tectonic evolution of a part of the tethyside orogenic collage the kargi massif northern turkey the central part of the rhodope pontide fragment one of the major tectonic units in turkey provides critical data for evaluating the cimmeride and alpide evolution of the mediterranean tethysides tectonic events that affected the central part of the rhodope pontide fragment since the end of the paleozoic generated east west trending belts with the event of every episode redeforming and partly obliterating the structures of previous episodes this evolution may be conveniently described in terms of three major episodes 1 two different realms of pre dogger oceanic rocks are present in the area the northern realm coincided with main branch of paleo tethys that was being actively destroyed by south dipping subduction the southern realm the karakaya ocean a back arc basin related to this subduction began opening by rifting of a retroarc carbonate platform during the permo triassic to the west a continental domain with sparse magmatism seperated the two oceanic areas toward the east the two oceans become united by the wedging out of the continental domain these two pre dogger oceans closed during the lias and their remnants were emplaced between the southern margin of laurasia and the fragments of the cimmerian continent 2 the second episode partly overlapped the first with rifting south of the cimmerian continent fragment during the lias this rifting was followed by a transgression which covered the ruins of the cimmeride orogenic belt by the malm this rifting concurrently led to the development of the northern branch of the neo tethys and a south facing atlantic type continental margin a southerly thickening sedimentary prism developed on this margin during the lias to early cretaceous interval 3 the floor of the northern branch of neo tethys began to be consumed along the north dipping subduction zone beneath the previosly constructed continental margin this convergent margin generated a magmatic arc to the north and to the south a subduction accretion complex present width of which approaches 80 km to the south of the subduction accretion complex yet another convergent margin is indicated by the presence of the remnants of an ensimatic arc the northern branch of the neo tethys was totally consumed in the late cretaceous to eocene interval

millennial scale northwest african droughts related to heinrich events and dansgaard oeschger cycles evidence in marine sediments from offshore senegal 1 we present a suite of new high resolution records 0 135 ka representing pulses of aeolian fluvial and biogenic sedimentation along the senegalese continental margin a multiproxy approach based on rock magnetic element and color data was applied on three cores enclosing the present day northern limit of the itcz a strong episodic aeolian contribution driven by stronger winds and dry conditions and characterized by high hematite and goethite input was revealed north of 13 n these millennial scale dust fluxes are synchronous with north atlantic heinrich stadials fluvial clay input driven by the west african monsoon predominates at 12 n and varies at dansgaard oeschger time scales while marine productivity is strongly enhanced during the african humid periods and marine isotope stage 5 from latitudinal signal variations we deduce that the last glacial itcz summer position was located between core positions at 12 26 and 13 40 n furthermore this work also shows that submillennial periods of aridity over northwest africa occurred more frequently and farther south than previously thought

alpine ophiolites product of low degree mantle melting in a mesozoic transcurrent rift zone abstract the alpine ophiolites and their original sedimentary cover upper jurassic radiolarite limestone with ophiolite breccia occur as dismembered tectonic slices often inverted by alpine tectonics or olistoliths among the calc schists cretaceous original petrologic features of the ophiolites are largely masked by the alpine metamorphism but are discernible by the relic mineral chemistry and petrographic reconstruction the residual peridotite is generally plagioclase lherzolite resembling those from oceanic fracture zones but spinel lherzolite is present at davos switzerland and peas france the latter is characterized by higher na 2 o 1 6 of clinopyroxene and lower cr 2 o 3 8 of spinel than the former 30 respectively and resembles subcontinental lherzolites geologic evidence indicates that these two lherzolites were together exposed on the jurassic ocean floor the mafic ultramafic cumulates are of plagioclase type characterized by the scarcity of orthopyroxene and high tio 2 avg 0 8 of clinopyroxene the basaltic volcanics are alkalic rather than tholeiitic in respect to their relic clinopyroxene which is as rich in tio 2 and na 7 o as that in hawaiian alkali basalt the alpine ophiolite association of lherzolitic residual peridotite plagioclase type cumulates and alkalic basalt is reasonable as cogenetic products of low degree partial melting in the mantle geologic and petrologic features of the alpine ophiolites are consistent with a geotectonic model postulating the generation of the alpine ophiolites in an early mesozoic rift zone cut by many transform faults

eastern turkish high plateau as a small turkic type orogen implications for post collisional crust forming processes in turkic type orogens abstract post collisional magmatism may be generated by extensive crustal melting in tibet type collisional environments or by falling out of slabs from under giant subduction accretion complexes in turkic type collisional orogens giving rise to decompression melting of the asthenospheric mantle replacing the removed oceanic lithosphere in turkic type post collisional magmatism the magmatic products are dominantly alkalic to peralkalic and greatly resemble those of extensional regions giving rise to much confusion especially in interpreting old collisional orogenic belts such magmatic regions are also host to a variety of economically valuable ore deposits including gold one place in the world where today active turkic type post collisional magmatism is present is the eastern anatolian high plateau produced after the terminal arabia eurasia collision in the late miocene the plateau is mostly underlain by the late cretaceous to oligocene east anatolian accretionary complex which formed south of the rhodope pontide magmatic arc this subduction accretion complex has been further shortening since the collision but it has also since been domed and became almost entirely covered by at least 15 000 km3 of volcanic rocks the volcanic rocks are calc alkalic in the north transitional in the middle and alkalic in the south of the plateau where the crust is thinnest today less than 38 km the volcanics are derived almost entirely from an enriched mantle the ages of the volcanics also become younger from north to south from about 11 ma to possibly 17th century ad we interpret the origin of the magmatic rocks as the result of decompression melting of the asthenospheric mantle sucked towards the exposed base of the east anatolian accretionary complex as the oceanic lithosphere beneath it fell out the lower density of the hot asthospheric material was the cause of the doming we believe that similar processes dominated the post collisional tectonics of such vast turkic type orogens as the altaids of central asia the late devonian early carboniferous lachlan belt in southeastern australia and the pan african collage of northeast afro arabia it is likely that archaean collisions were dominated by turkic type post collisional events rather than tibetan ones that only became common in the proterozoic

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

synsedimentary formation of ooidal ironstone an example from the jurassic deposits of se central iran abstract the study of the upper lower to middle jurassic badamu formation in se of central iran represents the first report of jurassic ooidal ironstone in iran and exhibits a record of ferruginous radial ooid dominated ironstone the studies showed that this chamosite or ferric oxide chamosite type ooidal ironstone is similar to minette type ironstones a shoreface zone in an epeiric or epicontinental sea is proposed for formation and accumulation of the ferruginous ooids and the ooidal ironstone fe derived from lateritic weathering has been transported into the shoreface zone via a fluvial current during the synsedimentary formation of the ooidal ironstone during the periods with moderate to lower energy or weakly agitated and quiet environmental conditions suspended ferruginous small radial ooids and fe were increased in the lower shoreface subzone higher environmental energy resulted in the development of large concentric ooids and limited distribution of fe in the upper shoreface subzone higher environmental energy also prevented the development of radial ooids and restricted fe precipitation on carbonate ooids variations in environmental energy in the middle shoreface subzone were unfavorable for increasing ferruginous ooids and fe in the environment although short term periods with weakly agitated and quiet or moderate to lower energy conditions could had been existed in the middle shoreface subzone but precipitation of fe on a few small radial ooids was allowed environmental energy in the shoreface zone was the major factor controlling precipitation of fe especially suspended river fe loads on the ooids however precipitation of dissolved fe could have been related to oxidizing conditions in the environment also the probable presence of fe oxidizing bacteria in the cortexes of the ferruginous small radial ooids indicates that bacteria could have possibly contributed to precipitation of fe on the ooids in a calm environment

jurassic granitoids in the northwestern sanandaj sirjan zone evolving magmatism in response to the development of a neo tethyan slab window abstract voluminous jurassic granitoids within the sanandaj sirjan zone ssz provide insight into the magmatic arc formed in the active margin of eurasia here we present new in situ zircon u pb whole rock major and trace element and sr nd isotopic data for the gorveh plutonic complex gpc of the northwestern ssz in iran six samples from the plutons within the gpc yielded zircon u pb ages that range from 151 to 146 ma these plutons can be subdivided into two groups based on their geochemistry group 1 rocks the mobarak abad diorites and the gorveh gabbros and diorites contain relatively high concentrations of the high field strength elements hfse nb ta zr and ti and have low th nb 0 20 0 56 and moderate sm yb ratios 1 51 2 32 low 87sr 86sr i values 0 70354 0 70622 and high end t values 2 3 5 4 these features indicate that the group 1 rocks formed from magmas derived from a subduction modified region of the subcontinental lithospheric mantle the group 2 plutons the bolban abad granites and the gorveh quartz monzonites have a type granites affinities including high k2o na2o and zr nb ce y concentrations and high feotot mgo and 10 000 ga al ratios these a type rocks are enriched in rb th and k and depleted in ba u nb ta sr p and ti the group 2 plutons have different sr nd isotopic compositions to each other indicating they were derived from different sources and record different igneous processes the gorveh quartz monzonites have high 87sr 86sr i ratios 0 70552 0 70617 negative end t values 1 1 to 5 4 and extremely low concentrations of mgo 0 32 0 35 wt suggesting they were derived from an igneous quartzo feldspathic crustal source in comparison the bolban abad granites have positive end t values and contain high concentrations of sio2 and low concentrations of mgo suggesting that they formed from group 1 magmas that subsequently underwent assimilation and fractional crystallization processes combining these new data with the results of previous research we conclude that this jurassic magmatism was the result of the formation of a slab window within the subducting neo tethys slab a process that caused the partial melting of overlying continental lithospheric material

hot spot activity and the break up of pangea abstract the mesozoic and cenozoic positions of the continents that formed pangea in the triassic jurassic were derived from paleomagnetic and intraplate volcanic data paleoclimatic observations such as reef and fossil flora distribution and geological observations major hot spots helped to determine the longitudinal position of pangea and to construct a model of plate motion during the pangean break up the position of the northern part of pangea was constrained using iceland and jan mayen hot spots the iceland hot spot was traced from its present day position to greenland in the paleocene to baffin bay in the late cretaceous to the alpha ridge in the early cretaceous to the chukchi borderland in the middle late jurassic to franz joseph land in the early jurassic the yenisei khatanga trough in the middle triassic and finally to west siberia in the late permian early triassic the hot spot activity is expressed by eastern and western greenland volcanics the siberian trap basalts and perhaps by alpha ridge and chukchi borderland volcanics the chukchi borderland volcanics are related to the early stage of the opening of the canadian basin the position of the southern part of pangea was constrained using the bouvet hot spot this hot spot was tracked from its present day position to western antarctica in the early cretaceous late jurassic and to south africa in the early jurassic late triassic this hot spot activity produced the ferrar and karoo volcanics the model of plate motion obtained agrees with other data on intraplate volcanics which are also related to hot spots at the time of the opening of the central atlantic the cape verde and canary island hot spots were located along the ocean s spreading axis the long lasting location of hot spot and associated mantle upwelling plumes could help to explain forces driving the tethys transit plates opening of the ligurian ocean and eastern mediterranean the european hot spots are related to the several mesozoic phases of rifting for example the rhine graben hot spot may have been located at the rifting axis in the north sea several authors have already discussed the contribution of hot spots to the opening of the south atlantic and indian oceans mantle plumes associated with hot spots play an active role in rifting and initial phases of spreading the lithospheric displacement caused by upwelling combined with sometimes remote collisional forces on the other side of continental plate may result in compression and basin inversion

plate tectonics and the evolution of the british isles this paper is an essay on the rationalism and difficulties of the geological corollaries of plate tectonics with illustrations particularly from the geological history of the british isles there is special emphasis on the mechanics of continental lithosphere in extension when basins are formed and in convergence whereby continental lithosphere thinned in extension is shortened and restacked to form orogenic belts an upper lithosphere stress concentrating elastic lid suffers extremely complex inhomogeneous deformations above a simpler viscoelastic lower lithosphere whose deformation drives strain and displacement in the elastic lid the very nature of plate tectonics as relative motion among a multi plate mosaic of plates provides a rational semi quantitative framework for large scale geological complexity in space and time the caledonian and hercynian history of the british isles may be explained semi quantitatively in terms of plate boundary evolution but it is not possible to define unequivocal solutions the rationalism of plate tectonics is exhibited on a smaller more direct field scale by many of the complexities of ophiolite complexes such as ballantrae scotland that can be explained as the effects of the evolution of ridge ridge transforms and by diachronous deformation and sedimentation in subduction accretion prisms such as the southern uplands deformational and sedimentary patterns in nw europe from triassic times to the present day resulted from a complex interaction of extensional events related to diachronous atlantic opening and compressional events related to sequential alpine shortening phases those widespread events are typical of intracontinental plate boundary systems and create great difficulties for the geologist wishing to decipher relative plate motion from geological sequences because huge areas much larger than the british isles must be analysed and synthesized before much sense can be extracted further difficulties arise from the palinspastic and preservation problems of convergent boundaries orogenic shortening telescopes and partially subducts facies belts most orogenic belts were assembled as a collage of displaced and rotated terrain elements that were subsequently flattened elongated and straightened to an apparent but deceiving simplicity therefore while plate tectonics provides a rational geological framework against which for example british basin evolution can be tested and explained we cannot yet use the geological effects of plate boundary evolution to reconstruct relative plate motion quantitatively

speculations on the mesozoic plate tectonic evolution of eastern china several orogenic belts transecting eastern china are the sites of former convergent plate margins although there have been varying views on the collisional framework of individual continental blocks styles of convergence at these zones and the timing of respective collisions a tectonic study of eastern china mongolia and the southern soviet far east indicates the collision of the south china block with a combined north china northeast china fold zone block in the late triassic early jurassic their collective suturing to eurasia in the late jurassic early cretaceous followed by the sikhote alin japan block in the mid to late cretaceous the evidence is as follows 1 a linear belt of late triassic early cretaceous granites and granodiorites trends east from the qinlingshan through the dabieshan to the huaiyang massif ophiolites flysch subduction zone melange a paired metamorphic belt indicating north dipping subduction and marine strata of carboniferous to late triassic age from the qinlingshan define the suture between the north and south china blocks 2 a sinuous belt of ultramafics blueschists silicic to intermediate magmatism and west and north vergent folds and thrusts trend from the west margin of the ordos basin through central inner mongolia and along the east great khingan range to the amur river coupled with a mid jurassic early creataceous unconformity a suturing of eastern chinese blocks to eurasia along this zone is suggested 3 a fold and thrust belt with ultramafics flysch blueschists and subduction zone melange along the ussuri river in northeast china indicates the suturing of the sikhote alin japan block to eurasia along a west dipping subduction zone in the mid to late cretaceous similarly a tectonic study of southern china and southeast asia has revealed a complex regional mosaic of suture bounded terrains which nucleated about the eastern western and southern margins of the yangtze craton during the late triassic and early jurassic the evidence is as follows 4 a north south trending belt of ophiolites blueschists calc alkaline volcanics and subduction zone melange including granites granodiorites and strongly deformed marine strata all of late triassic age exposed in the longmenshan of sichuan merge with the kekexilishan ophiolite zone into the ailaoshan tengtiaohe ophiolite and blueschist belt in central yunnan along which the songban ganzi complex and the shan thai malaya block join the craton and 5 a southeastern prolongation of the ailaoshan tengtiaohe belt bifurcates into the southeast trending konvoi zone of northern vietnam and the north south trending pak lay luang prabang zone of laos and eastern thailand zones of ophiolite calc alkaline volcanics and strong late triassic deformation they separate the indosinia and shan thai malaya blocks from the craton respectively these findings differ significantly from previous interpretations of a late paleozoic consolidation of south eastern asia as well as disputing the existence of a true pangea

mesozoic overthrust tectonics in south china whether south china was a platform or a para platform after the mid paleozoic deformation or whether the mountains of huanan south china form a mesozoic collision type orogen depends upon whether the ophiolite melange in the yangtze region is precambrian basement or was overthrust onto a foreland thrust belt during the mesozoic this paper presents evidence of tectonic windows under the melange and basement klippes above the melange in the yangtze region

petrogenesis and tectonic implications of the high k alamas calc alkaline granitoids at the northwestern margin of the tibetan plateau geochemical and sr nd hf o isotope constraints abstract the alamas granitoid pluton in the eastern part of the western kunlun orogen the northwestern margin of the tibetan plateau is composed of quartz diorite zircon separates from the pluton has sims u pb age of 446 ma rocks from the pluton have a narrow range of sio2 56 84 62 57 wt mgo 1 76 2 94 wt and total alkalis na2o k2o 5 14 9 59 wt and are metaluminous and high k calc alkaline to shoshonitic in composition they are enriched in light rare earth elements lrees with la yb n 14 25 and show weakly negative eu anomalies these rocks are relatively enriched in sr 472 676 ppm and ba 435 2388 ppm and depleted in nb ta th and ti their end t values range from 6 4 to 8 4 and 87sr 86sr i 0 7184 0 7200 zircons from the pluton show ehf t values of 1 4 to 8 8 and 18o 6 4 9 0 geochemical data indicate that the granitoids were likely derived from the reworking of an ancient deep crustal source influenced by a minor mantle derived component magmatic differentiation was dominated by the fractional crystallization of hornblende biotite and accessory minerals such as apatite allanite and fe ti oxides in summary the late ordovician alamas pluton is an i type granitoid that was emplaced in a post collisional environment suggesting that this tectonic stage had already initiated prior to 445 ma

structural aspects of suspect terranes and accretionary tectonics in western north america abstract the regional structural relationships between the cordilleran suspect terranes and cratonic north america and within the suspect terranes themselves are reviewed for eastern alaska western canada and united states and northern mexico a distinctive characteristic of the relationships between the suspect terranes and the north american craton is that although much modified by post accretionary strike slip faulting thrust faulting and extensional detachment faults the generally more distal and or oceanic terranes are frequently found as rootless nappes or thin thrust sheets sitting upon more inboard terranes or upon the craton itself the amount of tectonic transport implied is large strike slip faulting has been a very significant aspect of the structural evolution of the north american cordillera with total distributed displacements of over 2000 km documented on geologic relationships alone and much more implied by paleomagnetic evidence at least since later mesozoic time to the present movements were such that the terranes moved northward post accretionary intraplate telescoping along thrust faults transpressional strike slip faulting and delamination at mid to upper crustal levels were important processes in cordilleran tectonic evolution

plate tectonics and orogenic research after 25 years a tethyan perspective abstract orogeny the process by which the earth s prominent mountain ranges are constructed has been a central topic of interest in the earth sciences since at least the end of the 18th century the recognition that strains and displacements of very considerable magnitude occur along all of the three dimensions within an orogenic belt during its evolution has grown gradually during the last two centuries emphasis on primary vertical movements dominated the ideas on the nature of orogeny during the first half of the 19th century whereas compression and consequent uplift across mountain belts were believed to be the main cause for their origin during the subsequent one hundred years or so that were spent under the dominance of the fixist contraction theory mobilist tectonicians realised that continental drift in places also required motion along the trend of orogenic belts but this view did not gain general acceptance recognition of significant strike slip motion parallel or subparallel with mountain ranges evolved independently and mostly within the fixist camp by the 1960 s presence of important motions both along and across mountain belts had become common knowledge but no theoretical basis existed to account for them all

rbsr dating of the kohistan arc batholith in the trans himalaya of north pakistan and tectonic implications the kohistan arc batholith in northern pakistan is situated between the indus suture and the northern suture it belongs to the trans himalayan belt which continues eastwards as the ladakh arc batholith in northwest india and the kangdese batholith in southern tibet in kohistan the island arc consists upward sequence of the chilas stratiform complex of norites noritic gabbros and chromite layered dunites which formed in the sub arc magma chamber plutons of tonalite and diorite one of which has a rbsr isochron age of 102 12 ma the chalt volcanics of basaltic tholeiites succeeded by andesites to rhyolites and the yasin group sediments which formed in overlying intra arc basins and which contain albian aptian faunas all these rocks were deformed by major fold structures which are correlated with the formation of the northern suture ne sw trending basic jutal nomal dykes north of gilgit cross cut all the above structures and have a39ar 40ar hornblende age of 75 ma thus the northern suture must have formed in the period 102 12 to 75 ma with the island arc attached to the eurasian plate further northward subduction of the tethyan plate gave rise to an andean type batholith two plutons of which have rbsr isochron ages of 54 4 ma and 40 6 ma the dir utror group of calc alkaline lavas and sediments with eocene fossils are remnants of the volcanic cover of the andean type batholith we suggest that late cretaceous blueschists formed during subduction under the active continental margin and that continental collision and formation of the indus suture was in the eocene the batholith was intruded by layered aplite pegmatite sheets at 34 14 ma and 29 8 ma rbsr ages in post collisional times the87sr 86sr initial ratios of all the dated rocks range between 0 7039 and 0 7052

overview of tectonic settings related to the rifting and opening of mesozoic ocean basins in the eastern tethys oman himalayas and eastern mediterranean regions

preliminary conclusions of the royal society and academia sinica 1985 geotraverse of tibet the 1985 chinese british expedition to the tibetan plateau attempted to solve the question of the origin of the very thick crustal rocks in this region continuing northwards movement of the indian plate over the past 38 myr has given rise to severe folding and thrust faulting causing crustal thickening by internal deformation previous collisions of microplate terranes derived from gondwanaland occurred during mesozoic times but the kun lun terrane of northern tibet was already part of laurasia by the carboniferous

the impact of african aridity on the isotopic signature of atlantic deep waters across the middle pleistocene transition abstract a high resolution analysis of benthic foraminifera as well as of aeolian terrigenous proxies extracted from a 37 m long marine core located off the mauritanian margin spanning the last 1 2 ma documents the possible link between major continental environmental changes with a shift in the isotopic signature of deep waters around 1 0 0 9 ma within the so called mid pleistocene transition mpt time period the increase in the oxygen isotopic composition of deep waters as seen through the benthic foraminifera 18o values is consistent with the growth of larger ice sheets known to have occurred during this transition deep water mass 13c changes also estimated from benthic foraminifera show a strong depletion for the same time interval this drastic change in 13c values is concomitant with a worldwide 0 3 decrease observed in the major deep oceanic waters for the mpt time period the phase relationship between aeolian terrigeneous signal increase and this 13c decrease in our record as well as in other paleorecords supports the hypothesis of a global aridification amongst others processes to explain the deep water masses isotopic signature changes during the mpt in any case the isotopic shifts imply major changes in the end member 18o and 13c values of deep waters

k ar dating of some infra ophiolitic metamorphic soles from the eastern mediterranean new evidence for oceanic thrustings before obduction abstract k ar dating on separated minerals from the infra peridotitic metamorphic soles outcropping beneath various eastern mediterranean ophiolite complexes pindos nappe in greece lycian antalya beysehir hoyran mersin and pozanti karsanti nappes in the taurides belt turkey baer bassit nappe in syria shows that the main metamorphic recrystallization of those rocks occurred 25 30 m y prior to the tectonic emplacement of the ophiolite complexes onto a continental platform these data support the hypothesis of an early slicing of the oceanic crust before its obduction advocated recently by several authors

scale dependence of textural alignment in shales quantified using electron microscopy abstract textural alignment is the primary cause of anisotropy associated with the permeability and mechanical and acoustic properties of rocks although particle alignment in shales is known to be influenced by burial depth mineralogical composition and diagenetic history understanding how these influence image based measures of compaction at the micrometer to nanometer scale is not well understood here we use a novel method which combines scanning electron microscopy and image analysis algorithms to quantify clay alignment in sedimentary rocks for a given field of view the alignment intensity in shales from the podhale basin in poland is positively correlated with clay content based on image analysis however our analysis also shows that the intensity is strongly dependent on the spatial scale of imaging as magnification increases and the field of view in the sem images decreases the alignment intensity drops therefore in images with small fields of view clay alignment is determined by the presence of localized equant silicate mineral grains which effectively hinder the alignment process at the micrometer scale by contrast in images with larger fields of view alignment intensity is determined by the overall compaction state we also show that when the field of view is constant alignment intensity decreases with pixel density and we suggest a range of operating parameters required for obtaining optimum values for alignment intensity from sem images of shales

palaeocene oligocene tectonics of nw europe s u m m a r y during the palaeogene in nw europe a gallic subplate was very nearly created this plate was almost entirely bordered by plate boundaries although not of equivalent ages to the n the n atlantic began to open about 56 ma ago the alpine convergent boundary extended southwestwards to the pyrenean subduction zone on the ne side a n british pataeocene early eocene fracture dyke boundary appears to pass southeastwards across the north sea into a lutetian to burdigalian rift system there is an age disparity between the fracturing and nw dyke intrusion in britain and the fracturing and formation of the rhine graben the last dykes in britain ypresian were intruded not long before the important subsidence of the rhine graben earliest lutetian but their causes were different the british dyke fractures propagated southeastwards via the cleveland dyke towards the north sea whereas the rhine graben propagated northwards towards the zuider zee depression however the structures from both sides converged in the sole pit basin in the north sea with the result that a continuous though not coevally originated fracture system developed from nw scotland to the alps during the early tertiary there was a wide range of tectonic activity throughout nw europe fig 1 in nw britain lavas were extruded and plutonic complexes and extensive dyke swarms were intruded in the north sea tufts were deposited as a result of volcanism along the rockall faeroe rift reactivation of faults con trolled magmatism localization of gas fields devolatilization of coal and inversion of basins caused uplift deformation and erosion along the rhine graben doming subsidence and rift propagation took place in association with alka line volcanism and plutonism coevally with the meso alpine compressive deformations that oc curred in the alps faulting controlled sedimen tation patterns during inversion of basins in southern england the english channel northern france and holland old rift faults were reacti vated in the western approaches basin thrust faults formed in the southern bay of biscay contemporaneously with thrusting in the pyre nees and variscan massifs were tilted and uplifted along steep reverse faults the main tectonic activity lasted from about 60 ma to about 30 ma but it was diachronous generally earlier in the nw and later in the se the igneous activity in northern britain was related to the opening of the n atlantic in the late palaeocene but the formation of the rhine graben and the inversion of sedimentary basins in the north sea and near the english channel were caused by major orogenic events in the alpine region in the late eocene to mid oligocene frisch 1981 tectonic phenomena in the north sea provide the link between those in northern britain and those in the rhine graben the aim of this paper is to demonstrate that these movements from the atlantic and alpine sides almost fragmented the lithospheric plates of nw europe during the creation of a gallic sub block an earlier northern boundary from nw scotland to northern germany being transected by a later eastern boundary along the rhine graben

the ophiolites of the tibetan geotraverses lhasa to golmud 1985 and lhasa to kathmandu 1986 ophiolite belts are found in tibet along the zangbo banggong and jinsha river sutures and in the anyemaqen mountains the eastern extension of the kunlun mountains where studied the zangbo suture ophiolites are characterized by apparently thin crustal sequences 3 3 5 km an abundance of sills and dykes throughout the crustal and uppermost mantle sequences common intraoceanic melanges and unconformities and an n morb petrological and geochemical composition the ophiolites probably formed within the main neo tethyan ocean and the unusual features may be due to proximity to ridge transform intersections rather than to genesis at very slow spreading ridges as the current consensus suggests the banggong suture ophiolites have a supra subduction zone petrological and geochemical composition although at least one locality in the ado massif shows morb characteristics however it is also apparent that the dykes and lavas show a regional chemical zonation from boninites and primitive island arc tholeiites in the south of the ophiolite belt through normal island arc tholeiites in the central belt to island arc tholeiites transitional to n morb in the north the ophiolites could represent fragments of a fore arc island arc back arc complex developed above a jurassic northward dipping subduction zone and emplaced in several stages during convergence of the lhasa and qiangtang terranes the ophiolites of the jinsha river suture have a n morb composition where analysed but more information is needed for a proper characterization the anyemaqen ophiolites where studied have a within plate tholeiite composition and may have originated at a passive margin it is not however certain whether true oceanic lithosphere as opposed to strongly attenuated continental lithosphere existed in this region

magmatic evolution of the tethysides a guide to reconstruction of collage history abstract the magmatic evolution of the tethyside super orogenic complex is reviewed in five time frames and magmatic products are displayed on corresponding non palinspastic suture maps on these maps a one to one relationship between suture zones drawn mostly using non magmatic criteria such as stratigraphy and structural style except for ophiolites and plate boundary magmatism is seen subduction related magmatism is most persistent both spatially and temporally and forms the best magmatic guide to former sutures post collisional magmatism is the next in abundance but is less helpful in delineating sutures in detail ophiolites come third in abundance magmatism associated with the rifting is very sparse along the tethyside sutures taken all together magmatic rocks along the tethysides record continuous tectonic evolution and form superb guides to the detection of the primary orogenic collage components in the tethysides and to their distinction from the secondary orogenic collage components that form through intracontinental deformation it is surprising how well even the smallest of the primary orogenic collage components are delineated by magmatic arc rocks this contrasts with a common interpretation of the north american cordilleran orogenic collage

glacial water mass geometry and the distribution of δ13c of σco2 in the western atlantic ocean 1 oxygen and carbon isotopic data were produced on the benthic foraminiferal taxa cibicidoides and planulina from 25 new piston cores gravity cores and multicores from the brazil margin the cores span water depths from about 400 to 3000 m and intersect the major water masses in this region these new data fill a critical gap in the south atlantic ocean and provide the motivation for updating the classic glacial western atlantic 13c transect of duplessy et al 1988 the distribution of 13c of co2 requires the presence of three distinct water masses in the glacial atlantic ocean a shallow 1000 m southern source water mass with an end member 13c value of about 0 3 0 5 vpdb a middepth 1500 m northern source water mass with an end member value of about 1 5 and a deep 2000 m southern source water with an end member value of less than 0 2 and perhaps as low as the 0 9 values observed in the south atlantic sector of the southern ocean ninnemann and charles 2002 the origins of the water masses are supported by the meridional gradients in benthic foraminiferal 18o a revised glacial section of deep water 13c documents the positions and gradients among these end member intermediate and deep water masses the large property gradients in the presence of strong vertical mixing can only be maintained by a vigorous overturning circulation

geochemistry tectonic setting and metamorphism of mid triassic volcanic rocks of greece abstract mid triassic volcanic rocks are preserved in about eight localities in the nappes of the external hellenides and are also widespread in the pelagonian zone of central greece most have suffered low grade metamorphism immobile trace elements from 35 samples 80 major element analyses and pyroxene analyses show that these rocks are the product of an eastward dipping subduction zone the subducted ocean was perhaps analogous to the pennine zone of the alps the othris and pindos ophiolites are probably contemporaneous products of related back arc extension metamorphic mineral assemblages in volcanic rocks of the phyllite series of the external hellenides indicate high pressure low temperature conditions probably related to a second subduction that occurred prior to the main phase of hellenide nappe emplacement in the miocene

origin and assembly of the tethyside orogenic collage at the expense of gondwana land abstract the tethysides are a superorogenic complex flanking the eurasian continent to the south and consisting of the cimmerides and alpides products of palaeo and neo tethys respectively we here review their evolution mainly on the basis of new maps showing the distribution of sutures magmatic rocks certain palaeobiogeographically and palaeoclimatologically significant taxa and facies and fragments of pan african 900 450 ma orogenic system forming the basement of many tethyside blocks these are supplemented by palaeomagnetic data reported in the literature a fundamental tenet of this paper is that major sutures which contain ophiolite fragments represent tectonic sections between continental blocks where oceanic crust has been subducted palaeo tethys came into existence largely in late carboniferous time coevally it began to be consumed by both internal and peripheral subduction zones which continued into the permian some of these had been inherited from pre tethyan times in the later permian rifting subparallel with the northern margin of gondwana land began between the zagros and malaysia separating a cimmerian continent from n gondwana land and thus heralding the opening of neo tethys and other smaller oceans that were back arc basins of palaeo tethys this rifting possibly also extended farther west into crete and mainland greece however the north china block yangtze block huanan block the eastern moity of the qangtang block north tibet and annamia all originally pieces of the end proterozoic early palaeozoic gondwana land had already separated from it in pre late carboniferous times possibly during the devonian all of these blocks and the cimmerian continent were characterized by cathaysian floral elements in late palaeozoic time palaeomagnetic and palaeontological data showing the original gondwana land affinity of these continental blocks are supplemented by correlating late proterozoic early palaeozoic pan african sutures orogenic belts and sedimentary basin fragments across tethyside sutures late permian foraminiferal provinces are related to this palaeogeographical interpretation by triassic times most cimmeride subduction zones were already in existence the cimmerian continent accelerated its separation from gondwana land and locally in the late permian began disintegrating internally along the waser rushan pshart banggong co nu jiang mandalay ocean by late triassic time all of the chinese blocks except lhasa and annamia had collided with each other and with laurasia the resulting enormous orogenic collage had a soft cushion between itself and laurasia in the form of the enormous accretionary complex of the songpan ganzi this connection enabled laurasian land vertebrates to reach south east asia by late triassic time in late triassic to middle jurassic times most major cimmeride collisions were completed widespread aridity in central asia occurred in late jurassic time probably in the rain shadow of the newly formed cimmeride mountain wall neo tethyan subduction systems formed along the s margin of the cimmerides or within neo tethyan oceanic lithosphere during the jurassic most if not all were north or east dipping they continued the northerly migration of the tethyside blocks evolution of the tethysides influenced the distribution of marine and terrestrial organisms and affected sea level changes and patterns of atmospheric circulation during much of the mesozoic and cainozoic it is likely to have reflected the surface expression of a persistent trend in the large scale convective circulation in the mantle that continuously transported material northward into the tethyan domain

collapse of a hercynian tibetan plateau into a late palaeozoic european basin and range province the large width the importance of crustal shortening and the likelihood of thick crust in the late palaeozoic hercynian or variscan belt of southern europe suggest a tectonic evolution analogous to the evolution of the tibetan plateau and its neighbouring himalaya1 and of the altiplano and other high plateaux of the andes here we extend dewey and burke s analogy1 of the hercynian chain with tibet to include the subsequent stage of pervasive normal faulting and crustal extension and thinning which characterize the active tectonics of tibet parts of the high andes and the basin and range province of western north america it appears that normal faulting and rifting also characterize the late palaeozoic tectonic activity of the hercynian chain

magmatism and metamorphism in the ladakh himalayas the indus tsangpo suture zone abstract ladakh india provides a complete geological section through the northwestern part of the himalayas from kashmir to tibet within this section the magmatic metamorphic and geotectonic evolution of the northern himalayan orogeny has been studied using petrographic geochemical and isotope analytical techniques the beginning of the himalayan cycle was marked by large basaltic extrusions panjal trap of permian to lower triassic age at the northern margin of the gondwana continent indian shield these continental type tholeiitic basalts were followed by a more alkaline volcanism within the triassic to jurassic lamayuru unit of the gondwana continental margin lower jurassic to cretaceous oceanic crust and sediments ophiolitic melange s s accompany the triassic to cretaceous flysch deposits within the indus tsangpo suture zone the major structural divide between the indian shield high himalaya and the tibetan platform so far no relic of paleozoic oceanic crust has been found subduction of the tethyan oceanic crust during upper jurassic and cretaceous time produced an island arc represented by tholeiitic and calc alkaline volcanic rock series dras volcanics and related intrusives accompanied by volcaniclastic flysch deposits towards the tibetan continental margin subsequent to the subduction of oceanic crust large volumes of calc alkaline plutons trans himalayan or kangdese plutons intruded the tibetan continental margin over a distance of 2000 km and partly the dras island arc in the ladakh region the collision of the indian shield and tibetan platform started during the middle to upper eocene and caused large scale still active intracrustal thrusting as well as the piling up of the himalayan nappes the tectonically highest of these nappes is built up of oceanic crust and huge slices of peridotitic oceanic mantle spongtang klippe in the high himalayas the tectonic activity was accompanied and outlasted by a barrovian type metamorphism that affected triassic sediments of the kashmir nun kun synclinorium up to kyanite staurolite grade and the deeper seated units up to sillimanite grade cooling ages of micas are around 20 m y muscovite and 13 m y biotite towards the indus tsangpo suture zone metamorphism decreases with no obvious discontinuity through greenschist prehnite pumpellyite to zeolite grade remnants of possibly an eo himalayan blueschist metamorphism have been found within thrust zones accompanying ophiolitic melange in the suture zone

tectonic history of the western tethys since the late triassic the tectonic history of the western tethys since the late triassic is illustrated through a set of computer generated plate reconstructions which are based on a rigorous plate motions model of this region the model is constrained by the atlantic plate kinematics and on land geologic evidence and defines 13 tectonic phases spanning the time interval from the late ladinian 230 ma to the present the kinematics associated with the late triassic western tethyan rifts produced the detachment of a large composite fragment from the northern margin of gondwana it can be considered as the eastern propagation of the central pangea breakup during the early jurassic these rift zones became inactive while new zones of extension formed along the southern margin of eurasia the eastern margin of iberia and within the rifted northern gondwana fragment itself plate motions associated with the first two extensional centers can still be considered as an eastern branch of the central atlantic plate kinematics conversely the kinematic parameters of the latter rift result from the composition of the euler rotation describing the central pangea breakup and the euler pole of closure of the paleo tethys ocean the late triassic early jurassic rifting phases determined the formation of a number of independent microplates at the interface between africa and eurasia starting from the early cretaceous convergence between africa and eurasia triggered further deformation within the dispersed continental fragments and the formation of backarc basins at the active margins ultimately leading to an increase in the number of tectonic elements that were moving independently in the western tethyan region during the late cretaceous and the cenozoic the proposed tectonic evolution of the western tethys area is compatible with both global scale plate kinematics and geological constraints from on land data observed across the present day mosaic of displaced terranes surrounding the mediterranean region

magmatism at rifted continental margins the intense volcanism and uplift observed on many rifted continental margins forming basaltic seaward dipping reflector sequences is accompanied by the emplacement of a thick igneous section at depth partial melting by decompression of passively upwelling asthenosphere that is hotter than normal because it is near a hotspot explains both the thickness of the new igneous crust and the initial elevation of the rifted margins

tectonics of south china key to understanding west pacific geology abstract south china is not a post caledonian platform but a composite of orogenic belts this late proterozoic mesozoic orogen is a collage of three continental fragments the three blocks are yangzi huanan and dongnanya and they are separated by the banxi nanpanjiang formerly xianggangzhe and gunanhai suture zones yangzi was separated from gondwanaland during late precambrian when an open ocean called banxi was present between the two continents tectonic processes at an active margin during sinian and early paleozoic time led to the genesis of an accretionary wedge complex the banxi melange and the huanan flysch nappes on the northern margin of huanan which was then the northern margin of the gondwana continent reorganization of plates during the devonian suspended subduction at the huanan active margin huanan was uplifted and unconformably overlain by transgressive deposits of devonian and or carboniferous age a remnant ocean the nanpanjiang sea still existed between yangzi and huanan and deep water sedimentation continued both at the southern margin of yangzi and at the northwestern margin of huanan the latter again became an active margin during late paleozoic when permian and triassic flysch sediments were deposited in foredeeps ahead of advancing nappes huanan and yangzi collided during the triassic and resulted in the deformation of the passive margin of the latter to form the yangzi deformed belt huanan was separated from a more southerly continent dongnanya in the devonian by seafloor spreading which created a late paleozoic and early mesozoic ocean gunanhai ocean a continuously deposited sequence ranging in age from devonian to triassic was laid down on the southern passive margin of huanan on the other side of the ocean the dongnanya permian strata include glacial marine deposits of gondwanaland affinity dongnanya became separated from gondwanaland during the late permian when it marched northward to be reunited with huanan the huanan passive margin sequence was deformed by folding and overthrusting after the late mesozoic collision of huanan and dongnanya scattered outcrops of the suture melange resulting from this collision occur at a few localities in coastal fujian dongnanya in our model was the mesozoic continent of southeast asia peripheral to mainland asia this may or may not have been the eastern continuation of the microcontinent sibumasu siam thailand burma malaysia sumatra the collision of dongnanya and asia gave rise not only to the gunanhai melange of coastal fujian but also to the ophiolite melanges parts of which are present in taiwan tailuko the philippines and western malaysia

geochemical evidence for the origin of late triassic melange units in the oman mountains as a small ocean basin formed by continental rifting a useful tool to elucidate past tectonic environments is the geochemistry of volcanic and sedimentary rocks when used together the regional structural setting of the oman mountains indicates that deep water sediments and volcanic rocks formed adjacent to the rifted arabian margin in the late triassic near the axis of a narrow ocean basin of red sea type tholeiitic to trachytic extrusives formed seamounts associated with late triassic reefal build ups immobile trace element compositions point to a within plate origin the interbedded and overlying late triassic deep sea sedimentary cover comprises ribbon radiolarites and both distal siliclastic and calcareous turbidites that accumulated on an abyssal plain at least ca 180 km northeast of the arabian continent associated ferromanganiferous oxide sediments are interpreted as chemical precipitates derived from high temperature vents in the spreading axis of the young ocean basin pervasive regional subsidence took place during end triassic early jurassic time later in the cretaceous oceanic crust was consumed in a northeast dipping subduction zone morb type crust was subducted while late triassic volcanic edifices and sedimentary cover were accreted during eventual trench margin collision the semail ophiolite split into blocks allowing sub ophiolite melange rocks to be expelled upwards through corridors creating the batinah melange as the ophiolite nappe ploughed inboard over already thrust assembled abyssal plain sediments hawasina complex some duplexes were uplifted oversteepened overturned and then slid backwards onto the ophiolite to form the batinah sheets

upwelling intensification as part of the pliocene pleistocene climate transition a deep sea sediment core underlying the benguela upwelling system off southwest africa provides a continuous time series of sea surface temperature sst for the past 4 5 million years our results indicate that temperatures in the region have declined by about 10 c since 3 2 million years ago records of paleoproductivity suggest that this cooling was associated with an increase in wind driven upwelling tied to a shift from relatively stable global warmth during the mid pliocene to the high amplitude glacial interglacial cycles of the late quaternary these observations imply that atlantic ocean surface water circulation was radically different during the mid pliocene

superpositional tectonic patterns along the continental margin of the southeastern mediterranean a review abstract resurgent tectonic phases superimposed their structural imprints on the continental margin of the southeastern mediterranean this margin underwent a complete cycle from a margin of a marine basin in the paleozoic to the margin of the incipient neo tethys ocean in the early jurassic through the margin of the mature ocean in the late jurassic cretaceous and paleogene it became the margin of a desiccated marine basin in the late miocene and finally became the margin of a rejuvenated marine basin after the pliocene the margin was affected by folding faulting and intermittent volcanism during the early liassic early neocomian late cretaceous oligocene and early miocene the younger tectonic phases were superimposed on the older ones reactivating them in some places and masking them in others the geological characteristics and this complex tectonic history are masked effectively by the thick messinian evaporitic sequence this smoothing effect was further enhanced by the huge pile of sediments that has been deposited by the river nile since the early pliocene

sahel megadroughts triggered by glacial slowdowns of atlantic meridional overturning 1 the influence of the large scale ocean circulation on sahel rainfall is elusive because of the shortness of the observational record we reconstructed the history of eolian and fluvial sedimentation on the continental slope off senegal during the past 57 000 years our data show that abrupt onsets of arid conditions in the west african sahel were linked to cold north atlantic sea surface temperatures during times of reduced meridional overturning circulation associated with heinrich stadials climate modeling suggests that this drying is induced by a southward shift of the west african monsoon trough in conjunction with an intensification and southward expansion of the midtropospheric african easterly jet