from greenhouse to icehouse organic walled dinoflagellate cysts as paleoenvironmental indicators in the paleogene abstract dinoflagellates are an important component of the extant eukaryotic plankton their organic walled hypnozygotic cysts dinocysts provide a rich albeit incomplete history of the group in ancient sediments building on pioneering studies of the late 1970s and 1980s recent drilling in the southern ocean has provided a wealth of new dinocyst data spanning the entire paleogene such multidisciplinary studies have been instrumental in refining existing and furnishing new concepts of paleogene paleoenvironmental and paleoclimatic reconstructions by means of dinocysts because dinocysts notably exhibit high abundances in neritic settings dinocyst based environmental and paleoclimatic information is important and complementary to the data derived from typically more offshore groups as planktonic foraminifera coccolithophorids diatoms and radiolaria by presenting case studies from around the globe this contribution provides a concise review of our present understanding of the paleoenvironmental significance of dinocysts in the paleogene 65 25 ma representing earth s greenhouse icehouse transition this episode holds the key to the understanding of extreme transient climatic change we discuss the potential of dinocysts for the reconstruction of paleogene sea surface productivity temperature salinity stratification and paleo oxygenation along with their application in sequence stratigraphy oceanic circulation and general watermass reconstructions

fault reactivation and superimposed folding in a proterozoic sandstone volcanic sequence davenport province central australia abstract the davenport province is a fluvial to shallow marine sequence of weakly metamorphosed sandstone rhyolite and basalt deposited in an ensialic basin that was probably initiated by continental extension isopach ridges and cut offs suggest that synsedimentary normal and transfer faults controlled deposition these faults were reactivated as reverse faults and strike slip faults during deformation upright concentric folding took place in two episodes initially about nw trending axes and subsequently about generally ne trending axes balanced cross sections suggest that the folds die out downwards above the base of the sequence however units from well below the concentrically folded part of the sequence crop out in anticlinal cores indicating that there is no through going detachment horizon in the sequence this is consistent with the rarity of clayey silty and evaporitic sediments suitable for detachment hence the faults probably cut the entire sequence and enter the basement interference of the folds formed dome and saddle and crescentic patterns thrust sheets imbricate stacks and duplexes are not recognized in the province and layer parallel shortening is moderate about 28 these suggest a correspondingly low degree of earlier regional extension about 40 the folding may have been due to a sudden change in lithospheric plate velocity

the adjustment of mantle plumes to changes in plate motion the relative motion of hotspots and lithospheric plates implies a velocity shear in the underlying mantle causing horizontal advection of mantle plumes as they rise toward the lithosphere consequent tilting of plumes parallel to the direction of plate motion indicates that plumes must undergo a period of readjustment after the velocity vector for plate motion is altered thus the shape of bends in the surface tracks of hotspots resulting from changes in plate motion will reflect the plume adjustment laboratory experiments as well as computations using a simple theory developed in richards griffiths 1988 for the dynamics of continuous plume conduits demonstrate that the bend in the surface track has a radius of curvature approximately equal to the maximum horizontal deflection of the conduit thus the sharpness of the bend at an age of 43ma in the hawaiian emperor volcanic chain implies that the deflection of the underlying plume in that case was small 200 km this small deflection is expected for plumes carrying large buoyancy fluxes and it indicates that tilting of the conduit is unlikely to be sufficient to cause diapiric instability

niobium uranium evidence for early formation of the continental crust niobium uranium ratios in greenstone belt basalts and gabbros indicate that parts of the late archean mantle beneath western australia underwent a level of melt extraction resulting in formation of the continental crust comparable to that seen in the present mantle the implication is either that i the amount of continental crust that formed before 2 7 x 10 9 years ago was much greater than generally thought or ii crustal growth occurred by severe depletion of small volumes of the mantle rather than by moderate depletion of a large volume of mantle

the changing nature of mantle hotspots through time implications for the chemical evolution of the mantle the melting products of mantle plumes can place constraints on the geochemical structure of the mantle and show how it has varied through time the highest temperature melts komatiites and picrites derived from each plume are assumed to represent the best available sample of the thermal boundary layer from which the plume originates a survey of the geochemistry of picrites and komatiites shows that they underwent a fundamental change at about the archean proterozoic boundary archean komatiites have depleted or neutral geochemistry whereas most protero zoic and younger picrites have originated from enriched oib type mantle although rare depleted plumes persist to the tertiary e g gorgona if plumes originate from a thermal boundary layer near the core mantle interface the composition of this layer must have changed from a mixture of depleted and neutral mantle during the archean to mainly enriched mantle in the proterozoic this change coincides with a drop in the maximum mgo content of the ma

late eocene to early oligocene dinoflagellate cysts from the priabonian type area northeast italy biostratigraphy and paleoenvironmental interpretation abstract marginal marine late eocene through early oligocene dinoflagellate cyst successions have been studied from the type area of the priabonian stage northeast italy the results allow the recognition of dinoflagellate zones previously defined in bio and magnetostratigraphically well calibrated pelagic sequences from central italy thus providing a first order correlation of the priabonian stage to the standard chronostratigraphic scale with unprecedented precision within this detailed biostratigraphic framework successive shifts in the composition of the dinoflagellate cyst assemblages are interpreted in terms of sea level and sea surface temperature sst fluctuations the evaluation of ecologically and sedimentary controlled changes in the composition of dinoflagellate cyst assemblages in combination with sequence stratigraphic analysis allows the reconstruction of third and fourth order sea level fluctuations the third order sea level fluctuations appear to correspond to exxon cycles ta4 1 4 5 the stratigraphically and palaeoenvironmentally important late eocene dinoflagellate cyst species glaphyrocysta priabonensis sp nov is formally described

the structural development of the wessex basin the mesozoic and cenozoic structures of southern england form part of a system of disturbances which extends across the channel into northern france they are reviewed in the light of published data from south dorset basin development started with permo triassic rifting and is believed to have continued through the jurassic and early cretaceous with intermittent growth on deep seated listric normal faults probably associated with roll over anticlines this regime had ceased by the aptian and the basin passed through a period of relative stability in the late cretaceous followed by tectonic inversion and in the miocene by northwards compressive movement along the former normal faults this partially restored the earlier separative displacement at depth and probably accentuated pre existing roll over anticlines it was expressed in the surficial upper cretaceous palaeogene sequence as monoclinal flexuring the early rifting and the change from overall n s crustal stretching to compression may reflect plate tectonic events in the north atlantic region

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

on the reactivation of extensional fault systems in terranes that have undergone substantial extension three sets of faults dominate a shallow to steep dipping commonly rotational normal faults b a really extensive shallow dipping normal detachment faults and c steep dipping transfer faults that strike at high angles to the normal faults these fault systems may extend through a large fraction of the crust reactivation of these fault systems will depend primarily on the relative strengths of the faults shear zones and their host rock and their orientation in the prevailing stress field it is concluded that reactivation is generally mechanically favoured but that it will probably only take place when the fault shear zones are in near ideal orientations consideration of the tectonic setting of extended terranes and of the limited number of well described examples suggests that reverse thrust reactivation of the normal and detachment faults and wrench reactivation of transfer faults are the most likely styles examples of these styles are described from the bass strait basins of southeastern australia because extended terranes commonly underlie sedimentary basins for example on passive continental margins reactivation of extensional faults may be a key control on the tectonic evolution of such basins i e basin inversion

the proterozoic davenport province central australia regional geology and geochronology abstract the davenport province forms the southern part of the tennant creek inlier and lies to the north of the arunta inlier the oldest rocks exposed are turbidites and minor 1880 1870 ma old felsic volcanics of the warramunga group these were folded probably during the barramundi orogeny and intruded by granite before being overlain by the hatches creek group a sequence up to 10 000 m or more thick of fluvial to shallow marine sedimentary and bimodal volcanic rocks a rift sag model of basin development involving crustal extension in an intracratonic setting is suggested for this group interpretation of u pb zircon data for felsic volcanics in the lower part of the hatches creek group is hampered by problems of ancient pb inheritance but a preferred crystallization age of 1820 1810 ma is indicated two episodes of upright concentric folding about nw se and ne sw trending axes preceded emplacement of granite at 1660 ma or earlier rb sr whole rock data for felsic volcanics from the hatches creek group are tentatively interpreted to indicate a region wide hydrothermal alteration event at 1645 44 ma this alteration may have been associated locally with w cu bi and mo mineralization

magnetostratigraphy and small mammals of the late oligocene banovici basin in ne bosnia and herzegovina article i nfo a combined magnetostratigraphic and small mammal investigation was carried out to acquire better age control on the sedimentary infill of the banovici basin in bosnia and herzegovina although the dinarides occupy a crucial paleogeographic position bridging central europe and anatolia no detailed records of its small mammal fauna have been published until now a rich small mammal assemblage with over 500 molars was excavated from a section exposing marls and clays just underlying the basins main coal layer the fauna of this turija small mammal locality compares best with the uppermost oligocene to lowermost miocene localities from the european mp30 mn1 mammal zones and with anatolian zone b from central turkey a 160 m thick series of lacustrine sediments overlying the main coal layer in the nearby grivice section was sampled for paleomagnetic purposes the magnetostratigraphic pattern of the grivice section comprises a long reversed interval a subsequent short normal interval and another reversed interval the most logical correlation of this pattern to the late oligocene part of the geomagnetic polarity time scale gpts is to chrons c6cr c6cn 3n and c6cn 2r 24 to 23 2 ma this correlation implies a sedimentation rate of 20 cm kyr for the banovici basin and an age of approximately 24 ma for the turija mammal site the proposed correlation fits well with other magnetostratigraphically calibrated mammal records in western and central europe as well as anatolia and with the recalibrated ages of 24 95 0 05 ma and 24 72 0 04 ma for two basalt flows bracketing the enspel mp28 site in southern germany our results indicate that a limited exchange of fauna from central asia to western europe existed in late oligocene and miocene times

three dimensional geological modeling supports a revised burdigalian chronostratigraphy in the north alpine foreland basin precise age data are a basic prerequisite for the correlation of a sedimentary succession with the global time scale which in turn allows one to place its biotic and other data in a global context in the north alpine foreland basin nafb a patchy distribution of outcrops and uncertainties in the correlation of strata have led to conflicting age models in particular for deposits of lower miocene burdigalian age here we present a new three dimensional geological model of the se german sector of the nafb covering an area of 13 200 km2 which helps to resolve the discrepancies the dataset comprises lithostratigraphic information from 150 boreholes supplemented by magnetostratigraphic data from six outcrops computer based 3d modeling was conducted with the open source software qgis for a 500 m thick succession comprising units of the upper marine molasse the brackish oncophora fm and the upper freshwater molasse the results provide new insights pertaining to 1 the isochrony of strata 2 subsidence and 3 synsedimentary tectonics the new data permit us to reliably place the outcrops within the regional lithostratigraphic scheme thus enabling a new correlation of their magnetic polarities with the global time scale on this basis we propose a revised age model for the middle and upper burdigalian strata in the eastern part of the nafb which is supported by previously reported 87sr 86sr age data the model indicates that the succession is substantially up to 0 8 ma younger than earlier publications have suggested furthermore it implies that the two synsedimentary tectonic events discerned may both be related to the large scale tectonic movements that affected the nafb during the late burdigalian

pechenga ferropicrites and other early proterozoic picrites in the eastern part of the baltic shield ti poor komatiites and ti rich picrites exist among the widely occurring primitive mgo rich volcanics in the early proterozoic successions of the kola peninsula both have been encountered in the imandra varzuga belt but only picrites are found in the pechenga complex the picritic rocks are divided into two types the first type restricted to the kolasjoki and pilgujarvi suites and the tominga series is called ferropicrite due to its exceptionally high total iron content which usually exceeds 14 feotot the second type found in the south pechenga series and in the umba and panarechka suites has more typical iron content and geochemically resembles suisaarian picrites from southern karelia ferropicrites occur as pillow lavas lava breccias tuffs massive lavas and up to 50 m thick differentiated spinifex textured flows in addition to high feotot contents these rocks are characterized by low al2o3 contents and by high concentrations of tio2 zr and other incompatible elements they are strongly lree enriched and have immobile trace element characteristics similar to within plate transitional or alkali basalts it is shown that with regard to both the major and trace element compositions the parental magma of the ni bearing intrusions of the pilgujarvi suite is similar to ferropicrite suggesting that the emplacement of the intrusions took place contemporaneously with ferropicritic volcanism the high incompatible element contents of the ferropicritic rocks rule out a genetic relationship with associated tholeiitic basalts proposed geotectonic settings for the pechenga area including an intracratonic rift zone a passive continental margin an oceanic island arc a mature continental volcanic arc and a collisional suture zone are discussed the presence of ti rich picritic rocks does not support a subduction related environment

archean plate tectonics constraints and inferences the earth has been cooling since archean time the higher temperatures beneath archean ridges resulted in more partial melting which extended down to greater depths than at present the archean oceanic crust was much thicker 20 km than modern crust 5 km this inference is compatible with previous ideas of suspected oceanic crust the thicker oceanic crust in the archean tended to resist subduction similar to modern aseismic ridges but could not prevent it short lived episodes of intra arc spreading followed by cordilleran type compression may have produced archean greenstone belts archean meta tonalites represent a significant part of the continental crust and were probably produced by melting of the subducted slab which was favored by the higher mantle temperatures

the influence of alteration on the trace element and nd isotopic compositions of komatiites abstract to investigate the effects of hydrothermal alteration and metamorphism on the chemical and isotopic compositions of komatiites we studied samples from the alexo and texmont regions in the 2 7 ga abitibi belt of canada and from the weltevreden mendon and komati formations of the 3 2 3 5 ga barberton belt of south africa particular emphasis was placed on multiple samples from individual layered spinifex olivine cumulate flows the argument being that if these flows showed variations in the ratios of elements incompatible with olivine then these variations were most likely due to chemical mobility during alteration data for rare earth ree and high field strength elements hfse reveal anomalies non chondritic hfse ree ratios particularly in the most altered samples after taking into account the limits of analytical precision these anomalies are attributed to element mobility not to fractionation of high pressure minerals as suggested by other authors for abitibi komatiites in barberton komatiites three separate processes produced hfse anomalies element mobility during alteration crust assimilation in some samples and majorite fractionation the effects of the latter process were recognized from systematic relationships between hfse ree and al ti that coincided with calculated majorite fractionation trends initial nd isotopic compositions of the komatiites were determined by analysis of magmatic pyroxenes the initial nd of pyroxene from the 2 7 ga alexo komatiite is 3 8 slightly higher than in whole rocks 0 6 to 3 5 average 2 5 in the carbonatized texmont samples the range in initial nd is far greater 6 2 to 8 8 the variable initial nd values in the rocks are attributed to isotopic exchange of nd with surrounding rocks during early alteration and fractionation of sm nd during later events pyroxene from 3 3 ga barberton komatiitic basalt had initial nd of 2 3

the effects of chemical density differences on convective mixing in the earth s mantle subducted lithosphere and crust are chemically distinct from ambient mantle and some have hypothesized that the resulting anomalous buoyancy may affect mixing and in particular may cause chemical heterogeneity to persist to explore this effect two dimensional convective mixing calculations have been carried out boundary conditions mimicking features of plate kinematics have been imposed and negatively buoyant tracers placed beneath converging plate margins to simulate subduction the principal parameter of interest is the density anomaly ratio o t the ratio of the chemical to the thermal buoyancy for a density anomaly ratio appropriate to complete separation of the oceanic crust from the rest of the lithosphere it is marginally possible for the tracers to segregate into the warm region near the bottom and neutralize the thermal buoyancy there for internally heated flows and to segregate to the base of the uprising limbs of convection for bottom heated flows however for the more likely scenario of oceanic crust and lithosphere maintaining their layering after subduction there is neither segregation nor neutralization of thermal buoyancy the average residence time is the average time tracers remain in the box from subduction to sampling beneath ridges and could be increased by 4 from the passive case at the very most if the crust and lithosphere remain layered even in the more extreme case of separation of crust from lithosphere the residence times are only increased by 15 from the passive case

petrology of the keweenawan mamainse point lavas ontario petrogenesis and continental rift evolution the mamainse point formation forms a 5250 m thick sequence of flood basalts ranging from high mgo picrites and basalts to basaltic andesites that formed in the keweenawan 1100 ma midcontinent rift of north america this section is the most continuous and complete sequence of keweenawan lavas and records almost the entire igneous history of the rift the section consists of numerous stratigraphic groups of lavas that have distinct chemical characteristics a variety of petrogenetic processes including partial melting of different sources at decreasing depth periodically replenished tapped and fractionated magma chambers crustal contamination and polybaric fractionation were responsible for the evolution of the magmas the dominant petrogenetic process changes with stratigraphy which suggests that the physical evolution of the rift controlled which process operated at any time the initial fracturing of the crust presumably allowed high mgo liquids generated at depth to be erupted the early stage of rifting was apparently slow and allowed the formation of long lived magma chambers in which complex processes such replenishment mixing and assimilation contamination could occur the late stage of rifting however was apparently vigorous and allowed voluminous magmas from throughout the crust to be erupted these conclusions from the chemostratigraphy agree with the physical stratigraphy which indicates a change in tectonic style from an early stage of broad subsidence to a late stage of active graben formation

simulation of foreland basin stratigraphy using a diffusion model of mountain belt uplift and erosion an example from the central alps switzerland foreland basin stratigraphy can be considered as the result of three interacting processes thrust deformation which builds the tectonic load sedimentary and erosional processes which redistribute that load and the flexural response of the lithosphere the resultant stratigraphy of foreland basins is commonly composed of a small number of shallowing and coarsening upward cycles bounded by regional unconformities to understand the development of these unconformities we present a simple model of these three processes coupling an evolving thrust wedge on a linear elastic plate with erosion and sedimentation defined by the diffusion equation applied to topography our model demonstrates the development of regional unconformities without recourse to either eustasy or complex viscoelastic models for the continental lithosphere the model describes the thrust wedge foreland basin system in terms of four parameters 1 the effective elastic thickness of the foreland plate te 2 the sediment transport coefficient k 3 the thrust wedge advance rate 4 the surface slope of the thrust wedge the model is applied to the oligocene miocene north alpine foreland basin nafb of eastern switzerland the stratigraphy of the nafb can be simplified into two large scale shallowing upward cycles separated by an unconformity at the base of the burdigalian 22 ma geological information is taken from the nafb to estimate suitable values for the parameters listed above assuming a linear elastic lithospheric rheology the te value is estimated at 10 5 km from decompacted sediment columns data to constrain the sequential development of the thrust wedge come from structural geology the early stages 40 24 ma of compression involved a relatively low angle thrust wedge with an advance rate of approximately 2 4 mm yr at about 24 ma the wedge advance slowed down and thickened by underplating crystalline basement of the foreland plate the value for the transport coefficient has been estimated from previous studies prior to attempting to simulate the broad scale geometry of the nafb the role of each parameter was assessed individually the values for te and k are held constant throughout the simulation of the nafb at 7 5 km and 500 m yr respectively the geometry of the base burdigalian unconformity is reproduced by variations in the parameters describing the thrust wedge the surface slope angle of the wedge is increased from 2 5 to 4 over 0 2 m y and the rate of thrust advance is decreased from 2 mm yr to 0 2 mm yr during the thickening event between 24 and 23 8 ma the rejuvenation of the internal parts of the thrust load causes backtilting of the foreland basin sediments and after a time lag erosion of the distal stratigraphy over the forebulge so simulating the base burdigalian unconformity

timing of palaeoproterozoic au cu bi and w mineralization in the tennant creek region northern australia improved constraints via intercalibration of 40ar 39ar and u pb ages abstract the magnitude of systematic uncertainties inherent in comparisons of proterozoic 40ar 39ar and u pb based ages is illustrated via a consideration of age constraints on two contrasting styles of palaeoproterozoic mineralization in northern australia published 40ar 39ar ages for au cu bi mineralization in the tennant creek region range from 1825 to 1829 ma apparently younger than the timing of deformation bracketed by 207pb 206pb zircon ages of igneous rocks at 1850 ma despite this apparent age offset propagation of systematic uncertainties in the 40ar 39ar ages shows that they are indistinguishable from the 1850 ma zircon ages recalculation using revised estimates of the 40k decay constant and age of the 40ar 39ar standard shifts the 40ar 39ar ages to 1847 1851 ma in excellent agreement with 207pb 206pb zircon ages from tennant supersuite igneous rocks and with local geological constraints to the south of tennant creek in the davenport ranges numerous small w prospects have been interpreted to be coeval with post tectonic granites of the devils suite at 1710 1720 ma new 40ar 39ar ages from muscovite selvages on wolframite bearing veins range from 1697 to 1703 ma these ages shift to 1711 to 1717 ma when recalculated using revised estimates for 40k decay constant and age of standard material bringing them into agreement with existing 207pb 206pb zircon ages for devils suite granitoids and consistent with field and geochemical interpretations these examples highlight the importance of considering systematic sources of uncertainty when comparing radiometric ages determined via different isotopic methods particularly in the proterozoic and archean parts of the timescale ongoing work to calibrate the 40ar 39ar and u pb based timescales will further enhance the application of multiple isotopic methods to constrain geological processes

paleo oxygenation in tertiary epeiric seas evidence from dinoflagellate cysts the dinoflagellate cyst record from an epeiric setting in the oligocene of southwest germany exhibits major changes associated with variations in paleo oxygenation inferred from benthic foraminifera dinoflagellate cyst diversity and equitability strongly decline in the case of extreme oxygen depletion the relative abundance of the dinoflagellate cyst thalassiphora pelagica inversely correlates with the availability of benthic oxygen the t pelagica spectrum consists of different morphotypes which are considered to represent different stages within the cystal part of the t pelagica life cycle following the concept of benedek and gocht the distribution of these morphotypes is also linked to oxygen availability horizons with highest t pelagica abundances are characterized by the occurrence of t pelagica specimens representing an early stage in cyst formation a model is proposed that uses t pelagica as a proxy to assess oxygenation both at the sediment surface and in the water column the occurrence of t pelagica specimens representing an early stage in cyst formation is interpreted to reflect the extension of oxygen depletion upwards into the water column because t pelagica is an easily recognized globally distributed element of paleogene and early neogene dinoflagellate cyst assemblages the application of this model may be of importance in the reconstruction of paleo oxygenation in epeiric settings from these times its sensitivity to changes in benthic oxygenation can be as high as if not higher than that of the approach based on foraminifera moreover it is applicable to decalcified sediments

petrology and geochemistry of metabasalts from the 1 95 ga jormua ophiolite northeastern finland the jormua ophiolite exposes a unique fragment of red seatype oceanic crust formed in a setting related to continental break up 1950 ma ago two distinct types of basalt are represented the early dykes and the main basalt suite early dykes have fractionated h ree patterns oib like trace element patterns low 2j nb 6 and ejwfyss ga 0 6 indicative of their derivation from an enriched source the remaining dykes and all lavas belong to the second e morb like main basalt suite which is characterized by high mg number and cr contents flat ree patterns zr b 6 17 chondritic th ta and only moderately depleted isotopic signatures zjufi gs a 4 1 9j most main suite samples cannot be related solely by fractional crystallization to a common parental magma rather they represent distinct melt fractions that underwent variable amounts of chromite olivine plagioclase fractionation during ascent a significant part of the compositional diversity of the main basalt suite can be explained by mixing a depleted source with a relatively uniform proportion of an enriched component similar to that represented by the oib like early dykes it is probable that during the latest stages of continental rifting the oib type melts metasomatized the upper part of the depleted asthenospheric mantle which became the source of the main basalt suite soon after the old continental lithosphere was ruptured the complete absence of any evidence for a subduction related component in the basalts implies that jormua is not a back arc ophiolite

geochemistry of the siberian trap of the noril sk area ussr with implications for the relative contributions of crust and mantle to flood basalt magmatism the sequence investigated of the siberian trap at noril sk ussr consists of at least 45 flows that have been divided into six lava suites the lower three suites consist of alkalic to subalkalic basalts the ivakinsky suite overlain by nonporphyritic basalts the syverminsky suite and porphyritic and picritic basalts the gudchikhinsky suite the upper three suites are tholeiitic the uppermost 750 m of dominantly non porphyritic basalt belong to the mokulaevsky suite and are characterized by a nearly constant mg number 0 54 0 56 sio2 48 2 49 1 wt ce 12 18 ppm and ce yb 5 8 the underlying 1100 m of dominantly porphyritic basalt belong to the morongovsky and nadezhdinsky suites there is a continuous increase in sio2 48 1 55 2 wt ce 12 41 ppm and ce yb 5 18 from the top of the mokulaevsky to the base of the nadezhdinsky with little change in the mg number 0 53 0 59 mokulaevsky magmas have trace element signatures similar to slightly contaminated transitional type mid ocean ridge basalts the change in major and trace element geochemistry in the upper three suites is consistent with a decline in the degree of anatexis and assimilation of tonalitic upper crust by mokulaevsky magma the nadezhdinsky and underlaying lavas thicken within and thus appear to be related to an elongate basin centred on the noril sk talnakh mining camp the mokulaevsky and morongovsky lavas thicken to the east and appear to be related to a basin centred more than 100 km to the east of the noril sk region these magmas may have risen up out of a different conduit system

petrogenesis of the flood basalts from the early permian panjal traps kashmir india geochemical evidence for shallow melting of the mantle article i nfo the early permian panjal traps of northern india represent a significant eruption of volcanic rocks which occurred during the opening of the neotethys ocean basaltic basaltic andesites dacitic and rhyolitic rocks collected from guryal ravine and pahalgam show evidence for subaerial and subaqueous eruptions indicating that they are contemporaneous with the formation of a shallow marine basin the major and trace element geochemistry of the basalts is consistent with a within plate setting and there are basalts which have high ti tio2 n 2 0 wt and low ti tio2 b 1 8 wt compositions the high ti basalts are similar to oib whereas the low ti basalts are similar to continental tholeiites the identification of high and low ti basalts within the panjal traps is analogous to other large igneous provinces e g karoo deccan parana emeishan the sr nd isotopic values end t 5 3 to 1 3 isr 0 70432 to 0 71168 of both types of basalts overlap indicating that the rocks may have originated from the same ancient subcontinental lithospheric i e emii like mantle source tdm 2000 ma the two groups of basalts can be modeled by using a primitive mantle source and different degrees of partial melting where the high tirocksare producedby 1 partial melting of a spinel peridotite source where as the low ti rocks are produced by 8 partial melting trace elemental and isotope modeling indicatesthat some of the basalts assimilated 10 crustal material in contrast the basaltic andesites are likely formed by mixing be tween basaltic magmas and crustal melts which produced rocks with higher sio2 55 wt content and enriched isotopic signatures end t 6 1 isr 0 70992 the panjal trap volcanism was likely due to partial melting of the sclm within a passive extensional setting related to the rifting of cimmeria from gondwana contemporaneous volcanic and plutonic granitic rocks throughout the himalaya are probably not petrogenetically related but are likely part of the same regional tectonic regime

a model of ocean crust accretion for the superior province canada abstract one of the keys to understanding the origin of archaean greenstone belts lies in the geological relationships between mafic and ultramafic greenstones felsic to intermediate volcanic rocks and terrigenous sediments traditional models for greenstone belt evolution have been based on in situ stratigraphic relationships most of these models for example an oceanic island arc developed on oceanic basement back arc basins and the recently popular plume model predict concordant stratigraphic relationships among the various greenstone belt lithologies however rather than being depositional in nature several authors have indicated that many of the relationships between the different lithologies in greenstone belts are in fact tectonic suggesting an allochthonous origin for most greenstone sequences all of these latter models make analogies to phanerozoic tectonic processes involving accretion of oceanic materials with volcanism related to both plate subduction and rifting in this paper we have evaluated the geological relationships between volcanic rocks and sediments in three regions in the superior province where the accretion of oceanic material can be documented and direct comparisons are made to geological processes in phanerozoic accretionary complexes in the malartic area in the southeastern abitibi subprovince 3 to 4 km thick slices of komatiite and tholeiite with intercalated terrigenous sediment are tectonically imbricated and are overlain by calc alkaline volcanics which postdate tectonic stacking in both the larder lake region of the southwestern abitibi belt and in the beardmore geraldton belt at the south eastern limit of the wabigoon belt slices of iron rich tholeiite and chemical sediments of an oceanic origin are tectonically imbricated with terrigenous sediment the malartic val d or area is considered to be an example of accretion of an archaean oceanic plateau while the larder lake and the beardmore geraldton regions are potentially typical of accretion of normal oceanic crust in an arc environment phanerozoic accretion of oceanic crust is accompanied by a step back in subduction and in this paper we suggest that oceanic crust accretion may have been the principal mechanism by which the locus of subduction migrated towards the south of the superior province asthenospheric upwelling associated with the isolated sinking plate may have been responsible for widespread late magmatism this scenario requires that magmas be erupted through previously accreted volcanic plutonic and sedimentary material furthermore later ridge subduction will result in transpressional tectonics and eruption of mafic sequences over mature and immature volcano plutonic sequences the combined result of the plate tectonic scenario envisaged would result in the well described cyclic stratigraphy of many granite greenstone sequences

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

late permian mafic rocks identified within the doba basin of southern chad and their relationship to the boundary of the saharan metacraton the doba gabbro was collected from an exploration well through the cretaceous doba basin of southern chad the gabbro is composed mostly of plagioclase clinopyroxene and fe ti oxide minerals and displays cumulus mineral textures whole rock 40 ar 39 ar step heating geochronology yielded a late permian plateau age of 257 1 ma the major and trace elemental geochemistry shows that the gabbro is tholeiitic in composition and has trace element ratios i e la yb n 7 sm yb pm 3 4 nb y 1 zr y 5 indicative of a basaltic melt derived from a garnet bearing mantle source the moderately enriched sr nd isotopes i e i sr 0 70495 to 0 70839 nd t 1 0 to 1 3 fall within the mantle array i e oib like and are similar to other late permian plutonic rocks of north central africa i e i sr 0 7040 to 0 7070 the enriched isotopic composition of the doba gabbro contrasts with the more depleted compositions of the spatially associated neoproterozoic post pan african within plate granites the contrasting nd isotope composition between the older within plate granites and the younger doba gabbro indicates that different mantle sources produced the rocks and thus may mark the southern boundary of the saharan metacraton

genesis of flood basalts from eclogite bearing mantle plumes on the basis of the geochemistry of flood basalts we investigate the hypothesis that the source material for these magmas includes a significant eclogite component derived from ancient subducted oceanic crust entrained into mantle plumes at the core mantle boundary we present finite element models of the melting in starting plumes plume heads in both continental and oceanic regions starting plume models in which the plume heads are composed solely of pyrolite mantle are incapable of generating sufficient volumes of melt or any melt at all for reasonable plume head temperatures 1400 1600 c or lithosphere ages 6 25 100 m y this result is in contrast to the results of farnetani and richards 1994 whose models require higher plume head temperatures 1600 1700 c and a weak mobile lithosphere in order to generate volumes of melt similar to flood basalts plumes composed of 15 wt eclogite yield volumes of melt comparable to flood basalts in both oceanic and continental settings the maximum extent of melting in these models is about 10 30 this agrees with the model of hofmann and white 1982 who argued that ocean island basalts are derived from subducted oceanic crust if the embedded eclogite thermally reequilibrates with the surrounding plume material then melt volumes and total extents of melting increase by a additional factor of 2 3 a simple two dimensional rifting model for a slow spreading ocean basin underlain by a lens of hot plume material shows that plume related melting occurs over 50 60 m y a much longer period than for flood basalts

oceanic plateau model for continental crustal growth in the archaean a case study from the kostomuksha greenstone belt nw baltic shield abstract field studies combined with chemical and isotope data indicate that the kostomuksha greenstone belt in the nw baltic shield consists of two lithotectonic terranes one mafic igneous and the other sedimentary separated by a major shear zone the former contains submarine komatiite basalt lavas and volcaniclastic lithologies and the latter is composed of shelf type rocks and bif komatiitic and basaltic samples yield sm nd and pb pb isochron ages of 2843 39 and 2813 78 ma respectively their trace element compositions resemble those of recent pacific oceanic flood basalts with primitive mantle normalized nb th of 1 5 2 1 and nb la of 1 0 1 5 this is in sharp contrast with island arc and most continental magmas which are characterized by nb th la n 1 calculated initial nd isotope compositions nd t 2 8 to 3 4 plot close to an evolution line previously inferred for major orogens momo which is also consistent with the compositions of recent oceanic plateaux the high liquidus temperatures of the komatiite magmas 1550 c and their al depleted nature require an unusually hot 1770 c mantle source for the lavas 200 c hotter than the ambient mantle at 2 8 ga and are consistent with their formation in a deep mantle plume in equilibrium with residual garnet this plume had the thermal potential to produce oceanic crust with an average thickness of 30 km underlain by a permanently buoyant refractory lithospheric mantle keel nb u ratios in the komatiites and basalts calculated on the basis of th u pb relationships range from 35 to 47 and are thus similar to those observed in modern morb and oib this implies that some magma source regions of the kostomuksha lavas have undergone a degree of continental material extraction comparable with those found in the modern mantle the mafic terrane is interpreted as a remnant of the upper crustal part of an archaean oceanic plateau when the newly formed plateau reached the active continental margin its upper part collided with the sedimentary terrane but was too buoyant to subduct as a result the volcanic section of the plateau was imbricated and obducted thus becoming a new segment of continental crust the deeper zones were delaminated and tectonically underplated or subducted

isotopic and geochemical constraints on the evolution of the 1 93 1 79 ga svecofennian crust and mantle in finland the central part of the svecofennian domain in finland is characterized by thick lithosphere with crustal thickness up to 60 65 km major collisional events at 1 91 1 90 ga and 1 89 ga and a thrusting event at 1 86 1 84 ga the latter due either to continuation of shortening or to a separate intracrustal collision are considered the major causes of crustal thickening the origin of the svecofennian domain has been formerly attributed to mixing of depleted mantle melts with variable amounts of archaean crustal component via subduction but the new data and reinterpretation of older data imply that besides juvenile crust the svecofennian domain also contains older palaeoproterozoic crustal components no archaean component is found in the 1 93 1 91 ga gneissic tonalites and related felsic volcanics end t 1 4 which occur in the savo belt sb adjacent to the archaean craton the depleted mantle model ages vary from 1 94 ga to 2 33 ga with most values 2 0 ga the analysed syntectonic post thickening granitoids 1 89 ga in the sb exhibit positive end t values ranging from 2 8 to 3 0 pointing to a juvenile source the hypersthene granitoids are similar in age but show epsilon values around 1 which are only slightly lower than found in em derived mafic rocks the isotopic and geochemical data for the central finland granitoid complex and tampere schist belt indicate the occurrence of evolved thick crust 2 0 ga and associated lithospheric mantle already at 1 91 ga the southernmost part of the southern svecofennian in finland also shows evolved type crust 2 0 ga a tentative model for the development of crust and mantle is presented it includes the occurrence of continental nucleus surrounded by more juvenile island arc accretions two successive collisional events thickened both the crust and the subcontinental lithospheric mantle with the greatest relative amount of thickening occurring in the mobile zones convective removal of the lowermost parts of thickened lithosphere caused an upwelling of hot asthenosphere increase in the geotherm and melting of both the enriched subcontinental lithospheric mantle and the convective depleted asthenosphere mafic magmatism during the post thickening stage 1 89 ga thus varied from dominantly enriched to dominantly depleted mantle melts and was associated with variable crustal contamination the thickening of the lithosphere in the central part of the svecofennian domain was not followed by large extension suggesting that the thickness was already great 1 88 1 80 ga ago this together with the thick high velocity lower crust could have provided the isostatic balance needed to stabilize the thick crust

neodymium and lead isotope evidence for enriched early archaean crust in north america there has been considerable debate as to whether isotopically enriched continental crust was an important geochemical reservoir in earliest archaean times fundamental questions such as the volume of the early crust its composition and the effect of its formation and or recycling on mantle geochemistry have still to be answered the most direct clues to the nature of the earliest crust and mantle are obtained from the isotope systematics of old rocks1 5 here we report neodymium and lead isotope measurements and uranium lead zircon geochronology from archaean gneisses of the slave province northwest territories canada the gneisses contain zircons with cores older than 3 842 gyr one sample of tonalitic gneiss has a nd chondritic model age of 4 1 gyr and an end 3 7 gyr of 4 8 this is the oldest reported chondritic model age for a terrestrial sample and provides evidence for strongly enriched pre 3 8 gyr crust a reservoir complementary to the depleted mantle already in existence by 3 8 gyr before present2 6

exploration in a classic thrust belt and its foreland bavarian alps germany the foreland of the bavarian alps consists of the tertiary molasse and its mesozoic substratum so far 38 oil and gas fields have been found with reserves of 14 million bbl 2 million mt of oil and 140 bcf 4 billion m3 of natural gas the fields are associated predominantly with antithetic normal faults and to a lesser extent with low relief anticlines or a combination of such anticlines and shaling out sands the lower part of the molasse and its mesozoic substratum dip to the south under the alpine thrust units despite its depth of 7 000 m 23 000 ft and more the subthrust is still considered to be within the oil window and is the prime target for exploration in the bavarian alps other possible targets are the kalkalpin and helveticum thrust units the chances in the folded molasse and flysch thrust units are considered to be extremely low seismic exploration in the alps is difficult because of the rough topography thick quaternary sediments a deep weathering zone and heterogeneous seismic velocities in the thrust units and their quaternary cover

on the separation of relatively buoyant components from subducted lithosphere subducted oceanic lithosphere consists of a sandwich of component layers morb underlain by depleted parent mantle these layers give rise to intrinsic density contrasts within the slab which have been hypothesized to cause component separation after subduction we explore this process through numerical models of simple layered slabs and through convection models which incorporate large local chemical density contrasts we show that the components of positive and negative buoyancy density contrast component volume must be almost equal in order to cause significant component separation or to selectively block deep penetration of buoyant slab components in the transition zone published petrological constraints on component densities and relative volumes indicate that such buoyancy neutralization is not achieved in subducted slabs therefore component separation should not occur on the timescale associated with the sinking of slabs through the upper mantle and transition zone

implications of nb u th u and sm nd in plume magmas for the relationship between continental and oceanic crust formation and the development of the depleted mantle abstract the nb u and th u of the primitive mantle are 34 and 4 04 respectively which compare with 9 7 and 3 96 for the continental crust extraction of continental crust from the mantle therefore has a profound influence on its nb u but little influence on its th u conversely extraction of midocean ridge type basalts lowers the th u of the mantle residue but has little influence on its nb u as a consequence variations in th u and nb u with sm nd can be used to evaluate the relative importance of continental and basaltic crust extraction in the formation of the depleted sm nd enriched mantle reservoir this study evaluates nb u th u and sm nd variations in suites of komatiites picrites and their associated basalts of various ages to determine whether basalt and or continental crust have been extracted from their source region emphasis is placed on komatiites and picrites because they formed at high degrees of partial melting and are expected to have nb u th u and sm nd that are essentially the same as the mantle that melted to produce them the results show that all of the studied suites with the exception of the barberton have had both continental crust and basaltic crust extracted from their mantle source region the high sm nd of the gorgona and munro komatiites require the elevated ratios seen in these suites to be due primarily to extraction of basaltic crust from their source regions whereas basaltic and continental crust extraction are of subequal importance in the source regions of the yilgarn and belingwe komatiites the sm nd of modern midocean ridge basalts lies above the crustal extraction curve on a plot of sm nd against nb u which requires the upper mantle to have had both basaltic and continental crust extracted from it it is suggested that the extraction of the basaltic reservoir from the mantle occurs at midocean ridges and that the basaltic crust together with its complementary depleted mantle residue is subducted to the core mantle boundary when the two components reach thermal equilibrium with their surroundings the lighter depleted component separates from the denser basaltic component both are eventually returned to the upper mantle but the lighter depleted component has a shorter residence time in the lower mantle than the denser basaltic component if the difference in the recycling times for the basaltic and depleted components is 1 0 to 1 5 ga a basaltic reservoir is created in the lower mantle equivalent to the amount of basalt that is subducted in 1 0 to 1 5 ga and that reservoir is isolated from the upper mantle it is this reservoir that is responsible for the sm nd ratio of the upper mantle lying above the trend predicted by extraction of continental crust on the plot of sm nd against nb u

geological and geochemical evidence for variable magmatism and tectonics in the southern canadian cordillera paleozoic to jurassic suites greenwood southern british columbia the paleozoic and early mesozoic rocks of the greenwood mining camp in southern british columbia are a part of the quesnel terrane in the eastern part of the intermontane belt of the canadian cordillera upper paleozoic rocks include the knob hill group composed of oceanic tholeiitic basalts with la yb n 0 4 1 2 associated with deep ocean sedimentary rocks and serpentinites the attwood group that comprises island arc tholeiites with la yb n 1 4 and positive end values clastic sedimentary rocks and limestones and a unit of oceanic gabbros with la yb n 0 5 these lithologically defined units occur as tectonically emplaced slivers of oceanic crust probably produced during the closure of the slide mountain basin during the permian they are unconformably overlain by middle triassic calc alkaline volcanic and sedimentary rocks of the brooklyn group the brooklyn group volcanic rocks have characteristics of mature island arc rocks including la yb n 2 5 4 5 and positive end values the paleozoic r

variability of nb u and th la in 3 0 to 2 7 ga superior province ocean plateau basalts implications for the timing of continental growth and lithosphere recycling extensive volcanic sequences of tholeiitic basalts having near flat ree patterns and spatially associated with komatiites occur in many archean superior province greenstone belts they are considered to be fragments of intraoceanic volcanic plateaus derived from a plume basalts from the 2 9 3 0 ga lumby lake greenstone belt carefully screened for minimum alteration have variable nb u ratios of 36 to 58 least altered basalts from the abitibi and wawa greenstone belts also have variable nb u ratios of 25 to 50 and 28 to 42 respectively compared to an average value of 47 for modern ocean basalts in the abitibi suite nb u correlates positively with nb lapm but negatively with th lapm alteration can be ruled out as the cause of nb u variation as there are no correlations of nb u with loi or eu eu and nb u correlates with nb th in all three suites of basalts numerous lines of evidence indicate that crustal contamination can be eliminated as the cause of nb u variability especially for samples with nb u 36 high nb u ratios can be explained by recycling ocean crust processed through a subduction zone high nb u and nb lapm low th lapm into the mantle source of the basalts whereas low nb u ratios can be accounted for by recycling complementary subarc mantle lithosphere or continental crust low nb u and nb lapm high th lapm into the mantle source for the entire population of basalts th lapm ratios generally 1 may result from recycling the residue of slab derived archean type tonalites having high th la ratios nb u ratios as high as the 47 10 range of modern ocean basalts have been found in a 2 7 ga volcanic belt of the yilgarn craton taken together the results signify very early growth of the continental crust rather than episodic growth over several ga

nd isotopic evolution of the precambrian mantle abstract the most convincing examples of proterozoic oceanic crust assemblages occur in the circum superior fold belt of quebec nd isotope analyses of the chukotat and hellancourt basalts from this belt constrain the e nd of the early proterozoic depleted mantle reservoir at 4 to 6 along with data for phanerozoic ophiolites and modern morb the proterozoic basalts infer a constant rate of evolution of the depleted mantle at a rate of 2 2 e nd units per ga recalculation of sm nd isotopic data for archaean greenstone belts to their most reasonable pb pb or u pb ages suggests significantly greater isotopic heterogeneity in the archaean presumed uncontaminated tholeiites and al undepleted komatiites fall within the extrapolated growth limits for the proterozoic phanerozoic depleted mantle while al depleted komatiites adk in early archaean greenstone belts have significantly higher e nd the latter require derivation from a source whose isotopic composition evolved at 2 4 times the rate of the al undepleted komatiite and tholeiite source the isotopic heterogeneities noted for the archaean indicate a compositionally layered mantle by the late archaean the adk source was either isolated by declining geothermal gradients or had become remixed with the overlying mantle giving rise to the present depleted mantle reservoir

the evolution of the mantle s chemical structure abstract the geochemistry of flood basalts and their associated picrites and of komatiites and their associated basalts combined with a theoretical model for the structure of mantle starting plumes can be used to decipher key elements of the geochemical structure of the deep mantle and show how it has varied through time we argue that the thermal boundary layer above the core consisted mainly of depleted mantle similar to the present morb source during the archaean and this was largely replaced between 2 7 and 2 0 billion years ago by enriched mantle to form the oib source we suggest that this change in the nature of the hotspot source reflects a fundamental change in the dominant component of downward convection from cold plumes breaking away from beneath a stable lithosphere during the pre archaean to subduction of lithosphere in the archaean and post archaean mantles

on the dynamics of long lived plume conduits in the convecting mantle abstract long lived plumes responsible for mantle hotspots and associated chains of volcanism will be deflected from the vertical as they penetrate through a mantle which is overturning on much larger length scales in response to surface cooling recent experiments have shown that plume inclination leads to entrainment of surrounding material as a result of coupling between conduction of heat and laminar stirring we develop an analysis for entrainment and its effects on the structure and dynamics of continuing steady conduits in a mantle undergoing horizontal shear it is found that the stirring of surrounding mantle with plume source material can result in major modifications of the diameter temperature and mass flux in plumes this model is a departure from the axisymmetric vertical pipe models of plumes and predicts that the temperature in a defected plume will decrease with height thus reducing the plume temperature in the upper mantle while allowing a sizeable temperature difference at the source the cooling of plume conduits after they are deflected by plate motion can lead to a lower maximum temperature and lower mgo contents of basalts along some linear hotspot tracks compared with picrites from their associated flood basalt provinces the effects of entrainment depend primarily on the buoyancy or heat flux carried by the plume with greater cooling for smaller fluxes strong continuing plumes such as hawaii easter island macdonald tahiti and reunion are predicted to produce picrites having mgo contents comparable to the picrites found in continental flood basalt provinces weak plumes such as crozet bouvet and juan de fuca will give only cooler melts through entrainment long lived plumes can sample the whole depth of the mantle from the source to the surface and stirring within the plume leads to compositional zonation which may contribute to compositional heterogeneity in hotspot melts we also predict that plumes originating at the core mantle boundary and having buoyancy fluxes less than about3 103 n s 1 will not give surface hotspots in good agreement with the lower limit of hotspot buoyancy fluxes inferred from seafloor topography

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

tertiary volcanism in the kangerdlugssuaq region e greenland trace element geochemistry of the lower basalts and tholeiitic dyke swarms

the formation and fate of large oceanic igneous provinces abstract large igneous provinces are conspicuous features of late phanerozoic geology and include continental flood basal rifted continental margin volcanic sequences and oceanic plateaus the latter are formed in an environment which typically recycles back into the mantle on a time scale of 200 m y but because comparisons have recently been made between oceanic plateaus and precambrian greenstone belt sequences new questions arise about their formation their fate and their preservation here we review some critical aspects of three oceanic plateaus ontong java kerguelen and the caribbean colombian abducted plateau and comment on their make up and the factors governing their preservation with particular relevance to ancient terranes many large igneous provinces can be linked to mantle plumes where plumes ascend beneath spreading ridges their energy is transformed into a large melt volume producing over thickened plateau crust where the spreading rate is low in relation to magma supply the plateau may become subaerial e g iceland but with fast spreading the plateau remains submarine thicker lithosphere may result in plume incubation before magma extrusion and there are many intermediate situations where plumes could readily break through thin lithosphere oceanic or continental because magma supply exceeds extension rate plateaus may be characterised by thick sequences of flows and sills rather than the sheeted dykes typical of phanerozoic ophiolites precambrian greenstones could represent imbricated oceanic plateaus or plumes penetrating thin continental lithosphere the initial high temperature and the buoyant nature of the depleted refractory keel of plateaus contributes to their preservation relative to normal oceanic crust when they collide with active margins they choke the subduction zone causing subduction flip or backstep and the development of extensive talc alkaline arc volcanism on top of the plateau sequences however after x 100 m y they are potentially negatively buoyant so if fluids become available to promote tmnsfotmation of the deeper zones to eclogite they may be able to spontaneously subduct

pb and nd isotopic study of two archean komatiitic flows from alexo ontario isotopic analysis of two archean komatiitic flows from alexo ontario gives a pb pb isochron age of 2690 15 ma and a sm nd isochron age of 2752 87 ma these ages agree well with u pb zircon ages from underlying and overlying volcanics the variations in element ratios that define the isochrons were not produced during crystallization of the lavas the spread in u pb was caused by submarine alteration soon after eruption and the spread in sm nd resulted from a differences in the composition of the residue of melting and b contamination of the upper komatiite flow through thermal erosion of the lower flow the 147sm 144nd ratio of uncontaminated komatiite is 0 25 which reflects the depleted nature of its mantle source the th u ratio of about 3 4 is probably also representative of depleted mantle the initial nd of 2 44 0 51 indicates that the mantle depletion took place long before magma formation

physical models of regional fold superposition the role of competence contrast abstract physical models of superposed folds demonstrate that rheologic contrast strongly controls the style of fold interference rheologic contrast affects strain partitioning between layer parallel shortening and buckling and affects the development of structural anisotropies parallel to first generation folds laminates with insignificant competence contrast are characterized by circular to elliptical domes and basins type 1 interference in addition to folding the layers accommodate shortening by layer parallel thickening preferentially in the fold culminations laminates with significant competence contrast are characterized by buckled fold hingelines and axial surfaces type 2 interference the less competent layers thicken in the hinges the more competent layers maintain their initial thickness in plan view the f 1 hingelines are refolded in a lobate cuspate to box style whose axial traces form conjugate pairs the models are dynamically scaled to represent folds with km scale wavelengths the evolution of surface structures map patterns at different depths and equal area projections synthesized from structure contour maps were used for analysis comparable to regional analysis common to field studies gravitational body forces effectively damp the vertical amplitudes of both the first and second generation folds thereby enhancing the formation of type 2 interference

glacial eustasy during the cenozoic sequence stratigraphic implications a newly constructed composite oxygen isotope record is a proxy for eustasy that extends back to the cretaceous tertiary boundary and provides an independent test of sequence stratigraphic based eustatic curves the isotope record shows several eustatic episodes that are consistent with the geological record of ice sheet evolution the first evidence for the existence of an ice sheet in east antarctica occurs near the lower middle eocene boundary base of the lutetian stage there is no evidence for a large ice sheet on antarctica prior to this time however strata of this age are lacking over most of the continent the isotope curve also indicates that the ice sheet experienced phases of growth during the late eocene and middle oligocene followed by a decrease in volume in the early miocene the ross sea stratigraphic record indicates initial evolution of the west antarctica ice sheet during the early miocene by the middle miocene the ice sheet spread across the ross sea weddell sea and antarctic peninsula continental shelves the pliocene pleistocene record of glaciation in antarctica includes numerous glacial erosion surfaces on the continental shelf indicating repeated advance and retreat of both east and west antarctica ice sheets these volume changes in the antarctica ice sheet were in response to the rise and fall of sea level caused by expanding and contracting northern hemisphere ice sheets there is a reasonable correlation between eustatic curves derived from sequence stratigraphic studies and the composite oxygen isotope record since the middle eocene this correlation indicates that glacial eustasy has been the principal factor regulating stratal stacking patterns on a global scale since the middle eocene

crustal uplift in the alps why the drainage pattern matters introductionmountain belts grow by accretion ofupper crustal material into the orogen icwedge fig 1 materialenteringthewedge gets deformed uplifted andexhumed basedontheoreticalmodels willett et al 1993 concluded thatorogensadoptadistinctgeometrywitha low tapered pro wedge facing thesubduction plate and a high taperedretro side this geometry establishesbecauseorogenstendtobeonthevergeof gravitational failure davis et al 1983 according to these models thevelocity eld of the crust and hence the exhumation path of rock particles depends on erosion at the surface e g horton 1999 consequently anychanges in erosion rates potentiallyresult in a modi cation of the patternof strain rates of surface erosion however depend on climate exposedlithologies the size of the catchmentarea thegeometriccon gurationofthedrainage basin and rates of crustaluplift tucker and slingerland 1996 although the impact of these para meters on surface erosion has beenstudied extensively using numericalmodels tucker and slingerland 1997 ku hni and p ffner 2001 schlu neggeret al 2001 verylittleattentionhas been paid to the in uence of thedrainage geometry on the spatial pat tern of erosion and on strain the central swiss alps provide anexample where the present daydrainage geometry reveals a distinctpattern that is characterized by largeorogen parallel dispersal systems therhone and rhine rivers fig 2a that drain the core of the alps and smaller orogen normal rivers according to the conceptual modelsoutlined above it is reasonable tospeculate that the asymmetry of thealpine drainage pattern is likely toin uence the pattern of crustal up lift speci cally the highest rates ofstrain release are expected in therhone and rhine valleys becausethe large drainage basins of theserivers are likely to result in enhancederosional potentials this hypothesiswill be tested in the following chap ters this present paper evaluates thepotential controls of the asymmetriccon guration of the alpine drainagepattern on possible variations oferosional potential as well as onpresent day and glacial sedimentyield this information is then rela ted to the published pattern ofcrustal uplift allowing possible pos itive feedback between surface ero sion and lithospheric forcing to beidenti ed the intention is not todiscard other processes as possiblecontrols on the pattern of crustaluplift e g exural rebound afterremoval of the pleistocene ice sheet kahle et al 1997 but rather topropose an alternative model thatmight enrich the range of possibleinterpretations pattern of crustal upliftand seismic activityfigure 2 a displays the recent verti cal movements of the swiss alpscomputed for rst order levellingbenchmarks relative to the benchmarkat aarburg gubler et al 1981 kahleet al 1997 the geodetic data wascollected repeatedly between c 1905and 1990 measured uplift is inter preted in terms of crustal upliftbecause benchmarks have not experi enced erosion i e they record theshort term velocity eld of upper most crustal levels the data indicatea considerable increase of crustaluplift rates from the foreland tothe rear of the alps where currentrates of 1 mm yr

isotopic evidence for the origin of the manihiki and ontong java oceanic plateaus abstract pb nd and sr isotopic results for lavas of the cretaceous ontong java and manihiki oceanic plateaus fall well within the modern day oceanic island or hotspot field the data provide no evidence of old continental basements but indicate a major involvement of kerguelen type or em i like mantle in the sources of both plateaus which appear to have probably been formed at least in part by hotspots however the presently active hotspots that pacific plate reconstructions suggest might have been possible plateau sources lack kerguelen type isotopic compositions either these hotspots did not participate in the formation of the two plateaus or if they did kerguelen type material must have been volumetrically much more important early in their existence two hypotheses for the origins of these plateaus which involve hotspot sources are consistent with the sparse available geochemical geochronological and geophysical data the first holds that the plateaus formed cataclysmically in association with surfacing plume heads the second posits a relatively steady but robust hotspot at or near a ridge crest and requires a much longer period of formation a near ridge origin appears to be indicated by evidence that most of the pacific plateaus were built largely on relatively young ocean crust however we suggest that a near ridge origin is also compatible with the plume head concept in that plume heads appear very likely to become associated with spreading axes through their influence on rift propagation which should be substantially greater than for ordinary hotspots in either case the lack of hotspot tracks seamount chains attached to the two plateaus would be a consequence of ridge migration or rift propagation in a near ridge setting

the lower marine to lower freshwater molasse transition in the northern alpine foreland basin oligocene central switzerland south germany age and geodynamic implications the wilhelmine alpe section near immenstadt allgau south germany which represents one of the best continuously exposed outcrops within the northern alpine foreland basin has been analyzed for magnetostratigraphic and palynostratigraphic signals the section comprises the marine to terrestrial transition from lower marine umm to lower freshwater molasse usm sediments based on the correlation of the local magnetic pattern with the geomagnetic polarity timescale gpts and palynostratigraphic data an age of about 31 ma is suggested for the umm usm transition in the wilhelmine alpe section a comparison with coeval magnetostratigraphic sections from central and eastern switzerland indicates that the regression of the umm sea along the southern margin of the molasse basin occurred strongly heterochronously between 31 5 and 30 ma the heterochroneity is attributed to the deposition of fan delta and alluvial fan sediments which document that the overall marine conditions during the umm were accompanied by strong clastic input derived from the rising alps this clastic contribution had a much stronger influence on the depositional pattern than previously thought

effects of partial melting on the uranium decay series distinction between possible partial melting processes using stable incompatible trace elements depends largely on the mass fraction of liquid produced at low degrees of melting less than 5 both incompatible element concentrations and interelement fractionations are greater for batch fractional and accumulated fractional melting than for dynamic or continuous melting at higher degrees of melting uncertainty in initial source composition and small fractionations between incompatible elements make it unlikely that the most realistic melting models batch and dynamic could be discriminated however it is possible for these melting processes to be distinguished by the magnitude of radioactive disequilibria that is produced within the 238u decay series even for durations of partial melting much longer than the half lives of 230th and 226ra both230th 238u and 226ra 230th disequilibria can occur because of differences in the bulk solid liquid partition coefficients for these elements the magnitude of these disequilibria depends on melting process the 230th 232th of a dynamic melt can differ significantly from that of a batch melt and will not necessarily indicate the original 238u 232th of the source if morb is formed by a dynamic melting process then the th u of the upper mantle near ridge axes may be higher than that calculated from the 230th 232th of the basalts

depositional and structural evolution of a foreland basin margin in a magnetostratigraphic framework the eastern swiss molasse basin this integrated study of the sedimentology magnetostratigraphic chronology and petrography of the mostly continental clastics of the oligocene to miocene swiss molasse basin underpins a reconstruction of facies architecture and delineates relationships between the depositional evolution of a foreland basin margin and exhumation phases and orogenic events in the adjacent orogen a biostratigraphically based high resolution magnetostratigraphy provides a detailed temporal framework and covers nearly the whole stratigraphic record of the molasse basin 31 5 13 ma three transverse alluvial fan systems evolved at the southern basin margin they are characterized by distinct petrographic compositions and document the exhumation and denudation history of the growing eastern swiss alps enhanced northward propagation of the orogenic wedge is interpreted to have occurred between 31 5 and 26 ma during the period 24 19 ma intense in sequence and out of sequence thrusting took place as molasse strata were accreted to the orogenic wedge a third active tectonic phase possibly caused by backthrusting of the plateau molasse probably occurred between ca 15 and 13 ma fan head migration between 31 5 and 13 ma is probably controlled by the structural evolution of the thrust front due to molasse accretion and backthrusting

controls of erosional denudation in the orogen on foreland basin evolution the oligocene central swiss molasse basin as an example a high resolution three dimensional reconstruction of the 25 m y old central swiss molasse basin reveals two sedimentary domains separated by a 5 km wide flood plain the proximal domain of the basin attained a width of 20 km and its basement is steeply flexed 6 7 dip petrographic data indicate that it was filled by sediment from the rigi dispersal system derived from the central alps of eastern switzerland and by locally sourced bajadas in contrast the distal sedimentary domain located farther north was gently dipping 2 and was filled by the meandering lac leman and honegg dispersal systems chronological data reveal that sedimentation in the northern proximal part of the basin started at 27 ma when sediment supply to the basin started to increase deflection of the foreland plate at 25 ma is successfully simulated by flexural modeling of the thrust load and the sediment load the model reveals that the lac leman and honegg dispersal systems are located on a buried flexural bulge furthermore it shows that burial and suppression of the flexural bulge at 27 ma as well as an increase of the basin wavelength were controlled by the contemporaneous increase in the sediment supply rate of the rigi system the model presented suggests that the tectonic subsidence of the molasse basin was mainly controlled by tectonic events in the northern part of the orogen within 70 km distance from the tip of the orogenic wedge crustal thickening in this part of the orogen is reflected in the proximal molasse by sedimentary cycles characterized by an increase in the sediment accumulation rates up section and by the presence of locally sourced bajada fans at the top of each cycle although south vergent back thrusting along the insubric line 150 km south of the foreland basin contributed little to flexure it resulted in an increase of the sediment supply to the foreland basin this is reflected in the molasse by coarsening and thickening upward trends an increase of the basin wavelength basinward shifts of the depocenters of the dispersal systems and uplift and erosion of the proximal basin border

three fe ti oxide ore bearing gabbro granitoid complexes in the panxi region of the permian emeishan large igneous province sw china the permian 260 ma emeishan large igneous province of sw china contains three nearly identical gabbro granitoid complexes that host giant fe ti oxide deposits the fe ti oxide deposits are within the lower portions of evolved layered gabbroic intrusions and are spatially and temporally associated with a type granitic plutons the 264 3 ma taihe layered gabbroic intrusion hosts a large magmatic fe ti oxide deposit and is coeval with the taihe peralkaline a type granitic pluton which is dated at 261 2 ma within the a type granitic pluton are microgranular enclaves which have compositions intermediate between the gabbro and host granite primitive mantle normalized incompatible element plots show corresponding reciprocal patterns between the mafic and felsic rocks the chondrite normalized ree patterns show eu anomalies changing from positive eu eu 1 5 to 5 9 in the gabbroic intrusion to negative in the enclaves eu eu 0 4 to 0 6 and granites eu eu 0 2 to 0 5 whole rock end t values of the gabbroic intrusion end t 2 5 to 3 3 are similar to those of the enclaves end t 1 0 to 2 0 and granite end t 1 5 to 1 9 whereas the zircon ehf t values of the gabbro ehf t 8 1 0 8 are indistinguishable from those of the granites ehf t 9 2 1 0 suggesting that the parent magmas for all rock types originated from the same mantle source geochemical modeling indicates that the gabbros and granites can be generated by fractional crystallization of a common parental magma similar to high ti emeishan flood basalt the compositional jump from the gabbro to the enclaves is attributed to the crystallization of fe ti oxide minerals the results of this study and other studies suggest that the magmatic conditions for example pressure composition fo2 which led to the formation of at least three fe ti oxide bearing gabbro granitoid complexes were relatively common during the development of the emeishan large igneous province

flood basalts and ocean island basalts a deep source or shallow entrainment abstract basalts from continental flood basalts cfbs and intraplate or hotspot ocean islands are found to have distinct geochemical signatures this diversity in composition is generally believed to result from the upwelling plume entraining overlying reservoirs of shallow and intermediate depth mantle material during its ascent from the deep mantle here we present laboratory experiments and numerical model calculations which clarify that for a strongly temperature dependent viscosity like that of the mantle a rising plume head should be expected to bring up a surrounding sheath of deep mantle from its source region mixing between the central core of the plume and this sheath produces the whorl like structures noted in previous studies where they were typically attributed to thermal entrainment of surrounding ambient mantle but this is mainly the product of intermixing between neighboring parts of the plume s deep source material these results imply that the popular idea that mantle plumes should typically mix small fractions of deep primitive mantle material with much larger fractions of shallower depleted mantle needs to be critically reexamined

thorium uranium ratio of the earth through the study of komatiites and ophiolites remnants of the old mantle we review the evolution of the lead system in the depleted mantle through time using the 230th232th vs 208pb 206pb diagram 208pb 206pb radiogenic lead ratio we constrain the th u ratio of the bulk silicate earth to 4 2 this value is also in full agreement with ratios inferred from the study of the various types of meteorites using this value in the 208pb 206pb 143nd144nd diagram bulk earth plots on the oceanic basalt correlation line as previously guessed a comparison with sr and nd data shows that the th u fractionation correlates with that of rb sr and nd sm whereas the u pb fractionation is independent of other ratios

the thrust and shear zones of the moine thrust zone and the nw scottish caledonides the caledonian thrust belts of nw scotland exhibit many features typical of foreland fold and thrust belts the major structures propagated westwards towards the foreland the thin carapace of cambro ordovician sediments being accreted in imbricate and duplex zones beneath the major thrust sheets on a broad scale the rules of thrust geometry are obeyed but many local anomalies are found particularly in the assynt region in assynt several large fault bounded gravity spreading structures surge zones have been identified extensional faults have been traced into strike slip and then contractional faults there are zones of extensional and contractional imbricate faults and extensional and contractional ductile flow similar structures have been found along the length of the moine thrust belt suggesting that gravity spreading was an important mechanism during the later stages of movement within the main caledonian fold belt there are several major tectonic slides which are recognized by a local increase in intensity of deformation the majority are nw verging and were originally gently dipping in the highlands se of the great glen there are early major se verging recumbent folds associated with considerable crustal thickening reflected in high pressure 12kbar metamorphism many of the major slides may have formed by spreading away from this thickened crust the caledonian deformation and metamorphism occurred later in the moines and moine thrust zone than in the dalradian and grampian moines to the se of the great glen this may reflect different levels of deformation alternatively it may argue for a large strike slip displacement on the great glen fault juxtaposing crustal blocks which had experienced different ages of caledonian deformation and metamorphism the late movement on the moine thrust involved spreading from an area of thickened caledonian crust formed either by the stair step trajectory of the basal thrust to the moines and or by thickening due to intrusion of large masses of late caledonian granites

constraining the potential temperature of the archaean mantle a review of the evidence from komatiites the maximum potential temperature of the archaean mantle is poorly known and is best constrained by the mgo contents of komatiitic liquids which are directly related to eruptive temperatures however most archaean komatiites are significantly altered and it is difficult to assess the composition of the erupted liquid relatively fresh lavas from the saskmar suite belingwe greenstone belt zimbabwe 2 7 ga include chills of 25 6 wt mgo and olivines ranging to fo93 6 implying eruption at around 1520 c a chill sample from alexo township ontario also 2 7 ga is 28 wt mgo and associated olivines range to fo94 1 implying eruption at 1560 c however inferences of erupted liquids containing 32 33 wt mgo from lavas in the barberton greenstone belt south africa 3 45 ga and from the perseverance complex western australia 2 7 ga may be challenged on the grounds that they contain excess cumulate olivine or were enriched in mg during alteration or metamorphism re interpretation of olivine compositions from these rocks shows that they most likely contained a maximum of 29 wt mgo corresponding to an eruption temperature of 1580 c this composition is the highest liquid mgo content of an erupted lava that can be supported with any confidence the hottest modern magma on gorgona island 0 155 ga contained 18 20 mgo and erupted at circa 1400 c if 1580 c is taken as the temperature of the most magnesian known eruption then the source mantle from which the liquids rose would have been at up to 2200 c at pressures of 18 gpa corresponding to a mantle potential temperature of 1900 c these temperatures are in excess of the mantle temperatures predicted by secular cooling models and thus komatiites can only be formed in hot rising convective jets in the mantle this result requires that archaean mantle jets may have been 300 c hotter than the archaean ambient mantle temperature this temperature difference is similar to the 200 300 c temperature difference between present day jets and ambient mantle temperatures an important subsidiary result of this study is the confirmation that spinifex rocks may be cumulates and do not necessarily represent liquid compositions

ground melting and ocellar komatiites a lead isotopic study at kambalda western australia stratigraphically and geographically restricted ocellar komatiite flows at kambalda western australia appear to represent the products of ground melting of sulphidic sediments by komatiites in lava channels that localized the fe ni cu sulphide ores an immiscible sulphide liquid formed and gravitationally separated from the melted sediment xenomelt the resultant buoyant silicate liquid being partly or wholly assimilated by the turbulently convecting komatiite magma rarely the xenomelt gravitationally migrated to the top of flows and overflowed into the less turbulent lava levees where it collected to form a separate layer overlying a komatiitic layer within a single flow there was selective preservation of the hybrid felsic layer as an upper ocellar unit within an ocellar komatiite flow in lava levees flanking lava channels the ocellar unit is enriched in elements previously concentrated in the sediments and shows u th pb isotopic systematics akin to the underlying sediments moreover the partitioning relationships of u and pb between the immiscible xenomelt and sulphide liquid enhances the range of u pb ratios for components of the ocellar unit thus allowing sufficient spread of modern uranogenic pb isotopic ratios to form isochrons albeit imprecise ones the range and similarity of model th u data from these flows 2 8 3 9 and adjacent sulphidic sediments 2 3 4 4 mostly 2 8 3 9 contrasts with the generally invariable th u within kambalda ultrabasic basic flows 3 6 3 9 and further supports the ground melting hypothesis

accreted fragments of the late cretaceous caribbean colombian plateau in ecuador the eastern part of the western cordillera of ecuador includes fragments of an early cretaceous c123 ma oceanic plateau accreted around 85 80 ma san juan unit west of this unit and in fault contact with it another oceanic plateau sequence guaranda unit is marked by the occurrence of picrites ankaramites basalts dolerites and shallow level gabbros a comparable unit is also exposed in northwestern coastal ecuador pedernales unit picrites have lree depleted patterns high endi and very low pb isotopic ratios suggesting that they were derived from an extremely depleted source in contrast the ankaramites and mg rich basalts are lree enriched and have radiogenic pb isotopic

geochronology geochemistry and sr nd hf isotopic compositions of mafic ultramafic intrusions in the niubiziliang ni cu sulfide deposit north qaidam orogenic belt nw china implications for magmatic source geodynamic setting and petrogenesis abstract whole rock geochemical and sr nd isotopic data and zircon u pb hf isotopic data are reported for mafic ultramafic intrusions in the niubiziliang ni cu sulfide deposit located in the westernmost segment of the north qaidam orogenic belt nqob nw china ni cu sulfide ores of economic interest are found within these intrusions with the niubiziliang iii and iv intrusions hosting most of the ni cu mineralization the niubiziiang mafic ultramafic intrusions intrude biotite plagioclase gneiss of the paleoproterozoic dakendaban group zircons separated from the gabbro plagioclase bearing clinopyroxenite and olivine websterite yield u pb ages of 389 2 373 2 and 380 2 ma respectively indicating a genetic linkage with middle late devonian regional magmatism 390 370 ma gabbro zircons have ehf t values of 4 2 10 9 with older tdm1 ages 0 86 0 59 ga the niubiziliang intrusions contain many xenoliths of paleoproterozoic biotite plagioclase gneiss and the intrusions have similar trace element patterns with strong enrichment in liles e g k rb and th and moderate depletion in hfses e g nb ta p and ti the major and trace element characteristics and sr nd hf isotopic compositions indicate the parental magmas originated from a metasomatized asthenospheric mantle source that had previously been modified by subduction related fluids the magmas experienced significant crustal contamination both in the magma chamber and during ascent triggering sulfur over saturation that resulted in the deposition and enrichment of sulfides considering the tectonic evolution of this region we infer that the niubiziliang mafic ultramafic intrusions formed in a post collisional extensional setting related to delamination of the subcontinental lithosphere

implications of melting for stabilisation of the lithosphere and heat loss in the archaean abstract the increased depth and volume of melting induced in a higher temperature archaean mantle controls the stability of the lithosphere heat loss rates and the thickness of the oceanic crust the relationship between density distributions in oceanic lithosphere and the depth of melting at spreading centres is investigated by calculating the mineral proportions and densities of residual mantle depleted by extraction of melt fractions the density changes related to compositional gradients are comparable to those produced by thermal effects for lithosphere formed from a mantle which is 200 c or more hotter than modern upper mantle if archaean continental crust formed initially above oceanic lithosphere the compositional density gradients may be sufficient to preserve a thick archaean continental lithosphere within which the archaean age diamonds are preserved the amount of heat advected by melts at mid ocean ridges today is small but heat advected by melting becomes proportionally more important as higher mantle temperatures lead to a greater volume of melt and as the rate of production of oceanic plates increases archaean tectonics could have been dominated by spreading rates 2 3 times greater than now and with mantle temperatures between ca 1600 c and 1800 c at the depth of the solidus mid ocean ridge melting would produce a relatively thick but light refractory lithosphere on which continents could form protected from copious volcanism and high mantle temperatures

climate exposed source rock lithologies crustal uplift and surface erosion a theoretical analysis calibrated with data from the alps north alpine foreland basin system paleofloristic data imply that paleoclimate changed in the swiss alps at the oligocene miocene boundary from humid and hot conditions toward a climate with high temperature and low humidity the aridization is associated with a change in depositional pattern from alluvial fans to lakes and floodplains suggesting decreasing sediment discharge a further 25 40 decrease of sediment discharge occurred at ca 20 ma when the orogenic core of the alps became exposed to the surface we applied a surface processes model to explore potential controls on the pattern of sediment discharge and on the evolution of the alpine drainage basin the model is based on the presumption that the rates of fluvial incision into bedrock are proportional to shear stress exerted by the flowing water the model results imply that the paleoclimate change resulted in an instantaneous decrease of sediment discharge and a vertical topographic growth until steady state conditions between erosional and crustal mass flux are established however exposure of the crystalline core of the alps at ca 20 ma is likely to have resulted in the 25 40 decrease of sediment discharge and the reorganization of the drainage pattern from an orogen normal to an orogen parallel orientation of dispersion

geochemistry and petrogenesis of early carboniferous volcanic rocks in east junggar north xinjiang implications for post collisional magmatism and geodynamic process abstract early carboniferous volcanic rocks in the batamayineishan formation overlie unconformably the molasse deposits and the ophiolitic melanges and are restricted in narrow zones along both sides of the kalamaili orogenic belt in north xinjiang southern central asian orogenic belt these rocks demonstrate the post collisional setting in east junggar commenced in tournaisian and also mark an important transitional period from the final amalgamation to late paleozoic voluminous juvenile granitoids in east junggar the volcanic rocks are composed of basalt basaltic andesite andesite trachyte and rhyolite both mafic and felsic rocks are characterized by enrichments in large ion lithophile elements light rare earth elements and depletion in nb and ta low initial 87sr 86sr and high positive nd t three groups of mafic rocks have been identified shoshonitic group 1 has the highest mgo cao ni and cr and the lowest na2o al2o3 la ba la yb and ba th with primary magma features group 2 calc alkaline and high k calc alkaline mafic rocks have the lowest k2o p2o5 th and th nb and the highest tio2 and group 3 shoshonitic to potassic alkaline has the highest k2o p2o5 la ba la yb and th nb and the lowest tio2 the a type like felsic rocks were derived from the differentiation of the mafic magma geological and geochemical evidences indicate that the batamayineishan formation was generated from the process of slab breakoff detachment group 1 samples are produced by decompressional melting of the upwelling asthenosphere mainly composed of spinel and garnet 50 50 lherzolite which has been enriched by overlying metasomatized lithosphere during ascent group 2 is derived from 5 10 partial melting of shallower spinel bearing lithospheric mantle induced by the hot rising asthenosphere where the contribution of slab derived fluid is predominant low partial melting 3 5 of the mantle wedge and or thickened lithospheric mantle enriched by slab derived components generates group 3 slab breakoff as an important geodynamic process accounts for the post collisional magmatism between 343 5 ma 330 ma providing a model for post collisional crust mantle interaction in the caob