pull apart emplacement of the margeride granitic complex french massif central implications for the late evolution of the variscan orogen a microstructural magnetic fabric and gravity study is performed on the carboniferous margeride granitic complex that crops out in the central part of the variscan french massif central this complex consists of three facies namely a main porphyritic monzogranite a two mica granite and late leucogranite dykes and stocks in spite of local variations the magnetic lineation mainly trends nw se with a shallow plunge throughout the complex the magnetic foliation pattern is more complex with various strikes and generally a moderate dip this fabric pattern complies with the late orogenic nw se regional extensional deformation already recognised in this part of the variscan belt new gravity measurements complete available data and are used to build up a gravity map and a cross section with which four gravity minima are identified three gravity minima are interpreted as extensional fractures consistent with a nw se maximum stretching the last gravity low corresponds to an extensional jog between the two westernmost fractures the structural and gravity features of the complex are used to propose a feeding and emplacement model controlled by the regional late orogenic extensional tectonics the margeride complex is interpreted as a kilometre scale laccolith like pluton emplaced in a transtensional setting controlled by a nw se opening direction such a model strengthens the relationships between pluton emplacement and late orogenic collapse of the variscan belt

asynchronizing paleo pacific slab rollback beneath se china insights from the episodic late mesozoic volcanism abstract extensive late mesozoic igneous rocks in se china have been widely considered to be generated under the paleo pacific tectonic regime previous studies suggested a fundamental oceanward younging trend for the igneous rocks further implying a nw ward paleo pacific subduction model with gradual slab rollback new la icp ms zircon u pb dating results show that early stage of the late mesozoic volcanic sequences also known as the lower volcanic series in fujian province was formed in three episodes 160 148 ma for the changlin formation 145 130 ma for the nanyuan formation and 130 127 ma for the xiaoxi formation among which the second episode made the volcanism climax thus the entire lower volcanic series in fujian were formed earlier than those in zhejiang 140 118 ma displaying a ne ward younging trend parallel to the potential subduction belt besides in situ hf isotope analyses on dated zircons yield an ehf t range of 19 2 to 1 7 for the lower volcanic series in fujian the majority of the studied volcanic rocks have more radiogenic hf than that of the metamorphic basement requiring the involvement of juvenile components in their origin moreover the zircon ehf t value increases with time in each single area as well as the igneous rocks elsewhere in se china except for the changlin formation which shows very opposite isotopic varying trend increasing zircon ehf t values imply an increasing material contribution of contemporaneous underplated mantle derived magmas which was plausibly induced by gradual crustal decompression as well as the ne ward younging trend of the lower volcanic series it is also identified that the juvenile material contribution in the igneous petrogenesis gradually took place in the same direction along the coastal area thus here we propose an asynchronizing paleo pacific slab rollback model during 150 120 ma to account for the episodic magmatism and crustal extension in se china on the contrary decreasing zircon ehf t values of the changlin formation volcanics indicate that they were formed under enhanced crustal compression probably induced by slab advance on the other hand the upper volcanic series in fujian were formed during 110 88 ma coevally to those in zhejiang with more depleted zircon hf isotopic compositions than the lower volcanic series as well indicating that the entire coastal se china was under the back arc tectonic setting during that time

cretaceous ridge subduction along the lower yangtze river belt eastern china the lower yangtze river belt is one of the most important metallogenic belts in china the mechanisms responsible for ore genesis and the formation of related cretaceous igneous rocks such as adakite a type granitoid and nb enriched basalt remain controversial mesozoic granitoids in the lower yangtze river belt were mostly formed in the early cretaceous 140 125 ma and three granitoid belts the inner the south and the north have been defined according to petrological and geochemical characteristics previously based mainly on negative end and high initial sr isotope values the adakitic rocks were generally attributed to partial melting of thickened or delaminated lower crust both of which require crustal thickening mesozoic crustal thickening however is not supported by the development of extensional basins in the region from the late jurassic to cretaceous eastern china was closely associated with subduction of the pacific plate in the south and the izanagi plate in the north the midocean ridge mor between these two plates was drifting toward and likely subducting under the lower yangtze river belt a ridge subduction model can therefore explain the distribution of different magmatic rocks and ore deposits in the belt partial melting of subducting young hot oceanic slabs close to the ridge formed adakitic rocks the negative end values of adakitic rocks can be plausibly interpreted by mixing between adakitic magmas and enriched components in the lithospheric mantle and or crustal materials through afc process a slab window opened during ridge subduction as indicated by a type granitoids in the center of the inner belt nb enriched basalt found in the belt was likely formed by partial melting of a mantle wedge metasomatized by fluids released from the subducting slab at shallow depths

late mesozoic development of the southern qinling dabieshan foreland fold thrust belt central china and its role in continent continent collision abstract the southern qinling dabieshan foreland fold thrust belt is a key region in which to investigate the continental continental collision between north china and south china we provide structural evidence of a diachronous dextral strike slip thrusting deformation from the middle triassic in the east to the late late triassic in the west which indicates a northwestward oblique subduction and suturing of the south china plate under the north china qinling dabieshan plate we also identify a later phase of intracontinental deformation that included an orthogonal intracontinental collision and south vergent thrusting during the early and middle jurassic indentation of south china into the qinling dabieshan orogen and arc shaped extrusions of the southern qinling dabieshan foreland fold thrust belt from the late jurassic to the cretaceous our results reveal that the long term intracontinental collision rotated clockwise in a northwest southeast vergent direction in the late triassic to northeast east southwest west vergent in the late cretaceous this collision may have been driven by a north vergent subduction of the meso tethys ocean in the south west vergent subduction of the izanagi plate in the east and south vergent compression of the eurasia plate in the north

early mesozoic tectonics of the south china block insights from the xuefengshan intracontinental orogen abstract intracontinental orogens remain less understood than accretionary or collisional orogens that are related to plate margin interactions in the center of the south china block the xuefengshan belt provides a well exposed example of such an intracontinental orogen of early mesozoic age detailed field tectonic observations indicate that the xuefengshan belt can be divided into a western outer zone characterized by km scale box fold structures and an eastern zone separated from the western outer zone by the se dipping main xuefengshan thrust in the eastern zone nw verging folds coeval with a pervasive slaty cleavage and a nw se trending lineation are the dominant structures from west to east the dip of the cleavage surface exhibits a fan like pattern the bulk architecture of the xuefengshan belt results from polyphase deformation d 1 is characterized by a top to the nw ductile shearing d 2 corresponds to se directed back thrusting and folding d 3 consists of upright folds with vertical cleavage and lineation at depth a high strain zone characterized by greenschist facies metamorphic rocks and a top to the nw ductile shearing corresponds to a ductile decollement zone that accommodated the deformation of the neoproterozoic to early triassic sedimentary series kinematic compatibility suggests that the synmetamorphic ductile shearing was coeval with the d 1 event in the sedimentary cover the xuefengshan belt is interpreted as an early mesozoic intracontinental orogen which possibly originated from the se directed continental subduction of a piece of the south china block in response to northwestwards subduction of the pacific plate

petrogenesis and tectonic implications of early cretaceous s and a type granites in the northwest of the gan hang rift se china the gan hang rift trending at least 450 km in a ne sw direction is a part of a mesozoic basin and range province in southeastern china detailed shrimp zircon u pb chronology major and trace element and sr nd hf isotope data of three granitic plutons and a diabasic dike in the northwest of the gan hang rift are used to explore the origin of these granites and their relationship to the evolution of the gan hang rift shrimp zircon u pb dating shows that the granitic plutons and diabasic dike were emplaced in the early cretaceous 122 129 ma the tongshan and damaoshan plutons close to the gan hang rift consist mainly of weakly peraluminous granitic rocks which show a2 subtype affinity these granites have initial 87sr 86sr ratios of 0 7080 0 7103 end t values of 1 4 to 5 6 and ehf t in situ zircon values of 3 8 to 1 2 detailed elemental and isotopic data suggest that they were formed by partial melting of granulitized mesoproterozoic metamorphic basement including metasedimentary and metaigneous rocks in the shallow 30 km crust at a high temperature 823 c these pure crustal melts underwent crystal fractionation of biotite plagioclase and k feldspar forming the more felsic end member of granitic rocks the ehu pluton more distant from the gan hang rift is composed of two mica granites which are strongly peraluminous and are s type granites these granites have higher initial 87sr 86sr ratios 0 7162 0 7191 and lower end t 7 1 to 7 4 and ehf t 6 8 in situ zircon values than the tongshan and damaoshan granites detailed elemental and isotopic data suggest that the ehu granites were formed by partial melting of paleoproterozoic metasedimentary basement in the deep 40 50 km crust at a relatively low temperature 757 c with no fractional crystallization and mantle derived magma mixing in their evolution the coeval diabasic dike is calc alkaline and has initial 87sr 86sr ratio of 0 7085 end t value of 0 9 and ehf t in situ zircon value of 7 1 suggesting that it was derived from the asthenospheric mantle wedge that had been metasomatised via addition of subducted sediment derived melt with monazite in the residual phase the association of early cretaceous 122 129 ma s and a type granites in the northwest of the gan hang rift marks the onset of back arc extension or intra arc rift with ongoing extension the crust and lithospheric mantle became progressively thinned the upwelling of asthenosphere triggered partial melting of both metasedimentary and metaigneous rocks in the more thinned crust close to the gan hang rift forming the a type granitic magmas such as tongshan and damaoshan whereas partial melting of metasedimentary rocks in the less thinned crust farther from the gan hang rift formed the s type granitic magmas such as ehu the red sediments with the total thickness more than 10 000 m have been successively deposited in the gan hang rift valley since the late early cretaceous 105 ma suggesting that this region experienced the most back arc extension

the anisotropy of magnetic susceptibility in biotite muscovite and chlorite single crystals the anisotropy of magnetic susceptibility ams of single crystals of biotite muscovite and chlorite has been measured in order to provide accurate values of the magnetic anisotropy properties for these common rock forming minerals the low field ams and the high field paramagnetic susceptibility are defined for the high field values it is necessary to combine the paramagnetic deviatoric tensor obtained from the high field torque magnetometer with the paramagnetic bulk susceptibility measured from magnetization curves of the crystals this leads to the full paramagnetic susceptibility ellipsoid due to the anisotropic distribution of iron cations in the silicate lattice the ellipsoid of paramagnetic susceptibility which was obtained for the three phyllosilicates is highly oblate in shape and the minimum susceptibility direction is subparallel to the crystallographic c axes the anisotropy of the susceptibility within the basal plane of the biotite has been evaluated and found to be isotropic within the accuracy of the instrumental measurements the degree of anisotropy of biotite and chlorite is compatible with previously reported values while for muscovite is smaller than previously published values the shape of the chlorite ams ellipsoid for all the samples is near perfect oblate in contrast with a wide distribution of oblate and prolate values reported in earlier studies reliable values are important for deriving models of the magnetic anisotropy where it reflects mineral fabrics and deformation of rocks d 2003 elsevier science b v all rights reserved

multiple emplacement and exhumation history of the late mesozoic dayunshan mufushan batholith in southeast china and its tectonic significance 2 magnetic fabrics and gravity survey the late mesozoic magmatic province is a prominent feature of the south china block scb however the tectonic regimes associated with the magmatism are still elusive a combined anisotropy of magnetic susceptibility ams and gravity study has been carried out to determine the fabric patterns and shape at depth of the dayunshan mufushan composite batholith in the north central scb this is a companion paper to part 1 that presented the structural and geochronological data of this batholith the magnetic fabrics in the batholith interior predominantly reflect magma flow structures two distinct patterns of the magnetic lineations are defined around nne ssw and wnw ese trends for the early stage and late stage intrusions of the batholith respectively the gravity survey reveals that the early stage intrusion has a main feeder zone located below its northern part while several linear feeder zones trending nne ssw are inferred for the late stage intrusion integrating all results a two stage construction of the batholith with distinct tectonic regimes has been established it is concluded that the early stage intrusion experienced a southward magma transport during its emplacement partially assisted by far field compression from the north at ca 150 ma conversely the emplacement and exhumation of the late stage intrusion was accommodated by a nw se crustal stretching involving a lateral magma expansion above the multiple feeder zones likely corresponding to extensional fractures and ductile shearing during 132 95 ma localized mainly along the dayunshan detachment fault finally we discuss the geodynamic linkage between the paleo pacific subduction and the late mesozoic tectono magmatism in the scb

control of granite emplacement by regional deformation abstract a combination of structural measurements and gravity data is used to investigate the mode of granite emplacement in various tectonic settings the deepest zones revealed by gravity data provided those zones also show vertical lineations in outcrop are interpreted as magma feeder channels these structures or root zones reflect the flow pattern of the upwelling magma while being emplaced they can be interpreted within the regional stress field active during that time granites intruded during transcurrent shearing have only a few feeder channels these roots are not connected to the main shear zone all of them lie within locally extensional areas of the regional stress field this situation is also observed in granites emplaced during a compressional phase of deformation when emplaced during a regionally extensional phase the granitic plutons are very thin with many small deeper zones in which variously evolved granite types may be intrusive these regions vertically differentiated are also interpreted as feeder channels the different morphology at depth as well as the difference in the number of root zones suggest that deformation controls the emplacement of granitic plutons the root zones are oriented at a high angle to the direction of maximum compression when magma is emplaced in a plastic crust if the pluton is emplaced higher in a brittle crust then the roots are aligned with the maximum compression according to the andersonian theory of fracture the orientation of the root zone along the direction of extension in a plastic crust may be explained by shearing which acts as a valve mechanism for segregating magma in all the examples the geometry of the root zone relative to the major fault plane implies that the shear zone does not induce magma generation and that it does not serve as a guide for magma emplacement

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

early cretaceous extensional reworking of the triassic hp uhp metamorphic orogen in eastern china corresponding to the early mesozoic continental subduction between the north china block ncb and the south china block scb the tongbaishan hong an dabieshan sulu massifs are famous for their hp uhp metamorphism more than 50 of the hp uhp orogenic belt was significantly reworked by early cretaceous extensional tectonics this early cretaceous event with a fast cooling period at 130 120 ma superimposed on the early mesozoic hp uhp orogenic belt and intensively changed the architecture of this orogen each individual segment documents different early cretaceous extensional structures namely the central tongbaishan domain is a metamorphic core complex mcc represented by an a type non cylindrical antiform the central dabieshan domain is a typical cordilleran type migmatite cored mcc the southern sulu uhp domain is a wedge shaped structure exhumed by a simple detachment fault these late stage extensional structures expose the previous hp uhp orogenic belt as fragments along the ncb scb boundary the geodynamic setting of this early cretaceous extensional tectonics along the hp uhp orogen is a part of a 1000km scale crustal extension belt that is widespread in eastern eurasia continent from trans baikal to the central part of scb convective erosion or delamination of the mantle lithosphere might be considered as a possible mechanism for mantle removal

interpreting magmatic fabric patterns in plutons abstract most plutons have widespread magmatic fabrics the interpretation of which remains controversial we propose a method to constrain likely causes of fabric patterns the application of which indicates the following 1 preserved fabric patterns often form after chamber construction and only rarely provide information about ascent or emplacement 2 fabrics are poor recorders of total strain and are easily reset preserving only the last increment of strain during crystallization 3 in magmatic systems mechanically decoupled from host rocks patterns may result from strain during internally driven flow filter pressing or porous flow in relatively static chambers or by final increments of strain during emplacement 4 with greater emplacement depths fabric patterns increasingly reflect strain caused by regional deformation and 5 given that magmatic fabrics are easily reset and reflect only the last increment of strain of comparatively weak materials they may provide a relatively direct record of paleostress in orogenic belts

pluton dyke relationships in a variscan granitic complex from ams and gravity modelling inception of the extensional tectonics in the south armorican domain france abstract the carnac granitic complex south armorican domain western france was emplaced during late carboniferous times in the deepest variscan unit roofed by two major extensional shear zones through the acquisition and interpretation of field data anisotropy of magnetic susceptibility and gravity data emphasized by petrological and structural observations we address the emplacement model and possible magmatic processes involved between dyking to massive plutonism in a synkinematic context gravity modelling highlights an overall eastward thinning of the pluton and several deep zones in the western part of the complex interpreted as the pluton feeder zones the internal granitic fabric developed in a sub solidus state shows marked planar linear anisotropy consistent with a vertical shortening in the wnw ese regional stretching regime and eastward magma spreading this study documents the occurrence of numerous nne ssw trending dykes within the eastern part of the pluton suggesting that this granitic complex formed by the coalescence of dykes oriented perpendicular to the regional stretching direction and thus interpreted as large scale tension gashes the synkinematic character of the carnac complex intrusion recently dated at ca 319 6 ma thus times the inception of the late orogenic extensional deformation experienced in the whole south armorican domain

structure of late variscan millevaches leucogranite massif in the french massif central ams and gravity modelling results in the limousin area variscan leucogranitic plutons are spatially associated with normal faults and major strike slip shear zones that are a continuation of the south armorican shear zone our study focuses on the large n s trending millevaches granitic massif massif central france and intends to highlight through gravity modelling structural and anisotropy of magnetic susceptibility ams the massif structure at depth and to discuss the mode of emplacement of granites within a strike slip tectonic context the mica subfabric suggests that the magnetic foliations display a general nw se sub horizontal pattern on both sides of the n s pradines dextral wrench fault zone that deforms the core of the massif on 5 km width the magnetic lineation trend exhibits a sigmoidal pattern n s in the pradines fault zone and nw se on both sides of it which are consistent with a dextral wrench component the horizontal magnetic foliations and lineations are consistent with the thin granite laccolith model there is no significant imprint of the extensional variscan belt collapse on the internal fabric of millevaches granites than the tectonic dextral transcurrent movement prevailing in this area

structural development of the lower paleozoic belt of south china genesis of an intracontinental orogen abstract an extensive structural study of wuyishan and surrounding areas south china brings data on the structures formed prior to the devonian unconformity building the lower paleozoic belt an ordovician tectonic event was responsible for i south directed structures in the wuyishan proper and its southern border related to both thin and thick skinned tectonics accompanied by metamorphism and crustal thickening ii north directed structures to the west of ganjiang fault and north of jiangshan shaoxing fault where only thin skinned tectonics is visible the southward deformation accommodating the shortening includes emplacement of thrust sheets involving deep crustal material and some mantle peridotite and likely repetition of continental material both responsible for crustal thickening the orogeny was due to the underthrusting of the southern part of the south china block beneath the northern part of this block closing the pre existing nanhua rift created at around 850 800 ma and involving the rift sedimentary fill in the southward thrusting the syn metamorphic tectonic piling was followed by anatexis and granite emplacement at around 440 390 ma our new u th pb epma monazite dating gives ages of 433 9 ma and 437 5 ma early silurian for the main anatectic event and a younger age of 412 5 ma late silurian early devonian for a late re heating process the lower paleozoic belt is an intracontinental orogen without oceanic suture record it shows some similarities with the cenozoic european pyrenees chain with a strong asymmetry

a multidisciplinary study on the emplacement mechanism of the qingyang jiuhua massif in southeast china and its tectonic bearings part i structural geology ams and paleomagnetism during the cretaceous the south china block scb experienced a widely distributed extensional event including numerous plutons emplacement and basin opening investigations on the tectonic regime coeval with pluton emplacement and emplacement mechanism of the pluton remain relatively rare in the scb in order to address these questions a multidisciplinary approach including field structural and petrographic observations anisotropy magnetic susceptibility ams and paleomagnetic analyses was carried out on the qingyang jiuhua granitic massif which intrudes into the lower yangtze fold belt in the northeastern part of scb the qingyang jiuhua massif is composed of the granodioritic qingyang and monzogranitic jiuhua plutons dated by zircon u pb method at ca 142 ma and ca 131 ma respectively our structural observations show that the intrusion of the qingyang jiuhua massif does not modify the fold strike a weak ductile deformation of the country rocks and granitoid can be only observed in the boundary zone with limited contact metamorphism in the contact aureole of the massif the foliation follows the pluton contour and the mineral lineation is rare when present it exhibits a down dip attitude field and microstructural observations indicate isotropic magmatic textures in most parts of the massif the ams analysis of 93 sites reveals weak values for the anisotropy degree pj 0 for most of the measured samples two principal foliation patterns are identified horizontal foliations in the center of the plutons and vertical foliations on the boundaries magnetic lineation strike is largely scattered and weakly inclined at the scale of the entire massif the paleomagnetic investigations indicate that a the younger jiuhua pluton did not produce a remagnetization in the older qingyang pluton b no relative movement can be observed between these two plutons c the entire massif did not experience any important relative movement with respect to south china considering the paleomagnetic uncertainties integrating the newly obtained results with previous observations our study favors a permissive emplacement mechanism for the two plutons i e vertical magma intrusion into an opening space controlled by the nw se brittle stretching of the upper crust which is in agreement with a weak extensional regional tectonic framework of the scb

formation of the jurassic south china large granitic province insights from the genesis of the jiufeng pluton abstract the magma source and thermal regime for the formation of large granitic provinces lgps have been the subjects of much debate as mineralogical geochemical and isotopic variations of plutonic rocks preserve records of crustal thermal evolution the jiufeng pluton was comprehensively studied to gain insights into material and heat sources and tectonic process in the formation of the south china lgp sims zircon u pb analyses of a biotite granite 158 9 1 3 ma three two mica granites stage i 159 7 1 2 ma stage ii 158 6 1 2 ma and 156 8 1 2 ma and a muscovite granodiorite 159 0 4 4 ma of the jiufeng pluton suggest that they are coeval and were emplaced in a short period of no more than 5 3 m y limited variations in whole rock nd isotope end t 12 6 to 9 5 and zircon hf o isotopes ehf t 12 0 to 6 3 18 o 6 8 9 4 suggest that the jiufeng pluton was mainly derived from melting of a common metasedimentary source possibly with a minor basaltic contribution we consider the geochemical variations of the jiufeng pluton are primarily a result of incremental assembly of magma batches produced from rapid step like transition from fluid saturated to fluid absent melting of the source the muscovite granodiorite with high na 2 o 3 80 wt k 2 o na 2 o 1 is interpreted to have been produced by fluid saturated melting at low temperature 650 c high p 2 o 5 0 09 0 17 wt zircon saturation temperature t zr 769 816 c and la yb ratios 8 4 55 8 of the stage ii two mica granite support its formation from high temperature 800 c biotite dehydration melting whereas lower p 2 o 5 zr 685 742 c and la yb

ocean basin evolution and global scale plate reorganization events since pangea breakup we present a revised global plate motion model with continuously closing plate boundaries ranging from the triassic at 230 ma to the present day assess differences among alternative absolute plate motion models and review global tectonic events relatively high mean absolute plate motion rates of approximately 9 10 cm yr 1 between 140 and 120 ma may be related to transient plate motion accelerations driven by the successive emplacement of a sequence of large igneous provinces during that time an event at 100 ma is most clearly expressed in the indian ocean and may reflect the initiation of andean style subduction along southern continental eurasia whereas an acceleration at 80 ma of mean rates from 6 to 8 cm yr 1 reflects the initial northward acceleration of india and simultaneous speedups of plates in the pacific an event at 50 ma expressed in relative and some absolute plate motion changes around the globe and in a reduction of global mean plate speeds from about 6 to 4 5 cm yr 1 indicates that an increase in collisional forces such as the india eurasia collision and ridge subduction events in the pacific such as the izanagi pacific ridge play a significant role in modulating plate velocities

eastern asia and the western pacific timing and spatial distribution of rifting in china abstract a compilation of recognized rifts in china indicates that five distinct episodes of rifting have occurred these episodes occurred at approximately 2 3 ga 1 85 1 7 ga 1 0 0 7 ga 550 190 ma and 175 ma to present the timing and location of rifting has largely been a function of tectonic activity along the margins of the various chinese paleocontinents some of the oldest rocks in china are found in the sino korean and tarim cratons and most of the identified precambrian rifts are found within or on the margins of these cratons these cratons are viewed as the continental nucleus of china and fold belts and microcontinents have been accreted to this nucleus through time much of the locus of rifting has migrated away from this nucleus as has the location of the continental margin especially in the paleozoic beginning in the middle mesozoic rifting commenced along the entire eastern margin of china extension occurred concomitantly with subduction in a backarc setting this is the dominant cenozoic extensional regime in china and is still active today to date few cenozoic rifts have been found in western china where most tectonism is probably a consequence of the collision between india and china but it seems likely that more rifts will be discovered in this area as it becomes better studied

late jurassic early cretaceous continental convergence and intracontinental orogenesis in east asia a synthesis of the yanshan revolution abstract the basic tectonic framework of continental east asia was produced by a series of nearly contemporaneous orogenic events in the late middle jurassic to early cretaceous commonly the late mesozoic orogenic processes were characterized by continent continent collision large scale thrusting strike slip faulting and intense crustal shortening crustal thickening regional anatexis and metamorphism followed by large scale lithospheric extension rifting and magmatism to better understand the geological processes this paper reviews and synthesizes existing multi disciplinary geologic data related to sedimentation tectonics magmatism metamorphism and geochemistry and proposes a two stage tectono thermal evolutionary history of east asia during the late middle jurassic to early cretaceous ca 170 120 ma in the first stage three orogenic belts along the continental margins were formed coevally at ca 170 135 ma i e the north mongol okhotsk orogen the east paleo pacific coastal orogen and the west bangong nujiang orogen tectonism related to the coastal orogen caused extensive intracontinental folding and thrusting that resulted in a depositional hiatus in the late jurassic as well as crustal anatexis that generated syn kinematic granites adakites and migmatites the lithosphere of the east asian continent was thickened reaching a maximum during the latest jurassic or the earliest cretaceous in the second stage ca 135 120 ma delamination of the thickened lithosphere resulted in a remarkable 120 km lithospheric thinning and the development of mantle derived magmatism mineralization metamorphic core complexes and rift basins the middle jurassic early cretaceous subduction of oceanic plates paleo pacific meso tethys and mongol okhotsk and continent continent collision e g lhasa and qiangtang along the east asian continental margins produced broad coastal and intracontinental orogens these significant tectonic activities marked by widespread intracontinental orogeny and continental reconstruction are commonly termed the yanshan revolution movement in the chinese literature

intracontinental subduction a possible mechanism for the early palaeozoic orogen of se china the early palaeozoic orogen of se china consists of three lithotectonic elements from top to bottom a sedimentary upper unit a metamorphic lower unit and a gneissic basement the boundaries between these units are flat lying south directed ductile decollements the lower one is coeval with an amphibolite facies metamorphism m1 the belt is reworked by migmatite granite domes high temperature metamorphism m2 and granitic plutons related to post orogenic crustal melting we date here the syn m1 ductile shearing at 453 7 ma by u th pb method on monazite previous ages and our new 40 ar 39 ar ages of biotites and muscovites show that the metamorphic rocks experienced syn m2 exhumation from 440 to 400 ma the early palaeozoic orogen of se china is an intracontinental belt in which decollements accommodated the north directed subduction of the cathaysian continent this orogen is an example of intracontinental subduction that was not preceded by oceanic subduction

phanerozoic polyorogenic deformation in southern jiuling massif northern south china block constraints from structural analysis and geochronology abstract the structure of the jiuling massif has been investigated in order to delineate the polyorogenic deformation and discuss its geodynamic evolution and orogenic mechanisms detailed structural analysis indicates that the d1 event is characterized by top to the nnw ductile shearing with pervasive foliation and mineral and stretching lineation developed in the entire region compared with the d1 deformation d2 structures are localized in ductile shear zones with subvertical foliation and subhorizontal e w trending lineation indicating a dextral ductile shearing the d3 event marked by folds and thrusts mainly in a brittle domain modified the d1 structures by asymmetrical folds the dominant d4 structures are gravitational folds and normal faults corresponding to a later extension our new geochronological data suggest that the d1 event occurred between 465 and 380 ma with d2 dextral shearing at the end of this early paleozoic orogen and the d3 event has been constrained at 245 215 ma the final uplift of the jiuling massif by the d4 event can be correlated with the late mesozoic extension across the eastern south china block along with previous studies in the south china block the structural pattern of the jiuling massif elucidates the influence of the early paleozoic and early mesozoic intracontinental belts triggered by repeated reactivation of the jiangshan shaoxing fault combined with deformation to the south the early paleozoic belt shows a positive flower pattern with opposing kinematics rooted in the jiangshan shaoxing fault during the early mesozoic a general intracontinental belt was developed with uniform kinematics in both the jiuling massif and the xuefengshan belt possibly resulted from the far field effect of the triassic nw directed paleo pacific subduction

the liaonan wanfu metamorphic core complexes in the liaodong peninsula two stages of exhumation and constraints on the destruction of the north china craton 1 the liaodong peninsula early cretaceous extension province lep located in the eastern part of the north china craton ncc was highly extended in early cretaceous the liaonan and the wanfu metamorphic core complexes mccs are two most important elements in the province both are typical cordilleran type core complexes that are composed of a general three layered structure but show various differences shearing along the jinzhou detachment fault zone in the liaonan mcc generated ductile to brittle tectonites various deformation microstructures and several types of quartz c axis fabrics implying that the mcc has a complicated progressive exhumation history however medium temperature microstructures and fabrics are superimposed by low temperature features along the wanfu detachment fault zone suggesting a relatively short and simple exhumation history of the wanfu mcc magmatic zircon u pb geochronological dating reveals that the early cretaceous extension of the lep began with the initiation of shearing along the jinzhou detachment fault zone and exhumation of the liaonan mcc before ca 134 ma subsequently relatively slow cooling and exhumation of the lower plate accompanied a giant magmatic event from 130 ma to 120 ma exhumation of a new mcc i e the wanfu mcc was triggered by progressive extension after ca 120 ma rapid cooling and exhumation of the metamorphic lower plate of the two mccs are attributed to coeval detachment faulting along both the jinzhou and wanfu faults from 120 ma to 113 ma the extension ended at ca 107 ma these data provide reliable evidence that the two mccs were exhumed progressively and sequentially during two stages of crustal extension of the lep our results indicates that heterogeneous lithosphere extension and upper middle crust upper mantle detachment possibly due to the interaction between the paleo pacific and eurasian plates are among the most important processes during the early cretaceous thinning of the lithosphere tectonic extension may have led to the detachment in east asia which contributed to the thinning of the lithosphere and the destruction of the ncc a phase of uniform and consistent wnw ese extension was responsible for the generation of the extensional structures in the eastern asia continent and the destruction of the ncc

a multidisciplinary study of the emplacement mechanism of the qingyang jiuhua massif in southeast china and its tectonic bearings part ii amphibole geobarometry and gravity modeling abstract the late mesozoic geology of the lower yangtze area is characterized by extensional sedimentary basins numerous granitic plutons and several world class ore deposits regionaly the coeval tectonic and geodynamic framework remains inadequately investigated in order to provide a more comprehensive understanding of the regional tectonic context we performed a multidisciplinary study of the emplacement mechanism of the qingyang jiuhua massif it consists of the granodioritic qingyang and the monzogranitic jiuhua plutons u pb dated at 142 1 0 ma and 131 2 6 ma respectively biotite and amphibole cooling ages are 5 8 myr younger results deduced from field structural observation petrographic and magnetic fabrics paleomagnetism show that this massif was probably vertically emplaced by permissive intrusion coeval with weak regional extension however detailed information on the characteristics of this tectonic event and its emplacement depth was not documented therefore we carried out an investigation of amphibole geobarometry and gravity modeling in order to address the emplacement mode of the plutons within their regional tectonic framework amphibole data show that the qingyang jiuhua massif was emplaced in the upper crust at a depth between 5 and 12 km gravity modeling indicates that the massif is laccolithic it is characterized by several ne sw striking linear thickening zones that we interpret as the feeder roots of the massif these results led us to conclude that 1 the qingyang jiuhua massif was fed by vertical ne sw striking tension gashes 2 consequently the late early cretaceous tectonics in the study area was characterized by nw se extension

origin and tectonic significance of a mesozoic multi layer over thrust system within the yangtze block south china abstract in the yangtze block south china a well developed mesozoic thrust system extends through the xuefeng and wuling mountains in the southeast to the sichuan basin in the northwest the system comprises both thin and thick skinned thrust units separated by a boundary detachment fault the dayin fault to the northwest the thin skinned belt is characterized by either chevron anticlines and box synclines to the northwest or chevron synclines to the southeast the former structural style displays narrow exposures for the cores of anticlines and wider exposures for the cores of synclines thrust detachments occur along silurian fs and lower cambrian fc strata and are dominantly associated with the anticlines to the southeast this style of deformation passes gradually into one characterized by chevron synclines with associated principal detachment faults along silurian fs cambrian fc and lower sinian fz strata there are however numerous secondary back thrusts therefore the thin skinned belt is like the valley and ridge province of the north american applachian mountains the thick skinned belt structurally overlies the thin skinned belt and is characterized by a number of klippen including the xuefeng and wuling nappes it is thus comparable to the blue ridge province of appalachia the structural pattern of this thrust system in south china can be explained by a model involving detachment faulting along various stratigraphic layers at different stages of its evolution the system was developed through a northwest stepwise progression of deformation with the earliest delamination along lower sinian strata fz analyses of balanced geological cross sections yield about 18 1 21 total 88 km shortening for the thin skinned unit and at least this amount of shortening for the thick skinned unit the compressional deformation from southeast to northwest during late jurassic to cretaceous time occurred after the westward progressive collision of the yangtze block with the north china block and suggests that the orogenic event was intracontinental in nature

pattern and kinematic polarity of late mesozoic extension in continental ne asia perspectives from metamorphic core complexes 1 late mesozoic extension in ne asia resulted in the development of a large extensional province metamorphic core complexes mccs are the major features in this province and have 40ar 39ar ages of 130 110 ma for the mylonites and u pb zircon ages of 150 110 ma for the integral granitic intrusions based on this and previous studies this paper summarizes major characteristics of these mccs and recognizes a regional kinematic shear sense most mccs in the transbaikalia region sino mongolia border tract and the northwest central portion of the north china craton ncc show a top to the southeast se shear whereas those in the eastern and southern ncc locally underwent top to the northwest nw shear the three largest basins songliao huabei and ordos in north china are located in the transitional zone between domains of opposing shear sense we interpret the extension in the transbaikalia sino mongolia tract and northwestern part of the ncc to reflect late orogenic collapse of thickened crust following middle late jurassic collision along the okhotsk suture the southeastward extension is probably controlled by crustal scale top to the se tangential shear the transition from contraction to extension is marked by detachment faults that nucleated as extensional crenulation cleavage ecc i e c in sub horizontal ductile shear zones late in orogenic crustal thickening the combined effect of gravitational loading and thermal uplifting is considered to be the origin of the late or post orogenic collapse the top to the nw extension in the ne of the ncc might reflect antithetic sub extensional zones or mesozoic back arc extension as a far field effect of cretaceous pacific plate subduction

high resolution mantle tomography of china and surrounding regions 1 a high resolution p wave tomographic model of the crust and mantle down to 1100 km depth under china and surrounding regions is determined by using about one million arrival times of p pp pp and pcp waves from 19 361 earthquakes recorded by 1012 seismic stations the subducting pacific slab is imaged clearly as a high velocity zone from the oceanic trenches down to about 600 km depth and intermediate depth and deep earthquakes are located within the slab the pacific slab becomes stagnant in the mantle transition zone under east china the western edge of the stagnant slab is roughly coincident with a surface topographic boundary in east china the active changbai and wudalianchi intraplate volcanoes in northeast china are underlain by significant slow anomalies in the upper mantle above the stagnant pacific slab these results suggest that the active intraplate volcanoes in ne china are not hot spots but a kind of back arc volcano associated with the deep subduction of the pacific slab and its stagnancy in the transition zone under the mariana arc however the pacific slab penetrates directly down to the lower mantle the active tengchong volcano in southwest china is related to the eastward subduction of the burma microplate the subducting indian and philippine sea plates are also imaged clearly the indian plate has subducted down to 200 300 km depth under the tibetan plateau with a horizontal moving distance of about 500 km high velocity anomalies are revealed in the upper mantle under the tarim basin ordos and sichuan basin which are three stable blocks in china

magnetic fabric vs magnetite and biotite shape fabrics of the magnetite bearing granite pluton of gameleiras northeast brazil abstract a well defined orientation pattern has been evidenced by the magnetic fabric lineation and foliation of the granite pluton of gameleiras northeast brazil this magnetic fabric is roughly parallel to the magmatic fabric defined by the elongate mafic enclaves and by the planar arrangement of tabular k feldspar phenocrysts low field anisotropy of magnetic susceptibility properties as well as magnetic mineralogy studies and optical microscopy observations indicate that multidomian iron rich magnetite is the main carrier of the magnetic susceptibility and its anisotropy to examine in more detail the relationships between the magnetic and magmatic fabrics measurements of two dimensional shape fabrics of grain populations of biotite and magnetite have been carried out on a total of 500 grains using an automatic shape fabric analysis procedure on carefully selected and oriented thin sections angular departure has been calculated between the maximum elongation of the mineral shape fabric of the given grain population and the maximum susceptibility axis of the corresponding macroscopic specimen we found that the fabric of biotite correlates strongly with the magnetic fabric 7 8 the fabric of magnetite although not as well defined mainly because of its weak shape anisotropy still correlates with the magnetic fabric 12 18 origin of low field magnetic fabric in the pluton of gameleiras is concluded to be controlled principally by the statistical alignment of the long axes of inequant magnetite grains magnetic interactions between the grains of magnetite where they form clusters may account for the observed scattering or abnormal magnitudes of the magnetic anisotropy concerning the orientation of the magnetic fabric as the long dimensions of the magnetites preferentially align with the boundaries of the other minerals particularly biotite and titanite the magnetic fabric as defined by the grains of magnetite is concluded to approximate closely the magmatic fabric defined by the shape fabric of biotite however as no correlation exists between shapes or clustering of magnetites and strain undergone by the deforming magma the strength of the magnetic fabric in ferromagnetic granites cannot be used in a simple way as a measure of strain intensity

on granitoid emplacement and related structures a review granitoid magmas are emplaced at the upper crust under a complex interaction of gravitational processes and horizontal tectonics natural intrusive bodies show a wide variety of structural patterns that must be strongly related with their dynamic history of ascent and emplacement comparisons of natural structural patterns with experimental and numerical models are necessary to interpret in a coherent fashion the emplacement mechanism involved from natural experimental numerical and theoretical considerations one can reach important conclusions on the problem of ascent and final emplacement of granitoid magmas in the earth s crust the present paper reviews the principal emplacement mechanisms referred to in the literature that is doming diapirism ballooning stoping cauldron subsidence and dike propagation dike propagation is the most effektive process in magma transport from deep zones in the crust and upper mantle extensional fractures for dike propagation and magma ascent can be developed at depth in the crust according to the modern dike propagation theory in orogenic domains magmas reach the upper crust through narrow channels or dikes and are accumulated in a final reservoir in the form of a discordant supracrustal pluton or a ballooning pluton if during emplacement any regional deformation acts simultaneously

timing and processes of late mesozoic mid lower crustal extension in continental ne asia and implications for the tectonic setting of the destruction of the north china craton mainly constrained by zircon u pb ages from metamorphic core complexes abstract late mesozoic large scale crustal extension occurred across the northeastern section of continental asia and metamorphic core complexes mccs are the major features in this region this paper presents new zircon u pb and ar ar ages of biotite and hornblende for pre or syn mylonitized and post kinematic unmylonitized intrusions from these mccs and attempts to determine the timing of initiation duration and termination of the mid lower crustal ductile extension by integrating with geochronological data from previous studies these mccs have similar 40 ar 39 ar biotite and hornblende ages 130 to 110 ma mainly from 130 to 120 ma which are interpreted as the cooling ages and represent the timing of the late stage formation of the mccs zircon u pb ages for syn to post kinematic intrusions from some mccs suggest that the mid lower crustal extension probably initiated at ca 150 145 ma with a peak occurring 145 130 ma and continued to 130 120 ma which is older than that suggested by 40 ar 39 ar ages by 10 20 ma the early ductile extension most likely initiated in mongolia and northern china during the late jurassic 150 145 ma at a deeper level then propagated southwards 140 130 ma and finally developed throughout the entire continental ne asia 130 120 ma based on the timing and processes of the extension a geodynamic model and tectonic setting of the destruction of the north china craton ncc are proposed

tectonics of the middle triassic intracontinental xuefengshan belt south china new insights from structural and chronological constraints on the basal decollement zone in orogenic belts a basal decollement zone often develops at depth to accommodate the shortening due to folding and thrusting of the sedimentary cover in the early mesozoic intracontinental xuefengshan belt of south china such a decollement zone is exposed in the core of anticlines formed by the emplacement of the late orogenic granitic plutons our detailed multi scale structural analysis documents a synmetamorphic ductile deformation in the basal decollement the neoproterozoic pelite and sandstone and the intruding early paleozoic granites were deformed and metamorphosed into mylonites and orthogneiss respectively the metamorphic foliation contains a nw se stretching lineation associated with top to the nw kinematic indicators the ductile shearing of these high strained rocks can be correlated with nw verging folds and thrusts recognized in the neoproterozoic to early triassic sedimentary cover monazite u th pbtot chemical dating and zircon sims u pb dating provide age constraints of the ductile shearing between 243 and 226 ma and late orogenic granite emplacement around 235 215 ma in agreement with recent geochronological data these new results show that the xuefengshan belt is an early mesozoic orogen dominated by the nw directed shearing and thrusting at the southeastern boundary of the xuefengshan belt the chenzhou linwu fault separates the early mesozoic domain to the nw from the early paleozoic domain to the se the tectonic architecture of this belt was possibly originated from the continental underthrusting to the se of the south china block in response to northwest directed subduction of the paleo pacific plate

metamorphic core complex dynamics and structural development field evidences from the liaodong peninsula china east asia metamorphic core complexes mcc constitute remarkable features within wide rifts based on analogue and numerical modelling mcc dynamics and structural development are mainly controlled by first geothermal gradient second the compositional layering and after the strain rate and partial melting in the late mesozoic continental extension occurred in east asia leading to the development of mcc magmatism and extensional sedimentary basins based on an integrated study i e structural and finite strain analysis petrofabrics anisotropy of magnetic susceptibility ams and u pb on zircon dating this paper aims at constraining the tectonic evolution and deformation mechanisms in the south liaodong peninsula ne china the gudaoling massif is identified as a migmatitic mcc developed from upper jurassic ca 157 154 ma to early cretaceous ca 128 113 ma intrusion of the yinmawanshan synkinematic pluton early cretaceous to the south of the dome marks out the final stages of shearing along the gudaoling detachment zone and of exhumation of the mcc into the upper crust the gudaoling mcc and the yinmawanshan pluton stay in line with the coeval south liaodong mcc to the south all making a 140x30km wide extensional band formed during a regional e w to nw se crustal stretching the area shows a bi phased development with a slow and a fast stage which correspond to 1 crustal necking and 2 dome amplification exhumation stages according to published thermo mechanical modelling results finally the gudaoling mcc lower unit almost exclusively displays migmatites and anatectic granitoids of late jurassic and early cretaceous respectively occurrence of partial melting during earlier stages of extension seems controlling the initiation of mcc as a soft anomaly within lower crust in east asia a regional scale thermal event during jurassic cretaceous times may have significantly reduced the bulk lithosphere strength and locally induced partial melting emphasizing pervasive deformations and development of a wide rift system

late mesozoic compressional to extensional tectonics in the yiwulushan massif ne china and its bearing on the evolution of the yinshan yanshan orogenic belt part i structural analyses and geochronological constraints abstract with a cratonic nucleus the north china craton ncc experienced a complex tectonic evolution with multiphase compressional and extensional events during mesozoic times along the northern part of the ncc the yinshan yanshan fold and thrust belt was a typical intraplate orogen jurassic and cretaceous continental sedimentation magmatism widespread intraplate characterize the yinshan yanshan orogenic belt the geodynamic significance of these tectonic events is still in dispute in the western part of the liaoning province the yiwulushan massif crops out at the eastern end of the yinshan yanshan orogenic belt the yiwulushan massif presents an elliptical domal shape with a ne sw striking long axis the structural evolution of this massif brings new insights for the understanding of the mesozoic plutonic tectonic history of the ncc a multidisciplinary study involving structural geology geochronology anisotropy of magnetic susceptibility ams and gravity modeling have been carried out the presentation of the new results splits into two parts part i this paper deals with field and laboratory structural observations and presents the main geochronological results the ams gravity modeling data will be provided in a companion paper part ii the early compressional deformation d1 corresponds to a late jurassic to early cretaceous southward thrusting the subsequent deformation is related to the early cretaceous exhumation of the yiwulushan massif a detailed structural analysis allows us to distinguish several deformation events d2 d3 and d4 the cretaceous extensional structures such as syntectonic plutons bounded by ductile normal faults metamorphic core complexes and half graben basins are recognized in many places in east asia these new data from the yiwulushan massif constitute a link between transbaikalia mongolia north china and south china indicating that nw se extensional mesozoic tectonics occurred throughout the entire region

effects of jet flipping on local scour downstream of a sluice gate abstractthis study shows that a jet flipping phenomenon may occur during the scouring process downstream of a sluice gate the flipping consists of a digging and a filling phase the former scoured the bed and the latter refilled the scour hole the process is cyclical the phases changed intermittently and no equilibrium scour condition was reached the scour depth during digging reached a high of 60 more than the filling phase moreover the scour is more severe with jet flipping compared to no jet flip conditions generally the maximum scour depth for the digging and filling phases increases with the increase in the tailwater depth and densimetric froude number but decreases as the apron length increases the jet flipping frequencies increase with the increase in the densimetric froude number and apron length but decrease as the tailwater elevation increases the coand effect is the most significant factor that triggers the jet flipping phenomenon a review of relevant data from previous studies s

gravity inversion ams and geochronological investigations of syntectonic granitic plutons in the southern part of the variscan french massif central magnetic fabric analyses gravity inversion associated with chemical u th pb dating and structural observations are carried out to elucidate relationships between faulting and magmatic processes this multidisciplinary study has been undertaken on late carboniferous plutons situated in the southern part of the variscan french massif central fmc the glenat omps and boisset plutons crop out on both sides of the crustal scale sillon houiller fault shf the anisotropy of magnetic susceptibility ams measurements and structural observations show that i the plutons acquired their final structure during the magma crystallization and record a nw se maximum stretching trend ii in the boisset pluton post magmatic fabrics predominate with a nnw sse trending lineation the structural pattern deduced from the ams study is thus consistent with the nw se late orogenic extensional tectonic regime that has been documented in other parts of the fmc during late carboniferous the 3d geological modelling refined by 3d gravity inversion does not show any evidence of rooting of the granites along the shf therefore despite the apparent cartographic relationship between the shf and the three plutons our study does not support a genetic link between fault and plutons it also questions the existence of the shf in this part of the massif central at the time of pluton emplacement and emphasises the dominant role of the regional tectonic framework rather than local faulting as a factor controlling pluton emplacement

unusually dumbbell shaped guposhan huashan twin granite plutons in nanling range of south china discussion on their incremental emplacement and growth mechanism abstract the guposhan huashan twin plutons crop out as an unusually dumbbell shaped batholith within the middle mesozoic central nanling range granitic belt in south china the batholith was controlled by the east west striking yishan quannan deep crustal fault zone and spatially associated with two regional anticlinoria both plutons show concentric zonation patterns and are composed of two emplacement sequences separated by an approximately 9 m y magmatic lull the 160 163 ma niumiao lisong wanggao sequence and the 148 151 ma xinlu huamei sequence based on field mapping and observations geochronology and geochemistry in a proposed incremental emplacement and growth model the early magmas exploited physical discontinuities such as the contacts between rocks with contrasting competence and densities as well as saddles of regional scale anticlinoria assisted by dominant vertical heat flow and uplifting of the roofs the niumiao lisong wanggao sequence magmas were subsequently emplaced in an over accretion style as distinguishable increments during an approximately 3 m y span gravimetric data observations of field relationships as well as analysis of finite strain and country rock rheology confirm that the plutons became thickened considerably due to largely vertical assembling and growth and most of their space was created by vertical displacement of their country rocks interestingly the coeval twin plutons show contrasting development and growth modes during their assembling and thickening process the guposhan pluton was built on an outward building mode with a reverse zoning pattern while the huashan pluton on an inward building mode with a normal zoning pattern we propose that the contrasting modes largely resulted from their divergent thermal evolution path for example the incremental inward building mode helps retain thermal energy and maintain a high temperature in older units which facilitates injection of subsequent younger increment units of the same sequence within the older host units the independent newer sequence of xinlu huamei magmas however were controlled by shallow brittle faults developed in the older sequence units and emplaced as stocks and dykes by a dyke propagation mechanism

spatial and temporal pattern of erosion in the three rivers region southeastern tibet convergence and collision between india and eurasia have produced the tibetan plateau which stands 5 km high over a region of 3 million km2 its southeastern margin lies in the restraining bend between the sichuan basin and the eastern himalayan syntaxis in this region three parallel rivers the salween the mekong and the yangtze carve gorges up to 3 km deep along the longitudinal profiles large scale knickzones defined by very high steepness correspond to the gorges of the salween and mekong the yangtze instead has a nearly linear profile upstream from its first big bend new low temperature thermochronometric data reveal a complex pattern of erosion in the three rivers region from the salween in the west to the yangtze in the east the magnitude and rate of erosion decrease from south to north erosion rates exhibit variable gradients in space and time along the salween and the mekong a northward increase of erosion rate is followed by a decrease with additional distance to the north variations of erosion rate in time are characterized by a deceleration along the salween and a general deceleration with local acceleration along the mekong this pattern together with river profile analysis is best explained by active coupling between tectonics and river incision related to the indentation and northward migration of the corner of the indian continent

late jurassic early cretaceous closure of the mongol okhotsk ocean demonstrated by new mesozoic palaeomagnetic results from the trans baikal area se siberia summary we present new palaeomagnetic results from the transba ikal area se siberia from the mongol okhotsk suture zone the boundary between the amuria and siberia blocks in order to better constrain the time of closure of the mongol okhotsk ocean in the mesozoic we col lected 532 rock samples at 68 sites in six localities of basalts trachy basalts and andesites from both sides of the mongol okhotsk suture at unda river j 3 51 7 n 117 4 e kremljevka peak k1 51 8 n 117 5 e and torey lakes k 1 50 1 n 115 9 e on the southern side of the suture and at monostoy river j 1 51 1 n 106 8 e ingoda river k 1 51 2 n 112 2 e and bichura town k 1 50 6 n 107 6 e on the northern side progressive thermal demagnetization enabled us to resolve low ltc and high htc temperature components of magnetization at most sites jurassic palaeopoles computed from the htcs show a large discrepancy with respect to the apparent polar wander path of eurasia which we interpret in terms of 1700 2700 km of post late jurassic northward movement of amuria with respect to siberia although ge ological data suggest a middle jurassic closure of the mongol okhotsk ocean in the west trans baikal region our data give evidence of a large remaining palaeolatitude difference between the amuria and siberia blocks in contrast early cretaceous sites cluster remarkably well along a small circle which is centred on the average site location this implies the absence of post early cretaceous northward motion of amuria relative to siberia and demonstrates the pre early cretaceous closure of the mongol okhotsk ocean finally we interpret the very large tectonic rotations about local vertical axes evidenced by the small circle distribution of poles as arising both from collision processes and from left lateral shear movement along the suture zone due to the eastward extrusion of amuria under the effect of the collision of india into asia

isotopic and paleomagnetic constraints on the mesozoic tectonic evolution of south china in order to better constrain the paleogeographic evolution of south china we measured sm nd and rb sr isotopic compositions for 23 mesozoic granites that crop out throughout the area tightly grouped neodymium depleted mantle model ages 1 4 0 3 ga suggest the region is underlain by relatively homogeneous proterozoic crust and fail to define crustal provinces neither the isotopic nor geologic data suggest that a mesozoic suture exists however granites possessing anomalously high sm 8 ppm and nd 45 ppm concentrations relatively high initial epsilon neodymium 4 to 8 and high but variable initial 87sr 86sr 0 759 to 0 713 form a northeast trending zone that coincides with two prominent mesozoic basins southeast of the zone lie the majority of mesozoic intrusives and upper triassic to lower cretaceous extensional basins found in south china mesozoic paleomagnetic poles are well clustered northwest of the zone pre cretaceous poles southeast of it are discordant with respect to those from the northwest the only recognized tectonostratigraphic terrane in south china lies southeast of the zone the terrane is bordered by a northeast trending sinistral fault that was active in the mesozoic other faults in south china have similar attitudes ages and sense of shear together the observations suggest that the mesozoic tectonic regime in south china consisted of strike slip activity plus concomitant rifting as terranes or fragments of similar crust were transported north along sinistral faults the zone defined by the granites enriched in nd and sm demarcates displaced terranes to the southeast from relatively stable land to the northwest

geochemical and zircon u pb and hf isotopic study of the baijuhuajian metaluminous a type granite extension at 125 100 ma and its tectonic significance for south china abstract the early cretaceous baijuhuajian pluton is an a type granitic intrusion emplaced along the jiangshan shaoxing jssx fault zone in western zhejiang province se south china it intruded into a late jurassic volcanic basin bounded by proterozoic sandstone and siltstone the granite has a coarse grained porphyritic texture and is composed of alkali feldspar phenocrysts in a matrix of k feldspar quartz plagioclase and biotite the pluton has a clear a type geochemical signature e g it is metaluminous a cnk 1 0 and has high sio2 75 to 78 wt total alkalis na2o k2o 4 8 to 8 5 wt rare earth elements total ree 228 to 438 ppm hfse and fe feot feot mgo 0 87 to 0 97 on the other hand it is characterized by low cao 0 51 to 1 01 wt tio2 0 06 to 0 13 wt p2o5 below detection and sr 7 0 to 9 5 ppm the hypersolvus texture and fine grained groundmass indicate emplacement at a shallow depth shrimp u pb zircon dating yielded a weighted mean 206pb 238u age of 126 3 ma and zircons from this pluton give ehf t values between 0 52 and 4 24 suggesting the importance of juvenile material in the magma source most of the plutonic rocks in southeast china are calc alkaline subduction related intrusions of jurassic and cretaceous age previous studies suggested that the sparse a type granites in south china were emplaced in two episodes 190 155 ma in the inland area and 100 ma along the coastal area in se south china our study of the baijuhuajian granite together with the coeval suzhou a type granite 123 ma in jiangsu province defines an important extensional event between 125 and 100 ma considering the available data for the mesozoic igneous rocks in south china we suggest that subduction of the pacific plate in the mesozoic produced voluminous calc alkaline granitoids and volcanic rocks with a younging trend from west to east the a type granites were generated during periods of local extension at 190 155 ma and 125 100 ma perhaps corresponding to localized rollback of the pacific plate

late mesozoic compressional to extensional tectonics in the yiwulushan massif ne china and their bearing on the yinshan yanshan orogenic belt part ii anisotropy of magnetic susceptibility and gravity modeling abstract granitoids play an important role in deciphering both crustal growth and tectonic evolution of earth in the eastern end of the yinshan yanshan belt of north china craton the yiwulushan massif is a typical region that presents the tectonic evolution features of this belt our field work on the host rocks has demonstrated two phases of opposite tectonics compressional and extensional however the deformation is almost invisible in the intrusive rocks to improve the understanding of the tectonic evolution of the yiwulushan massif and the late mesozoic tectonics of east asia a multidisciplinary study has been carried out in this study anisotropy of magnetic susceptibility ams and gravity modeling have been applied on these jurassic plutons lushan jishilazi and guanyindong which intrude into the yiwulushan massif according to laboratory measurements and microscopic observations on thin sections the ams of the yiwulushan massif is characterized by secondary fabrics indicating that the intensive post solidus deformation has reset the primary magmatic magnetic fabrics a relatively gentle nw dipping magnetic foliation has been identified with two distinct groups of magnetic lineations of n34 e and n335 e orientations namely lm1 and lm2 relatively gravity modeling reveals a southward thinning of the massif with a possible feeding zone rooted in the northern part of the massif integrating all results from structural observation geochronological investigation ams measurement and gravity modeling two tectonic phases have been identified in the yiwulushan massif posterior to the jurassic 180 160 ma magmatism in the yinshan yanshan area the early one concerns a late jurassic early cretaceous 141 ma compressional event with a top to the south to southwest sense of shear the second one shows an early cretaceous 126 ma nw se ductile extensional shearing at that time sedimentary basins widened and jurassic plutons started to be deformed under post solidus conditions in fact the nw se trend of the maximum stretching direction is a general feature of east asian continent during late mesozoic

granite laccolith emplacement in the iberian arc ams and gravity study of the la tojiza pluton nw spain a laccolithic geometry is inferred from geological and geophysical data for the la tojiza granitic body emplaced in an extensional regime nearly circular in outcrop this 16 km diameter pluton one of the so called post tectonic granodioritic plutons of galicia is located in the northern variscan arc of iberia within the mondonedo nappe western asturian leonese zone from anisotropy of magnetic susceptibility measurements and extensive microstructural data the magnetic fabric of this pluton has been inferred to correlate with the magmatic fabric magnetic foliations strike ne sw and dip gently se magnetic lineations trend nw se and plunge gently se gravity data suggest that the pluton has the shape of a sheet with a maximum preserved thickness of 1 km and a root zone located at its eastern border this pluton is reinterpreted as late kinematic and emplaced during an extensional tectonic regime related to the ductile vivero normal fault it is suggested that the emplacement of most post tectonic plutons of this part of the variscan orogenic belt may have been controlled by the late extensional collapse of the iberian massif

zircon u pb geochronology and hf isotopic compositions of the mesozoic granites in southern anhui province china abstract mesozoic magmatism is extensively developed in eastern china along the eastern asian continental margin but the geochronological framework is not well understood for each area in this study seventy eight samples were collected from the mesozoic igneous rocks in south anhui province east central china in order to precisely constrain their formation ages and contributions to crustal growth zircon u pb age determination using ion probe and laser ablation techniques indicate that magmatism in the area can be divided into two separate stages at 150 136 and 136 120 ma respectively the first stage of granitic rocks not documented in other areas of eastern china is mostly composed of hornblende bearing i type granodiorite and monzogranite and is considered to be formed by slab tearing during pacific subduction the second stage of magmatism coeval to that in other areas of eastern china is characterized by the occurrence of a type granite and syenite and their volcanic counterparts although i type granitic rocks are also developed locally these rocks are mostly distributed around and within the fault controlled volcanic basins and some are closely related to detachment shearing indicating their formation in an extensional setting zircon hf isotopic analyses indicate that archean material might be present in the area but that the main episode of crustal growth took place during the mesoproterozoic with very little juvenile material in the mesozoic

mesozoic tectonic evolution of the southeast china block new insights from basin analysis abstract in order to better understand the mesozoic tectonic evolution of southeast china block secb in short this paper describes geological features of mesozoic basins that are widely distributed in the secb the analyzed data are derived from a regional geological investigation on various mesozoic basins and a recently compiled 1 1 500 000 geological map of mesozoic cenozoic basins two types of basin are distinguished according to their tectonic settings namely the post orogenic basin type i and the intracontinental extensional basin type ii the latter includes the graben and the half graben or faulted depression basins our studies suggest that the formation of these basins connects with the evolution of geotectonics of the secb the post orogenic basin type i was formed in areas from the piedmont to the intraland during the interval from late triassic to early jurassic and the formation of the intracontinental extensional basin type ii connects with an intracontinental crustal thinning setting in the late mesozoic the graben basin was generated during the middle jurassic and is associated with a bimodal volcanic eruption and the half graben or faulted depression basin filled mainly by the rhyolite tuff and sedimentary rocks during early cretaceous is occupied by the late cretaceous paleogene red colored terrestrial clastic rocks we noticed that the modern outcrops of numerous granites and basins occur in a similar level and the mesozoic granitic bodies contact with the adjacent basins by large normal faults suggesting that the modern landforms between granites and basins were yielded by the late crustal movement the modern basin and range framework was settled down in the cretaceous abundant sedimentary structures are found in the various basins from that the deposited environments and paleo currents are concluded during the late triassic early jurassic time the source areas were situated to the north and northeast sides of the outcrop region in this paper we present the study results on one geological and geographical separating unit and two separating fault zones the wuyi orogenic belt is a late mesozoic paleo geographically separating unit the ganjiang fault zone behaves as the western boundary of early cretaceous volcanic rocks and the zhenghe dapu fault zone separates the se china coastal late mesozoic volcanic sedimentary basins and the wuyi orogenic belt finally we discuss the geodynamic mechanisms forming various basins proposing a three stage model of the mesozoic sedimentary evolution

a short lived but significant mongol okhotsk collisional orogeny in latest jurassic earliest cretaceous abstract the central asian orogenic belt caob is one of the largest orogenic collages on earth which records a long history of accretionary orogenesis from the late proterozoic to the mesozoic although paleomagnetic studies have unanimously suggested that the major mongol okhotsk ocean in the eastern caob closed mainly during the late jurassic early cretaceous few conclusive and self consistent geological studies in central asia and east asia have confirmed the occurrence of this significant collisional orogeny based mainly on basin evolution in the caob tarim block north china block and qaidam qialian block during the late jurassic early cretaceous the present study proposes that a short lived but significant mongol okhotsk collisional event occurred in the latest jurassic earliest cretaceous the rapid closure of the mongol okhotsk ocean was possibly related to the clockwise rotation of siberia which was driven by the continental collision between the siberia craton and the kolyma omolon microcontinent in northeast russia in the late jurassic owing to the clockwise rotation of siberia and the subsequent closure of the mongol okhotsk ocean a number of areas to both the east and the west of the mongol okhotsk suture underwent uplift and erosion including lake baikal transbaikalia sayan ranges altai shan junggar basin turfan basin tarim basin tian shan mohe basin great xing an ranges hailar tamsag basin erlian basin songliao basin east gobi basin yingen basin bei shan yin shan yan shan belt ordos basin chaoshui yabulai basin and northern qaidam basin forming a giant fold and thrust belt in northern china and mongolia the collision probably caused a prominent gap between late jurassic and early cretaceous magmatic episodes in the eastern caob and north china block after lasting for about 10 million years the main portion of the mongol okhotsk ocean closed forming a northeast trending suture during the early cretaceous a remnant ocean still remained in the easternmost part of the mongol okhotsk ocean and the regional compressional setting in east asia related to the closure of the mongol okhotsk ocean changed to escape tectonics driven by the collision of the lhasa block at the southern margin of asia strike slip faulting triggered gravitational collapse of upper crust and delamination of lower crust and mantle lithosphere that was thickened during the mongol okhotsk collision resulting in the development of a continental scale extensional setting in east asia associated with rift basins metamorphic core complexes and igneous rocks

metamorphic core complexes vs synkinematic plutons in continental extension setting insights from key structures shandong province eastern china abstract continental extension is an important geodynamical process mostly diagnostic of a peculiar behaviour of the crust accommodated by geological structures that highlight these specific conditions in the crust though a metamorphic core complex mcc reveals a much stronger crustal extension than a synkinematic pluton the nature of those two structures implies different implications in terms of crustal extension mechanisms and geodynamic significations in eastern asia a major continental extensional event occurred during mesozoic and early cenozoic times the resulting various extensional events described in previous studies consist in large intracontinental basins important volcanism emplacement of plutons in the upper crust and exhumation of mccs an efficient description of mccs and plutons in eastern asia is essential to discriminate important differences of those two structures in terms of strain amount undergone by continental crust and geodynamic significations in that way an integrated structural and geophysical study has been realized to precisely discriminate the structure in the jiaodong peninsula shandong province eastern china previously regarded as simple sheared plutons a succession of three main stages have been identified all pertaining to a nw se extensional setting 1 the exhumation of the late jurassic early cretaceous linglong mcc below the se dipping linglong detachment fault 2 the emplacement of the guojialing syntectonic pluton below the n dipping extensional guojialing intracrustal shear zone 130 124 ma and 3 a penetrative brittle normal faulting associated with gold mineralizations 120 ma as a result the maximum amount of extension in jiaodong peninsula characterized by partially melted lower to middle crust upward into the linglong mcc should be revised to late jurassic early cretaceous period

is the jurassic yanshanian intraplate tectonics of north china due to westward indentation of the north china block terra nova 24 333 338 2012 abstract the northern mountains of beijing are the type locality of the mesozoic yanshanian orogen our structural study emphasizes the importance of dextral strike slip for the formation of this intracontinental belt the south and north directed thrusts are positive flower structures rooted in strike slip faults this transpressional tectonics developed from late triassic to late jurassic early cretaceous through three deformation phases coeval with syntectonic sedimentation separated by two transtensional episodes coeval with magmatism the late jurassic early cretaceous tectonic event is also recognized in several places of north china strike slip faulting controls the deformation of the northern border of the north china block in the yanshan yinshan belt simultaneously east directed thrusts and folds develop along n s elongated ranges in helanshan shanxi highlands and taihangshan the jurassic tectonics of the north china block is interpreted in a unitary way as the result of the westward underthrusting of the north china block below the alashan block

intracontinental deformation in a frontier of super convergence a perspective on the tectonic milieu of the south china block abstract since mesozoic the south china region has been located at the center of a triangular area surrounded by westward subduction of the pacific plate northward subduction of the india plate beneath the eurasia plate and collision of the north and south china blocks along the central china orogen this region thus marks the frontier of a super convergent regime within the super convergence domain the compressional structures in the center of the south china block are mainly characterized by shortening thrusting and decollement the block underwent inhomogeneous rejuvenation of the pre existing crust and lithospheric structures through reactivation of late stage activities the surface deformation of the xuefeng intracontinental tectonic system within the south china block is possibly derived from intraplate tectonics particularly the distinct magmatism between the west and east limbs of the xuefeng precambrian uplift is a possible response of the rheological structure of the lithosphere the deep structures as revealed from tomographic studies show marked difference of lithosphere architecture between the eastern and western sub blocks of the south china block these data illustrate the long term mosaic of multi block convergence which led to present day inhomogeneity in the continental lithosphere in south china a comparison of the intensive contraction of the south china block with the distinct features of rifting of the north china block brings out the contrasting structural and tectonic signatures developed in the same frontier of one of the largest super convergent systems on the globe during the mesozoic to cenozoic

cretaceous tectonic evolution of south china a preliminary synthesis abstract the cretaceous tectonic evolution of south china is characterized by widespread extensional basin and dome generation voluminous magma intrusion eruption and associated polymetallic mineralization all of which are of world wide interests that have stimulated the attention of geologists for more than half a century due to the lack of a comprehensive understanding of regional tectonic evolution and geodynamics of south china many controversies regarding the nature and origin of these features remain this paper attempts to make a review by synthesizing existing structural petrological geochronological and geochemical data of the cretaceous structures and magmas which guides us to propose a three stage tectono thermal evolutionary history of south china during the cretaceous period the earliest cretaceous 145 137 ma tectonic stage was characterized by syn orogenic shortening deformation and metamorphism under a nw se compressional setting which generated voluminous porphyry cu au ore bearing adakitic rocks in the lower yangtze river belt and gneissic granites in the coastal area its geodynamic origin was interpreted as combined effects of the pacific izanagi ridge subduction beneath the lower yangtze river belt and the pacific subduction beneath the cathaysia block this syn orogenic shortening stage was followed by a post orogenic stage 136 86 ma comprising two episodes of alternate extensional and shortening events the extension 136 118 ma in the earlier episode was dominated by a nw se extensional regime it led to significant taphrogenesis manifested by large scale extensional basins and voluminous magma intrusion eruption this extensional event was associated with a combination of the slab window opening during the ridge subduction and the rollback of the subducted pacific slab the subsequent nw se transpressional event led to cessation of active marginal magmatism and resulted in tectonic inversion of previous rift basins this transpression lasted from 117 ma to 108 ma and its driving mechanism was attributed to the collision between the eastern asian margin and the west philippine block the extension 107 86 ma in the later episode was dominated by a wnw ese extensional regime which led to a second phase of basin subsidence and produced numerous a and i type granites and bimodal volcanoes the tectonic regime then changed at ca 85 ma to wnw ese oriented transpression causing inversion of the late cretaceous rift basins and cessation of extension related magmatism the change of stress field from wnw ese extension to wnw ese transpression was related to the variations of the subducted slab dynamics i e the transition from either the ese ward retreat to wnw ward subduction of the pacific slab or the alternate stress during slab break off during the latest cretaceous a drastic change of regional tectonic stress orientation from wnw ese to n s occurred in south china the third stage predominated by n s extension was developed which caused a new phase of regional crustal subsidence along the youngest e w trending extensional structures

u pb zircon geochemical and sr nd hf isotopic constraints on age and origin of jurassic i and a type granites from central guangdong se china a major igneous event in response to foundering of a subducted flat slab abstract the mesozoic geology of se china is characterized by widespread jurassic to cretaceous igneous rocks consisting predominantly of granites and rhyolites and subordinate mafic lithologies however the tectonic regime responsible for the inland jurassic granites remains controversial we report here u pb zircon ages geochemical and sr nd hf isotopic data for the nankunshan alkaline granite and the fogang granitic batholith in central guangdong mineralogical and geochemical features suggest that the fogang and nankunshan rocks are i and aluminous a type granites respectively shrimp u pb zircon analyses yield consistent ages ranging from 159 3 ma to 165 2 ma for four samples from the fogang batholith and an age of 158 5 ma for the nankunshan granite the fogang granites having i sr 0 7098 0 7136 e nd t 4 3 to 12 2 and e hf t 11 5 to 3 1 for the magmatic zircons were derived from paleoproterozoic mafic intermediate igneous protolith with minor addition of mantle derived magmas the nankunshan rocks have relatively lower i sr 0 706 0 708 higher e nd t 0 3 to 2 4 and e hf t 5 7 to 1 1 for the magmatic zircons and some oib like trace element ratios they were likely generated through extensive fractional crystallization of mantle derived alkaline parental magma associated with crustal assimilation these 160 ma i and a type granites in central guangdong were emplaced coeval with the widespread 165 155 ma i and a type granites and syenites following the initiation of intraplate basaltic and or bimodal igneous magmatism at 180 170 ma in the adjacent regions we interpret these jurassic igneous rocks as anorogenic magmatism formed during a major igneous event in response to foundering of an early mesozoic subducted flat slab beneath se china continent

an early extensional event of the south china block during the late mesozoic recorded by the emplacement of the late jurassic syntectonic hengshan composite granitic massif hunan se china continental scaled extension is the major late mesozoic jurassic and cretaceous tectonic event in east asia characterized by faulting magmatic intrusions and half grabens in an area with a length of 5000 km and a width of 1000 km numerous studies have been conducted on this topic in the south china block scb however the space and time ranges of the compressional or extensional regimes of the scb during the jurassic are still unclear partly due to the lack of structural data the emplacement fabrics of granitic plutons can help determine the regional tectonic background in this study a multidisciplinary approach including anisotropy of magnetic susceptibility ams macro and micro structural analyses quartz c axis preferred orientation gravity modeling and monazite epma dating was conducted on the hengshan composite granitic massif in scb that consists of the triassic nanyue biotite granitic pluton and the late jurassic baishifeng two mica granitic pluton the magnetic fabrics are characterized by a consistent nw se oriented lineation and weakly inclined foliation a dominant high temperature deformation with a top to the nw shear sense is identified for both plutons the deformation increasing from the center of the baishifeng pluton to its western border is associated to the development of the west hengshan boundary fault whbf the gravity modeling shows a saw tooth shaped ne sw oriented structure of the baishifeng pluton which may be considered as ne sw oriented tension gashes formed due to the nw se extension all results show that the triassic nanyue pluton was deformed under post solidus conditions by the whbf coeval with the emplacement of the late jurassic baishifeng pluton all these observations comply with the nw se extensional tectonics coeval with the emplacement of the baishifeng pluton which argues that the nw se crustal stretching started since the late jurassic at least in this part of the scb