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

contemporary tectonic stress in northeastern iberia new results from borehole breakout analysis abstract in situ data on the orientation of tectonic stress in northern iberia are scarce fault slip and focal mechanism data show a variety of stress orientations difficult to interpret in terms of a general stress pattern breakout analysis performed on oriented caliper logs from 17 hydrocarbon exploration wells has provided new data on the orientation of tectonic stress in northeastern iberia the southern pyrenees the ebro basin and the northwestern valencia trough the majority of breakout data indicate a rather consistent ne sw to ene wsw directional signature of the larger horizontal principal stress orientation this orientation differs significantly from the nw western european mean stress orientation it is consistent with depth within the first 4 7 km of both the sedimentary fill and the sedimentary basins substrate and also within allochthonous thrust sheets in the southern pyrenees and in the underlying autochthonous and parautochthonous foreland basin sediments

deposition and imbrication of a 2670 2629 ma supracrustal sequence in the indin lake area southwestern slave province canada new u pb age data from the southwestern slave province demonstrate that units of the indin lake supracrustal belt form an imbricated structural stack the oldest rocks of the belt are undated mafic volcanic flows of the hewitt lake group that are crosscut by a 2670 ma felsic sill itself coeval with mafic through felsic volcanic rocks of the 2668 2671 ma leta arm group the youngest rocks of the belt are 2647 2629 ma turbidites and felsic volcanic rocks of the unconformably overlying chalco lake group tonalite orthogneiss of the adjacent cotterill gneiss complex is 2680 ma suggesting that it does not represent in situ basement to the supracrustal belt intercalation of the older hewitt lake and leta arm groups with the younger chalco lake group is interpreted to result from d1 imbrication and folding between 2629 and 2609 ma the age of a crosscutting tonalite intrusion subsequent d2 folding and regional low pressure metamorphism occurred between 2609 ma and ca 2590 ma d3 normal faulting between the

kinematics and geochronology of multistage ductile deformation along the eastern alxa block nw china new constraints on the relationship between the north china plate and the alxa block abstract the ne trending bayanwula shan lang shan is an important tectonic belt lying between the north china plate ncp to the east and the alxa block to the west an understanding of its nature and the timing of deformation are essential to understand the relationship between the ncp and the alxa block two phases of ductile deformation have been observed in this belt large scale top to the west ductile thrusting characterized the early deformation in the bayanwula shan lang shan nearly east west trending quartz stretching lineations and lineations formed by amphibole and biotite are well developed different types of sheath and oblique folds with east west trending fold hinges are also developed in the region the shear strain of this ductile thrust is up to 17 the ductile deformation may have resulted from the top to the west thrusting of the northern part of the ncp over the alxa block and may have occurred ca 351 ma biotite 40 ar 39 ar age later ductile deformation was expressed as ne trending sinistral shear along the entire bayanwula shan lang shan and likely occurred ca 250 ma biotite and muscovite 40 ar 39 ar ages this shear may have resulted from the collision between the yangtze and north china plates to the south during the triassic combined with recently obtained detrital zircon u pb ages for the area the ductile deformation events in the eastern alxa block indicate that the block may not have been part of the ncp at least before the end of the devonian both blocks were located in the paleo asian ocean during the paleozoic and collided or amalgamated with each other at the end of the devonian

dynamic processes from plate subduction to intracontinental deformation insights from the tectono sedimentary evolution of the zhaosu tekesi depression in the southwestern chinese tianshan abstract the zhaosu tekesi depression located to the northern margin of the south tianshan orogenic belt stb is an ideal region for investigating paleozoic orogenic processes and later modifications of the southwestern chinese tianshan in this study we employ 2d seismic and borehole data revealing the geological structure and tectonic evolution of the zhaosu tekesi basin and we report new zircon u pb geochronological and hf isotopic data and whole rock elemental and sr nd isotopic analytical results of the late silurian magmatic rocks as well as the whole rock trace element data of the early carboniferous carbonate rocks borehole data wells zc1 and zc2 and seismic imaging suggest that the zhaosu tekesi depression is composed of silurian lower carboniferous permian jurassic upper cretaceous and paleogene strata with six unconformity surfaces c 1 a s 3 p 1 c 1 a p 2 t p 1 j p 2 t k 2 d j and e k 2 d the silurian strata exhibit southward thrust structures and consist of basalt andesite dacite tuff minor muddy limestone and diorite intrusions zircon shrimp u pb dating for the well zc1 diorites yield a crystallization age of 422 ma with positive e hf t values 3 3 10 6 the diorites and andesites are calc alkaline and show enrichment in liles such as th and u and depletion in nb ta and ti relatively high initial 87 sr 86 sr ratios 0 703376 0 706737 and low e nd t values 0 5 to 1 4 and high th yb 0 29 0 35 and ta yb 1 31 1 58 ratios indicating their formation in an active continental margin setting the lower carboniferous strata mainly comprise oolitic limestone bioclastic micritic limestone and interbedded calcareous siltstone high sr ba 1 31 2 33 v sc 10 4 18 6 and low th u 0 01 0 23 ratios suggest that they were potentially deposited in the mixing near shore marine environment these results in combination with previous data indicate that the southwestern chinese tianshan is controlled by northward subduction of the south tianshan ocean during the late silurian to early carboniferous the presence of the lower permian terrigenous strata and rift basin structure constrains the timing of collision between the ctb kyb and the tarim craton as the late carboniferous thrusting reactivation and syn tectonic growth strata in the basin suggest that the southwestern chinese tianshan experienced late cretaceous compression and neogene modifications after the late permian post faulting thermal subsidence triassic peneplanation and jurassic regional subsidence intracontinental subduction due to distant effect is the main tectonic control on the miocene rapid uplift of the tianshan the zhaosu tekesi basin evolution sheds light on dynamic processes of the southwestern chinese tianshan from plate subduction to intracontinental deformation

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

a tale of amalgamation of three permo triassic collage systems in central asia oroclines sutures and terminal accretion the central asian orogenic belt records the accretion and convergence of three collage systems that were finally rotated into two major oroclines the mongolia collage system was a long n s oriented composite ribbon that was rotated to its current orientation when the mongol okhotsk orocline was formed the components of the kazakhstan collage system were welded together into a long single composite arc that was bent to form the kazakhstan orocline the cratons of tarim and north china were united and sutured by the beishan orogen which terminated with formation of the solonker suture in northern china all components of the three collage systems were generated by the neoproterozoic and were amalgamated in the permo triassic the central asian orogenic belt evolved by multiple convergence and accretion of many orogenic components during multiple phases of amalgamation followed by two phases of orocline rotation

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

the europium anomalies in banded iron formations and the thermal history of the oceanic crust abstract the ree rare earth element patterns in banded iron formations bif s from various sites in north america two sites in finland and three drillcores of the hamersley basin in western australia have been investigated using icp ms inductively coupled plasma mass spectrometry and inaa instrumental neutron activation analysis the results of this study as well as existing data show trends in the eu anomalies bif s older than 2 5 ga exhibit positive eu anomalies while younger ones exhibit none in chemical sediments between 2 5 and 2 3 ga both trends can be found because of the worldwide distribution of bif s this feature can be considered as a global change in the conditions controlling ree mobilization consideration of the redox conditions of bif formation in comparison with the calculated and measured eu redox equilibria suggests that a eu anomaly points to a dominantly hydrothermal source of ree s with solution temperatures above 175 c we suggest that the disappearance of the eu anomaly towards the proterozoic marks a change from mainly high temperature hydrothermal to dominantly low temperature alteration

geochemical and nd pb isotopic systematics of late archean granitoids southwestern slave province canada constraints for granitoid origin and crustal isotopic structure new geochemical and nd pb isotopic data for 2 62 2 59 ga granitoids from the southwest slave province are used to determine the source s of granitoid magmas to evaluate the role of pre 2 8 ga basement during this magmatism and to refine the existing nd pb isotopic structure of the western slave province the pb isotopic data require crust older than 3 2 ga as a granitoid protolith whereas the nd isotopic data require input from juvenile crustal material this discrepancy is explained if the granitoid protoliths are mixtures of ancient basement and 2 7 ga juvenile crust in varying proportions specifically granitoids from the southwestern slave province require 10 30 basement whereas granitoids from other parts of the western slave province require 50 incorporation of basement as a protolith may be achieved indirectly by assimilation of basement during juvenile 2 7 ga magmatism or directly during 2 62 2 59 ga magmatism the granitoid isotopic data suggest that indirect basement input was

crustal thinning in the southwestern iberia margin the mode of crustal thinning in the southwestern margin of the iberian peninsula is investigated along a transect that extends from onshore iberia to the eastern end of the horseshoe abyssal plain on onshore areas the crustal structure has been deduced using wide angle seismic reflection data whereas offshore we have used coincident steep and wide angle reflection data along a ne sw oriented seismic profile that extends from cape san vicente to the horseshoe abyssal plain in addition 2d gravity modelling has been performed to validate the crustal structure deduced from seismic data our model results reveal that the crust undergoes a strong but continuous thinning from 31 km onshore iberia to less than 15 km in the horseshoe abyssal plain and that thinning occurs over horizontal distances of about 120 km

moho topography beneath the iberian western mediterranean region mapped from controlled source and natural seismicity surveys abstract the complex tectonic interaction processes between the european and african plates at the western mediterranean have left marked imprints in the crustal architecture of this area particularly concerning the lateral variations in crustal thicknesses the detailed mapping of such variations is hence of large interest as it provides a major constraint to geophysical and geodynamic modeling at different scales controlled source seismic profiling and receiver functions from natural seismicity are widely considered as major tools to constrain moho topography we compile here the moho depths determined from a comprehensive number of both types of seismic surveys to end up with a new 3d moho depth map of the iberian peninsula its continental margins and north morocco since the 1970s the lithospheric structure beneath this study area has been extensively investigated using multichannel normal incidence seismic reflection and refraction wide angle reflection profiling in the last few years some high resolution surveys at sea and inland have been acquired the latter ones involving 1000 land stations on the other hand the topoiberia iberarray experiment has triggered the investigations on crustal and lithospheric structure using natural seismicity providing a homogeneous spatial resolution never achieved before the availability of good quality results from both methodologies in a common area provides an excellent opportunity to check the consistency of the moho depth estimations the integration of both datasets has resulted in a new high resolution map of the crustal thickness variations the final grid evidences large moho topography variations including crustal imbrication in the pyrenean range a large and relatively undisturbed variscan massif in the center of iberia and a probable delamination process beneath the gibraltar arc the crustal thicknesses vary from 15 km in continental margins up to values exceeding 50 km beneath the pyrenees or the rif cordillera

crustal structure of a nonvolcanic rifted margin east of newfoundland a detailed seismic refraction study was undertaken of the rifted continental margin southeast of the grand banks of newfoundland along a preexisting deep multichannel seismic reflection profile the primary objectives were to determine the topography of moho across the margin to detect any evidence for crustal underplating and to determine the position of the continent ocean boundary data were obtained from nine ocean bottom seismometers deployed along the profile with an array of six 1000 inch3 air guns as the source two additional refraction lines were shot parallel to structure over the continental slope the resultant velocity model is well constrained and consistent with the observed gravity the crust thins abruptly beneath the continental slope to a major structural transition identified as the continent ocean boundary there is no significant magmatic underplating of the thinned continental crust seaward of the transition the seismic velocity structure is characterized by a 50 km wide zone of intermediate velocity 7 2 7 6 km s beyond this is a thin 3 5 km crust with low velocity 4 5 5 km s and high gradient over mantle with a velocity of 7 7 km s increasing to 8 0 km s the zone of intermediate velocity is interpreted to be highly serpentinized mantle material beneath a very thin crust suggesting little or no magmatic activity during and immediately after rifting this interpretation is consistent with results from adjacent and conjugate margins

final amalgamation of the central asian orogenic belt in ne china paleo asian ocean closure versus paleo pacific plate subduction a review of the evidence abstract the central asian orogenic belt caob evolved through complex closure of the paleo asian ocean from the neoproterozoic to the late phanerozoic this caused the chinese cratons to collide with eurasia and led to the formation of the world s largest phanerozoic orogenic belt ocean closure commenced in the west and was completed in the east near changchun closure of the paleo asian ocean in ne china was along the solonker xar moron changchun yanji suture and this was likely completed in the late permian although associated activity continued into the triassic there was an overlap in the latest permian early triassic between terminal activity associated with paleo asian ocean closure and the onset of tectonism associated with subduction of the paleo pacific plate this switch in geodynamic setting occurred at 260 250 ma and is reflected by a relaxing of north south directed compression and the onset of east west directed processes related to paleo pacific subduction by the early jurassic events associated with the westward advance of the paleo pacific plate dominated leading to extensive development of i type granites as far inland as the great xing an range from 140 ma the paleo pacific plate retreated eastward resulting in an extensional setting in the early cretaceous the effects of which were enhanced by regional thinning of the lithosphere commonly attributed to delamination throughout this period the eastern asian margin was tectonically complex the north south oriented jiamusi khanka bureya block was rifted away from the eastern margin of the caob in the late triassic but was then re united in the jurassic by westward advancing subduction that affected both the western and eastern margins of the block accretionary complexes continued to evolve in the cretaceous along the whole eastern margin of asia with final accretion of the nadanhada terrane part of the sikhote alin accretionary terrane with the caob at 130 ma followed by the emplacement of s type granites

tertiary strike slip faulting in southeastern mongolia and implications for asian tectonics abstract geologic maps have long portrayed the late cretaceous recent geologic history of southeastern mongolia as tectonically quiescent we present new data based on outcrop observations that indicate the northeast trending east gobi fault zone egfz was reactivated in the cenozoic as a sinistral strike slip fault system inversions of cenozoic fault slip data imply that faulting was associated with north northwest subhorizontal shortening and east northeast subhorizontal extension we propose that faulting is tertiary in age based on published interpretations of seismic reflection data which reveal that the mid cretaceous 100 95 ma unconformity is deformed by strike slip faults and based on field observation of strike slip faults and fracture sets that cut upper cretaceous and cenozoic strata but lack evidence for neotectonic activity published seismicity maps also appear to argue against significant quaternary faulting within the egfz these new data may lend credence to published models proposing a middle miocene or older kinematic linkage between the egfz and the altyn tagh fault in china the recognition that the egfz has a history of left lateral displacement in both the early mesozoic and cenozoic means that currently available estimates of offset based on displaced paleozoic rocks constrain total offset only this reactivation history supports the notion that inherited lithospheric structures are important in controlling the location and thus modes of intracontinental deformation in asia as a function of collisional far field effects and evolving boundary conditions of the pacific margin

did the altyn tagh fault extend beyond the tibetan plateau abstract the pre miocene northeastern termination of altyn tagh fault is a critical outstanding problem for understanding the mechanics of cenozoic deformation resultant from the indo asian collision and mechanisms of tibetan plateau formation structures beyond the widely accepted ne end of the altyn tagh fault near the town of yumen are needed in order to accommodate strike slip deformation related to plate like lateral extrusion tectonics but structures with the necessary slip magnitudes and histories have not been identified we report on a series of newly recognized and documented e to ene striking faults within the alxa block ne of the tibetan plateau that are visible on remotely sensed images and confirmed by field studies these structures are demonstrably left lateral faults based on offset geology and kinematic indicators such as striae and s c fabrics in fault gouge the faults have post cretaceous offsets of at least tens to possibly 150 km but limited post miocene displacement constrained by offset sedimentary basins these characteristics suggest that strike slip faults of the alxa region have a similar structural history as the central eastern altyn tagh fault and can provide a mechanism for accommodating oligocene early miocene extrusion along the altyn tagh fault

geology and u pb geochronology of the neoarchean snare river terrane tracking evolving tectonic regimes and crustal growth mechanisms u pb zircon crystallization ages determined by isotope dilution thermal ionization mass spectrometry id tims and laser ablation microprobe inductively coupled plasma mass spectrometry lam icp ms for 13 intrusive units in the neoarchean snare river terrane srt provide tight constraints on the timing of crust formation and orogenic evolution seven metaluminous plutons were emplaced over 80 ma from ca 2674 to 2589 ma whereas six peraluminous bodies were emplaced in a 15 ma interval from ca 2598 to 2585 ma a detrital zircon study yielded an age spectrum with peaks correlative with known magmatic events in the slave province with the ca 2635 ma age of the youngest detrital zircon population providing a maximum estimate for the onset of sedimentation this age contrasts with evidence for pre 2635 ma sedimentation elsewhere in the srt indicating that sedimentation was protracted and diachronous evolution of the srt can be subdivided into four stages i 2674 2635 ma formation of a metal

thin crust at the western iberia ocean continent transition and ophiolites western iberia is bounded by a nonvolcanic rifted continental margin made up of three apparently independent segments the age of breakup decreases from south to north seismic refraction and reflection profiles and magnetic and gravity data from each segment show a consistent pattern of geophysical observations across the ocean continent transition oct zone which is a few tens of kilometers wide we emphasize here the discovery of thin 2 4 km oceanic crust underlain by 7 6 km s 1 material within the oct the available evidence favors the suggestion that the 7 6 km s 1 layer is serpentinized peridotite and that the thin oceanic crust is primarily the result of a poor magma supply for a few million years immediately after continental breakup this thin crust may be the source of some ophiolites which exhibit thin crustal sections and continental margin affinities

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

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

seismic images and evolution of the iberian crust in the pyrenees abstract crustal images of the pyrenean range based on coincident wide angle and vertical deep seismic profiles are presented in this study wide angle experiments in the thickened iberian crust along strike lines have provided no evidence for reflections from interfaces between layers of different velocities in the middle or lower crust in contrast reversed sounding on a line across the pyrenean range shows wide angle reflectivity its unusual reflectivity pattern points to widespread impedance contrasts of localized continuity embedded in the crust this observation is consistent with a new image of vertical incidence reflectivity obtained by coherency weighted migration of the coincident ecors traverse this profile shows numerous discontinuous and dipping reflections from the middle to the base of the crust the reflections dip towards the axis of the range the upper limit and the attitude of the reflective zones are so strongly variable that the crust can hardly be envisaged as the simple continuous three layered model previously used for geodynamical reconstructions the reflective zones cannot be brought back to the horizontal image of a layered lower crust of proposed extensional origin by plate unbending to the pre alpine stage the dipping reflections may result from compressional deformation as the south pyrenean deep crustal domain has undergone both hercynian and alpine orogenic deformation if part of the present reflectivity pattern was due to alpine remobilization the corresponding lower crustal deformation would have spread out thereby accomodating convergence by thickening even outside the axial zone where the upper crustal deformation appears to be concentrated

geochemical sr nd and zircon u pb hf isotopic studies of late carboniferous magmatism in the west junggar xinjiang implications for ridge subduction abstract voluminous granitic intrusions are distributed in the west junggar nw china and they can be classified as the dioritic rocks charnockite and alkali feldspar granite groups the dioritic rocks sio 2 50 4 63 8 wt are calc alkaline and mg enriched average mgo 4 54 wt mg 0 39 0 64 with high sr y ratios average 21 2 weak negative eu average eu eu 0 80 and pronounced negative nb ta anomalies their sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7035 0 7042 e nd t 4 5 7 9 e hf t 14 1 14 5 show a depleted mantle like signature these features are compatible with adakites derived from partial melting of subducted oceanic crust that interacted with mantle materials the charnockites sio 2 60 0 65 3 wt show transitional geochemical characteristics from calc alkaline to alkaline with weak negative eu average eu eu 0 75 but pronounced negative nb ta anomalies sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7037 0 7039 e nd t 5 2 8 0 e hf t 13 9 14 7 also indicate a depleted source suggesting melts from a hot juvenile lower crust alkali feldspar granites sio 2 70 0 78 4 wt are alkali and fe enriched and have distinct negative eu and nb ta anomalies average eu eu 0 26 low sr y ratios average 2 11 and depleted sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7024 0 7045 e nd t 5 1 8 9 e hf t 13 7 14 2 these characteristics are also comparable with those of rocks derived from juvenile lower crust despite of the differences in petrology geochemistry and possibly different origins zircon ages indicate that these three groups of rocks were coevally emplaced at 305 ma a ridge subduction model can account for the geochemical characteristics of these granitoids and coeval mafic rocks as the slab window opened upwelling asthenosphere provided enhanced heat flux and triggered voluminous magmatisms partial melting of the subducting slab formed the dioritic rocks partial melting of the hot juvenile lower crust produced charnockite and alkali feldspar granite and partial melting in the mantle wedge generated mafic rocks in the region these results suggest that subduction was ongoing in the late carboniferous and thus support that the accretion and collision in the central asian orogenic belt took place in north xinjiang after 305 ma and possibly in the permian

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

subduction induced mantle heterogeneity beneath eastern tianshan and beishan insights from nd sr hf o isotopic mapping of late paleozoic mafic ultramafic complexes abstract extensive late paleozoic mafic ultramafic complexes in the eastern tianshan and beishan regions in the southern central asian orogenic belt caob represent an ideal opportunity to assess the characteristics of the mantle in this area whole rock nd sr and zircon hf o isotopic data from the mafic ultramafic complexes of the jueluotage belt middle tianshan massif and beishan rift were compiled for isotopic mapping end t and 87sr 86sr i values range from 5 5 to 9 5 and 0 702 to 0 706 respectively in the jueluotage belt and from 2 to 11 and 0 703 to 0 710 respectively in the middle tianshan and beishan zircon ehf t and 18o values are in the range of 6 0 17 0 and 4 14 8 00 respectively in the jueluotage belt 1 8 and 5 17 6 30 respectively in the middle tianshan and show large variations of 1 17 and 4 12 respectively in the beishan spatially the 87sr 86sr i ratios of mafic ultramafic complexes increase from the jueluotage belt and beishan to the middle tianshan while the end t values show an opposite trend and reflect a mixing character between depleted mantle and em ii reservoirs zircon ehf t and 18o values exhibit an apparent southward decreasing and increasing trends respectively from the jueluotage belt through middle tianshan to beishan rift their corresponding hf model ages increase continuously from north to south these observations demonstrate that the mantle sources beneath the eastern tianshan and beishan are compositionally heterogeneous which presumably result from significant and variable degrees of subduction related modification we therefore suggest that the late paleozoic subcontinental lithospheric mantle beneath the jueluotage belt was modified by slab derived melts and fluids during the subduction of the junggar ocean whereas the mantle beneath the beishan rift was influenced mainly by slab derived fluids during the subduction of the south tianshan ocean the unique similarities of the middle tianshan mafic ultramafic complexes reflect a two stage modification of their mantle source namely the sequential subduction of the early south tianshan ocean followed by subduction of the junggar ocean thus the subduction induced mantle heterogeneity beneath the caob is probably regional in scope

re os isotopic ages of pyrite and chemical composition of magnetite from the cihai magmatic hydrothermal fe deposit nw china the eastern tianshan orogenic belt of the central asian orogenic belt and the beishan terrane of the tarim block nw china host numerous fe deposits the cihai fe deposit 90 mt at 45 6 fe in the beishan terrane is diabase hosted and consists of the cihai cinan and cixi ore clusters ore minerals are dominantly magnetite pyrite and pyrrhotite with minor chalcopyrite galena and sphalerite gangue minerals include pyroxene garnet hornblende and minor plagioclase biotite chlorite epidotite quartz and calcite pyrite from the cihai and cixi ore clusters has similar re os isotope compositions with 14 to 62 ppb re and 10 ppt common os pyrrhotite has 5 to 39 ppb re and 0 6 ppb common os pyrite has a mean re os model age of 262 3 5 6 ma n 13 in agreement with the isochron regression of 187os vs 187re the re os age 262 ma for the cihai fe deposit is within uncertainty in agreement with a previously reported rb sr age 268 25 ma of the hosting diabase indicating a genetic relationship between magmatism and mineralization magnetite from the cihai deposit has mg al ti v cr co ni mn zn ga and sn more elevated than that of typical skarn deposits but both v and ti contents lower than that of magmatic fe ti v deposits magnetite from these two ore clusters at cihai has slightly different trace element concentrations magnetite from the cihai ore cluster has relatively constant trace element compositions some magnetite grains from the cixi ore cluster have higher v ti and cr than those from the cihai ore cluster the compositional variations of magnetite between the ore clusters are possibly due to different formation temperatures combined with regional tectonic evolution of the beishan terrane the re os age of pyrite and the composition of magnetite indicate that the cihai fe deposit may have derived from magmatic hydrothermal fluids related to mafic magmatism probably in an extensional rift environment

sulige field in the ordos basin geological setting field discovery and tight gas reservoirs abstract the sulige gas field in central ordos basin is one of the largest gas fields in china with about 860 7 billion cubic meters bcm of proven gas reserve this field covers an area of about 500 km 2 with largely tight clastic reservoirs the upper carboniferous lower permian deltaic sandstones and upper permian lacustrine mudstones deposited in a stable cratonic tectonic setting form an effective petroleum system that is favorable for gas accumulation in large scale stratigraphic lithologic traps based on wire line log response drilling stem test and core data a generalized classification system was developed for the gas reservoirs of the sulige field thus type i and ii rocks are capable of gas production without natural and or artificial fracturing type iii rocks are capable of gas production after artificial fracturing and type iv rocks are too tight to produce at commercial rates even with artificial fracturing the discovery of the sulige field not only adds large gas reserve in the ordos basin but strongly indicates the overlook potential of both tight sandstone reservoir rock and relatively porous and permeable sweet spots in it

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

segmentation and inversion of the hangjinqi fault zone the northern ordos basin north china abstract this paper deals with the segmentation and inversion of the hangjinqi fault zone hfz which is the dominant structure in the northern part of the ordos basin in north china hfz was reactivated during the late triassic and obliquely inverted during the middle jurassic shortening subsurface geological mapping and structural analysis were carried out to determine the segmentation and kinematic history of the deformation the hfz was a left stepping fault zone and was made up of three segments the porjianghaizi fault pf wulanjilinmiao fault wf and sanyanjing fault sf which are separated by two relay ramps two distinct phases can be identified in its structural evolution 1 during the late triassic compressional deformation the hfz was characterized by shortening and thrusting to the north and 2 during the middle jurassic phase the hfz was oblique to the extensional fault trends the reverse faults were reactivated as dextral strike slip faults as a result of transtensional inversion the inversion ratio of the hfz indicates an increase in deformational degree from east to west over the whole region the first deformation stage resulted from the n s compression between the south china and north china plates during the late triassic the second deformation stage of compression was related to the west northwestward subduction of the paleo pacific plate during the middle jurassic in the jurassic deformation framework the hfz may be interpreted as an accommodation structure parallel to the yanshan yinshan orogenic belt developed in the northern ordos area

geochemistry of the archean yellowknife supergroup the archean yellowknife supergroup slave structural province canada is composed of a thick sequence of supracrustal rocks which differs from most archean greenstone belts in that it contains a large proportion 80 of sedimentary rocks felsic volcanics of the banting formation are characterized by hree depletion without eu anomalies indicating an origin by small degrees of partial melting of a mafic source with minor garnet in the residua granitic rocks include synkinematic granites hree depleted low 87sr86sr i post kinematic granites negative eu anomalies high 87sr86sr i and granitic gneisses with ree patterns similar to the post kinematic granites sedimentary rocks turbidites of the burwash and walsh formations have similar chemical compositions and were derived from 20 mafic intermediate volcanics 55 felsic volcanics and 25 granitic rocks jackson lake formation lithic wackes can be divided into two groups with group a derived from 50 mafic intermediate volcanics and 50 felsic volcanics and group b characterized by hree depletion derived almost exclusively from felsic volcanics ree patterns of yellowknife sedimentary rocks are similar to other archean sedimentary ree patterns although they have higher lanybn these patterns differ significantly from typical post archean sedimentary ree patterns supporting the idea that archean exposed crust had a different composition than the present day exposed crust

a deep seismic crustal transect from the ne iberian peninsula to the western mediterranean the crustal structure of the transition from the ne iberian peninsula to the western mediterranean is characterized by large scale variations related to its successive compressional and extensional cenozoic tectonics the spanish esci spanish acronym for seismic studies of the iberian crust program recorded 450 km of deep vertical reflection profiles on land and at sea to complete a first crustal transect from the pyrenees to the south balearic basin highly energetic north dipping reflections imaged by combining the etude continentale et oceanique par reflexion et refraction sismique ecors and esci land profiles mark a crustal thickening toward the pyrenees the reflective band becomes horizontal beneath the catalan coastal ranges and the moho is found at 11s no evidence of crustal thinning affecting the iberian mainland is observed the transition to the mediterranean basin is constrained by complementary onshore offshore recordings forward modeling combined with multichannel processing of the wide angle data set shows that a strong thinning of the continental crust by a factor of 2 over a distance of 50 km affects both the catalan and balearic flanks of the valencia trough this thinning affects mainly the deep crust characterized by low velocities of 6 4 6 5 km s suggesting a removal of part of the lower crust during the rifting processes velocities of 7 8 7 9 km s are interpreted in the uppermost mantle with no evidence of underplating the moho appears as a marked discontinuity throughout the transect the nw part of the balearic promontory is characterized by a thick and reflective crust from 12 to 30 km depth an intensely fracturated evaporitic sequence is seen for 100 km along the south balearic basin the seismic images of the continent ocean transition are disturbed by these prominent sediments however the deep reflectivity suggests a crustal thinning moho at 6 7 s to be related to the middle miocene present extensional tectonics

coincident seismic reflection refraction studies of the continental lithosphere a global review nearly 50 coincident seismic reflection refraction studies to depths of at least the moho provide an improved understanding of the continental lithosphere some conclusions include the following 1 a transparent upper crust a common observation on vertical reflection profiles cannot generally be correlated with velocity gradients or low velocity zones rather a commonly transparent upper crust may be explained by short wavelength steeply dipping features in the brittle upper crust and to a lesser degree by signal contamination from source generated noise 2 the reflective lower crust in extensional terranes appears to be characterized by a high average seismic velocity 6 6 7 3 km s and to consist of laminated high and low velocity layers with typical thicknesses of 100 200 m 3 landward dipping reflectors observed in the middle to lower crusts of convergent zones have been identified as paired high and low velocity slabs which represent oceanic crust and mantle accreted via underplating to the continental margin 4 the crust mantle boundary may differ sufficiently when imaged with vertical incidence and wide angle data to justify the retention for the present of the concept of separate reflection and refraction mohos while there is good evidence that these features are coincident within measurement uncertainties in most regions recently recorded data from the basin and range admit the possibility for noncoincidence in that area 5 upper mantle reflections which cannot be migrated into the lower crust remain rare despite isolated unequivocal examples thus the upper mantle appears to be relatively homogeneous at seismic reflection wavelengths and to lack the laminations inferred for the lower crust the wide angle method will likely provide the most reliable information on the velocity structure and physical state of this portion of the lithosphere for some years to come 6 there appear to be clear and consistent basic differences between convergent and extensional terranes which have been identified from coincident experiments these differences may be sufficiently universal to infer the tectonic history of poorly exposed terranes 7 no truly three dimensional coincident experiment i e including three dimensional migration has been conducted but some three dimensional data have been collected using both methods measurements of attenuation poisson s ratio and anisotropy within the crust using coincident data sets remain frontiers

crustal and velocity structure of the valencia trough western mediterranean part i a combined refraction wide angle reflection and near vertical reflection study abstract in november 1988 a 2 ship seismic survey was carried out in the valencia trough western mediterranean to collect normal incidence seismic reflection profile data and six expanding spread profiles along the continental shelf and trough velocity depth solutions derived from the analysis of the esp show a pattern that corresponds with that of continental type crust except in the northeastern part of the basin where oceanic type crust is present along the iberian margin and centre of the trough the seismic data show a well developed neogene and quaternary sequence with velocities ranging from 1 6 to 4 8 km s 1 whereas along the mallorca margin these deposits are absent at the base of the neogene there is a tertiary unconformity which marks the top of the pre rift mesozoic carbonate platform the carbonates which have velocities from 5 4 to 5 9 km s 1 thin towards the axis of the trough along both margins the upper and middle crust is characterised by an almost transparent layer of variable thickness 6 9 km with velocities ranging from 6 1 to 6 4 km s 1 dipping reflections are observed however in the southern part of the trough these reflections are the result of a locally thick mesozoic sequence a 6 km thick highly reflective lower crust with velocities from 6 4 to 7 0 km s 1 is seen beneath the iberian margin along the mallorca margin the lower crust is variably reflective with a velocity of about 6 6 km s 1 in the centre of the trough the total crustal thickness is less than 14 km which thickens asymmetrically towards the flanks reaching values of 20 21 km off shore the iberian peninsula and 23 km in the mallorca margin the seismic moho increases in depth from 13 to 16 km in a ne sw direction relatively low upper mantle velocities from 7 6 to 8 0 km s 1 have been found throughout the basin indicating a slightly anomalous uppermost mantle or a crust mantle transitional layer in the northeast the velocities for the sediments are similar to those described above however the crustal velocity solution can be characterised by three velocity gradient layers ranging from 4 8 6 1 6 1 6 7 to 6 7 7 2 km s 1

early paleozoic ca 465 ma eclogites from beishan nw china and their bearing on the tectonic evolution of the southern central asian orogenic belt abstract the beishan orogen is situated in the southern margin of the central asian orogenic belt caob it is connected with the tianshan orogen to the west and the mongolia xing anling orogen to the east this orogen exhibits a well preserved neoproterozoic to late paleozoic sequence containing ophiolites accretionary complexes and eclogites in this paper we provide new geochronological data for eclogites and their country paragneiss to constrain the tectonic evolution of the beishan orogen u pb zircon dating of eclogites yielded two concordant ages 886 4 ma for the cores and 465 10 ma for the rims the zircon cores display bright luminescence with homogeneous or planar sector zoning intermediate th u ratios 0 17 0 47 and hree enriched patterns with negative eu anomalies eu eu 0 16 0 58 these features coupled with their formation temperatures of 680 710 c suggest that the core age represents the time of an earlier upper amphibolite facies metamorphic episode by contrast the zircon rims have dark luminescence with homogeneous or fir tree zoning low th u ratios 12 kbar for the eclogite facies metamorphism on the other hand zircon dating of a paragneiss near the eclogite produced two major groups of ages ca 1450 ma for a magmatic event and ca 900 ma for a metamorphic event it seems that eclogites and their country rocks have suffered the same earlier metamorphic episode based on the available age data obtained for eclogites ophiolites and granitoids from the beishan area a tectonic scenario can be constructed as follows the ca 1000 ma oceanic crust representing by protoliths of the eclogites was involved in early neoproterozoic orogenesis to have formed a microcontinental block during the early paleozoic island arcs and microcontinental blocks were progressively accreted to the northern margin of the tarim craton dunhuang block this was followed by the closure of paleoasian ocean and final amalgamation between the tarim craton and kazakhstan block in the late paleozoic

the archean deep marine environment turbidite architecture of the burwash formation slave province northwest territories the slave province is an archean craton that contains an unusually large proportion of preserved sedimentary rock including thick turbidite successions the burwash formation is exposed in the southern slave province between the anton and sleepy dragon basement massifs at the base of the succession volcanics and clastic metasedimentary rocks of the raquette lake formation record initiation of the basin in a rifted arc environment these are overlain by thin black slates representing a transgression followed by well over 5 km of burwash formation metamorphosed turbiditic sandstones and slates interspersed with thin felsic tuff layers lateral correlation within the formation is possible using airphotos and recognizable tuff units burwash formation sandstones include thinly bedded units displaying bouma sequences and thicker bedded units with scour and fill structures and stratification bands characteristic of dense sediment gravity flows the sedimentary rocks are organized in architectural elements that include channel fill sandstones and conglomerates muddy levees interchannel sandstones resembling high amplitude reflection packages harps described from modern fans and possible depositional lobes the overall sedimentary architecture was probably controlled by events in the tectonically active source area or areas the archean turbidites resemble their phanerozoic and modern analogues although they show less voluminous levees and are generally less organized than large modern passive margin fans which probably have no equivalents in the archean

crustal and velocity structure of the valencia trough western mediterranean part ii detailed interpretation of five expanded spread profiles abstract a dense grid of single ship and two ship multichannel seismic profiles has been used to study the crustal structure of the valencia trough western mediterranean five full esps expanded spread profiles and one half esp esp 2 with a total shot receiver range up to 100 km were shot as part of the survey the interpretation of 5 esps is discussed here in detail these esps were analyzed by matching travel time and amplitude variations in both t and p domains the final travel time fit of the synthetic to real data is good enough to be confident that seismic phases have been correctly identified the esps show clear seismic arrivals up to ranges of 100 km middle and lower crustal arrivals and wide angle moho reflections can be seen in all profiles the esps are located on the catalan continental shelf and the mallorca margin esps 6 5 3 and in the trough esps 2 4 7 in the northeast part of the trough esp 2 the crustal velocity structure is of the oceanic type where typical velocities of oceanic layers 2 and 3 are recognised in the centre of the trough the results from esp 4 based only on travel time modelling indicate that a thinned continental crust 14 km thick underlies that part of the trough thickening towards the southwest where the crust is 16 km thick beneath esp 7 in the catalan and balearic margins the total crustal thicknesses are about 20 and 23 km respectively the upper and middle crusts are characterised by velocities ranging from 5 9 6 0 to 6 4 km s whereas in the lower crust velocities vary from 6 4 to 7 0 km s an anomalous low uppermost mantle velocity is found beneath the basin that ranges from 7 6 km s in the catalan margin to 8 0 km s in the southeast part of the trough

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

accretionary and collisional orogenesis in the south domain of the western central asian orogenic belt caob abstract the central asian orogenic belt caob was the result of long lived multi stage tectonic evolution including proterozoic to paleozoic accretion and collision mesozoic intracontinental modification and cenozoic rapid deformation and uplift the accretionary and collisional orogenesis of its early history generated a huge orogenic collage consisting of diverse tectonic units including island arcs ophiolites accretionary prisms seamounts oceanic plateaus and micro continents these incorporated orogenic components preserved valuable detailed information on orogenic process and continental crust growth which make the caob a key region to understanding of continental evolution mantle crust interaction and associated mineralization the western caob refers to the west region in north xinjiang of china and circum balkash of kazakhstan with occurrences of the spectacular kazakhstan orocline and its surrounding mountain belts because orogenic fabrics of this part mostly preserve their original features caused by the interactions among the southern siberian active margin in the north and the tarim craton in the south the western caob can be regarded as an ideal region to study the processes of the accretionary and collisional orogenesis and associated mineralization since a large number of researchers have been working on this region research advances bloom strikingly in a short time period therefore we in this special issue focus on these new study advances on the south domain of the western caob including the kazakhstan collage system tianshan orogenic belt and beishan region and it is anticipated that this issue can draw more attention from the international research groups to be interested in the studies on orogenesis of the caob

neoarchaean banded iron formation within a 2620 ma turbidite dominated deep water basin slave craton nw canada abstract banded iron formations bifs are iron and silica rich chemical metasedimentary rocks that span much of the archaean and early palaeoproterozoic rock record given that bifs precipitated from ambient seawater they provide an important archive of earth s ancient marine environments within the slave craton occurs one of the largest and well preserved achaean turbidite basins in the world interstratified within the greywacke mudstone turbidites is bif that is commonly associated with ferruginous sediments and locally with a felsic tuff bed field investigations were conducted at four bif locations from west to east across the slave craton these bifs are composed of micro and mesobands of quartz and magnetite with various amounts of metamorphic grunerite ferro actinolite ferro hornblende as well as greenalite and stilpnomelane the latter two are interpreted as retrograde metamorphic phases as the bif was deposited in a deep water turbidite dominated basin s the chemical component was occasionally diluted by clastic detritus as illustrated by highly variable al 2 o 3 content and al 2 o 3 tio 2 and cr ni that are similar to the clastic sediments on a layer by layer basis the iron content in the bif ranges from as low as 4 wt and up to 75 wt fe 2 o 3 whereas the sio 2 content ranges from 20 wt to as high as 94 wt this together with a hyperbolic germanium silicon distribution suggests that continentally sourced silica comprised the background precipitation which was periodically interrupted by pulses of hydrothermally sourced iron high temperature hydrothermal fluids contributed with an average shale normalized positive eu anomaly of 1 92 which indicates input of reduced eu 2 to the ambient seawater the bif shows characteristic aqueous fluid derived rare earths yttrium fractionation pr yb 3 to ce 4 the age and trace element signature of the felsic tuff is similar to arc type volcanic rocks potentially linking the bif and turbidite basin s to the defeat suite plutons ca 2630 2620 ma the slow chemical rainout behind bif deposition presumably took place on the basin plain adjacent to a steep submarine ramp where turbidite currents and slump generated debris flows sourced and triggered by the tectonically active arc terrain episodically delivered clastic sediment

mineral deposits in the archean slave structural province lithological and tectonic setting abstract the slave structural province an archean granite greenstone terrane covering 190 000 km 2 in the northwest territories of canada has an extensive history that goes back to earth s oldest preserved rocks it has been subjected to archean and proterozoic orogenies the former built the craton the latter destroyed or reworked large areas reducing the slave to its present size a mounting body of data suggests that primitive sialic crust had a major influence on the evolution of most slave rock types supracrustals of the slave province comprise abundant greywacke mudstone 80 and subordinate volcanics 15 that form two distinct types of volcanic belt yellowknife type dominated by tholeiitic basalts and hackett river type dominated by felsic intermediate calc alkaline pyroclastics slave volcanogenic massive sulphides have high silver lead zinc contents rare element pegmatites are associated with highly evolved granitoids in four widespread parts of the province and gold deposit types define zones trending northeasterly across the craton conglomerates quartz sandstones and quartz arenites formed in a shelf environment define a northerly trending zone locally coincident with a long lived fault and a major lead isotope boundary which together demarcate a western part of the slave containing old basement tectonic models for the slave comprise two types those that attempt to explain its formation in plate tectonic terms and those that suggest formation by rifting of primitive sialic crust with volcanics and granitoids derived largely by melting of that crust many features of the slave seem more easily explained by a rift related model rather than by plate tectonics

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

rare earth element systematics of the chemically precipitated component in early precambrian iron formations and the evolution of the terrestrial atmosphere hydrosphere lithosphere system irrespective of provenance age and metamorphic grade the chemically precipitated component in early precambrian 2 3 ga iron formations ifs displays smyb cn 1 which reflects the corresponding ratios of contemporaneous seawater in conjunction with end if end shale jacobsen and pimentel klose 1988a b this rare earth element ree signature reveals that the ree distribution in early precambrian ifs must be explained by mixing between a marine bottom and a surface water component and that the rees and by analogy the fe cannot be derived from weathering of a continental source eusm n ratios of detritus free ifs are controlled by the marine bottom water component where eusm cn 1 results from ree input from high temperature high t hydrothermal fluids which received their ree signature during high t alteration of oceanic crust at spreading centres decreasing eusm cn of ifs with decreasing depositional age reflect the vaning importance of high t compared to low t 200 c hydrothermal fluids hence oceanic crust alteration for the ree budget of marine bottom waters and are related to decreasing upper mantle temperatures smyb n ratios of detritus free ifs are controlled by the marine surface water component whose ree distribution is affected by the same processes which operate today in the entire ocean mixing calculations reveal that smyb cn in early precambrian marine surface waters was significantly lower than it is today to explain this difference two mechanisms are discussed on the basis of higher pco2 and lower po2 levels of the precambrian atmosphere 1 1 higher co32 in precambrian river and coastal waters and 2 2 redox cycling of the rees at an oceanwide chemocline which separated anoxic and oxic water masses however these processes are synergetic and could have operated in unison the ree distribution in precambrian ifs is described as a result of mixing in a multicomponent system where high t and low t hydrothermal fluids contributed to the marine bottom water and ree input from the dissolved ree pool in river waters after some modification in estuaries dominated the ree distribution in marine surface waters after mixing of bottom and surface waters their combined ree load was quantitatively scavenged onto precipitating iron oxihydroxides or iron carbonates and deposited with the chemically precipitated component after addition of variable amounts of clastic detritus this eventually resulted in the ree distribution observed in precambrian ifs

joint inversion of local regional and teleseismic data for crustal thickness in the eurasia africa plate boundary region summary a new map for the moho discontinuity eam02 in the eurasia africa plate boundary region is presented reliable results have also been obtained for the southern and eastern mediterranean basin the northern african coasts and the eastern atlantic ocean regions only occasionally considered in studies on the mediterranean region the moho topography model is derived from two independent sets of constraints information contained in the fundamental and highermode rayleigh waves obtained from waveform modelling is used to constrain the moho depth between estimates of crustal thickness taken from published reflection and refraction surveys gravity studies and receiver function analysis strong lateral variations in the moho topography have been observed in the mediterranean sea confirming the complex evolution of this plate boundary region in the west the moho discontinuity has been found at 15 20 km depth suggesting extended and at least in some locations oceanic crust while in the east the crust is on average 25 30 km thick there it is interpreted either as mesozoic oceanic or thinned precambrian continental crust covered by thick sedimentary deposits standard continental crust 30 35 km is observed along the eastern part of the northern african coast while to the west a rapid change from a relatively deep moho down to 42 km below the atlas mountain range to the thin crust of the southwestern mediterranean sea has been found the crust beneath the eastern north atlantic ocean can be up to 5 km thicker compared with standard oceanic crust 6 km the crust has been interpreted to be heterogeneous as a consequence of irregular magma supply at the mid atlantic ridge in addition serpentinization of the submoho mantle could contribute to the imaging of apparently anomalous thick oceanic crust in europe the presence of crustal roots 45 km beneath the major mountain belts has been confirmed while thin crust 25 km has been found beneath extensional basins comparing the obtained moho topography and moho depth computed assuming isostatic compensation at 60 km depth shows that most of the mediterranean and eastern atlantic region appears to be in isostatic equilibrium the large positive residuals observed for the eastern mediterranean are likely to be due to overestimating crustal thickness owing to the thick sediment deposits present

progressive accretionary tectonics of the beishan orogenic collage southern altaids insights from zircon u pb and hf isotopic data of high grade complexes abstract high grade gneissic rocks of the southern altaids in central asia bear one of the key elements needed to solve a basic architectural controversy in accretionary orogenesis high grade gneissic rocks widely crop out across the beishan orogenic collage southern altaids mapped as the beishan complex or dunhuang group which was previously regarded as neoarchean to paleoproterozoic crystalline basement but without systematic field geochronological and geochemical data this study provides field geochemical and zircon u pb and hf isotopic data for gneissic rocks from the beishan complex geochemical data show that the gneissic rocks were probably formed in a subduction environment most zircons from the beishan complex are magmatic origins because their th u ratios are mostly in the range of 1 0 0 1 they show age peaks at 494 ma 464 ma and 375 ma with a few precambrian ages zircon hf isotopic compositions indicate that both old and juvenile materials were involved in the formation of the beishan complex which is different from the basement rocks of the tarim craton both in radiometric age and hf isotopic composition all these data thus do not support a uniform precambrian basement in the beishan area as previously proposed but instead the beishan complex probably formed in a subduction accretion environment combined with regional geology and geochemical data we conclude that the beishan orogenic collage was characterized by progressive long lived accretionary tectonics during late precambrian to paleozoic time therefore shedding light on the controversy regarding the architecture of accretionary orogens

u pb ages of detrital zircon of the paleozoic sedimentary rocks new constraints on the emplacement time of the hegenshan ophiolite ne china abstract the hegenshan ophiolite in the solonker hegenshan belt is the largest ophiolite in the central asian orogenic belt caob despite its significance in constraining regional tectonic evolution the emplacement time of the hegenshan ophiolite is still under debate in this study we provide new detrital zircon ages of the paleozoic sedimentary rocks that unconformably overlie the wusinihei ophiolite northeastern part of the hegenshan ophiolite to constrain the lower limit emplacement time of the hegenshan ophiolite the zircon ages obtained from the paleozoic sedimentary rocks range from 298 8 to 363 7 ma and show bimodal distribution at 300 320 ma peak at 308 ma and 320 360 ma peak at 330 ma the age group of 300 320 ma coincides with the age range of the volcanic rocks of the late paleozoic gegenaobao formation the age group of 320 360 ma with a peak at 330 ma may be linked to local mafic ultramafic rocks of the hegenshan ophiolite accordingly we suggest that the emplacement time of the hegenshan ophiolite should have occurred earlier than the deposition of the gegenaobao formation most likely during the time between 308 and 330 ma instead of the silurian devonian or mesozoic as previously considered

deep crustal configuration of the valencia trough and its iberian and balearic borders from extensive refraction and wide angle reflection seismic profiling abstract new constraints on the crustal structure of the valencia trough and its iberian and balearic borders have been obtained from a seismic experiment carried out in autumn 1989 a total of 290 sea shots were fired along three refraction wide angle reflection profiles recorded by 10 ocean bottom seismometers obss in the valencia trough and 110 land stations distributed on both flanks the huge amount of recorded data in each profile has been integrated and jointly interpreted to obtain a single 2 d velocity depth model along each line the geometry and deep crustal structure of the basin are fairly well defined in terms of laterally inhomogeneous layered media the sedimentary cover shows a layer of consolidated sediments overlain by neogene and quaternary sediments with a velocity of 5 1 km s interpreted as mesozoic carbonates at the northwestern side of the valencia trough a fairly homogeneous crystalline basement is found on all profiles at a mean depth of 4 km below sea level with a velocity of 6 0 6 1 km s the observed critical distances and arrival times of the moho reflected waves are firm evidence supporting a thinning of the crust towards the centre of the basin the moho rises up to 13 km in the axis of the trough in the nw se and e w profiles and up to 15 km in the ne sw profile moreover the analysis of the well defined pmp phase shows differences between the deep structure of the flanks and the trough respectively a sharp velocity contrast at the moho from a fairly uniform lower crust of 6 4 km s velocity is proposed in both flanks in contrast to a high velocity gradient within the lower crust interpreted near the axis of the trough an anomalous upper mantle velocity of 7 8 km s fits the data throughout the valencia trough and mallorca with slightly higher values of 7 9 km s required towards the continent these velocity values do not favour an interpretation in terms of underplating the geometry of the moho discontinuity shows a gradual deepening towards the iberian peninsula in contrast to a more abrupt slope towards mallorca depth values of 30 32 km typical of a continental crust are inferred about 40 km inland on the iberian peninsula in agreement with the crustal thicknesses already reported for the iberian range and the ebro basin the present crustal thinning resulting from rift episodes in the valencia trough and the iberian border is thus confirmed and constrained from our seismic results of the deep crust and upper mantle

fluid inclusion and noble gas studies of the dongping gold deposit hebei province china a mantle connection for mineralization the dongping gold deposit 100 t au occurs about 200 km inboard of the northern margin of the north china craton the deposit is mainly hosted by syenite of a middle paleozoic alkalic intrusive complex that was emplaced into late archean basement rocks both groups of rocks are intruded by late jurassic to early cretaceous crustal melt granite dikes and stocks some within a few kilometers of the deposit the gold ores were deposited during this latter magmatic period at about 150 ma a time that was characterized by widespread regional north south compression that formed the east west trending yanshan deformational belt the ores include both the telluride mineral bearing low sulfide quartz veins and the highly k feldspar altered syenite with most of the resource concentrated in two orebodies 1 and 70 fluid inclusion microthermometry indicates heterogeneous trapping of low salinity e g 5 7 wt nacl equiv fluids that varied from a few to 60 mole percent nonaqueous volatile species laser raman spectroscopy confirms that the vapor phase in these inclusions is dominated by co2 but may be comprised of as much as 9 mole percent h2s and 20 mole percent n2 methane concentrations in the vapor phase are consistently 1 mole percent the variable phase ratios are consistent with fluid immiscibility during ore formation fluid inclusion trapping conditions are estimated to be 250 to 375 c and 0 6 to 1 0 kbar helium isotope studies of fluid inclusions in ore stage pyrites indicate 3he 4he ratios of 2 1 to 5 2 ra ra 1 4 10 6 for air for orebody 1 and 0 3 to 0 8 ra for orebody 70 the former data suggest that at least 26 to 65 percent mantle helium occurs in the fluids that deposited the veins in orebody 1 the lower values for orebody 70 which is characterized by a more disseminated style of gold mineralization are interpreted to reflect an increased interaction of ore fluids with surrounding crustal rocks which may have contributed additional 4he to the fluids a mantle source for at least some of the components of the gold forming fluid is consistent with upwelling of hot asthenosphere and erosion of as much as 100 to 150 km of cool archean lithosphere beneath the craton during this time the dongping deposit is located along the 100 km wide north south gravity lineament which marks the western border of the thinned crust as both regional metamorphism of mesoproterozoic and younger cover rocks and widespread granite magmatism also occurred at ca 150 ma it is unclear as to whether one or both of these also contributed fluid and or metals to the hydrothermal system importantly these new data suggest that economically significant gold deposits of similar mineral style and fluid composition which are scattered along the margins of the craton may all be products of a fluid originally partly sourced within the mantle

carboniferous granitic plutons from the northern margin of the north china block implications for a late palaeozoic active continental margin we report four late palaeozoic zircon sensitive high resolution ion microprobe shrimp u pb ages for granitic plutons from the inner mongolia palaeo uplift on the northern margin of the north china block these cast a new light on the poorly understood tectonic history of the northern margin of the north china block and the central asian orogenic belt during the late palaeozoic the plutons have for a long time been considered to belong to the early precambrian basement of the north china block our new shrimp u pb zircon dating of four plutons at longhua daguangding boluonuo and hushiha has yielded intrusive ages of 311 2 ma 324 6 ma 302 4 ma and 310 5 ma respectively geochemical data suggest that these granitoids have a calc alkaline subduction related i type signature indicating the existence of an andean style continental arc along the northern margin of the north china block during the late palaeozoic our results also indicate that the palaeo asian ocean still existed during latest carboniferous earliest permian time and that the final collision between the southern mongolia composite terranes and the north china block occurred later than c 290 ma we suggest that the northern margin of the north china block was an active continental margin and the inner mongolia palaeo uplift is a deeply exhumed mid crustal root of a late palaeozoic andean style continental arc

early paleozoic to devonian multiple accretionary model for the qilian shan nw china abstract the qilian shan located in the northern tibetan plateau of nw china has an excellent record of early to middle paleozoic subduction accretion that resulted from convergence between the alax and qaidam blocks but there is no consensus about its detailed tectonic history this paper summarizes its tectonic divisions and discusses its tectonic evolution from the cambrian to devonian the belt has the following tectonic divisions in the far north the southern passive margin of the alax block is juxtaposed against an early cambrian to ordovician marianan type intra oceanic arc north qilian the north qilian arc is separated from a japanese type arc central qilian to the south by a high pressure metamorphic belt composed mostly of ophiolitic slices and oceanic crusts the central qilian is bounded to the south by a wide melange zone south qilian consisting of ophiolitic slices and continental margin sequences farther to the south lies the oulongbuluk microcontinent that is separated from the qaidam block and farther south by an ultrahigh pressure metamorphic belt tectonostratigraphic analysis together with geochemical geochronological and geophysical data indicates a complex evolution by subduction accretion processes from the cambrian to the devonian before final amalgamation and docking to the northern alax block this model solves the long lasting discussion on the polarity of subduction in paleozoic time this multiple subduction accretion history sheds light on the continuity of paleozoic sutures along the qilian shan and the nature of the altyn tagh fault and thus contributes to an improved understanding of the tectonic architecture of central asia

image of the moho across the continent ocean transition u s east coast strong wide angle reflections from the moho were recorded by ocean bottom seismic instruments during the 1988 carolina trough multichannel seismic experiment in an area where the moho is difficult to detect with vertical incidence seismic data prestack depth migration of these reflections has enabled the construction of a seismic image of the moho across the continent ocean transition of a sedimented passive margin the moho rises across the margin at a slope of 10 12 from a depth of about 33 km beneath the continental shelf to 20 km beneath the outer rise this zone of crustal thinning defines a distinct 60 70 km wide continent ocean transition zone we interpret the moho in the carolina trough as a jurassic feature formed by magmatic intrusion and underplating during the rifting of pangea

the dunhuang block is a paleozoic orogenic belt and part of the central asian orogenic belt caob nw china abstract the dunhuang region is located in the east of the tarim craton and has long been considered as part of a precambrian terrane of the tarim or north china craton aiming to further understand the tectonic evolution of the dunhuang region and its relations with the adjacent tectonic units this study is to investigate the metamafic rocks in the sanweishan area dunhuang region and in turn to summarize the tectono thermal events and re examine the tectonic characteristics of the region the metamafic rocks in the sanweishan area include three types of amphibolites they are garnet clinopyroxene amphibolites garnet free amphibolites and garnet mica amphibolites the whole rock major and trace element compositions suggest that their protoliths probably are e morb type tholeiitic basalts with some crustal addition the la icp ms zircon u pb data of the garnet clinopyroxene amphibolite and the garnet free amphibolite yielded lower intercept ages of 410 11 ma and 403 8 ma respectively while the garnet mica amphibolite gave three metamorphic ages of 441 3 ma 408 12 ma and 370 2 ma three suites of metamorphic conditions have been estimated at 1 14 1 55 gpa and 458 548 c 0 67 0 82 gpa and 590 600 c and 0 47 0 66 gpa and 557 586 c for garnet clinopyroxene amphibolite using different mineral thermobarometers which illustrate a collisional orogeny related clockwise p t path integrated with the study on the high pressure granulite in the mogutai area it can be inferred that the metamafic rocks in the sanweishan area obviously experienced amphibolite facies metamorphism at ca 410 400 ma and they could also underwent high grade metamorphism at ca 440 430 ma therefore we suggest that the e w trending collisional orogeny of ca 440 400 ma could sweep cross the dunhuang region the dunhuang region records multiple tectono thermal events occurring at ca 3 1 2 5 ga ca 2 3 1 6 ga ca 440 400 ma and ca 370 310 ma however compared with the limited exposure of the precambrian complex the early silurian late carboniferous ca 440 310 ma orogeny related metamorphic and magmatic rocks are widely distributed in the dunhuang region making up nearly 70 of the total exposure thus it is proposed that the geological bodies in the dunhuang region were mainly produced by a paleozoic orogeny and represent a paleozoic orogen while the limited precambrian complex probably is a microcontinental fragment or part of the margin of the tarim or north china cratons that was involved in this paleozoic orogeny during the early silurian late carboniferous we infer that the dunhuang orogenic belt is part of the central asian orogenic belt caob and thus the caob probably extends beyond the beishan orogenic collage and southward into the dunhuang region

crustal structure beneath the iberian peninsula and surrounding waters a new compilation of deep seismic sounding results abstract we present here an updated compilation of crustal parameters beneath the iberian peninsula and surrounding waters inferred from extensive deep seismic exploration performed in the last three decades firstly the more relevant experiments developed in both terrestrial and marine domains are revisited together with the corresponding seismic velocity depth models published that reveal the characteristics of the different tectonic domains sampled including oceanic zones continental margins orogenic and rift belts the results are summarized in three crustal transects sketched along representative directions secondly we compiled the moho depths along all the available seismic profiles to produce a geo referred database this database has then been interpolated using kriging algorithms to obtain a continuous moho depth model in the form of a regular grid file this integrative moho map for a key area in the western mediterranean documenting the interaction between european and african plates can contribute to constrain further regional studies either at crustal lithospheric or mantle scales

slowing extrusion tectonics lowered estimate of post early miocene slip rate for the altyn tagh fault abstract determination of long term slip rate for the altyn tagh fault is essential for testing whether asian tectonics is dominated by lateral extrusion or distributed crustal shortening previous slip history studies focused on either quaternary slip rate measurements or pre early miocene total offset estimates and do not allow a clear distinction between rates based on the two the magmatic and metamorphic history revealed by shrimp zircon dating of clasts from miocene conglomerate in the xorkol basin north of the altyn tagh fault strikingly matches that of basement in the southern qilian shan and northern qaidam regions south of the fault this match requires that the post early miocene long term slip rate along the altyn tagh fault cannot exceed 10 mm year supporting the hypothesis of distributed crustal thickening for post early miocene times this low long term slip rate and recently documented large pre early miocene cumulative offset across the fault support a two stage evolution wherein asian tectonics was dominated by lateral extrusion before the end of early miocene and since then has been dominated by distributed crustal thickening and rapid plateau uplift

multichannel seismic image of the crustal thinning at the ne iberian margin combining normal and wide angle reflection data the seismic image of the moho at the transition from the ne iberian peninsula to the western mediterranean sea is investigated using coincident steep and wide angle reflection data merged into a single stacked and migrated section the multichannel analysis of onshore offshore large aperture data provides new insight on the deep structure in areas where the steep sections lack penetration and reveals that the crust undergoes a strong but continuous thinning along the flanks of the valencia trough the combined seismic image indicates that almost half of the continental crust 14 15 km is lost in less than 60 km horizontal distance

river fluxes of dissolved silica to the ocean were higher during glacials ge si in diatoms rivers and oceans two centric marine diatom species thalassiosira oceanica and thalassiosira antarctica were grown in batch cultures to determine the incorporation of germanium ge and silicon si into siliceous shells opal the results were modeled as ge si isotope fractionation during exponential growth diatoms take up and incorporate ge si from solution without major discrimination against ge during stationary phase growth near silica limitation the antarctic species t antarctica discriminates slightly against ge but integrated ge si opal produced over the latter portion of the growth cycle is indistinguishable from the initial solution ratio these results confirm experiments using radioactive 68ge that showed absence of fractionation during diatom silica uptake azam and volcani 1981 in contrast to two box ocean models that invoke 50 ge discrimination by diatoms to explain the observed excess surface ocean germanium concentration murnane and stallard 1988 froelich et al 1989 and late pleistocene ocean sediment ge si opal records mortlock et al 1991 runs of a 10 box ocean ge and si model pandora with 50 discrimination reproduce the excess surface ocean ge but introduces curvature into the deep ocean ge versus si relationship that is not observed in the oceans thus 50 fractionation is not supported by either cultures or models if diatoms do not fractionate ge si then late pleistocene ge si opal variations in piston cores are caused not by changes in local biosiliceous production and silica utilization mortlock et al 1991 but rather by whole ocean changes in ge si seawater the marine ge si opal record of the last 450 kyr can be modeled as transient oceanic responses to instantaneous continental climate transitions consistent with the chemical weathering model of murnane and stallard 1990 glacial periods are characterized by lower continental weathering intensity lower ge si riv and two fold higher dissolved silica river fluxes marine ge si opal records thus contain a history of ocean silica chemistry that reflect rapid global changes in continental weathering

crustal structure of an intraplate thrust belt the iberian chain revealed by wide angle seismic magnetotelluric soundings and gravity data abstract the iberian chain is a cenozoic intraplate thrust belt located within the iberian plate unlike other belts in the iberia peninsula the scarcity of geophysical studies in this area results in a number of unknowns about its crustal structure the iberian chain crust was investigated by means of a ne sw refraction wide angle reflection seismic transect and two magnetotelluric profiles across the chain oriented along the same direction the seismic profile was designed to sample the crust by means of three shots designed to obtain a reversed profile the resulting velocity depth model shows a moderate thickening of the crust toward the central part of the profile where crustal thickness reaches values above 40 km thinning toward de sw tajo and ne ebro foreland basins the crustal thickening is concentrated in the upper crust the seismic results are in overall agreement with regional trends of bouguer gravity anomaly and the main features of the seismic model were reproduced by gravity modeling the magnetotelluric data consist of 39 sites grouped into two profiles with periods ranging from 0 01 s to 1000 s dimensionality analyses show significant 3d effects in the resistivity structure and therefore we carried out a joint 3d inversion of the full impedance tensor and magnetic transfer functions the mesozoic and cenozoic basins along the chain are well characterized by shallow high conductive zones and low velocities elongated conductors reaching mid crustal depths evidence the presence of major faults dominating the crustal structure the results from the interpretation of these complementary geophysical data sets provided the first images of the crustal structure of the iberian chain they are consistent with a cenozoic shortening responsible of the upper crust thickening as well as of the uplift of the iberian chain and the generation of its present day topography

nitrogen isotope compositions of the upper triassic chang 7 shale ordos basin north china implications for depositional redox conditions abstract nitrogen isotope 15n analysis has been used to evaluate depositional redox conditions in well preserved sedimentary systems however fewer n isotope studies have been performed to reconstruct the redox conditions in lacustrine shales in this paper we report the 15n data from the upper triassic chang 7 shale of the ordos basin north china sedimentary 15n values are significantly higher in the chang 73 and the lower part of the chang 72 submembers zone a average 9 4 1 3 than in the upper part of the chang 72 and the chang 71 submembers zone b average 5 4 1 5 previous geochemical measurements i e u and toc p of both zones show wide redox variation however the average values of these redox proxies are relatively higher in zone a than those of zone b combined with lithological aspects of the chang 7 shale our study suggest that zone a was mainly deposited under suboxic bottom water conditions whereas zone b deposits accumulated within an oxygenated water column stable organic carbon isotopes 13corg and total nitrogen tn values of the two zones display little variation that could be attributed to changing organic matter sources and or post depositional alteration of 15n thus we suggest that variations in 15n throughout the chang 7 shale primarily reflect differences in depositional redox conditions and 15n values of the shale that can provide important details regarding the depositional history of unconventional resource plays

moho and lower crustal reflectivity beneath a young rift basin results from a two ship wide aperture seismic reflection experiment in the valencia trough western mediterranean summary we present new images of the lower crust and moho beneath the valencia trough a young rift basin in the western mediterranean these images were obtained from a two ship wide aperture reflection experiment and show several features not distinguishable on previously available conventional single ship reflection profiles the moho which was previously only seen intermittently can now be confidently traced throughout the basin we have constructed a present day depth to moho map and estimated the degree of crustal thinning for the whole basin crustal thinning is at a maximum in the centre of the basin where values reach 3 15 0 25 at the margins of the basin the value decreases to 1 5 0 1 the reflective character of the lower crust and moho is different beneath different parts of the basin we have been able to correlate these differences with the amount of stretching we therefore interpret the variations of the observed lower crustal reflectivity as having been caused by the most recent neogene stretching event that opened the valencia trough along the iberian margin there is well developed lower crustal reflectivity consisting of 1 2s two way time twt of 1 4 km long near horizontal reflectors underlain by a more continuous although not significantly stronger reflector interpreted to be the reflection moho offshore this lower crustal reflective unit thins rapidly such that it is undetectable 40 50 km from the coastline where the crust has been stretched by a factor of 1 7 0 1 as the lower crustal reflectivity becomes undetectable the reflection moho becomes a robust continuous event where exceeds 2 4 0 2 however the moho is a weak event and difficult to trace we infer that either the extension itself or associated melting significantly weakened or even destroyed the lower crustal reflectivity in the centre of the basin and enhanced the moho where extension was moderate the balearic margin is somewhat anomalous in that there appears to have been flexural loading of the crust due to thrusting and folding that occurred at the same time as extension in the valencia trough the lower crust shows evidence of weak but locally variable lower crustal reflectivity it is possible that the lower crustal reflectivity was preserved simply because the moho was flexed downward and so decompression and hence melting of the upper mantle was restricted this suggests that the melting itself rather than the extension is the primary mechanism of lower crustal modification

the origin of early archean banded iron formations and of continental crust isua southern west greenland abstract a long standing geological debate is whether the fe component of precambrian banded iron formations bifs was derived from continental sources or had its origin in hydrothermal alteration of the oceanic crust recent ree and nd and fe isotope studies of iron rich layers in bifs have provided strong supporting evidence for the hydrothermal origin of the fe component pillow basalts from the 3800 3700 ma isua greenstone belt west greenland underwent significant enrichment of fe up to 21 wt fe2o3 and some chalcophile elements e g pb 5 4 7 2 ppm prior to a late archean tectonothermal metamorphic event recorded in the region published and new pb pb isotopic data from the isua pillow basalts and the spatially associated bifs suggest that the enrichment of fe and pb and deposition of the bifs took place between 3800 and 3600 ma similar pb isotopic compositions in pillow basalts and bifs are consistent with a common source of pb for both supracrustal rocks iron and pb were probably scavenged from the oceanic crust and were re deposited on the ocean floor by hydrothermal fluids suggesting that high temperature hydrothermal alteration of early archean oceanic crust played an important role in the deposition of the isua bifs geochemical lithological and structural data from the isua greenstone belt are all collectively consistent with a convergent margin geodynamic setting the isua bifs are spatially and temporally associated with boninitic and island arc picritic pillow basalts given the observations that cenozoic boninites and picrites tend to form in an intra oceanic arc forearc setting we suggest that the isua bif boninite and bif picrite associations were also deposited in a similar geodynamic setting the original pillow basalt bif stratigraphy was dismembered and transposed throughout the belt during early to late archean tectonothermal events the high temperature hydrothermal alteration of the oceanic crust produced significant hydrothermal discharge with large quantities of fe necessary to contribute to the deposition of the isua bifs the hydrothermal alteration may have resulted from the opening of an asthenospheric window developed as a consequence of ridge subduction beneath an early archean arc forearc region the ridge subduction model can also explain the origin of the contemporaneous tonalite trondhjemite granodiorite ttg intrusions in the isua region partial melting of laterally accreted and thickened oceanic crust under amphibolite to eclogite metamorphic conditions by upwelling of asthenospheric windows may have produced ttg melts the diapirically upwelling ttg melts formed the early archean continental crust and the complementary eclogitic residues contributed to the sub continental lithospheric mantle

the valencia trough western mediterranean an overview abstract the valencia trough located between the spanish mainland and the balearic islands corresponds to a late oligocene to recent sedimentary basin that is characterized by a highly attenuated continental crust its northwestern part evolved in response to a late oligocene to early miocene rifting phase that was followed by a period of post rift subsidence and a late miocene pleistocene phase of renewed crustal extension and magmatism its southeastern part is characterized by late oligocene to mid miocene compressional deformation and subsequent extension in the axial parts of the basin the crust has been thinned by a factor of 3 whereby the lower crust is either absent or very thin upper crustal extension appears to be considerably smaller than indicated by the crustal configuration of the basin a major problem in understanding the evolution of the valencia trough is the lack of knowledge about the physical state and structure of the lithosphere before late oligocene extension evolution of the northwestern part of the basin appears to be linked with the development of the oceanic provencal basin middle miocene compressional deformation of the balearic fold belt forming the southeastern margin of the valencia trough appears to have interfered with the tensional evolution of the latter which only resumed after crustal shortening in the balearic domain had given way to extensional tectonics during the late miocene

the seismic moho structure of shatsky rise oceanic plateau northwest pacific ocean abstract oceanic plateaus are large igneous provinces formed by extraordinary eruptions that create thick oceanic crust whose structure is poorly known owing to the lack of deep penetration seismic data multichannel seismic mcs reflection and wide angle refraction data allow us to show moho structure beneath a large part of the shatsky rise oceanic plateau in the northwest pacific ocean moho reflectors in the two data sets can be connected to trace the interface from the adjacent abyssal plain across much of the interior the reflectors display varied character in continuity shape and amplitude similar to characteristics reported in other locations beneath normal crust the moho is observed at 13 km depth 7 km below the seafloor in mcs data and disappears at 20 km depth 17 km below the seafloor beneath the high plateau moho at the distal flanks dips downward towards the center with slopes of 0 5 1 increasing to 3 5 at the middle flanks seismic moho topography is consistent with airy isostasy confirming this widely applied assumption data from this study show that crustal thickness between the massifs in the interior of the plateau is nearly twice normal crustal thickness despite the fact that this crust records apparently normal seafloor spreading magnetic lineations the moho model allows improved estimates of plateau area 5 33 10 5 km 2 and volume 6 90 10 6 km 3 the latter assuming that the entire crust was formed by shatsky rise volcanism because the massifs formed at spreading ridges this study is unique in showing moho depth and structure over an extraordinarily large area beneath an oceanic plateau giving insight to plateau structure and formation

subduction related metasomatic mantle source in the eastern central asian orogenic belt evidence from amphibolites in the xilingol complex inner mongolia china abstract the central asian orogenic belt caob formed mainly in the paleozoic due to the closure of the paleo asian oceanic basins and accompanying prolonged accretion of pelagic sediments oceanic crust magmatic arcs and precambrian terranes the timing of subduction accretion processes and closure of the paleo asian ocean has long been controversial and is addressed in a geochemical and isotopic investigation of mafic rocks which can yield important insight into the geodynamics of subduction zone environments the xilingol complex located on the northern subduction accretion zone of the caob mainly comprises strongly deformed quartzo feldspathic gneisses with intercalated lenticular or quasi lamellar amphibolite bodies an integrated study of the petrology geochemistry and geochronology of a suite of amphibolites from the complex constrains the nature of the mantle source and the tectono metamorphic events in the belt the protoliths of these amphibolites are gabbros and gabbroic diorites that intruded at ca 340 321 ma with positive ehf t values ranging from 2 89 to 12 98 their t dm1 model ages range from 455 to 855 ma and peak at 617 ma suggesting that these mafic rocks are derived from a depleted continental lithospheric mantle the primitive magma was generated by variable degrees of partial melting of spinel bearing peridotites fractionation of olivine clinopyroxene and hornblende has played a dominant role during magma differentiation with little or no crustal contamination the mafic rocks are derived from a late neoproterozoic depleted mantle source that was subsequently enriched by melts affected by slab derived fluids and sediments or melts with a sedimentary source rock the carboniferous mafic rocks in the northern accretionary zone of the caob record a regional extensional event after the early paleozoic subduction of the paleo asian ocean both addition of mantle derived magmas and recycling of oceanic crust played key roles in significant late carboniferous ca 340 309 ma vertical crustal growth in the caob amphibolite facies metamorphism p 0 34 0 52 gpa t 675 708 c affected these mafic rocks in the xilingol complex at ca 306 296 ma which may be related to the crustal thickening by northward subduction of a forearc oceanic crust beneath the southern margin of the south mongolian microcontinent the final formation of the solonker zone may have lasted until ca 228 ma

crustal structure beneath broad band seismic stations in the mediterranean region summary we have analysed receiver functions to derive simple models for crustal structure below 12 broad band seismological stations from the midsea project and 5 permanent broad band stations in the mediterranean region including northern africa to determine an accurate moho depth we have reduced the trade off between crustal velocities and discontinuity depth using a new grid search method which is an extension of recently published methods to determine crustal thickness in this method the best fitting synthetic receiver function containing both the direct conversion and the reverberated phases is identified on a model grid of varying moho depth and varying poisson s ratio the values we found for moho depth range from around 20 km for intra oceanic islands and extended continental margins to near 45 km in regions where the eurasian and african continents have collided more detailed waveform modelling shows that all receiver functions can be well fit using a 2 or 3 layer model containing a sedimentary layer and or a mid crustal discontinuity on comparing our results with moho maps inferred from interpolated reflection and refraction data we find that for some regions the agreement between our receiver function analysis and existing moho maps is very good while for other regions our observations deviate from the interpolated map values and extend beyond the geographic bounds of these maps

how did the alxa block respond to the indo eurasian collision the cenozoic deformation of the alxa block resulted directly from the evolution of the northern qinghai tibetan plateau however many data show that the deformation occurred only in the middle late miocene our studies show that the altyn tagh fault did not pass through the alxa block on the contrary it went along the southern boundary of the jintai huahai basin linking with the helishan southern longshoushan fault due to important tectonic events in the northern qinghai tibetan plateau during the middle late miocene time the northern plateau underwent rapid uplift and the plateau compressed the hexi corridor region resulting in a change from ns trending to ew trending structures in the jinta huahai basin and in the development of compressive structures in the beishan the southern alxa fault underwent right lateral movement and in the northern and central parts of the block ns trending tertiary extensional structures formed these basins controlled by tertiary faults are similar to basins developed by lateral extrusion with a strong foreland and weak limited boundaries the authors suggest that a regional conjugate fault system resulted from nearly ns trending compression from the qinghai tibetan plateau during the miocene and pliocene in the alxa block and southern mongolia and due to the control of early structures in these regions most brittle faults reactivated earlier ductile faults nw se faults along the altai mountain and ne sw faults to the southeast in mongolia consist of a conjugate fault system to the north the altyn tagh fault and southern helishan longshoushan fault comprise a conjugate fault system to the south the beishan and jinta huahai basin occupied the convergent area between these two sets of faults the compression controlled the tertiary deposition and led to the development of the cenozoic jinta huahai basin the alxa block bounded by these two sets of faults moved eastwards which resulted in the development of cenozoic compressive structures to the west of helan shan and superimposed early ductile shear zones along the northeastern and southwestern boundaries of the alxa block respectively this model could explain the cenozoic deformation occurring in and around the alxa region

lateral variations in the deep crustal structure at the iberian margin of the valencia trough imaged from seismic reflection methods abstract new seismic images of crustal structure have been obtained in an area of western europe where attenuation of the continental crust by a factor of two has been reported previously the deep lateral variations in velocity and reflectivity at the transition between the northeastern iberian peninsula and the valencia trough are investigated with three seismic reflection data sets first results of the spanish deep vertical reflection esci profile on the iberian mainland extend the crustal image of the ecors pyrenees profile to the mediterranean coast a highly reflective zone about 15 km thick is imaged in the lower crust beneath the ebro basin and the catalan coastal ranges where the moho is located at 29 km depth this deep reflective pattern appears to be disrupted by the effects of low velocity sediments in the neogene reus basin but continuity of the principal horizons is inferred from illuminations of the area using sources and receivers at wide angle offsets high quality recordings up to 50 km inland of air gun shots from the marine esci profile provide a complementary image of the structure on land and show that the major crustal thinning toward the valencia trough starts near the shoreline the lateral variability of the crust is investigated using a second profile displaced 30 km to the northeast independent refraction and reflection interpretation techniques applied to this profile which consisted of densely spaced shots resulted in consistent depth sections with crustal thinning beginning further inland thinning of 10 km takes place within 60 km at the catalan margin moho reflected waves pmp have been processed as far traces of a deep reflection profile the processing included common midpoint cmp sorting stacking and migration the inferred lateral variations in deep crustal structure near the catalan coast are consistent with the bouguer gravity field they may be related to episodes of extensional block faulting that have affected the catalan valencian domain since paleogene times

oxygen overshoot and recovery during the early paleoproterozoic abstract during the lomagundi event ca 2 22 to 2 06 ga marine carbonates recorded the largest and longest uninterrupted positive carbon isotope excursion the earliest extensive marine sulfate evaporites were deposited and the average ferric iron to total iron expressed as fe 2 o 3 fe fe2o3 ratio of shales increased dramatically at the end of the lomagundi event the first economic sedimentary phosphorites were deposited and the carbon isotope values of marine carbonates returned to 0 vpdb thereafter marine sulfate evaporites and phosphorites again became scarce while the average fe 2 o 3 fe fe2o3 ratio of shales decreased to values intermediate between those of the archean and lomagundi age shales we propose that the large isotopic and chemical excursions during the lomagundi event were caused by a positive feedback between the rise of atmospheric o 2 the weathering of sulfides in the pre 2 3 ga continental crust and the flux of phosphate to the oceans cf holland 2002 the rise in the terrestrial phosphate flux led to an increase in the burial rate of organic carbon and a major transfer of oxygen from the carbon to the sulfur cycle the end of the lomagundi event was probably caused by a decrease in the terrestrial phosphate flux related to the weathering of low pyrite sediments that were deposited during the lomagundi event the rate of deposition of organic matter and the precipitation of sulfate evaporites decreased the isotopic and chemical excesses of the lomagundi event were eliminated and the ocean atmosphere system entered the period frequently called the boring billion

continentally derived solutes in shallow archean seawater rare earth element and nd isotope evidence in iron formation from the 2 9 ga pongola supergroup south africa abstract the chemical composition of surface water in the photic zone of the precambrian ocean is almost exclusively known from studies of stromatolitic carbonates while banded iron formations ifs have provided information on the composition of deeper waters here we discuss the trace element and nd isotope geochemistry of very shallow water if from the pongola supergroup south africa to gain a better understanding of solute sources to mesoarchean shallow coastal seawater the pongola supergroup formed on the stable margin of the kaapvaal craton 2 9 ga ago and contains banded iron formations ifs that represent the oldest documented superior type iron formations the ifs are near shore pure chemical sediments and shale normalized rare earth and yttrium distributions reysn exhibit positive lasn gdsn and ysn anomalies which are typical features of marine waters throughout the archean and proterozoic the marine origin of these samples is further supported by super chondritic y ho ratios average y ho 42 relative to older isua ifs 3 7 ga from greenland and younger kuruman ifs 2 5 ga also from south africa the pongola ifs are depleted in heavy rare earth elements hree and appear to record variations in solute fluxes related to sea level rise and fall sm nd isotopes were used to identify potential sediment and solute sources within pongola shales and ifs the nd t for pongola shales ranges from 2 7 to 4 2 and nd t values for the coeval iron formation samples range 1 9 to 4 3 are generally indistinguishable from those of the shales although two if samples display nd t as low as 8 1 and 10 9 the similarity in nd isotope signatures between the shale and iron formation suggests that mantle derived rey were not a significant nd source within the pongola depositional environment though the presence of positive eu anomalies in the if samples indicates that high t hydrothermal input did contribute to their rey signature isotopic mass balance calculations indicate that most 72 of the nd in these seawater precipitates was derived from continental sources if previous models of fe nd distributions in archean ifs are applied then the pongola ifs suggest that continental fluxes of fe to archean seawater were significantly greater than are generally considered

constraining the crustal root geometry beneath northern morocco abstract consistent constraints of an over thickened crust beneath the rif cordillera n morocco are inferred from analyses of recently acquired seismic datasets including controlled source wide angle reflections and receiver functions from teleseismic events offline arrivals of moho reflected phases recorded in rifsis project provide estimations of the crustal thicknesses in 3d additional constraints on the onshore offshore transition are inferred from shots in a coeval experiment in the alboran sea recorded at land stations in northern morocco a regional crustal thickness map is computed from all these results in parallel we use natural seismicity data collected throughout topoiberia and picasso experiments and from a new rifsis deployment to obtain receiver functions and explore the crustal thickness variations with a h grid search approach this larger dataset provides better resolution constraints and reveals a number of abrupt crustal changes a gridded surface is built up by interpolating the moho depths inferred for each seismic station then compared with the map from controlled source experiments a remarkably consistent image is observed in both maps derived from completely independent data and methods both approaches document a large crustal root exceeding 50 km depth in the central part of the rif in contrast with the rather small topographic elevations this large crustal thickness consistent with the available bouguer anomaly data favors models proposing that the high velocity slab imaged by seismic tomography beneath the alboran sea is still attached to the lithosphere beneath the rif hence pulling down the lithosphere and thickening the crust the thickened area corresponds to a quiet seismic zone located between the western morocco arcuate seismic zone the deep seismicity area beneath western alboran sea and the superficial seismicity in alhoceima area therefore the presence of a crustal root seems to play also a major role in the seismicity distribution in northern morocco

the receiver structure beneath the ebro basin iberian peninsula abstract a model of the lithospheric structure of the eastern sector of the ebro basin has been obtained by means of an inversion of the receiver functions from teleseismic events recorded by one of the institut d9estudis catalans stations located at the northeastern iberian peninsula the technique consists of the isolation and modeling of the amplitude and travel time of the direct p teleseismic wave reverberations in a local structure and has been used for events recorded at poblet station pob placed in the southeast margin of the basin from 1994 to 1997 the analysis of pob records shows the influence of large scale heterogeneities in the wave pulses coming from azimuths crossing the catalan coastal ranges whereas a simple medium response is obtained when teleseismic waves only sample the basin the inversion of such simple receiver functions in terms of a flat layered media 2 to 3 km layer thicknesses along with a comparison of the results with previous and independent works enables us to obtain a crustal and upper mantle velocity structure for the ebro basin the preferred solution models show a strong near surface velocity gradient a nearly constant p velocity upper crust of about 5 9 km sec and a sharp velocity contrast at the crust mantle boundary located at 32 km depth overlayed by a lower crust consisting of a significant positive velocity gradient the upper mantle shows an average p velocity of about 8 0 km sec