an E–W directed shortening of the Arabian–Nubian Shield due to collision between East and West Gondwana at ∼670–610 Ma ago. This analysis indicates the following: (1) The Nakasib Suture is dominated by flattening strain with the flattening plane of the associated strain ellipsoid oriented at 21°/77°SE. This flattening deformation is interpreted to be associated with nappe emplacement from north to south. (2) Some regions along the Nakasib Suture are characterized by constriction strain that might be due to refolding of the early nappes about NE-trending axes. (3) The Oko Shear Zone is characterized by constriction strain, with the XY plane of the strain ellipsoid oriented at 171°/68°E. The strain ellipsoid associated with the Oko Shear Zone manifests superimposition of E–W shortening on the NE-trending fold and thrust belt associated with the Nakasib Suture. (4) The tectonic strain of the Oko Shear Zone, superimposed over the structures of the Nakasib Suture, is characterized by a strain ellipsoid whose flattening plane is oriented at 21°/49°W. The strain ellipsoid of the tectonic strain has a major axis with a quadratic elongation of 3.6 and an orientation of 357°/25°, an intermediate axis with a quadratic elongation of 1.2 and an orientation of 231°/30°, and a minor axis with a quadratic elongation of 0.25 and an orientation of 115°/18°. This suggests that the post-accretionary deformation of the Arabian–Nubian Shield was superimposed as a NW–SE directed shortening that created early N–S shortening zones and late NW-trending sinistral strike-slip faults.     

Structural–Kinematic Parageneses and Dynamic Evolution Model of the Baltic–Mezen Shear Zone in the Phanerozoic,the Northwestern Part of the East European Platform

Geotectonics - The tectonic features of the Baltic–Mezen shear zone that developed along the border of the Fennoscandian Shield and the Russian Plate in the north of the East European...     

Analysis of the Ore-Controlling Structure of Ductile Shear Zone Type Gold Deposit in Southern Beishan Area, Gansu, Northwest China

The ductile shear zone-type gold deposit is a kind that both the ore-forming mechanism and ore-controlling factors are closely related to the ductile shear zone and its evolution. Ductile shear zone develops in Beishan area, Gansu of Northwest China, and develops especially well in the south belt. The controls of the ductile shear zone on gold deposits are as follows. (1) The regional distribution of gold deposits (and gold spots) is controlled by the ductile shear zone. (2) The ductile-brittle shear zone is formed in the evolution process of ductile shear zone and both are only ore-bearing structures and con- trol the shape, attitude, scale, and distribution of mineralization zones and ore-bodies. (3) Com- presso-shear ductile deformation results in that the main kind of gold mineralization is altered mylonite type and the main alteralization is metasomatic. (4) Ore-bearing fracture systems are mainly P-type ones, some D-type and R-type ones, but only individual R’-type and T-type ones. (5) Dynamic differen- tiation and dynamic metamorphic hydrothermal solution resulting from ductile deformation is one of the sources of ore-forming fluid of gold mineralization, and this is identical with that ore-forming ma- terials are mainly from metamorphic rocks, and ore-forming fluid is mainly composed of metamorphic water, and with the fluid inclusion and geo-chemical characteristics of the deposit. (6) There is a nega- tive correlation between the gold abundance and susceptibility anisotropy (P) of the altered mylonite samples from the deposit, which shows that the gold mineralization is slightly later than the structural deformation. All above further expound the ore-forming model of the ductile shear zone type of gold deposits.     

Moho depth determination beneath the Zagros Mountains from 3D inversion of gravity data

Due to its geological and economic importance, the Zagros Mountains have been investigated by many researchers during the last decades. Nevertheless, in spite of all the studies conducted on the region, there are still some controversial problems concerning the structure of the Zagros Mountains, including crustal depths, demanding more insights into understanding the crustal constraints of the region. Accordingly, we have conducted a gravity study to determine Moho depth map of the Zagros Mountains region, including its major structural domains from the coastal plain of the Persian Gulf to central Iran. The employed data are the densest and most accurate terrestrial gravity data set observed until now with the precision of 5 μGal and resolution of 5 arc-minute by 5 arc-minute. To image Moho depth variations, gravity inversion software GROWTH2.0 is used, proposing the possibility to model stratified structures by means of a semi-objective exploratory 3D inversion approach. The obtained results reveal the crustal thickness of ~?30–35 km underneath the southwestern most Zagros Fold-Thrust Belt increasing northeastward to 48 km. The maximum Moho depth is estimated ~?62 km below the Zagros Mountains belt along the Main Zagros Thrust. Northeast of the study area, an average crustal thickness of 46 km is computed beneath Urumieh–Dokhtar magmatic arc and central Iran.     

Three-Dimensional P-Wave Velocity Structure of the Crust of North China

Since the Xingtai (邢台) earthquake in 1966,China Earthquake Administration has carried out a survey campaign along more than thirty deep seismic sounding (DSS) profiles altogether about twenty thousand kilometers long in North China to study the velocity structure of the crust and the upper mantle in this region,and has obtained a great number of research findings. However,these researches have not provided a 3D velocity structure model of the crust of North China and cannot provide seismic evidence for the study of the deep tectonic characteristics of the crust of the whole region. Hence,based on the information from the published data of the DSS profiles,we have chosen 14 profiles to obtain a 3D velocity structure model of North China using the vectorization function of the GIS software (Arc/Info) and the Kriging data gridding method. With this velocity structure model,we have drawn the following conclusions: (1) The P-wave velocity of the uppermost crust of North China changes dramatically,exhibiting a complicated velocity structure in plane view. It can be divided into three velocity zones mainly trending towards north-west. In the research area,the lowest-velocity zones lie in the Haihe (海河) plain and Bohai (渤海) Bay. Although the geological structure of the sedimentary overburden in the study area is somewhat inherited by the upper crust,there are still several differences between them. (2) Generally,the P-wave velocity of the crust increases with depth in the study area,but there still exists local velocity reversion. In the east,low-velocity anomalies of the Haihe plain gradually disappear with increasing depth,and the Shanxi (山西) graben in the west is mainly characterized by relatively low velocity anomalies. Bounded by the Taihang (太行) Mountains,the eastern and western parts differ in structural trend of stratum above the crystalline basement. The structural trend of the Huanghuaihai (黄淮海) block in the east is mainly north-east,while that of the Shanxi block and the eastern edge of the Ordos block is mainly north-west. (3) According to the morphological features of Moho,the crust of the study area can be divided into six blocks. In the Shanxi block,Moho apppears like a nearly south-north trending depression belt with a large crustal thickness. In the southern edge of the Inner Mongolia block and the south of the Yanshan (燕山) block,the Moho exhibits a feature of fold belt,trending nearly towards east-west. In the eastern edge of the Ordos block,the structure of Moho is relatively complex,presenting a pattern of fold trending nearly towards north-west with alternating convexes and concaves. Beneath the Huanghuaihai block,the middle and northern parts of the North China rift zone,the Moho is the shallowest in the entire region,with alternating uplifts and depressions in its shape. For the anteclise zone in the west of Shandong (山东) Province,the Moho is discontinuous for the fault depression extending in the north-west direction along Zaozhuang (枣庄) -Qufu (曲阜).