tectonothermal evolution of the mayuan assemblage in the cathaysia block implications for neoproterozoic collision related assembly of the south china craton the mayuan assemblage in the cathaysia block southeast china consists of felsic paragneiss pelitic schist greenschist amphibolite marble calcsilicate and quartzite that underwent three episodes of deformation d1 d3 and four episodes of metamorphism m1 m4 in the early neoproterozoic the m1 assemblage consists of mineral inclusions defining an early foliation s1 within porphyroblasts represented by chlorite muscovite biotite plagioclase quartz in pelitic schist and actinolite chlorite epidote albite quartz enclosed in amphibolite m2 coincides with the development of the regional schistosity s2 and represents the formation of the porphyroblasts and growth of matrix minerals resulting in development of prograde metamorphic zones chlorite biotite garnet staurolite and kyanite zones m3 is simultaneous with the third phase of deformation d3 and produced sillimanite bearing mineral assemblages in pelitic schist and hornblende bearing assemblages in amphibo lite the last metamorphic episode m4 gave rise to the retrogressive assemblage chlorite muscovite in pelitic rock and actinolite chlorite epidote in amphibolite the sequence of mineral assemblages and history of metamorphic reactions built from the petrogenetic grid of pelites suggest a near isothermal decompression clockwise p t path for the mayuan pelitic schists using the tweequ software program the garnet biotite thermometer and garnet muscovite biotite plagioclase barometer yield p t conditions for m1 of 5 5 to 6 0 kb and 450 to 500 c and conditions for the garnet staurolite and kyanite zones of m2 of 6 0 to 7 0 kb and 550 to 600 c 6 0 to 7 5 kb and 600 c and 11 0 to 11 5 kb and 600 c the p t conditions of m3 were estimated at 570 to 625 c and 4 0 to 4 5 kb using the muscovite biotite thermometer and hornblende plagioclase geothermo barometer the garnet chlorite thermometer yields temperatures of 300 to 400 c for m4 but the pressures of m4 cannot be quantitatively estimated because of the lack of a suitable geobarometer these p t estimates also define an near isothermal decompression clockwise p t path which is involved in initial crustal thickening followed by rapid exhumation and final cooling and is related to amalgamation of the cathaysia and yangtze blocks to form the south china craton

cambrian depositional history of the zanskar valley region of the indian himalaya tectonic implications a well preserved cambrian section in the zanskar valley of northern india has previously been interpreted to record the transition from a passive to an active tectonic margin related to cambrian ordovician orogenesis this interpretation has been used to support the tectonostratigraphic interpretation of other successions across the tethyan himalaya our detailed paleoenvironmental analysis significantly revises the tectonic and depositional history of these cambrian deposits no definitive record of impending cambrian ordovician orogenesis is recorded in these late middle cambrian rocks a critical transition from an 125 m thick stromatolite bearing carbonate deposit the karsha formation into shale and sandstone of the kurgiakh formation was interpreted to represent tectonically induced drowning of a carbonate platform siliciclastic strata of the kurgiakh formation were thought to record deep water flysch deposition in a tectonically active foreland basin next to an arc trench system this interpretation was based on sandstone beds with classic bouma sequences we show that these event beds in the kurgiakh instead contain hummocky cross stratification quasi planar lamination and combined flow ripple stratification all of which reflect deposition in shallow marine storm influenced environments thus although the karsha carbonate platform may have been drowned it did not culminate in deep sea flysch deposition and this in turn eliminates a major line of evidence linking kurgiakh deposition to the onset of cambrian ordovician orogenesis other aspects of cambrian ordovician deposits of northern india also shed doubt on the proposed link between kurgiakh sedimentation and the cambrian ordovician orogenic event first our improved biostratigraphic database suggests that the transition from the karsha carbonate to the kurgiakh formation may have predated the main phase of cambrian ordovician orogenesis as recorded by overlying ordovician molasse by as much as 20 30 my second published data from the ordovician molasse indicate northward paleocurrents which are parallel to those recorded by siliciclastic deposits of the parahio formation below the karsha and thus are at odds with standard models of foreland basin development for the cambrian ordovician event our sedimentological analysis of depositional cycles of the parahio formation indicates that these strata record storm influenced environments from offshore marine to shoreface to fluvial settings this is at odds with previous paleoenvironmental interpretations that ranged from deep sea flysch to intertidal deposits paleocurrent data for marine and fluvial facies of the parahio formation in both zanskar and the spiti valley to the south indicate northeast sediment transport this supports the view that the parahio and overlying carbonate of the karsha formation record the ancient northern passive margin of india during the cambrian and that these strata may be distal equivalents of the younger cambrian deposits of the lesser himalaya

paleozoic to mesozoic deformation of eastern cathaysia a case study of the chencai complex zhejiang province eastern china and its tectonic implications

metamorphic rocks in central tibet lateral variations and implications for crustal structure insights about lateral variations in the age composition and structure of the central tibetan crust are provided by geologic investigations of metamorphic rocks in the qiangtang terrane previous studies have shown that a tectonic melange of triassic age with blueschist and eclogite bearing blocks within a greenschist facies matrix is exposed over an e w distance of 600 km in the central qiangtang terrane new mapping shows that the melange extends over a n s distance of 150 km nearly to the trace of the early mesozoic jinsha suture in the north the melange exposed structurally beneath upper paleozoic to mesozoic strata in the footwalls of early mesozoic normal faults is composed mostly of paleozoic metasedimentary and crystalline rocks these findings support the hypothesis that a large part of the central and northern qiangtang terrane crust is composed of supracrustal rocks the duguer range

lhasa terrane in southern tibet came from australia the u pb age and hf isotope data on detrital zircons from paleozoic metasedimentary rocks in the lhasa terrane tibet defi ne a distinctive age population of ca 1170 ma with e hf t values identical to the coeval detrital zircons from western australia but those from the western qiangtang and tethyan himalaya terranes defi ne an age population of ca 950 ma with a similar e hf t range the ca 1170 ma detrital zircons in the lhasa terrane were most likely derived from the albany fraser belt in southwest australia whereas the ca 950 ma detrital zircons from both the western qiangtang and tethyan himalaya terranes might have been sourced from the high himalaya to the south such detrital zircon connections enable us to propose that the lhasa terrane is exotic to the tibetan plateau system and should no longer be considered as part of the qiangtang greater india tethyan himalaya continental margin system in the paleozoic reconstruction of the indian plate as current models show rather it should be placed at the northwestern margin of australia these results provide new constraints on the paleogeographic reconstruction and tectonic evolution of southern tibet and indicate that the lhasa terrane evolved as part of the late precambrian early paleozoic evolution as part of australia in a different paleogeographical setting than that of the qiangtang greater india tethyan himalaya system

early paleozoic orogenesis along gondwana s northern margin constrained by provenance data from south china abstract the cambrian ordovician boundary unconformity in the southern part of the south china craton is spatially and temporally related to coeval orogenic activity along the indian margin of east gondwana detrital zircon age spectra from strata above and below the unconformity range in age from 3580 450 ma with peaks in the late mesoproterozoic and neoproterozoic the patterns are similar to time equivalent sequences elsewhere in south china and together with regional facies relationships and paleocurrent data indicate derivation from a gondwana source the disconformity at the base of the ordovician succession forms part of a regional break that has also been documented in the himalaya qiangtang lhasa sibumasu and western australia all these successions have similar detrital zircon age spectra suggesting derivation from common source s in south china the effects of this tectonic event are relatively mild and are represented by a local disconformity at the base of the ordovician succession but elsewhere in north gondwana this event is marked by an angular unconformity with metamorphism of older units and relatively widespread magmatic activity south china was likely located in a distal location to the northeast of the pulse of tectonic activity which was focused in the himalaya region and was close to the continent ocean boundary between northern gondwana and the proto tethys

discovery of middle late devonian and early permian magmatic events in east asia and their implication for the indosinian orogeny in south china insights from the sedimentary record

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

geochemical characteristics of organic rich shale upper yangtze basin implications for the late ordovician early silurian orogeny in south china abstract the tectonic sedimentary environment of the upper yangtze basin in china underwent significant changes from the late ordovician to early silurian the widely distributed shale formed during this interval has recorded key information about paleo ocean conditions analysis of major trace and rare earth elements was conducted on 135 shale samples collected from five wells in the study area the covariation of the mo u enrichment factors indicates that the upper yangtze basin in the wufeng stage was characterized by sedimentary differentiation the southern basin was dominated by a reducing bottom water environment under euxinic conditions the northern and central basins were dominated by a suboxic anoxic environment up to the early longmaxi stage the upper yangtze basin formed a relatively uniform sedimentary environment with gradually increasing oxygen content in the bottom water the paleoproductivity index indicates that marine organisms flourished in the wufeng to early longmaxi stages making an important contribution to the siliceous content in organic rich shale subsequently terrestrial siliciclastics gradually dominated analysis of paleogeography rare earth elements and weathering indicates that chuanzhong and qianzhong uplifts in the wufeng stage were the provenance area of the upper yangtze basin with a relatively high weathering degree weathering decreased in the beginning of the longmaxi stage and then gradually increased with the xuefeng uplift becoming the main provenance areas after the early longmaxi stage controlled by stress transfer from the continuous syn caledonian kwangsian orogeny in eastern south china the upper yangtze basin and xiangzhong basin were squeezed and pushed up successively the uplift separated the open shallow sea and formed a relatively limited deep water shelf environment resulting in a relatively euxinic bottom water environment which was favorable for organic matter accumulation the tectonic sedimentary evolution of the upper yangtze basin in the late ordovician to early silurian proposed in this study indicates that the formation of organic enriched shale was not only related to glacio eustatic change but also strongly controlled by tectonic activities in south china

age provenance and tectonic setting of neoproterozoic to early paleozoic sequences in southeastern south china block constraints on its linkage to western australia east antarctica abstract zircon u pb hf isotopes and whole rock geochemical data are applied to investigate the age provenance and tectonic setting of the late neoproterozoic and overlying sequences from the cathaysia block of south china in order to understand the tectonothermal evolution of this block and its linkage to supercontinents new zircon u pb ages in combination with previous fossil data indicate the lower and upper louziba groups were deposited between 655 and 635 ma cryogenian and 635 and 542 ma ediacaran respectively low cia 65 57 and high icv 1 64 1 05 values for the louziba sandstones are suggestive of immature sediments that experienced mild to weak chemical weathering in their source areas in contrast early paleozoic sandstones have higher cia 80 69 and lower icv 0 95 0 84 values indicative of mature sediments with intensive chemical weathering despite the different degrees of chemical weathering all sedimentary samples show positive correlation between tio2 and al2o3 and negative correlation in plot of la yb n vs zr but lack any correlation between la yb n and p2o5 these data demonstrate sorting of resistant minerals such as rutile monazite and zircon did not affect the chemical signatures of sampled detritus consequently geochemical features in combination with detrital age spectra indicate that the louziba group and overlying sequences record sedimentation in collisional and passive settings with the detritus being derived dominantly from intermediate to felsic igneous rocks late mesoproterozoic ca 1300 1000 ma detritus in the lower louziba group were probably derived primarily from the windmill islands bunger hills 1300 1050 ma and albany fraser belt musgrave 1300 1050 ma regions in east antarctica western australia increasing input of early neoproterozoic 990 950 ma detritus in the uppermost louziba group indicates a greater contribution from the eastern ghats ca 990 950 ma in eastern india and north prince charles mountains ca 990 950 ma in east antarctica this source change was synchronous with the formation of the kuunga 560 530 ma and pinjarra 560 520 ma orogens during assembly of gondwana the sediments of the huanglian formation of the uppermost louziba group likely recorded this event overlying paleozoic sequences then were deposited on passive margin with detritus being derived dominantly from an early neoproterozoic source e g eastern india and east antarctica

the dynamic evolution of the palaeozoic geography of eastern asia abstract new palaeogeographical reconstructions are presented for eleven time intervals through the palaeozoic of the eastern asia region from the middle cambrian at 510 ma to the end of the permian at 250 ma they centre on the continental blocks of north china south china and annamia indochina and their relationships with northeastern gondwana which was united to form part of pangea from the late carboniferous onwards also shown is the continent of tarim during the lower palaeozoic as well as the hutag uul songliao and khanka jiamasu bureya terranes both of which straddle the russian mongolian and chinese borders today from silurian times onwards we conclude that annamia and south china were united as a single continent throughout the lower palaeozoic and early devonian and were translocated by major strike slip faulting along the northeastern gondwana margin during that period from off afghanistan to outboard of the sibumasu and australian sectors of the superterrane they left the gondwana marginal area together during the lower devonian opening of the palaeotethys ocean but very shortly afterwards they themselves divided into the two separate continental blocks that we recognise today not to reunite until the triassic the various cambrian to permian rocks found in japan largely represent active volcanic arcs which originally lay to the southeast of south china although the carboniferous was more quiescent there the neotethys ocean opened during the permian dividing sibumasu and the tibetan terranes from gondwana and the palaeotethys ocean started to close progressively in the upper palaeozoic as most of the east asian continents and smaller terranes moved towards siberia the positions of the various continents and terranes have been deduced from a mixture of palaeomagnetic and faunal data the positions of large igneous provinces and kimberlites and the need to provide kinematic continuity between maps of successive ages however many uncertainties remain

paleozoic tectonic setting and paleogeographic evolution of the qin fang region southern south china block detrital zircon u pb geochronological and hf isotopic constraints

cambrian ordovician orogenesis in himalayan equatorial gondwana an early paleozoic tectonic event the kurgiakh orogeny has long been known from the western tethyan himalaya and it is conspicuously recorded by an angular unconformity between cambrian marine shelf deposits and coarse ordovician conglomerate as well as widespread granitic plutons although the lateral extent of this event is poorly known two regions in the central and eastern himalaya the kumaon of india and klippen of bhutan contain conglomerate units that may be correlative with this event in the kumaon the ralam formation which contains a basal polymictic conglomerate unit with detrital zircon grains as young as ca 512 ma is overlain by sandstone containing the arthropod walking trace diplichnites gouldi an ichnospecies known from ordovician and younger strata the late early cambrian trilobite redlichia sp recovered from the underlying martoli group indicates that the conglomerate is post early cambrian and based on stratigraphic analysis also likely records the kurgiakh orogeny in central bhutan the lower paleozoic succession contains conglomerate units with monomictic quartz sandstone clasts and less mature sandstone matrix these units have also been assigned to a variety of ages but rocks that apparently underlie these strata contain the furongian late cambrian zonal trilobite kaolishania granulosa here we show that detrital zircon ages of a conglomerate clast no grains younger than ca 781 ma are distinctly older than those of the matrix which contains a large peak at 490 500 ma thus these conglomerate units are also potential lateral correlatives of the ordovician conglomerate deposits of the western himalaya our paleontological and geochronologic data suggest that the kumaon and bhutan conglomerate units and possible equivalents in tibet correlate with those of the western himalaya this increases their known along strike extent by over 1000 km bringing the total extent to 1200 km thus the kurgiakh orogeny spanned much of the himalayan orogen along the northern gondwanan margin and reflected a widespread tectonic reorganization of the peri gondwanan realm to andean type margins recent work indicates that it also likely extended into the lhasa block and both the north and south china blocks this event was associated with widespread ca 500 ma arc related plutonism recorded in the himalaya and with abundant detrital zircon grains of that age in younger himalaya strata based on current fossil data the duration of the hiatus associated with the cambrian ordovician boundary which separates pre and postdeformational strata ranges from 22 to 36 m y available geochronological data do not provide greater constraint if the event was close in age to the depositional age of the ordovician molasse then the main phase of tectonic uplift erosion and deposition may have been much later than the presumed cambrian ordovician boundary interval age of ca 485 ma

geologic correlation of the himalayan orogen and indian craton part 2 structural geology geochronology and tectonic evolution of the eastern himalaya despite being the largest active collisional orogen on earth the growth mechanism of the himalaya remains uncertain current debate has focused on the role of dynamic interaction between tectonics and climate and mass exchanges between the himalayan and tibetan crust during cenozoic india asia collision a major uncertainty in the debate comes from the lack of geologic information on the eastern segment of the himalayas from 91 e to 97 e which makes up about one quarter of the mountain belt to address this issue we conducted detailed field mapping u pb zircon age dating and 40ar 39ar thermochronology along two geologic traverses at longitudes of 92 e and 94 e across the eastern himalaya our dating indicates the region experienced magmatic events at 1745 1760 ma 825 878 ma 480 520 ma and 28 20 ma the first three events also occurred in the northeastern indian craton while the last is unique to the himalaya correlation of magmatic events and age equivalent lithologic units suggests that the eastern segment of the himalaya was constructed in situ by basement involved thrusting which is inconsistent with the hypothesis of high grade himalaya rocks derived from tibet via channel flow the main central thrust in the eastern himalaya forms the roof of a major thrust duplex its northern part was initiated at ca 13 ma while the southern part was initiated at ca 10 ma as indicated by 40ar 39ar thermochronometry crustal thickening of the main central thrust hanging wall was expressed by discrete ductile thrusting between 12 ma and 7 ma overlapping in time with motion on the main central thrust below restoration of two possible geologic cross sections from one of our geologic traverses where one assumes the existence of pre cenozoic deformation below the himalaya and the other assumes flat lying strata prior to the india asia collision leads to estimated shortening of 775 km 76 strain and 515 km 70 strain respectively we favor the presence of significant basement topography below the eastern himalaya based on projections of early paleozoic structures from the shillong plateau i e the central shillong thrust located 50 km south of our study area since northeastern india and possibly the eastern himalaya both experienced early paleozoic contraction the estimated shortening from this study may have resulted from a combined effect of early paleozoic and cenozoic deformation

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

palaeozoic and mesozoic tectonic evolution and palaeogeography of east asian crustal fragments the korean peninsula in context east and southeast asia comprises a complex assembly of allochthonous continental lithospheric crustal fragments terranes together with volcanic arcs and other terranes of oceanic and accretionary complex origins located at the zone of convergence between the eurasian indoaustralian and pacific plates the former wide separation of asian terranes is indicated by contrasting faunas and floras developed on adjacent terranes due to their prior geographic separation different palaeoclimates and biogeographic isolation the boundaries between asian terranes are marked by major geological discontinuities suture zones that represent former ocean basins that once separated them in some cases the ocean basins have been completely destroyed and terrane boundaries are marked by major fault zones in other cases remnants of the ocean basins and of subduction accretion complexes remain and provide valuable information on the tectonic history of the terranes the oceans that once separated them and timings of amalgamation and accretion the various allochthonous crustal fragments of east asia have been brought into close juxtaposition by geological convergent plate tectonic processes the gondwana derived east asia crustal fragments successively rifted and separated from the margin of eastern gondwana as three elongate continental slivers in the devonian early permian and late triassic late jurassic as these three continental slivers separated from gondwana three successive ocean basins the palaeo tethys meso tethys and cenotethys opened between these and gondwana asian terranes progressively sutured to one another during the palaeozoic to cenozoic south china and indochina probably amalgamated in the early carboniferous but alternative scenarios with collision in the permo triassic have been suggested the tarim terrane accreted to eurasia in the early permian the sibumasu and qiangtang terranes collided and sutured with simao indochina east malaya in the early middle triassic and the west sumatra terrane was transported westwards to a position outboard of sibumasu during this collisional process the permo triassic also saw the progressive collision between south and north china with possible extension of this collision being recognised in the korean peninsula culminating in the late triassic north china did not finally weld to asia until the late jurassic the lhasa and west burma terranes accreted to eurasia in the late jurassic early cretaceous and proto east and southeast asia had formed palaeogeographic reconstructions illustrating the evolution and assembly of asian crustal fragments during the phanerozoic are presented d 2005 international association for gondwana research published by elsevier b v all rights reserved

cambrian biostratigraphy of the tal group lesser himalaya india and early tsanglangpuan late early cambrian trilobites from the nigali dhar syncline precise biostratigraphic constraints on the age of the tal group are restricted to 1 a basal level correlative with the anabarites trisulcatus protohertzina anabarica assemblage zone of southwest china 2 a level near the boundary of the lower and upper parts of the tal group correlative with the early tsanglangpuan stage drepanuroides zone and 3 an interval low in the upper part of the tal group correlative with later in the tsanglangpuan stage palaeolenus zone these correlations are based on small shelly fossil and trilobite taxa other chronostratigraphic constraints include the marked negative 13 c isotopic excursion coincident with the transition from the krol group to the tal group this excursion is used as a proxy for the precambrian cambrian boundary in several sections worldwide and if applied to the lesser himalaya indicates that the boundary is at or just above the base of the tal group the upper parts of the tal group may be of middle or late cambrian age and might form proximal equivalents of sections in the zanskar spiti region of the tethyan himalaya both faunal content and lithological succession are comparable to southwest china furthering recent arguments for close geographic proximity between the himalaya and the yangtze block during late neoproterozoic and early cambrian time trilobites from the uppermost parts of the sankholi formation from the nigali dhar syncline are described and referred to three taxa one of which drepanopyge gopeni is a new species they are the oldest trilobites yet described from the himalaya

paleo marine environments of the early cambrian yangtze platform abstract previous studies have generated conflicting interpretations of the nature of oceanographic conditions on the early cambrian yangtze platform we investigated paleo environmental conditions there through an analysis of major elements and redox sensitive trace elements in the lower cambrian niutitang formation from two new drillcores ry1 and ry2 in guizhou province high toc and excess ba concentrations record elevated productivity in the study area redox variation was documented based on enrichment factors efs for redox sensitive trace elements mo u v cr ni and co and c org p ratios these proxies indicate that the lower unit of the niutitang formation was deposited under fluctuating but mostly anoxic conditions whereas the upper unit was deposited under predominantly suboxic conditions mo toc ratios suggest persistently moderate to strong basinal restriction although mo u covariation patterns show no evidence for operation of a particulate shuttle despite the semi restricted anoxic nature of the deep watermass episodic deep water oxygenation events occurred within the yangtze platform sea and longer term secular variation may record eustatic influences additionally patterns of trace metal enrichment record evidence of submarine hydrothermal inputs we propose an integrated paleoenvironmental model to account for the observed productivity redox and hydrographic patterns in the early cambrian yangtze platform on the basis of paleo marine environmental analysis we ascribe the scarcity of trace fossils in the bottom to oxygen deficiency and deny considering the songlin biota as the onset of the intense biogenic evolution after global atmospheric and oceanic oxygenation

architecture and evolution of accretionary orogens in the altaids collage the early paleozoic west junggar nw china understanding the development and evolution of accretionary orogens is crucial for characterizing continental crust growth in time and space in the altaids tectonic collage of central asia conflicting geodynamic models have been proposed about the paleozoic evolution during which juvenile continental crust has been extensively formed this study focuses on west junggar nw china a key region that has not been extensively studied yet a multidisciplinary approach including detrital zircon provenance study geochemistry and field structural analysis provides new constraints on the evolution of three early paleozoic units of west junggar oceanic subduction predominates in early paleozoic time as indicated by ophiolitic melanges fore arc volcaniclastic turbidites and magmatic arc suites however the development of an olistostrome supplied by a sub contemporaneous carbonate platform the occurrence of continental molasse and the chronology and geochemistry of magmatic events reveal interruptions of subduction discrete collisions of volcanic arcs and micro continent are inferred from the tectonic structure of the belt which displays allochthonous units rooted in the suture zone early paleozoic magmatic and sedimentary rocks of west junggar bear some resemblance to contemporaneous analogues in eastern kazakhstan and suggest a lateral connection on the basis of these new results we propose a geodynamic evolution in four steps 1 early to middle ordovician intra oceanic subduction forming island arcs 2 late ordovician tectonic accretion of island arcs against the kazakhstan continental margin 3 early to middle silurian resumption of subduction and active continental margin magmatism and 4 late silurian early devonian micro continent collision this scenario consistent with the formation of the kazakhstan block supports a model of multiple episodic accretions for the altaids tectonic collage rather than a continuous continental margin evolution these results also suggest that in central asia continental crust growth results from alternating vertical and horizontal growth episodes throughout the paleozoic

preliminary stratigraphic and structural architecture of bhutan implications for the along strike architecture of the himalayan system article i nfo preliminary mapping and stratigraphic correlation of lesser himalayan rock in eastern bhutan using field characteristics u pb detrital zircon dating and nd geochemistry define the first order stratigraphic architecture of the indian passive margin sequence in the eastern himalaya we use this new image of the lateral and vertical relationships of the original stratigraphy to determine the structural framework of the eastern himalayan fold thrust belt in bhutan we propose that lesser himalayan rock in bhutan can be divided into lower lesser himalayan rocks with a paleoproterozoic detrital zircon signal and upper lesser himalayan rocks with detrital zircon signals of 1000 500 ma the 500 ma detrital grains are from rocks in the frontal portions of the fold thrust belt north of the main boundary thrust as well as directly in the footwall of the main central thrust in the hinterland our preliminary stratigraphic study coupled with mapping allows us to construct a composite balanced cross section through the kuru chu valley in eastern bhutan which provides the first image of the geometry and amount of shortening through bhutan the main frontal thrust tilts a 6 km of thick section of neogene foreland basin deposits these deposits are separated from folded and faulted upper lesser himalayan rocks by the main boundary thrust north of the main boundary thrust we propose two duplex systems the southernmost duplex contains 9 repeated sections of upper lesser himalayan units including the permian gondwana sequence and the cambrian or younger baxa group the northernmost duplex is located in the footwall of the main central thrust and is comprised of two repeated sections of the proterozoic shumar and daling formations the southern boundary of this northern duplex is the shumar thrust which acts as a roof thrust for the southern duplex system and may correlate to the ramgarh thrust in nepal and india we propose that the development of the duplex systems passively folded overlying tethyan and greater himalayan rocks minimum shortening for this part of the himalayan fold thrust belt is 359 km all of which has occurred from 22 ma to present defining a long term shortening rate of 16 mm yr

cambrian stratigraphy and depositional history of the northern indian himalaya spiti valley north central india recent work on himalayan tectonics indicates that prior to the cenozoic collision of india and asia an enigmatic cambrian ordovician event may have strongly infl uenced the regional geology of the himalaya stratigraphic and sedimentological analyses of well preserved cambrian deposits are critical for understanding the nature of this early tectonic event and its infl uence on the later tectonic evolution of the himalaya the parahio formation defi ned herein of the parahio valley spiti region in the tethyan himalaya of india is the best biostratigraphically resolved section of cambrian strata in the entire himalaya this formation consists of 1350 m of dominantly siliciclastic deltaic deposits the formation ranges from uppermost lower cambrian lungwangmiaoan stage to middle middle cambrian hsuchuangian stage representing a time span of 5 10 m y it contains numerous mediumscale shoaling cycles that range from storminfl uenced offshore deposits to thick trough cross bedded fl uvial facies many thin carbonate beds with abundant trilobite fossils directly overlie the fl uvial facies and represent transgressive systems tract deposits the cycles are interpreted to have resulted from delta lobe switching based on a lack of systematic stratigraphic changes in cycle or facies thicknesses this paleoenvironmental reconstruction contradicts previous interpretations of this unit that range from deep sea fl ysch to shallow marine tidalites in addition our paleoenvironmental analysis and paleocurrent data suggest that the uppermost lower to middle cambrian deposits of the lesser and tethyan himalaya are parts of the same ancient northward prograding fl uvial deltaic depositional system of the paleo tethys margin of india an angular unconformity with overlying ordovician conglomeratic rocks has considerable local relief with meter scale scours and a valley fi ll 100 m thick the scours have northeast southwest orientations which parallel both the paleocurrents in the underlying parahio formation as well as published paleocurrent readings from the coarse red beds of the overlying ordovician strata the cambrian ordovician unconformity is of regional extent and our recent biostratigraphic database indicates that the minimum hiatus associated with the unconformity in spiti is 15 m y our sedimentological analysis and associated paleocurrent data from the parahio formation along with additional data from units both above and below the unconformity indicate that published models portraying foreland basin development at this time with southward directed thrusting are problematic an alternate possibility that uplift took place south of the tethyan himalaya is also problematic because no published stratigraphic or structural evidence exists for such an uplift to the south for either the greater or the lesser himalaya lithotectonic zones

a two stage model for the formation of the granite greenstone terrains of the kalgoorlie norseman area western australia abstract accurate dating of granites and greenstones from the kalgoorlie norseman area shows that there is considerable overlap in time between the formation of granites and greenstones however within any given area the oldest granites are always younger than the oldest greenstones it is suggested that the long lived 100 ma thermal anomaly in the uppermost asthenosphere that gave rise to the greenstones is also responsible for regional metamorphism and widespread crustal melting leading to the production of granites the time lag between greenstone and granite formation reflects the time taken to conduct heat from the mantle to the lower crust anatexis of the lower crust produces a layer of light granitic magma that may temporarily stop the ascent of basaltic magma where this happens the basaltic magma ponds at the base of the crust supplying additional heat and accelerating the melting process eventually the granitic melt layer becomes unstable and rises within the crust areas intruded by granite become topographic highs with topographic lows forming above areas where light melt has been withdrawn from the lower crust when the topographic highs protrude above sea level they are eroded to shed sediments into the topographic lows which contain the basaltic greenstones thus the change from a basalt dominated lower greenstone succession to a sediment and felsic volcanic dominated upper whitestone sequence seen in many granite greenstone terrains results from the intrusion of granitic magma into the upper crust the crust of the kalgoorlie norseman area formed in two distinct stages separated by a time break of up to 700 ma the first stage involved the formation of primitive crust probably above subduction zones this primitive crust was characterized by low rb sr and high sm nd ratios the second stage resulted from a major change in mantle convection that produced thermal upwellings in the mantle below the continents this led to the eruption of basaltic magmas and to metamorphism and anatexis of the lower crust to produce granitic magmas melting reset the rb sr sm nd and u pb systems giving the impression that the second event which occurred at 2 7 ga was a major crust forming event it was in fact a crustal reworking process although some basalt was added to the crust at this time although the two stage history of the kalgoorlie norseman area is clearly seen in the lead isotopes and in zircons it is not readily discernible from the rb sr and sm nd systematics due to mantle like ratios of those elements in the primitive crust

linking the indochina block and gondwana during the early paleozoic evidence from u pb ages and hf isotopes of detrital zircons abstract to constrain the paleoposition of indochina within gondwana during the early paleozoic we performed in situ u pb and hf isotope analyses on detrital zircons from three river sediment samples in the truong son belt of the indochina block the age distributions yield dominant neoarchean 2 5 ga mesoproterozoic 1 7 1 4 ga grenvillian 0 95 ga and pan african 0 65 0 5 ga age groups and minor paleo to meso archean zircons hf isotope compositions of zircons for each age group exhibit large ranges of ehf t suggesting that the zircon host rocks have diverse sources the oldest hf model ages for zircons of neoarchean grenvillian and pan african age group yield 3 7 ga or older while those of mesoproterozoic age group show 3 3 ga the remarkable similarity of age distribution and hf isotope compositions among detrital zircons of indochina and those of tethyan himalaya western cathaysia and qiangtang suggests that indochina was located outboard of qiangtang and south of south china in the indian margin of gondwana during the early paleozoic our results are consistent with the paleontological correlations of east gondwana margin during the early paleozoic

detrital zircon u pb ages and hf isotopes of lower middle devonian to middle jurassic sandstones in the qinfang basin southern south china block constraints on provenance and tectonic setting abstract the affinity and tectonic setting of the qinfang basin have long been an enigma here we conduct detrital zircon u pb geochronology and hf isotope analysis of lower middle devonian to middle jurassic samples from the basin along with published age data and paleontology from the basin and its east and west sides this study constrains the sediment provenance and tectonic setting of the basin sandstones of devonian jurassic exhibit an age distribution with major age groups at 2700 2300 ma absence for jurassic 1200 800 ma 600 400 ma and 334 246 ma for jurassic alone zircons peak 2500 ma were sourced from the yangtze block while 1200 800 ma grains may originate from the yunkai massif and jiangnan orogen 600 400 ma zircons came from yunkai massif and or cambrian to silurian sediments in the basin carboniferous early triassic 334 246 ma zircons were probably supplied by an older sequence in the great youjiang basin and the darongshan shiwandashan igneous rocks the qinfang basin and its two sides are considered to share a common east gondwana source in silurian the similarity in age spectra between silurian devonian samples in the basin and ailaoshan belt suggests that the basin is a remnant of the paleo tethyan ailaoshan ocean however the distinct age distribution between the basin and indochina block shows that the basin is not a part of the indochina block by middle jurassic following the final closure of the ailaoshan ocean in the late triassic the adjacent western area may be still tectonically active thereby feeding the basin with detritus of early permian

stratigraphic correlation of cambrian ordovician deposits along the himalaya implications for the age and nature of rocks in the mount everest region the depositional age and stratigraphic correlations of metamorphosed and variably deformed rocks of mount everest are poorly known because of limited recovery of diagnostic fossils detailed study of cambrian and ordovician strata from along the length of the himalaya has produced a coherent stratigraphy that stretches from northern india to tibet our work also demonstrates that the north col formation rocks everest series between the qomolangma and lhotse detachments of the south tibetan detachment system still locally preserve sedimentary textures and primary stratigraphy that match those within cambrian strata similar to 1100 km to the west in northern india this demonstrates a coherency of depositional systems and stratigraphic architecture for cambrian deposits along much of the himalaya tethyan margin it also allows for the first time identification of precise depositional ages of several units in the everest region in particular the yellow band carbonate and directly underlying siliciclastic strata which are both shown to be late middle cambrian in age detrital zircon data presented herein for a sample from these siliciclastic strata contain a similar age spectrum to those from middle cambrian strata in northern india as well as grains as young as ca 526 ma both of which support the depositional age and continuity of depositional systems along the length of the himalaya highly fractured rocks of the ordovician lower chiatsun group in the hanging wall of the south tibetan detachment system in nyalam 75 km to the west of everest correlate with ordovician strata of the mount qomolangma formation on mount everest our correlations indicate that the base of the summit pyramid of everest the foot of the third step is composed of a 60 m thick white weathering thrombolite bed the top of this ancient microbial deposit crops out only 70 m below the summit of mount everest

using isotopic and chronologic data to fingerprint strata challenges and benefits of variable sources to tectonic interpretations the paro formation bhutan himalaya 1 we combine detrital zircons dz and epsilon neodymium nd signatures with field mapping in the paro formation in western bhutan dz age spectra are strongly variable and display signatures that have been used to uniquely identify both greater himalayan gh and lesser himalayan lh strata dz age peaks from six quartzite samples require sources for 0 5 0 8 1 2 1 4 1 7 1 8 and 2 5 ga zircons in the paro formation the youngest 0 5 ga zircons argue for a cambrian maximum deposition age two samples have a youngest 1 8 ga peak typically attributed to paleoproterozoic lh rocks a 450 ma crystallization age from two granite samples constrains the minimum deposition age as ordovician new nd signatures from six detrital samples from the paro formation show significant variation with lithology schists have nd 0 values between 12 0 and 16 9 while quartzite values vary between 18 8 and 24 5 these data imply that the paro formation was derived from both young and old sources with dz and nd values obtained from the same quartzite samples requiring old detritus while the nd values obtained from interbedded schist require younger detritus using published isotopic and chronologic definitions of himalayan strata schist rich layers wouldbeconsideredgh whiletheinterbeddedquartzite

mode of strain release of three graben associated earthquakes the nature of graben associated earthquakes has been investigated through a study of three earthquake sequences using various seismological geophysical and geological data the three earthquake sequences are the hsingtai china earthquakes maximum magnitude ms 7 2 of march 1966 the nevada earthquakes maximum m 7 1 of 1954 and the new madrid earthquakes maximum ms 8 8 of 1811 1812 these earthquake sequences represent a unique mode of strain release away from plate margins and have bearings on earthquake hazard evaluation a number of similarities have been observed during these earthquake sequences for instance all the three sequences are characterized by large multiple shocks or swarms which last from several weeks to several months among the large multiple shocks in each sequence the largest one usually is not the first one migrations of epicenters were observed during the hsingtai and the new madrid earthquakes significant amount of strike slip motion was associated or inferred with all these three sequences in particular for the hsingtai earthquakes the swarm of multiple shock activities is probably due to induced instability or strong coupling and interactions among a series of adjacent faults in a heterogeneous graben the strike slip motion can be explained by a change or a rotation of stress orientation after the formation of a graben the graben events are similar to the earthquake swarms associated with spreading oceanic ridges but with significant differences available data indicate that the recurrence intervals for the three graben earthquake sequences studied are of the order of thousands of years

placing limits on channel flow insights from the bhutan himalaya article i nfo article history along the majority of the himalayan orogenic belt high grade metamorphic rocks of the greater himalaya gh are separated from lower grade metasedimentary rocks by two shear zones the basal top to the south main central thrust mct and the upper top to the north south tibetan detachment std seemingly pervasive ductile deformation within gh rocks and a lack of exposed hanging wall or footwall cut offs for the mct and std have permitted models that predict 100 s of km of coeval displacement on both structures extruding the gh section via gravitational loading from tibet and focused erosion along the himalayan topographic front we present new mapping stratigraphic columns mineral assemblages u pb zircon ages and a suite of structural data from gh and tethyan himalayan th rocks in eastern and central bhutan our observations highlight significant 250 300 c along strike changes in the deformation temperature conditions recorded by gh rocks in eastern bhutan th rocks are separated from gh rocks displaying ubiquitous partial melt textures by a top to the north sense shear zone correlated with the std however in central bhutan limited partial melt textures are only present near the base of the gh section a biotite muscovite garnet mineral assemblage persists from 0 2 to 3 km above the mct through th rocks and distinct lithologies interfinger at the gh th contact which suggests that th strata are in depositional contact above gh strata the same strata exposed on both sides of the std limits slip along this structure to 20 km thin section scale top to the south shear is focused in a 2 3 km thick zone above the mct while the overlying 11 km thick section displays top to the north shear suggesting asymmetric channel behavior strained quartz grains allow us to quantify 23 34 km of top to the north shear our data suggest that the gh th section in central bhutan acted as a cool viscous low displacement channel and when compared to sections in eastern bhutan highlights dramatic changes in temperature conditions viscosity and possibly displacement along strike however we suggest that the erosion rates needed to remove low viscosity material limit channel flow even in regions containing significant partial melt and that the magnitude of channel flow is small compared to the total mass balance of the system

kwangsian crustal anatexis within the eastern south china block geochemical zircon u pb geochronological and hf isotopic fingerprints from the gneissoid granites of wugong and wuyi yunkai domains abstract gneissoid granites were traditionally thought to be the components of the precambrian basement in the eastern south china block but twenty four gneissoid granite samples from the wugong northern wuyi southern wuyi and yunkai domains gave zircon u pb ages of 424 456 ma 410 457 ma 426 438 ma and 415 450 ma respectively the existence of the abundant kwangsian gneissoid granites which have incorrectly been assigned to the precambrian basement indicates that the so called precambrian stratigraphic sequence of the cathaysia block should be termed complex rather than group these gneissoid granites are peraluminous granites with relatively high al 2 o 3 mgo tio 2 feot and cao na 2 o but low cao al 2 o 3 mgo feot and rb sr ratios they exhibit strongly negative ba sr nb p and ti and positive pb anomalies in primitive mantle normalized diagrams and have initial 87 sr 86 sr ratios ranging from 0 70924 to 0 72935 and negative end t values from 6 4 to 11 4 similar to those of paragneissic enclaves and precambrian sedimentary rocks in the eastern south china block the zircons crystallized in the kwangsian gneissoid granites gave ehf t values ranging from 2 4 to 19 4 with the peak at 4 almost all grains clustering at 1 to 12 and hf model ages of 1 1 2 1 ga the synthesis of these geochemical and in situ zircon hf isotopic data indicates that the kwangsian gneissoid granites dominantly originated from proterozoic metapelitic and meta igneous components with insignificant input of juvenile mantle derived magmas a model of two stage crustal anatexis is proposed for the kwangsian granitic magma in the eastern south china block with 1 the formation of 460 430 ma granite through the breakdown of hydrous minerals under the condition of the doubly thickened crust and 2 the generation of the 430 400 ma granite accompanying promoted melting along a path of isothermal decompression due to the increasing thermal weakening for the collapse of the thickened crust the crustal anatexis has probably occurred in an intracontinental tectonic regime that was geodynamically linked to the far field response to the assembly of the australian indian plate with the cathaysia block during middle paleozoic kwangsian time

documenting basin scale geometry and provenance through detrital geochemical data lessons from the neoproterozoic to ordovician lesser greater and tethyan himalayan strata of bhutan abstract detrital zircon dz ages augmented with end 0 and 13 c isotopic values from 18 new and 22 published samples collected from lesser himalayan lh greater himalayan gh and tethyan himalayan th rocks in bhutan support deposition of 7 km of sedimentary rock in late cambrian ordovician time and provide a stratigraphic framework for the pre collisional indian margin youngest gh dz grains become younger upsection from 900 ma to 477 ma youngest dz grains in th samples are 490 460 ma both the lh jaishidanda formation fm and the lh baxa group overlie paleoproterozoic lh rocks the jaishidanda fm exhibits distinct populations of youngest dz peaks 475 550 ma and 800 1000 ma the baxa group manas pangsari and phuntsholing formations contains youngest dz peaks at both 500 525 ma and 0 9 1 0 ga however most samples from the baxa group in western bhutan contain no grains younger than 1 8 ga samples from the lh paro fm which sits directly under the mct in western bhutan have youngest dz peaks at 0 5 0 8 1 0 1 7 1 8 ga end values generally match dz spectra with samples that contain old youngest grain populations corresponding to more negative end signatures the paro fm is an exception where end 0 values from quartzite samples are quite negative 19 to 24 whereas the end 0 values from interbedded schist contain younger detritus 12 to 17 13 c values from the jaishidanda paro and manas formations have 13 c values 1 8 to 6 suggestive of deposition over late neoproterozoic to ordovician time 13 c values from the pangsari fm vary from 2 8 to 1 8 compatible with deposition in the early to middle neoproterozoic the young latest cambrian ordovician grains preserved in th gh and lh rocks suggest that the late cambrian ordovician orogeny documented in gh rocks throughout the orogen served as a significant sediment source in bhutan

origin and pre cenozoic evolution of the south qiangtang basement central tibet abstract the qiangtang terrane in central tibet with its high pressure rocks is a key area to unravel the evolution of the paleo tethys we present a new 1 50 000 map of the rongma area in the central qiangtang terrane of which relatively little is known so far we show that the qiangtang metamorphic belt can be separated in a paleozoic autochthonous basement and an overlying allochthonous thrust stack of subduction melange that contains high pressure rocks and permian sediments detrital zircon ages youngest peak at 591 ma and an 470 ma granite intrusion age constrain the age of the qiangtang basement to be between the late precambrian to middle ordovician this is in agreement with the observed unconformity between basement and overlying mid late ordovician strata this unconformity and the zircon age spectra are comparable to the himalaya area the qiangtang terrane must thus have been part of the gondwana supercontinent during the early paleozoic the occurrence of the late triassic eclogite and glaucophane bearing schists in the central qiangtang terrane indicates the existence of a suture zone between the north and south qiangtang terranes before the late triassic this suture zone resulted from closure of the paleo tethys between the north and south qiangtang terranes late triassic syn collisional granites and rapidly exhumed high pressure rocks resulted from the closure of this ocean and final amalgamation of the re united qiangtang terranes with eurasia collision of the lhasa and qiangtang terranes is correlated with north dipping reverse faults in the south qiangtang terrane which were probably related to exhumation of the qiangtang basement and formation of the qiangtang culmination

early palaeozoic orogenesis along the indian margin of gondwana tectonic response to gondwana assembly abstract shrimp u pb dating of zircons from a peralkaline s type lesser himalayan granite from the kathmandu region nepal indicate an age of emplacement of 475 ma the granites along with metasedimentary xenoliths show a similar signature of inherited detrital zircon ages ranging from archaean to early palaeozoic with prominent late mesoproterozoic grenvillian and neoproterozoic pan african age peaks and have a maximum age of 500 ma based on the youngest detrital grains deformation structures in xenoliths are truncated by the granite and along with the granites are assigned to a cambro ordovician orogenic event herein termed the bhimphedian orogeny that can be traced across the himalaya from pakistan to the eastern himalaya and possibly extends west into afghanistan we interpret the orogeny as being related to andean type orogenic activity on the northern margin of the indian continent following gondwana assembly the magmatic arc was associated with andesitic and basaltic volcanism and was active from c 530 to 490 ma the arc activity overlaps with and is succeeded by regional deformation crustal melting and s type granite emplacement that extended to 470 ma orogenic activity was driven by coupling across the plate margin either during on going subduction or through accretion of microcontinental ribbons possibly represented by the lhasa and qiangtang blocks it represents the termination of an orogenic cycle termed the north indian orogen that commenced with neoproterozoic rifting and passive margin development and terminated with the bhimphedian orogeny this was succeeded by a return to passive margin setting along the northern indian margin of gondwana which continued until the cenozoic himalayan orogeny

reappraisal of the ages of neoproterozoic strata in south china no connection with the grenvillian orogeny the jiangnan fold belt separates the yangtze and cathaysia blocks in south china and has long been considered grenvillian in age in order to place south china in central rodinia it consists of deformed early neoproterozoic strata that are unconformably overlain by undeformed late neoproterozoic strata and intruded by undeformed and unmetamorphosed granitic plutons zircons from the early neoproterozoic strata yield u pb ages as young as 830 ma and one granitic pluton has a zircon u pb age of ca 827 ma the 830 ma mafic rocks along the southeastern margin of the yangtze block have arc affinity geochemical characters whereas mafic rocks younger than 830 ma have typical ocean island basalt oib like compositions thus we suggest that the early neoproterozoic strata were deposited on an active continental margin prior to amalgamation of the yangtze and cathaysia blocks at ca 830 ma the overlying late neoproterozoic strata were deposited in the intracontinental rifted nanhua basin at 820 730 ma and probably reflect backarc spreading above the long lived 950 735 ma oceanic subduction zone along the northern and western margin of the yangtze block this model is consistent with the secular tectonic evolution of south china during the neoproterozoic the jiangnan fold belt is therefore not a grenvillian feature as previously suggested and there is no evidence to place south china in central rodinia instead we believe that south china was located in a marginal position relative to this supercontinent

rate and state frictional and healing behavior of carbonate fault gouge explained using microphysical model classical rate and state friction rsf laws are widely applied in modeling earthquake dynamics but generally using empirically determined parameters with little or no knowledge of or quantitative account for the controlling physical mechanisms here a mechanism based microphysical model is developed for describing the frictional behavior of carbonate fault gouge assuming that the frictional behavior seen in lab experiments is controlled by competing processes of rate strengthening intergranular sliding versus contact creep by pressure solution by solving the controlling equations derived from kinematic and energy entropy balance considerations and employing a microphysical model for rate strengthening grain boundary friction plus standard creep equations for pressure solution we simulate typical lab frictional tests namely velocity stepping and slide hold slide test sequences for velocity histories and environmental conditions employed in previous experiments the modeling results capture all of the main features and trends seen in the experimental results including both steady state and transient aspects of the observed behavior with reasonable quantitative agreement to our knowledge ours is the first mechanism based model that can reproduce rsf like behavior in terms of microstructurally verifiable processes and state variables since it is microphysically based we believe that our modeling approach can provide an improved framework for extrapolating friction data to natural conditions

provenance of rift sediments in a composite basin mountain system constraints from petrography whole rock geochemistry and detrital zircon u pb geochronology of the paleocene shashi formation southwestern jianghan basin central china the paleocene shashi formation the host formation of potassium rich brines and solid potash in the jianghan basin central china is characterized by rift sediments in a composite basin mountain system however the provenance of the paleocene jianghan basin remains poorly understood a combined study of petrographic examination geochemical analysis and zircon u pb dating was conducted to characterize the provenance of the shashi formation the petrographic analysis of the shashi formation reveals that these samples have predominantly quartz lithic compositions with minor contributions from other minerals whole rock geochemical data indicate that this formation experienced only slight to moderate alteration the chondrite normalized rare earth element ree patterns of these samples are essentially parallel to those of the upper continental crust ucc as their negative eu anomalies and th sc and la th ratios suggest an average source composition similar to that of the ucc these values together with plots of their th u vs th ratios suggest that certain samples have undergone moderate degrees of weathering and sedimentary recycling the analysis of major and trace elements i e la th hf th sc zr sc la y zr sc tio2 fe2o3t mgo al2o3 sio2 fe2o3t mgo indicates that their sedimentary sources were felsic rocks from an active continental margin or a continental arc containing a minor amount of recycled sediments derived from a passive continental margin the shashi formation contains detrital zircons that primarily have u pb ages of 2 5 1 8 ga 725 965 425 475 310 350 and 200 260 ma these results reveal that the pre ordovician zircons are derived from recycled sediments of the yangtze block and thus originate from the surrounding ranges the magmatic rocks of the south qinling belt and the jiangnan orogen are responsible for supplying the ordovician to triassic detrital zircons these provenance data combined with paleocurrent analyses can be used to identify multiple source characteristics in the jianghan basin during the early paleocene most clastic material was derived from a southern source during the late paleocene northern and western sources contributed most of the detritus to the basin the supply of clastic material was thus broadly variable and widely dispersed throughout the basin this study shows that the architecture of the region and the rifting process exerted important influences on this sediment dispersal

geologic and u pb geochronologic evidence for early paleozoic tectonism in the dadeldhura thrust sheet far west nepal himalaya abstract the dadeldhura thrust sheet inm western nepal consists of proterozoic lower paleozoic sedimentary and plutonic rocks and their metamorphic equivalents that rest structurally on proterozoic strata of the lesser himalayan sequence although regional metamorphism and ductile deformation were widespread during tertiary thrust emplacement relicts of early paleozoic tectonism are preserved locally new field and geochronologic studies together with the findings of previous workers indicate that this early paleozoic tectonism included 1 regional metamorphism to at least garnet grade 2 regional folding of a thick metamorphic sequence into a broad east west trending syncline 3 outcrop scale folding of metasedimentary rocks 4 emplacement of cambro ordovician granitic bodies during and after the metamorphism and deformation 5 uplift and erosion of the metamorphic sequence with garnet grade rocks locally exposed at the surface and 6 derivation of ordovician conglomeratic sandstones from the early paleozoic orogen similar records of metamorphism deformation and uplift erosion have been found in other regions of the himalaya indicating that rocks of the dadeldhura thrust sheet were originally involved in a regionally extensive orogenic system future tectonic models of himalayan orogenesis must accommodate this early paleozoic event