lithospheric thickness beneath the dabie shan central eastern china from s receiver functions summary p and s receiver functions obtained from a portable array of 34 broad band stations in east central china provide a detailed image of the crust mantle and lithosphere asthenosphere boundaries lab in the dabie shan and its adjacent areas clear s to p converted waves produced at the lab show a thin lithosphere beneath the whole study area based on our results the thickest lithosphere of 72 km is observed beneath the southern part of the area within the yangtze craton whereas beneath the north china platform the lithosphere is only 60 km thick s receiver functions also reveal in good agreement with p receiver functions a maximum depth of the moho beneath the dabie shan orogen at approximately 40 km furthermore we interpret the structural difference at 32 latitude as the probable location of

inverted metamorphism in the intracontinental ductile shear zone during himalayan collision tectonics collision tectonics in the himalaya ca 50 ma remobilized the proterozoic basement and its cover of the indian plate into the higher himalayan crystalline belt this belt displays an inverted metamorphism progressing from a garnet to a sillimanite k feldspar zone toward higher topographic and structural levels because previously hypothesized mechanisms are not sufficient to explain inverted metamorphism we propose that ductile shear displacements along ubiquitous closely spaced s c shear planes may have caused metamorphic inversion across a 15 20 km thick southwest vergent higher himalayan shear zone

higher himalaya in the bhagirathi section nw himalaya india its structures backthrusts and extrusion mechanism by both channel flow and critical taper mechanisms the higher himalayan crystalline hhc in the bhagirathi river section india on fieldwork reveals two extensional ductile top to n ne shear sub zones the south tibetan detachment system and the basal detachment besides a preceding top to s sw ductile shear a top to n ne brittle shear was identified as backthrusts from the hhc except its northern portion that occur repeatedly adjacent to numerous top to s sw brittle shears as fore thrusts the northern portion of the hhc the gangotri granite exhibits infrequent total six extensional and compressional brittle shear senses the backthrusts could be due to a low friction between the lower boundary of the hhc i e the main central thrust zone and the partially molten hot rock materials of the hhc subduction of the eurasian plate towards s sw below the indian plate more extensively in the garhwal sector could be the second possible reason presence of two ductile extensional shear sub zones may indicate channel flow or several exhumation mechanisms of the hhc in a shifting mode similar to mukherjee et al in int j earth sci 101 253 272 2012 the top to s sw extensional brittle shear exclusively within the upper northern portion of the hhc and a top to s sw brittle shear within the remainder of it is a possible indicator of critical taper deformation mechanism thus this work provides the field evidences of possibly both channel flow and critical taper conditions from a higher himalayan section besides that by larson et al geol soc am bull 122 1116 1134 2010

microstructures and strain variation across the footwall of the main central thrust zone garhwal himalaya india abstract the microstructural variation with a progressive change in the strain pattern are described in the rocks occurring across the footwall of the main central thrust mct in an area of the garhwal himalaya in the western garhwal himalaya the mct has brought upper amphibolite facies metamorphic rocks southward over the greenschist facies rocks of the lesser himalaya the progressively increasing flattening strain towards the mct changes either to plane strain or in some cases to constrictional strain this change in strain is well recorded in the microstructures the zone dominated by flattening strain is expressed as bedding parallel mylonites the grain reduction in this zone has occurred by dynamic recrystallization and quartz porphyroclasts were flattened parallel to the mylonite zone the maximum finite strain ratio observed in this zone is 2 2 1 8 1 the zone where the flattening strain changes either to plane strain or constrictional strain record an increase in finite strain ratio up to 3 8 1 9 1 this zone represents deformation fabrics like s c microstructures simultaneously developed during mylonitization in an intense ductile shear zone the above zone is either near the mct or adjacent to crystalline klippen occupying the core of the synforms in the footwall of the mct the microstructural evolution and the finite strain suggest that the mct has evolved as the result of superposition of southward directed simple shear over the flattening strain the simple shear has played an active role in the rapid translation which followed the mylonitization at deeper levels

mesozoic large scale lateral extrusion rotation and uplift of the tongbai dabie shan belt in east china the tectonic convergence between the south china block and qinling belt that occurred in mesozoic time was not homogeneous along strike it was mainly concentrated in the shengnongjia and hannan domes which formed a pair of indentors along which the south china block penetrated the qinling belt as a consequence the tongbai dabie shan belt the eastern part of the qinling orogenic belt may have undergone large scale eastward extrusion clockwise rotation and uplift as indicated by its boundary deformation features it was originally located beneath the narrowest part of the qinling belt where it underwent ultrahigh pressure metamorphism in the early triassic

tectonic evolution of kishtwar window with respect to the main central thrust northwest himalaya india abstract the crystalline rocks of the himalaya high himalayan crystallines central crystallines etc are bounded to the north by the south tibetan detachment zanskar shear zone trans himadri fault system at the top and to the south by the main central thrust mct at the base to the south and tectonically below the mct lies the lesser himalaya comprising low grade meta sedimentary thrust sheets and the crystallines klippen north and tectonically above the zanskar shear zone lies the sedimentary succession of the tethys himalaya however there is a different geological set up to the northwest of beas valley which defies this normal set up i e the tethyan rocks either lie in contact with the lesser himalayan thrust sheets or they to a large extent surround the high himalayan crystallines hhc this unique set up is documented around areas comprising chamba kishtwar and zanskar regions in the northwest himalaya the lesser himalayan imbricated sequence extending progressively into deeper structural level beneath and crops out as kishtwar window within the hhc this tectonic contact along the window was described as the mct however the present study of the window indicates that its western side is bounded by a high angle ne dipping reverse fault kishtwar thrust the structural analyses reveal that this fault is younger than the mct and is interpreted here as breach thrust present observations suggest that the rocks of the window and the northeasterly lying hhc have been folded and exhumed together as an antiform along the kishtwar thrust the hhc occurring south of the kishtwar window have actually been thrust over the lesser himalayan thrust sheet this contact is observed and marked for the first time and is accordingly described as the main central thrust as per convention along the length of the himalayan belt northeast and in the hangingwall of the mct the repetition of the contact of the lesser himalayan rocks and the hhc is interpreted here as the deeper part of the mct brought to the surface by kishtwar thrust

spatial variations of ductile strain in fold and thrust belts from model to nature abstract fold and thrust belts ftbs accommodate tectonic convergence through folding and faulting of crustal rocks during a collisional event between two continental plates although evidence of distributed deformation is common in ftbs that usually leads to continuous foliations and regionally occurring ductile structures of multiple orders it has rarely been given much attention assuming that zones of localized deformation like shear zones and brittle faults accommodate the entire amount of tectonic convergence this study presents 3d laboratory scale models using viscous thin sheet as crustal layers to understand the evolution of ductile strain in a tectonic wedge we varied the degree of mechanical coupling at the basal decollement i e weak versus strong to investigate this issue at constant convergence velocity in all experiments to avoid the influence of rate dependence on viscous rheology our results reveal that the strength of basal decollement controls the mode of wedge growth and hence the strain pattern particularly towards the hinterland the weak decollement models yield a zone of constriction towards the central part of hinterland explaining the occurrence of isolated patches of l tectonites and cross folds in ftbs while the strong decollement condition allows gravity driven flow to be active in the hinterland leading to orogen parallel recumbent folds in contrast both weak and strong decollement models produce deformation that characterises the commonness of pervasive hinterland dipping ductile fabrics towards the mountain front we correlate our findings to show that spatio temporal variations in basal coupling are responsible for varying occurrence of ductile structures in ftbs

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

deformation metamorphism and imbrication of the indian plate south of the main mantle thrust north pakistan south of the main mantle thrust in north pakistan rocks of the northern edge of the indian plate were deformed and metamorphosed during the main southward thrusting phase of the himalayan orogeny in the hazara region between the indus and kaghan valleys metamorphic grade increases northwards from chlorite zone to sillimanite zone rocks in a typically barrovian sequence metamorphism was largely synchronous with early phases of the deformation the metamorphic rocks were subsequently imbricated by late north dipping thrusts each with higher grade rocks in the hanging wall than in the footwall such that the metamorphic profile shows an overall tectonic inversion the rocks of the hazara region form one of a number of internally imbricated metamorphic blocks stacked after the metamorphic peak on top of each other during the late thrusting this imbrication and stacking represents an early period of post himalayan uplift

the tectonostratigraphy and structure of the eastern nepal himalaya six years of geological research in eastern nepal has resulted in a new geological map of the eastern nepal himalaya which includes the region stretching from the sikkhim border in the east to the kathmandu valley in the west and from the summits of the higher himalaya in the north to the ganges plain in the south this research has permitted the determination of the tectonostratigraphy and structure of one section of the central himalayan arc south of the tibetan plateau the eastern nepal himalaya can be divided into three distinct thrust bound tectonic packages 1 the higher himalayan thrust sheet composed of the higher himalayan crystallines 2 the lesser himalayan thrust sheet composed of the lesser himalayan series and 3 the sub himalayan imbricate zone composed of sedimentary rocks belonging to the siwalik group the higher himalayan thrust sheet of eastern nepal has been thrust over the lesser himalayan metasediments a minimum of 140 km and possibly as much as 175 210 km along the main central thrust mct the lesser himalayan thrust sheet is overlain by the mct and is underlain by the main detachment fault mdf and the main boundary thrust mbt out of sequence thrust faults in the hanging wall of the mbt have breached and offset the presently inactive mct the sub himalayan imbricate zone is an emergent imbricate fan bounded by the mbt to the north and the main frontal thrust mft to the south and is underlain by the mdf which lies at a depth of between 5 km and 7 km a balanced cross section constructed across the higher lesser and sub himalaya of eastern nepal shows that the eastern nepal himalayan orogenic wedge has undergone a minimum of between 210 and 280 km of horizontal north south tectonic shortening since the initiation of the mct the lesser and sub himalaya have absorbed 70 km of north south shortening by thrusting along the basal mdf of which the sub himalayan imbricate zone has accommodated 25 km the sun kosi thrust has accommodated about 10 km and the mbt has accommodated the remaining 35 km of shortening since the initiation of the mct between 15 ma and 25 ma shortening across the eastern nepal himalaya has occurred at an average rate of 8 4 18 6 mm per year the structural geometry of the eastern nepal himalaya suggests an overall piggyback sequence of thrusting with motion transferred from the mct to the underlying mdf and its emergent splay thrust the mbt and with the mbt rotated to its present steep orientation by imbricate thrusting within the sub himalaya

the three dimensional electrical structure and metallogenic prospect of the ning nanjing wu wuhu basin and the southern adjacent area in eastern china abstract the ning nanjing city wu wuhu city volcanic basin which is famous for its large scale high grade ore deposits is a typical iron ore district in the middle lower reaches of the yangtze river metallogenic belt mlrymb the southern part of the basin is primarily covered by quaternary strata few in depth studies have been conducted on this area due to the increasing demand for mineral resources and the controversy over regional tectonic models some fundamental geological issues still need to be addressed these questions include defining the characteristics of the geological structures and the mineralization prospects therefore a three dimensional 3d magnetotelluric survey was conducted in this area to obtain a deep 3d electrical model it was found that the yangtze fault cjf and the main thrust fault mtf in the upper crust have the same deep middle crust to upper mantle setting and part of the thrust tectonics of the metallogenic belt the southern boundary of the ning wu basin is located on a fault belt boundary fault bf with a northeast east or east west strike influenced by the mtf in the southern part of the study area bounded by the bf and mtf the southeastern part of the study area exhibits signs of uplift with the recent discovery of a large porphyry copper deposit to the southeastern part of the study area which is located in an uplift structure similar to the one south of the ning wu basin this local uplift could have potential mineralization prospects based on a previous mineralization theory developed for this belt

evaluation of a regional strain gradient in mylonitic quartzites from the footwall of the main central thrust zone garhwal himalaya india inferences from finite strain and ams analyses abstract the footwall of the main central thrust mct zone along the bhagirathi valley comprises a wide zone of mylonitic quartzite and deep level tectonites the systematic variation of finite strain parameters es k and v in the mylonites indicates heterogeneous deformation which is determined to vary between simple shear and non coaxial flattening type in such a strain regime the outer boundary of the quartz clasts are no longer preserved thus leading to an error in finite strain measurement in order to supplement the finite strain studies anisotropy of magnetic susceptibility ams analyses were carried out on the mylonitic quartzites a systematic variation in degree of anisotropy p with distance from the mct is documented and is interpreted to be tectonic in origin based on these results it is concluded that p can be used as a strain intensity gauge at least on an outcrop scale where a systematic variation in p values from one part of the outcrop to the other can be established however the quantitative relation between principal axes of finite strain ellipsoid and ams axes magnitude of principal susceptibility difference k1 and k3 and finite strain magnitude e1 ln 1 e1 and e3 ln 1 e3 were related by a logarithmic relationship with a correlation coefficient of 0 844

rayleigh wave tomography of china and adjacent regions 1 this paper presents a tomographic study on the s wave velocity structure of china and adjacent regions group velocity dispersions of fundamental rayleigh waves along more than 4000 paths were determined with frequency time analysis the study region was divided into a 1 1 grid and velocities in between grid nodes were calculated by bilinear interpolation the occam s inversion scheme was adopted to invert for group velocity distributions this method is robust and allows us to use a fine grid in model parameterization and thus helps to restore a more realistic velocity pattern checkerboard tests were carried out and the lateral resolution was estimated to be 4 6 in china and its eastern continental shelves the resulting group velocity maps from 10 to 184 s showed good correlation with known geological and tectonic features the pure path dispersion curves at each node were inverted for shear wave velocity structures the three dimensional velocity model indicates thick lithospheres in the yangtze and tarim platforms and hot upper mantles in baikal and western mongolia coastal area and continental shelves of eastern china and indochina and south china sea regions the tibetan plateau has a very thick crust with a low velocity zone in its middle beneath the crust a north dipping high velocity zone mimicking a subducting plate reaches to 200 km in depth and reaches to the kunlun mountains northward in northern tibet a low velocity zone immediately below the moho extends eastward then turns southward along the eastern edge of the plateau until it connects to the vast low velocity area in indochina and the south china sea

timing of the north south china collision from new middle to late mesozoic paleomagnetic data from the north china block we collected paleomagnetic samples from 43 sites of middle and upper jurassic and cretaceous rocks along a 250 km wide east west trending belt located north of the suture between the north and south china blocks in each case laboratory treatment isolates dual polarity antipodal magnetizations that are demonstrably pre folding the paleomagnetic poles show very good coherence with coeval poles from other localities from the north china block ncb suggesting the apparent polar wander path apwp from the ncb is not distorted by post middle jurassic local vertical axis block rotations a comparison of the ncb apwp with that from the south china block suggests that collision continued after the early jurassic and ended no later than the late jurassic a cusp in the ncb apwp coincides with a marked angular unconformity that separates middle jurassic from upper jurassic and younger sediments together these observations indicate that the scissor like collision and associated deformation ceased at the middle to late jurassic boundary 159 ma the time of rapid 1 ma relative angular velocity between the two plates 225 to 190 ma coincides with a peak in u pb and ar ar dates obtained from metamorphic rocks in the suture the combined late jurassic and cretaceous paleomagnetic data from both blocks suggest that the united china plate traveled 2000 km north relative to eurasia during the late jurassic before docking in the cretaceous this younger collision event is also marked by a cusp on the cretaceous part of the eurasian apwp

degree of magnetic anisotropy as a strain intensity gauge in a saturated finite strain zone abstract anisotropy of magnetic susceptibility ams analyses were carried out on mylonitic quartzites from the footwall of the main central thrust zone garhwal higher himalaya india a systematic increase in degree of magnetic anisotropy p with distance from the main central thrust is documented even in a saturated finite strain zone on the basis of correlation between orientation of principal ams axes and multidomain magnetite and the mylonitic foliation and lineation direction strain related recrystallization was inferred to be the cause of development of the magnetic fabric based on these results it is concluded that if p is not controlled by k m then the former can be used as a strain intensity gauge supplementary material three dimensional finite strain parameters and rock magnetic and ams datasets are available at http www geolsoc org uk sup18328

jiangnan orogen in south china developing from divergent double subduction abstract the jiangnan orogen is considered as a continent continent collisional belt resulting from the closure of a meso neoproterozoic ocean separating the southeastern margin of the yangtze block from the northwestern margin of the cathaysia block recent data indicate the existence of early neoproterozoic 1000 825 ma volcanic arc assemblages on both sides of the orogen suggesting that the ocean lithosphere between the yangtze and cathaysia blocks must have undergone divergent double sided subduction during the period of 1000 825 ma the divergent double subduction eventually resulted in the closure of the ocean basin at 825 ma leading to the soft collision of the yangtze and cathaysia blocks to form the jiangnan orogen without involvement of continental deep subduction high grade metamorphism of continental crust and uplift exhumation of high grade metamorphic rocks shortly after the collision the initial detachment of the ocean lithosphere from the overlying crust and sedimentary sections induced underplating of mantle magmas triggering partial melting of accretionary wedge strata to form some peraluminous s type granites in the period 825 815 ma finally the sinking of the oceanic slab pulled down the overlying strata to form some basins in which the banxi group and its equivalent strata including bimodal volcanic rocks were formed in the period 815 750 ma

deep electrical structure and geological significance of tongling ore district abstract the tongling ore district is one of the most important cu polymetal producers in the lower yangtze river metallogenic belt of china to understand deep crust structure beneath the tongling ore district more comprehensively and objectively and to explore important scientific issues such as characteristics of lithospheric evolution the relationship between the crustal structure and metallogenesis it is necessary to study the variation of crust electrical structure in the study area for this purpose six magnetotelluric mt sounding profiles have been completed by sinoprobe 03 project based on the imaging results of the six profiles electrical conductivity structure of the crust has been analyzed the result shows that the geology beneath the profiles is characterized by layering in vertical direction and block division in horizontal direction the high resistivity features are related to the transparent zones detected at the upper crust in reflection seismic surveys the absence of reflectivity from the depths indicates a homogeneous zone which could often be related to granitic intrusions combining the regional tectonic evolution mt and other geophysical data we suggest that the high resistivity bodies crossing the tongling uplift reveals the center of magma activity and the layering in vertical direction in the lower crust reveals a normal lower crust model and propose a small scale underplating model which is different with both the partial melting of a thickened or delaminated lower continental crust model and the oceanic ridge subduction the underplating might be related to small scale lithospheric or lower crust delamination controlled by deep fault during the early cretaceous the result of this mt survey is important for constraining lithospheric evolution and metallogenesis in the lower yangtze metallogenic belt such as collision of north china craton and yangtze craton underplating models deep structure mechanisms of ne sw trending rift as well as graben basin and probable metallogenesis

where is the north china south china block boundary in eastern china the qinling dabieshan belt results from the collision of the north china and south china blocks the eastern extension of the belt the sulu area consists of several stacked units from top to bottom these are 1 a weakly metamorphosed slate sandstone unit of neoproterozoic paleozoic age 2 a gneiss quartzite unit metamorphosed under high pressure p conditions 3 a marble amphibolite unit 4 an ultrahigh p unit that was very deeply subducted and then exhumed and 5 a migmatitic dome resulting from the melting of ultrahigh p rocks all these units underwent the same deformation which was characterized by a top to the northwest extensional ductile shearing during their exhumation granulite facies restites in the migmatite are not significantly different from granulitized eclogites derived from the ultrahigh p rocks the migmatite exhibits the same structural and petrologic evolution as that determined for the other units the resemblance of petrologic and structural features and the lack of ocean basin rock shows that the boundary between the north china block and south china block must be placed north of the sulu area

exhumation history of the nw indian himalaya revealed by fission track and 40ar 39ar ages new fission track and ar ar geochronological data provide time constraints on the exhumation history of the himalayan nappes in the mandi beas valley tso morari transect of the nw indian himalaya results from this and previous studies suggest that the sw directed north himalayan nappes were emplaced by detachment from the underthrusted upper indian crust by 55 ma and metamorphosed by ca 48 40 ma the nappe stack was subsequently exhumed to shallow upper crustal depths 10 km by 40 30 ma in the tso morari dome northern section of the transect and by 30 20 ma close to frontal thrusts in the baralacha la region from the oligocene to the present exhumation continued slowly metamorphism started in the high himalayan nappe prior to the late oligocene high temperatures and anatexis of the subducting upper indian crust engendered the buoyancy driven ductile detachment and extrusion of the high himalayan nappe in the zone of continental collision late extrusion of the high himalayan nappe started about 26 ma ago accompanied by ductile extensional shearing in the zanskar shear zone in its roof between 22 and 19 ma concomitant with thrusting along the basal main central thrust to the south the northern part of the nappe was then rapidly exhumed to shallow depth 10 km between 20 and 6 ma while its southern front reached this depth at 10 5 ma

geometrical analysis of mesoscopic shear zones in the crystalline rocks of mct zone of garhwal higher himalaya abstract the crystalline rocks of the mct zone of garhwal higher himalaya exhibit well preserved mesoscopic shear zones majority of these shear zones are of ductile and brittle ductile type with both sinistral and dextral sense of movement detailed analysis of mesoscopic shear zones reveals that sinistral shear zones exhibit a strike variation from nne to ene and dextral shear zones exhibit variation from nnw to wnw directions thus forming a conjugate pair the bisectors of statistically preferred orientations of the two sets of the shears indicate that they generated due to nne ssw horizontal compression these dextral and sinistral shear zones exhibit strike slip geometry developed during progressive ductile shearing

sm nd and rb sr isotopic chronology and cooling history of ultrahigh pressure metamorphic rocks and their country rocks at shuanghe in the dabie mountains central china abstract ultrahigh pressure metamorphic uhpm rocks at shuanghe area in the dabie mountains occur as a uhpm block within the regional granitic gneiss a sm nd isochron defined by garnet omphacite rutile from coesite bearing eclogite yield an age of 226 3 3 2 ma another sm nd isochron defined by garnet two phengites form uhp gneiss yield a similar age of 226 5 2 3 ma the two consistent sm nd ages defined by three uhpm minerals suggest that nd isotopic equilibrium between uhpm minerals in these rocks during uhp metamorphism has been achieved this may correspond to the age of peak metamorphism with an average metamorphic temperature of 800 c it is demonstrated that the retrograde metamorphism of uhpm rocks occurred in an open chemical system whereas the nd and sr isotopic systematics of uhpm minerals may have remained closed nd and sr isotopic disequilibrium between uhpm minerals and retrograde metamorphic minerals has been observed therefore the tie line of garnet or phengite and whole rock containing retrograde metamorphic minerals gives an old sm nd age and a young rb sr age respectively with no geologic significance however a rb sr ages of 219 0 6 6 ma defined by phengite garnet from a uhpm gneiss indicates that the uhpm rocks at shuanghe cooled down to 500 c at that time it suggests that the uhpm rocks at shuanghe experienced the first rapid cooling during 226 219 ma on the other hand the rb sr ages of 174 7 8 to 169 2 3 3 ma defined by retrograde minerals amphibole or biotite with closure temperature ranging from 450 to 300 c and intensely retrograded metamorphic rocks reflect a second rapid cooling during this time interval this is consistent with the rapid cooling time of 190 170 ma of the orthogneiss in the dabie su lu uhpm belt obtained by ar ar dating method in contrast to uhpm rocks the nd isotopic composition of the garnet in granitic gneiss has been reset during retrograde metamorphism and is in equilibrium with those of retrograde epidote and biotite which yields a sm nd isochron age of 213 5 ma indicating the retrometamorphic time corresponding to amphibolite facies in addition the biotites from the granitic gneiss yield rb sr ages of 171 173 ma similar to those of the uhpm rocks these data suggest that the country rocks granitic gneiss may have a similar cooling history to the uhpm rocks at shuanghe two stages of rapid cooling of uhpm rocks at shuanghe may correspond two stages of fast uplift the initial rapid uplifting and cooling of uhpm rocks during 226 219 ma may be caused by compression tectonics during subducting time of the continental crust whereas the later rapid cooling may reflect the exhumation of the entire subducted continental crust by extension during the early middle jurassic

ductile thrusting in the himalayas shear sense criteria and stretching lineations the most useful criteria used for the determination of the sense of shear can be found in the huge himalayan ductile shear zone known as m c t they allow the determination of the sense of shear in relation with an unequivocal southward directed intracontinental thrust the various criteria analyzed at different scales of observation show the permanence of the shear flow regime throughout the metamorphic evolution however most of the observed shear criteria are due to late nappe emplacement postdating the climax of the metamorphism the most striking feature is the stretching lineation which is roughly normal to the thrust trace all along the belt it parallels everywhere the shear direction and thus indicates the transport direction of the nappes the observed radial pattern of stretch trajectories implies that the local motion does not correspond to the average plate convergence

molybdenite re os and albite 40ar 39ar dating of cu au mo and magnetite porphyry systems in the yangtze river valley and metallogenic implications abstract the area of the middle lower yangtze river valley eastern china extending from wuhan hubei province to western zhenjiang jiangsu province hosts an important belt of cu au mo and fe deposits there are two styles of mineralization i e skarn porphyry stratabound cu au mo fe deposits and magnetite porphyry deposits in several nne trending cretaceous fault bound volcanic basins the origin of both deposit systems is much debated we dated 11 molybdenite samples from five skarn porphyry cu au mo deposits and 5 molybdenite samples from the datuanshan stratabound cu au mo deposit by icp ms re os isotope analysis nine samples from the same set were additionally analyzed by ntims on re os results from the two methods are almost identical the re os model ages of 16 molybdenite samples range from 134 7 2 3 to 143 7 1 6 ma 2 the model ages of the five samples from the datuanshan stratabound deposit vary from 138 0 3 2 to 140 8 2 0 ma with a mean of 139 3 2 6 ma their isochron age is 139 1 2 7 ma with an initial os ratio of 0 7 8 1 mswd 0 29 these data indicate that the porphyry skarn systems and the stratabound deposits have the same age and suggest an origin within the same metallogenic system albite 40 ar 39 ar dating of the magnetite porphyry deposits indicates that they formed at 123 to 125 ma i e 10 20 ma later both mineralization styles characterize transitional geodynamic regimes i e the period around 140 ma when the main ns trending compressional regime changed to an ew trending lithospheric extensional regime and the period of 125 115 ma of dramatic ew trending lithospheric extension

reflection seismic imaging of the lujiang zongyang volcanic basin yangtze metallogenic belt an insight into the crustal structure and geodynamics of an ore district abstract five intersecting reflection seismic profiles were shot over the lu zong volcanic basin one of the seven ore districts in the middle and lower yangtze river metallogenic belt in east china the seismic data reveal diversified reflective patterns in the upper crust which are interpreted as horsts and grabens in general the middle crust presents both thrust related structures and boudinage and pinch and well structure on different profiles suggesting a strong structural heterogeneity which developed due to a change from compressional to extensional movement below the central part of a volcanic basin the lower crust shows an obvious reflective anisotropy both at the crust mantle and the brittle ductile transition in the crust this feature may be explained by magma system emplaced along a ne trending fault or weak zone at different depths the tan lu fault zone is present as a northwest dipping fault extending to the middle crust the lower crust beneath the tan lu fault and peripheries is possibly of dabie affinities the lu zong volcanic basin is clearly defined as an asymmetric graben with master faults on both the east and north flanks a contrasting reflection character suggests a division of the basin into northern and southern parts separated by a concealed fault the relatively thin volcanic sequences inferred to be 800 1000 m in thickness cover triassic and paleozoic sequences at depth that provide an exploration target in the northern part for deep porphyry and skarn deposits the dominant deformation reflects nw se oriented compression and extension formed during the yanshanian orogenic episode the contraction deformation is characterized by thrust faults with imbrication of crustal slices whereas the extension deformation is featured by normal faults regional detachments and basins underplating of mantle derived magma and a multi layer magma intrusion model are proposed to explain both seismic features and the ne striking lu zong volcanism and igneous intrusions in the region

geothermobarometry and development of inverted metamorphism in the darjeeling sikkim region of the eastern himalayan abstract the darjeeling sikkim region provides a classic example of inverted himalayan metamorphism the different parageneses of pelitic rocks containing chlorite biotite garnet staurolite kyanite sillimanite plagioclase and k feldspar are documented by a variety of textures resulting from continuous and discontinuous reactions in the different zones microprobe data of coexisting minerals show that xmg varies in the order garnet staurolite biotite chlorite white mica is a solid solution between muscovite and phengite garnet is mostly almandine rich and shows normal growth zoning in the lower part of the main central thrust mct zone and reverse zoning in the upper part of the zone chemographical relations and inferred reactions for different zones are portrayed in afm space in the low grade zones oriented chlorites and micas and rolled garnets grew syntectonically and were succeeded by cross cutting chlorites and micas and garnet rims in the upper zones sillimanite kyanite and staurolite crystallized during a static inter kinematic phase p t contitions of metamorphism estimated through different models of geothermobarometry are estimated to have been 580 c for the garnet zone to a maximum of 770 c for the sillimanite zone the preferred values of pressure range from 5 0 kbar to 7 7 kbar models to explain the inverted metamorphism include overthrusting of a hot high himalayan slab along a c 5 km wide ductile mct zone and the syn or post metamorphic folding of isograds

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

thermal model for the zanskar himalaya abstract crustal thickening along the northern margin of the indian plate following the 50 ma collision along the indus suture zone in ladakh caused widespread high temperature medium pressure barrovian facies series metamorphism and anatexis in the zanskar himalaya metamorphic isograds are inverted and structurally telescoped along the main central thrust mct zone at the base of the high himalayan slab along the zanskar valley at the top of the slab isograds are the right way up and are also telescoped along northeast dipping normal faults of the zanskar shear zone zsz which are related to culmination collapse behind the miocene himalayan thrust front between the mct and the zsz a metamorphic anatectic core within sillimanite grade rocks contains abundant leucogranite granite crustal melts of probable himalayan age a thermal model based on a crustal scale cross section across the zanskar himalaya suggests that m1 isograds developed during early himalayan barrovian metamorphism were overprinted during high grade mct related anatexis and folded around a large scale recumbent fold developed in the hanging wall of the mct

3 d velocity model beneath the middle lower yangtze river and its implication to the deep geodynamics abstract a mineralization zone exists in the middle and lower yangtze river mlyr region previous studies have shown that this zone might be caused by the rich ejection of magma in the mesozoic we have applied the teleseismic tomography method to determine a 3 d p wave velocity structure of the mantle down to 500 km depth beneath this region by using 14 740 p wave arrival times collected from 519 teleseismic events recorded by 46 portable and 47 static seismic stations located in this region the relative residual times used for the tomography are calculated by the modified multi channel cross correlation method which increases not only the efficiency but also the data precisions up to 0 01 s the grid space is set as 1 1 horizontally and 50 100 km vertically our tomographic results show that the lithosphere with high velocity anomalies is separated into two parts one locates above the depth of 100 km and the other at the depths between 250 km and 400 km and the asthenosphere with low velocity anomalies locates between these two parts of lithosphere which indicates that the lithosphere delaminated together with the asthenosphere upwelling the detachment of lithosphere might relate to the subduction of the western pacific slab therefore our study provides clear evidence to geoscientists for understanding the deep dynamic process beneath this region and a three stage geodynamic process is developed based on the new evidence

deformational and strain patterns of an intracontinental collision ductile shear zone an example from the higher garhwal himalaya the higher himalayan metamorphic belt of garhwal appears to have evolved in a major 15 20 km wide ne dipping ductile shear zone of the overthrust type due to intracontinental crustal shortening during cenozoic continental collision within the indian plate out of the four major distinct deformation phases the most widespread d2 deformation phase is marked by a prominent s2 foliation axial planar to the reclined and recumbent f2 folds a coaxial ne plunging l2 stretching lineation and syntectonic growth of index metamorphic minerals these deformational structures are developed during the sw directed d2 ductile shear deformation irrespective of the orientation of the s2 foliation and thrust zones regional strain patterns in quartzite of the lesser himalayan garhwal group beneath the main central thrust mct and the central crystalline zone using rf data on quartz clasts and aggregates reveal that the maximum values of finite strain es are attained within narrow mylonitic shear zones within the broad ductile shear zone during its progressive translation towards higher structural levels in late d2 and d3 deformation phases the shortening was accommodated by development of thrust sheets with the mct and jutogh thrust surfaces coinciding with zones of mylonite and maximum strain at higher levels ductile shear zones became compressional brittle ductile zones which resulted in the development of sw verging f3a and f3b folds during the southward migration of the thrust sheets in the lesser himalaya

constraints on the jurassic swing of the apparent polar wander path for the north china block summary in order to better constrain the mesozoic apparent polar wander path for the north china block late triassic and early jurassic sandstones in the ordos basin and early jurassic sandstones in the datong area were collected at 37 sites thermal demagnetization isolated a high temperature magnetization component with unblocking temperatures of 500 580 c from five sites of the early jurassic yongdingzhuang formation in the datong area palaeomagnetic directions from the five sites yielded a positive fold test at the 95 per cent confidence level these directions combined together with previously reported ones give a new early jurassic mean direction d 358 0 i 50 6 95 4 4 and corresponding palaeomagnetic pole 81 5 n 302 5 e a95 5 5 the palaeomagnetic pole is regarded as representative of the north china block because this pole is derived from two localities in the undisputed part of the north china block comparisons of the early jurassic pole for the north china block with coeval ones for the south china block identify significant differences between the poles from the two regions at early jurassic times the north china block was at the peak of rotational motion associated with the collision of the north and south china blocks the north china block underwent a counterclockwise rotation of 12 32 with respect to the south china block since the early jurassic at least 17 per cent of the total rotational motion remaining at that time

reconciling records of ice streaming and ice margin retreat to produce a palaeogeographic reconstruction of the deglaciation of the laurentide ice sheet abstract this paper reconstructs the deglaciation of the laurentide ice sheet lis including the innuitian ice sheet from the last glacial maximum lgm with a particular focus on the spatial and temporal variations in ice streaming and the associated changes in flow patterns and ice divides we build on a recent inventory of laurentide ice streams and use an existing ice margin chronology to produce the first detailed transient reconstruction of the ice stream drainage network in the lis which we depict in a series of palaeogeographic maps results show that the drainage network at the lgm was similar to modern day antarctica the majority of the ice streams were marine terminating and topographically controlled and many of these continued to function late into the deglaciation until the ice sheet lost its marine margin ice streams with a terrestrial ice margin in the west and south were more transient and ice flow directions changed with the build up peak phase and collapse of the cordilleran laurentide ice saddle the south eastern marine margin in atlantic canada started to retreat relatively early and some of the ice streams in this region switched off at or shortly after the lgm in contrast the ice streams draining towards the north western and north eastern marine margins in the beaufort sea and in baffin bay appear to have remained stable throughout most of the late glacial and some of them continued to function until after the younger dryas yd the yd influenced the dynamics of the deglaciation but there remains uncertainty about the response of the ice sheet in several sectors we tentatively ascribe the switching on of some major ice streams during this period e g m clintock channel ice stream at the north west margin but for other large ice streams whose timing partially overlaps with the yd the drivers are less clear and ice dynamical processes rather than effects of climate and surface mass balance are viewed as more likely drivers retreat rates markedly increased after the yd and the ice sheet became limited to the canadian shield this hard bed substrate brought a change in the character of ice streaming which became less frequent but generated much broader terrestrial ice streams the final collapse of the ice sheet saw a series of small ephemeral ice streams that resulted from the rapidly changing ice sheet geometry in and around hudson bay our reconstruction indicates that the lis underwent a transition from a topographically controlled ice drainage network at the lgm to an ice drainage network characterised by less frequent broad ice streams during the later stages of deglaciation these deglacial ice streams are mostly interpreted as a reaction to localised ice dynamical forcing flotation and calving of the ice front in glacial lakes and transgressing sea basal de coupling due to large amount of meltwater reaching the bed debuttressing due to rapid changes in ice sheet geometry rather than as conveyors of excess mass from the accumulation area of the ice sheet at an ice sheet scale the ice stream drainage network became less widespread and less efficient with the decreasing size of the deglaciating ice sheet the final elimination of which was mostly driven by surface melt

thermobarometric constraints on the thermal history of the main central thrust zone and tibetan slab eastern nepal himalaya the main central thrust mct south of mt everest in eastern nepal is a 3 to 5km thick shear zone separating chlorite bearing schist in the lower plate from sillimanite bearing migmatitic gneiss in the overlying tibetan slab the metamorphic grade increases through the mct zone toward structurally higher levels previous workers have suggested that either post or synmetamorphic thrust movement has caused this inversion of metamorphic isograds in an effort to quantify the increase in grade and to constrain proposed structural relations between metamorphism and slip on the fault four well calibrated thermobarometers were applied to pelitic samples collected along two cross strike transects through the mct zone and tibetan slab results show an increase in apparent temperature up section in the mct zone from 778 k to 990 k and a decrease in temperature to 850 k in the lower tibetan slab which is consistent with synmetamorphic thrust movement a trend in calculated pressures across this section is less well defined but on average decreases up section with a gradient of 28mpa km resembling a lithostatic gradient pressure temperature paths for zoned garnets from samples within the mct zone modelled using the gibbs method show a significant decrease in temperature and a slight decrease in pressure from core to rim which might be expected for upper plate rocks during synmetamorphic thrust movement samples from the uppermost tibetan slab yield higher temperatures and pressures than those from the lower tibetan slab which may be evidence for later resetting of thermobarometers by intrusion of the large amounts of leucogranite at that structural level

the lower crust conductor from nanjing ning wuhu wu area in the middle and lower reaches of yangtze river preliminary results from 3d inversion of magnetotelluric data abstract a series of long magnetotelluric mt profiles across the ning wu basin and adjacent areas were deployed by sinoprobe 03 deep drilling project and 3d exploration technology integration project from 2011 to 2012 broadband mt data were acquired at 349 sites with a frequency range of 0 0003 320 hz preliminary three dimensional inversion of this dataset revealed a striking feature of extensive areas with high conductivity in the lower crust combined with regional heat flow results relative water content was calculated from the resistivity model suggesting that a local melt or shear fracture may exist in the lower crust which is likely to be hot and weak the location of the upper mantle uplift and yangtze river deep fault were hence inferred from the electrical properties furthermore the yangtze river fault may be composed of a number of minor faults

u pb zircon ages constrain the architecture of the ultrahigh pressure qinling dabie orogen china new shrimp and tims zircon ages 40ar 39ar ages and eclogite locations contribute significantly to our understanding of the ultrahigh pressure dabie shan 1 the geographic extent of the yangtze craton that was subducted to ultrahigh pressure extends to the northern edge of the dabie shan 2 the northern half of the dabie shan is a magmatic complex intruded over a 10 myr interval between 137 and 126 ma that accommodated 100 n s stretching of the pre existing collisional architecture 3 granitic orthogneisses and enclosing ultrahigh pressure paragneisses have indistinguishable zircon populations the population of triassic zircon ages ranges from 219 to 245 ma leading us to question the prevailing assumption that 219 ma zircons formed at ultrahigh pressure and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation

geometry classification and kinematics of s c and s c fabrics in the mushandike area zimbabwe planar fabrics in small dextral shear zones in the mushandike area of the zimbabwe archaean craton consist of a sigmoidal primary foliation s displaced by discrete shears c or c these planar fabrics can be divided into porphyroclastic megacrystic and banded types depending mainly on the values of the angles between s and c or c surfaces the structures that define s surfaces and the spacing of c or c surfaces the values of the angles between the foliations are proposed as the basis for a general classification of planar fabrics in shear zones the values of the angles depend on microstructures and strain microstructural evidence shows that slip did not occur on s surfaces but only on c or c surfaces a model for the development of the fabrics proposes that s surfaces lie parallel to the xy plane of the local finite strain ellipsoid in domains of simple shear between c or c surfaces c surfaces may form in the orientation of a coulomb failure surface the field evidence suggests that an increase in bulk strain is at least partly accommodated by an increase in local strain in domains between c or c surfaces as the s surfaces rotate with the local finite strain ellipsoid

the subduction and continental collision of the north china and yangtze blocks magnetotelluric evidence from the susong anqing section of western anhui china

geologic framework and tectonic evolution of the qinling orogen central china abstract the geologic framework of the qinling orogen was built up through interplay of three blocks the north china block including the north qinling the south qinling and the south china block separated by the shangdan and mianlue sutures the shangdan suture resulted from middle paleozoic collision of the north china block and the south qinling the mianlue suture resulted from late triassic collision of the south qinling and the south china block present upper crust of the qinling is structured dominantly by thrust fold systems the north qinling displays thick skinned deformation with crystalline basement involved whilst the south qinling is characterized by thin skinned thrusts and folds detached above the lower sinian two types of precambrian basement crystalline and transitional are defined according to lithology and metamorphic grade and different in age stratigraphic and sedimentary architecture is characterized by distinct zonation the qinling orogen experienced a prolonged continental divergence and convergence between blocks during the period from late neoproterozoic to early paleozoic times the south qinling was the northern margin of the south china block and the north qinling was the southern margin of the north china block separated by a proto tethyan qinling ocean the north qinling evolved into an active margin when the proto tethyan qinling ocean subducted northward during ordovician time collision of the south and north qinling took place in middle paleozoic along the shangdan suture synchronous with the collision rifting occurred at the southern rim of the south qinling and was followed by the opening of the paleo tethyan qinling ocean during the late paleozoic resulting in the splitting of the south china block from the south qinling collision of the south qinling and the south china block came about in the late triassic along the mianlue suture the late triassic collisional orogeny caused extensive fold and thrust deformation and granitoid intrusions throughout the qinling and led to final amalgamation of the north and south china blocks

fossil oceanic subduction zone beneath the western margin of the trans north china orogen magnetotelluric evidence from the luliang complex abstract the trans north china orogen tnco is a major collisional belt along which the western and eastern blocks of the north china craton were amalgamated through subduction accretion collision tectonics during paleoproterozoic the luliang complex along the western margin of the tnco is thought to be the trace of an ancient suture zone here we employ magnetotelluric array to build a lithosphere scale high resolution 3 d resistivity model for this region between 36 n and 38 n e w trending slices of the resistivity model show relatively high conductivity 10 100 m lithospheric mantle of the western block that extends eastward beneath the relatively high resistivity lithosphere of the tnco at depths from 80 to 180 km we interpret this east dipping upper mantle conductivity contrast as the lithosphere scale suture between the western block and tnco that formed through eastward subduction the enhanced conductivity of the fossil oceanic subduction zone might be a reflection of the reduction in grain size of olivine grains in mantle peridotite by the localized shearing stress associated with the subduction combined with the interconnecting high conductivity minerals along grain boundaries formed through metasomatism by slab derived fluids similar subduction related boundaries are absent in the resistivity models beneath the northern luliang complex near 39 n where prominent high conductivity anomalies indicating asthenosphere upwelling and lower crustal underplating are seen such mantle architecture could result from the subduction of an oceanic ridge or slab window as also proposed for the origin of the subduction related luyashan charnockite suite in this region our data also suggest that the phanerozoic tectonic and magmatic events in this region only locally modified the precambrian lithospheric tectonic framework without total destruction we propose a 3 d model to illustrate the contrasting paleoproterozoic tectonic evolution between the northern and southern domains of the luliang complex and the tnco during the transition stage between the termination of oceanic subduction and the subsequent continental collision

development of major structures across the northwestern himalaya india the himalaya originated as a result of subduction of the tethyan oceanic crust that lay between india and tibet followed by continent continent collision a petrotectonic assemblage of the subduction complex consisting of the shergol ophiolitic melange the nidar ophiolite the nindam flysch formation and blue schist rocks were obducted over the indian margin during initial collision of the indian continent with tibet in the middle eocene the collision was followed by continued convergence of india against tibet resulting in crustal shortening both in the indian and the tibetan margins this crustal shortening produced regional metamorphism and polyphase deformation of the higher himalaya zone the tso morari crystallines and the pangong tso group continued convergence produced major folds nappes and thrusts of the higher tethys and lesser himalayas on the indian plate and the indus shyok and karakoram zones of the trans himalaya the major structures include the main karakoram thrust zanskar thrust tso morari dome zanskar synform sum dome kashmir chamba nappe panjal thrust and mbt the compressional phase was followed by an uplift phase in the early miocene which formed a foredeep to the south in front of the uplifted cordillera the siwalik group sediments were deposited in this foredeep and were later folded and faulted the folds and thrusts in the outer himalaya belt are younger than those of other tectonic zones of the himalaya

a tectono genetic model for porphyry skarn stratabound cu au mo fe and magnetite apatite deposits along the middle lower yangtze river valley eastern china abstract the middle lower yangtze river valley metallogenic belt yrb situated along the northern margin of the yangtze craton is characterized by porphyry skarn stratabound cu au mo fe deposits in the areas of uplift and magnetite apatite deposits in cretaceous fault basins following detailed field investigations and a review of published data we recognize two episodes of magmatism and mineralization in the yrb 1 156 137 ma high k calc alkaline granitoids associated with 148 135 ma porphyry skarn stratabound cu au mo fe deposits and 2 135 123 ma shoshonitic series associated with 134 9 122 9 ma magnetite apatite deposits a type granitoids and associated alkaline volcanic have a small age range from 126 5 to 124 8 ma and are temporally spatially and genetically associated with the second episode the geodynamic history of the yrb did not experience the paleozoic to mesozoic lithospheric thickening that took place in the north china craton this process is inferred to be linked to partial melting of the delaminated lower crust at high pressures resulting in the development of c type adakitic rocks the petrochemical and sr nd isotopic data show that both the shoshonitic series and a type granitoids are quite different from adakites with only some of the k calc alkaline granitoids having adakitic signatures previous ore genesis models were established based on an assumed relationship with adakites and a continuous tectono thermal evolution from 150 to 100 ma all data obtained for the middle lower yangtze river region consistently show that the tan lu regional strike slip fault zone initiated at 233 6 to 225 6 ma from the collision between the north china and yangtze cratons and was reactivated at ca 160 ma the tan lu fault was caused by the oblique subduction of the izanagi plate which along the yrb the low angle subducted slab and the overlying crust was disrupted or broken due to the disharmonious movement of the two blocks the high k calc alkaline granitoids magmas were derived from melting of the subducted slab with some input of crustal material these magmas were emplaced at the intersections between ne and ew trending faults and formed porphyry skarn stratabound cu au mo fe deposits between 156 and 137 ma after 135 ma the subducted plate changed its direction of motion to northeast now running parallel to the eurasian continental margin and leading to large scale continental extension the shoshonitic series and subsequent a type granitoids magmatism and the development of magnetite apatite ores in the yrb took place in both fault basins and ne trending rifts between 135 and 124 ma

thrust tectonics crustal shortening and the structure of the far eastern nepal himalaya balanced and restored structural sections across the far eastern nepal himalaya have been constructed in order to determine the structure and evolution of the himalayan orogenic wedge and the amount of tectonic shortening the region has undergone since the initiation of thrusting along the main central thrust mct the far eastern nepal himalaya is comprised of three distinct thrust bound tectonic packages the higher himalayan crystalline thrust sheet the lesser himalayan metasediment thrust package and the sub himalayan imbricate fan the higher himalayan crystallines consisting of kyanite and sillimanite bearing gneisses intruded by the miocene jannu leucogranites have been thrust over the lesser himalayan metasediments along the mct for a distance of 140 km to 175 km the lesser himalayan metasediments are a 12 km thick unit consisting primarily of phyllites metaquartzites and mylonitic augen gneisses in which garnet biotite and chlorite metamorphic zones are exposed in progressively deeper structural levels the lesser himalayan metasediment thrust package is underlain by a decollement the main detachment fault mdf which lies at a calculated depth of between 6 and 10 km underneath the mahabharat lekh and at a calculated depth of 20 to 25 km north of the tamar khola dome the tamar khola dome overlies a footwall ramp along the mdf where the mdf cuts upsection through the lesser himalayan metasediments the lesser himalayan thrust package probably has an internal structure aproximating a hinterland dipping duplex with the mct and the mdf corresponding to the roof and floor thrusts respectively both the tamar khola thrust an out of sequence breach thrust and the main boundary thrust mbt are splay thrusts off of the mdf the sub himalaya consisting of nonmetamorphosed sedimentary rocks displays an emergent imbricate fan geometry and is underlain by the southern continuation of the mdf which lies at a depth of 5 5 km to 6 km beneath the siwalik hills folding and thrusting within the lesser himalayan thrust package and the sub himalayan imbricate fan have accomodated 45 to 70 km of tectonic shortening total north south shortening across the higher lesser and sub himalaya of far eastern nepal south of the tibetan plateau has been of the order of 185 km to 245 km and has occurred at an average rate of 7 4 mm to 15 3 mm per year since the initiation of the mct between 16 and 25 ma

crustal and uppermost mantle velocity structure and its relationship with the formation of ore districts in the middle lower yangtze river region abstract in this study we conduct ambient noise tomography and teleseismic two plane wave tomography to probe the crustal and uppermost mantle structures in the middle lower yangtze river region the data used include 14 months from july 2012 to august 2013 of continuous vertical component ambient noise data and 219 teleseismic earthquakes recorded at 138 broadband seismic stations from chinese provincial networks and 19 temporary seismic stations deployed by china university of geosciences beijing first we apply ambient noise tomography to the collected ambient noise data to generate rayleigh wave group and phase velocity maps at 5 42 s periods and two plane wave tomography method to earthquake data to generate intermediate to long period phase velocity maps at 20 143 s periods we then combine the short to intermediate period velocity maps from ambient noise and the intermediate to long period phase velocity maps from earthquake data to generate broadband phase velocity maps at periods from 5 to 143 s by inverting these 5 143 s rayleigh wave velocity maps we construct a 3 d shear wave velocity model from the surface to 250 km depth in the middle lower yangtze river region the 3 d model shows that in the upper crust the basin regions including the jianghan hehuai subei hefei and nanyang basins are all featured with low velocities and the mountain regions with high velocities in the uppermost mantle a low velocity zone at 100 200 km depth is observed beneath the middle lower yangtze river metallogenic belt moreover our tomographic results show that the ningwu and ningzhen ore districts are clearly characterized by the strongest low velocity anomaly in the uppermost mantle at 70 200 km depth the depth extent of the low velocity zone becomes shallower and the amplitude of low velocity anomaly becomes larger from the southwest jiurui ore district to northeast ningwu ore districts the change pattern of the low velocity zone in the depth extent and the amplitude is consistent with the fact that peak ages of magmatic events along the middle lower yangtze river metallogenic belt progressively become younger and younger from 148 ma in the southwest to 125 ma in the northeast the observed low velocity zone may represent the cooling hot upper mantle which was partially molten in the past resulting from partial melting of the paleo pacific plate or of an enriched mantle source induced by the westward subduction of the paleo pacific plate the upwelling of the mantle derived magmas may result in the formation of these granitic rocks and coeval ores deposits along the middle lower yangtze river metallogenic belt

extrusion vs duplexing models of himalayan mountain building 1 discovery of the pabbar thrust confirms duplex dominated growth of the northwestern indian himalaya since mid miocene ongoing himalayan growth is generally thought to be dominated by duplexing and or extrusion processes these models may be tested by reconstructing himalayan fold thrust belt growth since the middle miocene however our knowledge of basic structural geometry remains too fragmentary to resolve the issue even in areas with rich stratigraphic diversity such as the northwestern indian himalaya in this region a primary outstanding question involves the uncertain relationship of the berinag thrust and the tons thrust structures with displacements of 80 km and 40 km respectively the uncertain geometry and kinematics allow for the complete range of duplexing or extrusion processes for the integrated kinematic history since the middle miocene to address this issue field mapping and kinematic analysis were performed to reconstruct the deformation of the lesser himalayan sequence in the northwest indian himalaya our results reveal a new discovery a 450 m thick top to southwest shear zone termed the pabbar thrust the pabbar thrust placed the outer lesser himalayan sequence the tons thrust hanging wall directly on the berinag group the berinag thrust hanging wall this discovery requires that the berinag thrust and tons thrust are in fact the same structure and discrete duplexing processes dominated growth of the northwest indian himalaya for the past 10 15 million years along strike extension of these kinematics and corresponding geometries is consistent with the observed orogenic framework and resolves a stratigraphic continuity problem across the india west nepal border where prior work suggests that structures are continuous but stratigraphy does not match

structure of the chamba nappe and position of the main central thrust in kashmir himalaya abstract the chamba nappe composed of an approximately 8 km thick sequence of late precambrian to jurassic age rocks is located between the higher himalaya crystallines hhc and the lesser himalayan lh formations of panjal imbricate zone piz in the kashmir himalaya to the south the panjal thrust demarcating the base brings the chamba nappe rocks over the panjal imbricate zone to the north the chamba nappe rocks lie over the metamorphic hhc along the south dipping chenab normal fault cnb a pervasive stretching lineation defined by a mineral lineation stretched pebbles and felspar phenocrysts plunges ne nne and occurs on the foliation cleavage surface this lineation is related to southward displacement of the chamba nappe the chamba nappe is folded by regional scale fold viz the chamba tandi and bharmor synclines and the tisa anticline these nw se trending folds structures were developed synchronously with southward thrusting of the chamba nappe the chamba nappe results from southwestward sliding of cover rocks from their basement hhc due to topographic uplift the main central thrust mct in kashmir himalaya is different from that of the kumaun and nepal himalaya the mct vaikrita thrust does not extend west of the beas river but it is exposed in the rampur window and the kishtwar window separating the hhc from the underlying lh rocks southward propagation of the mct from the window zone up cutting the overlying hhc is transferred to the panjal thrust which transports the chamba nappe to the south over the lesser himalayan formations

strain analysis of a northern apennine shear zone using deformed marble breccias abstract the alpi apuane region of the northern apennines appears to have been deformed within a large scale low angle shear zone with an overthrust sense of movement the presence of mineral stretching lineations folds progressively rotated into the x strain direction and schistosities which intersect the nappe boundaries at small angles suggest that a component of shear strain occurred during the deformation the strain ratios and orientations on two dimensional sections have been determined from deformed marble breccias reduction spots and oncalites data from three or more non perpendicular non principal sections have been combined to determine the finite strain ellipsoids at 33 sites within the shear zone the finite strains have been separated into components of simple shear longitudinal strain and volume change algebraic expressions have been derived and graphs constructed which enable components of and and and to be determined directly from a knowledge of strain ratio r within the shear zone and the angle between the principal strain direction and the shear zone boundary the alpi apuane data indicate that neither simple shear alone nor simple shear with volume change can satisfactorily explain the observed strains consideration of simple shear plus longitudinal strain leads to a general relationship in which the value of shear increases and the values of longitudinal strain change along a sw ne profile across the zone integration of the resulting shear strain distance curves gives a minimum displacement of 4 km within the shear zone combination of the finite strains with the total time of deformation known from k ar studies leads to average strain rates from 1 4 to 9 6 10 15 sec 1 a characteristic flat ramp flat geometry initially formed the boundaries of what was later to develop into the overthrust shear zone and deformation of the underlying crystalline basement is believed to have occurred by ductile shearing estimates of 21 crustal shortening for the region suggest that the crustal thickness prior to deformation was approximately 20 km in this part of the northern apennines

the caledonian skerrols thrust sw scotland microstructure and strain abstract strain measurements from quartz grain shapes in deformed jura quartzite around loch skerrols islay sw scotland indicate that a strain gradient is associated with the loch skerrols thrust which separates the jura quartzite from the bowmore sandstone progressive strain is associated with microstructural changes in the form of grain refinement through to dynamic recrystallization the maximum observed ratio of intracrystalline strain is 1 9 1 3 0 4 1 1 the x axis of the finite strain ellipsoid is close to the mesoscopic lineation in quartz mylonites in the thrust belt and the xy plane is sub parallel to the foliation the apparent flattening strain may be explained by superposition of simple shear thrust regime on initial flattening strain associated with the earlier stages of progressive development of the islay anticline this implies that the loch skerrols thrust is comparable to a break thrust strain induced quartz c axis preferred orientation increases in intensity as the thrust is approached at a higher strain the fabric is a type i cross girdle pattern with a point maximum within 20 of z fabric asymmetry both internal and with respect to an external frame defined by the principal strain directions attests to a westward transport of dalradian jura quartzite on the thrust

palaeomagnetic evidence and tectonic origin of clockwise rotations in the yangtze fold belt south china block fold axis strikes in the yangtze fold belt of the south china block scb undergo significant changes over distances of 1000 km this large scale variation provides an ideal opportunity to test the oroclinal bending hypothesis using palaeomagnetic methods which we have attempted by drilling the lower triassic daye formation limestones in western hubei province thermal demagnetization isolated two components in most samples a low unblocking temperature component 400 c unblocks univectorially towards the origin the htc passes the mcfadden fold test with an overall mean tilt corrected direction of dec 255 inc 24 n 7 95 9 scanning electron microscopy sem observations suggest an early diagenetic possible bio chemical origin for the magnetic extracts dominated by ti poor magnetite rock magnetic data show no evidence that the htc has been affected by tectonic or compaction strain our data together with previously published results suggest a general clockwise rotation pattern in the middle yangtze fold belt which is probably related to the collision between the north and scbs comparison of palaeomagnetic rotations with fold axis trends in the fold belt suggests that about 30 clockwise rotation occurred in the middle reaches of the yangtze river while a 15 difference in fold axis trends would be due to initial variation within the fold belt however since little is known about the timing of the clockwise rotation whether the middle yangtze fold belt is an orocline awaits further studies

the himalayan main central thrust pile and its quartz rich tectonites in central nepal as a consequence of the india asia collision a deeply rooted crustal slice has been moved southwards over the northern boundary of the indian plate at the present level of observation the main central thrust pile a rock pile of several kilometers thickness is affected by shear deformation which imprinted a flat foliation and a constant n s trending lineation these tectonites were deformed under a wide range of metamorphic conditions grading from lower greenschists up to the sillimanite field the m c t shear zone is studied from the view point of the quartz rich rocks over 100 samples helped to define a microstructural zonation roughly parallel to the thermal one 35 samples particularly rich in quartz have been used for the preferred orientation study of their c axes and a few of them for their a axes strong lattice orientations when undisturbed by mainly secondary recrystallization attest to large plastic strains in quartz the c axes are mostly distributed along unequally populated crossed girdles which may evolve into an inclined single girdle the a axes concentrate perpendicular to the more populated girdle concerning quartz the following is suggested 1 1 the deformation has a dominant shear component responsible for the dominant stretching lineation it could be simple shear in case of inclined single girdles 2 2 one set of a axes line up parallel to the shear direction slip on various planes in the a direction may be an important deformation mechanism 3 3 asymmetry in the lattice preferred orientations could be used to predict the sense of shear for the himalayan thrusting in more than 80 of cases

crustal structure beneath the middle lower yangtze metallogenic belt in east china constraints from passive source seismic experiment on the mesozoic intra continental mineralization abstract to understand the formation and the tectonic process of the mesozoic middle lower yangtze metallogenic belt ymb the sinoprobe program deployed a quasi linear passive source seismic array across the belt we performed receiver function profiling and measurement of shear wave splitting parameters with the collected data our results show that the moho depth varies significantly along the profile and that a mantle uplift exists right beneath the ymb we also found that the lower crust of the ymb is different from that of its adjacent areas in structure on the receiver function profile it possesses seismic anisotropy with direction roughly parallel to the belt our sks skks shear wave splitting results also show similar belt parallel azimuthal anisotropy right beneath the ymb we interpret the seismic anisotropy in the lower crust of the ymb as the result of mineral crystal alignment caused by melting and belt parallel flow in the mesozoic ore forming process besides we observed a nearly south dipping converter extending from shallow to lower crust beneath the hefei basin which most possibly resulted from the mesozoic crustal extension we interpret the mantle uplift and the crustal extensional structures to be consequences of asthenospheric upwelling during the mesozoic ore forming process our results suggest that the lower crust of the ymb was most likely one part of the multi level metallogenic magma system in the mesozoic magmatism and mineralization processes and the formation of the metallogenic belt to be a result much similar to the mash melting assimilation storage and homogenization cf hildreth and moorbath 1988 richards 2003 process first the asthenospheric upwelling resulted in a crustal extensional environment then the melts from the upwelling asthenosphere intruded into the lower crust of the ymb and assimilation occurred when they mixed with in situ lower crustal materials which led to the formation of adakitic like magma the adakitic like magma rose up along extensional structures and reacted with country rocks to form mineral deposits

paleomagnetic study of upper jurassic rocks from the sichuan basin tectonic aspects for the collision between the yangtze block and the north china block abstract a paleomagnetic study has been carried out on late jurassic redbeds in the sichuan basin at the northern part of the yangtze block upper jurassic brownish red sandstones and brownish red siltstones were collected in the penglaizhen formation at 26 sampling sites around jiangyang city 30 4 n 104 5 e thermal demagnetization isolated a high temperature magnetization component with a maximum unblocking temperature of about 690 c the primary nature of magnetization acquisition is ascertained through a 95 confidence level positive fold test as well as a positive reversal test the tilt corrected overall mean direction of 24 sites is d 25 9 i 26 7 95 4 7 leading to a paleomagnetic pole situated at 61 3 n 222 7 e a95 4 2 this pole position agrees with two other late jurassic poles from the sichuan basin indicating that this basin has behaved as a rigid block since late jurassic these three poles from the sichuan basin therefore provide a characteristic late jurassic pole for the stable part of the yangtze block 64 3 n 231 2 e a95 6 7 comparison of the late jurassic pole of the yangtze block with the contemporaneous pole for the north china block ncb identifies a significant difference implying that the relative movement between the yangtze block and the ncb continued during and probably after the late jurassic paleomagnetic and geological evidence reveals that the yangtze block collided first with the ncb during the early permian and continued to penetrate into it while undergoing a clockwise rotation until the early cretaceous

ages of ultrahigh pressure metamorphism and protolith orthogneisses from the eastern dabie shan u pb zircon geochronology the dabie shan contains two rare geologic features one the occurrence of ultra high pressure metamorphic rocks peak p 2 7 gpa uhp in the east central part of this belt and two some of these uhp rocks are characterized by anomalously 18o depleted oxygen isotopic compositions geochronological data are presented that constrain the age of the uhp metamorphism the ages of protoliths and potentially the age of hydrothermal alteration believed to be responsible for the anomalous oxygen isotopic compositions the age of uhp metamorphism in the eastern dabie shan is dated at 218 5 1 7 ma using u pb in zircon separated from host gneisses this age is identical within uncertainty with a less precise age using u pb in zircon separated from uhp eclogite 225 5 3 6 ma and with previously published ages from uhp eclogites of the dabie shan map relationships strongly support an in situ as opposed to exotic orgin of the uhp eclogites in this region the identical ages of uhp metamorphism in the gneisses and eclogites provides additional support for this interpretation the age of the protolith gneisses is dated as 772 5 9 5 ma based on the upper intercept of the zircons which accords well with other dates of granitic gneisses in the qinling belt the association of mafic and felsic magmatism is interpreted to reflect rifting along the northern margin of the yangtze block at this time these upper intercept ages may date the hydrothermal circulation of 18o depleted meteoric water which gave rise to the anomalously 18o depleted uhp protoliths of the dabie shan and sulu region the coincidence of these protolith ages with the early sinian glaciation of south china is consonant with the suggestion that the highly anomalous oxygen isotopic compositions in this region date from this time the upper intercept age of the eclogite is imprecisely determined at 447 82 79 ma but is younger than the host gneisses and is not characterized by anomalous oxygen isotopic compositions

himalayan main central thrust and its implications for himalayan inverted metamorphism abstract within the scheme of south directed orogenic polarity in the himalayan region the main central thrust mct developed at an intermediate stage between the collisional tectonics at the indus tsangpo suture in the north and the latest underthrusting at the main boundary thrust mbt in the south the proposition that the mct marks the base of the high himalayan central crystalline zone against the inner sedimentary belt of the lower himalayas creates confusion in its definition and its location in kashmir and especially in the eastern himalayas the problem can be resolved by defining the mct as the basal thrust of the crystalline nappe sequences either rooted at or detached as klippe from the central crystalline zone characteristically the nappe sequences show inverted metamorphism which generally starts at the basal thrust the redefined mct from the chlorite biotite grades and reaches the kyanite sillimanite grades with migmatites and anatectic granites in the highest tectonic levels the metamorphic rank of both the substrate and the overthrust rock units in the vicinity of the mct is generally low the inverted metamorphism may be explained by intracontinental underthrusting and resultant downbowing of the isotherms the redefined mct does not appear to represent the thrust zone along which the continental underthrusting was initiated it developed to the south of this zone as a possible sole thrust of the nappe stack of the crystalline rocks that overrode the parautochthonous sedimentary sequences of the lower himalayas the sequence of events associated with the mct would probably be repeated in the mbt

selecting and weighting spatial predictors for empirical modeling of landslide susceptibility in the darjeeling himalayas india in this paper we created predictive models for assessing the susceptibility to shallow translational rocksliding and debris sliding in the darjeeling himalayas india by empirically selecting and weighting spatial predictors of landslides we demonstrate a two stage methodology 1 quantifying associations of individual spatial factors with landslides of different types using bivariate analysis to select predictors and 2 pairwise comparisons of the quantified associations using an analytical hierarchy process to assign predictor weights we integrate the weighted spatial predictors through multi class index overlay to derive predictive models of landslide susceptibility the resultant model for shallow translational landsliding based on selected and weighted predictors outperforms those based on all weighted predictors or selected and unweighted predictors therefore spatial factors with negative associations with landslides and unweighted predictors are ineffective in predictive modeling of landslide susceptibility we also applied logistic regression to model landslide susceptibility but some of the selected predictors are less realistic than those from our methodology and our methodology gives better prediction rates although previous predictive models of landslide susceptibility indicate that multivariate analyses are superior to bivariate analyses we demonstrate the benefit of the proposed methodology including bivariate analyses

insight into modification of north china craton from seismological study in the shandong province 1 the study region is located in the southeast edge of the north china craton ncc which is an archean craton that underwent removal and modification of the lithospheric mantle during the late mesozoic cenozoic teleseismic data from the ncisp 1 experiment were used to image the velocity structures in lithosphere and to investigate the upper mantle anisotropy seismological results revealed that the present lithosphere consists of two parts with different structural features throughout including crustal fabric lithospheric velocity stratification and anisotropic patterns the formation of the contrasting structure is attributable to one of two possibilities the trans lithospheric tanlu fault zone tlfz operated as an asthenospheric upwelling channel and facilitated the ncc reactivation or the tlfz separates the study region into two lithospheric blocks with the ancient lithospheric materials remaining within the rejuvenated lithospheric mantle these two possibilities suggest that thermal chemical erosion may have played critical roles in the ncc reactivation

spatial analysis of mutual fault fracture and slope controls on rocksliding in darjeeling himalaya india in this paper the fry analysis was applied to study the spatial patterns in terms of trends of rockslides proportion analysis was applied to study the spatial associations between rockslides and slope aspects and distance distribution analysis was applied to study the spatial associations of rockslides with different sets of faults fractures based on trends in a study area in the darjeeling district india the results of applications of these spatial analytical techniques support a proposition of mutual fault fracture and slope controls on rocksliding in different parts of the study area deep seated rockslides are associated with a ne trending faults fractures and sse facing slopes b nw trending faults fractures and ene facing slopes and c nne trending faults fractures and se or ene facing slopes whereas shallow translational rockslides are associated with a nne trending faults fractures and se nw wnw or wsw facing slopes b wnw trending faults fractures and sw facing slopes c nnw trending faults fractures and ese or sw facing slopes and d nw trending faults fractures with sw facing slopes creating and integrating spatial evidence layers representing mutual fault fracture and slope controls on rocksliding can be achieved satisfactorily via application of evidential belief functions a predictive map of rockslide susceptibility derived by using slope aspects slope inclinations and proximity to the faults fractures that are spatially associated with rockslides is superior to that derives by using slope aspects slope inclinations and proximity to all faults fractures the proposed analytical methods are suitable for first pass regional scale assessment of rockslide susceptibility