post collisional southeastern beishan granites geochemistry geochronology sr nd hf isotopes and their implications for tectonic evolution abstract bandaoshan granites are exposed in the southeastern beishan area in the central part of the central asia orogenic belt caob la icp ms u pb zircon dating indicates that bandaoshan granites were emplaced into precambrian basement at 285 4 ma and their geochemistry indicates that they are alkali rich potassium high granites initial end values 4 3 to 2 7 and ehf values 2 7 to 0 7 suggest that bandaoshan granites were derived from mantle derived melt and an upper continental crustal or sedimentary component the latter plays a significant role in their genesis in combination with regional geology the early permian bandaoshan pluton is interpreted to form in a post collisional environment in the southeastern beishan area late carboniferous qiaowan granites early permian yin aoxia granites and middle permian xijianquanzi granites are also considered as post collisional granites and together with bandaoshan granites indicate that the region was in a post collisional stage from the end of late carboniferous to middle permian

constraints on the lateral strength of slabs from three dimensional dynamic flow models if the viscosity of subducted lithosphere is described purely by temperature and pressure it should remain considerably more viscous than the surrounding material as it descends through the upper mantle many dynamic models of the long wavelength geoid associated with slabs require an increase in viscosity from the upper to the lower mantle but assume the perturbation to the lateral viscosity structure arising from the slabs themselves can be ignored previous studies indicate that strong and localized viscosity variations should have a dramatic influence on the geoid we present 3d finite element models of the regional geoid of the western pacific subduction zones slab buoyancies and viscosities are defined using the distribution of seismicity the geoid is very sensitive to the lateral strength of the slab very viscous slabs penetrating a low viscosity mantle generate significant 10 50 m long wavelength geoid lows opposite to the geoid high which is observed over slabs to obtain a geoid high comparable to that observed the lower mantle viscosity must be 60 200 times greater than the upper mantle viscosity and the slab must be in contact with the lower mantle these strict requirements suggest that slabs have been weakened and cannot act as stress guides from the deep mantle to the surface

u pb shrimp and 40ar 39ar geochronological constraints on the evolution of the xingxingxia shear zone nw china a triassic segment of the altyn tagh fault system the development of structures and their age along the segment of the altyn tagh fault system and the eastward extension of the tianshan orogenic belt remain speculative recent investigations on the structural framework granitic intrusions and metamorphic rocks in the eastern tianshan and adjacent areas show that the ne striking xingxingxia sinistral ductile shear zone nw china is subparallel to the altyn tagh fault zone and is superposed on the eastern tianshan orogenic belt u pb zircon sensitive high resolution ion microprobe shrimp dating and muscovite biotite and k feldspar 40 ar 39 ar thermochronology indicate that sinistral shear along the xingxingxia shear zone initiated at 240 235 ma broadly at the same time as initial formation of the altyn tagh fault zone but later than initiation of dextral strike slip motion along ew trending eastern tianshan orogenic belt at 270 245 ma formation of the xingxingxia ductile shear zone was associated with gondwanaland convergence along the southern margin of the eurasian continent during the late permian early triassic

final amalgamation of the tianshan and junggar orogenic collage in the southwestern central asian orogenic belt constraints on the closure of the paleo asian ocean abstract the tianshan and junggar orogenic collage occupies the southwestern part of the central asian orogenic belt and was assembled by collision accretion of several continental blocks and island arcs during late paleozoic early mesozoic time following the consumption of the junggar and south tianshan oceanic basins in the western segment of the paleo asian ocean considerable and continuing controversy has surrounded when where and how this orogenic collage was eventually amalgamated which plays a crucial role in understanding the formation of the central asian orogenic belt this review synthesizes the most recent data and geologic evidence that place critical constraints on the closure history of the oceanic domains a comprehensive data compilation indicates that the formation of ophiolites along major sutures in the tianshan and junggar region lasted to the early carboniferous for the kalamaili suture zone to the mid carboniferous for the west junggar region and the north tianshan suture zone and to the late carboniferous for the south tianshan suture zone ca 325 310 ma ultra high pressure metamorphism along the western part of the south tianshan suture occurred nearly synchronously with the collision between the tarim craton and the central tianshan yili block the oceanic closure and collision events were sequentially followed by widespread and intensive a type granitic and bimodal magmatism which commenced in the mid carboniferous along the kalamaili belt and in the latest carboniferous to earliest permian in other regions contemporaneously large scale strike slip shearing took place along major faults subparallel to main sutures with a dominant dextral sense and deformation time since the carboniferous permian transition available data and geological evidence support an eastward propagating scissor like closure of the western segment of the paleo asian ocean along the south tianshan and north tianshan suture zones over a period from the mid to end carboniferous with the south tianshan belt representing the final suturing site in the early permian the tarim mantle plume likely merely affected the southwestern part of the assembled tarim and tianshan region as indicated by regional discrepancy in type and geochemistry of early permian a type granitoids the amalgamation of the tianshan and junggar orogenic collage was associated with the tectonic bending of the kazakhstan orocline leaving a piece of trapped oceanic crust in its middle part beneath the present junggar basin

the liuyuan volcanic belt in nw china revisited evidence for permian rifting associated with the assembly of continental blocks in the central asian orogenic belt the basaltic pillow lavas in the liuyuan region of nw china considered to be part of an ophiolitic suite have been central to the models on tectonic setting evolution and timing of the final closure of the palaeo asian ocean new field evidence on the sedimentary units associated with the basalts reveals comparable sequences in the northern and southern flanks of the liuyuan volcanic belt with coarse to fine sediments from periphery to the centre the dacites and rhyolites formed coevally with the pillow basalts the pillow basalts are interlayered with lacustrine sandstone claystone and clayey lake deposits detrital zircons from these sediments yield zircon u pb ages of 291 285 ma andesites dacites and rhyolites from the basaltic sequence yield u pb ages of 280 277 ma similar to the 282 280 ma ages of gabbros that intrude the pillow lavas all these rocks cover the 460 440 ma granite and greenschist basement and have been intruded by gabbros of c 272 ma age with subsequent 230 227 ma north south contractional thrusting and folding the data from our study are incompatible with the existing models that consider the basalts as part of an ophiolitic suite along the northern continental margin of china from west to east the tarim dunhuang alxa and north china cratonic areas all show evidence for regional extension through rifting during early middle permian time these rift features and basaltic eruptions occurred coevally with the assembly of various microcontinental blocks against the siberian craton at c 300 250 ma synchronous with amalgamation of the central asian orogenic belt caob on the northern side of the liuyuan rift these events were also broadly synchronous with formation of the global supercontinent pangea

the liuyuan complex in the beishan nw china a carboniferous permian ophiolitic fore arc sliver in the southern altaids the tectonic history and time of closure of the palaeo asian ocean of the altaids are issues of lively current debate to address these issues this paper presents detailed geological petrological and geochemical data of the liuyuan complex lc in the beishan region in nw china located in the southernmost altaids in order to constrain its age origin and tectonic setting the lc mainly comprises massive basalts pillow basalts basaltic breccias gabbros and ultramafic rocks together with cherts and tuffs most prominent are gabbros and large volumes of basaltic lavas these mafic rocks have high tio 2 contents flat rare earth element ree patterns and show high field strength elements hfses similar to those of mid ocean ridge basalts morb the mafic rocks exhibit positive e nd t 6 6 9 0 values representing magmas derived from the mantle but these basic rocks are also enriched in th relative to rees and are systematically depleted in nb ta ti relative to rees there is also a large range in initial 87 sr 86 sr 0 7037 0 7093 all these variables indicate that mantle derived magma was contaminated by fluids and or melts from a subducting lithospheric slab and formed in a supra subduction zone ssz setting a gabbro intruded in the complex was dated by la icp ms on 20 zircons that yielded a 206 pb 238 u weighted average age of 286 2 ma considering the fact that all these basalts are imbricated against permian tuffaceous sediments and limestone we propose that the lc formed as an ophiolite in a fore arc in carboniferous permian time this indicates that the palaeo asian ocean still existed at 286 2 ma in early permian time and thus the time of closure of the palaeo asian ocean was in or after the late permian

reassessment of continental growth during the accretionary history of the central asian orogenic belt we argue that the production of mantle derived or juvenile continental crust during the accretionary history of the central asian orogenic belt caob has been grossly overestimated this is because previous assessments only considered the palaeozoic evolution of the belt whereas its accretionary history already began in the latest mesoproterozoic furthermore much of the juvenile growth in central asia occurred in late permian and mesozoic times after completion of caob evolution and perhaps related to major plume activity we demonstrate from zircon ages and nd hf isotopic systematics from selected terranes within the caob that many neoproterozoic to palaeozoic granitoids in the accreted terranes of the belt are derived from melting of heterogeneous precambrian crust or through mixing of old continental crust with juvenile or short lived material most likely in continental arc settings at the same time juvenile growth in the caob occurred during the latest neoproterozoic to palaeozoic in oceanic island arc settings and during accretion of oceanic island arc and precambrian terranes however taking together our data do not support unusually high crust production rates during evolution of the caob significant variations in zircon ehf values at a given magmatic age suggest that granitoid magmas were assembled from small batches of melt that seem to mirror the isotopic characteristics of compositionally and chronologically heterogeneous crustal sources we reiterate that the chemical characteristics of crustally derived granitoids are inherited from their source s and cannot be used to reconstruct tectonic settings and thus many tectonic models solely based on chemical data may need re evaluation crustal evolution in the caob involved both juvenile material and abundant reworking of older crust with varying proportions throughout its accretionary history and we see many similarities with the evolution of the sw pacific and the tasmanides of eastern australia

rare earth elements in the permian phosphoria formation paleo proxies of ocean geochemistry abstract the geochemistry of deposition of the meade peak member of the phosphoria formation mpm in southeast idaho usa a world class sedimentary phosphate deposit of permian age that extends over 300 000 km2 is ascertained from its rare earth element ree composition ratios of ree al2o3 suggest two sources seawater and terrigenous debris the seawater derived marine fraction identifies bottom water in the phosphoria sea as o2 depleted denitrifying suboxic most of the time and seldom sulfate reducing anoxic this interpretation is supported by earlier research that showed progressively greater ratios in the marine sediment fraction of cr ni v ni mo ni relative to their ratios in seawater for which marine cr v and mo can have a dominantly o2 depleted bottom water source and ni a photic zone largely algal source the water chemistry was maintained by a balance between bacterial oxidation of organic matter settling through the water column determined largely by primary productivity in the photic zone and the flux of oxidants into the bottom water via advection of seawater from the open ocean samples strongly enriched in carbonate fluorapatite the dominant ree host mineral have variable er sm tm sm and yb sm ratios their distribution may represent greater advection of seawater between the phosphoria sea and open ocean during deposition of two ore zones than a center waste and greater upwelling of nutrient enriched water into the photic zone however the mean rate of deposition of marine ni a trace nutrient of algae and po43 a limiting nutrient indicate that primary productivity was probably high throughout the depositional history an alternative interpretation of the variable enrichments of er tm and yb relative to sm is that they may reflect temporally variable carbonate alkalinity of open ocean seawater in permian time a more strongly negative ce anomaly for all phosphatic units than the ce anomaly of modern pelletal phosphate is further indicative of an elevated o2 concentration in the permo carboniferous open ocean as proposed by others in contrast to the depletion of o2 in the bottom water of the phosphoria sea itself the oceanographic conditions under which the deposit accumulated were likely similar to conditions under which many sedimentary phosphate deposits have accumulated and to conditions under which many black shales that are commonly phosphate poor have accumulated a shortcoming of several earlier studies of these deposits has resulted from a failure to examine the marine fraction of elements separate from the terrigenous fraction

dynamics of retreating slabs 2 insights from three dimensional laboratory experiments 1 a laboratory analogue of a three layer linear viscous slab upper mantle lower mantle system is established in a silicone putty honey and crystallized honey tank experiment the same setup as in the numerical investigation part 1 is used we focus on the interaction of the slab with the induced passive mantle flow by widely varying the mantle volume flux boundary conditions in our numerical experiments the lateral volume flux was set to zero in interpreting the results relative to the real earth the base of the box is taken as the bottom of the mantle convection system while the lateral boundaries may be associated with the presence of other nearby slabs dynamic force equilibrium assessed on the basis of an analytical review of forces is described for four different phases 1 the subduction initiation instability 2 the accelerating dynamic free fall phase of the slab 3 the dynamic interaction with the 660 km discontinuity and 4 a final phase of steady state trench retreat phase 3 is an important feature not observed in the numerical experiments this highly dynamic phase of interrupted trench retreat can therefore be attributed to boundary conditions on mantle volume flux on the basis of integration constants of force equilibrium in phases 2 and 4 we identify two different classes of volume flux one in which the lateral boundary can be considered open and the other class where it is closed closed boundary condition cases are obtained if any of the lateral box boundaries are 600 km away from the slab assuming a one to one relation between trench retreat and back arc spreading enigmatic observations of episodic opening of back arc basins can be explained by our experimental observations

40ar 39ar thermochronological evidence for formation and mesozoic evolution of the northern central segment of the altyn tagh fault system in the northern tibetan plateau to better constrain the probable timing of formation and evolution of the altyn tagh sinistral strike slip system in the mesozoic a 40ar 39ar thermochronological study has been carried out in the north central segment of the altyn tagh fault system the northern margin of the qaidam basin and the eastern kunlun orogenic belt muscovite biotite and k feldspar separated from mylonite granite pegmatite and metamorphic rocks have been analyzed the range of 40ar 39ar data and structural evidence indicate that a peak metamorphic event in terranes bordering the altyn tagh fault system occurred between 450 and 420 ma at ca 250 230 ma there is evidence for initial sinistral strike slip shearing sinistral strike slip deformation occurred later along the altyn tagh fault system at 165 160 ma and 100 89 ma respectively cooling histories in the northern margin of the qaidam basin and the eastern kunlun orogenic belt show that these areas also experienced rapid cooling ca 250 230 ma as was the case for the early altyn tagh fault system this regional tectonic and cooling process indicates that the initial formation of the altyn tagh sinistral slip fault system occurred in latest permian early triassic time and was coupled with or related to suturing in the northern margin of the qaidam basin and the kunlun orogenic belt cooling events along the altyn tagh fault system between 165 and 160 ma and 100 89 ma were accompanied by differential closure along the bangong lake nujiang suture zone in its eastern and western sectors during the middle late jurassic and early cretaceous respectively zhao et al 2001 1 wang et al 2002 2 1 ref 48 2 ref 41

haskell 1935 revisited the haskell 1935 value of 1021 pa s for mantle viscosity is a classic and enduring constraint on the rheology of the earth s interior we revisit this inference using spherically symmetric self gravitating maxwell viscoelastic earth models our inference is based on both forward and inverse analyses of decay times associated with uplift at two sites considered by haskell angerman river sweden and oslo norway rather than the raw relative sea level rsl data at these sites we demonstrate that predictions of the decay time associated with the angerman river data are insensitive to variations in both the late pleistocene ice load history and the lithospheric thickness of the earth model the predictions at oslo are sensitive to both these inputs and hence decay times at this site provide a remarkably robust constraint on mantle viscosity we derive a constraint on the average viscosity of the mantle of 0 65 1 10 1021 pa s where the average resolved by the data encompasses a region which extends from the base of the lithosphere to a depth of near 1400 km this indicates that many previous analyses which have invoked the haskell value of 1021 pa s as a constraint on the average upper mantle i e above 670 km depth viscosity alone have misinterpreted the resolving power of the inference furthermore our analysis indicates that a number of apparently contradictory inferences of viscosity based on fennoscandian data satisfy the new rigorous interpretation of the haskell constraint finally we demonstrate how the ambiguity in the upper mantle lower mantle viscosity contrast associated with the haskell average may be reduced by invoking decay time constraints estimated from rsl curves in hudson bay a preliminary inversion of decay times at angerman river and richmond gulf in hudson bay suggests a contrast of approximately an order of magnitude between the average viscosities of the upper mantle and the top 1000 km of the lower mantle however a conclusive analysis in this regard must await the determination of consistent decay time estimates for the hudson bay region

variability in flow and temperatures within mantle subduction zones 1 a series of laboratory experiments is used to model three dimensional aspects of flow in subduction zones and the consequent temperature variations in the slab and overlying mantle wedge the effects of longitudinal rollback and slab steepening components of motions are considered along with different thicknesses of the over riding lithosphere the results show that the style of plate sinking influences the evolution of subduction zones both in terms of the speed orientation and temperature of flow in the overlying wedge and in terms of the temperatures at the surface of the descending slab in the simplest case of longitudinal sinking without rollback motion velocities in the mantle wedge are 30 40 of the downdip plate speed return flow paths in the shallow mantle wedge are nearly horizontal for slow slab speeds and steepen with higher slab speeds and there is no mass flux around the edges of a slab segment of finite width rollback subduction leads to flow both around and beneath the sinking slab with larger velocities in the wedge up to 150 of the slab speed and flow focused toward the center of the plate segment rollback subduction in which either the trench migrates or the plate steepens with time induces shallow and steep wedge return flow trajectories respectively the thermal evolution of the plate is strongly influenced by sinking style and rate highest temperatures are along the edges of the slab for longitudinal sinking but along the centerline of the slab segment for rollback motion slab surface temperatures when scaled to the mantle are higher than those given by previous two dimensional numerical models and are consistent with recent observational and experimental data on melt compositions

end permian to mid triassic termination of the accretionary processes of the southern altaids implications for the geodynamic evolution phanerozoic continental growth and metallogeny of central asia the altaids is one of the largest accretionary orogenic collages in the world with the highest rate of phanerozoic continental growth and significant metallogenic importance it is widely accepted that subduction related orogenesis of the altaids started in the late precambrian and gradually migrated southward present coordinates however it is uncertain when and how the building of the altaids was finally completed based on structural geology geochemical geochronological and paleomagnetic data this paper presents late paleozoic to early mesozoic accretionary tectonics of two key areas north xinjiang in the west and inner mongolia in the east together with neighboring mongolia the late paleozoic tectonics of north xinjiang and adjacent areas were characterized by continuous southward accretion along the wide southern active margin of siberia and its final amalgamation with the passive margin of tarim which may have lasted to the end permian to early mid triassic in contrast in inner mongolia and adjacent areas two wide accretionary wedges developed along the southern active margin of siberia and the northern active margin of the north china craton which may have lasted to the mid triassic the final products of the long lived accretionary processes in the southern altaids include late paleozoic to permian arcs late paleozoic to mid triassic accretionary wedges composed of radiolarian cherts pillow lavas and ophiolitic fragments and high pressure ultrahigh pressure metamorphic rocks permian alaskan type zoned mafic ultramafic complexes intruded along some major faults of the tien shan we define a new tarim suture zone immediately north of the tarim craton that is probably now buried below the tien shan as a result of northward subduction of the tarim block in the cenozoic the docking of the tarim and north china cratons against the southern active margin of siberia in the end permian to mid triassic resulted in the final closure of the paleoasian ocean and terminated the accretionary orogenesis of the southern altaids in this part of central asia this complex geodynamic evolution led to formation of giant metal deposits in central asia and to substantial continental growth

application of the ce anomaly as a paleoredox indicator the ground rules much attention has been paid in recent years to the potential application of the ce anomaly measured in various marine phases as a paleoceanographic indicator of widespread marine anoxia in this paper we present and discuss results from recent studies of present day rare earth element ree distributions and hence ce anomaly distributions in the marine environment which are particularly pertinent to paleoceanography subsequently we review and discuss the validity of the recent literature in which ce anomalies have been employed as paleoredox indicators

origin of permian gabbroic intrusions in the southern margin of the altai orogenic belt a possible link to the permian tarim mantle plume abstract zircon u pb dating and hf isotopic compositions whole rock and mineral geochemistry as well as field and petrographic observations are reported for the permian gabbroic intrusions in the southern margin of the altai orogenic belt similar zircon ages 280 ma of these intrusions to that of the permian tarim mantle plume suggest a possible link to the tarim plume olivine grains from the intrusions have fo contents ranging from 69 to 81 and nio from 0 05 to 0 21 wt indicating the crystallization from evolved mafic magmas clinopyroxene grains from the intrusions have 0 3 to 1 5 wt tio 2 and display a rift to arc like affinity the intrusions are tholeiitic and have slightly lree depleted to lree enriched chondrite normalized rare earth element ree patterns zircons from the intrusions have e hf t values higher than 8 0 whereas the whole rock e nd t values range from 2 7 to 7 0 rocks from the intrusions have nb la of 0 2 to 0 8 consistent with the derivation from a depleted sub lithospheric mantle with variable degrees of crustal contamination the presence of primary hornblende and or biotite in the rocks indicates metasomatism by slab derived fluids melts in the mantle source we propose that upwelling of asthenospheric mantle in the permian tarim mantle plume along the irtish suture zone resulted in partial melting of the sub continental lithospheric mantle based on a comparison between the ni cu sulfide bearing kalatongke complex and un mineralized intrusions we suggest that ni cu sulfide mineralized intrusions in the altai orogenic belt are characterized by the enrichments of lrees and liles and by the presence of primary water bearing minerals

u pb ages and hf o isotopes of zircons from late paleozoic mafic ultramafic units in the southern central asian orogenic belt tectonic implications and evidence for an early permian mantle plume abstract u pb age and hf o isotope data of zircons from the mafic ultramafic complexes and their related rhyolites and granites in the eastern tianshan and beishan regions were obtained by sims and la icpms the hf o isotopic composition and hf model age of the zircons are closely related to their tectonic setting and formation age the zircons from the mafic ultramafic complexes in the bogeda haerlike and jueluotage belts have higher ehf t 8 17 and lower 18o 4 6 whereas most of the zircons in the gabbros granites and rhyolite from the middle tianshan massif and beishan rift display lower ehf t 0 8 and higher 18o 5 8 the positive ehf t and relatively lower 18o values suggest that these mafic ultramafic complexes were derived from depleted mantle which was subjected to subduction related modification processes the hf isotopic composition of zircons in the granites has revealed that the growth of juvenile crust was also very significant in the southern margin of the central asian orogenic belt during the paleozoic the hf model ages of the analyzed zircons together with the regional geology suggest that the beishan area had a northward subduction possibly from ca 900 ma to ca 400 ma whereas the eastern tianshan had a south directed subduction most likely from ca 600 ma to ca 310 ma additionally zircons with ca 280 ma u pb ages display wider and more scattered ehf t and 18o variations than the relatively older and younger ones which further support the early permian mantle plume model

trench parallel flow beneath the nazca plate from seismic anisotropy shear wave splitting of s and sks phases reveals the anisotropy and strain field of the mantle beneath the subducting nazca plate cocos plate and the caribbean region these observations can be used to test models of mantle flow two dimensional entrained mantle flow beneath the subducting nazca slab is not consistent with the data rather there is evidence for horizontal trench parallel flow in the mantle beneath the nazca plate along much of the andean subduction zone trench parallel flow is attributale utable to retrograde motion of the slab the decoupling of the slab and underlying mantle and a partial barrier to flow at depth resulting in lateral mantle flow beneath the slab such flow facilitates the transfer of material from the shrinking mantle reservoir beneath the pacific basin to the growing mantle reservoir beneath the atlantic basin trenchparallel flow may explain the eastward motions of the caribbean and scotia sea plates the anomalously shallow bathymetry of the eastern nazca plate the long wavelength geoid high over western south america and it may contribute to the high elevation and intense deformation of the central andes

late cretaceous ca 84 ma compressive deformation of the stable platform of northeast africa egypt far field stress effects of the santonian event and origin of the syrian arc deformation belt large scale northeast trending asymmetric folds and associated contractional faults seismically induced slump structures and strike slip and extensional faults of santonian age occur within the eastern desert of egypt 150 km south of the syrian arc mobile belt the region also displays a locally developed angular unconformity at the base of the flat lying campanian limestone these features demonstrate that the deformation event responsible for the syrian arc also led to northwest southeast shortening in central egypt ca 84 ma the late santonian shortening corresponds precisely with a brief period of compression that swept across the entire african plate coeval with a significant change in the poles of opening of the north atlantic this santonian event is a prominent example of the role that far field compressional stresses can play in stable intraplate settings

mega fold interference patterns in the beishan orogen nw china created by change in plate configuration during permo triassic termination of the altaids abstract kilometer size fold interference patterns in the beishan orogenic collage boc in the southernmost altaids formed by fold superimposition in fossiliferous permian sedimentary rocks first phase f 1 upright and almost north trending folds were refolded by e to ene trending f 2 folds whose axial planes and axes are vertical or subvertical from east to west there is a regional change in style of interference patterns from lobate cuspate to crescent to mushroom shape this variation is accompanied by a westward decrease in the f 2 interlimb angle and related to a higher percentage of coarse grained clastic rocks suggesting a dependence of the f 2 deformation on lithology axial planar slaty cleavages are well developed in f 1 and poorly developed in f 2 folds the superposed folds mainly underwent flexural slip and flexural flow folding to give rise to the lobate cuspate pattern and to the crescent pattern caused by flattening and flexural flow folding where the sediments were unconsolidated and enriched in fluids the two folding events are interpreted to result from a major change in plate configuration that caused the inversion of an inter arc basin during the final amalgamation of the boc in the latest permian to early to mid triassic the two folding events bracketed between a maximum detrital zircon age of

multiple rind structure in pillow lava as an indicator of shallow water multiple rind structure is common among shallow water pillows with diameters larger than about 1 m in oamaru new zealand on the columbia plateau usa and elsewhere a rind consists of sideromelane tachylyte and tachylytic basalt a multiple rind is a concentric set of repeated rinds in various forms e g a portion of a broken rind thrust under another part a series of short and detached subparallel rinds or a pouch shaped depression transitions and combinations of these three forms are common multiple rind structure develops at any part of the pillow perimeter but does not cover the pillow completely it is always accompanied by a rupture in the outermost rind up to 13 rinds have been observed but two to four rinds are most common the multiple rind structure is formed by implosion resulting from condensation of exsolved h2o when h2o condenses a pressure difference between the interior and exterior of a pillow is created above a certain threshold pressure difference the outer skin of a pillow is torn at weak points such as radial joints and thrusts under the neighboring skin buckles to form a pouch shaped depression or produces some variation of these one set of multiple rinds is thus formed further exsolution and condensation of h2o in solidifying pillows may cause development of additional rinds h2o exsolution and condensation and subsequent implosion are limited to low pressure environments so that multiple rind structure is characteristic of shallow water pillow lava

postcollisional eastward extrusion and tectonic exhumation along the eastern tianshan orogen central asia constraints from dextral strike slip motion and 40ar 39ar geochronological evidence abstractpostcollisional extrusion and tectonic evolution in the eastern tianshan orogenic belt etob remains poorly known especially the mechanism of dextral strike slip motion and associated tectonic exhumation to better constrain this development a structural and 40ar 39ar geochronological study was carried out on a syndextral strike slip intrusion the jueluotag batholith as well as on other granitic plutons in the etob 40ar 39ar analyses of hornblende biotite k feldspar and plagioclase from quartz mica diorite granodiorite and dioritic porphyry dykes were used to construct cooling histories of the etob hornblendes have cooling ages of 277 272 ma similar to the syntectonic granitic intrusions but biotite ages are 261 254 ma along the syndextral strike slip pluton from east to west the dextral strike slip motion cuts through 268 ma dioritic porphyry dikes as well from these data we conclude that dextral strike slip motion occurred from 270 to 245 ma based on the syntectonic granitic intr

geodynamics of rifting and implications for hydrocarbon habitat abstract rifting activity is probably mainly governed by frictional forces exerted on the base of the lithosphere by the convecting sub lithospheric upper mantle combined with deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system and by far filed stresses controlling the interaction of lithospheric plates moving relative to each other mantle plumes rising from the deep mantle do not appear to be a major driving force of rifting therefore it is questioned whether a distinction between active and passive rifts is still justified lithospheric extension triggers by decompression partial melting of the lower lithosphere and upper asthenosphere magmatic processes contribute materially to thinning of the subcrustal lithosphere and probably also of the lower crust this could explain the frequently observed discrepancy between upper crustal extension by faulting and lower crustal attenuation as defined by refraction and deep reflection seismic data the duration of the rifting stage of palaeorifts and passive margins is highly variable depending on plate interaction crustal separation can be achieved after as little as 7 ma and as much as 280 ma of rifting activity the availability of crustal discontinuities does not appear to play a significant role in the time required for crustal separation to be achieved the structural style of rifts is controlled by the physical state of the lithosphere the mode and amount of its extension and the lithological composition of pre and syn rift sediments subsidence of rift basins is controlled by the balance between isostatic adjustment of the crust to lithospheric stretching and uplift induced by mechanical and thermal attenuation of the subcrustal lithosphere post rift basin subsidence is mainly governed by the decay of rift induced thermal anomalies densification of the lower crust and intra plate stresses provide overprinting effects in view of this it is unlikely that quantitative analyses of post rift basin subsidence give a direct measure of lithospheric extension tectonically active rifts palaeorifts and passive margin basins contain major hydrocarbon provinces their hydrocarbon charge can rely exclusively on pre rift syn rift or post rift sedimentary sequences or a combination thereof maturation of source rocks can be achieved during the syn and or post rift stage of basin evolution during rifting conductive and convective heat transfer accounts for elevated geothermal gradients these play an important role in the maturation of pre and syn rift source rocks thermal gradients decrease asymptotically during the post rift stage

controls of the structure of subducted slabs numerical simulations of subducting slabs are formulated in which the shape and dip of the slab are determined by the dynamics of the flow rather than imposed a priori the dip of slabs is a function of the time since the initiation of subduction slabs fold develop a kink in dip and thicken on entry into a high viscosity lower mantle comparison of the simulations with seismic observations suggest that the lower mantle is at least 10 30 times more viscous than the upper mantle

unconformity bounded upper paleozoic megasequences in the beishan region nw china and implications for the timing of the paleo asian ocean closure abstract the upper paleozoic sedimentary sequence in the beishan region northwest china provides a complete record of the complex amalgamation and extension history of the central asian orogenic belt caob for this part of china integrating newly obtained stratigraphic paleontological and radiometric data this paper reviews the upper paleozoic stratigraphic sequences of the beishan region the upper paleozoic strata here are divided into three megasequences ms they are in ascending order devonian mississippian guadalupian and lopingian megasequences ms1 ms2 and ms3 separated by two major unconformities each indicative of a huge sedimentary hiatus ms1 is made up of continental and epicontinental marine deposits in the pre rift stage ms2 of transgressive marine and volcanic strata in the syn rift stage and ms3 of continental sediments in the inversion stage the major unconformity between ms1 and ms2 is interpreted to represent the regional uplift and erosion caused by the final amalgamation of caob in the beishan region the syn rift ms2 represented by transgressive marine and volcanic strata in the beishan region can be well correlated with pennsylvanian guadalupian megasequences in inner mongolia where they rest on a major interregional unconformity this correlation suggests the closure of the paleo asian ocean in beishan and inner mongolia before the carboniferous followed by the development of a synchronous marine rift megasequence in the southern part of the caob

paleozoic multiple accretionary and collisional processes of the beishan orogenic collage in the paper by xiao and others american journal of science v 310 p 1553 1594 1 please substitute figure 16 reproduced here for figure 16 that appears between pages 1573 and 1574 the original figure 16 2 was printed at an improper size figure 3 1 lookup volpage 310

geochemistry and geochronology of acidic rocks in the beishan region nw china petrogenesis and tectonic implications abstract the dacites and granites from the beishan region are characterized by highly fractionated ree enrichments in lile and depletion in nb ta and sr the dacites contain zircons with ages ranging from 265 to 748 ma these features suggest that they were probably derived from the older lower continental crust and were later on subjected to significant crustal assimilation the rhyolites diorites and diabases all of which share similar geochemical features such as trace element patterns zircon u pb ages of 280 ma positive ehf t and end t high initial 87sr 86sr i and 18o values were most likely derived from the juvenile crustal sources and subsequently experienced fractional crystallization and crustal contamination these acidic igneous rocks except dacites and granites and coeval mafic ultramafic intrusions form a bimodal igneous series this implies that the beishan region was a late paleozoic rift probably developed in association with the early permian mantle plume activity in the tarim basin

diagenetic constraints on the use of cerium anomalies as palaeoseawater redox proxies an isotopic and ree study of cambrian phosphorites seawater rare earth element ree distributions show heavy ree enrichment and cerium depletion but these characteristics are only rarely found in ancient marine authigenic minerals such as carbonate fluorapatite and calcite this enigma may be due to changing ree distributions in seawater through time or to post depositional redistribution of ree differentiating between these two factors is however far from straightforward as their relative importance is likely to vary and evidence is often ambiguous in this article we present ree as well as strontium and sulphur isotope data for basal cambrian phosphorites of south china which confirm the emerging rule of thumb that granular phosphorites seldom reveal extreme departures from the seawater norm and tend to undergo therefore less diagenetic alteration of ree spectra than skeletal apatites and other phosphate types this study reveals stratigraphic trends in ce anomalies ce ce that could be taken to imply the preservation of a palaeoenvironmental signal however further investigation of our data shows that such trends in phosphorites can also be caused by post depositional alteration which tends to 1 increase ree contents 2 reverse ce depletion 3 alter eu anomalies and 4 concentrate preferentially the middle ree mree further alteration which may be due to weathering has led to hree depletion and positive la and y anomalies tetrad effect this is the first study where ceanom and phosphate bound sulphate s isotope values could be compared for one particular stratigraphic section which provides us with an additional tool for the interpretation of diagenetic redox conditions least altered phosphorites from meishucun that yield a strongly negative ceanom constrain early cambrian seawater 34s to about 33 we conclude that phosphorite ceanom primarily records variations in the local conditions of diagenesis while outlining some special cases in which diagenetic redox conditions may broadly reflect local bottom water redox conditions

toroidal mantle flow around the calabrian slab italy from sks splitting 1 sks wave birefringence is likely caused by ongoing and or fossil mantle flow splitting parameters in the southern tyrrhenian subduction zone define a quite complex pattern of fast directions with large delay times up to 2 7 s fast directions are prevalently trench parallel below the slab and rotate to trench normal at the western edge of the slab depicting a ring around it we interpret these anisotropic patterns also observed in laboratory experiment as toroidal mantle flow induced by rollback subduction

phosphorite associations on seamounts in the tropical southwest pacific ocean abstract recent surveys have revealed two distinct types of submarine phosphorite on seamounts between latitudes 6 22 s in the southwest pacific ocean the more widely distributed type comprising carbonate fluorapatite and calcite of typical marine facies is found on a number of isolated seamount peaks at depths between 1000 1650 m a markedly different assemblage in which dolomite is associated with carbonate fluorapatite has been discovered at depths of 550 1100 m on two flat topped guyots on the northern margin of the fiji plateau the latter assemblage is strongly reminiscent of the insular type of phosphorite that occurs subaerially on several tropical pacific islands and atolls it is possible that similar phosphorite deposits may lie beneath biogenic carbonate sediments on a series of shallow banks in line with the guyots the mineral associations geochemistry textures and geological settings indicate that the guyot phosphorites may well represent submerged insular deposits while the phosphorites on the isolated seamounts are entirely of marine origin

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

the western central asian orogenic belt a window to accretionary orogenesis and continental growth abstract the architecture of accretionary orogens is a key to understand continental growth here we present an overview of the orogenic components and their amalgamation in the western central asian orogenic belt caob the caob records the convergence and interactions among various types of orogenic components including the japan type mariana type and alaska aleutian type arc systems as well as the active marginal sequences of the siberia craton which incorporated wide accretionary complexes and accreted arcs and terranes during construction of the caob the kazakhstan arc chain was characterized by multiple subduction whereas the northern fringe of the tarim craton remained mostly as a passive margin the multiple convergence and accretions among these various orogenic components generated huge orogenic collages in the late paleozoic and even in the early triassic involving parallel amalgamation circum microcontinent amalgamation and oroclinal bending the preservation of trapped basins played a significant role in orogenesis with some parts of the oceanic plate being subducted and others behaving as rigid units the orogenesis in the caob was long lived lasting for more than 800 m y involving multiple subduction and long continuous accretion and featuring the complexity of accretionary orogenesis and continent growth

morphology and distribution of lava flows on mid ocean ridges a review abstract both pillow lavas and sheet flows are important features of oceanic volcanism at mid ocean ridges submersible surveys of the neovolcanic zone and off axis areas of these complex seafloor volcanic terrains conducted during the past two decades have uncovered a wide array of basaltic surface forms with pillow basalts displaying bulbous spherical and elongate tubular patterns and sheet flows exhibiting smooth lobate rippled wrinkled ropy whorly hackly and jumbled configurations the genesis of pillow lavas and sheet flows appears to be coupled to variations of spreading rates within the plate boundary zone while pillow lavas predominate at slow spreading centers i e mid atlantic ridge sheet flows prevail along the active volcanic zones at intermediate i e galapagos rift and juan de fuca ridge fast and superfast i e east pacific rise spreading centers yet both types of flows are present at each setting the distribution of pillow lavas and sheet flows along a ridge segment is related to the topographic gradient of the ridge axis hence the formation of sheet flows tends to be favored by the development of topographic highs between ridge segment boundaries where the crustal lid is thinnest and rifting directly taps the underlying magma chamber fissure eruptions here release copious amounts of surface fed fluid lavas which decrease in volume down the topographic gradient away from the bathymetric high of the ridge segment lava flows become more distal and channelized thereby fostering greater concentrations of tube fed pillow flows toward the segment boundaries the type of lava flow morphologies encountered at any specific site on the seafloor is contingent upon the effusion rate cooling rate and slope as demonstrated by laboratory simulations

crystal chemical controls on rare earth element concentrations in fossil biogenic apatites and implications for paleoenvironmental reconstructions abstract fossil biogenic apatites display rare earth element ree patterns which have been proposed to reflect environmental and biological controls on past seawater composition these patterns can be separated in two groups 1 patterns that are similar to those of open ocean and epicontinental waters and 2 patterns that exhibit a strong enrichment in intermediate ree bell shaped patterns because ree trapping in biogenic apatites occurs mostly post mortem it is essential to understand the crystal chemical inorganic factors that can influence the fractionation of ree between apatite and water a model is proposed by which partition coefficients of ree between apatite and water for a substitution mechanism can be extrapolated from mineral melt partition data to seawater or sedimentary basin conditions these are compared with available experimental partition coefficients for ree adsorption mechanism and used to discuss ree patterns in fossil biogenic apatites calculations of ree patterns for apatites at equilibrium with water show that the bell shaped patterns in fossil apatites can be explained by fractionation with seawater or continental fluids at low temperature under crystal chemical control involving a substitution mechanism and in the context of extensive or late diagenesis in that case the equilibrium composition of the fluid cannot be unequivocally determined from observed ree concentrations in fossil apatites this type of sample is inappropriate to discuss the past environmental or biological controls on ree concentrations in the ocean other sample compositions suggest little to moderate alteration of the original oceanic to freshwater pattern through the adsorption mechanism in the context of weak or early diagenesis and may be used with caution for that purpose

structure age and tectonic development of the huoshishan niujuanzi ophiolitic melange beishan southernmost altaids abstract the huoshishan niujuanzi ophiolitic melange hno is located near the central part of the beishan orogenic belt in the southernmost altaids the hno consists of ultramafic rocks cumulate gabbros gabbros plagiogranites diorites diabases basalts andesites rhyolitic volcaniclastic rocks and siliceous sedimentary rocks many of which are in a schist matrix gongpoquan group geochemical data of the mafic rocks indicate a calc alkaline or a mixture of calc alkaline and tholeiitic rocks with negative nb ta and positive pb ba and la anomalies suggesting formation in an island arc or supra subduction zone setting a gabbro from a block in the melange in the niujuanzi area has a zircon age of 435 0 1 9 ma and a plagiogranite with an age of 444 3 1 9 ma and another gabbro from the huoshishan area has an age of 410 5 3 7 ma the schist matrix has a zircon age of 512 5 3 ma and contains silurian devonian and carboniferous fossils thus the melange formed in the late carboniferous or later our structural analysis of fault planes in the hno the crenulation cleavages s 2 of the schist and fold axial planes of early permian sandy limestone quartz veins and late permian sandstones indicates that the melange underwent a north to south compression and the orientation of stretching lineations slickensides and fold hinge lines implies that the hno experienced top to the north or northwest movement the entire planar and linear structural data set suggests that the subduction polarity was probably to the south in the late paleozoic the emplacement age of the hno was probably near the end permian based on the age of the youngest rocks in the ophiolitic melange and by the presence of a late permian unconformity from our work integrated with published regional data we outline a comprehensive geodynamic model for the central boc

brasiliano crustal structure and the tectonic setting of the parnaiba basin of ne brazil results of a deep seismic reflection profile a 1430 km deep crustal seismic reflection profile of the parnaiba basin shows an asymmetric structured western margin and a gently dipping eastern margin the 3 km thick phanerozoic sedimentary section overlies a pronounced planar regional unconformity that crosses three precambrian blocks with differing seismic facies the amazonian araguaia block the parnaiba block and the borborema block the blocks are separated by steep crustal scale boundaries across which seismic facies change abruptly in the west the ophiolitic metasedimentary rocks of the araguaia group overlie the amazonian craton both craton and metasediments terminate eastward against a steep east dipping fault zone defining the amazonian araguaia block and parnaiba block boundary reactivation of this neoproterozoic margin in the late triassic and late jurassic early cretaceous folded and elevated basement and basin over 2 km a second crustal boundary defines the eastern margin of the parnaiba block with the neoproterozoic borborema block this boundary is interpreted as the extension of the transbrasiliano shear zone these data demonstrate that the basement of the parnaiba basin was formed during brasiliano orogenesis by west directed collision related thrusting succeeded by lateral accretion along steep crustal scale boundaries after formation of a post brasiliano peneplain the parnaiba basin developed seamlessly across three very different crustal blocks and appears to have been significantly larger than its present outline no extensive underlying rift system is evident suggesting that basement structure had little to do with basin formation but that episodic reactivation of the boundary zones and basement fabric has controlled the structuring and preservation of the basin

uranium oxidation state and related variations in geochemistry of phosphorites from the negev southern israel abstract study of the u iv fraction in mishash phosphorites of the negev reveals an apparently paradoxical trend the u iv fraction is very low less than 10 in the pristine organic rich laminated phosphorites while this proportion considerably increases up to 60 in the recycled mechanically reworked granular phosphorites which deposited under more oxidizing conditions this u iv trend seems to differ from that reported in other phosphate sequences elsewhere and is also quite unexpected in view of the sediment patterns and the specific formational conditions of the two phosphorite facies in addition a lower u iv in the analyzed granular phosphorites is coupled with a lower rare earth elements ree p 2 o 5 and y p 2 o 5 ratio a higher cd and zn concentration less negative ce ce ce and eu eu eu anomalies a lower hree dy er enrichment and a lower f p 2 o 5 and higher co 2 f ratio in francolites this u iv trend in the two negev phosphorite facies may be explained by varying time spans of apatite growth changes in sediment burial rates and principally different residence times of the phosphate fraction in the suboxic apatite forming diagenetic zone near the sediment water interface the associated varying concentrations of rees y cd and zn in phosphorites and of f and co 2 in francolites in the two granular phosphorite facies are apparently collectively regulated by the same sedimentary dynamics during phosphate deposition it is presumed that the u iv trend observed in the negev phosphorites could be detected in other phosphate sequences as well if pristine phosphorites are carefully identified and selectively analyzed

two major cenozoic episodes of phosphogenesis recorded in equatorial pacific seamount deposits seamount phosphorites have been recognized since the 1950s but this is the first study to provide an in depth exploration of the origin and history of these widespread deposits representative samples from equatorial pacific cretaceous seamounts were analyzed for chemical mineralogical and stable isotope compositions the phosphorites occur in a wide variety of forms but most commonly carbonate fluorapatite cfa replaced middle eocene and older carbonate sediment in a deep water environment 1000 m element ratios distinguish seamount phosphorites from continental margin plateau and insular phosphorites uranium and thorium contents are low and total rare earth element ree contents are generally high ree ratios and shale normalized patterns demonstrate that the rees and host cfa were derived from seawater strontium isotopic compositions compared with inferred cenozoic seawater curves define two major episodes of cenozoic phosphatization late eocene early oligocene 39 34 ma and late oligocene early miocene 27 21 ma three minor events are also indicated the major episodes occurred at times of climate transition the first from a nonglacial to glacial earth and the second from a predominantly glacial to warm earth the paleoceanographic conditions that existed at those times initiated and sustained development of phosphorite by accumulation of dissolved phosphorus in the deep sea during relatively stable climatic conditions when oceanic circulation was sluggish fluctuations in climate sealevel and upwelling that accompanied the climate transitions may have driven cycles of enrichment and depletion of the deep sea phosphorus reservoir as temperature gradients in the oceans increased antarctic glaciation expanded and oceanic circulation and upwelling intensified expansion and intensification of the oxygen minimum zone may have increased the capacity for midwater storage of phosphorus supplied by dynamic upwelling around seamounts however the bottom waters never became anoxic during the phosphogenic episodes fluctuations in the ccd and lysocline co2 fluxes and changes in bottom water circulation and temperatures may have bathed the seamount carbonates in more corrosive waters which coupled with increased supplies of dissolved phosphorus promoted replacement processes the late eocene early oligocene phosphogenic episode recorded in seamount deposits is not matched by large phosphorite deposits in the geologic record whereas the late oligocene early miocene episode and middle miocene event are matched by large deposits distributed globally the seamount phosphorites are exposed at the surface of the seamounts and have been for most of the neogene and oligocene the phosphorites do not show signs of etching that would indicate substantial undersaturation of seawater phosphate with respect to cfa mass balance calculations indicate that about 5 4 19 1012 g of p2o5 are locked up in equatorial pacific seamount phosphorites that amount is equivalent to about 2 7 years of the present annual input from rivers

from continental rifting to seafloor spreading insight from 3d thermo mechanical modeling abstract continental rifting to seafloor spreading is a continuous process and rifting history influences the following spreading process however the complete process is scarcely simulated using 3d thermo mechanical coupled visco plastic numerical models we investigate the complete extension process and the inheritance of continental rifting in oceanic spreading our modeling results show that the initial continental lithosphere rheological coupling decoupling at the moho affects oceanic spreading in two manners 1 coupled model a strong lower crust mechanically couples upper crust and upper mantle lithosphere generates large lithospheric shear zones and fast rifting which promotes symmetric oceanic accretion i e oceanic crust growth and leads to a relatively straight oceanic ridge while 2 decoupled model a weak ductile lower crust mechanically decouples upper crust and upper mantle lithosphere generates separate crustal and mantle shear zones and favors asymmetric oceanic accretion involving development of active detachment faults with 3d features complex ridge geometries e g overlapping ridge segments and curved ridges are generated in the decoupled models two types of detachment faults termed continental and oceanic detachment faults are established in the coupled and decoupled models respectively continental detachment faults are generated through rotation of high angle normal faults during rifting and terminated by magmatism during continental breakup oceanic detachment faults form in oceanic crust in the late rifting early spreading stage and dominates asymmetric oceanic accretion the life cycle of oceanic detachment faults has been revealed in this study

geochemistry of trace elements and their relations with organic matter in kuh e sefid phosphorite mineralization zagros mountain iran abstract the kuh e sefid phosphorite is located in zagros fold thrust belt in the ramhormoz area northeast of khuzestan province phosphorite mineralization occurred as phosphorite lenses of eocene age hosted by the pabdeh formation the aim of this study is to investigate the role of organic matter in trace metal portioning during phosphorite mineralization in the kuh e sefid deposit field observations and petrographic studies indicate that the host rock of mineralization is mainly consists of shale marl and limestone with textures varying from wackestone to packestone phosphoritic components such as authigenic apatite pellets intraclasts fish skeletal fragments and micro fossils are present moreover non phosphorite minerals including calcite glauconite pyrite iron oxide and to a lesser extend montmorillonite with microcrystalline quartz occurred in phosphorite lenses nasc normalized patterns of rees showed the enrichment of heavy rare earth elements hree and negative ce anomaly with co existence of slightly negative and positive eu anomalies represent a transitional change from oxic to anoxic sedimentation conditions during mineralization importantly ni co versus v cr ratios and v v ni diagrams confirmed dysoxic to anoxic conditions for the hydrocarbon bearing phosphorite and hosted shales while non hydrocarbon bearing phosphorites exhibit oxic conditions the ftir studies revealed qualitative information about the component bonds and nature of the organic matter and mineral organic bonds like oh carboxylic oh carboxylic acid c o c c alkaline group ch2 c c aromatic ch aliphatic and aromatic stretching associated with phosphate mineralization it seems that there is a link between a relatively high concentration of trace metals i e rees u co zn ni v sr and mo and phosphorites enriched in organic matter especially those of aromatic hydrocarbon groups

effective petroleum source rocks of the world stratigraphic distribution and controlling depositional factors six stratigraphic intervals representing one third of phanerozoic time contain petroleum source rocks that have provided more than 90 of the world s discovered original reserves of oil and gas in barrels of oil equivalent the six intervals are 1 silurian generated 9 of the world s reserves 2 upper devonian tournaisian 8 of reserves 3 pennsylvanian lower permian 8 of reserves 4 upper jurassic 25 of reserves 5 middle cretaceous 29 of reserves and 6 oligocene miocene 12 5 of reserves this uneven distribution of source rocks in time displays no obvious cyclicity and the factors that controlled the formation of source rocks vary from interval to interval maps that show facies structural forms and petroleum source rocks were prepared for this study analysis of the maps indicates that several primary factors controlled the areal distribution of source rocks their geochemical type and their effectiveness i e the amounts of discovered original conventionally recoverable reserves of oil and gas generated by these rocks these factors are geologic age paleolatitude of the depositional areas structural forms in which the deposition of source rocks occurred and the evolution of biota the maturation time of these source rocks demonstrates that the majority of discovered oil and gas is very young almost 70 of the world s original reserves of oil and gas has been generated since the coniacian and nearly 50 of the world s petrole m has been generated and trapped since the oligocene the term petroleum as used in this study includes oil gas condensate and gas but does not include bitumen and other solid and semisolid oil derivatives

influence of trench width on subduction hinge retreat rates in 3 d models of slab rollback subduction of tectonic plates limited in lateral extent and with a free trailing tail i e free subduction is modeled in a three dimensional 3 d geometry the models use a nonlinear viscoplastic rheology for the subducting plate and exhibit a wide range of behaviors depending on such plate characteristics as strength width and thickness we investigate the time evolution of this progressive rollback subduction measure the accompanying return flow in the upper mantle and quantify the plate kinematics due to the 3 d geometry flow is allowed to accompany slab rollback around the lateral edges of the slab the toroidal component as opposed to 2 d geometry where material is forced to flow underneath the slab tip the poloidal component a simple force balance is provided which relates the speed of backward trench migration to the resistive forces of generating flow and weakening the plate our results indicate most of the gravitational energy of the system i e the negative buoyancy of the subducting slab is converted into a toroidal flow 69 a much smaller amount goes into weakening the plate 18 and the remaining amount goes into driving flow parallel to displacement of the slab 13 for the trench widths w we investigate 1500 km a maximum trench retreat rate occurs for trenches 600 km wide which is attributed to the interaction between a plate of finite width and the induced flow which has a lengthscale in the horizontal direction these numerical results quantitatively agree with comparable 3 d laboratory experiments using analogue models with a purely viscous plate material schellart 2004a 2004b including correlations between increasing retreat rate with increasing plate thickness trench width for maximum retreat rate 500 km and estimated amount of slab buoyancy used to drive rollback induced flow 70 several implications for plate tectonics on earth result from these models such as rollback subduction providing a physical mechanism for ephemeral slab graveyards situated above the more viscous lower mantle and endothermic phase transition prior to a flushing event into the lower mantle mantle avalanche

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

geochemistry and petrogenesis of 270 ma ni cu pge sulfide bearing mafic intrusions in the huangshan district eastern xinjiang northwest china implications for the tectonic evolution of the central asian orogenic belt abstract the central asian orogenic belt caob is composed of remnants of microcontinents island arcs and ophiolites the presence of voluminous phanerozoic a type granites suggest that continental growth occurred through addition of juvenile crust and the presence of mafic ultramafic intrusions provides direct evidence for a mantle contribution the origin of the mafic ultramafic intrusions in the huangshan area eastern xinjiang has been debated because of the lack of precise age data or trace element and isotopic analyses we have studied the geology petrology geochemistry and geochronology of two of the intrusions the huangshanxi and huangshandong intrusions both are typical layered mafic ultramafic intrusions composed of diorite troctolite norite gabbronorite gabbro wehrlite websterite and lherzolite with systematic geochemical variations that are consistent with the following crystallization sequence ol chr ol pyx chr cpx pl and hbl bt igneous zircons separated from a diorite in the huangshanxi intrusion yield a 269 2 ma shrimp u pb age the rocks are enriched in rb ba u k sr relative to th and lree depleted in nb ta relative to th and exhibit positive end values 6 7 to 9 3 ni cu pge sulfide mineralization occurs in the lower parts of the intrusions primarily strata bound within wehrlite or lherzolite and locally stratiform along the margins of the intrusions the parental magmas appear to have been derived from a mantle source that was previously contaminated by subduction of oceanic crust and to have been variably contaminated by upper continental crustal components the intrusions appear to have formed by plume related activity within a continental setting likely related to post orogenic extension following carboniferous amalgamation of arc related terranes in the caob this plume activity was most likely also responsible for providing heat sources to generate large amounts of a type granites across the caob

time dependent subduction dynamics driven by the instability of stagnant slabs in the transition zone abstract seismic tomographic images indicate that subducted slabs are commonly deflected within the transition zone and that accumulated slab material may be subject to localized flux into the lower mantle these observations are consistent with numerical models of mantle flow in which slab flux through phase transition at 660 km depth is impeded by the combination of rheological contrast and thermodynamic effects we use a suite of 2d numerical experiments to explore the impact of slab avalanches on plate boundary dynamics the experiments are initiated at the onset of the avalanche and two distinct classes of model geometry are considered in the first the stagnant deflected slab within the transition zone is connected to subduction at the surface of the model domain in this case we find that the location of the avalanche relative to the trench governs whether the slab dip steepens or shallows in response to the instability at 660 km depth these experiments show a relatively modest impact on plate dynamics at the surface in the second class of models the stagnant slab material is not connected to the shallow subduction and the former is interpreted as a remnant of either a relic subduction event or an accumulation of material from a distant zone of convergence in this case the impact on plate boundary dynamics is more profound for example when the avalanche is positioned under or forward of the island arc it drives a significant retreat of the trench in contrast an avalanche that initiates under the back arc leads to a thrusting of the island arc over the retro plate and a reversal in the polarity of subduction we test the influence of various boundary conditions on the evolution of these models and consider several different rheological parameterizations for the lithosphere and mantle the models are intended as general templates for the impact of slab avalanches however we discuss the potential connection of these results to various zones of convergence along the western pacific margin

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

sedimentology geochemistry and origin of phosphatic chalks the upper cretaceous deposits of nw europe santonian lower campanian and lower maastrichtian phosphatic chalks in northern france southern england and belgium are europe s largest sedimentary phosphatc deposits the stratigraphy sediment ology petrography mineralogy and geochemistry of the lithofacies are reviewed and new data presented depositional and diagenetic models for phosphatic chalk deposits are developed using published experimental work and from observations of modern high and low productivity phosphogenic systems it is concluded that phosphatic chalks were deposited in well oxygenated current swept environments phosphatization required a delicate balance to be maintained between moderate organic carbon and carbonate sedimentation rates reduced bulk sediment accumulation rate and an enhanced rate of bioturbation precipitation of carbonate fluorapatite francolite accompanied the bacterially mediated decomposition of organic matter occurring within centimetres of the sediment seawater interface and taking place preferentially within microbial bodies and coatings in addition to the organically derived component pore water phosphate levels were enhanced by phosphate absorbed on ferric oxyhydroxides which was liberated during iron reduction mineralization was probably a dominantly post oxic process but occurred in a thick sediment mixed layer in which marine organic matter was undergoing intense mixed aerobic and anaerobic microbial degradation phosphogenesis occurred predominantly on the ne margins of the anglo paris basin where shallower sea floors and suitable palaeoceanographic conditions prevailed phosphogenic episodes were limited by sea level fluctuations which controlled the effectiveness of the erosional currents that formed and maintained the phosphatic basins and may have stimulated local productivity

quantifying the net slab pull force as a driving mechanism for plate tectonics 1 it has remained unclear how much of the negative buoyancy force of the slab fb is used to pull the trailing plate at the surface into the mantle here i present three dimensional laboratory experiments to quantify the net slab pull force fnsp with respect to fb during subduction results show that fnsp increases with increasing slab length and dip up to 8 12 of fb making fnsp up to twice as large as the ridge push force the remainder of fb is primarily used to drive rollback induced mantle flow 70 to bend the subducting plate at the trench 15 30 and to overcome shear resistance between slab and mantle 0 8

the influence of trench migration on slab penetration into the lower mantle abstract a two dimensional numerical convection model in cartesian geometry is used to study the influence of trench migration on the ability of subducted slabs to penetrate an endothermic phase boundary at 660 km depth the transient subduction history of an oceanic plate is modelled by imposing plate and trench motion at the surface the viscosity depends on temperature and depth a variety of styles of slab behaviour is found depending predominantly on the trench velocity when trench retreat is faster than 2 4 cm a the descending slab flattens above the phase boundary at slower rates it penetrates straight into the lower mantle although flattening in the transition zone may occur later leading to a complex slab morphology the slab can buckle independent of whether it penetrates or not especially when there is a localised increase in viscosity at the phase boundary flattened slabs are only temporarily arrested in the transition zone and sink ultimately into the lower mantle the results offer a framework for understanding the variety in slab geometry revealed by seismic tomography

an experimental study of subduction and slab migration results of fluid dynamical experiments on the behavior of subducted slabs are presented that address two important characteristics of subduction slab penetration through the transition zone and horizontal slab migration cold negatively buoyant molded slabs of concentrated sucrose solution with viscosities of 3 5 106 p were introduced into a more dilute two layered sucrose solution representing the upper and lower mantle the transition zone was modeled by a step increase in both density and viscosity ratios of slab thickness to upper layer depth the relative viscosities in each layer and the effective rayleigh number were close to mantle values the initial configuration consisted of two plates separated by a trench gap with one plate attached to a dipping slab simulating a developing subduction zone with an overriding plate two different boundary conditions were used at the trailing ridge end of the subducting plate 1 fixed end corresponding to zero ridge motion and 2 free end corresponding to zero ridge resistance based on results from 15 experiments with various combinations of upper layer lower layer and slab densities u l s and viscosities we find that slab penetration depth is sensitive to both the normalized slab density anomaly r s l s u and the dip angle three penetration regimes are recognized r 0 5 in which the slab sinks into the lower layer without distortion 0 2 r 0 5 which permits limited penetration the amount dependent on dip angle in this regime the slab develops a thick root just beneath the transition zone and r 0 2 in which the slab is deflected by the transition and little or no penetration occurs retrograde slab motion and trench migration occurred in nearly every case with the overriding plate moving parallel to but faster than the trench causing the gap between subducting and overriding plates to close with time prograde trench motion was never observed and the trench remained stationary only in the extreme case free ridge condition with equal upper and lower layer densities these results suggest that the transient extension found in many back arc basins may be a direct consequence of the tendency for retrograde subduction and the amount of extension may be governed in part by the degree of slab penetration through the transition zone

plate tectonic characteristics during the early paleozoic in beishan near the sino mongolian border region china abstract the tectonic units during the early paleozoic in the beishan area comprise the northern margin of the eastern end of the tarim plate and the northern flank of the mingshui hanshan microplate these two paleoplates were separated by an ophiolitic melange belt all of them underwent the tectonic evolution typical of the continental margins of atlantic andean and west pacific ocean types the continental margin was an open continental shelf and a continental slope from the early cambrian to the early ordovician at the southern flank of the continental shelf an isolated shallow sea basin developed separated from the shelf by an upwarping zone its northern flank shibanjing xiaohuangshan is an ocean basin north of this ocean basin is the mingshui hanshan microcontinent or microplate this area is situated in a tensional tectonic environment it shows the characteristics of a passive atlantic type margin during the middle and late parts of the early ordovician the continental crust on the southern fringe of the shallow sea basin was pulled open and a narrow ocean rift formed in the mid ordovician this area underwent a period of considerable development in which extension and subduction existed simultaneously the southern fringe of the continental margin came to the end of the passive margin on its northern side a marginal volcanic zone with calc alkali volcanism formed in the late ordovician silurian the oceanic crust was folded and subducted southward and the hengluanshan xichanjing marginal basin developed into an interarc basin where the hongliuhe niujuazi backarc basin and the baiyunshan dongqiyishan volcanic arc had formed with associated granitic magma at the beginning of the devonian the tarim plate finally came into collision with the mingshui hanshan microplate and the ocean basin closed forming the mingshui shibanjing xiaohuangshan suture belt there were volcanic molasse basins on both sides of this orogenic zone from the late paleozoic the beishan area was incorporated into an intraplate mobile zone

the hongliuhe fold and thrust belt evidence of terminal collision and suture reactivation after the early permian in the beishan orogenic collage northwest china abstract the early permian strata of the hongliuhe formation are located in the beishan orogenic collage a tectonically significant region between the tianshan orogen and the southern mongolian accretionary system of the central asian orogenic belt also called the altaids stratigraphic reconstruction of the formation shows a general fining upwards sequence of rhythmically interbedded sandstones and siltstones with thick basal successions of conglomerates all unconformably overlying an earlier mid paleozoic volcanic assemblage the clast lithotypes of conglomerates successively change from being polymictic metamorphic tectonites at the base to a nearly monomictic granitoids mid section illustrating the unroofing sequence of an arc related provenance the intra oceanic gongpoquan composite arc that comprises most of the central beishan fits the clast lithotype profile of the conglomerates and is the most probable provenance the hongliuhe formation stratigraphy experienced a thin skinned fold and thrust belt style of deformation that has a consistent southward vergence deformation in the lower stratigraphic levels created thrust imbrication of formation scale blocks each separated by steep north over south brittle ductile shear zones the blocks are internally deformed with large scale folding with one significant block containing a sheath fold 15 km in breadth the upper stratigraphic levels accommodated strain through low angle thrust ramping shear folding imbricate fanning and duplexing this indicates that the hongliuhe formation was syn orogenically deposited in a progressively deepening terrestrial to nearshore environment within the gongpoquan composite arc our observations and data indicate that the basin overlies an earlier paleozoic suture zone this implies that the northern half of the gongpoquan composite arc overrode the southern half in a fault system reactivation centered about this zone of weakness and created a stratigraphic and structural response consistent with a subduction or collision driven fold and thrust belt its southward structural vergence is supporting evidence of a north dipping subduction related fault system under the northern gongpoquan the timing of both the genesis of the hongliuhe formation and this deformation event coincides with the subduction of the liuyuan ocean or docking of the dunhuang block further south at the southern margin of the gongpoquan composite arc

growth of pillow lobes evidence from pillow lavas of hokkaido japan and north island new zealand pillow lavas of cretaceous and neogene age well exposed in hokkaido japan and those of neogene age in north island new zealand provide evidence of their mode of growth some pillow lobes diverge by formation of longitudinal surface spreading cracks the others grow by formation of transverse spreading cracks leaving one or more crusts ending toward the front of the lobes and constrictions therefore it is possible to determine the direction of growth of pillow lobes by tracing the crusts and or constrictions

evolution of tethyan phosphogenesis along the northern edges of the arabian african shield during the cretaceous eocene as deduced from temporal variations of ca and nd isotopes and rates of p accumulation abstract the evolution of tethyan phosphogenesis during the cretaceous eocene is examined to try to explain fluctuations of phosphogenesis through time and whether or not they reflect long term changes in ocean circulation or in continental weathering twenty seven time stratigraphic phosphate levels in various tethyan sites covering a time span of about 90 myr from the hauterivian to the eocene were analyzed for 44 ca 42 ca and 143 nd 144 nd in their carbonate fluorapatite cfa fraction p and ca accumulation rates and bulk sedimentation rates were quantified throughout the cretaceous eocene negev sequence to examine how changes in 44 ca 42 ca and 143 nd 144 nd are reflected in the intensity of phosphogenesis a clear cut change occurs in e nd t and 44 ca and in the rates of p and ca accumulation and bulk sedimentation through the time analyzed e nd t is much lower in the hauterivian lower cenomanian 12 8 to 10 9 than in the upper cenomanian eocene 7 8 to 5 9 much lower 44 ca values occur in the hauterivian turonian 0 22 to 0 02 than in the coniacian eocene 0 23 to 0 40 p accumulation rates in the negev steeply increase from 2 k yr 1 in the albian coniacian to 1500 mol cm 2 k yr 1 in the campanian whereas a strong decrease is concomitantly recorded in the rates of ca accumulation and bulk sedimentation in addition distinct e nd t values are shown by the phosphorites of the negev 6 7 to 6 4 and egypt 9 1 to 7 6 during the campanian and by those of the negev 7 8 to 6 3 and north africa 10 1 to 8 9 during the maastrichtian eocene the culmination of p accumulation rates in the negev during the campanian occurring with a high in e nd t and 44 ca and a low in sedimentation rates indicates that paleoceanographic and paleogeographical factors mostly governed phosphorite accumulation in this area the abrupt e nd t rise after the cenomanian is attributed to increased incursion of pacific radiogenic water masses into the tethys driven by the late cretaceous global sea level rise the connection between north and south atlantic the global post santonian cooling and the progressive widening of the caribbean threshold all acting in combination to significantly intensify the tethyan circumglobal current tcc it also reflects a weakening of the continental nd signal due to a reduction of exposed landmasses caused by increased flooding of continental shelves high 44 ca values at those times also point to a decrease in weathering ca 2 fluxes and expansion of carbonate sedimentation in shelves both enriching seawater with isotopically heavy ca 2 deep ocean circulation intensified by the post santonian cooling of high latitudes increased p inventory in the tethys basin whereas the strengthened tcc and the folded shelf likely resulted in coastal and topographically induced upwelling supplying p rich intermediate waters to southeastern tethys shelves only in the paleocene eocene following major changes in global circulation produced by narrowing of tethys and widening of the atlantic did phosphogenesis shifts its locus of high intensity to the western atlantic tethys and west african atlantic coasts this change in paleocirculation is expressed by distinctly differing e nd t in the middle east and the north and west african phosphorites suggesting different oceanic p sources and current systems for these two major groups of phosphorites our nd isotope results further suggest a weaker tcc during the mid cretaceous becoming more intense in late cretaceous times they also point to the north pacific ocean as major source of deep water formation for the intermediate deep waters in the tethys basin during the late cretaceous

on the relationships between slab dip back arc stress upper plate absolute motion and crustal nature in subduction zones 1 statistical analysis of modern oceanic subduction zone parameters such as the age of a downgoing plate or the absolute plate motions is performed in order to investigate which parameter controls the dip of a slab and conversely what the influence of slab geometry is on upper plate behavior for that purpose parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge plateau seamount subduction on the basis of tomographic images slabs that penetrate through or lie on the 670 km discontinuity are also identified the results of the statistical analysis are as follows 1 back arc stress correlates with slab dip i e back arc spreading is observed for deep dips deeper than 125 km larger than 50 whereas back arc shortening occurs only for deep dips less than 30 2 slab dip correlates with absolute motion of the overriding plate the correlation is even better when the slab lies on or even more penetrates through the 670 km discontinuity 3 slabs dip more steeply by about 20 on average beneath oceanic overriding plates than beneath continental ones 4 slabs dip more steeply on average by about 10 near edges 5 slab dip does not correlate with the magnitude of slab pull the age of subducting lithosphere at the trench the thermal regime of the subducting lithosphere the convergence rate or the subduction polarity east versus west the present study provides evidence that the upper plate absolute motion plays an important role on slab dip as well as on upper plate strain retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates one can infer that low slab dips correlate well with compression in continental advancing upper plates whereas steep dips are often associated with extension in oceanic retreating upper plates excess weight of old slabs is often counterbalanced by other forces probably asthenospheric in origin such as lateral mantle flow near slab edges or anchor forces to determine slab dip

geochronology and geochemistry of early permian mafic ultramafic complexes in the beishan area xinjiang nw china implications for late paleozoic tectonic evolution of the southern altaids abstract the hongshishan and pobei mafic ultramafic complexes outcrop in the beishan area in the southernmost altaids they consist of olivine gabbros dunites and pyroxenites zircons from olivine gabbros from the hongshishan complex yield a u pb age for emplacement of 281 8 2 6 ma the olivine gabbros have low sio 2 47 08 48 66 tio 2 0 14 0 29 mno 0 07 0 09 and k 2 o 0 06 0 09 but high mg values 81 t fe 2 o 3 4 93 6 29 the hongshishan and pobei mafic ultramafic complexes are characterized by negative anomalies in high field strength elements zr hf nb ta which we interpret as arc related signatures they also have spikes in large ion lithophile fluid mobile elements ba u pb sr and high mobile immobile element ratios i e primitive mantle normalized sr nd ba la 1 which are attributed to an aqueous fluid component in the source zircon hf isotopes indicate that the source magma was derived either from a depleted mantle with some crustal contamination or from an enriched mantle these data are interpreted to indicate that the hongshishan and pobei mafic ultramafic complexes were emplaced in the early permian subduction related environment which suggests that the paleo asian ocean in the southernmost part of altaids did not close until after early permian

sm nd in marine carbonates and phosphates implications for nd isotopes in seawater and crustal ages this study explores the possibility of establishing nd isotopic variations in seawater over geologic time calcite aragonite and apatite are examined as possible phases recording seawater values of nd modern biogenic and inorganically precipitated calcite and aragonite from marine environments were found to have nd concentrations of from 0 2 to 70 ppb showing that primary marine caco 3 contains little ree and that nd ca is not greatly enhanced relative to seawater during carbonate precipitation very young marine limestone and dolomite containing no continental detritus have 200 ppb nd all the carbonates are lree enriched 0 16 sm nd 0 45 modern and very young atlantic and pacific carbonates have nd in the range of shallow atlantic and pacific seawater respectively implying that they derive their ree from local seawater the nd in well preserved carbonate fossils is 4 10 4 ppb much greater than in their modern counterparts but like the high values found for carbonates in other studies we believe the high ree contents at the 500 ppb level in some detritusfree carbonates are due to ree rich fe hydroxide in on the carbonate in favorable cases such material may record seawater nd values however introduction of extraneous ree may obscure the original isotopic composition of pure caco 3 because of its very low intrinsic primary ree abundance modern biogenic apatite is also shown to have very low ree content 150 ppb nd but appears to quickly scavenge ree from seawater inorganically precipitated apatite from phosphorites has high concentrations of seawater derived ree young phosphorite apatite from the atlantic and pacific oceans has nd in the range of the seawater from these oceans older apatite samples of similar age from different localities bordering common oceans record similar values of nd t sedimentary apatite has sr t values in good agreement with the curves for 87 sr 86 sr of seawater as a function of time individual conodonts from a single formation yield the same sr t and nd t other workers have shown that sedimentary apatite preserves seawater ree patterns these characteristics suggest that sedimentary apatite can be used to determine nd t in ancient seawater the seawater values so inferred range between 1 7 and 8 9 over the last 700 my and lie in the range of modern seawater showing no evidence for drastic changes high values of seawater nd t in the triassic and latest precambrian may correlate with the breakup of large continental landmasses the initial nd t 15 0 of a 2 ae old phosphorite implies the presence of 1 5 ae old continental crust at 2 ae ago the approach outlined here can be used to constrain the age of the exposed crust as a function of time

dynamic processes controlling evolution of rifted basins abstract the extension of the lithosphere controlling the development of rifted basins is driven by a combination of plate boundary forces frictional forces exerted on the base of the lithosphere by the convecting asthenosphere and deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system although mantle plumes are not a primary driving force of rifting they play an important secondary role by weakening the lithosphere and by controlling the level of rift related volcanic activity a distinction between active and passive rifting is only conditionally justified the extension of the lithosphere depending on its rate and magnitude and the potential temperature of the asthenosphere can cause by adiabatic decompression partial melting of the lower lithosphere and upper asthenosphere in rift systems the level and timing of volcanic activity is highly variable the lack of volcanic activity implies passive rifting an initial passive rifting stage can be followed by a more active one during which magmatism plays an increasingly important role magmatic destabilization of the moho may account for the frequently observed discrepancy between upper and lower crustal extension factors combined with evidence for thermal thinning of the mantle lithosphere this suggest that the volume of the lithosphere is not necessarily preserved during rifting as advocated by conventional stretching models the structural style of rifts is controlled by the rheological structure of the lithosphere the availability of crustal discontinuities that can be tensionally reactivated the mode orthogonal or oblique and amount of extension and the lithological composition of pre and syn rift sediments simple shear extension prevails in rifts that subparallel the structural grain of the basement pure shear extension is typical for rifts cross cutting the basement grain pre existing crustal and mantle lithospheric discontinuities contribute to the localization of rift systems the duration of the rifting stage of extensional basins is highly variable stress field changes can cause abrupt termination of rifting in major rift systems progressive strain concentration on the zone of future crustal separation entails abandonment of lateral rifts depending on constraints on lateral block movements crustal separation can be achieved after as little as 9 my and as much as 280 my of rifting activity syn rift basin subsidence is controlled by isostatic adjustment of the crust to mechanical stretching of the lithosphere its magmatic inflation and thermal attenuation of the mantle lithosphere post rift basin subsidence is governed by thermal reequilibration of the lithosphere asthenosphere system deep seated thermal anomalies related to syn rift pull up of the asthenosphere lithosphere boundary have decayed after 60 my by about 65 and after 180 my by about 95 the magnitude of post rift subsidence is a function of the rift induced thermal anomaly and crustal density changes the potential temperature of the asthenosphere and initial water depths intraplate stresses can have an overprinting effect on post rift subsidence stretching factors derived from post rift subsidence analyses must be corrected for such effects

geochemical mineralogical and isotopic signatures of the semikan west kasrik turkish phosphorites from the derik mazidagi mardin area se anatolia the geochemistry and mineralogy of a condensed section 6 m thick of the west kasrik member coniacian santonian near the uplifted northern flank of the mardin derik anticline south eastern turkey was studied the only deposit exploited in turkey is found in this area the sediment textures as well as the mineralogical and the geochemical results collectively suggest that these recycled phosphorites accumulated in areas of intensive very early diagenesis of the sediments in highly oxic bottom waters and almost no detrital apport the total concentration of redox sensitive trace metals is very low 600 g g in addition the structural co2 and f p2o5 are unusually high 5 wt and 0 14 respectively ree distribution shows a clear seawater pattern with a strongly negative ce anomaly 0 20 0 02 and heavy ree enrichment lun lan 1 50 0 12 however their total concentration is very low the low ree contents quite unexpected in recycled phosphorites are explained by the scarcity of terrigenous components which leads to minimal incorporation of rees from detrital clastic phases in the cfa fraction weathering previously suggested as responsible for the high p enrichment of the mazidagi phosphorites is rather improbable in the samples we studied in view of their high structural co2 high f p2o5 high la nd n and la sm n ratios and high sr p and ca p of separated cfa fractions that all negate post depositional weathering more likely the high p enrichment of some rocks p2o5 content reaches 34 was produced by sedimentary and early diagenetic processes acting in oxygenated areas of starved sedimentation on and around tectonic highs

distribution of rare earth and some trace elements in tertiary phosphorites from the senegal basin and their weathering products abstract the distribution of rare earth and some trace elements is studied in the eocene phosphate deposits of farim saliquinhe lam lam and n diendouri ouali diala from the mauritania senegal guinea basin the depth of weathering varies between the three deposits the strong weathering of lam lam is marked by the leaching of apatites ca phosphate and the appearance of epigenetic crandallites al ca phosphate in the void spaces that have resulted from leaching of apatites and then of crandallites ferruginous and kaolinitic fillings are epigenised in millisites al ca phosphate all the studied phosphates have high ree ba and sr abundances and present shale normalized patterns with a negative ce anomaly of variable intensity clay minerals of sediments smectites and of weathered rocks kaolinites are characterized by low ree and high v zn cu cr sc and zr abundances they have no ce anomaly the comparison of the ree patterns and of the trace element abundances lead to three important conclusions at first the genesis of al ca phosphates is marked by specific leaching of heavy ree relative to the light ones then millisites have a ree abundance and a ce anomaly less pronounced than those of apatites and crandallites they show also v zn cu cr sc and zr abundances like clay minerals these facts can be related to their epigenetic formation from clay minerals last but not least the negative ce anomaly of epigenetic crandallites follows the same trend as in apatites therefore this anomaly cannot constitute a proof of the primary character for apatites if such variations should be related to the paleogeographic setting of studied deposits they must be considered with prudence

diagenetic trends of fluorine concentration in negev phosphorites israel implications for carbonate fluorapatite composition during phosphogenesis an electron probe and chemical study of bulk phosphorite samples and separated constituents from various negev deposits was carried out together with xrd ftir spectroscopy and textural analysis the results allow a better understanding of the distribution of fluorine in these upper cretaceous phosphorite sequences and shed light on variations in the composition of the carbonate fluorapatite cfa phase during phosphogenesis two facies are recognized 1 a pristine microbially generated phosphorite facies 2 a recycled peloidal and biodetrital facies fluorine distribution in the negev phosphorites is facies controlled f p2o5 is much lower in the pristine facies 0 090 0 107 than in the recycled facies 0 107 0 120 in addition f p2o5 varies considerably between the various constituents of the phosphate fraction f poor francolites f p2o5 as low as 0 080 co exist with f rich francolites f p2o5 as high as 0 135 in the same phosphorite bulk sample a lower f p2o5 in francolite is associated with higher cd and zn concentrations in the phosphorite an increase in fe rich smectites in the clay fraction and the presence of structural oh in the francolite the lower f p2o5 ratios in the pristine facies are attributed to high organic deposition rates during the formation of these matted sediments leading to rapid burial of the in situ forming cfa this is possibly coupled with diffusion of f from sea water into bottom sediments being hampered by microbial mat coatings these conditions resulted in o2 depleted porefluids inducing the precipitation of cd rich zn sulphides and the formation of fe rich smectites f enrichment probably takes place when the earlier formed f poor primary cfa is relocated close to the sea floor and bathed with interstitial sea water solutions of higher f concentrations oxidation and removal of the sulphide bound cd and zn apparently occurred together with enrichment in f of the francolite combining chemical data with xrd and ftir results suggests a multistage growth for the negev phosphate constituents in shifting formational sites and porefluids of varying f concentrations this multiphase growth is reflected in the patchy distribution of f in the negev constituents and might explain the inverse correlation between mean co2 f and f p2o5 ratios of the analysed phosphorites in the two facies it also suggests that cfa or an amorphous precursor initially formed with some oh groups in the apatite structure which were subsequently substituted by f ions in recycled francolite through re equilibration with porefluids of higher f concentrations

geochemistry of cadmium in the negev phosphorites cadmium cd in the negev phosphorites as in many other phosphorites is the most enriched trace element relative to average shale nevertheless similar to many other phosphorites cd is either not correlated to p2o5 or has a slightly negative correlation with it there is a clear correlation between the cd content the textural and the compositional characteristics of the phosphorites and the structural position of the phosphate deposits well developed synclinal sections comprised of many thin strata of laminated non disturbed pelletal phosphorites have much higher cd concentrations than the condensed anticlinal sections which consist of a few amalgamated thick beds of massive reworked phosphorites cd is in general poorly related to the present organic matter content but has a good correlation with organic matter in unoxidized beds of the synclinal sections cd is strongly correlated to zn in the relatively undisturbed beds of the synclinal sections but shows no correlation in the highly reworked beds of anticlinal sections the source of cd is upwelled seawater that is enriched in both p and cd this is the basis of the relationship between phosphogenesis and cd concentration reducing conditions during decomposition of organic matter immobilize cd in the pore fluids of the bottom sediments precipitation of cd rich sphalerite and adsorption on organic matter appear to be the main processes for its initial speciation and concentration in the negev phosphorites cd is concentrated mainly in laminated non disturbed microbially generated phosphorites where authigenic apatites are also found cd does not as a rule replace ca in the structure of authigenic primary sedimentary apatites this finding has practical applications since it makes possible the production of a phosphate concentrate with a much lower cd concentration than the original phosphorite exposure to oxic conditions leads to mobilization of the sulfide bound cd scouring activity bioturbation and recycling and transportation of stirred up phosphate corpuscles in an oxidizing environment appear to be effectual for removing cd oxidation alone without fabric disruption and mobilization of the phosphate grains does not significantly lower cd concentrations since the released cd may be almost immediately readsorbed on the various components of the phosphorite such as iron oxyhydroxides organo clay fractions and apatites the cadmium concentration and the cdzn ratio of a phosphorite may help to determine whether a phosphorite is pristine or not and to unravel its diagenetic history

kinematics and flow patterns in deep mantle and upper mantle subduction models influence of the mantle depth and slab to mantle viscosity ratio 1 three dimensional fluid dynamic laboratory simulations are presented that investigate the subduction process in two mantle models an upper mantle model and a deep mantle model and for various subducting plate mantle viscosity ratios sp m 59 1375 the models investigate the mantle flow field geometrical evolution of the slab sinking kinematics and relative contributions of subducting plate motion and trench migration to the total rate of subduction all models show that the subducting plate is always moving trenchward resulting from slab pull furthermore all deep mantle models show trench retreat as do upper mantle models in the initial stage of subduction before slab tip transition zone interaction upper mantle models with a low sp m 66 217 continue to show trench retreat after interaction upper mantle models with a high sp m 378 709 show a period of trench advance after interaction followed by trench retreat upper mantle models with a very high sp m 1375 show continued trench advance after interaction the difference in trench migration behavior and associated slab geometries is attributed to both sp m and the mantle depth to plate thickness ratio tm tsp which both affect the slab bending radius to mantle thickness ratio rb tm four subduction regimes can be defined regime i with rb tm 0 3 trench retreat slab draping and a concave trench regime ii with 0 3 rb tm 0 5 episodic trench migration slab folding and a concave trench regime iii with rb tm 0 5 trench advance slab rollover geometries and minor trench curvature and regime iv with rb tm 0 8 trench retreat slab draping and a rectilinear trench in all models slab parallel downdip motion induces poloidal mantle flow structures in addition trench retreat and rollback motion of the slab induce quasi toroidal return flow around the lateral slab edges toward the mantle wedge rollback induced poloidal flow around the slab tip is not observed in any of the experiments finally comparison between the slab geometries observed in the upper mantle models and slab geometries observed in nature imply that the effective viscosity ratio between slab and ambient upper mantle in nature is less than 103 and of the order 1 7 102 with a best estimate of 1 3 102

palaeozoic late collisional strike slip deformations in tianshan and altay eastern xinjiang nw china in central asia thrusts and shear zones resulting from palaeozoic accretional events were reworked by e w trending ductile strike slip faults during late palaeozoic early mesozoic time in the tianshan range microstructures and quartz c axis fabrics show a main dextral shearing associated with sinistral localized shear zones the relationship between these conjugate structures indicates a nnw sse trending bulk shortening in the chinese altay mountains the existence of type microstructures in an important sinistral mylonitic zone infers high rates of deformation this shear zone is bordered by a late dextral ductile fault synchronous with a granite emplacement field evidence and datings from the literature provide chronological constraints in the late carboniferous the sinistral mylonitic deformation took place in the erqishi irtysh shear zone in the northeastern part of xinjiang and in kazakhstan during the early permian a regional dextral event occurred in the tianshan range and under the whole of northern xinjiang

vertical and horizontal strain partitioning of the central tianshan nw china evidence from structures and 40ar 39ar geochronology abstract as a major tectonic collage within the central asia orogen belt caob the chinese tianshan provides a natural laboratory for studying structures resulting from the amalgamation of different tectonic units detailed field and micro tectonic observations along two orogen perpendicular transects identify several structural domains with greatly variable structural styles that were generated at different structural levels but are now juxtaposed geological features combined with 40 ar 39 ar geochronologic data suggest the variable structural styles of the domains developed simultaneously during ca 320 300 ma kinematic reconstruction and deformation temperature estimation taking 40 ar 39 ar thermochronological data into account suggest both horizontal and vertical extrusion of material of the central tianshan magmatic arc assemblage ctma crust which is the expression of heterogeneous strain partitioning during orthogonal continental convergence such a kinematic scenario is quite compatible with and satisfactorily interpreted in the light of the results of available analogue deformation experiments and numerical modeling the structure styles with orthogonal symmetry of both the ctma and the southern tianshan fold and thrust belt revealed in this study in conjunction with previously published paleomagnetic data from the tarim plate suggest that the diachronous collision of the illi central tianshan micro continent with the tarim plate may have caused the tarim block rotation