nonuniform response of the major surface currents in the nordic seas to insolation forcing implications for the holocene climate variability 1 high resolution sediment cores from the voring plateau the north iceland shelf and the east greenland shelf have been studied to investigate the stability of major surface currents in the nordic seas during the holocene results from diatom assemblages and reconstructed sea surface temperatures ssts indicate a division of the holocene into three periods the holocene climate optimum 9500 6500 calendar cal years bp the holocene transition period 6500 3000 cal years bp and the cool late holocene period 3000 0 cal years bp the overall climate development is in step with the decreasing insolation on the northern hemisphere but regional differences occur regarding both timing and magnitude of sst changes sites under the direct influence of the norwegian atlantic current and the irminger current indicate sst cooling of 4 5 c from early holocene to present compared to 2 c recorded under the east greenland current superimposed on the general holocene cooling trend there is a high frequency sst variability which is in the order of 1 1 5 c for the voring plateau and the east greenland shelf and 2 5 3 c on the north iceland shelf

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

the ryonchon podiform chromitite from the chongjin ophiolite northern korea peninsula abstract ryonchon chromitite is located structurally in the chongjin area the northern part of the korean peninsula which crops out at chongjin terrane the eastern end of the jilin yanbian hambuk fold belt we made on site geological survey of this deposit and conducted the analyses on the investigated chromitite by means of microscopes single crystal x ray diffraction sxrd and electron probe micro analysis epma to elucidate the phase composition the chemical composition and the texture and structure of the chromitite the chromian spinels from the study area are separated into four groups according to chemical composition of spinel as ordinary spinel group minerals are the studied chromites have different compositions according to their occurrences which is not owing to the precipitation of different pararental melts but the subsolidus re equilibration during the hydrous alternation the massive semi massive unaltered chromite in chromitite from dunite have consistently low cr cr

profiles of ocean island coral reefs controlled by sea level history and carbonate accumulation rates modern and preserved coral reefs on islands exhibit a broad range of forms from actively accreting fringing and barrier reefs to terraces preserved by drowning or subaerial exposure darwin s canonical model of reef development proposes an evolutionary sequence of reef forms as a volcanic island ages and subsides from fringing reef to lagoon bounding barrier reef to atoll compiled data from modern systems show however that many islands do not follow this sequence implying that reefs are shaped by more than island subsidence alone we show that the diversity of modern reef morphology arises from the combined effects of island subsidence coral growth and glacial sea level cycles a model for the evolution of a reef elevation profile over the past 400 k y reveals that different combinations of reef accretion rate and island vertical motion produce a variety of forms that matches the observed distribution of modern reefs this match occurs only if the model is driven by pleistocene sea level oscillations few modern environments have the right combination of conditions to produce the darwinian atoll progression

madagascar s escape from africa a high resolution plate reconstruction for the western somali basin and implications for supercontinent dispersal accurate reconstructions of the dispersal of supercontinent blocks are essential for testing continental breakup models here we provide a new plate tectonic reconstruction of the opening of the western somali basin during the breakup of east and west gondwana the model is constrained by a new comprehensive set of spreading lineaments detected in this heavily sedimented basin using a novel technique based on directional derivatives of free air gravity anomalies vertical gravity gradient and free air gravity anomaly maps also enable the detection of extinct mid ocean ridge segments which can be directly compared to several previous ocean magnetic anomaly interpretations of the western somali basin the best matching interpretations have basin symmetry around the m0 anomaly these are then used to temporally constrain our plate tectonic reconstruction the reconstruction supports a tight fit for gondwana fragments prior to breakup and predicts that the continent ocean transform margin lies along the rovuma basin not along the davie fracture zone dfz as commonly thought according to our reconstruction the dfz represents a major ocean ocean fracture zone formed by the coalescence of several smaller fracture zones during evolving plate motions as madagascar drifted southwards and offshore tanzania is an obliquely rifted rather than transform margin new seismic reflection evidence for oceanic crust inboard of the dfz strongly supports these conclusions our results provide important new constraints on the still enigmatic driving mechanism of continental rifting the nature of the lithosphere in the western somali basin and its resource potential

structures in podiform chromite from the maqsad district sumail ophiolite oman about 15 chromite bodies have been recognized in the maqsad area of the oman ophiolite the occurrence in this area of three chromite bodies within the cumulate sequence must be integrated into the classification of cassard et al 1981 which presently explains only those pods lying in the uppermost mantle sequence plastically deformed harzburgites and dunites the occurrence of chromite bodies within the cumulates and the abundance of chromite in the maqsad area are related to the exceptional magmatic activity and the unusual plastic flow pattern particular to this area it was probably a feeding zone along the oceanic spreading center sitting on top of a mantle diapir

plastic deformation and recrystallization of garnet a mechanism to facilitate diffusion creep elongate and deformed garnets from glenelg nw scotland occur within a thin shear zone transecting an eclogite body that has undergone partial retrogression to amphibolite facies at circa 700 c optical microscopy back scattered electron imaging electron probe microanalysis and electron back scatter diffraction reveal garnet sub structures that are developed as a function of strain subgrains with low angle misorientation boundaries occur at low strain and garnet orientations are dispersed around rational crystallographic axes across these boundaries towards high strain areas boundary misorientations increase and there is a loss of crystallographic control on misorientations which tend towards random in high strain areas a polygonal garnet microstructure is developed the garnet orientations are randomly dispersed around the original single crystal orientation some garnet grains are elongate and ca rich garnet occurs on the faces of elongate grains oriented normal to the foliation commonly the garnet grains are admixed with matrix minerals and where in contact with other phases garnet is well faceted we suggest that individual garnet porphyroclasts record an evolution from low strain conditions where dislocation creep and recovery accommodated deformation through increasing strain where dynamic recrystallization occurred by subgrain rotation to highest strains where recrystallized grains were able to deform by diffusion creep assisted grain boundary sliding with associated rotations

deformation of chromite s e m investigations abstract the scannning electron microscope operated in the back scattered electron mode has been applied in a microstructural analysis of chromites samples from the oman vourinos greece and tiebaghi new caledonia ophiolites displaying a variety of mantle flow and emplacement structures have been examined and compared with undeformed chromite from the bushveld intrusion and deformed chromite from the high grade metamorphic fiskenaesset complex channelling contrast and electron channelling patterns provide information on grain boundaries subgrains inclusions and cracks as well as defects in the chromite lattice these microstructures suggest that dislocation mechanisms are active in chromite deforming under mantle conditions recovery is quick whereas nucleation is slow and accordingly recrystallization of chromite occurs by migration of grain boundaries driven either by strain energy or by interfacial energy increasing the grain size during deformation dislocation mechanisms however only provide a secondary mode of deformation compared to brittle fracturing and passive rotation of chromite grains and fragments in the weaker silicate matrix diffusion accommodated flow may also be significant

advanced insights into magmatism and volcanism of the mozambique ridge and mozambique basin in the view of new potential field data summary a new plate tectonic model is presented which describes the emplacement of the mozambique ridge off southeast africa as the result of long lasting volcanic activity 140 122 ma during the initial opening between africa and antarctica thus an oceanic origin for the mozambique ridge is proposed this model is based on a new and systematic high resolution magnetic anomaly data set acquired across the mozambique ridge and throughout the mozambique basin data from the mozambique basin allow the identification of mesozoic magnetic anomalies from m0r to m26 124 61 155 3 ma with previously unmatched accuracy small scale fracture zones are recognized by offsets of magnetic anomalies in the westernmost part of the basin additionally a bend in the major fracture zones f and e between m17r and m18n 142 84 144 04 ma and a recognized sinusoidal change in spreading direction with an amplitude of about 15 indicate that the basin experienced several small scale changes in spreading direction through time a maximum change in spreading direction to almost 0 at around m11n 135 69 ma is followed by a short lived increase in spreading half rate from 23 5 km ma 1 to about 27 5 km ma 1 in the time frame from m10r to m9n 134 30 132 83 ma we propose that this is related to the initial opening of the south atlantic ocean represented by the onset of seafloor spreading between the falkland plateau and africa in the conjugate georgia and natal basins across the mozambique ridge high amplitude magnetic anomalies at major structural boundaries suggest that the different plateaus of the ridge were formed at different times a simple 2 d gravity and magnetic model for the ridge supports the hypothesis of multiple volcanic episodes which formed the ridge though long lasting volcanic activity between about 140 and 122 ma together with new and verified rotation parameters from the mozambique basin and its conjugate the riiser larsen sea antarctica a series of plate tectonic reconstructions are presented which demonstrate when and how the different parts of the ridge evolved through time

deformation fabric and microstructures in ophiolitic chromitites and host ultramafics sultanate of oman harzburgite and dunite from the oman ophiolite display a strong olivine fabric and microstructures typical for high temperature mantle deformation the syntectonic magmatically formed chromitites occurring throughout the mantle sequence vary from almost undeformed resembling chromitite from stratiform complexes to highly deformed usually the interstitial silicates in the deformed ore possess a fabric similar to those of the host rocks whereas a weak or poikilitic fabric is observed on relatively undeformed deposits chromite grains are elongated and flattened defining strong foliations and lineations in disseminated ore and pull apart like structures perpendicular to the lineation form in the coarser massive ore no preferred crystallographic orientation of chromite has been detected with the x ray texture goniometer and it is proposed that chromite mainly deforms by passive rotation in the weaker silicate matrix cataclasis and diffusion creep

was a change in thermohaline circulation responsible for the little ice age long term 800 year and short term 20 year estimates of the rate of deep water formation in the southern ocean are in serious disagreement this disagreement suggests to me that the rate of deep water formation may have been larger during the little ice age lia than it is now

mesozoic and early cenozoic sediment influx and morphology of the mozambique basin abstract the mozambique basin is one of the oldest extensional sedimentary basins developed along the eastern african margin the basin hosts a continuous record of sediments since the jurassic separation of antarctica from africa the objectives of this study were to extend the regional stratigraphic framework north of the zambezi delta into the deep abyssal plains and review the early evolution of the mozambique basin using nine multi channel seismic reflection profiles we identify six major stratigraphic units that were deposited in jurassic early cretaceous late cretaceous paleogene neogene and quaternary times mesozoic sedimentation rates of 5 10 cm kyr and 1 3 cm kyr during the paleogene are calculated in the deeper basin the presence of shales in neighbouring wells on the shelf implies an euxinic environment in the rapidly subsiding basin until early cretaceous times the mesozoic sediments have a high seismic velocity that exceeds 4 5 km s except in a distinct early cretaceous low velocity 3 7 km s zone that may indicate the presence of undercompacted overpressured shales in spite of the fact that the zambezi catchment was much smaller in pre miocene times the high late cretaceous sedimentation rates can be attributed to rapid denudation of the african continent after a major tectonic uplift episode at approximately 90 ma increased sediment influx into the basin from the zambezi in late cretaceous times resulted in the formation of an elongated submarine fan lobe into the mozambique channel north of beira high strong north south bottom currents commenced within the channel in late cretaceous times forcing the aggradation of sediments on the southern flank of the lobe in addition we observe several current controlled sediment deposits in the deeper basin that are influenced by north south bottom currents low paleogene sedimentation rates are attributed to a sediment starved basin during a relative quiet tectonic phase onshore

cataclastic deformation mechanism for the development of core mantle structures in amphibole core mantle structures in mineral grains have typically been interpreted as having formed during crystal plastic deformation optical scanning electron and transmission electron microscope investigations of what appear to be core mantle structures in amphibole from the cheyenne belt southeastern wyoming indicate that subgrainlike structures herein subgrains formed as a result of cataclastic rather than crystal plastic deformation optical and scanning electron microscope evidence to support a cataclastic origin include 1 the angular shape of subgrains with boundaries commonly parallel to 110 cleavage 2 the wide variety of subgrain sizes 3 the compositional zonation of the amphibole subgrains with actinolitic hornblende interiors overgrown by magnesio hornblende and 4 the observation that some of the undulatory extinction in porphyroclast cores is due to slip along cleavage planes whether dislocations observed in the amphibole grains were formed prior to cataclastic deformation work hardening or formed in response to crack propagation is unknown the implication of these observations is that core mantle structures in other minerals may at least in some cases have also formed during cataclastic deformation rather than during recovery and or recrystallization therefore optical identification of core mantle structures is not a priori evidence for a crystal plastic deformation mechanism

growth and demise of cenozoic isolated carbonate platforms new insights from the mozambique channel seamounts sw indian ocean although long term evolutions of isolated shallow water carbonate platforms and demise episodes leading to guyot formation have been the subject of numerous studies during the last decades their driving processes are still the subject of active debates the mozambique channel sw indian ocean is characterized by several flat topped seamounts ranging from 11 s to 21 s in latitudes based on a comprehensive geomorphologic study and on dredged samples analysis we show that these features correspond to tropical isolated shallow water carbonate platforms coupling strontium isotopy and foraminifera biostratigraphy well constrained chronostratigraphy results indicate that shallow water carbonate production started in the mozambique channel during distinct cenozoic periods ranging from paleocene to early miocene our data also demonstrate that these carbonate platforms were subsequently characterized by different evolutions locally marked by tectonic and rejuvenated volcanism while some of them kept developed until present days forming modern carbonate systems some others were drowned during late neogene and subsided to form guyots although different factors can be discussed tectonic and volcanism appear as good potential triggers for demise episodes during late miocene early pliocene times chronology and location of this geodynamical activity tend to emphasize influence of east african rift system until southern mozambique channel

marine geology of the sodwana bay shelf southeast africa abstract the geostrophic current controlled northern zululand shelf displays a unique assemblage of interesting physical sedimentological and biological phenomena the shelf in this area is extremely narrow 3 km and is characterised by submarine canyons coral reefs and steep gradients on the continental slope three submarine canyons occur in the study area and are classified as mature or youthful phase canyons depending on the degree to which they breach the shelf these canyons originated as mass wasting features which were exploited by palaeo drainage during sea level regressions shelf lithology is dominated by a series of coast parallel patch coral reefs which have colonised beachrock and aeolianite sequences that extend semi continuously from 5 to 95 m and delineate late pleistocene palaeocoastline events the unconsolidated sediment on the shelf is either shelf sand mainly terrigenous quartz grains or bioclastic sediment large scale subaqueous dunes commonly form in the unconsolidated sediment on the outer shelf due to the agulhas current flow these dunes occur as two distinct fields at depths of 35 to 70 m the major sediment transport direction is towards the south but occasional bedload parting zones exist where the bedform migration direction changes from south to north

a paleoclimate record of the last 17 600 years in stalagmites from the b7 cave sauerland germany abstract petrographical and geochemical parameters of stalagmites from the b7 cave in iserlohn letmathe northern rhenish massif nw germany record late and postglacial climate changes temperature and or precipitation fabrics and microfacies of the stalagmite profiles lead to a differentiation of four hierarchies of rhythms clastic layers in the stalagmites are caused by flooding events and are time markers twenty four tims th u age dates provide a time calibration of stalagmite growth phases one stalagmite reveals an early growth period between 17 6 and 16 7 ka bp between 9 6 and 5 5 ka bp atlantic episode of the holocene the growth rate of the stalagmites was higher than before and after this time with dominant light porous microfacial laminae and high 18 o and 13 c values representing partly kinetic fractionation effects this part of the holocene is interpreted as a mainly warm episode with frequent interruptions of dripping within the past 4 ka the profiles with predominant dark compact facies reveal low isotopic values which may be interpreted as a temperature proxy record the stalagmite records resemble records from an irish stalagmite correlation with the 14 c record from european tree rings suggests that colder periods in the north atlantic were accompanied by drier winters in central europe

deglacial changes in ocean circulation from an extended radiocarbon calibration temporal variations in the atmospheric concentration of radiocarbon sometimes result in radiocarbon based age estimates of biogenic material that do not agree with true calendar age this problem is particularly severe beyond the limit of the high resolution radiocarbon calibration based on tree ring data which stretches back only to1 2 about 11 8 kyr before present bp near the termination of the younger dryas cold period if a wide range of palaeoclimate records are to be exploited for better understanding the rates and patterns of environmental change during the last deglaciation extending the well calibrated radiocarbon timescale back further in time is crucial several studies attempting such an extension using uranium thorium dated corals3 4 5 and laminae counts in varved sediments6 7 8 9 show conflicting results here we use radiocarbon data from varved sediments in the cariaco basin in the southern caribbean sea to construct an accurate and continuous radiocarbon calibration for the period 9 to 14 5 kyr bp nearly 3 000 years beyond the tree ring based calibration a simple model compared to the calculated atmospheric radiocarbon concentration and palaeoclimate data from the same sediment core suggests that north atlantic deep water formation shut down during the younger dryas period but was gradually replaced by an alternative mode of convection possibly via the formation of north atlantic intermediate water

podiform chromitites of the tari misaka ultramafic complex southwestern japan as mantle melt interaction products petrological and geochemical data for chromian spinel from the tari misaka ultramafic complex southwest japan are discussed in this complex the spinels form a chromitite pod coated by a dunite envelope and are collectively enclosed by clinopyroxene bearing harzburgite chromitite and its dunite envelope are distinctly different from the surrounding harzburgite in terms of the chromian spinel chemistry the ti content is five to ten times higher in spinels of the enveloped chromitite than in those of the harzburgite although the cr ratio cr cr al the atomic ratio for both is almost constant or varies only slightly 0 4 0 5 this eliminates a comagmatic origin for the harzburgite dunite chromitite association spinel chemistry for both major and trace elements is almost the same in the chromitite and its dunite envelope except for the mg ratio and the co and mn contents which have a changed subsolidus as a function of the spinel mode thus the dunite and chromitite could be comagmatic but fractional crystallization of the dunite after or before chromitite deposition is not possible the chemistry of the chromian spinel and the mode of occurrence of the dunite chromitite association are consistent with an origin by interaction of an exotic melt with wall rock harzburgite the dunite may be of replacive origin the exotic melt would react with the harzburgite at the new contact to form dunite and a secondary si rich melt the melt thus formed could mix with a successive inflow of a more primitive melt the mixed melt would be expected to have chromite chromian spinel as the only liquidus phase precipitating chromitite next to the replacive dunite the dunite envelope podiform chromitite is most commonly formed in the mantle beneath island arcs the spread in cr ratios of spinel in podiform chromitites is almost comparable with that of spinel in primitive arc related magmas the presence of chromitite in the upper mantle beneath the japan arcs is consistent with this interpretation

plastic deformation and post deformation annealing in chromite mechanisms and implications plastic deformation in chromite is not frequently reported in literature we present a detailed microstructural analysis of this mineral from the massive chromitite of the neoarchaean sittampundi complex southern india the study reveals intracrystalline plasticity is dominantly active in this mineral and it produces distinctive features corresponding to at least two different microstructural regimes the regime 1 is deformation related and it commenced with recovery of strained grains and formation of new grains corresponding to subgrain rotation recrystallization this was followed by nucleation of strain free new grains in regions of high strain the regime 2 appears to be post deformational and dominantly temperature controlled producing distinctive features of static annealing of already deformed grains this regime corresponds to grain boundary migration recrystallization resulting coarsening of strain free facetted new grains at the expense of high dislocation density subgrains the resultant micro features resemble closely what is known as abnormal grain growth not yet documented for chromites these coarse grains in places also feature accommodation of deformation by displaying very low angle subgrain boundaries the movement of high angle grain boundaries in regime 2 through precursor strained grains provided high diffusivity paths for the rapid exchange of components producing compositional heterogeneity between grains dominated by deformation features and facetted new grains these microstructural observations coupled with chemical heterogeneity provide new directions in interpreting the deformation mechanism of chromite and its annealing history at the post deformation stage

climate change and solar variability what s new under the sun the sun has an obvious effect on climate since its radiation is the main energy source for the outer envelopes of our planet nevertheless there is a long standing controversy on whether solar variability can significantly generate climate change and how this might occur this is a crucial issue not only in the field of paleoclimatology but also for predicting the future of the earth s climate which will be subject to perturbations by anthropogenic greenhouse gases indeed if climate changes due to the sun were large and rapid this would make it more difficult to extract the anthropogenic effects from precise records of instrumental data over the past century hence sun climate relationships have never been so controversial as today forming a debate that often escapes the scientific arena here we provide a review of this problem by considering changes on different time scales from the last million years up to recent decades in doing so we also critically assess recent claims that the variability of the sun has had a significant impact on global climate the different studied records also illustrate the multi disciplinary nature of this difficult problem requiring knowledge in several fields such as astronomy and astrophysics atmospheric dynamics and microphysics isotope geochemistry and geochronology as well as geophysics paleoceanography and glaciology overall the role of solar activity in climate changes such as the quaternary glaciations or the present global warming remains unproven and most probably represents a second order effect although we still require even more and better data the weight of evidence suggests that solar changes have contributed to small climate oscillations occurring on time scales of a few centuries similar in type to the fluctuations classically described for the last millennium the so called medieval warm period 900 1400 a d followed on by the little ice age 1500 1800 a d

the indus yarlung zangbo ophiolites from nanga parbat to namche barwa syntaxes southern tibet first synthesis of petrology geochemistry and geochronology with incidences on geodynamic reconstructions of neo tethys the purpose of this first synthesis is to summarize findings on the yarlung zangbo suture zone yzsz ophiolites and discuss still remaining problems the yzsz studied for almost 30 years and is the youngest of the sutures recognized on tibet plateau it is now acknowledged that the yzsz is a complex assemblage of sedimentary metamorphic and igneous rocks created during and shortly after the collision between india and eurasia the ages of the various lithologies span a time interval from the jurassic to the middle miocene with some permian and devonian exotic blocks from melange zone the yzsz is characterized by ophiolitic complexes and ophiolitic melange the ophiolites are of two types non dismembered and dismembered sections the non disturbed sections although tectonically reworked are observed along the segment from dazhuqu to jiding in xigaze area and spontang ophiolite the dismembered sequences are found in various locations such as nidar kiogar jungbwa saga sangsang xigugabu and luobusa the incomplete stratigraphic log could be connected to intraoceanic or orogenetic origins the ophiolites are distributed into two groups of ages the luobusa zedang and kiogar sequences being jurassic lower cretaceous whereas all other sequences are of lower cretaceous age compilation of geochronological data suggest that some ophiolite sequences might have evolved for over more than 70 my from their initial genesis to obduction which occurred around 70 90 my ago ophiolites differ in terms of petrological and geochemical aspects however they were all generated in suprasubduction zone and more specifically in arc few fore arc and back arc settings synthesis of more than 700 geochemical analyses show variable mixing of components from n morb type to iat cab and to oib end members the jurassic ophiolites show the maximum of arc component while the lower cretaceous ones show little to strong mixing in addition most ophiolites were created in short lived 30 my basins and generated close to the eurasiatic continental margin we propose that ladakh tibet ophiolites were generated in a suprasubduction context similar to mariana arc interarc and back arc or tonga lau system the variable arc signature of these ophiolites is directly related to their initial position within the suprasubduction system

formation of podiform chromitites by melt rock interaction in the upper mantle high cr and high al podiform chromitites are derived from melts formed by high and low degrees of partial melting in the upper mantle respectively the close association of the chromitites regardless of compositions with dunites and harzburgites can be explained by interaction between melts and harzburgites or depleted lherzolites in the upper mantle continuous reaction between melt and wallrock modifies the compositions of the magmas towards the precipitation of chromite alone under upper mantle conditions the modified magmas would have higher silica and lower magnesia and would be extracted into the crust or erupted leaving chromite pods in situ with dunite rinds that grade to harzburgites or lherzolites

mesozoic exhumation of the southern cape south africa quantified using apatite fission track thermochronology southern africa s topography is distinctive an inland plateau of low relief and high average elevation is separated from a coastal plane of high relief and low average elevation by a steeply dipping escarpment the origin and evolution of this topography is poorly understood because unlike high plateaus elsewhere its development cannot be easily linked to present day compressional plate boundary processes understanding the development of this regional landscape since the break up of gondwana is a first order step towards resolving regional epeirogenesis we present data that quantifies the timing and extent of exhumation across the southern cape escarpment and coastal plane using apatite fission track analysis afta of 25 outcrop samples and 31 samples from three deep boreholes kw1 67 sa1 66 cr1 68 outcrop fission track aft ages are cretaceous and are significantly younger than the stratigraphic ages of their host rocks indicating that the samples have experienced elevated paleotemperatures mean track lengths vary from 11 86 to 14 23 m the lack of cenozoic apatite ages suggests that major cooling was over by the end cretaceous the results for three boreholes situated seaward south of the escarpment indicate an episode of increased denudation in the mid late cretaceous 100 80 ma an earlier episode of increased denudation 140 120 ma is identified from a borehole north of the escarpment thermal modelling indicates a history involving 2 5 3 5 km of denudation in the mid late cretaceous 100 80 ma at a rate of 175 to 125 m ma the aft data suggest that less than 1 km of overburden has been eroded regionally since the late cretaceous 80 ma at a rate of 10 to 15 m ma but do not discount the possibility of minor in relative amplitude episodes of uplift and river incision through the cenozoic the reasons for rapid denudation in these early and mid cretaceous episodes are less clear but may be related to epeirogenic uplift associated with an increase in mantle buoyancy as reflected in two punctuated episodes of alkaline intrusions e g kimberlites across southern africa and contemporaneous formation of two large mafic igneous provinces 130 and 90 ma flanking its continental margins because cenozoic denudation rates are relatively minimal epeirogenic uplift of southern africa and its distinct topography cannot be primarily related to cenozoic mantle processes consistent with the lack of any significant igneous activity across this region during that time

the offshore east african rift system structural framework at the toe of a juvenile rift the cenozoic east african rift system ears extends from the red sea to mozambique here we use seismic reflection and bathymetric data to investigate the tectonic evolution of the offshore branch of the ears the data indicate multiple and time transgressive neotectonic deformations along 800km of the continental margin of northern mozambique we observe a transition from a mature rift basin in the north to a juvenile fault zone in the south the respective timing of deformation is derived from detailed seismic stratigraphy in the north a 30km wide and more than 150km long n s striking symmetric graben initiated as half graben in the late miocene extension accelerated in the pliocene causing a continuous conjugate border fault and symmetric rift graben coevally the rift started to propagate southward which resulted in a present day 30km wide half graben approximately 200km farther south since the pleistocene the rift has continued to propagate another 300km where the incipient rift is reflected by subrecent small scale normal faulting estimates of the overall brittle extension of the matured rift range between 5 and 12km with an along strike southward decrease of the extension rate the offshore portion of the ears evolves magma poor similar to the onshore western branch the structural evolution of the offshore ears is suggested to be related to and controlled by differing inherited lithospheric fabrics preexisting fabrics may not only guide and focus extension but also control rift architecture

collisional emplacement history of the naga andaman ophiolites and the position of the eastern indian suture abstract dismembered late mesozoic ophiolites occur in two parallel belts along the eastern margin of the indian plate the eastern belt closely following the magmatic arc of the central burma basin coincides with a zone of high gravity it is considered to mark a zone of steeply dipping mafic ultramafic rocks and continental metamorphic rocks which are the locus of two closely juxtaposed sutures in contrast the western belt which follows the eastern margin of the indo burma range and the andaman outer island arc broadly follows a zone of negative gravity anomalies here the ophiolites occur mainly as rootless subhorizontal bodies overlying eocene oligocene flyschoid sediments two sets of ophiolites that were accreted during the early cretaceous and mid eocene are juxtaposed in this belt these are inferred to be westward propagated nappes from the eastern belt emplaced during the late oligocene collision between the burmese and indo burma andaman microcontinents ophiolite occurrences in the andaman islands belong to the western belt and are generally interpreted as upthrust oceanic crust accreted due to prolonged subduction activity to the west of the island arc this phase of subduction began only in the late miocene and thus could not have produced the ophiolitic rocks which were accreted in the late early eocene

the initial gondwana break up a synthesis based on new potential field data of the africa antarctica corridor abstract the initial opening of the africa antarctica corridor in the heart of gondwana is still enigmatic due to missing information on the origin of major crustal features and the exact timing of the onset of the first oceanic crust in the jurassic therefore in 2014 new ship borne magnetic data were systematically acquired in the northern mozambique basin and across beira high which we merged with all accessible magnetic data in the mozambique basin herein distinct magnetic lineations are observed which allow a refined identification of a whole set of jurassic magnetic spreading anomalies constraining the timing of the onset of oceanization beginning at m38n 2n 164 1 ma in combination with high resolution potential field data from the conjugate antarctic margin well expressed fracture zones can be traced throughout the africa antarctica corridor and allow the precise rotation of antarctica back to africa the initial fit depicts striking continuations of onshore tectonic features across the plate boundaries taking onshore aeromagnetic data of both margins into account within a tight gondwana fit the beira high can be restored along the major sinistral namama orvin shear zone of the east african antarctic orogen the beira high represents a continental block which was detached from antarctica by 157 ma at the latest simultaneously the antarctic plate cleared the area of the mcp however the crustal nature of the southern mcp remains ambiguous the northern natal valley and the mozambique ridge consist of thick oceanic crust being emplaced between m26r m18n 157 1 144 ma and m18n m6n 144 131 7 ma respectively about the half of this crust was won from the antarctic plate by a series of southwards directed ridge jumps to the northern boundary of the explora wedge a refined kinematic break up model constrained by the most extensive magnetic dataset is presented describing consistently the initial opening of the africa antarctica corridor and the somali basin

grain scale plastic deformation of chromite from podiform chromitite of the naga manipur ophiolite belt india implication to mantle dynamics abstract the mantle peridotites of the naga manipur ophiolite belt india host concordant podiform chromitite bodies that preserve various magmatic and deformation structures magmatic structures preserve evidence of a gradual transition from nodular to massive varieties nodular types are least affected by deformation deformation structures restricted mainly in massive types include intracrystalline features of brittle as well as crystal plastic processes within chromite brittle deformation is preserved as grain fracturing and intense microbrecciation along narrow zones forming protocataclasite crystal plastic deformation has generated chromite subgrains and new grains in response to recovery and recrystallization processes subgrains are polygonal in shape and the boundaries between them are commonly straight forming a mosaic with most meeting at triple junctions subgrains are commonly internally zoned and the mosaics formed by them correlate well with the compositional maps subgrains show high al at their cores and high fe at their boundaries in response to differential diffusive chemical re equilibration we discuss how this compositional resetting related to deformation is significant in various geologic studies related to tectonic setting and thermobarometric evolution of an area deformation mechanisms are likely to have started with crystal plastic processes and later switched over to the brittle regime the preservation of all grain scale deformation structures in the chromitite bodies of the naga manipur ophiolite belt is suggestive of mantle dynamics vertical accretion and horizontal movement in particular

linking source and sink evaluating the balance between onshore erosion and offshore sediment accumulation since gondwana break up south africa abstract the geomorphic origin and evolution of the tectonically unique interior highland of southern africa the kalahari plateau and its flanking low lying coastal planes remain largely unresolved because of a lack of regional quantitative analyses of its uplift and erosion history here we focus on the southern cape south africa and link onshore denudation based on new apatite fission track thermochronology results to offshore sediment accumulation using abundant well data and a seismic reflection profile we attempt to relate source and sink in order to resolve some first order issues concerning timing of the exhumation and development of the topographic features of southern africa the volume of sediment accumulated off south africa s south coast is calculated using 173 wells and a seismic reflection profile a total uncompacted sediment volume of 268 500 km3 accumulated off south africa s south coasts since 136 ma in the outeniqua and southern outeniqua basins accumulation volumes and rates were highest in the early cretaceous 48 800 104 km3 at 8150 km3 ma from 136 to 130 ma and 57 500 104 km3 at 5750 km3 ma from 130 to 120 ma and mid late cretaceous 83 700 104 km3 at 3200 km3 ma from 93 to 67 ma volumes and accumulation rates were lowest for the early mid cretaceous 47 400 104 km3 at 1750 km3 ma from 120 to 93 ma and the cenozoic 31 200 104 km3 at 450 km3 ma from 67 to 0 ma although our analysis shows that the accumulated volume of offshore sediments does not match the calculated volume of onshore erosion as quantified through apatite fission track thermochronology e g tinker j h de wit m j brown r 2008 mesozoic exhumation of the 439 southern cape south africa quantified using apatite fission track thermochronology tectonophysics doi 10 1016 j tecto 2007 10 009 the timing of increased sediment accumulation closely matches the timing of increased onshore denudation this suggests that the greatest volumes of material were transported from source to sink during two distinct cretaceous episodes and that the processes driving onshore denudation decreased by an order of magnitude during the cenozoic

the separation of madagascar and africa identification of a sequence of east west trending magnetic anomalies of mesozoic age in the western somali basin helps define the position of madagascar in the gondwana reconstruction the anomalies are symmetric about ancient ridge segments and are flanked to the north and south by the jurassic magnetic quiet zone the motion of madagascar relative to africa was from the north and began in the middle jurassic about the same time as the initial breakup of gondwanaland sea floor spreading ceased when madagascar assumed its present position in the early cretaceous

geomorphology and sedimentology of a modern isolated carbonate platform the glorieuses archipelago sw indian ocean abstract the study of modern carbonate systems commonly helps in improving facies interpretation in fossil reefs and in providing analogues of sediment distribution depending on the specific platform configuration i e rimmed shelves and isolated carbonate platforms this paper deals with a geomorphological and sedimentological study of the glorieuses archipelago an isolated carbonate platform located between the northern tip of madagascar and mayotte the dataset consists of digital terrain model satellite imagery and box sediment samples analyses of grain size and composition of carbonate grains are used to characterize the distribution and heterogeneity of sediment accumulated on the isolated platform main results show that the glorieuses archipelago is organized in distinctive morphological units including a reef flat developed along the windward side an apron and a semi enclosed 12 m and up to 15 m water depth the lack of carbonate mud in sediments deposited on the archipelago can be explained by the direct connection between the lagoon and the open ocean the main carbonate grains include halimeda segments coral fragments large benthic foraminifers red algae and molluscs according to the shape and the position of intertidal sandwaves the current arrangement of moderately sorted fine to medium sands appears to be strongly influenced by tidal currents the in situ sediment production accumulation and transport on the platform finally contribute to carbonate sand export to distinct deep marine areas depending on wind regimes and currents

sub milankovitch climatic cycles in holocene stalagmites from sauerland germany calcitic stalagmites from caves in the sauerland germany prove the existence of sub milankovitch cycles in precipitation during the last 6000 yr the 18o record dated with th u is interpreted as an indicator of paleohumidity spectral analysis of 18o from 6000 a bp up to the recent top of a stalagmite from the atta cave yields statistically significant peaks at 1450 117 64 and 57 a additionally we find a good correlation of the stalagmite s 18o and 14c from european tree rings the 1450 a cycle in the stalagmite probably is analogous to the pervasive millennial scale climate cycle described by bond et al science 278 1997 1257 1266 294 2001 2130 2136 derived from the amount of ice rafted debris in deep sediments from the north atlantic our results suggest that the centennial to millennial shifts observed in the north atlantic are accompanied by synchronous shifts of the climate in northern and central europe which most probably can be attributed to solar irradiation variations

somali basin chain ridge and origin of the northern somali basin gravity and geoid low the northern somali basin located between chain ridge and the horn of africa north of 4 n is characterized by a distinct 5 m geoid low and by large negative gravity anomalies the boundaries of the basin are marked by steep gradients in both gravity and geoid basement in northern somali basin is 1 2 km deeper than on the carlsberg ridge flank to the southeast or on the sheba ridge flank to the north with a sharp discontinuity across the boundary of the basin the western somali basin to the south was created in the late jurassic and early cretaceous by the movement of madagascar away from africa reinterpretation of the magnetic anomalies in the western somali basin shows that they record both limbs of a mid ocean ridge that was active by m22 time kimmeridgian and died soon after mo aptian magnetic and gravity data allow the relict ridge crest to be traced from davie ridge near the african coast to the dhow vlcc ars fracture zone complex at 50 e davie ridge is a transform fault connecting the western somali basin spreading center with a similar age spreading center in the mozambique basin the dhow vlcc ars complex can be shown to continue north of the 4 n bathymetric high separating the northern and western somali basins and to intersect the african margin near 7 n the northern somali basin thus appears to be the third of a series of oceanic basins separated by long transform faults created during movement between east and west gondwanaland the original northern somali basin was split apart by the northward motion of india in the late cretaceous and chain ridge formed along the new boundary thermal and gravity modeling shows that the flexure resulting from differential subsidence across chain ridge combined with the difference in lithospheric thermal structure late jurassic vs early tertiary on either side of it explains well the amplitude and shape of the observed geoid step and gravity anomalies across chain ridge the geoid step up from the basin up to the african coast can be modeled as an edge effect between the juxtaposed jurassic oceanic and african continental crust thus the geoid and gravity low over the northern somali basin results from the superposition of a continental edge effect anomaly and the fracture zone edge effect anomaly

reconstruction of madagascar and africa evidence from the davie fracture zone and western somali basin new seismic reflection gravity and magnetic data from offshore east africa allow the davie fracture zone to be traced from 11 s to its intersection with the kenyan coast at 2 s constraining the relative motion of madagascar and africa seasat derived free air gravity anomalies and slope rise positive magnetic anomalies observed in shipboard data help to locate the continent ocean boundaries cob off the shore of east africa and madagascar seismic reflection data further document a diapirprovince off madagascar presumably conjugate to that observed off kenya and somalia the dhow and very large crude carrier vlcc basement ridges are complex features and do not appear to be simple fracture zones owing their existence entirely to the separation of madagascar and africa from these data we determine a predrift fit of madagascar and africa involving a 14 2 rotation of madagascar to africa about a pole at 10 n 150 e the geometry of the reconstruction adheres to seismic and potential field data indicating the oceanic nature and extent of the comoros basin and of the somali basin between kenya and the seychelles and it does not conflict with onshore or offshore stratigraphy timing of the opening of the western somali basin is constrained by mesozoic marine magnetic anomalies and extrapolation to the interpreted cob and occurred between approximately 165 and 130 ma

evolution of the glorieuses seamount in the sw indian ocean and surrounding deep somali basin since the cretaceous abstract little is known about the geological history of the glorieuses seamount including basic information about its age and origin related to the regional evolution of the southern tip of the somali basin this study focused on describing and reconstructing the long term stratigraphic evolution of the glorieuses seamount sw indian ocean to identify the mechanisms that have occurred through time to finally shape the emerged modern islands distinct terrace levels currently submerged along the flanks of the seamount and surrounding seamounts have already been interpreted as resulting from successive carbonate development and back stepping episodes over the last 62 myr new isotopic and biostratigraphic dating on the flanks of the seamount coupled with sequence stratigraphic interpretation of seismic profiles acquired in the adjacent basin provide new constraints for the late cretaceous and cenozoic vertical evolution of the seamount topped by carbonate platforms and sedimentation in the surrounding deep basin even if starved steep slopes prevent a straightforward source to sink continuity between the platform and the basin domains our findings propose a consistent chronostratigraphic framework for the identified seismic markers and sequences in the deep basin and discuss a long term geological model that includes the main driving factors behind deposition volcanic events subsidence vs uplift phases climate and hydro dynamism changes and their quantitative impact on the evolution of the isolated carbonate sedimentary system our results show that i the glorieuses volcanic seamount emerged from two successive late cretaceous magmatic pulses firstly during the turonian then during the maastrichtian ii at least two potential uplift phases are recognized during the tertiary paleogene and or the eocene and tortonian iii basinal sedimentation recorded an abrupt change probably related to major regional hydro dynamical changes in late eocene times in the western indian ocean iv the export of sediments from the platform towards the basin numerous gravity flow processes is strongly enhanced after the mid miocene and is probably linked to the onset of the asian monsoon winds and bipolar circulation finally the glorieuses seamount although located in the vicinity of the comoros islands appears to have a much longer history and is geologically more comparable to the nearby seychelles this long term study has enabled us to associate the glorieuses seamount with the sse nnw madagascar seychelles alignment rather than with the comoro hot spot evolution

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

persistent influence of the north atlantic hydrography on central european winter temperature during the last 9000 years 1 a prominent feature of the n atlantic s holocene climate is a series of centennial and millennial shifts in ocean surface hydrography bond et al 2001 oppo et al 2003 here we compare a climate reconstruction from the central alps for the last 9 000 years derived from the oxygen isotope record in stalagmites with the percentage of hematite stained grains in n atlantic sediments an indicator for the proportion of ice bearing surface water from north of iceland the excellent match between the two curves proves that meteorological conditions in the alpine region responded synchronously to changes in the hydrography of the n atlantic during winter in addition the speleothem record suggests significant multi centennial and millennial variability of n european winters throughout the past 9 000 years that might have been incompletely recorded in tree ring archives

holocene history of drift ice in the northern north atlantic evidence for different spatial and temporal modes 1 we present new high resolution proxy data for the holocene history of drift ice off iceland based on the mineralogy of the 2 mm sediment fraction using quantitative x ray diffraction these new data bolstered by a comparison with published proxy records point to a long term increasing trend in drift ice input into the north atlantic from 6 to 5 ka toward the present day at sites influenced by the cold east greenland current this feature reflects the late holocene neoglacial or cooling period recorded in ice cores and further terrestrial archives on greenland in contrast a decrease in drift ice during the same period is recorded at sites underlying the north atlantic drift which may reflect a warming of this region the results document that holocene changes in iceberg rafting and sea ice advection did not occur uniformly across the north atlantic centennial scale climate variability in the north atlantic region over the last 4 kyr is linked to the observed changes in drift ice input increased drift ice may have played a role in the increase of cold intervals during the late holocene e g the little ice age cooling

cretaceous palaeopositions of the falkland plateau relative to southern africa using mesozoic seafloor spreading anomalies summary mesozoic seafloor spreading anomalies mo m10 have been identified in the natal valley between the south eastern african margin and the mozambique ridge these identifications confirm that seafloor spreading north and south of the falkland agulhas fracture zone fafz began simultaneously the natal valley anomalies are offset 1300km by the fafz from their equivalents in the southern cape basin their positions confirm that the tugela ridge marks the continent ocean boundary cob in the natal valley we have computed successive falkland plateau palaeopositions using natal valley and georgia basin anomalies these show that the offset in spreading ridges at the fafz remained 1300km long from m10 to mo time by anomaly 34 time the offset was 1270 km therefore no major ridge jumps had occurred by then dating mo as 108 myr bp and anomaly 34 as 80 myr bp the average halfspreading rate immediately south of the fafz for the cretaceous normal polarity epoch is 4 2 cm yr using this we date a the change in early pole of rotation at 105 myr b a reconstruction which juxtaposes salt boundaries in the brazil and angolan basins at 103 7 myr c final separation of the falkland plateau from southern africa at 98 3 myr d the formation of the oceanic northern part of the agulhas plateau at 97 3 90 7 myr comparison of magnetic data with implied cob positions in the southernmost cape and argentine basins and the georgia basin suggests continental separation began 122 1 27 myr bp with undeformed magnetic anomalies dating from 122 myr m10 these dates are consistent with micropalaeontological and sedimentological data around southern africa

is there continental crust underneath the northern natal valley and the mozambique coastal plains 1 to draw conclusions about the crustal nature and history of the natal valley and the mozambique ridge systematic potential field data were obtained during the aistek iii cruise with r v pelagia in 2009 this paper presents and interprets the results of that expedition the new magnetic data reveal a pattern of linear magnetic spreading anomalies nw se trending in the southwestern part of the mozambique ridge and e w trending on its central part the ariel graben which separates the mozambique ridge from the northern natal valley is represented by a pronounced negative anomaly in the magnetic as well as the free air gravity field the northern natal valley bears a complicated pattern of anomalies with mainly sw ne trends in the northern natal valley no indications for a continent ocean boundary between continental crust in the north and oceanic crust in the south exist either in the free air gravity or in the magnetic field the magnetic wavelengths of the mozambique coastal plains are similar to those of the northern natal valley and the mozambique ridge particularly in the gravity data the mozambique coastal plains the northern natal valley and the mozambique ridge appear as one continuous geological province we interpret the region from the mateke sabi monocline in the north to the southwestern tip of the mozambique ridge as mainly being floored by thickened oceanic crust

development of the mozambique and ruvuma sedimentary basins offshore mozambique two major sedimentary basins have been identified on the continental margin of mozambique mozambique basin and ruvuma basin the formation of the basins is related to the break up of gondwana and opening of the western part of the indian ocean the basins are relatively young having developed discordantly to the structural plan of gondwana sedimentary basins the history of the formation of the east african continental margin sedimentary basins within mozambique has been studied on the basis of the present day concept of gondwana break up and madagascar s drift with respect to africa two stages in the history of the east african basins can be recognized late gondwana and post gondwana the late gondwana stage 303 157 ma is typified by sedimentary and volcanogenic rocks of the karoo group the post gondwana stage 157 0 ma corresponds to the period of active gondwana break up and the formation of indian ocean marginal basins the mozambique basin occupies both central and southern parts of the coastal plain of mozambique extending onto the continental shelf and slope the sedimentary fill is composed of upper jurassic cretaceous and cenozoic rocks which discordantly overly the karoo basalts the upper jurassic occurs as continental red beds mostly distributed within buried grabens cretaceous rocks occur as terrigenous sediments of continental and marine genesis cenozoic deposits are of predominantly marine and deltaic origin the ruvuma basin situated in the north of mozambique is part of an extended east african marginal basin which includes parts of the coastal plains and continental margins of tanzania and kenya the basement of the basin is composed of crystalline and metamorphic rocks of pre cambrian age the sedimentary fill is represented by continental terrigenous karoo sediments marine and lagoonal jurassic deposits and marine and deltaic rocks of cretaceous and cenozoic age the main difference between the sedimentary fill of the mozambique and ruvuma basins is related to the changing sedimentary environment in the period of gondwana break up jurassic and early cretaceous when the drift of madagascar and sea floor spreading took place and the marine basin gradually extended southwards between the east african margin and madagascar from the late cretaceous on the sedimentary environment within both basins was similar the mozambique and ruvuma basins forming a single system on the east african continental margin the main stages of the basins development are illustrated by a series of palaeogeographic reconstruction maps

solid sediment load history of the zambezi delta abstract we calculate the solid sediment load history of the zambezi river for the last 120 ma i e middle cretaceous recent times our starting point is a detailed grid of two dimensional seismic reflection profiles calibrated by well log information after conversion from two way travel time to depth we made simple assumptions about the compaction history in order to construct isochore maps of solid sediment load and yield as a function of geological time our results show that solid sediment load has varied by 1 order of magnitude over the last 120 ma there have been three periods of elevated flux the first period occurred in late cretaceous times 90 65 ma and was synchronous with the rapid denudation of southern africa recognised by for example apatite fission track modelling the second period occurred in oligocene times 34 24 ma during proposed rapid regional uplift of southern africa the youngest phase of elevated flux started in late miocene times 10 ma and has continued to increase rapidly through to the present day a large proportion of this increase can be attributed to a doubling of the size of the zambezi catchment during the pliocene at other times we suggest that load variations can be used to bound the uplift history of the region which encompasses the catchment finally changes in the shape of the zambezi delta through time suggest that the mozambique current which sweeps through the mozambique channel from northeast to southwest initiated at the start of the miocene

an 40ar 39ar geochronological and electron microprobe investigation of an archean pyroxenite and its bearing on ancient atmospheric compositions a detailed 40ar 39ar step heating geochronological study of pyroxene mineral separates from the munro warden ultramafic sill in the archean abitibi greenstone belt of the canadian superior province yields a precise primary age of 2703 11 ma 1 this is in excellent agreement with previous u pb zircon dates of 2700 2712 ma for the enclosing felsic volcanics and a sm nd age of 2622 60 ma for the sill electron microprobe analyses coupled with the argon evolution patterns demonstrate that deuteric amphibole is responsible for this primary age whereas the volumetrically dominant diopside shows anomalously young ages an isotope correlation plot reveals two components of contaminating argon with differing 40ar 36ar ratios in distinctly different sites one component trapped during the deuteric formation of amphibole has an 40ar 36ar ratio that is significantly less 258 3 than the present day atmospheric value of 295 5 this may be an indication of the atmospheric ratio 2 7 ga ago the other comp

creep of garnet in eclogite mechanisms and implications abstract the rheological properties of subducting rocks represent a fundamental parameter in the dynamics of subduction zones making robust predictions about these properties and the general strength evolution of subducting plates is hampered by a relatively restricted understanding of the mechanisms by which rocks deform at high and ultrahigh pressure this uncertainty is relates to the discrepancy between experimental and field based observations of eclogite deformation and in particular the role of garnet to further investigate this important aspect we performed a textural and micro structural investigation applying optical microscopy element mapping and electron backscatter diffraction on deformed garnet polycrystals from an eclogite mylonite the results were compared to those from a study of undeformed polycrystals that formed from a supercooled frictional melt at hp conditions the mylonites polycrystals are flattened parallel to the main high pressure foliation in the rock and individual grains were shortened by an average 15 although dislocation creep is commonly presumed to dominate garnet straining in eclogites no record of this mechanism was found instead the garnet grains have dissolution surfaces indicative of deformation by intergranular pressure solution the observations provide compelling evidence for the role of fluids and syn tectonic porosity in the weakening of garnet a supposedly rigid eclogite component such weakening represents a crucial step in the fundamental feedback loop between fluid ingress metamorphism and near instantaneous competence loss in rocks undergoing deep subduction

a model of plate kinematics in gondwana breakup summary an accurate model of relative plate motions in gondwana breakup is based on visual fitting of seafloor isochrons and fracture zones fz from the riiser larsen sea and mozambique basin used predictively the model precisely locates kinematic markers in the west somali basin which allows the conclusion that the spreading centres in the west somali and mozambique basins and the riiser larsen sea formed parts of the boundary between the same two plates the locations of fz and less well defined isochrons from neighbouring regions are also consistent with their formation on other lengths of this same boundary and with its relocation from the west somali basin and northern natal valley to the west enderby basin and lazarev sea during chron m10n small independently moving plates thus played no role in the breakup of this core part of gondwana in an inversion procedure the data from these areas yield more precise finite rotations that describe the history of the two plates separation breakup is most simply interpreted to have occurred in coincidence with karoo volcanism and a reconstruction based on the rotations shows the lebombo and mateke sabi monoclines and the mozambique and astrid ridges as two sets of conjugate volcanic margins madagascar s pre drift position can be used as a constraint to reassess the positions of india and sri lanka in the supercontinent

gondwana breakup no evidence for a davie fracture zone offshore northern mozambique tanzania and kenya plate tectonic reconstructions assume a major inactive transform fault the davie fracture zone in the west somali basin along which madagascar is thought to have migrated southwards following gondwana breakup in the mesozoic based on the interpretation of reflection seismic data we show that the walu ridge offshore kenya and the kerimbas basin offshore northern mozambique are tectonically unrelated to the southward motion of madagascar and correlate with late cretaceous volcanism and inversion in kenya and the evolution of the east african rift system respectively offshore tanzania geophysical data do not show basement structures indicating the presence of a major transform fault these results challenge the commonly supported transform margin concept and imply a more southerly pre breakup position of madagascar within gondwana opening of the west somali basin by sw propagating oblique rifting and seafloor spreading is proposed

reconstruction of temperature in the central alps during the past 2000 yr from a δ18o stalagmite record the precisely dated isotopic composition of a stalagmite from spannagel cave in the central alps is translated into a highly resolved record of temperature at high elevation during the past 2000 yr temperature maxima during the medieval warm period between 800 and 1300 ad are in average about 1 7 8c higher than the minima in the little ice age and similar to present day values the high correlation of this record to d 14 c suggests that solar variability was a major driver of climate in central europe during the past 2 millennia d 2005 elsevier b v all rights reserved

age progressive volcanism in the comores archipelago western indian ocean and implications for somali plate tectonics abstract the comores islands together with the tertiary volcanic province of northern madagascar form a sublinear trend of alkali olivine basalt shield volcanoes across the northern entrance of the mozambique channel potassium argon dating of shield building lavas confirms an eastward increase in age of volcanism along the chain consistent with a hotspot origin for the lineament the velocity of the somali plate over the mantle magma source is 50 mm yr we use the distribution of ages along the comores madagascar chain in conjunction with existing age data for the reunion mascarene plateau hotspot track to model the absolute motion of the somali plate for the last 10 m y we calculate the relative motion across the east african rift by subtracting the somali plate absolute motion from african plate absolute motion during this period the model predicts 320 km of total separation across the east african rift during the past 10 m y which is greater than has been estimated from surface geological evidence the geometry of older portions of the comores and reunion trends indicates that there was no significant relative motion between the african and somali plates prior to about 10 m y ago

a cretaceous and jurassic geochronology an integrated geomagnetic polarity and geologic time scale for the jurassic and cretaceous periods is presented based on various methods according to the availability of definitive isotopic ages an age calibrated sea floor spreading model is used to interpolate the ages of the kimmeridgian to barremian and the campanian to maestrichtian stages numerical age estimates for the aptian to santonian stage boundaries follow published isotopic age determinations the hypothesis of equal duration of ammonite zones is employed as a vernier to apportion time for the hettangian to oxfordian stages the new scale results in ages of 208 ma for the base of the jurassic 144 ma for the jurassic cretaceous boundary and 66 5 ma for the top of the cretaceous the integrated biostratigraphic magnetostratigraphic and geochronometric record serves as a working hypothesis for geologic correlation of jurassic and cretaceous strata

complexity of holocene climate as reconstructed from a greenland ice core glaciochemical time series developed from summit greenland indicate that the chemical composition of the atmosphere was dynamic during the holocene epoch concentrations of sea salt and terrestrial dusts increased in summit snow during the periods 0 to 600 2400 to 3100 5000 to 6100 7800 to 8800 and more than 11 300 years ago the most recent increase and also the most abrupt coincides with the little ice age these changes imply that either the north polar vortex expanded or the meridional air flow intensified during these periods and that temperatures in the mid to high northern latitudes were potentially the coldest since the younger dryas event

geology basin analysis and hydrocarbon potential of mozambique and the mozambique channel abstract the phanerozoic structural and stratigraphic history of mozambique can be separated into a karoo and a post karoo phase outcrops of karoo rocks limited to the zambezi valley and small outliers in the mozambique belt to the north show an eastward facies change marked by an increased argillaceous content and the appearance of carbonates the wide coastal plain and immediate offshore of southern mozambique is marked by extensional horsts and grabens with a mesozoic fill covered by cenozoic sediments little is known of lower stratigraphic horizons in this coastal province there is a facies transition from continental clastics in the northwest and west to more lacustrine and marine deposits to the southeast and east upwards the succession passes into the cenozoic without a noticeable break in the offshore in the north seismic lines help define two basins the zambezi delta basin and the mozambique channel basin separated by the beira high in the south several offshore grabens have been defined one of which the south mozambique graben has been examined here by an integrated basin analysis technique the technique which uses available depositional structural and thermal data provides information on maturation and the timing of hydrocarbon maturation the analysis showed variation in the extension rate with time and two phases of rifting the greatest rate of extension accumulation and subsidence between 96 and 76 ma depends upon interpretational assumptions during this time interval fault blocks were systematically tilted domino fashion this event broadly coincides with the rifting between madagascar and antarctica a period of plate reorganization the earlier rifting phase was probably related to the strike slip separation of madagascar from africa modelling heat flow suggests that the later event is the more important from the maturation hydrocarbon standpoint the offshore and potentially the deeper onshore post karoo basins have traps reservoirs and seals with the presumed source rock within lower cretaceous or jurassic strata there is at the present time no means to evaluate the potential of the karoo rocks

phanerozoic plate boundaries in mainland se asia the himalayas and tibet recently available geological data on tibet yunnan and burma facilitate recognition of phanerozoic subduction systems and two continental fragments of gondwanaland which extend through mainland se asia a cambrian subduction system and 5 mesozoic early caenozoic collision belts are identified in the region indochina eastern thailand and central tibet accreted to china in the early triassic western se asia and s tibet separated from gondwanaland in the permian or early triassic and collided with asia in the late triassic and the western burma island arc system collided with asia in the jurassic upper triassic flysch and schist in the eastern indoburman ranges were accreted to western burma in the jurassic early cretaceous during eastward subduction and formation of a back arc thrust belt following collision of greater india with arc complexes to the n in the cretaceous and with tibet in the early eocene western burma moved northwards about 450 km along the sagaing namyin ridge thrust transform fault formation of the 3 major belts of tin granites and the porphyry copper deposits in the region took place in specific tectonic settings within the subduction systems and collision belts

sensitivity of the lln climate model to the astronomical and co2 forcings over the last 200 ky the louvain la neuve climate model there referred to as the lln 2 d model has been used extensively to simulate the northern hemisphere ice volume under both the insolation and co2 forcings the period analysed here covers the last 200 ky first sensitivity analyses to constant co2 concentration were performed the model was accordingly forced by insolation changes only the co2 concentration being kept constant to respectively 210 250 and 290 ppmv results show that the simulated ice volume variations are comparable to the geological reconstructions only when the co2 concentration is low 210 ppmv and that the sensitivity of the simulated northern hemisphere ice volume to co2 is not constant through time second three co2 reconstructions were used to force the lln 2 d model in addition to insolation results show 1 a better agreement with the specmap oxygen isotope time series in particular as far as the amplitude of the signal is concerned and 2 that the simulated northern hemisphere ice volume is not very sensitive to the slight differences between these three reconstructions

coexistence of compositionally heterogeneous chromitites in the antalya isparta ophiolitic suite sw turkey a record of sequential magmatic processes in the sub arc lithospheric mantle abstract the antalya isparta region in southwestern turkey is well known for its large ophiolitic peridotite exposures which host various chromite orebodies these are small sized massive to disseminated in texture chromitites occurring in the form of lenses or veinlets and commonly surrounded by dunite envelopes of variable thickness chromitite seams from the antalya mantle suite are both high and intermediate cr varieties cr 0 56 0 83 whereas chromitites in the isparta mantle sequence are exclusively cr rich cr 0 75 0 85 in situ minor and trace element abundances obtained by la icp ms analyses of unaltered cr spinel from the cr rich chromitites are comparable to those reported in cr spinel of chromitites from typical fore arc peridotite complexes however minor and trace element concentrations in cr spinel from intermediate chromitites are dissimilar to those acquired from cr spinels of chromitites from well known back arc basin derived ultramafic massifs calculation of parental magma compositions indicates that both types of chromitites share a common parentage with progressively fractionating arc related melts the studied chromitites are characterized by a systematic enrichment in ipge os ir and ru 41 317 ppb with respect to ppge rh pt and pd 3 49 ppb resulting in negatively sloping chondrite normalized pge patterns that are less fractionated in intermediate chromitites their noble mineral assemblage is vastly dominated by tiny 10 m euhedral laurite crystals followed by subsidiary irarsite and trivial amounts of os ir alloy grains pgm grains are not encountered in the intermediate chromitites potentially due to crystallization resulting from pge poor melt laurite is os poor and exhibits a narrow range of os for ru substitution ru ru os 0 75 0 99 however the concomitance of laurite and millerite in the cr rich chromitites of the mutual antalya isparta mantle suite is in favour of their precipitation from an os depleted melt characterized by local and rapid variations of fs2 prior to or coevally with cr spinel crystallization moreover the presence of amphibole inclusions in cr spinel indicates that the melt triggered chromitite genesis potentially involved a hydrous component overall data suggest that investigated orebodies were produced by a successively fractionating arc derived melt that generated compositionally distinct chromitites at two different pseudo stratigraphic levels within the antalya isparta arc type mantle suite

a geotectonic history of the northern mozambique basin including the beira high a contribution for the understanding of its development abstract the interpretation of a comprehensive two dimensional 2d seismic reflection data set discloses several rifting events for a typical passive rifted margin two major phases have been identified rift i phase is in agreement with the initial rifting of gondwana postulated for the early jurassic this rift resulted in break up stage 1 governed by a north eastern motion of antarctica along the reactivated pebane shear zone indications for lava flows were identified in the seismic profiles along the offshore zambezi depression they form a link between early rifting the initial break up and early sea floor spreading the nature of these lava flows may be associated with the emplacement of thick volcanic dykes during post rift magmatism that occurred when the antarctica plate with the beira high drifted dextrally from the west to east until the mid jurassic times it is more likely that the break up in stage 1 resulted from high tension due to strain relief of rift i phase above all the extensional deformation occurred in a narrow rift mode therefore the v shape of the offshore zambezi depression suggests a possible rift failure structure this is best explained by the reeves and de wit model 2000 this model postulates that the motion of antarctica changed to southward direction at around 170 ma consequently a rift jump from the offshore zambezi depression in the northwest to the south eastern edge of the beira high occurred and rift ii phase may have started leading to the break up in stage 2 during this stage rift ii phase the extension migrated towards the east thus thinning the crust and exhuming the sub continental mantle in the continental ocean transition zone the rift ii phase shows a sequence of half graben morphologies confining the syn rift infill that is subdivided in three units syn rift i syn rift ii and rift sag all three units appear to have developed under minor extensional regimes in the crust evidenced by gently dipping low angle detachment faults with respect to the presence of the two rift phases it is deduced that break up and sea floor spreading are diachronous within rift i and rift ii segments

dynamic recrystallization of feldspar a mechanism for ductile shear zone formation feldspar aggregates experimentally deformed in the dislocation creep regime undergo dynamic recrystallization because recovery is difficult due to the limited climb of dislocations recrystallizing aggregates have a lower strength because of the cyclic production of small strain free grains and they develop a strong preferred orientation consistent with that observed in mylonites thus recrystallization accommodated dislocation creep may be responsible for the grain size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones

uplift history of a transform continental margin revealed by the stratigraphic record the case of the agulhas transform margin along the southern african plateau the south and southeast coast of southern africa from 28 s to 33 s forms a high elevated transform passive margin bounded to the east by the agulhas falkland fracture zone affz we analysed the stratigraphic record of the outeniqua and durban thekwini basins located on the african side of the affz to determine the evolution of these margins from the rifting stage to present day the goal was to reconstruct the strike slip evolution of the agulhas margin and the uplift of the inland high elevation south african plateau the agulhas transform passive margin results from four successive stages 1 rifting stage from late triassic to early cretaceous 200 134 ma punctuated by three successive rifting episodes related to the gondwana breakup 2 wrench stage 134 131 ma evidenced by strike and dip slip deformations increasing toward the affz 3 active transform margin stage 131 92 ma during which the falkland malvinas plateau drifts away along the affz with an uplift of the northeastern part of the outeniqua basin progressively migrating toward the west 4 thermal subsidence stage 92 0 ma marked by a major change in the configuration of the margin onset of the shelf break passive margin morphology two main periods of uplift were documented during the thermal subsidence stage of the agulhas margin 1 a 92 ma short lived margin scale uplift followed by a second one at 76 ma located along the outeniqua basin and 2 a long lasting uplift from 40 to 15 ma limited to the durban thekwini basin this suggests that the south african plateau is an old upper cretaceous relief 90 70 ma reactivated during late eocene to early miocene times 40 15 ma

denudation along the atlantic passive margin new insights from apatite fission track analysis on the western coast of south africa apatite fission track aft data from two traverses across the great escarpment of the western coast of south africa are used to reconstruct the tectonic evolution and denudation history of this sector of the atlantic passive margin fission track ages range between 180 and 86 ma modelling of this data identifies two distinct cooling events the first event between 160 and 138 ma is recorded only by the rocks above the escarpment in the karoo area and is tentatively linked to post karoo magmatism 180 ma thermal relaxation the second between 115 and 90 ma results instead from a tectonically induced denudation episode responsible for the removal of up to 2 5 km of crust across the coastal zone in front of the escarpment and less than 1 km on the elevated interior plateau based on these results it is suggested that the cretaceous is the time when most of the elevated topography of southern africa was generated with only a minor cenozoic contribution

metamorphic evolution of sulphide rich chromitites from the chernichevo ultramafic massif se bulgaria abstract the upper mantle rocks of the metamorphosed ophiolite of chernichevo rhodope metamorphic complex in southern bulgaria host small chromite ores with unusual mineralization of base metal sulphides rich in platinum group elements mineralogical and chemical data indicate that after their formation in the mantle the chernichevo s chromite ores were modified by the intrusion of an alkaline mafic melt which resulted in the precipitation of a suite of metasomatic minerals sulphides calcite apatite and ilmenite accompanied by an increase in feo tio2 ga zn vn mn and especially ti and fe2o3 contents in the chromite the degree of chemical modification and abundance of metasomatic minerals are positively correlated and mark the extent of reaction of the chromitite with the intruding melt sulphide segregation promoted the concentration of high amounts of pges up to 3661 ppb particularly pt and pd yielding chromite ores with a typical flat to positive sloped chondrite normalized pattern subsequently the chromite ores were deformed and metamorphosed together with their host rocks at ultra high pressure uhp 2 5 gpa 1200 c to be later retrograded under eclogite and finally hydrous amphibolite facies conditions giving rise to three microstructural types metamorphism of the most metasomatized i e sulphide rich chromitites at temperatures 700 c within the conditions of uhp and eclogite facies resulted in the formation of 1 non porous recrystallized chromite consisting of a granoblastic microstructure made up of coarse grained blasts and finer grained chromite neoblasts in contrast hydrous metamorphism on the less metasomatized i e sulphide poor chromitite under the conditions of amphibolite facies ca 482 483 c resulted in the formation of 2 partly altered chromite characterized by unaltered cores surrounded by fe2 rich and al depleted porous chromite containing abundant clinochlore and 3 porous chromite corresponding to a chromite that was entirely transformed by the metamorphic alteration to fe2 rich and al depleted porous chromite during metamorphism magmatic ni fe cu sulphides originally formed during the metasomatic event in the mantle were altered resulting in a major leaching of cu rich sulphides leading to significant remobilization of pt and pd

compositional changes of minerals associated with dynamic recrystallizatin the rate of compositional and isotopic exchange between minerals may be enhanced significantly if the rock is deformed simultaneously the enhanced exchange rate may result from a reduction in grain size shorter distance for volume diffusion dissolution and growth of grains by diffusion creep pressure solution or the movement of high angle grain boundaries through strained grains during recrystallization in the dislocation creep regime the migration of high angle grain boundaries provides high diffusivity paths for the rapid exchange of components during recrystallization the operation of the latter process has been demonstrated by deforming aggregates consisting of two plagioclases an1 and an79 at 900 c 1 gpa confining pressure and a strain rate of 2x10 6s 1 the polygonal recrystallized grains were analyzed using an analytical transmission electron microscope and have a variable but often intermediate composition at the conditions of these experiments the volume interdiffusion rate of nasi caal is too slow to produce any observable chemical change and microstructural chemical relations indicate that the contribution from diffusion creep was insignificant except for initially fine grained 2 10 m aggregates these results indicate that strain induced recrystallization can be an effective mechanism for enhancing the kinetics of metamorphic reactions and for resetting the isotope systematics of minerals such as feldspars pyroxenes and amphiboles

a precisely dated climate record for the last 9 kyr from three high alpine stalagmites spannagel cave austria 1 here we use the 18o time series of three stalagmites from the high alpine spannagel cave austria which grew in small distance from each other to construct a precisely dated continuous 18o curve for the last 9 kyr comnispa this speleothem record is not influenced by effects of kinetic isotope fractionation due to the low temperatures in the cave thus the variations in 18o can be interpreted in terms of past climate change comnispa exhibits several oscillations within the holocene and their timing and duration is in agreement with that documented by other alpine archives

significance of chromian spinels from the mantle sequence of the andaman ophiolite india paleogeodynamic implications abstract the mantle section of the andaman ophiolite between rutland island in the south and north andaman in the north has been studied the restitic peridotite of rutland island is comprised of depleted harzburgite to clinopyroxene bearing harzburgite whereas that in middle and north andaman is mostly less depleted lherzolite dominated mantle chromian spinels from the mantle section of the andaman ophiolite belong to four major groups group 1 spinels from rutland chromitite pods characterized by high cr cr cr a1 atomic ratio 0 71 group 2 spinels from north andaman chromitite pods with two subgroups 2a characterized by high cr 0 70 0 79 and 2b characterized by medium cr 0 48 0 51 and high mg mg mg fe 2 atomic ratio 0 74 0 77 group 3 residual accessory grains from rutland peridotite characterized by medium cr 0 37 0 55 and low mg 0 41 0 54 and group 4 residual accessory grains from middle and north andaman peridotites characterized by low cr 0 09 0 23 and high mg 0 71 0 81 group 1 chromian spinels of rutland island and group 2a of north andaman are likely to have crystallized from boninitic melts formed by relatively high degrees of partial melting whereas group 2b varieties from north andaman are from tholeiitic melts formed by lower degrees of melting the chemistry of the residual accessory chromian spinels groups 3 and 4 suggests that the mantle peridotites of rutland island towards the south are similar to fore arc peridotites of suprasubduction zone environments whereas those of north andaman are less depleted this spatial and or temporal directional change in spinel compositions may reflect variations linked to the melting history where the same sliver of oceanic mantle underwent different styles of melting in different tectonic settings at different points in time the coexistence of both chromitite types with high and low cr spinels in the same area from north andaman possibly reflects temporal variations of separate melt intrusions produced through specific melting stages it is inferred that a similar geodynamic setting to the present day also existed during the cretaceous period with a fore arc at rutland island and a back arc towards the north at north andaman

global surface temperatures over the past two millennia 1 we present reconstructions of northern and southern hemisphere mean surface temperature over the past two millennia based on high resolution proxy temperature data which retain millennial scale variability these reconstructions indicate that late 20th century warmth is unprecedented for at least roughly the past two millennia for the northern hemisphere conclusions for the southern hemisphere and global mean temperature are limited by the sparseness of available proxy data in the southern hemisphere at present

syndeformational recrystallization dynamic or compositionally induced dynamic recrystallization in the strict sense of the term is the reconstitution of crystalline material without a change in chemical composition driven by strain energy in the form of dislocations driving potentials additional to internal strain energy may contribute to the recrystallization of naturally deformed minerals which form solid solutions such as feldspar amphiboles and pyroxenes if they change their composition during recrystallization to estimate the relative importance of these driving potentials the chemical composition of porphyroclasts and recrystallized grains of plagioclase clinopyroxene and hornblende have been investigated in samples from a high grade shear zone of the ivrea zone italy the plagioclases show two different recrystallization microstructures bulging recrystallization at grain boundaries and discrete zones of recrystallized grains across porphyroclasts probably involving fracturing deformation took place under amphibolite facies conditions on a retrograde p t path porphyroclast and recrystallized compositions from bulging recrystallization microstructures differ only in their or content and yield a g between mean host grain and mean recrystallized grain composition at fixed p t conditions of approximately 5 joules 10 4 m3 extreme compositional variations yield approximately 60 j 10 4 m3 the increase of free energy due to dislocations calculated for common glide systems in plagioclase are on the order of 100 joules 10 4 m3 for high values of dislocation densities of 1014 m 2 thus the effect of chemically induced driving energies on grain boundary velocity appears small for mean compositions but may be as great as that of deformational energies for larger chemical differences in the other type of microstructure porphyroclasts and recrystallized grains in discrete zones differ in their anorthite content the maximum g induced by the compositional disequilibrium is on the order of 100 j 10 4 m3 this maximum value is of the same magnitude as the g derived from high dislocation densities of 1014 m 2 the resulting combined g is approximately twice as high as for deformational g alone and heterogeneous nucleation may become a feasible recrystallization mechanism which is evident from the microstructures the recrystallization mechanism depends on the nature of the driving potential grain boundary migration gbm and heterogeneous nucleation can release gibbs free energy induced by compositional disequilibrium whereas this is not likely for subgrain rotation therefore only gbm and heterogeneous nucleation may link metamorphism and deformation so that syndeformational recrystallization may represent a transitional process ranging from dynamic recrystallization to metamorphic reaction

geomorphologie et origine des bancs du nord est du canal de mozambique ocean indien occidental geomorphology and origin of the reef banks of the north eastern mozambique channel western indian ocean abstract the comores archipelago is extended to the east by several emerged glorieuses geyser and submerged zelee leven cordeliere reef banks these topographic features have been investigated by echosounding and magnetometry and volcanic rocks have been sampled on their slopes geyser all these data give some information about the origin of the banks 1 1 the bathymetry suggests that the banks are coral limestones and biogenic carbonate sediments capping the top of the volcanic structures 2 2 the magnetic anomalies reflect volcanic basement and are generally localised on the summit or upon the slopes of the banks 3 3 the hyaloclastic rocks sampled on the slopes of the geyser reef bank are related with subaerial or hydroexplosive volcanism the comores archipelago and the banks of the northeastern mozambique channel mark the boundary between the mesozoic oceanic somali basin and the continental substratum of the comores basin the basaltic magma appears to have been intruded along the north west trending fracture zones related with southward movement of madagascar relative to the african mainland

holocene forcing of the indian monsoon recorded in a stalagmite from southern oman a high resolution oxygen isotope record from a thorium uranium dated stalagmite from southern oman reflects variations in the amount of monsoon precipitation for the periods from 10 3 to 2 7 and 1 4 to 0 4 thousand years before the present ky b p between 10 3 and 8 ky b p decadal to centennial variations in monsoon precipitation are in phase with temperature fluctuations recorded in greenland ice cores indicating that early holocene monsoon intensity is largely controlled by glacial boundary conditions after approximately 8 ky b p monsoon precipitation decreases gradually in response to changing northern hemisphere summer solar insolation with decadal to multidecadal variations in monsoon precipitation being linked to solar activity

a 1052 year tree ring proxy for alpine summer temperatures a june august alpine temperature proxy series is developed back to ad 951 using 1 527 ring width measurements from living trees and relict wood the reconstruction is composed of larch data from four alpine valleys in switzerland and pine data from the western austrian alps these regions are situated in high elevation alpine environments where a spatially homogenous summer temperature signal exists in an attempt to capture the full frequency range of summer temperatures over the past millennium from inter annual to multi centennial scales the regional curve standardization technique is applied to the ring width measurements correlations of 0 65 and 0 86 after decadal smoothing with high elevation meteorological stations since 1864 indicate an optimal response of the rcs chronology to june august mean temperatures the proxy record reveals warm conditions from before ad 1000 into the thirteenth century followed by a prolonged cool period reaching minimum values in the 1820s and a warming trend into the twentieth century this latter trend and the higher frequency variations compare well with the actual high elevation temperature record the new central alpine proxy suggests that summer temperatures during the last decade are unprecedented over the past millennium it also reveals significant similarities at inter decadal to multi centennial frequencies with large scale temperature reconstructions however deviating during certain periods from h h lamb s european north atlantic temperature history

structural classification of chromite pods in southern new caledonia a structural classification of podiform chromite orebodies from southern new caledonia results in a division of deposits into three major types discordant subconcordant and concordant with penetrative structures foliation and lineation in the enclosing peridotite discordant deposits are very irregular in shape and clearly crosscut banding and foliation subconcordant deposits are generally tabular in shape and lie within 10 degrees to 25 degrees in strike and or dip to the foliation concordant deposits are also tabular and lie parallel to foliation in peridotite pyroxene lineation in host peridotite always indicates the elongation direction of these deposits within subconcordant and discordant deposits chromite lineations are often oblique to those in the surrounding rocks they follow local variations in the orebody shape and indicate possible deposit extensions at depth the classification of chromite bodies corresponds to three stages of increasing deformation as evidenced by chromite ore textures discordant deposits which are the least deformed are characterized by primary textures such as the nodular one the foam texture the chromite net and the occluded silicate texture within subconcordant and mainly concordant deposits massive disseminated and antinodular ores show evidence of strong deformation the lack of geochemical distinctions between the different deposit types supports such an hypothesis these chromite deposits in ophiolitic harzburgites are thought to have been formed beneath an oceanic spreading ridge if the discordant pods are considered as representative of the original situation it is proposed that the chromite has crystallized and has been dynamically concentrated along steep conduits traversing the enclosing harzburgite and feeding a magma chamber next the chromite enriched pipes are caught up by plastic deformation in the mantle and tectonically reoriented toward the foliation the chromite deposits in the massif du sud are located within a domain about 1 5 km thick in the harzburgites and dunite zones this domain is limited upward by the first cumulates and gabbros and downward by the transition to a different plastic flow regime in the harzburgites finally some guides to chromite prospecting and exploration are cited applicable at different geologic scales and based on lithologic and structural criteria

neoarchean suprasubduction zone arc magmatism in southern india geochemistry zircon u pb geochronology and hf isotopes of the sittampundi anorthosite complex abstract the sittampundi anorthosite complex sac in southern india is one of the well exposed archean layered anorthosite gabbro ultramafic rock associations here we present high precision geochemical data for the various units of sac coupled with zircon u pb geochronology and hf isotopic data for the anorthosite the zircon ages define two populations the older yield a concordia age of 2541 13 ma which is interpreted as the best estimate of the magmatic crystallization age for the sittampundi anorthosite a high grade metamorphic event at 2461 15 ma is suggested by the upper intercept age of the younger zircon population a neoproterozoic event at 715 180 ma resulted in pb loss from some of the metamorphic zircons the magmatic age of the anorthosite correlates well with the timing of crystallization of the arc related 2530 ma magmatic charnockites in the adjacent salem block while the metamorphic age is synchronous with the regional metamorphic event the geochemical data suggest that the rocks were derived from a depleted mantle source sub arc mantle metasomatism of slab derived fluids and subsequent partial melting produced hydrous aluminous basalt magma the magma fractionated at depth to produce a variety of high alumina basalt compositions from which the anorthositic complex with its chromite rich and amphibole rich layers formed as cumulates within the magma chamber of a supra subduction zone arc the coherent initial176hf 177hf ratios and positive ehf values 1 7 4 5 of the magmatic zircons in the anorthosite are consistent with derivation of a rather homogeneous juvenile parent magma from a depleted mantle source our study further confirms that the southern part of the dharwar craton was an active convergent margin during the neoarchean with the generation and emplacement of suprasubduction zone arc magmas which played a significant role in continental growth

new u pb zircon ages of plagiogranites from the nagaland manipur ophiolites indo myanmar orogenic belt ne india we report new u pb zircon ages and whole rock chemistry of plagiogranites from the nagaland manipur ophiolites exposed in the indo myanmar orogenic belt ne india which represents the southeastern extension of the indus yarlung tsangpo suture of neo tethyan origin the plagiogranites sio 2 57 and 76 wt are characterized by high sodium na 2 o 5 9 7 2 wt and low potassium k 2 o 0 1 0 6 wt coupled with low rb 1 8 9 5 ppm low rb sr n sm n 0 50 1 13 relative to heavy ree sm n yb n 0 59 0 88 with low ree and incompatible trace element contents that indicate derivation from a depleted mantle source high ratios of large ion lithophile elements to high field strength elements with pronounced nb ta and ti depletions furthermore suggest the presence of the subduction component in the source region zircon u pb geochronology yielded mean 206 pb 238 u ages between 116 4 2 2 and 118 8 1 2 ma that record crustal formation in the early cretaceous collectively the data indicate that the nagaland manipur ophiolites are broadly coeval and geochemically comparable with neo tethyan ophiolites elsewhere in the indus yarlung tsangpo suture

deformation related recrystallization processes abstract recrystallization is a common microstructural transformation that occurs during deformation metamorphism and diagenesis of rocks studies on minerals and rock analogues have demonstrated that a wide range of recrystallization mechanisms can occur the range of mechanisms is related to the various ways in which two basic processes grain boundary migration and new grain boundary formation combine to transform the microstructure two recent papers drury et al 1985 urai et al 1986 have proposed different schemes for the description of recrystallization mechanisms the purpose of this paper is to provide a unified framework for the description of mechanisms recrystallization mechanisms are divided into three main types rotation mechanisms which principally involve the formation of new grain boundaries migration mechanisms which principally involve grain boundary migration and general mechanisms which involve both basic processes a further distinction is made on the basis of the continuity of the microstructural transformation with respect to time each of the three main types of mechanism can be divided into a number of sub types depending on whether the processes of grain boundary migration new grain boundary formation and new grain formation occur in a discontinuous or continuous manner with respect to time as the terms continuous and discontinuous have been used in the metallurgical literature to signify the spatial continuity of the microstructural transformation the terms discontinuai and continual are used to refer to the temporal continuity of the transformation it is recommended that the following aspects should be specified if possible in a general description of recrystallization mechanisms 1 1 how do the basic processes combine to transform the microstructure 2 2 if new grain development occurs what is the development mechanism and does new grain formation occur in a continual or discontinuai manner 3 3 if grain boundary migration is involved in the transformation what is the migration mechanism i e fast solute escape migration slow solute loaded migration fluid assisted migration etc and is migration a continual or discontinuai process the application of the unified scheme is illustrated by reviewing studies that have provided detailed information on the recrystallization mechanisms involved the complicating effects of solid solution impurities dispersed second phase particles and grain boundary fluid films are also considered and it is demonstrated that variations in content of these types of impurity can significantly effect the types of recrystallization that occur in a given material

fluid present deformation aids chemical modification of chromite insights from chromites from golyamo kamenyane se bulgaria abstract chemical signatures of chromitites are commonly used to track the evolution of the earth s mantle however chemical modification during deformation may have important implications for the interpretation of chromites signatures here we describe the details of how deformation promotes chemical modification in chromite physicochemical characteristics of the chromites were quantified by measuring crystallographic orientation relationships using electron back scattered diffraction ebsd and electron microprobe analysis emp chromites show porphyroclastic textures with coarse grained porphyroclasts ca 0 2 5 mm and fine grained neoblasts 3 and cr and lower mg values than the cores of large grains we interpret f1 and f2 to represent chromite recrystallized by heterogeneous nucleation and subgrain rotation recrystallization respectively crystallographic preferred orientation cpo and misorientation data on the well developed low angle subgrain boundaries in coarse grains and f2 grains indicate that deformation in chromite was accommodated mainly by dislocation creep with the dominant activation of the 111 slip system the retrograde p t exhumation path predicted by thermodynamic and chemical modeling suggests that these fine grained chromites were produced when the initial chromitites reacted with oxidizing fluids during retrograde metamorphism 1 0 gpa and 500 700 c our results show that deformation in the dislocation creep regime in a chemically open system has induced chemical modification and homogenization within chromite aggregates as well as strain localization this close physicochemical link offers new avenues of interpreting the chemical signatures of chromites utilizing their microstructurally controlled variation or lack thereof

palaeo oceanography deepwater variability in the holocene epoch the conversion of surface water to deep water in the north atlantic results in the release of heat from the ocean to the atmosphere which may have amplified millennial scale climate variability during glacial times1 and could even have contributed to the past 11 700 years of relatively mild climate known as the holocene epoch 2 3 4 here we investigate changes in the carbon isotope composition of benthic foraminifera throughout the holocene and find that deep water production varied on a centennial millennial timescale these variations may be linked to surface and atmospheric events that hint at a contribution to climate change over this period

bottom current controlled sedimentation a synthesis of the contourite problem abstract an overview of the main items concerning deep bottom current controlled deposits is presented these include the definition of contourites their processes of deposition and facies characteristics contourite drift types and seismic patterns the interplay of turbidity and bottom current processes and the correlation between bottom current processes and global changes in climate sea level from this discussion several of the more significant problems are highlighted which we believe should be priority targets for future research 1 identification of contourites especially in ancient series where they are interbedded with turbidites and other facies 2 refinement of the numerous parameters used as tracers of palaeocirculation patterns 3 understanding the interaction between the different types of bottom currents and also between bottom current and other deep water processes 4 distinction in the sedimentary record between the effects of short time scale bottom current variations due to regional causes like benthic storms and geological scale current fluctuations related to climatic or astronomic control 5 understanding the control of glacial interglacial climatic cycles on the nature and rate of contourite deposition