late paleozoic closure of the ob zaisan ocean along the irtysh shear zone nw china implications for arc amalgamation and oroclinal bending in the central asian orogenic belt the largest accretionary orogen in the world the central asian orogenic belt has evolved through the assembly of various oceanic and continental blocks understanding the processes associated with the development of this orogenic belt relies on precise recognition of the boundaries between various terranes one such major suture zone which records the collisional history of the siberian marginal arc system chinese altai with intra oceanic arc systems east west junggar is the irtysh shear zone in nw china the spatial continuity and the tectonic nature of this shear zone are still poorly understood but its development has supposedly made a significant impact on the architecture of the western central asian orogenic belt and the formation of the kazakhstan orocline here we provide new insight into the evolution of this shear zone based on detrital zircon ages hf isotope composition structural data and ar ar age constraints on the timing of deformation our results show a major discrepancy in detrital zircon populations and hf model ages across the southern chinese altai and the northern east west junggar thus allowing us to map the exact location of the tectonic boundary detrital zircon data constrain the initial closure of the ob zaisan ocean to the late carboniferous 323 ma and new structural and ar ar geochronological data shed light on the subsequent collisional processes we propose that the collisional zone between the chinese altai and the east west junggar was initially subjected to crustal thickening at ca 323 295 ma followed by orogen parallel extension ca 295 ma and transpressional deformation ca 286 253 ma the closure of the ob zaisan ocean in nw china postdates the initial phase of oroclinal bending in the western central asian orogenic belt thus indicating that oroclinal bending initiated during subduction based on our new constraints and other available geological data we suggest that the early stage of oroclinal bending was likely driven by slab rollback

depleted mantle source for the ulungur river a type granites from north xinjiang china geochemistry and nd sr isotopic evidence and implications for phanerozoic crustal growth abstract the ulungur river granites from north xinjiang china display two petrological trends 1 a peralkaline trend composed of alkali amphibole and alkali pyroxene bearing granites and dykes and 2 an aluminous trend represented by alkali granites monzogranites and aluminous granite dykes within the peralkaline granites rb sr whole rock isochron dating yielded an age of 300 ma for the emplacement of the peralkaline granites and 270 ma for the aluminous granites the ulungur river granites share all the geochemical features common to a type granites of the world the ulungur river granites are post orogenic and emplaced in a post collisional extensional environment the peralkaline and aluminous rocks have indistinguishable nd isotopic compositions they have unusually positive and relatively uniform nd t values ranging from 5 1 to 6 7 for the peralkaline rocks at 300 ma and from 5 5 to 5 9 for the aluminous rocks at 270 ma the initial 87sr 86sr ratios cannot be determined precisely by the whole rock isochron technique mainly because most samples are highly radiogenic and have high rb sr ratios in addition the large variation in sr isotopic compositions can partly be explained by crustal contamination during magma differentiation the nd isotopic data clearly indicate that both the peralkaline and aluminous rocks were derived from a long lived depleted mantle reservoir the formation of the ulungur river a type granites is hypothesized as follows 1 a depleted mantle was first metasomatised then subjected to partial melting leading to the generation of basic magmas now represented by the kalatongke mafic ultramafic complex in the ulungur river area 2 the basic magmas have undergone high degrees of fractional crystallisation with separation of biotite hornblende plagioclase potassium feldspar and other accessory phases such as apatite and ilmenite with the residual magmas forming the peralkaline granites during the magmatic differentiation the liquids were probably contaminated by crustal materials as evidenced by nb depletion and nd sr isotopic systematics on the other hand the formation of aluminous granites involved a replenishment of new mantle derived magmas into the differentiating magma chamber and followed by fractional crystallisation with retention of hydrous phases hornblende and biotite the petrological evolution of the ulungur river granites is thus from peralkaline to calc alkaline and not from calc alkaline to peralkaline as more often observed elsewhere the ulungur river granites in common with the voluminous granitoids occurring in the central asian orogenic belt represent a significant juvenile addition of the continental crust in the phanerozoic

sedimentary record of mesozoic deformation and inception of the turpan hami basin northwest china the turpan hami basin is a major physiographic and geologic feature of northwest china yet considerable uncertainty exists as to the timing of its inception its late paleozoic and mesozoic tectonic history and the relationship of its petroleum systems to those of the nearby junggar basin to address these issues we examined the late paleozoic and mesozoic sedimentary record in the turpan hami basin through a series of outcrop and subsurface studies mesozoic sedimentary facies regional unconformities sediment dispersal patterns and sediment compositions within the turpan hami and southern junggar basins suggest that these basins were initially separated between early triassic and early jurassic time prior to separation upper permian profundal lacustrine and fan delta facies and triassic coarse grained braided fluvial alluvial facies were deposited across a contiguous junggar turpan hami basin permian through triassic facies were derived mainly from the tian shan to the south as indicated by northward directed paleocurrent directions this is consistent with the sedimentary provenance of triassic sandstone mean qm29f29lt42 qp23lvm49lsm28 and qm51p25k24 and conglomerate 32 granitic clasts in the northern turpan hami basin we interpret a relative increase in quartz and feldspar concentration and a relative decrease in volcanic lithic grains in the northern turpan hami basin to reflect unroofing in the tian shan and exposure of late paleozoic granitoid rocks in addition two basinwide unconformities of late permian early triassic and early triassic middle late triassic age attest to deformation within the turpan hami basin and associated continued uplift and erosion of the tian shan

the jeffers brook diorite granodiorite pluton style of emplacement and role of volatiles at various crustal levels in avalonian appinites canadian appalachians small appinite plutons ca 610 ma outcrop in the peri gondwanan avalon terrane of northern nova scotia with different structural levels exposed field mapping shows that the jeffers brook pluton is a laccolith emplaced along an upper crustal thrust zone likely in a dilational jog in a regional dextral strike slip system the oldest rocks are probably mafic sills which heated the area facilitating emplacement of intermediate magmas cross cutting relationships show that both mafic and intermediate magmas were supplied throughout the history of pluton emplacement the modal composition mineral chemistry and bulk chemistry of gabbro diorite tonalite granodiorite and granite have been studied in the main plutonic phases dykes and sills and mafic microgranular enclaves as with the type appinites in the scottish caledonides the pluton shows evidence of high water content the dominance of hornblende locally within pegmatitic texture vesicles and irregular felsic patches in enclaves and late aplite dykes analyzed mafic microgranular enclaves are geochemically similar to larger diorite bodies in the pluton tonalite granodiorite is distinct from the diorite in trace element geochemistry and radiogenic isotopes elsewhere to the east similar rocks of the same age form vertically sheeted complexes in major shear zones hornblende chemistry shows that they were emplaced at a deeper upper crustal level this implies that little of the observed geochemical variability in the jeffers brook pluton was developed within the pluton the general requirements to form appinites are proposed to be small magma volumes of subduction related magmas that reach the upper crust because of continual heating by mafic magmas moving through strike slip fault pathways and trapping of aqueous fluids rather than venting through volcanic activity

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

a new triassic shortening extrusion tectonic model for central eastern asia structural geochronological and paleomagnetic investigations in the xilamulun fault north china abstract at the northern margin of the north china block ncb the xilamulun fault xmf is a key belt to decipher the tectonic evolution of central eastern asia as it records the paleozoic final closure of the paleo asian ocean and localizes a late triassic intracontinental deformation in this study structural analysis 40 ar 39 ar dating and paleomagnetic studies were performed to investigate the kinematics of the xmf and to further discuss its triassic geodynamic significance in the central eastern asia framework after the paleozoic central asian orogenic evolution the structural analyses reveal two phases of ductile deformation the first one d1 which displays n verging and e w trending folds is related to the early paleozoic collisional event between the ncb and the songliao hunshandake block shb the second phase d2 displays a high angle foliation and a pervasive sub horizontal e w stretching lineation with kinematic criteria indicative of dextral strike slip shearing the 40 ar 39 ar dating on mylonitic granite places the main shearing event around 227 209 ma this d2 shearing is coeval with that of the dextral strike slip bayan obo chifeng fault bcf and the chicheng fengning longhua fault to the south which together constitute a dextral shearing fault system on the northern margin of the ncb during the late triassic the paleomagnetic study performed on the middle permian guangxingyuan pluton located between the xmf and bcf documents a local clockwise rotation of this pluton with respect to the ncb and shb our multidisciplinary study suggests an nnw sse shortening and strike slip shearing dominated tectonic setting on the northern margin of the ncb during the late triassic combining the contemporaneous dextral strike slip movements of the xmf and bcf in northern china and the sinistral strike slip movement of east gobi fault egf in southeastern mongolia with the large scale tectonic framework a late triassic nnw sse shortening eastward extrusion tectonic model for central eastern asia is firstly proposed the nnw sse shortening results in the eastward extrusion of the continental wedge bounded by the bcf and egf which is accommodated by the different kinematic patterns of the southern xmf and bcf and northwestern egf bounding faults this shortening extrusion tectonic framework is tentatively interpreted as the result of the far field forces associated with three late triassic lithosphere scale convergences in east asia i northward intracontinental subduction between the ncb and south china block ii collision of the qiangtang block with the qaidam block and iii southward subduction of the mongol okhotsk ocean beneath the mongolia block

seismic and aseismic weakening effects in transtension field and microstructural observations on the mechanics and architecture of a large fault zone in se tibet

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

laser probe 40ar 39ar studies of the peace river shocked l6 chondrite abstract samples of matrix and melt glass from the shocked l6 chondrite peace river have been subjected to 40ar 39ar analysis using both stepped heating and localised outgassing by a laser probe stepped heating studies of three distinct samples of matrix gave indistinguishable and unusually well defined low temperature 40ar 39ar plateaux corresponding to an age of 450 30 ma in an attempt to investigate whether or not this plateau age is meaningful i e the time of a major collision event involving the parent body or is an artefact resulting from partial degassing during the fragmentation of the parent body which commenced the exposure of peace river to cosmic rays 40 ma ago we have used a laser probe to analyse ar from single 80 m diameter laser pits which release gas from a few micrograms of sample on this scale no areas have ages younger than the plateau age supporting the view that it is not an artefact measurements on the melt glass gave variable and meaningless ages in some cases greater than the age of the solar system due to retention of 40ar and loss of k during melting followed by rapid quenching this observation indicates the danger of attempts to date impact melt glass directly the observation that argon has been retained by the glass and lost by the matrix can be understood in terms of a range of plausible annealing histories

isotope systematics and shock wave metamorphism iii k ar in experimentally and naturally shocked rocks the haughton impact structure canada this is a report on the first comparative study of the effects of experimental and natural shock on the k ar system first mineral separates from a 451 plus minus 1 ma old gneiss experimentally shocked up to 60 gpa were investigated by stepwise heating sup 40 ar sup 39 ar technique compared with an unshocked samples the shocked samples have lower sup 40 ar sup 39 ar ages and the diffusion properties have changed depending on peak shock pressure and mineralogic composition total resetting of the k ar clock did not occur secondly to compare these results with the effects of natural shock samples from the approximately 23 ma old haughton impact crater devon island canada with different shock stages were investigated up to shock stage 3 no total resetting of the k ar clock was observed but severe argon losses depending on shock pressure result in sup 40 ar sup 39 ar plateau ages which are incorrect in terms of the true crater age yet they provide an upper limit to the time of the cratering event

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

lateral extrusion in the eastern alps part 1 boundary conditions and experiments scaled for gravity lateral extrusion encompasses extensional collapse gravitational spreading away from a topographic high in an orogenic belt and tectonic escape plane strain horizontal motion of wedges driven by forces applied to their boundaries in the eastern alps it resulted from 1 an overall northerly compression apulia against eurasia 2 a strong foreland bohemian massif 3 lack of constraint along a lateral boundary carpathian region and 4 a previously thickened gravitationally unstable thermally weakened crust eastern alpine orogenic belt six indentation experiments reproduce lateral extrusion at lithospheric scale the models have two to four lithospheric layers with a mohr coulomb rheology for the upper and a viscous rheology for the lower crust the lithosphere rests upon a low viscosity asthenosphere a broad indenter a narrow deformable area and a weakly constrained eastern margin fullfill as closely as possible conditions in the eastern alps indentation produces both thickening in front of the indenter and escape of triangular wedges lateral variations in crustal thickness become attenuated by gravitational spreading the overall fault pattern includes domains of reverse strike slip oblique normal and pure normal faults strike slip faults in conjugate sets develop serially the narrow width of the deformable area and the strength of the foreland determine the angles between the sets gravitational spreading produces a rhombohedral pattern of oblique and pure normal faults along the unconstrained margin opposite the unconstrained margin the indenter front shows thrusts and folds intersecting with the conjugate strike slip sets a triangular indenter favors spreading high velocity of indentation favors escape high confinement limits lateral motion inhibits spreading and favors thickening lateral extrusion in the eastern alps is best modeled by 1 a weak lateral confinement 2 a broad and straight indenter 3 a narrow width of the deformable area and 4 a rigid foreland crustal thickening lateral escape and gravitational spreading all contribute to the overall deformation

evolution of the turfan basin chinese central asia the turfan basin extends approximately 550 km east west by 100 km north south within northwest china and is located between branches of the intracontinental tien shan orogenic belt at present it is an intermontane foreland basin loaded by southward transported thrust sheets at its northern margin this deformation is a long distance manifestation of the india asia collision but the orientation and location of cenozoic structures is strongly influenced by the pre cenozoic history of the region the exposed basement marginal to the turfan basin consists of volcanics granitoids volcaniclastics and deepwater sedimentary rocks associated with the north tien shan island arc and subduction accretion complexes north of the north tien shan fault this arc had accreted to the central tien shan by late carboniferous to early permian times the north tien shan fault represents a major boundary between paleozoic arcs and accretionary complexes to the north and proterozoic basement of the central tien shan and tarim block to the south postcollisional thrusting appears to have provided a tectonic load for lower permian subsidence and sedimentation early postcollisional strata are turbidites but there was a shoaling of facies until the deposition of permo triassic conglomerates no marine sediments have been deposited in the region since the paleozoic at least two mesozoic episodes of compressional deformation are recorded at the northern margin of the basin which was an intermontane foreland basin during this era lower jurassic fluvial and lacustrine clastic rocks lie above a regional unconformity an oligocene unconformity marks the onset of coarse clastic sedimentation in the cenozoic and probably indicates the start of major compressional deformation related to the india asia collision sedimentation rates have increased through the cenozoic the neotectonics of the turfan basin are dominated by a southward directed thrust system emergent within the basin at the fire mountains range southward directed thrusting appears to have been a feature of mesozoic age tectonics and possibly the late paleozoic tectonics of the basin the most important neotectonic structures in the turfan basin follow older structural trends

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

mesozoic reactivation of the beishan southern central asian orogenic belt insights from low temperature thermochronology abstract the beishan orogenic collage boc is located in the southeast of the central asian orogenic belt caob and formed during final consumption of the palaeoasian ocean in the late palaeozoic this study applies low temperature thermochronology to constrain the meso cenozoic history of the boc apatite fission track and u th sm he data obtained for granitoid samples along a north south transect through the boc indicate three distinct phases of exhumation during 1 the late triassic early jurassic 225 180 ma 2 early cretaceous 130 95 ma and 3 late cretaceous early palaeogene 75 60 ma samples from the northern boc reveal a more profound early cretaceous signal and a weaker late triassic early jurassic signal than those in the southern boc a possible explanation for this discrepancy is the presence of crustal scale fault zones in the northern boc that are interpreted to have undergone repeated reactivation throughout the mesozoic exposing deeper exhumed sections of the boc these faults may thus have acted as a major control on exhumation in the region this pattern is consistent with results from elsewhere in the caob such as in the tianshan and the altai where regional widespread exhumation occurred since the early cretaceous while major fault zones record localised exhumation during the late cretaceous and cenozoic late cretaceous early palaeogene cooling ages were found only in the south of the boc suggesting that exhumation at that time was more localised and did not reach the northern margins of the study area the meso cenozoic cooling events described here are thought to be related to the progressive closure of the palaeo tethys ocean to the south associated collision and accretion of micro continental blocks and island arcs at the southern eurasian margin are interpreted to have induced more widespread reactivation and exhumation in central asia than previously anticipated extending to the northern margin of the tarim craton this observation hence refines the existing tectonic history models for central asia

39ar40ar age and petrology of chico large scale impact melting on the l chondrite parent body our studies of the 105 kg chico l chondrite show that it contains 60 impact melt and the largest volume of impact melt recognized in stony meteorites we suggest that it is part of a much larger dike complex that formed when chondritic impact melt was intruded into host chondrite during a large if not catastrophic impact on the l chondrite parent body at about 0 5 ga petrologic and 39ar 40ar dating studies were made on several lithologies including the massive melt zone host chondrite and melt chondrite boundaries for the purpose of studying the melting and thermal histories associated with impacts on small bodies and their effects on the k ar chronometer the chondritic host is shocked to stage s6 and contains pockets and veins of melt there are no unmelted clasts in the interior of the melt coalesced metal troilite nodules reach up to 2 cm in size melt near the contact with the host chondrite contains numerous clasts and quenched more rapidly metal troilite textures suggest cooling rates of 0 1 c s in the interior of the melt dike during crystallization secondary kamacite rims indicate cooling at 0 01 1 c y over the range of 700 500 c consistent with an impact heated volume of up to a kilometer in thickness compositions of olivines and pyroxenes are generally similar in melt and chondritic host reflecting rapid crystallization not metamorphic equilibration the interior melt shows an overall depletion in k whereas the melt near the boundary is enriched in k the 39ar 40ar release spectra during stepwise heating of both melt and chondrite samples can be divided into two parts based on ar diffusion properties and k ca ratios the low temperature high k ca phase of both melt and host chondrite show ages of 0 54 0 78 ga ages of the high temperature low k ca phase of the melt are comparable or higher 0 61 1 35 ga whereas those of the host chondrite are even higher 0 87 1 86 ga due to lesser degrees of degassing isochron plots for several melt samples suggest an age of 0 53 ga and the presence of variable amounts of excess 40ar not completely degassed by the impact even this age however is significantly higher than the previously reported rb sr isochron age of 0 467 015 ga the apparent retention of radiogenic 40ar in the chico impact melt in spite of its relatively large size absence of clasts and moderately slow cooling rate below 700 c raises questions as to the reliability of using melts for 39ar 40ar dating of meteoritic impact events

continental growth of northwest china the northwest corner of china is the most interior part of central asia and includes parts of the siberian tarim and kazakhstan plates where the junggar and tarim basins are separated by the east west trending tianshan range a preliminary terrane map divides the area into thirteen separate areas which are classified into four types l continental 2 oceanic crust 3 island arc 4 composite the boundaries for these terranes are faults that have been previously defined either by geologic mapping or holocene activity passive consolidation of the southern margin of paleo asia is marked by accretion and subduction of the paleo tethys oceanic basins and by development of volcanic arcs paleomagnetic data indicate that the major plates and associated smaller terranes did not reach final consolidation until the permian or later the stratigraphic record reveals development of intracontinental basins in the late carboniferous followed by the pan asian thermal event that gave rise to widespread intrusion of a type granites of late paleozoic age the frontal collision of india along the southern border of asia in the paleocene reactivated nearly all of the older major strike slip and thrust faults formed during the late paleozoic consolidation of paleo asia the complex nature of the continental growth of central asia has produced and destroyed many varieties of sutures and reconstruction of the original plate and terrane configurations therefore is enigmatic

palaeozoic collisional tectonics and magmatism of the chinese tien shan central asia abstract the chinese tien shan range is a palaeozoic orogenic belt which contains two collision zones the older southern collision accreted a north facing passive continental margin on the north side of the tarim block to an active continental margin on the south side of an elongate continental tract the central tien shan collision occurred along the qinbulak qawabulak fault southern tien shan suture the time of the collision is poorly constrained but was probably in in the late devonian early carboniferous we propose this age because of a major disconformity at this time along the north side of the tarim block and because the youshugou ophiolite is imbricated with middle devonian sediments a younger probably late carboniferous early permian collision along the north tien shan fault northern tien shan suture accreted the northern side of the central tien shan to an island arc which lay to its north the north tien shan arc this collision is bracketed by the middle carboniferous termination of arc magmatism and the appearance of late carboniferous or early permian elastics in a foreland basin developed over the extinct arc thrust sheets generated by the collision are proposed as the tectonic load responsible for the subsidence of this basin post collisional but palaeozoic dextral shear occurred along the northern suture zone this was accompanied by the intrusion of basic and acidic magmas in the central tien shan late palaeozoic basic igneous rocks from all three lithospheric blocks represented in the tien shan possess chemical characteristics associated with generation in supra subduction zone environments even though many post date one or both collisions rocks from each block also possess distinctive trace element chemistries which supports the three fold structural division of the orogenic belt it is unclear whether the chemical differences represent different source characteristics or are due to different episodes of magmatism being juxtaposed by later dextral strike slip fault motions because the southern collision zone in the tien shan is the older of the two the tarim block sensu stricto collided not with the eurasian landmass but with a continental block which was itself separated from eurasia by at least one ocean the destruction of this ocean in late carboniferous early permian times represented the final elimination of all oceanic basins from this part of central asia

a 4 gyr shock age for a martian meteorite and implications for the cratering history of mars analyses of meteorites that originated on mars provide important insights into the geological and atmospheric evolution of the planet such analyses have hitherto been restricted to relatively young martian rocks1 the oldest martian meteorites have an age of approximately 1 3 billion years but the recently recognized2 martian meteorite allan hills 84001 which is distinct from the other martian meteorites2 4 shows evidence for a much older age5 6 here we report an analysis of the shock alteration history of this meteorite based on argon isotope dating from which we derive a shock age of 4 0 0 1 billion years the age and geological history of this meteorite suggest that it came from the heavily cratered noachian age terrains of mars s southern hemisphere and it may thus provide an absolute chronology for this region of the planet independent of that inferred from the cratering record the shock age of the meteorite also coincides with that of the so called lunar cataclysm a relatively short period during which many of the craters on the moon are believed to have formed supporting the idea7 that intense bombardment was widespread throughout the inner solar system between 3 9 and 4 1 billion years ago

shock effects on the k ar system of plagioclase feldspar and the age of anorthosite inclusions from north eastern minnesota abstract artificial shock pressures up to 52 5 gpa have no influence on the k ar system of plagioclase feldspar the 40ar 39ar analysis of feldspar labradorite an67 from anorthosite of north eastern minnesota shocked up to 45 gpa in vacuo to prevent massive entrapment of atmospheric argon shows that the age spectra and the argon diffusion properties remain unaltered similar feldspar samples labradorite an51 from nain labrador shocked in air up to 52 5 gpa and dated by the conventional k ar method also yield the same age as unshocked samples but with a higher atmospheric argon contribution the minnesota anorthosite has an 40ar 39ar age of 1075 10 m y no information on a possible previous history of the anorthosite became apparent

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

relations between surface faulting and granite intrusions in analogue models of strike slip deformation analogue models investigating the emplacement of granitic magmas at upper crustal levels during strike slip deformation are presented in this paper variations in parameters such as ratio between horizontal displacement dv and vertical magma upraising iv and the number of feeding points were considered during the experiments model results show that a strong interaction between surface faulting and granite intrusions develops during deformation on one side magma intrusion influences the sequence of structures development and the final fault pattern on the other side magma emplacement is strongly influenced by the syn intrusion lateral displacement dt which in turn reflects variations in dv and iv for high dt values i e high displacement velocity or low injection rate the importance of the horizontal displacement with respect to the vertical magma upraising results in asymmetric and drop shaped plutons with a sheared tail elongating in the sense of the strike slip displacement in this case the intrusion long axis tracks the long axis of the strain ellipsoid for low dt values i e low displacement velocity or high injection rate the strike slip displacement is strongly subordinated to the vertical magma upraising and pluton emplacement is controlled by development of tensional shears at the surface in these conditions the long axis of the magmatic bodies lies in the quadrants of compression of the strike slip deformation and intrusions are apparently dragged in an opposite sense to that imposed by the transcurrent deformation

shock effects and argon loss in samples of the leedey l6 chondrite experimentally shocked to 29 70 ga pressures abstract shock recovery experiments between 29 and 70 gpa were made on the leedey l6 chondrite to determine the possible loss of radiogenic ar caused directly by passage of shock waves as opposed to purely thermal loss during prolonged post shock cooling experiments were made in vacuum to prevent shock implantation of terrestrial air and following neutron irradiation the 39 ar 40 ar ratios were measured by stepwise temperature release petrographie examination of experimentally shocked leedey showed predominantly solid state effects in the form of increasing disaggregation of component minerals with increasing shock pressure but very little melting even at the highest shock levels the melting behavior of the minor feldspar phase was difficult to document but likely was complete by the higher shock levels the 39ar 40ar release patterns were complex for both shocked and reference leedey probably because of a natural shock history for the meteorite at ca 3 8 gy the average 39ar 40ar age for shocked leedey decreased with increasing shock levels from essentially no change at 29 gpa to a 20 decrease in age at 60 gpa the feldspar phases degas at lower extraction temperatures and show even greater lowering of 39ar 40ar age with increasing shock pressure the sample shocked at 69 gpa shows less mechanical disruption than samples shocked to lesser pressures and also shows a smaller loss of ar compared to samples shocked at 45 and 60 gpa we conclude that observed ar loss is caused by extensive grain fracturing and heating during shock and that greater ar loss at low extraction temperatures is enhanced by feldspar melting none of the experiments duplicated the magnitude of ar loss commonly seen in naturally shocked materials that lack evidence of pervasive melting we conclude that large scale gas loss in shocked but unmelted samples requires that the materials reside for prolonged periods of time at elevated temperatures in a relatively hot impact formation either inside or outside the crater cavity

successive granitic magma batches during pluton emplacement the case of cabeza de araya spain we present a dynamic scenario of granite emplacement in the upper introduction crust for the pluton of cabeza de araya extramadura spain various mechanisms have been suggested to account for based on interpretations of gravity data internal structures and the emplacement of granitic bodies they range from geochemical variations the three dimensional shape of the pluton forceful intrusion via diapirism cruden 1988 or dyking is derived from the inversion of gravity data and the magma flow clemens mawer 1992 to permitted intrusion assisted structures are obtained by mapping the rock anisotropy of magnetic by deformation hutton 1982 castro 1985 guinesusceptibility the deepest zones of the pluton which also correspond berteau et al 1987 hutton et al 1990 between these to vertical lineations are interpreted as feeder zones from which modes ballooning brun et al 1990 nested diapirs magma upwelled these feeders form small distinct areas connected bouchez diot 1990 paterson vernon 1995 to the pluton s floor 6 km below the present surface level we propose stoping paterson fowler 1993 and also recognition that the two main feeders of the pluton represent tension gashes that of the ubiquity of syntectonic magmatism hutton 1997 formed at the base of the brittle crust and served as conduits for call for a competition depending on the crustal level the magma that was ready to be tapped in the southern half of of observation between mechanisms driven by magma the pluton granite type a is cross cut by type b in the magmatic pressure and those involving tectonic accommodation state these magmas show a continuum in their chemistries attributed evidence for the original segregation and ascent of to progressive differentiation a fine grained leucocratic granite more felsic magmas now frozen as plutons is difficult to evolved chemically cross cuts the surrounding facies and crops out observe in the field as a consequence these processes above the southern feeder zone it is interpreted as a late magma have to be inferred from petrological and rheological batch coming from a different source that intruded into the models of both the magma and country rock e g cruden incompletely crystallized surrounding facies the normal petrographic 1988 weinberg podladchikov 1994 stages of magma zoning of the massif of cabeza de araya is therefore viewed as a generation and transport have however little influence succession of continuous and discontinuous infillings sequentially on the mode of the final ponding of the magma which permissive and forceful of increasingly evolved magma batches into in turn is not directly observable in the field in addition a reservoir that opened at varying rates within the brittle crust to considerations from regional geology information from

precise k ar 40ar 39ar rb sr and u pb mineral ages from the 27 5 ma fish canyon tuff reference standard the accuracy of ages measured using the 40ar 39ar technique is affected by uncertainties in the age of radiation fluence monitor minerals at present there is lack of agreement about the ages of certain minerals used as fluence monitors the accuracy of the age of a standard may be improved if the age can be measured using different decay schemes this has been done by measuring ages on minerals from the oligocene fish canyon tuff fct using the k ar 40ar 39ar rb sr and u pb methods k ar and 40ar 39ar total fusion ages of sanidine biotite and hornblende yielded a mean age of 27 57 0 36 ma the weighted mean 40ar 39ar plateau age of sanidine and biotite is 27 57 0 18 ma a biotite feldspar rb sr isochron yielded an age of 27 44 0 16 ma the u pb data for zircon are complex because of the presence of precambrian zircons and inheritance of radiogenic pb zircons with 207pb 235u 0 4 yielded a discordia line with a lower concordia intercept of 27 52 0 09 ma evaluation of the combined data suggests that the best age for fct is 27 51 ma

characteristics and dynamic origin of the large scale jiaoluotage ductile compressional zone in the eastern tianshan mountains china abstract the jiaoluotage ductile compressional zone jdcz in the eastern tianshan mountains china developed in volcanic and sedimentary rocks that were deposited in a carboniferous back arc basin situated between the jungar plate and middle tianshan terrane the ductile compressional zone 500 km long and 20 50 km wide consists of a number of east trending vertical foliation zones that display an augen geometry in plan view and a fan like geometry in transverse profiles ductile compressional deformation has produced pervasive foliation radial stretching lineations contemporaneous folds and various macroscopic to microscopic symmetric structures within the jdcz rotations of most clasts and porphyroclasts are bidirectional the preferred orientation patterns of the optical axes of calcite and quartz display axial or orthorhombic symmetry three kinds of strain patterns have been determined in the jdcz apparent flattening plane strain and apparent constriction the overall strain character of the jdcz is cream cake style the volcanic and sedimentary rocks within the jdcz were metamorphosed into greenschist facies with temperature of 400 600 c and pressure of 300 400 mpa development of the jdcz is interpreted as due to n s coaxial compression caused by the collision between the tarim plate and the middle tianshan arc jiaoluotage basin jungar plate system during the permian between 270 and 290 ma

topographic evolution of the tianshan mountains and their relation to the junggar and turpan basins central asia from the permian to the neogene abstract the modern tianshan mountains and their surrounding basins have mainly been shaped by the far field effects of the cenozoic india asia collision however precollision topographic evolution of the tianshan mountains and its impacts on the junggar and turpan basins remain unclear due to the scarcity of data detrital zircon u pb dating of 14 new and 23 published samples from permian to neogene strata in the northern western tianshan mountains northern and southern bogda mountains and central turpan basin are combined with sedimentary characteristics lithofacies petrofacies and paleocurrent data to investigate the temporal and spatial changes in sediment provenances based on the age characteristics of the source rocks in the tianshan mountains the detrital zircons are divided into three groups pre carboniferous zircons mainly from the central tianshan mountains carboniferous to permian zircons mainly from the north tianshan and bogda mountains and mesozoic zircons mainly from syn depositional volcanic activity the topographic evolution of the tianshan mountains and their relation to the junggar and turpan basins can be generally divided into six stages 1 positive relief tianshan and bogda mountains and a rifted marine basin formed during the early permian to early middle permian following late carboniferous orogenesis as evidenced by interbedded alluvial fan conglomerates and postcollisional extension related volcanic rocks along the basin margins by marine deposits far from the basin margins and by the predominance of carboniferous to permian detrital zircons 2 fluvial to lacustrine deposits in the modern southern junggar and turpan basins are characterized by abundant pre carboniferous zircons and consistently northward flowing paleocurrents indicating the submergence of the bogda mountains and a contiguous junggar turpan continental depression basin during the late middle permian to the triassic 3 the bogda mountains began to uplift in the early jurassic resulting in opposing paleocurrent directions a sudden increase in sedimentary lithic detritus and the dominance of carboniferous to permian detrital zircons along the southern and northern margins of this range 4 in contrast to the uplift of the bogda mountains the other parts of the tianshan mountains experienced gradual peneplanation from the early jurassic to the middle jurassic as confirmed by widespread fluvial to lacustrine deposits even inside the modern tianshan mountains and by the dominance of pre carboniferous detrital zircons 5 the dominance of carboniferous to permian zircons in the southern junggar basin suggests the west tianshan mountains were uplifted during the late jurassic while the dominance of pre carboniferous zircons in the central turpan basin indicates continuous peneplanation in the eastern tianshan mountains 6 the initial shape of the tianshan mountains junggar basin turpan basin system was constructed in the late jurassic but was modified in the cenozoic by the india asia collision resulting in much higher western tianshan and bogda mountains low eastern tianshan mountains and well developed foreland basins these cenozoic changes were recorded by the rapid cooling of apatites the dominance of carboniferous to permian zircons in the southern junggar basin and northern turpan basin and the dominance of pre carboniferous zircons in the central turpan basin

constraining the intracontinental tectonics of the sw central asian orogenic belt by the early permian paleomagnetic pole for the turfan hami block in order to better understand the kinematics and geodynamics of the late paleozoic to early mesozoic transition from accretionary to intracontinental tectonics in the southwest central asian orogenic belt we performed a paleomagnetic study on lower permian strata in the turpan hami basin nw china magnetite and hematite are shown to be the principal magnetic remanence carriers a typical sedimentary fabric that has not been modified by postdepositional deformation is recognized via magnetic fabric study stable characteristic remanent magnetizations all of reverse magnetic polarity are revealed by step demagnetizations remanence age is determined to be of early permian affinity based on a fold test remanence behavior of minerals and geochronologic data with inclination shallowing corrections the first early permian paleomagnetic pole for the turpan hami block is acquired as 75 7 n 276 3 e a 95 5 7 and n 10 sites comparisons of this new pole with published ones from neighboring cratonic blocks provide kinematic constraints on the tectonic transition and subsequent intracontinental tectonics of this region the turpan hami and yili blocks have kept their relative positions since the early permian significant latitudinal movement of 14 7 7 2 occurred between the turpan hami and south junggar blocks and could be mainly accommodated by late paleozoic crustal shortening of at least 530 km across an intra arc basin in the bogda belt the turpan hami block has experienced large magnitude relative rotations of 41 1 7 1 a nd 21 2 9 4 with respect to tarim and south junggar since the early permian respectively corresponding to dominant intracontinental tectonics characterized by large magnitude strike slip displacements along megashear zones

accretionary tectonics of back arc oceanic basins in the south tianshan insights from structural geochronological and geochemical studies of the wuwamen ophiolite melange

structural evolution of the irtysh shear zone northwestern china and implications for the amalgamation of arc systems in the central asian orogenic belt the nw se irtysh shear zone is a major tectonic boundary in the central asian orogenic belt caob which supposedly records the amalgamation history between the pen siberian orogenic system and the kazakhstan south mongolia orogenic system however the tectonic evolution of the irtysh shear zone is not fully understood here we present new structural and geochronological data which together with other constraints on the timing of deformation suggests that the irtysh shear zone was subjected to three phases of deformation in the late paleozoic d 1 is locally recognized as folded foliations in low strain areas and as an internal fabric within garnet porphyroblasts d 2 is represented by a shallowly dipping fabric and related similar to nw se stretching lineations oriented sub parallel to the strike of the orogen d2 foliations are folded by similar to nw se folds f 3 that are bounded by a series of mylonite zones with evidence for sinistral reverse kinematics these fold and shear structures are kinematically compatible and thus interpreted to result from a transpressional deformation phase d 3 two samples of mica schists yielded youngest detrital zircon peaks at similar to 322 ma placing a maximum constraint on the timing of d 1 d 3 deformation a similar to ne sw granitic dyke swarm similar to 252 ma crosscuts d 3 fold structures and mylonitic fabrics in the central part of the shear zone but is displaced by a mylonite zone that represents the southern boundary of the irtysh shear zone this observation indicates that the major phase of d 3 transpressional deformation took place prior to similar to 252 ma although later phases of reactivation in the mesozoic and cenozoic are likely the late paleozoic deformation d 1 d 3 at similar to 322 252 ma overlaps in time with the collision between the chinese altai and the intra oceanic arc system of the east junggar we therefore interpret that three episodes of late paleozoic deformation represent orogenic thickening d 1 collapse d 2 and transpressional deformation d3 during the convergence between the chinese altai and the east junggar on a larger scale late paleozoic sinistral shearing d3 together with dextral shearing farther south accommodated the eastward migration of internal segments of the western caob possibly associated with the amalgamation of multiple arc systems and continental blocks during the late paleozoic c 2015 elsevier ltd all rights reserved

syn tectonic emplacement of the late mesozoic laojunshan granite pluton in the eastern qinling central china an integrated fabric and geochronologic study abstract the porphyritic monzogranite of laojunshan eastern qinling in central china forms a w e elongated pluton characterized by a varying abundance of cm scale potassium feldspar kfs megacrysts with or without plagioclase mantles mafic microgranular enclaves mmes and associated mafic dykes the measurements of the shape preferred orientation of kfs megacrysts and the tiling of megacrystic pairs throughout the pluton suggest that these megacrysts constitute magmatic foliations subparallel to the pluton s margin possibly resulting from eastward non coaxial magmatic flow felsic mafic magma mixing and mingling were important during magma emplacement as indicated by abundant mmes coeval mafic dykes and disequilibrium textures within kfs megacrysts quantitative analyses of the crystal size distribution and the in situ chemistry of kfs megacrysts revealed that the temperature fluctuation in the deep magma chamber and the resultant textural coarsening are the most likely important mechanisms for the growth of kfs megacrysts the syn tectonic and two main stage emplacement of the pluton in a sinistral transtensional regime is inferred by the mechanical coupling between magmatic submagmatic solid state fabrics in the pluton and wall rock deformations as well as zircon u pb dating results which produce eastward younging emplacement ages spanning from 122 ma to 119 ma combined with other studies our work supports the idea that the qinling dabie orogen was in a sinistral transtensional regime during the early cretaceous which may correspond to the eastward extrusion tectonics in eastern china

structural evolution of the eastern segment of the irtysh shear zone implications for the collision between the east junggar terrane and the chinese altai orogen northwestern china the tectonic evolution of the central asian orogenic belt caob involved multiple episodes of arc accretions collisions but the reconstruction of these tectonic processes remains relatively poorly constrained evidence for a collision between the intra oceanic island arc system of the east junggar terrane and the active margin of the siberian craton chinese altai orogen is recorded in the eastern segment of the irtysh shear zone northwestern china field observations from the qinghe area show that this segment of the irtysh shear zone consists of four nw se sinistral mylonitic zones and domains bounded by these mylonitic zones show variable fold patterns in the northern part of the irtysh shear zone southern chinese altai orogen three generations of structures d d are recognized the earliest generation of foliation s is only recognized locally and is transposed to the orientation of the dominant d foliation s the latter is associated with a shallowly plunging stretching lineation which is sub parallel to the nw se hinge of d folds these third generation folds d show a steeply dipping axial plane trending nw se in the southern part of the irtysh shear zone northern east junggar terrane the structural pattern is simpler and only involves a single generation of penetrative foliation s new u pb detrital zircon and ar ar data provide constraints on the timing of collision and deformation these results suggest that the collision occurred after the early carboniferous with the timing of sinistral shearing constrained to the early middle permian the combination of sinistral shearing and nw se d folds likely represents an episode of transpressional deformation which was driven by oblique collision between the east junggar terrane and the chinese altai orogen during the early middle permian the occurrence of original sub horizontal s foliation and associated orogen parallel stretching lineation might indicate that following the initial collision after the early carboniferous the southern chinese altai orogen was subjected to orogen parallel extension in a larger scale context sinistral kinematics along the irtysh shear zone in northwestern china and northeastern kazakhstan together with coeval dextral strike slip deformation farther south might reflect an eastward escape of orogenic materials possibly in response to the permian convergence of the siberian baltic and tarim cratons

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

kilometer scale refolded folds caused by strike slip reversal and intraplate shortening in the beishan region china 1 spectacular refolded folds clearly visible from space deform permian turbidite successions in a 180 km2 fault bounded compartment within the beishan region of northwestern china the folds formed after final terrane amalgamation of the central asian orogenic belt and record an important sequence of intraplate deformation events in central asia during the late permian jurassic the fold interference patterns are dominantly type 2 refolds with kilometer scale wavelengths they are bounded to the north and south by regional e w faults with multiple displacement histories as revealed by crosscutting ductile and brittle fabrics and kinematic indicators indicating opposing senses of motion the proximity of the refolded folds to the main bounding faults strongly suggests that the fold interference structures were caused by strike slip fault reactivation and shear related bending of f1 folds combined with components of n s compression across the compartment faulting within the folded compartment is limited to an array of right lateral strike slip faults which appear to have helped accommodate room problems during the refolding event the driving force for the deformation was likely to be distant including regional block collision to the south large scale rotations between the siberian and north china cratons and plate boundary convergence along the eastern chinese margin our study demonstrates that continental interior regions characterized by terrane collages are susceptible to reactivation by distant plate margin effects and that polyphase reactivation of major strike slip and oblique slip faults can generate refolding within upper crustal basinal compartments in addition the fault bounded basin containing the refolded folds in the beishan region is surrounded by older more complexly deformed crystalline basement therefore the deformed basin sequence represents a unique compartmental archive of the permian triassic history of intraplate deformation north of tibet

late paleozoic to early triassic multiple roll back and oroclinal bending of the mongolia collage in central asia abstract the architecture and mechanics of an orogen can be understood in terms of a system of collages that are characterized by a complex assemblage of multiple components but the fundamental paleogeographic framework and the tectonic relationships between the different components are often insufficiently defined because of unavailable data the central asian orogenic belt caob provides an ideal opportunity to address the fundamental framework of paleogeography and tectonic relationships between the diverse and many components in this huge collage in this paper we review several lines of available evidence which enable us to propose a new tectonic model of huge roll back in the formation of the accretionary tectonics of the mongolian collage in central asia in the early paleozoic the mongolia collage comprised the southern siberian and the tuva mongol oroclines the siberia craton and the mongolia collage jointly formed a giant tadpole shape within the paleo asian and panthanlassic oceans its head siberia was to the south and the tail tuva mongol to the northwest the structures and tectonic zonation of the mongolia collage are characteristic of multiple arcs which have been separately described in detail in different segments southwards from the southern siberia east sayan west sayan gorny altai chara via the lake zone junggar tianshan gobi altai beishan alxa to the manlay hegenshan baolidao solonker segments almost all segments underwent early paleozoic to permian or even triassic frontal subduction and accretion while rifting in the late carboniferous to permian or triassic occurred in the outward oceanward westward advancing mongolian collage therefore we suggest that a huge complex roll back active from the carboniferous to permian or even to late triassic facilitated the formation of the mongolian collage the outward multiple roll back process was compatible and almost coeval with the start of the tuva mongol orocline and rotation of the siberian craton as confirmed by paleomagnetic and structural data during the roll back processes an archipelago paleogeography was formed behind the frontal subduction and accretion in which independent arcs or terranes were amalgamated or collided to form composite arcs or terranes either simultaneously or at slightly different times the roll back process was affected by the collision of the kazakhstan collage along the chara and karamay zones in the early permian the collision of the tarim craton along the south tianshan zone in the early permian the collision of the dunhuang block along the liuyuan zone in the early permian triassic the collision of the alxa block along the qugan qulu zone in the permian and the collision of the north china craton along the solonker zone in the middle late triassic the tectonic styles and architecture of accretionary orogenic belts like the caob are characterized both by the amalgamation of multiple terranes and by oroclinal bending the systematic anatomy of the multiple roll back processes and their interactions with the adjacent collages shed light on the evolving orogenic architecture and the crustal accretionary history of orogens

middle cambrian to permian subduction related accretionary orogenesis of northern xinjiang nw china implications for the tectonic evolution of central asia abstract middle cambrian to permian subduction related records are widely distributed in northern xinjiang which can be grouped into the chinese altay east junggar eastern tien shan west junggar yili and tarim domains by integrating paleogeographic and geological data we suppose that the chinese altay east junggar eastern tien shan domain was more closely located to siberia while the west junggar and yili domains occupied an intermediate position near the kazakhstan block in the early paleozoic paleoasian ocean distribution of andean type magmatic arcs island arcs accretionary wedges ophiolitic slices and or microcontinents shows an archipelago paleogeography forming a huge accretionary active margin sequences the tarim domain was on the opposite side of the early paleozoic paleoasian ocean remaining passive margin these tectonic units drifted northwards and approached the southern active margin of the siberian craton in the late paleozoic leading to termination of the paleoasian ocean and formation of a complicated orogenic collage between siberian craton and the tarim block between the end permian and triassic these multiple accretion processes significantly contributed to the lateral growth of central asia

geochronology petrogenesis and tectonic implications of triassic granitoids from beishan nw china abstract five triassic granitic plutons have been identified from the beishan orogen by zircon u pb dating that defines two age groups 240 238 ma and 225 217 ma they belong to the high k calc alkaline and shoshonitic series and have alkalic and alkali calcic characteristics and are weakly peraluminous metaluminous these granitoids are mainly highly fractionated i type granite and a type granite they have high i sr values 0 7049 0 7096 and negative e nd t values of 0 9 to 2 8 with nd model ages of 0 89 1 13 ga suggesting a crustal contribution they also exhibit relatively high zircon e hf t values 1 3 to 6 1 and two stage hf model ages of 1 34 0 87 ga suggesting a mixture of mantle and various amounts of crustal components the mantle component probably originated from underplated magmas produced by partial melting of lithospheric mantle metasomatized by fluids or melts possibly derived from pre existing subducted materials the mid triassic 240 238 ma pluton has relatively high sr y 15 9 41 8 ratios and mgo 1 22 3 77 contents with negligibly to weakly negative eu anomalies eu 0 57 0 80 indicating adakite like features this was likely the result of partial melting of crustal components in response to voluminous underplated mantle derived magmas probably linked to lithospheric delamination and asthenospheric upwelling the late triassic 217 225 ma granitic plutons show strong negative anomalies of eu ba nb ta sr p and ti these were likely the result of fractional crystallization and crustal contamination for the granitic magma emplaced in a post orogenic setting

cat mountain a meteoritic sample of an impact melted asteroid regolith cat mountain is a new ordinary chondrite impact melt breccia that contains several shocked chondrule bearing clasts of l5 material these clasts are surrounded by a total impact melt of similar composition material which appears to have cooled over a period of a few thousand years probably within a melt breccia lens in the bottom of a large 1 km diameter crater on an l chondrite asteroid noble gas isotopes indicate that the sample was involved in at least two different impact events approximately 880 and 20 myr ago following the 4 55 ga accretion of primitive chondritic material the 880 ma event is responsible for the impact breccia texture of the sample and the 20 ma event reduced the sample to a meter sized object we also infer that another impact occurred between 880 and 20 ma possibly the 500 ma event recorded in many other l chondrites to jettison the material from the asteroid belt into an orbit that evolved into an earth crossing trajectory the shock metamorphic processes that occurred at 880 ma redistributed the opaque phases in the meteorite and altered the crystalline characteristics of silicate phases this reduced the reflectance of the l5 material and decreased the amplitude of its spectral absorption features these characteristics are consistent with the spectral characteristics of some c class asteroids and suggest that some dark asteroids that appear to belong to the c class could be covered with shocked ordinary chondrite material if one assumes that cat mountain came from the same asteroid as other l chondrites with the same cosmic ray exposure age then the juxtaposition of these different materials suggests asteroids are rubble piles which are heterogeneous on a scale less than 100 m furthermore the structural integrity of cat mountain and other l chondrites suggests the strengths of asteroid rubble piles are limited by fractures and contrasting material properties and are thus inherently weak in a ram pressure regime produced when they enter a planetary atmosphere however in a regime where the asteroid is the target of impact fragmentation rather than the projectile the added porosity of a rubble pile structure will compensate for the presence of fractures and absorb a large amount of the impact energy in this case the structural integrity of the asteroid may appear to be the same as a previously unshocked chondritic material

lateral extrusion in transpression zones the importance of boundary conditions the homogeneous transpression strain model formulated by sanderson and marchini journal of structural geology 6 449 458 1984 has proved to be a useful tool in the analysis of complex three dimensional deformation patterns however some of the boundary conditions introduced in the mathematical model may be unrealistic and unnecessarily restrictive in this paper a strain matrix for unconfined transpression and transtension is derived in which material is allowed to move not only vertically but also horizontally in and out of the deforming zone parallel to its length lateral extrusion three end member plane strain components are defined wrench simple shear pure shear in xy lateral stretch and pure shear in yz vertical stretch these biaxial strains can be viewed as the apices of a strain triangle for unconfined transpression or transtension the edges of the triangle correspond to triaxial pure shear non coaxial biaxial lateral extrusion and the triaxial confined transpressional or transtensional strain of sanderson and marchini during unconfined transpression the orientation and in particular the geometry k value of the finite strain ellipsoid depends upon not only the amount of shortening across the zone and the amount of strike slip parallel to the zone but also upon the ratio of vertical to lateral stretch this can present serious difficulties when attempting to use finite strains to infer the direction and magnitude of zone boundary displacements examples of transpression zones in which there is evidence of a component of lateral extrusion are described from sw cyprus and central scotland these examples illustrate that antithetic strike slip shearing is a kinematic requirement of laterally unconfined transpression implying that synchronous shear sense indicators may give opposite senses of movement in shear zones specific geometric and mechanical boundary conditions together with internal fault zone rheologies may favour the lateral extrusion of material kinematic partitioning can occur to form fault bounded domains in which end member biaxial and or non coaxial strains are developed analysis of such domains can give a clearer understanding of regional scale triaxial deformations these findings illustrate the importance of establishing displacement boundary conditions when qualitatively or quantitatively analysing crustal deformation zones

late paleozoic tectonic evolution of the paleo asian ocean in the northern alxa block nw china the northern alxa block occupies a key position in the southern margin of central asian orogenic belt caob and records late paleozoic subduction and closure processes of the paleo asian oce

dunhuang tectonic belt in northwestern china as a part of the central asian orogenic belt structural and u pb geochronological evidence abstract the dunhuang tectonic belt dtb is located about 100 km south of the beishan tianshan orogen in the central asian orogenic belt in nw china it was previously considered as a part of the tarim or north china craton detailed structural analyses reveal two episodes of deformation in the central dtb d1 and d2 d1 is a north side up reverse shear and d2 a dextral strike slip mineral assemblages microstructures and quartz c axis patterns indicate that d1 deformation took place under amphibolite facies conditions 500 to 600 c and d2 mostly under greenschist facies conditions 300 450 c u pb zircon dating of eight granitoid intermediate intrusions mostly dikes with well constrained cross cutting relationships with the d1 and d2 structures and an amphibolite gneiss indicates that d1 deformation took place before ca 349 ma and most likely at ca 406 ma and d2 between ca 249 ma and ca 241 ma the dtb has a structural metamorphic and magmatic signature in the paleozoic mesozoic that is typical of an orogenic belt it shares a similar geological history with the beishan tianshan orogen and is likely a part of the central asian orogenic belt the dtb and the beishan tianshan orogen might represent two separate paleozoic mountain belts that developed more or less synchronously on the south and north sides respectively of the last vestige of the paleo asian ocean before its terminal closure in the permian the d1 reverse shearing in the dtb is interpreted to be related to a silurian devonian terrane accretion collision and the d2 dextral strike slip to post accretional collisional movement among terranes in late permian middle triassic time

closing history of the southern tianshan oceanic basin western china an oblique collisional orogeny abstract the tianshan tien shan range is an important paleozoic collisional orogenic belt and the key to understand the central asia tectonic evolution this paper integrates our research results with the existing chinese and international literature on sedimentology geochemistry isotopic geochronology paleontonology and paleomagnetism of the tianshan and tarim regions to propose that the oblique collision may have played an important role in the late paleozoic closing of the southern tianshan oceanic basin as a result of the sinian latest proterozoic younger than 800 ma continental extension and rifting process the tarim and yili blocks separated from their parent continent in the late cambrian ordovician the southern tianshan oceanic crusts between the two blocks subducted northward beneath the southern margin of the yili block in the silurian during the devonian early carboniferous the tarim block rapidly drifted to the north and rotated about 46 clockwise this process induced the collision of the yili micro continent with the eastern segment present geographical position of the tarim continent in the late devonian and the southern tianshan oceanic crust evolved to be a west facing remnant oceanic basin during the late carboniferous early permian the tarim block located within an almost constant latitude range rotated about 26 clockwise with respect to the yili micro continent which ultimately closed the remnant oceanic basin in a scissors like manner from east to west and completed the tarim yili collision subsequent a type subduction of the tarim continental crust and lithosphere scale sinistral shearing generated a magmatic arc on the southern margin of the tarim yili suture zone the late permian early triassic clastics deposited in a peripheral foreland basin developed above the arc

paleozoic accretionary and collisional tectonics of the eastern tianshan china implications for the continental growth of central asia this paper deals with the various tectonic units in the chinese eastern tianshan orogenic collage in the central asian orogenic belt and discusses the paleozoic geological history of the several periods of accretion and collision of archipelago systems lying between the tarim and southern angaran continental margins the late ordovician silurian to early devonian eastern tianshan archipelago was characterized by a the harlik dananhu subduction system with a s dipping polarity in the north b a southerly n dipping subduction system beneath the central tianshan arc in the middle and c the south tianshan ocean against tarim in the south during the devonian to early carboniferous n dipping subduction led to the harlik dananhu arc and the kanggurtag forearc basin accretionary complex in the early to mid carboniferous the magmatic front associated with the n dipping subduction beneath the dananhu harlik arc migrated southwards forming the yamansu arc constructed upon the kanggurtag accretionary forearc by the late carboniferous the dananhu harlik arc was attached northwards to the angaran margin resulting in lateral enlargement of the angaran continent in the latest carboniferous to early permian a multiple soft collision left wide suture zones in the south that include the ophiolite strewn aqikkuduk shaquanzi and kumishi accretion collision complexes which were stitched by early permian post collisional plutons by re defining and re interpreting the various tectonic terranes this paper presents a new improved model for the paleozoic evolution of this part of central asia

geometry kinematics and tectonic models of the kazakhstan orocline central asian orogenic belt abstract the central asian orogenic belt caob is one of the largest accretionary orogens on earth and is characterized by the occurrence of tight oroclines kazakhstan and tuva mongolian oroclines the origin of these large scale orogenic curvatures is not quite understood but is fundamentally important for understanding crustal growth and tectonic evolution of the caob here we provide an outline of available geological and paleomagnetic data around the kazakhstan orocline with an aim of clarifying the geometry kinematics and geodynamic origin of the orocline the kazakhstan orocline is evident in a total magmatic image and can be traced by the continuation of high magnetic anomalies associated with the devonian volcanic belt and the late devonian to carboniferous balkhash yili arc paleomagnetic data show 112 126 clockwise rotation of the northern limb relative to the southern limb in the late devonian to early carboniferous as well as 15 28 clockwise rotation of the northern limb and 39 40 anticlockwise rotation of the southern limb relative to the hinge of the orocline during the late carboniferous to permian we argue that the kazakhstan orocline experienced two stage bending with the early stage of bending late devonian to early carboniferous 112 126 driven by slab rollback and the later stage late carboniferous to permian 54 68 possibly associated with the amalgamation of the siberian tarim and baltic cratons this new tectonic model is compatible with the occurrence of rift basins the spatial migration of magmatic arc and the development of large scale strike slip fault systems during oroclinal bending

u pb and 207pb 206pb ages of zircons from basaltic eucrites implications for early basaltic volcanism on the eucrite parent body we have undertaken petrologic and shrimp u th pb isotopic studies on zircons from basaltic eucrites yamato y 75011 y 792510 asuka a 881388 a 881467 and padvarninkai with different thermal and shock histories eucritic zircons are associated with ilmenite in most cases and have subhedral shapes in unmetamorphosed and metamorphosed eucrites some zircons in highly metamorphosed eucrites with granulitic texture occur alone in pyroxene and typically have rounded to subrounded shapes due to recrystallization superchondritic zr hf ratios of eucritic zircons indicate that they crystallized from incompatible element rich melts after crystallization of ilmenite concentrations of uranium and thorium in zircons in the unmetamorphosed eucrite y 75011 are higher than those in metamorphosed eucrites the u pb systems of eucritic zircons are almost concordant but some zircon grains show reverse discordance radiogenic lead loss up to 48 from zircons is observed in the shock melted eucrite padvarninkai the 207pb 206pb ages of zircon in y 75011 4550 9 ma n 5 are nearly identical within analytical uncertainty to the ages of zircons from the metamorphosed eucrite y 792510 4545 15 ma n 13 the highly metamorphosed eucrites a 881388 4555 54 ma n 5 and a 881467 4558 13 ma n 8 and the shock melted eucrite padvarninkai 4555 13 ma n 18 the averaged 207pb 206pb age of zircon from five eucrites analyzed in this study is 4554 7 ma 95 confidence limits n 49 indistinguishable from the averaged u pb age 4552 9 ma of the same samples because of the high closure temperature of lead in zircon tclosure 1050 c with a cooling rate of 0 2 c yr the 207pb 206pb ages of eucritic zircon do not represent metamorphic ages but crystallization ages of extrusive lavas this fact strongly suggests that volcanism of the eucrite parent body occurred at a very early stage of the solar system history 7 20 ma after cai formation 4567 2 0 6 ma thus basaltic eucrites crystallized from parental magmas within a short interval following the differentiation of their parent body the u pb ages of eucritic zircons are older than the u pb sm nd and rb sr ages of some basaltic eucrites which is consistent with differences in closure temperatures of each isotopic system and suggests that thermal and shock metamorphism affected the isotopic systems of pyroxene plagioclase and phosphates

paleozoic multiple accretionary and collisional tectonics of the chinese tianshan orogenic collage abstract subduction related accretion in the junggar balkash and south tianshan oceans paleo asian ocean mainly in the paleozoic gave rise to the present 2400 km long tianshan orogenic collage that extends from the aral sea eastwards through uzbekistan tajikistan kyrgyzstan to xinjiang in china this paper provides an up to date along strike synthesis of this orogenic collage and a new tectonic model to explain its accretionary evolution the northern part of the orogenic collage developed by consumption of the junggar balkash ocean together with paleozoic island arcs northern ili issyk kul and chatkal located in the west which may have amalgamated into a composite arc in the paleozoic in the west and by addition of another two roughly parallel arcs dananhu and central tianshan in the east the western composite arc and the eastern dananhu and central tianshan arcs formed a late paleozoic archipelago with multiple subduction zones the southern part of the orogenic collage developed by the consumption of the south tianshan ocean which gave rise to a continuous accretionary complex kokshaal kumishi which separated the central tianshan in the east and other paleozoic arcs in the west from cratons tarim and karakum to the south cross border correlations of this accretionary complex indicate a general southward and oceanward accretion by northward subduction in the early paleozoic to permian as recorded by successive southward juxtaposition of ophiolites slices of ophiolitic melanges cherts island arcs olistostromes blueschists and turbidites which are mainly paleozoic in age with the youngest main phase being late carboniferous permian the initial docking of the southerly tarim and karakum cratons to this complicated late paleozoic archipelago and accretionary complexes occurred in the late carboniferous early permian in the eastern part of the tianshan and in the late permian in the western part which might have terminated collisional deformation on this suture zone the final stages of closure of the junggar balkash ocean resembled the small ocean basin scenario of the mediterranean sea in the cenozoic in summary the history of the altaids is characterized by complicated multiple accretionary and collisional tectonics

synkinematic mafic ultramafic sheeted intrusions emplacement mechanism and strain restoration of the permian huangshan ni cu ore belt eastern tianshan nw china abstract whilst petrology geochemistry and metal content of small mafic ultramafic ni cu bearing complexes have been extensively studied their structural controls and emplacement mechanisms are still poorly documented this study addresses the last two points with the huangshan ni cu ore belt chinese eastern tianshan as a case study the huangshan intrusions are early permian a period when the tianshan orogenic belt recorded major right lateral wrench tectonics characterized by crustal scale shear zones detailed mapping petro structural analysis and strain rate calculation within and around the intrusions allow us to establish that the huangshan ni cu bearing mafic ultramafic complexes are not layered intrusions instead they emplaced by injection of several mafic ultramafic magma batches within kilometre scale tension gashes generated by permian dextral shearing and should be considered as synkinematic sheeted intrusions finite strain analysis across the huangshan kangguer shear zone provides rather high shear strain rates 4 5 considering the location and alignment of the ni cu bearing mafic ultramafic bodies along regional shear zones throughout eastern tianshan it appears that wrench tectonics most likely controlled and focussed the intrusion of parent magmas as a consequence rifting related to post orogenic extension is not required to account for permian magmatic features of the tianshan belt finally the huangshan ni cu bearing mafic ultramafic intrusions are neither parts of an ophiolitic suture nor of a dunite cored alaskan type ore deposit as postulated in some previous studies in the light of these new results we believe that structural controls and emplacement mechanisms of many ni cu sulphides deposits hosted by small intrusions particularly funnel shaped ones should be re evaluated from a structural and geophysical point of view

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

paleozoic tectonic evolution of the tianshan orogen northwestern china abstract the chinese tianshan orogenic belt which extends west east for about 1500 km is one of the most important paleozoic orogenic belts in central asia it includes parts of the tarim yili central tianshan and junggar plates which are microcontinental blocks with precambrian basements and meso cenozoic depositional covers the tarim plate is separated from the yili central tianshan plate by the southern central tianshan suture early carboniferous ophiolitic melanges and blueschists are widespread in this suture and are believed to be produced by the collision between a passive continental margin on the north side of the tarim plate and an active continental margin on the south side of the yili central tianshan plate the collision occurred at the end of early carboniferous the yili central tianshan and the junggar plates are separated by the northern central tianshan suture also considered as an early carboniferous collisional zone along which the middle ordovician ophiolitic melanges blueschists and late devonian early carboniferous ophiolites are exposed in early paleozoic times the subduction of the paleo junggar ocean took place in an active margin on the north side of the yili central tianshan plate along the northern central tianshan suture at the end of early carboniferous the north side of the yili central tianshan plate collided with the north tianshan arc along this suture the paleo oceanic crust in the tianshan and adjacent region was entirely consumed at the end of early carboniferous meanwhile the tarim yili central tianshan and junggar plates were accreted to eurasia epicontinental sediments were then deposited throughout the tianshan region during the middle late carboniferous times in the early permian tianshan began to uplift with accompanying volcanic eruption and granitic intrusion the uplift was accelerated in the late permian and continental molasse was extensively deposited

shocked meteorites argon 40 argon 39 evidence for multiple impacts to contribute to the understanding of the impact history of asteroids we performed a high resolution 40ar 39ar study of ten moderately to highly shocked chondrites which we selected according to the shock classification given by stoffler et al 1991 two recent shocked chondrite falls and two highly shocked eucrites completed our sample suite when possible we separated impact melt from host rock for separate analysis in total we studied 28 samples from 14 meteorites in some cases atmospheric ar that we associate with terrestrial weathering was identified and corrected for the ages we obtained range between 100 ma and 4 1 ga and are clearly distinct from primordial ages that correspond to solar system formation we reproduced the previously reported cluster of l chondrite ages 500 ma the most prominent result of our study is that in the case of chondrites melts generally are older than host rocks or melt embedded unmolten rocks to solve this apparent paradox we propose that the melt forming event which was the most severe shock episode in the history of these meteorites has not been the only occasion affecting their k ar systems at least one later impact metamorphism must have occured the response of the k ar clock to this second event was more severe in the host rock than in the previously in the first event generated melt veins and pockets because of different ar retention rates hence impact metamorphism on meteorite parent bodies indeed was a multistage process extending in time over billions of years

models of lithospheric scale deformation during plate collision effects of indentor shape and lithospheric thickness abstract analogue models designed to test the effects of lithosphere thickness and indentor shape during continental collision indicate that strength and density differences between lithosphere types control structural vergence during collision models presented here include layers of putty and sand to form four three and two layer lithosphere types craton arc and oceanic equivalent respectively which collide during unidirectional shortening in most cases the stronger thicker four layer lithosphere does not deform but simply acts to bulldoze weaker material aside deformation concentrated in the thinnest layers consistently verges away from the indenting continent irrespective of indentor shape lateral strike slip component deformation occurs in the early stages of most models even with simple unidirectional shortening and is enhanced by highly oblique indentor shapes the effects of extrusion during collision tested by repeating models with and without a free edge which allows extrusion consistently indicate that with the ability to extrude structures are smaller and more numerous without extrusion fewer larger structures tend to develop two geological examples from the precambrian part of the tasman orogenic belt and the modern day collision at the leading edge of australia illustrate aspects of the models

initial geometry and paleoflow reconstruction of the yamansu skarn related iron deposit of eastern tianshan china from paleomagnetic and magnetic fabrics investigations this study aims to uses paleomagnetic and anisotropy of magnetic susceptibility ams methods to recognize the initial deposit position and to track the paleoflow at the origin of an iron skarn related deposit the yamansu deposit is located in eastern tianshan this province has a substantial mining potential for fe cu skarn cu ni and v ti orthomagmatic deposits and orogenic au lodes recent publication dates the yamansu deposit at 323 ma and uses this deposit to define a model of submarine volcanogenic iron oxide svio deposits hou et al 2014a b in the proposed model the yamansu stratoid ore bodies formed horizontally within the central venting part of a large submarine composite volcano in a back arc environment however at yamansu the ore bodies are tilted vertically by the large scale dextral strike slip faulting during the permian the carboniferous positions are thus overprinted and unknown understanding the initial geometry and structures of the yamansu deposit is then necessary to test and or improve the proposed svio model field observations evidence an emplacement of the basalt as sill and their alteration into skarn magnetic mineralogy results show that the magnetic remanence and susceptibility are carried by magnetite paleomagnetic measurements on massive garnet skarn and basaltic sill distinguish two groups of samples the higher coercivity samples give a mean remanence direction that is compatible with regional late carboniferous directions after bedding correction this result indicates that both the basaltic sill and the massive garnet skarn were horizontal when formed the ams results agree with the paleomagnetic data because the magnetic foliations after bedding correction are horizontal and the well defined magnetic lineation suggests the existence of a horizontal fluid flow during metasomatism these results indicate that the basaltic sill horizontally intruded the carboniferous limestones intercalated within the volcanic pile metasomatic iron rich fluid probably ascended along a pre existing fault and was injected horizontally into the limestone and sill and then caused extensive skarnization and deposited the magnetite before tilting due to the permian right lateral strike slip

tectonic evolution of the chinese tianshan orogen from subduction to arc continent collision insight from polyphase deformation along the gangou section central asia the central asian orogenic belt as the largest accretionary orogen on earth is an ideal candidate to study the geodynamics of convergent plate boundaries through a prolonged period the evolution of this orogen has been explained by different tectonic models which incorporated one or a combination of the following mechanisms lateral stacking of arc systems along major shear zones arc amalgamation oroclinal bending and trench migration here we elucidate major mechanisms responsible for the tectonic evolution of the central asian orogenic belt focusing on the chinese tianshan orogen in the southern central asian orogenic belt structural observations from the 50 km long gangou section show evidence of polyphase deformation the earliest episode of orogen parallel sinistral shearing constrained to the early devonian ca 399 ma by syn deformational intrusions was possibly controlled by oblique subduction this was followed by an episode of ne sw contractional deformation dated at ca 356 ma 40ar 39ar age of syn deformational hornblende and likely associated with an episode of trench advance the third stages of deformation during the latest carboniferous and permian involved ne sw contraction orogen parallel extension and dextral transpression our new geochronological data constrain the timing of orogen parallel extension to ca 303 293 ma and confirm that dextral activation along shear zones occurred during the permian the results highlight the role of trench migration oblique tectonics and syn collisional orogen parallel extension in the build up of the central asian orogenic belt and contribute to the pre collisional reconstruction of this orogenic system

granite intrusion along strike slip zones in experiment and nature granite intrusion in the upper crust along a crustal scale shear zone has been modelled by injecting a newtonian fluid low viscosity silicone putty into a sandpack containing a ductile layer of silicone putty which acted as a weak level along which the injected material could spread the strike slip regime was obtained using two mobile rigid basal plates sliding horizontally boundary conditions were chosen in order to analyse the influence of thicknesses of the brittle cover and of the volume of injected material on the pattern of the intrusion experiments showed that 1 the presence of a soft layer between two competent units allowed the formation of laccolithic intrusions by lateral expansion in the emplacement site 2 intrusions are elongate and their long axis tends to track the principal stretching direction associated with the strike slip regime 3 this direction controls the emplacement beginning with the first stages of injection 4 the lateral expansion of the intrusion is locally controlled by arrays of riedel faults formed in the overburden 5 the control of faults on the intrusion pattern increases with increasing brittle ductile ratio i e with increasing depth and or decreasing thickness of the weak layer 6 intrusions are sigmoidal or lozenge shaped in horizontal view 7 the greater the brittle ductile ratio the less sigmoidal are the intrusions 8 feeding pipes shifted with respect to the strike slip zone result in asymmetric intrusions with the development of a sheared tail trailing behind the intrusion examples of leucogranites of the south armorican shear zone south brittany france emphasize that our experiments can explain the geometry of many syntectonic granites emplaced along strike slip zones they further shed some light on mechanisms of pluton intrusion in the upper crust

late paleozoic strike slip shear zones in eastern central asia nw china new structural and geochronological data 1 new structural studies and 40ar 39ar dating in northwest china provide information about late paleozoic strike slip motions subsequent to accretional events which built eastern central asia during the paleozoic two principal areas were affected by these large transcurrent motions first in the tianshan range main east west ductile shear zones are dextral and coeval with an eastward decreasing greenschist retrograde metamorphism associated biotites give ages ranging from 290 ma to 245 ma the earlier n110 shearing occurred in western tianshan while the last one dextral in whole tianshan occurred 250 245 myr ago second in the chinese altay region several nw se shear zones structured the area the main motion is sinistral and occurred along the erqishi zone at 280 290 ma it is followed by a complex succession of dextral and sinistral shearing episodes leading to the northwestward structuring dated at 245 ma of a metamorphic zone that was folded during a compressive event

structural characterization of the katebasu gold deposit xinjiang china tectonic correlation with the amalgamation of the western tianshan abstract the western tianshan orogen is one of the world s most important gold provinces it hosts numerous world class gold deposits along the southern and northern margins of the middle tianshan most deposits occur within large scale fault zones and or ductile brittle deformation zones that were formed during the paleozoic evolution of the paleo asian ocean thereby indicating a correlation between orogenic tectonics and the gold mineralization however the correlation is often poorly constrained and remains controversially discussed the katebasu gold deposit in xinjiang china is located on the northern margin of middle tianshan and is hosted by secondary fault zones to the south of the north nalati fault the main period of gold mineralization is suggested to be the late carboniferous which is broadly coeval with the late paleozoic amalgamation of the western tianshan therefore the katebasu gold deposit provides an excellent opportunity to search for possible correlations between structural processes at a deposit scale and orogenic processes at a regional scale to better understand the tectonic controls on large scale gold mineralization in the western tianshan in this paper we report detailed structural mapping and field investigations on the fault geometries and kinematics of the katebasu gold deposit that permit inversion of the tectonic stress field our structural data combined with results of previous studies of the regional tectonics allow us to reconstruct the structural history of the katebasu gold deposit to look for possible correlations with the orogenic processes in the western tianshan and to determine their control on gold mineralization the first stage of deformation d1 was top to the north thrusting that was caused by a roughly n s compression and is represented by ene wsw striking thrust faults d1 was likely related to the closure of the south tianshan ocean and the resulting collision between the tarim craton and middle tianshan yili block the second stage of deformation d2 was dextral strike slip motion along the ene wsw striking faults that was caused by the wnw ese stress field and that corresponds to permian dextral wrenching in the tianshan orogen subsequent ne sw compression led to the third deformation event d3 that mainly formed nw se striking reverse faults and nne ssw striking dextral faults

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

some observational evidence on the effect of atmospheric forcing on tidal variability in the upper delaware bay until quite recently most oceanographic research relating to the tidal variability in an estuary has ignored the effect of atmospherically forced motions on the tidal motions mechanisms exist however through which high frequency wind waves and low frequency subtidal variability can interact nonlinearly with the tide and modify the tidal response of an estuary significantly a set of current and sea level observations in the upper delaware bay provides observational evidence which suggests that tidal variability in the interior of the bay was appreciably modified during two moderately strong atmospheric events