subduction zone trench migration slab driven or overriding plate driven abstract subduction zones on earth and their associated trenches and hinges migrate with respect to the overriding plate as indicated by overriding plate deformation e g backarc extension or backarc shortening and migrate with respect to hotspot and no net rotation reference frames global or absolute reference frames three geodynamic models exist that attempt to explain how absolute trench migration velocity correlates with overriding plate deformation in one model trench and hinge migration result from lateral migration of the slab with trench retreat causing overriding plate extension and trench advance causing overriding plate shortening in the second model trench migration is forced by the overriding plate with trenchward motion causing trench retreat and overriding plate shortening and motion away from the trench causing trench advance and overriding plate extension in the third model the trench and subduction hinge are thought to be static while overriding plate extension shortening is accommodated by landward trenchward overriding plate motion in this paper the conflicting geodynamic predictions made by these three models are tested by using global kinematic calculations of trench migration velocity v t overriding plate velocity v op and overriding plate deformation in different global reference frames the dependence between v t and overriding plate deformation as well as v op and overriding plate deformation was investigated correlation coefficients r and confidence limits were calculated using a quantitative approach with overriding plate deformation velocity v opd using a semi quantitative approach with an overriding plate strain classification approach and using a spearman rank correlation approach for the quantitative approach the correlation between v t and v opd is consistently positive r 0 33 0 68 where trench retreat corresponds to extension and statistically significant at 95 confidence level for all but one reference frame for v op and v opd the correlation is consistently negative but much less significant r 0 29 to 0 17 and statistically not significant at 95 confidence level for all reference frames these findings indicate that trench migration results predominantly from lateral migration of the slab rather than from overriding plate motion such lateral slab migration is most likely driven by the negative buoyancy force of the slab itself as demonstrated by geodynamic models

quartz recrystallization regimes c axis texture transitions and fluid inclusion reequilibration in a prograde greenschist to amphibolite facies mylonite zone ribeira shear zone se brazil abstract the crystal plastic behavior of quartz mylonites from the ribeira shear zone se brazil a major strike slip structure that was active during a prograde metamorphic phase related to the neoproterozoic brasiliano pan african orogeny was investigated using a multi method approach geothermobarometry results indicate deformational conditions ranging from 300 to 630 c and 500 700 mpa a strong correlation between mapped metamorphic zones and a dominance of different dynamic recrystallization mechanisms of quartz occurs within the mylonite zone bulging recrystallization blg dominates within the chlorite zone between 300 and 410 c subgrain rotation recrystallization sgr operates within the biotite zone from 410 to 520 c and grain boundary migration recrystallization gbm dominates in the garnet zone above 520 c the development of quartz c axis textures is mainly governed by temperature and dynamic recrystallization mechanisms textures from blg zone mylonites are characterized by maxima around z sgr zone mylonites display single girdles or asymmetric type i crossed girdles and gbm zone mylonites comprise maxima around y and intermediate between x and z the scarcity or absence of water bearing fluid inclusions in quartz mylonites from the sgr and gbm zones which are dominated by carbonic inclusions suggests water deficient conditions whereas blg zone mylonites are dominated by water bearing inclusions this evidence indicates that water was available in the protoliths but has been eliminated with increasing deformation and deformation temperature no effect of the water content variation on the quartz microstructural and recrystallized grain size evolution was detected and little influence on c axis texture development was observed most of the fluid inclusion densities were reequilibrated during the shear zone exhumation history recording a decompression in the range of 300 500 mpa while microstructural reequilibration effects related to the prograde metamorphism are largely preserved fluid inclusion microstructures and densities from two sgr zone samples preserved evidence for a near isothermal compression within the interior of the ribeira shear zone during the prograde metamorphism

provenance and tectonic setting of the paleoproterozoic metasedimentary rocks from the liaohe group jiao liao ji belt north china craton insights from detrital zircon u pb geochronology whole rock sm nd isotopes and geochemistry abstract geochronological geochemical and isotopic studies were carried out on the paleoproterozoic metasedimentary rocks from the jiao liao ji belt north china craton to constrain their provenance and tectonic setting as well as to evaluate the effects of weathering and sedimentary processes on the source rock signature u pb isotopic dating using the la icp ms method on zircons from the paleoproterozoic metasedimentary rocks reveals that the depositional age can be constrained in the period between 1 98 and 1 9 ga geochemical data reveal low intermediate degrees of weathering of the source compositionally immature and poorly sorted detailed analysis of detrital zircon u pb geochronology whole rock sm nd isotope and geochemistry show that these metasedimentary rocks are derived from a mixed provenance the predominant derivation is from metavolcanic rocks of the liaohe group and paleoproterozoic gneissic granitoids there is also an input of archean continental crust the metasedimentary rocks were probably deposited in a back arc basin within an arc system thus supporting the paleoproterozoic arc continent collision model as already proposed by some authors

reconstruction of northeast asian deformation integrated with western pacific plate subduction since 200 ma abstract the configuration and kinematics of continental deformation and its marginal plate tectonics on the earth s surface are intrinsic manifestations of plate mantle coupling the complex interactions of plate boundary forces result in plate motions that are dominated by slab pull and ridge push forces and the effects of mantle drag these interactions also result in continental deformation with a complex basin mountain architecture and evolution the kinematics and evolution of the western pacific subduction and northeast asian continental margin deformation represent a first order tectonic process whose nature and chronology remains controversial this paper implements a deep time reconstruction of the western pacific subduction continental accretion or collision and basin mountain deformation in northeast asia since 200 ma based on a newly revised global plate model we use gplates software to examine strain recovery geological and seismic tomography constraints for the western pacific plate subduction and sequentially backward rotations of deforming features the results indicate a nw se oriented shortening from 200 to 137 ma a nww see oriented extension from 136 to 101 ma a nearly n s oriented extension and uplift with a short term nww see oriented compressional inversion in northeast china from 100 to 67 ma and a nw se and nearly n s oriented extension from 66 ma to the present day the western pacific oceanic plate subducted forward under east asia along mudanjiang honshu island during the jurassic and the trenches retreated to the sikhote alin north shimanto and south shimanto zones from ca 137 128 ma ca 130 90 ma and in ca 60 ma respectively our time dependent analysis of plate motion and continental deformation coupling suggests that the multi plate convergent motion and ocean continent convergent orogeny were induced by advance subduction during the jurassic and earliest cretaceous our analysis also indicates that intra continent rifting and back arc extension were triggered by trench retreat during the cretaceous and that the subduction of the oceanic ridge and arc were triggered by trench retreat during the cenozoic therefore reconstructing the history of plate motion and subduction and tracing the geological and deformation records in continents play a significant role in revealing the effects of complex plate motions and the interactions of plate boundary forces on plate mantle coupling and plate motion intracontinental deformation coupling

formation and evolution of the tancheng lujiang wrench fault system a major shear system to the northwest of the pacific ocean abstract in the northwestern peri pacific tectonic belt on the continental margin of east asia there is a series of major faults trending ne nne i e the tancheng lujiang wrench fault system this fault system extends more than 5000 km with a width of 800 1000 km in the continent and a general strike of n30 e on the basis of the results of special research on the major faults and a lot of new information about the fault system the authors discuss early mesozoic triassic jurassic period sinistral transcurrent faulting in the different regions it is suggested that in the eastern continent of east asia the e w trending tectonic pattern of the paleozoic and beginning of the mesozoic appear in the nuclei of the sino korean continent massif and the yangtze continent massif these ancient continent massifs accreted both southwards and northwards which finally led to their collision and suturing to the angura continent massif later this tectonic framework was changed greatly by the tancheng lujiang wrench fault system along which left lateral horizontal displacement of 1000 1500 km took place since the mesozoic the dislocation and deformation of horizontal displacement of the tancheng lujiang fault system produced a series of structures trending ne nne in the continental margin which have greatly transformed the ancient structures of the preceding mesozoic this is an important diktyogenese in the continent of east asia during the horizontal displacement motion of these faults the folds nappes and decollement structures pull apart basins and subductions within the continent occurred in the continent at the same time some large sinistral ductile shear zones of horizontal displacement appeared the depth of the deformed horizon of ductile shear which belongs to simple shear was controlled by the tectonic element and distance away from the continental margin whilst the ductile shear deformation occurred the fault dynamo thermal metamorphism and the fault remelting took place and magma activity appeared within the continent in the mesozoic the fault zones used to be high energy zones and recently became the seismic belts within the continent formation of the tan lu fault system was due to the collision between the southern and northern blocks its large sinistral horizontal displacement in the mesozoic was closely related to the kula plate moving northwards therefore the continent disintegrated by shearing which produced some new terranes the shear edge belt was formed between the continent and ocean the change of relative motion between the continent and pacific ocean plate transformed the tan lu fault system into first extension and then compression clarifying the characteristics of the tan lu fault system is an important key in understanding the evolution of the east asia continent since the mesozoic

flat slab subduction trench suction and craton destruction comparison of the north china wyoming and brazilian cratons abstract the mechanisms of growth and destruction of continental lithosphere have been long debated we define and test a unifying plate tectonic driving mechanism that explains the numerous petrological geophysical and geological features that characterize the destruction of cratonic lithospheric roots data from three archean cratons demonstrate that loss of their roots is related to rollback of subducted flat slabs some along the mantle transition zone beneath the cratons during flat slab subduction dehydration reactions add water to the overlying mantle wedge as the subducting slabs roll back they suck in mantle material to infill the void space created by the slab roll back and this fertile mantle becomes hydrated the roll back causes concomitant lithospheric thinning of the overlying craton so the flux of newly hydrated mantle material inevitably rises causing adiabatic melting generating new magmas that gradually destroy the roots of the overlying craton through melt peridotite reactions calculated fluxes of new mantle material beneath cratons that have lost their roots range from 2 7 trillion to 70 million cubic kilometers which is sufficient to generate enough melt to completely replace the affected parts of the destroyed cratons cratonic lithosphere may be destroyed in massive quantities through this mechanism warranting a re evaluation of continental growth rates with time

formation of the wulong gold deposit liaodong gold province ne china constraints from zircon u pb age sericite ar ar age and h o s he isotopes abstract the wulong deposit is the largest quartz vein type gold deposit in the eastern liaoning province of china the gold ore bodies are hosted by mesozoic biotite granites the mineralization occurs as veins with lenticular shapes laser ablation icp ms zircon u pb dating was used to date the biotite granite granite porphyry and fine grained diorites at the wulong deposit yielding ages of 155 4 0 9 ma 154 2 1 2 ma and 123 1 0 9 ma respectively a sericite sample from the gold ore yields a 40ar 39ar plateau age of 122 8 0 8 ma mswd 0 09 the calculated 18ow values for the quartz range from 4 8 to 8 6 and the dw values of fluid inclusion waters in quartz vary from 98 to 77 the h o isotope data indicates that ore forming fluids could be of a magmatic origin the 34s values of metallic sulfides range from 1 1 to 2 4 with an average of 1 8 suggesting that the sulfur is derived from a magmatic source the 3he 4he ratios of fluid inclusions in pyrite range from 0 36 to 0 65 ra with an average of 0 50 ra the mantle helium involved in the ore forming fluid ranges from 3 9 to 7 2 indicating a crustal fluid source the wulong gold deposit was formed at ca 120 ma and it has a genetic association with contemporaneous fine grained diorite the wulong deposit was formed during lithospheric thinning of the eastern north china craton during the early cretaceous

the xiaoqinling metamorphic core complex a record of early cretaceous backarc extension along the southern part of the north china craton

a review of the western part of the altaids a key to understanding the architecture of accretionary orogens abstract the altaids is one of the largest accretionary collages in the world and the tectonic styles of the accretionary processes have been interpreted in several ways including as an amalgamation of multiple terranes as a result of oroclinal bending of a long single arc or as a caledonian continental collision based on recent tectonostratigraphic analyses together with paleomagnetic data the tectonic styles of the neoproterozoic to paleozoic accretionary processes of the altaids are discussed the western altaids is the main focus of the study which was mainly composed of several independent linear components such as arcs and microcontinents with proterozoic basement and cover rocks various kinds of arcs existed in the paleo asian ocean including a complicated type of arc alaskan type which is a combination of the japan and mariana type intra oceanic arcs and the cordillera type continental arcs these linear components rotated and collided with each other with multiple subduction polarities which could have been an important result of multiple linear element amalgamation and which has contributed greatly to the architecture of the eurasian continent the basic tectonic styles of the altaids can be summarized as arc arc collision oroclinal bending and large scale rotation and multiple subductions with a complicated archipelago paleogeography these basic features of accretionary orogens in general can be attributed to the amalgamation of complicated multiple linear elements some mesozoic to cenozoic accretionary orogens in the world are also characterized by processes of multiple linear element amalgamation more attention should be paid to the multiple linear element amalgamation of ancient accretionary orogens which will shed light on lateral and vertical continental growth

analysis of fault slip inversions do they constrain stress or strain rate fault slip data commonly are used to infer the orientations and relative magnitudes of either the principal stresses or the principal strain rates which are not necessarily parallel or equal at the local scale of an individual fault the shear plane and slip direction define the orientations of the local principal strain rate axes but not in general the local principal stress axes at a large scale the orientations of p and t axes maxima for sets of fault slip data do not provide accurate inversion solutions for either strain rate or stress the quantitative inversion of such fault slip data however provides direct constraints on the orientations and relative magnitudes of the global principal strain rates to interpret the inversion solution as constraining the global principal stresses requires that 1 the fault slip pattern must have a characteristic symmetry no lower than orthorhombic 2 the material must be mechanically isotropic and 3 there must be a linear constitutive relationship between the global stress and the global strain rate isotropic linear elastic constitutive equations are appropriate to describe the local deformation surrounding an individual slip discontinuity fault slip inversions however constrain the characteristics of a large scale cataclastic flow which is described by constitutive equations that are probably but to an unknown degree anisotropic and nonlinear such material behavior would not strictly satisfy the requirements for the stress interpretation thus at the present state of knowledge fault slip inversion solutions are most reliably interpreted as constraining the principal strain rates

triassic southeastward subduction of north china block to south china block insights from new geological geophysical and geochemical data abstract lithospheric subduction prior to the assembly of the south china and north china blocks is traditionally considered to be directed northward however some critical geological and geochemical data cannot be reconciled with this northward subduction this paper presents new lines of evidence against the traditional models and proposes a new and revolutionary tectonic model to explain the distribution and exhumation of high pressure hp ultrahigh pressure uhp metamorphic rocks of the dabie sulu belt we emphasize the following 1 the triassic tectonic environment of the southern margin of the north china block was passive not active based on the stratigraphy 2 in the southern margin of the north china block no arc magmatism was recorded 3 many paleoproterozoic slices of jiaobei affinity of the jiao liao ji belt in the north china block were located in the triassic sulu orogen 4 many 1 85 ga metamorphic zircons are preserved in the dabie sulu high pressure ultra high pressure hp uhp metamorphic rocks 5 the geometric asymmetry of many structural patterns in the hp uhp slices indicates top to the northwest thrusting during the exhumation of hp uhp slices 6 blueschists occur in the south of the uhp eclogite slices 7 in the eastern segment of the north qinling orogen no components with an affinity of the south china block have been found along the shangdan suture of the qinling orogen has been recorded an apparent northward subduction we consider that the suture is just a lateral subduction zone rather than a major collisional zone along the shangdan suture the rarity of i type plutonism can be attributed to a transform type continental margin the bureya jiamusi khanka block has an affinity to the south china block based on its similarity regarding the paleozoic history of deformation and triassic blueschist metamorphic facies metamorphism the bureya jiamusi khanka block could be the northern extension of the dabie sulu belt and this gigantic belt could be interpreted as an orocline related to the southeastward subduction of the north china block beneath the greater south china block

out of sequence thrusting in polycyclic thrust belts an example from the mesozoic yanshan belt north china craton the ew trending yanshan belt an intraplate fold thrust belt located in the northern north china craton that has experienced several episodes of deformation widely separated in time is characterized by out of sequence thrusts according to detailed mapping in the central yanshan belt five geometric and stratigraphic criteria used to aid in determining whether a thrust has an out of sequence geometry or not can be recognized they are 1 unconformable relationships 2 inclination of fault surfaces 3 irregular changes in apparent offset along strike 4 short fault length relative to apparent offset and 5 in sequence geometry with the help of these criteria two generations of out of sequence thrusts that postdate the original in sequence thrusting in the central yanshan belt are recognized the ancestral southward verging fold and thrust belt that formed prior to 180 ma was deformed and cut by two younger generations of faults that are probably more deeply rooted and are constrained to between 172 165 ma and 152 135 ma a series of thrusts with opposite vergence formed during the last period resulting in abundant abnormal field relationships such as younger on older thrust relations fold truncation and cutting down section the nature and occurrence of faults in the yanshan belt implies that superimposed deformation a common feature in polycyclic orogenic belts is a mechanism for the generation of out of sequence thrusting this adds to mechanisms already described in the literature such as maintaining constant critical taper at an orogenic scale inhibition of the deformation front and lateral changes in the nature of the decollement horizons

the enigmatic yinshan fold and thrust belt of northern china new views on its intraplate contractional styles the east to east northeast trending yinshan belt lies within north china extending westward at least 1100 km from china s eastern coast to inner mongolia this intraplate jurassic cretaceous belt underwent contractional and normal faulting folding and contemporaneous terrestrial sedimentation and magmatism current views on its contractional deformational style favor relatively limited thick skinned faulting of archean basement and cover units these views are challenged however by recent discoveries in the eastern part of the belt of south directed ductile nappe formation and large displacement 40 45 km thin skinned northward thrust faulting both involving archean and younger rock units collision of the siberian and north china plates upon closure of a jurassic and early cretaceous mongolo okhotsk ocean more than 800 1100 km to the north may have been responsible for yinshan north south contraction some patterns of contraction e g jurassic cretaceous ductile nappe formation appear to have been influenced by a superposed magmatic regime related to westward subduction of a pacific basin plate beneath the north china plate

mesozoic extensional structures of the fangshan tectonic dome and their subsequent reworking during collisional accretion of the north china block the fangshan area sw of beijing lies at the junction of the nne trending taihang mountain range and the southeastern portion of the yanshan intraplate orogenic belt and has undergone at least five stages of deformation mid to late triassic extensional deformation d 1 is represented by the formation of the fangshan tectonic dome during se directed extensional tectonics this deformation was later modified by nnw directed thrusting in the late triassic d 2 and wnw directed thrusting in the late jurassic d 3 these d 1 d 3 structures were deformed by the arcuate beiling syncline d 4 which formed during the intrusion of the 133 128 ma fangshan pluton d 5 deformation is represented by cretaceous to quaternary nne striking high angle normal faults the ages of these events demonstrate that the principal ene trending tectonic framework of the yanshan intraplate orogenic belt was established mainly in the mid to late triassic the indosinian event the jurassic to cretaceous yanshanian deformation with thick skinned thrusting was associated with intrusion of numerous plutons and strongly modified the older structures the tectonic evolution of the fangshan tectonic dome was linked genetically with the collision of the north china block and the siberian craton and the later collision of the north and south china blocks

influence of the subducting plate velocity on the geometry of the slab and migration of the subduction hinge abstract geological observations indicate that along two active continental margins east asia and mediterranean major phases of overriding plate extension resulting from subduction hinge retreat occurred synchronously with a reduction in subducting plate velocity in this paper results of fluid dynamical experiments are presented to test the influence of the velocity of the subducting plate on the hinge migration velocity and on the geometry of the slab results show that hinge retreat decreases with increasing subducting plate velocity in addition phases of hinge retreat alternate with phases of hinge advance for relatively high subducting plate velocities due to interaction of the slab with the bottom of the box simulating the upper lower mantle discontinuity such slab kinematics could explain the episodic behaviour of back arc opening observed in convergent settings the geometry of the slab and the kinematics of subduction are significantly affected by the velocity of the subducting plate three subduction modes with accompanying slab geometry can be recognized a relatively low subducting plate velocity is accompanied by relatively fast hinge retreat with backward sinking of the slab and a backward draping slab geometry with increasing subducting plate velocity hinge migration is relatively small resulting in subvertical sinking of the slab and a folded slab piling geometry for a very high velocity the hinge migrates forward resulting in forward oriented subduction vectors and a forward draping slab geometry

tectonic significance of fault slip data abstract the statistical analysis of populations of minor faults is commonly used by structural geologists working in areas of brittle rock it is based on measurements of fault attitude direction and sense of slip at individual sampling localities results are classically interpreted as indicators of stress or strain rate fields assuming a homogeneous stress or strain rate tensor respectively however fault patterns are expected to vary with time because of displacements rigid rotations and internal strains which generally occur along boundaries of fault bounded blocks as deformation proceeds thus in the general situation where early faults accumulate displacements and rigid rotations and where new faults develop during progressive deformation fault slip data can be rather complex and variable in space and reflect neither local stress or strain rate tensors nor finite strains and finite rotations in a simple way we use two examples of faulted regions to illustrate the spatial variability of fault slip data this can be due to local complications at the edges of fault blocks or to complex kinematic conditions at regional boundaries such complications may make it difficult to deduce a consistent and simple pattern of either stresses or strain rates instead our results suggest that information contained in fault slip data can be pertinent to finite deformation in particular the principal axes that we have deduced from the analysis of fault slip data are consistent with the finite displacements at block boundaries that we have calculated by numerical restoration

destruction of the eastern north china craton in a backarc setting evidence from crustal deformation kinematics abstract the eastern north china craton ncc west of the pacific ocean has a long history of rifting from early cretaceous to the end of paleogene when the craton experienced destruction and significant thinning of the lithosphere the cause and geotectonic environment for this destruction remain controversial we use a set of field observation including basin patterns fault geometry and kinematics fault plane slip data dike distribution and fabric data from metamorphic core complexes to determine the kinematic evolution of the deformation in the eastern ncc during this period we show that the principal extension direction in the eastern ncc evolved from wnw ese in the earliest middle early cretaceous via nw se in the latest early cretaceous to nearly n s in the late cretaceous paleogene the movement history from late mesozoic to early cenozoic of the oceanic plates in the pacific ocean with respect to the eastern ncc is available from previous studies the izanagi plate first subducted nearly orthogonally wnw wards during earliest middle early cretaceous time and then moved obliquely nnw wards in the latest early cretaceous while the relaying pacific plate moved generally northwards from the late cretaceous to paleogene both the movement direction of oceanic plates and the principal extension direction of the continental deformation rotated clockwise we suggest that such a correlation can be explained by the eastern ncc being in a backarc setting in the cretaceous paleogene period the results support the backarc extensional model for the destruction and significant lithospheric thinning of the eastern ncc

40ar 39ar dating of strike slip motion on the tan lu fault zone east china abstract ne trending sinistral ductile shear belts in the southern part of the tan lu fault zone east china are exposed along the eastern edge of the northern dabie belt and southern zhangbaling belt three hornblende three phengite and 12 biotite separates from greenschist facies mylonites in the shear belts were dated by the 40ar 39ar method in this study a hornblende plateau age of 143 3 ma is interpreted as representing the time of sinistral motion on the zhangbaling part of the tan lu fault zone seven biotite samples from the same part yield 40ar 39ar cooling ages from 137 to 125 ma a phengite plateau age of 138 8 ma from the eastern edge of the dabie belt is considered as being close to the time of deformation in the tan lu strike slip zone five biotite 40ar 39ar ages from 120 to 110 ma and one phengite age of 121 ma from the same area represent cooling ages after the sinistral faulting it is concluded on the basis of these 40ar 39ar ages that the sinistral ductile shear belts in the southern part of the tan lu fault zone were produced in earliest early cretaceous this sinistral motion on the tan lu fault zone is considered to be related to high speed highly oblique subduction of the pacific plate at this time

mesozoic and cenozoic tectonic evolution of the shiquanhe area of western tibet 1 in the shiquanhe area of far western tibet mid cretaceous strata lie unconformable on ophiolitic melange and jurassic flysch associated with the bangong nujiang suture zone on the basis of our mapping and geochronologic studies we suggest that these cretaceous strata were shortened by 57 over a north south distance of 50 km during late cretaceous early tertiary time the late cretaceous narangjiapo thrust placed permian strata 20 km over ophiolitic melange and cretaceous strata north of the narangjiapo thrust 40 km of shortening was accommodated by the late cretaceous early tertiary south directed jaggang thrust system that involves jurassic flysch and cretaceous strata and roots into a decollement within ophiolitic melange the most recent shortening was accommodated to the south of the narangjiapo thrust along the north dipping shiquanhe thrust the shiquanhe thrust cuts flat lying 22 6 0 3 ma volcanic rocks and underlying folded tertiary nonmarine strata in its footwall and was likely active during slip along the oligocene gangdese thrust system of southern tibet ophiolitic melange and structurally overlying jurassic flysch near shiquanhe are interpreted to represent remnants of a subduction accretion complex and forearc basin respectively that were obducted southward onto the margin of the lhasa terrane during late jurassic early cretaceous closure of the bangong nujiang ocean subsequent imbrication of the obducted sheet could have produced the two east west trending belts of ophiolitic melanges separated by 100 km in western tibet late cretaceous early tertiary thin skinned shortening may have been accommodated in the deeper crust by northward underthrusting and duplexing of lhasa terrane rocks beneath the obducted ophiolitic melange and the qiangtang terrane to the north

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

the tectonic expression slab pull at continental convergent boundaries examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style topographic elevation denudation metamorphism postcollisional convergence and foredeep basin geometry and facies the distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary at subduction boundaries where the rate of overall plate convergence is less than the rate of subduction termed here retreating subduction boundaries the transmission of horizontal compressive stress across the plate boundary is small and regional deformation of the overriding plate is by horizontal extension the tectonic expression of these retreating subduction boundaries includes topographically low mountains little erosion or denudation low grade to no metamorphism little to no involvement of crystalline basement in shortening little to no postcollisional convergence anomalously deep foredeep basins and a protracted history of flysch deposition within the adjacent foredeep basin analysis of deflection and gravity data across three retreating subduction boundaries apennine carpathian and hellenic systems shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km carpathians 50 and 150 km apennines and 50 and 250 km hellenides the total mass anomalies represented by the slabs are approximately 3 1012 6 1012 and 12 1012 n m respectively the slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland and partially supported by dynamic viscous stresses in the asthenosphere at subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction termed here advancing subduction boundaries the transmission of horizontal compressive stress across the plate boundary is large and regional deformation of the overriding plate is by horizontal shortening the tectonic expression of these advancing subduction boundaries includes topographically high mountains antithetic thrust belts large amounts of erosion and denudation exposure of high grade metamorphic rocks at the surface extensive deformation of crystalline basement to midcrustal depths protracted postcollisional convergence tens of millions of years and a protracted history of molasse deposition within the adjacent foredeep basins analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere western to eastern and southern alps and himalayas shows that the thrust sheets have been translated for great distances over the foreland lithosphere relative to the point at which the subduction forces are applied thus obscuring any flexural and gravity signals from the subducted slab however it appears that far field stresses presumably related to global plate motions drive most of the convergent motion across these subduction boundaries the concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved appendix b is available with entire article on microfiche order from the american geophysical union 2000 floridaavenue n w washington d c 20009 document t92 004 2 50 payment must accompany order

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

mesozoic extensional tectonics in eastern asia the south liaodong peninsula metamorphic core complex ne china abstract in the north china block the south liaodong peninsula massif is an elliptical metamorphic core complex mcc with a long axis trending ne sw in cross section view it is asymmetric with a steeply dipping northwestern flank and a gently dipping southeastern flank it consists of three lithotectonic units a gneissic migmatite unit a paleo to meso proterozoic micaschist slate unit and a neoproterozoic to mesozoic sedimentary cover three deformation events related to extensional tectonics are distinguished in the study area d1 is a ductile deformation related to the exhumation of the mcc the following event d2 corresponds to the development of recumbent folds formed during the early exhumation of the mcc and the youngest event d3 corresponds to brittle normal faulting that controlled the opening of a cretaceous continental half graben basin a pre d1 event characterized as northward verging is interpreted as the result of n s shortening that occurred in the late triassic during the fin

craton destruction and related resources craton destruction is a dynamic event that plays an important role in earth s evolution based on comprehensive observations of many studies on the north china craton ncc and correlations with the evolution histories of other cratons around the world craton destruction has be defined as a geological process that results in the total loss of craton stability due to changes in the physical and chemical properties of the involved craton the mechanisms responsible for craton destruction would be as the follows 1 oceanic plate subduction 2 rollback and retreat of a subducting oceanic plate 3 stagnation and dehydration of a subducting plate in the mantle transition zone 4 melting of the mantle above the mantle transition zone caused by dehydration of a stagnant slab 5 non steady flow in the upper mantle induced by melting and or 6 changes in the nature of the lithospheric mantle and consequent craton destruction caused by non steady flow oceanic plate subduction itself does not result in craton destruction for the ncc it is documented that westward subduction of the paleo pacific plate should have initiated at the transition from the middle to late jurassic and resulted in the change of tectonic regime of eastern china we propose that subduction rollback and retreat of oceanic plates and dehydration of stagnant slabs are the main dynamic factors responsible for both craton destruction and concentration of mineral deposits such as gold in the overriding continental plate based on global distribution of gold deposits we suggest that convergent plate margins are the most important setting for large gold concentrations therefore decratonic gold deposits appear to occur preferentially in regions with oceanic subduction and overlying continental lithospheric destruction modification growth

la icp ms u pb zircon ages of the liaohe group in the eastern block of the north china craton constraints on the evolution of the jiao liao ji belt abstract as a major lithostratigraphic unit of the palaeoproterozoic jiao liao ji belt in the eastern block of the north china craton the liaohe group consists of an assemblage that is transitional from a lower arkose and volcanic rich sequence langzishan li eryu and gaojiayu formations through a middle carbonate rich sequence dashiqiao formation to an upper argillaceous sequence gaixian formation tectonically associated with the liaohe group are large volumes of granitoid rocks named the liaoji granite whose emplacement age has been dated at about 2 1 2 2 ga regionally the liaohe group and the associated liaoji granite extend along a nee trending belt that is flanked by 2 45 2 55 ga basement gneisses u pb zircon data and cathodoluminescence cl imaging are used to constrain the maximum depositional and metamorphic ages of the liaohe group which are critical in understanding the tectonic setting and history of the jiao liao ji belt cathodoluminescence images reveal that most detrital zircon grains from the liaohe group are characterized by concentric oscillatory zoning with comparatively low luminescence and high th u ratios suggesting magmatic origin but minor zircon grains have structureless overgrowth or recrystallization rims that are characterized by low th u ratio indicating metamorphic origin results of this study show that the detrital zircon grains of the langzishan formation the lowest sequence of the liaohe group have concordant u pb ages ranging from 2 05 to 2 24 ga suggesting that the liaohe group was deposited at some time after 2 05 ga although most overgrowth or recrystallization rims are too narrow to be analyzed some rims yield concordant or upper intercept ages around 1 93 ga interpreted as the peak metamorphic age of the liaohe group detrital zircon grains from the dashiqiao formation in the middle sequence of the liaohe group have concordant u pb ages ranging from 2 25 to 2 53 ga some of which are similar to those of the basement gneisses in the area this study indicates that the liaohe group was deposited after the emplacement of the liaoji granite and the latter may be a major component of the provenance for the langzishan formation in contrast the protoliths of the middle sequence of the liaohe group were most probably derived from the basement gneisses these new zircon ages for the liaohe group and recent data for the liaoji granite combined with lithological structural and metamorphic considerations support a rifting model for the evolution of the jiao liao ji belt

a paleomagnetic study from the late jurassic volcanics 155 ma north china implications for the width of mongol okhotsk ocean abstract a new paleomagnetic study has been carried out on the tiaojishan formation effusives in the chengde area north china block ncb the age of the tiaojishan formation volcanics was recently dated from 153 ma to 161 ma by u pb and 40 ar 39 ar a total of 154 samples from 16 sites within the tiaojishan formation were collected detailed stepwise thermal demagnetization allows us to isolate two components from 77 oriented drill core samples a low temperature component identified in most samples falls close to the present local earth field direction characteristic remanent directions obtained from high temperature pass reversal test yield a pole position at 240 3 e 59 9 n with 95 6 8 combined with coeval paleopoles obtained from the siberia block sib these results show a significant paleolatitude displacement between the ncb and sib which indicates that the mongol okhotsk ocean was still open with 3000 km width in its eastern part at ca 155 ma

plate kinematics slab shape and back arc stress a comparison between laboratory models and current subduction zones a combination of statistical studies on present day subduction zones and three dimensional 3d laboratory models is performed with the aim to clarify the way that plate kinematics control the geometry of the slab and the overriding plate deformation in subduction zones in 3d laboratory models the analogue of a two layer linearly viscous lithosphere upper mantle system is achieved by means of silicon putty glucose syrup tank experiment the subducting and overriding plate velocities are systematically changed by exploring the variability field of natural plate kinematics both statistical and modelling approaches recognize the importance of overriding plate motion on subduction process behavior 1 trenches migrate at a rate close to the overriding plate motion but always move slower than the overriding plates the mechanism at work is a direct consequence of slab anchoring opposed by both lithosphere and mantle viscous resistance and is responsible for overriding plate deformation and slab geometry variability 2 an overriding plate shortens when the overriding plate moves toward the trench and conditions that are favourable for overriding plate extension are created when the overriding plate moves away from the trench 3 shallow and steep dips are found if the overriding plate moves toward and away from the trench respectively

contrasting late carboniferous and late permian middle triassic intrusive suites from the northern margin of the north china craton geochronology petrogenesis and tectonic implications two contrasting intrusive suites have been identified from the northern margin of the north china craton a late carboniferous dioritegranodiorite suite mainly made up of quartz diorite diorite granodiorite tonalite and hornblende gabbro and a late permian middle triassic suite of granitoid intrusions consisting of monzogranite syenogranite and quartz monzonite plutons from the late carboniferous suite exhibit variable sio2 contents and calc alkaline or high k calc alkaline metaluminous geochemical features most have low negative whole rock nd t values where t is the crystallization age of 17 1 to 11 5 and zircon hf t values of 38 3 to 11 2 indicating that they were derived mainly from anatectic melting of the ancient lower crust with some involvement of mantle materials however an older pluton in the suite exhibits higher nd t values of 11 5 to 9 9 nd model ages of 1 82 1 64 ga lower initial 87sr 86sr ratios of 0 7046 0 7048 and it contains some zircon grains that are characterized by high negative to positive zircon hf t values of 8 7 to 1 2 indicating strong involvement of juvenile materials derived from the lithospheric mantle the late carboniferous plutons are interpreted as subduction related and to have been emplaced in an andean style continental margin arc during the southward subduction of the paleo asian oceanic plate beneath the north china craton rocks from the late permian middle triassic intrusive suite display geochemical signatures ranging from highly fractionated i type to a type they exhibit higher zircon hf t values of 14 9 to 6 7 whole rock nd t values of 10 6 to 8 8 and younger hf and nd model ages than most of the late carboniferous plutons indicating that they could have been produced by extreme fractional crystallization of hybrid magmas resulted from mixing of coeval mantle and crust derived melts they are interpreted as postcollisional postorogenic granitoids linked to lithospheric extension and asthenosphere upwelling due to slab break off and subsequent sinking after final collision and suturing of the mongolian arc terranes with the north china craton these two contrasting intrusive suites suggest that the final closure of the paleo asian ocean and collision between the mongolian arc terranes and the north china craton occurred during the late permian and these events were followed by postcollisional postorogenic extension large volume magmatism and significant continental growth no significant syncollisional crustal thickening high pressure metamorphism or s type granitoid magmatism occurred during the collision process

paleoproterozoic structural evolution of the southern segment of the jiao liao ji belt north china craton abstract the paleoproterozoic jiao liao ji belt separates the eastern block of the north china craton into two small sub blocks the northern longgang and the southern rangrim blocks however it still remains unknown or controversial about the subduction polarity collisional deformation and kinematics between two sub blocks the southern segment of the belt consists of the paleoproterozoic fenzishan and jingshan groups and paleoproterozoic high pressure mafic granulites and serpentinites blocks which are located in the jiaodong complex all of which are separated from the jiaodong complex of neoarchean ttg gneisses by stz1 ductile shear zones structural analysis in this study indicates that most of the rocks in all the units of the southern segment of the jiao liao ji belt underwent three distinct episodes of folding d1 to d3 and two stage of ductile thrust shearing stz1 coeval to d1 and d2 stz2 between d2 and d3 the d1 deformation formed penetrative axial planar foliations s1 bedding parallel ductile shear zone mineral stretching lineations l1 and rarely preserved small isoclinal d1 folds in the jingshan and fenzishan groups in the jingshan group however penetrative deformational transposition resulted in stacking of sedimentary compositional layers which are separated by bedding parallel ductile shear zones stz1 at a period of about 1956 ma to 1914 ma the kinematic indicators of stz1 in the jingshan group with resultant prograde peak metamorphism up to granulite facies grade and the fenzishan group with peak metamorphism up to amphibolite facies grade indicate nw directed compression d2 resulted in crustal thickening with retrograded medium pressure granulite facies grade at about 1914 1893 ma the d2 deformation produced nw verging asymmetric and recumbent folds interpreted to have resulted from basement involved thicken skin structures the jiaodong complex was also involved into the development of wnw verging asymmetric tight folds associated with d2 in the jingshan and the fenzishan groups ongoing collision led to the development of transpressional ductile shearing stz2 forming the transpressional taipingzhuang dextral ductile shear zone between the jingshan group and the southern archean complex and the transpressional tading xiadian sinistral ductile shear zone between the jingshan group and the northern archean complex all three lithotectonic units were superposed during the late d3 deformation with amphibolite facies metamorphism the d3 deformation developed wnw trending open to tight upright folds at about 1893 1875 ma the structural pattern resulting from superimposition of d2 and d3 is a composite synform in the fenzishan and jingshan groups the structural events of d1 and stz1 and d2 and stz2 deformation were possibly responsible for fast syn collisional exhumation of the high pressure mafic granulites the structural patterns and deformational history of the fenzishan and jingshan groups suggest a southeastward directed oblique subduction beneath the northwestern margin of the rangrim block and that the final scissor shaped closure of the rift led to collision between the two blocks to form the coherent basement of the eastern block of the north china craton

plate motions slab dynamics and back arc deformation back arc extension or compression is often related to a particular behavior of the slab dip change and or forward backward migration with respect to the upper plate slabs act either passively when anchored in the deep mantle whereas back arc deformation accommodates the backward or forward motion of the upper plate with respect to the trench or actively when slab pull increasing with slab age forces the trench to rollback in addition to these two mechanisms i e slab anchor and slab rollback numerous observations support the existence of dynamic mantle flow that can exert an overpressure on one side of the slab causing its forward or rearward migration with respect to the arc based on a compilation of upper plate absolute motion trench absolute motion back arc deformation rate upper plate strain regime and slab age for all oceanic subduction zones excluding any kind of collision with continents arcs or plateaus we have examined how the combined effects of these parameters can account for the observed back arc deformations our main results are 1 a global correlation exists between upper plate absolute motion and back arc deformation i e back arc extension when upper plate retreats and vice versa 2 there are as many advancing trenches as retreating ones with trench motion globally limited to 50 mm y 1 furthermore there is no positive correlation between trench retreat and slab age and 3 upper plate absolute motion often fails to explain the back arc deformation rates and the trench motions observed at several subduction zones as tonga new hebrides sandwich or ryukyu we propose that the trench migration of these subduction zones are on the influence of mantle flows we conclude that spontaneous trench rollback related to slab pull is negligible with respect to upper plate motion back arc deformation regime is mostly controlled by the upper plate absolute motion relative to a partly anchored slab trench motion is limited to 50 mm y 1 even if locally mantle flows force slab to move

early mesozoic tectonic settings of the northern north china craton abstract this paper aims to reconstruct early mesozoic tectonic settings of the northern north china craton based on investigations of triassic middle jurassic successions and age determinations of some lithostratigraphic units and dikes cutting tilted early mesozoic strata it used to be regarded that the northern north china craton experienced fold thrust deformations in the triassic to early jurassic based on the occurrences of unconformity beneath triassic lower jurassic units synorogenic conglomerate and late triassic thrusting we revisited early mesozoic stratigraphy of the xiabancheng basin in the middle yanshan belt to restore tectonic environments during that period of time detrital zircon data reveal that the liujiagou and ermaying formations are actually of middle and late triassic ages respectively field observations show that the ermaying and xingshikou formations are conformable making up a sequence from fluvial to alluvial fan facies it is also noticeable that lower middle jurassic xingshikou through xiahuayuan succession is typified by fining and deepening upward depositional trend and contains abundant volcaniclastic rocks indicative of continued subsidence of the xiabancheng basin in a volcanic setting all the results cast doubts on the long held view that the first phase of shortening in the northern north china craton happened in the late triassic or early jurassic the angular unconformity beneath upper jurassic tiaojishan volcanics registered shortening in the northern north china craton which plausibly took place around 170 ma on account of zircon u pb ages of two dikes penetrating deformed middle triassic lower jurassic strata and ages of tiaojishan volcanics above the unconformity given the widespread occurrence of magmatism rift basins and metamorphic core complexes during the triassic and early jurassic it follows that the northern north china craton was more likely under an extensional setting during the early mesozoic

timing of deformation and location of the eastern liaoyuan terrane ne china constraints on the final closure time of the paleo asian ocean abstract the liaoyuan terrane lt in ne china formed as a result of accretion during the final closure of the paleo asian ocean between the north china and siberian cratons however the timing of the deformation and the location of the eastern lt remain controversial here we present structural data zircon u pb age dating and hf isotopic data to constrain the kinematics and timing of deformation and the southern boundary of the eastern lt in relation to the ne sw striking yilan yitong fault yyf one of two branches of the continental scale tan lu fault zone in ne china pre jurassic rocks in the eastern lt were widely involved in ductile deformation within wnw ese striking thrust belts the microstructures in these shear zones indicate a top to the ssw sense of shear and deformation temperatures of 450 500 c the northern margin of the north china craton ncc was also affected by the nne ssw contraction as shown by similar ductile deformation in the archean gneisses and wnw ese trending folds in the mesoproterozoic sedimentary cover the ages of crystallization in deformed and undeformed igneous rocks in the lt constrain the deformation to an interval between 241 and 193 ma u pb dating of zircons in syn kinematic migmatitic rocks indicates an age of 234 229 ma for the deformation we consider this middle triassic age to represent the final closure time of the paleo asian ocean along the eastern lt and this timing is consistent with the eastwards scissor style closure model lithological variations zircon age spectra ehf t values and model ages confirm the eastward extension of the lt across the yyf the data constrain the boundary between the lt and ncc to be roughly along the liao river to the west of the yyf and along the northern edge of the exposed archean gneisses to the east the boundary follows the bayan obo chifeng kaiyuan fault which was active as a thrust thrusting of material to the ssw during the middle triassic orogeny the newly determined boundary location indicates a 35 km sinistral displacement along the yyf

tectonic evolution of early mesozoic sedimentary basins in the north china block abstract there have been lots of debates regarding tectonic evolution of sedimentary basins in the north china block during the early mesozoic it has been widely held that the north china block was strongly affected by the collision of the north and south china blocks on the south and the terrane accretion on the north and thus early mesozoic basins of various scales in the interior and peripheries of the north china block resulted from crustal shortening based on extensive and detailed field investigations of stratigraphic sequences sedimentary processes and basin tectonics this study shows that the north china block manifested itself in general as a stable domain during the early mesozoic with rifting transcurrent faulting and magmatism occurring mainly in peripheral regions the north china block behaved as a big intracratonic basin in the triassic albeit its depositional area shifted to the west in mid late triassic time the westward migration of depocenter is assumed the consequence of uplifting of the eastern part of the north china block in response to the collision of the north and south china blocks along the tongbai dabie sulu orogenic belt and coeval transpression of the tanlu fault zone sedimentary basins had not developed until the early jurassic in the regions north of the tongbai dabie orogen and plausibly formed as the result of crustal stretching related to southeastward exhumation of the dabie high pressure metamorphic rocks the collision of the north and south china blocks led to south directed displacement of the east qinling orogen but few geological records exist indicative of mid late triassic thrusting in the southern margin of the western north china block instead some evidence is suggestive of extensional or transtensional deformations such as the existence of large normal faults controlling subsidence and sedimentation of middle late triassic basins and the presence of synsedimentary normal faults in basin sequences both the east and west qinling experienced rapid uplifting in the late triassic coeval with large volume granitic magmatism late triassic uplift and exhumation of the qinling was considered as the result of asthenospheric upwelling and voluminous granitic injections vertical motion of the northern east qinling might have triggered normal faulting along its northern edge and extensional subsidence of the southern margin of the western north china block broad denudation occurred simultaneously in the northern margin of the north china block from the late triassic to early jurassic in association with magmatism and development of a few small rift basins asthenospheric uprising is regarded as the cause for late triassic magmatism regional surface uplift and supracrustal stretching triassic subsidence and sedimentation in the interior of the north china block was punctuated by crustal uplifting possibly driven by mantle upwelling at the transition from the triassic to jurassic the whole north china block then resumed low rate subsidence from the late early jurassic and plausibly developed into a broad intracratonic basin in the middle jurassic characterized by widespread distribution of shallow water lacustrine and meandering river facies associations the sagging of the whole north china block is thought of as the consequence of thermal contraction in the aftermath of mantle upwelling backarc extension related to the paleo pacific plate subduction might have magnified subsidence of the eastern north china block resulting in relatively thick accumulation of middle jurassic strata this study restores tectonic and sedimentary history of the early mesozoic basins in different portions of the north china block and advances models to illustrate how the basins of diverse origin were generated and filled in different stages

petrological and geochemical constraints on the evolution of the alkaline complex of koraput in the eastern ghats granulite belt india abstract the koraput alkaline complex in the high grade eastern ghats belt india is synkinematically emplaced in a pull apart structure and far from the bastar cratonic margin the suite comprises four distinct members namely mafic syenite felsic syenite nepheline syenite and perthite syenite fe rich orthopyroxene rims on olivine in mafic syenite indicate iron enrichment in the early stage of differentiation with plagioclase of andesine composition it could be described as alkali norite the plutonic equivalent of hawiite felsic syenite with both alkali feldspars and plagioclase of oligoclase composition could be described as two feldspar syenite the plutonic equivalent of mugearite albitic rims on nepheline indicates subsolvus reaction chemical trends in amphiboles and plagioclase feldspars progressively more ferroan and more sodic respectively are strong indications of mineral fractionation in the koraput suite chemical trends in the variation diagrams are compatible with feldspar fractionation in the koraput suite a weak fe enrichment trend in the early stage of differentiation as observed in the afm diagram is compatible with that of the alkali basalt series nb anomalies both positive and negative are indicative of crustal contamination as expected in synkinematic plutons in terms of gondwana assembly and break up the alkaline complexes are supposed to represent rift related magmatism along the continental margin in spite of petrological evidence of the magmatic character of the koraput complex anorogenic setting is contra indicated by mesoscopic and microscopic fabrics more akin to synkinematic intrusion during f 2 folding in the host country rocks the proterozoic alkaline complexes in the eastern ghats belt could alternatively trace the path of moving gondwana continent over mantle plumes

rapid change from compression to extension in the north china craton during the early cretaceous evidence from the yunmengshan metamorphic core complex abstract the yunmengshan metamorphic core complex ymmcc is situated in the northern part of the eastern north china craton ncc the main structures in the ymmcc area include the arcuate sihetang shear zone shtsz and the ne striking shuiyu shear zone sysz we present structural and geochronological evidence to show that the shtsz was a top to the ssw thrusting zone formed between 140 and 137 ma the sysz is an extensional top to the se shear zone developed between 131 and 114 ma in association with intense magmatism and the development of the huairou rifted basin in the hanging wall ductile shearing on the sysz was followed by isostatic rebound from 114 ma to the end of the early cretaceous and then was replaced by brittle normal faulting the normal displacement on the sysz and subsequent isostatic rebound led to the formation of the ymmcc the mechanism of formation is consistent with a rolling hinge model of detachment faulting and core complex formation the structural evolution of the yunmengshan area in common with the whole eastern ncc thus records a rapid switch from a nne ssw shortening regime to a nw se extension one in the earliest cretaceous the intense nw se extension of the early cretaceous as recorded by the ymmcc was associated with peak lithospheric thinning in the eastern ncc we attribute the shortening to the final closure of the mongol okhotsk ocean between the siberian craton and the north china south mongol block and the extension to the back arc deformation related to the subduction of the izanagi plate in the pacific ocean we suggest that the yunmengshan area illustrates how back arc compression can be driven by the overriding plate motion while connection of the overriding plate to a larger continent favors subduction related back arc extension

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

petrogenesis of late triassic granitoids and their enclaves with implications for post collisional lithospheric thinning of the liaodong peninsula north china craton abstract major and trace element sr nd and hf isotope zircon u pb and hf isotope data are reported for mafic microgranular enclaves and host granitoids from the xiuyan pluton in the liaodong peninsula of the eastern north china craton in order to investigate their sources petrogenesis and tectonic implications the zircon u pb age of the enclaves 211 1 ma is identical to that of the host granite 210 1 ma establishing that the mafic and felsic magmas were coeval the enclaves have high mgo concentrations at low silica contents and positive ehf t values they are enriched in large ion lithophile and light rare earth elements and depleted in high field strength elements which can be explained as melts derived from a depleted asthenospheric mantle with some crustal contamination the host granitoids have high sio2 contents variable initial 87sr 86sr ratios and highly negative end t and ehf t values suggesting they were mainly derived from an ancient crustal source heated by the input of asthenospheric mantle derived magmas field observations geochronology geochemistry sr and nd isotopic and zircon hf isotopic compositions point to a complex petrogenesis where asthenospheric mantle and crust derived magma mixing was coupled with crystal fractionation thus explaining the genetic link between enclaves and host rocks identification of an asthenospheric mantle component in the xiuyan granitoids suggests asthenospheric mantle upwelling and decompressional melting of the mantle indicating that post collisional lithospheric thinning in the liaodong peninsula was initiated in the late triassic

mesozoic strike slip movement of the dunhua mishan fault zone in ne china a response to oceanic plate subduction abstract the ne striking dunhua mishan fault zone dmfz is one of two branches of the continental scale sinistral tan lu fault zone in ne china the field data presented here indicate that the ca 1000 km long dmfz records two phases of sinistral faulting the structures produced by these two phases of faulting include ne sw striking ductile shear belts and brittle faults respectively mylonite hosted microstructures and quartz c axis fabrics suggest deformation temperatures of 450 c 500 c for the ductile shear belts combining new zircon u pb dates for 14 igneous rock samples analyzed during this study with the geology of this region indicates these shear belts formed during the earliest early cretaceous this phase of sinistral displacement represents the initial formation of the dmfz in response to the northward propagation of the tan lu fault zone into ne china a phase of early cretaceous rifting was followed by a second phase of sinistral faulting at 102 96 ma as evidenced by our new u pb ages for associated igneous rocks combining our new data with the results of previous research indicates that the dfmz records a four stage cretaceous evolutionary history where initial sinistral faulting at the beginning of the early cretaceous gave way to rifting during the rest of the early cretaceous this was followed by a second phase of sinistral faulting at the beginning of the late cretaceous and a second phase of local rifting during the rest of the late cretaceous the cretaceous evolution of the dmfz records the synchronous tectonic evolution of the ne china continent bordering the pacific ocean two phases of regional n s compression generated the two phases of sinistral faulting within the dmfz whereas two stage regional extension generated the two phases of rifting the two compressive events were the result of the rapid low angle subduction of the izanagi and pacific plates whereas the two stage extension was caused by the roll back of these respective plates the final closure of the mongol okhotsk ocean at the beginning of the early cretaceous intensified the synchronous compression in ne china causing the northward propagation of the tan lu fault zone

origin ascent and oblique emplacement of magmas in a thickened crust an example from the cretaceous fangshan adakitic pluton beijing abstract the fangshan pluton in the western hills of beijing records a history of emplacement from primarily vertical ascent to oblique injection along a detachment fault thus the pluton provides a classic example answering the room problem this pluton and the nanjiao apophyse were produced by cretaceous magmatism that formed the regionally extrusive east china mesozoic igneous province the fangshan pluton includes two intrusive phases a 136 ma monzodiorite phase intruded by a 131 ma quartz monzodiorite phase rocks from both phases have chondrite normalized ree patterns enriched in lree and depleted in hree they have primitive mantle normalized trace element patterns with distinct negative nb and ti anomalies and high sr 1067 1348 ppm and low y 8 17 ppm with high sr y ratios 71 137 these features are characteristic of adakites and suggest that the magmas were derived by partial melting of the lower part of a thickened continental crust the pluton has magmatic foliations and lineation indicative of the intrusion of the earlier magmas slightly obliquely from se to nw the magmatic fabrics have enclave ratios rfmag rp of 3 0 3 5 asymmetric solid state deformational fabrics and strains rs for the northwestern 3 5 4 0 and southeastern parts 1 5 2 0 again suggest a later oblique injection of magmas from se to nw both the fangshan pluton and nanjiao apophyse suggest that magmas migrated most probably along the pre existing detachment fault such an emplacement is also supported by the aeromagnetic polarization data and strong deformation of the beiling syncline thus magma emplacement in the fangshan pluton probably involved creation of space by both faulting and wallrock strain

what drove late mesozoic extension of the northern china mongolia tract the northern china mongolia tract exhibited a tectonic transition from contractional to extensional deformation in late mesozoic time late middle to early late jurassic crustal shortening is widely thought to have resulted from collision of an amalgamated north china mongolia block and the siberian plate but widespread late late jurassic early cretaceous extension has not been satisfactorily explained by existing models some prominent features of the extensional tectonics of the northern china mongolia tract are 1 late jurassic voluminous volcanism prior to early cretaceous large magnitude rapid extension 2 overlapping in time of contractional deformation in the yinshan yanshan belt with development of extensionrelated basins in the interior of the northern china mongolia tract and 3 widespread occurrence of alkali granitic plutonism extensional basins and metamorphic core complexes in the early cretaceous a new explanation is advanced in this study for this sequence of events the collision of amalgamated north china mongolia with siberia led to crustal overthickening of the northern china mongolia tract and formation of a high standing plateau subsequent breakoff at depth of the north dipping mongol okhotsk oceanic slab is suggested as the main trigger for late mesozoic lithospheric extension of that tract slab breakoff resulted in mantle lithospheric stretching of the adjacent northern china mongolia tract with subsequent ascent of hot asthenosphere and magmatic underplating at the base of the crust collectively these phenomena triggered gravitational collapse of the previously thickened crust leading to late late jurassic early cretaceous crustal extension and importantly coeval contraction along the southern margin of the plateau in the yinshan yanshan belt the proposed model provides a framework for interpreting the spatial and temporal relationships of distinct processes and reconciling some seemingly contradictory phenomena such as the synchronous extension of northerly terranes during major contraction in the neighboring yanshan yinshan belt d 2003 elsevier b v all rights reserved

late mesozoic tectonic evolution and kinematic mechanisms in the daqing shan at the northern margin of the north china craton abstract the yinshan belt that lies along the northern edge of the north china craton is an intraplate deformational belt that developed in the late mesozoic the daqing shan is located in the eastern part of this belt and has attracted increasing attention because of its large scale thrust structures and the hohhot metamorphic core complex the structural dynamics and the tectonic relationships between compressional and extensional regimes in this region remain controversial our work indicates marked differences in the late mesozoic deformation events that affected the western and eastern parts of the daqing shan compressional structures in the western part of the range are characterized by high angle symmetrical thrusts consisted of s and n dipping thrust faults whereas the eastern part is characterized by low angle thrusting most notificeably the thrusting of meso neoproterozoic northwestwards across late jurassic early cretaceous strata field mapping has revealed that the symmetric thrusting in the western part is overlain by the low angle thrusting that is developed in the eastern part and the latter is overprinted by the hohhot metamorphic core complex therefore the daqing shan area underwent complex tectonic processes during the late mesozoic which can be summarized as d1 late middle jurassic to pre cretaceous symmetrical thrusting d2 late jurassic to early cretaceous low angle thrusting and d3 early cretaceous extensional development of a metamorphic core complex fault slip data indicate spatial and temporal variations in the tectonic stress field the symmetrical thrusting was the result of n s compression and the low angle thrusting was the result of nw se compression these structures may respectively be related to the far field effects of the closure of the mongolia okhotsk ocean north of the daqing shan and the subduction of the pacific plate beneath eastern asia the formation of the hohhot mcc could correlate with the extension of lithosphere in the east north china craton the characteristics of late mesozoic deformation in the daqing shan indicate that tectonic processes in this intraplate deformation belt were controlled by the interaction of multiple adjacent plates resulting in temporally distinct modes of deformation kinematics and dynamic mechanisms and which cannot reasonably be interpreted in terms of a single tectonic mechanism or dynamic model

continental subduction and exhumation of uhp rocks structural and geochronological insights from the dabieshan east china in the dabieshan the available models for exhumation of ultrahigh pressure uhp rocks are poorly constrained by structural data a comprehensive structural and kinematic map and a general cross section of the dabieshan including its foreland fold belt and the northern dabieshan domain foziling and luzenguang groups are presented here south dabieshan consists from bottom to top of stacked allochtons 1 an amphibolite facies gneissic unit devoid of uhp rocks interpreted here as the relative autochton 2 an uhp allochton 3 a hp rock unit susong group mostly retrogressed into greenschist facies micaschists 4 a weakly metamorphosed proterozoic slate and sandstone unit and 5 an unmetamorphosed cambrian to early triassic sedimentary sequence unconformably covered by jurassic sandstone all these units exhibit a polyphase ductile deformation characterized by i a nw se lineation with a top to the nw shearing and ii a southward refolding of early ductile fabrics the central dabieshan is a 100 km scale migmatitic dome newly discovered eclogite xenoliths in a cretaceous granitoid dated at 102 ma by the u pb method on titanite demonstrate that migmatization post dates hp uhp metamorphism ductile faults formed in the subsolidus state coeval to migmatization allow us to characterize the structural pattern of doming along the dome margins migmatite is gneissified under post solidus conditions and mylonitic ultramylonitic fabrics commonly develop the north and west boundaries of the central dabieshan metamorphics i e the xiaotian mozitan and macheng faults are ductile normal faults formed before late jurassic early cretaceous a cretaceous reworking is recorded by synkinematic plutons north of the xiaotian mozitan fault the north dabieshan domain consists of metasediments and orthogneiss foziling and luzenguang groups metamorphosed under greenschist to amphibolite facies which never experienced uhp metamorphism a rare n s trending lineation with top to the south shearing is dated at 260 ma by the 40ar 39ar method on muscovite this early structure related to compressional tectonics is reworked by top to the north extensional shear bands the main deformation of the dabieshan consists of a nw se stretching lineation which wraps around the migmatitic dome but exhibits a consistently top to the nw sense of shear the central dabieshan is interpreted as an extensional migmatitic dome bounded by an arched top to the nw detachment fault this structure may account for a part of the uhp rock exhumation however the abundance of amphibolite restites in the central dabieshan migmatites and the scarcity of eclogites found only in a few places argue for an early stage of exhumation and retrogression of uhp rocks before migmatization this event is coeval to the n s extensional structures described in the north dabieshan domain recent radiometric dates suggest that early exhumation and subsequent migmatization occurred in triassic liassic times the main foliation is deformed by north verging recumbent folds coeval to the south verging folds of the south dabieshan domain an intense cretaceous magmatism accounts for thermal resetting of most of the 40ar 39ar dates a lithosphere scale exhumation model involving continental subduction synconvergence extension with inversion of southward thrusts into nw ward normal faults and crustal melting is presented

temporal and spatial variations of mesozoic magmatism and deformation in the north china craton implications for lithospheric thinning and decratonization abstract mesozoic triassic cretaceous magmatic rocks and structural deformation are widely distributed in the north china craton ncc and are crucial to understanding the timing location and geodynamic mechanisms of lithospheric thinning and decratonization of the ncc our new geochronological geochemical and structural data combined with previously published results on mesozoic magmatic rocks and deformational structures in the ncc indicate a temporal and spatial migration of magmatism and deformation from its margins to its cratonal interior triassic and early jurassic igneous rocks are only distributed along the northern southern and eastern margins of the ncc in contrast cretaceous magmatic rocks are widely distributed in whole eastern and central parts of the ncc there is a younging trend for mesozoic magmatic rocks from the northern and eastern parts yanshan jiaodong peninsula and liaodong to the central part of the ncc taihangshan mesozoic deformation in the ncc exhibits a similar migration trend from craton margins to its inland areas triassic early jurassic deformation mainly occurred in the margins of the ncc and transformed from compression during the early middle triassic to extension during the late triassic to early jurassic in its northern margin middle late jurassic to earliest cretaceous deformation is widely distributed in the ncc and exhibited non unique contractional directions usually perpendicular to boundaries of the ncc and its ordos block indicating that it was likely controlled by multiple tectonic regimes during the middle late jurassic to earliest cretaceous early cretaceous deformation was characterized by near unique nw se extension that was likely controlled by unique geodynamic regime that probably related to the far field effect of cretaceous paleo pacific plate subduction the above mentioned temporal and spatial migrations of mesozoic magmatic rocks and deformation indicate that lithospheric thinning and decratonization of the ncc was diachronous and complex the lithospheric thinning and decratonization of the ncc initially started from its northern and eastern margins as a result of post collisional post orogenic lithospheric delamination during the middle late triassic and then spread to the interior of the craton during the late mesozoic interactions of the surrounding orogenesis and the small size of the ncc may have played important roles on its late mesozoic lithospheric thinning and decratonization

evolution of the yiwulushan metamorphic core complex from distributed to localized deformation and its tectonic implications 1 the yiwulushan metamorphic core complex ysmcc is situated in the northern part of the eastern north china craton ncc that experienced significant lithospheric thinning in an extensional regime during the late mesozoic we present structural and geochronological evidence to show that the ysmcc underwent nne ssw extension in the late jurassic and wnw ese extension in the early cretaceous we show that the ysmcc started with a sub horizontal shear zone referred to as the yiwulushan detachment shear zone ydsz in the late jurassic between 157 and 147 ma with a top to the ssw shear sense the ydsz was subjected to limited doming probably toward the end of its formation the nne striking wnw dipping waziyu normal shear zone wzysz was formed along the western margin of the core complex during the early cretaceous and truncates the earlier ydsz the whole core complex underwent doming in a rolling hinge manner in the end of the latest early cretaceous thereafter the ysmcc was overprinted by brittle normal faults along its western margin in the late cretaceous development of the initially flat lying ydsz is associated with diffuse mid crustal flow whereas the formation of the rolling hinge type wzysz is characterized by localized deformation the late jurassic more diffuse deformation has a rather constant top to the ssw shearing and produced a variety of shear related structures such as recumbent isoclinal folds and transposition foliation the ysmcc evolution also demonstrates that the late mesozoic extension along the northern part of the eastern ncc started as early as the late jurassic rather than the previously proposed early cretaceous

timing of the yanshan movement evidence from the jingxi basin in the yanshan fold and thrust belt eastern china evidence for the jurassic cretaceous yanshan movement orogeny was first identified in the jingxi basin a part of the yanshan fold and thrust belt yftb along the northern margin of the north china craton the disconformity or angular unconformity within the jurassic volcano sedimentary successions in the basin was interpreted to have resulted from a period of incipient shortening during the yanshan movement commonly known as event a however the absolute ages of both event a and associated jurassic units remain controversial here we present the results of zircon u pb geochronology for volcanic and clastic rocks from the jurassic units in the jingxi basin the results provide better constraints on the depositional ages of formations in the basin 1 early to early middle jurassic 175 167 ma for the nandaling formation 2 middle middle jurassic for the yaopo formation 3 late middle jurassic for the longmen formation 4 early late jurassic for the jiulongshan formation and 5 late late jurassic 157 149 ma for the tiaojishan formation stratigraphic and geochronological correlations indicate that the jiulongshan formation and the tiaojishan formation are of different depositional ages in different regions the longmen formation in the jingxi basin is equivalent to the haifanggou formation in the eastern yftb two sedimentary cycles separated by a sedimentary hiatus shown as disconformity and local angular unconformity have been identified in the jurassic sequences of the yftb depositional ages of the rocks above and below the disconformity which lies between the yaopo and longmen formations in the jingxi basin constrain the timing of event a to 167 2 ma late middle jurassic this event is the first phase of shortening deformation in eastern china induced by the paleo pacific plate subduction we propose that the shortening period was relatively transient in contrast to previous suggestions of long duration shortening

post cratonization deformation processes and tectonic evolution of the north china craton abstract the timing of events during the tectonic evolution of the north china craton ncc and the east china region is debated extension compression transpression and shearing occurred throughout the ncc since 320 ma these main structural stages and the temporal evolution of the structural framework and kinematics are summarized we present here data from multiple structural and magmatic stages related to temporally varying kinematics and deformation mechanisms during the evolution of the post cratonization of the ncc the timing of each stage is constrained using isotopic dating along with the timing of magmatic intrusion dyke intrusion and basin formation associated with tectonic transformation of the ncc age data indicate distinct structural and magmatic stages at 1600 1500 1300 1100 445 315 270 200 170 155 130 110 75 65 and 25 20 ma relating to basin formation and continental uplift or collapse these stages define the tectonic evolution and structural transformation of the ncc and associated de rooting reactivation reworking and orogenic growth the tectonic setting and movement direction of adjacent plates changed multiple times during the various stages the stages are as follows separation of the ncc from the gondwana supercontinent and northwards drift formation of the central asian orogenic belt caob and northwards subduction of the yangtze plate resulting in closure of the qinling ocean the rapid initial formation of the western pacific plate and subsequent subduction and final closure of the mongol okhotsk ocean combined with closure of the bangong nujiang suture zone and southwards movement of the siberian continent as well as thinning of the north china cratonic lithosphere moreover temporal and spatial changes in deformation magmatism and basin formation as well as kinematic and mechanical transformations occurred in response to the changing tectonic setting and movement direction of surrounding plates these various factors resulted in a complex structural evolution of the ncc since 1 8 ga

effects of bed compaction on scour at piers in sand clay mixtures abstractscour at piers embedded in a bed of cohesive sediment mixture is investigated to quantify the effects of bed compaction experiments were conducted in a large rectangular laboratory flume with natural clay sediment collected from a stream located at the chilean coastal range nonintrusive and high resolution topographic measurements of scoured bed in the vicinity of piers were performed during the experimental runs with an installation of a laser distance sensor lds that was traversed by the precision step motors measurements were taken by the lds in different azimuthal half planes to study the spatiotemporal variation of the topography of evolving scour holes experiments were conducted over 40 h until an advanced stage of scour was reached with the approaching flow having a bed shear stress equaling 95 of the shields critical condition for the initiation of motion of sand in a sand clay mixture results show that the bed is successively scoured in three different forms chunks of aggregate