association of permian gabbro and granite in the langshan southern central asian orogenic belt age origin and tectonic implications abstract the permian magmatism in the alxa region provides critical geological evidence to constrain the geodynamic processes in the southern central asian orogenic belt the shouji batholith is located in the southern langshan eastern alxa region and contains monzogranites k feldspar granites and gabbros new zircon u pb age data show that the shouji batholith formed at 282 268 ma the shouji granites have high na2o k2o nb and zr contents and high 10000 ga al feot feot mgo yb nb and yb ta ratios similar to the values of a2 type granites the shouij granites exhibit enrichment in light rare earth elements lree and large ion lithophile elements lile depletion in nb ti and sr and enriched isotopic compositions these shouji granites were generated by partial melting of dehydrated tonalite trondhjemite granodiorite rocks of the diebusige complex in the lower crust the shouji gabbros are sodium rich and low k calc alkaline rocks they are enriched in lree and lile depleted in high field strength elements and have enriched isotopic compositions these shouji gabbros were generated by partial melting of a lithospheric mantle source metasomatized by melts derived from an ancient continental crust in an active continental margin roll back of the enger us subducting slab accounts for generations of the shouji a2 type granites and gabbros the zircon u pb ages of carboniferous permian magmatism show a younging trend toward the northwest in the southern alxa region evidence from late paleozoic magmatism and arc derived sediments indicates that the southern alxa switched from an active continental margin to an intra oceanic arc during the early carboniferous late permian as a result of retreat of the subducting enger us slab

geochronology geochemistry and origins of the paleozoic triassic plutons in the langshan area western inner mongolia china abstract the langshan area is the northeastern part of the alxa block and adjacent to the inner mongolia da hinggan orogenic belt imdob to the north and geochronological and geochemical studies of the langshan plutons would be helpful for unraveling the relationship of the magmatism in the langshan area with that in the imdob based on zircon u pb ages presented in this study and in published papers five magmatic stages are recognized in langshan area late silurian 418 ma carboniferous 328 304 ma early permian 294 272 ma late permian 260 254 ma and middle late triassic 245 227 ma the late silurian two mica granite is peraluminous and has a source from ancient continental crust indicating a collision event the late early carboniferous mafic plutons are subalkaline show appinitic affinities and may originate from the subduction modified continental lithospheric mantle the late early permian granodiorite is weakly peraluminous and high k calc alkaline and may be derived from ancient continental crust while the late early permian alkaline gabbro diorite displays em i like sr nd isotopic signatures indicating an enriched subcontinental lithospheric mantle source the late permian granodiorite is calc alkaline and peraluminous and may result from the partial melting of ancient lower continental crust whereas the late permian leuconorite belongs to low k tholeiitic series and has a cumulate origin from an enriched mantle source with the input of juvenile mantle materials the paleozoic triassic magmatic stages in the langshan area are consistent with the main magmatic periods in the northern ncc which are the late silurian to middle devonian 418 386 ma and early carboniferous to late triassic 342 211 ma with three magmatic stages of carboniferous permian and triassic especially the carboniferous to late triassic magmatic period in the northern ncc are closely comparable with that in the imdob indicating the two domains might be involved in similar tectonic processes during late paleozoic

the alashan terrane did not amalgamate with north china block by the late permian evidence from carboniferous and permian paleomagnetic results abstract rock magnetic and paleomagnetic studies have been carried out on the early carboniferous limestones and the late permian purple sandstones sampled in the eastern alashan terrane alt northwest of china two components were isolated from the early carboniferous limestone by thermal progressive demagnetisation a low unblocking temperature component ltc of recent origin a pre folding medium temperature component mtc the paleomagnetic pole is 13 1 n 11 0 e a 95 7 0 that is probably the result of the hydrothermal fluids from the qilian orgenic belt acquired during the late carboniferous early permian also two components were separated from the late permian purple sandstone by thermal progressive demagnetisation the ltc with the recent viscous remanent magnetisation and the higher temperature component htc revealed from three sections which has passed a regional fold test at the 95 probability level and reversal test suggesting a primary characteristic magnetisation the corresponding paleomagnetic pole is 27 2 n 18 8 e a 95 12 0 the apparent polar wander path including early carboniferous late carboniferous early permian late permian and early middle triassic poles of the alt is significantly different with those of the ncb comparison of the apwps between the alt and ncb shows a strong similarity if the apwp of hexi corridor alashan rotated counterclockwise around an euler pole at 44 n 84 e by 32 then the coeval apw path of the alt overlaps to that of the ncb this result indicates that the alt migrated to the ncb after the early middle triassic along a tectonic boundary located between helanshan mountain and zhuozishan mountain and finally amalgamated to the ncb before the early cretaceous

the late archean history of the wyoming province as recorded by granitic magmatism in the wind river range wyoming abstract the wyoming province a small ca 500 000 km 2 archean craton is the most southwestern of the archean provinces in north america it is composed primarily of late archean potassium rich granitic rocks in contrast to many other archean provinces rocks of tonalite trondhjemite affinity are rare over most of the province and are restricted to rocks older than 2 8 ga field petrologic geochemical and isotopic study of the late archean granites exposed in the wind river range have allowed us to identify at least four periods of potassic calc alkalic magmatism at 2 8 2 67 2 63 and 2 55 ga granitic rocks of these ages appear to be widespread across the wyoming province the oldest calc alkalic granites of the wind river range emplaced at ca 2 8 ga appear to be derived predominantly from pre existing crust however nd isotopic data suggest that these granites cannot be the product solely of partial melting of older tonalitic gray gneisses during at least two other periods of plutonism at 2 67 and 2 63 ga generation of the wind river range batholiths involved the incorporation of substantial amounts of isotopically juvenile material either from depleted mantle or young continental crust the information presented below as well as data available from elsewhere in the wyoming province is interpreted to suggest that the wyoming province unlike other archean cratons is not composed of a tectonic amalgamation of smaller exotic terranes although the wyoming province did experience crustal addition in archean time it was not by lateral accretion but by incorporation of mantle derived melts into large granitic batholiths

grain size variations in pelagic carbonate oozes from the walvis ridge se atlantic ocean odp site 1265 a low resolution miocene record of carbonate sedimentation and preservation abstract a record of variations in winnowing and dissolution was established for miocene 19 7 ma deep sea sediments at site 1265 from the walvis ridge south atlantic in this study we apply a new preservation index based on the abundance of coarse calcareous silt fraction 10 63 m the miocene record of this silt preservation index corresponds well with the conventional foraminiferal preservation index and variations in sand content preservation ranging from good to moderate during the miocene indicates that site 1265 is located above and close to the foraminiferal lysocline grain size mass accumulation rates of carbonate and organic carbon records indicate that changes in productivity and carbonate production play a minor role in variations of calcium carbonate concentration the miocene variation in carbonate content is caused mostly by a combination of winnowing and dissolution significant decreases of preservation at 17 3 16 3 13 8 13 0 11 6 and 10 4 ma coincide with miocene glacial events mi events suggesting that dissolution was caused by an increase of southern component water scw influence which occurred as a response to the intensification of antarctic ice sheet development the scw acts as the major deep water source during the global miocene cooling at 10 4 ma a change to overall better preservation points to a weakening of scw that occurred as a response to the initiation or strengthening of north atlantic deep water nadw

tectonics and hydrogeology of accretionary prisms role of the decollement zone abstract at convergent margins the decollement zone comprises the plate boundary and is marked by profound structural disharmony by changes in stress orientation and by a discontinuity in plate velocity the decollement zone initiates in a weak sediment layer typically a low permeability hemipelagic mud lying below a more rapidly deposited stronger more permeable trench turbidite sequence at the deformation front trench turbidites tend to be offscraped whereas the finer grained hemipelagic and pelagic sediments are more likely to be underthrust offscraped materials may undergo only limited burial whereas underthrust deposits can be deeply buried undergo high pressure metamorphism and are more likely to be preserved in the stratigraphic record burial rates associated with underthrusting are high exceeding 20 km my with high rates of underthrusting sediment descending below the decollement zone is probably buried faster than it can dewater resulting in emplacement of relatively high porosity deposits at depth fluids flowing from modern decollement zones have migrated substantial distances laterally and tapped deep sources the episodicity of flow suggests pumping of fluid by cycles of dilation and flow to the decollement zone followed by failure and fluid expansion along the decollement zone possible pressure gradients below the decollement zone allow flow upward into it while maintaining minimum effective stress along it the thickness of sediment underthrust beneath the decollement zone determines whether these deposits are emplaced by diffusive underplating with stratal disruption and efficient dewatering or by coherent underplating with the formation of macroscopic duplexes and transfer of fluid to the base of the accretionary prism deformation mechanisms affecting accreted sediment depend on depth of entrainment into the decollement zone and duration of residence

the impact of analogue material properties on the geometry kinematics and dynamics of convergent sand wedges abstract simulation of geodynamic processes in sandbox experiments requires analogue materials with a deformation behaviour that reproduces the deformation mechanisms of typical crustal rocks we present data on the frictional strength of different sand types employing static and dynamic shear tests the sand types analysed are characterised by an elastic frictional plastic mechanical behaviour with a transient strain hardening and strain softening phase prior to transition to stable sliding this is in conflict with the standard assumption of an ideal cohesionless coulomb material with constant frictional properties the influence of the identified transient material properties on the kinematics growth mechanisms and internal deformation patterns of convergent sand wedges results in characteristic wedge segments which vary depending on material compaction between wedges with well defined segments i e frontal deformation zone frontal imbrication zone and internal accumulation zone with straight slopes and wedges with a continuous convex topographic profile for most materials only the frontal part of the wedge is critical during experimental runs taper and strength of the wedge segments can be shown to be controlled by the frictional properties of active faults wedge segmentation is controlled by a bulk strength increase toward the rear of the wedge due to fault rotation in mechanically less favourable orientations and plastic material hardening the limit between the frontal critical parts of a wedge and internal stable parts is largely controlled by a critical state of stress upon which either renewed failure or fault inactivation occurs on this basis we suggest that critical taper analysis of wedges must be restricted to specific kinematic segments comparison of the experimental results with the nankai accretionary wedge suggests that our interpretation also applies to natural convergent wedges moreover we provide constraints for the selection of adequate granular analogue materials to simulate typical crustal rocks in natural convergent wedges

isotope provinces mechanisms of generation and sources of the continental crust in the central asian mobile belt geological and isotopic evidence abstract the available geological geochronological and isotopic data on the felsic magmatic and related rocks from south siberia transbaikalia and mongolia are summarized to improve our understanding of the mechanisms and processes of the phanerozoic crustal growth in the central asian mobile belt camb the following isotope provinces have been recognised precambrian t dm 3 3 2 9 and 2 5 0 9 ga at the microcontinental blocks caledonian t dm 1 1 0 55 ga hercynian t dm 0 8 0 5 ma and indosinian t dm 0 3 ga that coincide with coeval tectonic zones and formed at 570 475 420 320 and 310 220 ma continental crust of the microcontinents is underlain by or intermixed with juvenile crust as evidenced by its isotopic heterogeneity the continental crust of the caledonian hercynian and indosinian provinces is isotopically homogeneous and was produced from respective juvenile sources with addition of old crustal material in the island arcs or active continental margin environments the crustal growth in the camb had episodic character and important crust forming events took place in the phanerozoic formation of the camb was connected with break up of the rodinia supercontinent in consequence of creation of the south pacific hot superplume intraplate magmatism preceding and accompanying permanently other magmatic activity in the camb was caused by influence of the long term south pacific plume or the asian plume damping since the devonian

calculation of coccolith volume and its use in calibration of carbonate flux estimates abstract estimates of coccolith volume help in determining total coccolith carbonate fluxes and the relative contributions of various coccolith species it is argued here that the best approach to deriving such coccolith volumes is to calculate shape factors k s for each species separately based on reconstruction of cross profiles and to determine mean sizes from measurement of size variation in the sample of interest then volume k s l 3 values of k s are given for the most important extant coccolith species based on reconstruction of cross sections followed by calculation of a volume of rotation using an iterative routine implemented in an image analysis package substantial errors ca 50 are inevitable in such calculations but the resultant data are nonetheless of value coccolith assemblages from north atlantic 47 n 20 w jgofs 1989 sediment trap samples are analysed as a test case calculated coccolith pic fluxes constitute 30 80 of the chemically determined total pic fluxes

distribution of recent nannofossils beneath the benguela system southwest african continental margin abstract one hundred and forty eight surface samples recovered from the southwest african continental margin 35 17 s have been examined for their nannofossil content coccolith abundance is high in sediments from the deep water areas but declines toward the coast under the main areas of upwelling five distinct bathymetric oceanographic assemblages can be characterized by their nannofossil content coastal upwelling orange bank intermediate southern deep water and northern deep water angola current these distinctions are based on the distribution of eight ecologically significant taxa in order of decreasing abundance they are gephyrocapsa oceanica helicopontosphaera kamptneri cyclococcolithina leptopora coccolithus pelagicus syracosphaera div sp thoracosphaera div sp rhabdosphaera div sp and umbilicosphaera sibogae sea surface temperature and salinity variations which correlate with the extent and intensity of the main upwelling cells are thought to be the most important controls on coccolith distribution although nutrient availability and reduction of surface water transparency may be of local importance the exclusion of three stratigraphical index species for the quaternary emiliania huxleyi gephyrocapsa caribbeanica and g ericsonii from the quantitative analysis will permit the future application of these results to paleoceanographic reconstructions of the benguela system in pleistocene core sections

field geology geochronology and geochemistry of mafic ultramafic rocks from alxa china implications for late permian accretionary tectonics in the southern altaids abstract the time of termination of orogenesis for the southern altaids has been controversial systematic investigations of field geology geochronology and geochemistry on newly discriminated mafic ultramafic rocks from northern alxa in the southern altaids were conducted to address the termination problem the mafic ultramafic rocks are located in the bijiertai honggueryulin and qinggele areas stretching from west to east for about 100 km all rocks occur high grade gneisses as tectonic lenses that are composed of peridotite pyroxenite gabbro and serpentinite most of which have undergone pronounced alteration i e serpentinization and chloritization geochemically the rocks are characterized by uniform compositional trends i e with low sio2 contents 42 51 52 21 wt and alkalinity na2o k2o 0 01 5 45 wt mostly less than 0 8 wt and enrichments in mgo 7 37 43 36 wt with mg 52 75 91 87 as the rocks have been strongly altered and have a wide range of loss on ignition loi 0 44 14 07 wt values they may have been subjected to considerable alteration by either seawater or metamorphic fluids the ree and trace element patterns show a relatively fractionated trend with lile enrichment and hfse depletion similar to that of t morb between n morb and e morb indicating that the parental melt resulted from the partial melting of oceanic lithospheric mantle overprinted by fluid alteration of island arc origin the ultramafic rocks are relics derived from the magma after a large degree of partial melting of oceanic lithospheric mantle with superposed island arc processes under the influence of mid ocean ridge magmatism la icp ms u pb zircon ages of gabbros from three spots are 274 3 ma mswd 0 35 306 3 ma mswd 0 49 262 5 ma mswd 1 2 respectively representing the formation ages of the mafic ultramafic rocks therefore considering other previously published data we suggest that the mafic ultramafic rocks were products of south dipping subduction most probably with a slab window caused by ridge subduction of the paleo asian ocean plate beneath the alxa block in the late carboniferous to late permian before the ocean completely closed this sheds light on the controversial tectonic history of the southern altaids and supports the concept that the termination of orogenesis was in the end permian to triassic

middle to late miocene oxygen isotope stratigraphy of odp site 1085 se atlantic new constrains on miocene climate variability and sea level fluctuations abstract the middle miocene 18o increase represents a fundamental change in earth s climate system due to a major expansion and permanent establishment of the east antarctic ice sheet accompanied by some effect of deepwater cooling the long term cooling trend in the middle to late miocene was superimposed by several punctuated periods of glaciations mi events characterized by oxygen isotopic shifts that have been related to the waxing and waning of the antarctic ice sheet and bottom water cooling here we present a high resolution benthic stable oxygen isotope record from odp site 1085 located at the southwestern african continental margin that provides a detailed chronology for the middle to late miocene 13 9 7 3 ma climate transition in the eastern south atlantic a composite fe intensity record obtained by xrf core scanning odp sites 1085 and 1087 was used to construct an astronomically calibrated chronology based on orbital tuning the oxygen isotope data exhibit four distinct 18o excursions which have astronomical ages of 13 8 13 2 11 7 and 10 4 ma and correspond to the mi3 mi4 mi5 and mi6 events a global climate record was extracted from the oxygen isotopic composition both long and short term variabilities in the climate record are discussed in terms of sea level and deep water temperature changes the oxygen isotope data support a causal link between sequence boundaries traced from the shelf and glacioeustatic changes due to ice sheet growth spectral analysis of the benthic 18o record shows strong power in the 400 kyr and 100 kyr bands documenting a paleoceanographic response to eccentricity modulated variations in precession a spectral peak around 180 kyr might be related to the asymmetry of the obliquity cycle indicating that the response of the dominantly unipolar antarctic ice sheet to obliquity induced variations probably controlled the middle to late miocene climate system maxima in the 18o record interpreted as glacial periods correspond to minima in 100 kyr eccentricity cycle and minima in the 174 kyr obliquity modulation strong middle to late miocene glacial events are associated with 400 kyr eccentricity minima and obliquity modulation minima thus fluctuations in the amplitude of obliquity and eccentricity seem to be the driving force for the middle to late miocene climate variability

neogene carbonate burial in the pacific ocean 1 i have compiled caco3 mass accumulation rates mars for the period 0 25 ma for 144 deep sea drilling project and ocean drilling program drill sites in the pacific in order to investigate the history of caco3 burial in the world s largest ocean basin this is the first synthesis of data since the beginning of the ocean drilling program sedimentation rates caco3 contents and bulk density were estimated for 0 5 myr time intervals from 0 to 14 ma and for 1 myr time intervals from 14 to 25 ma using mostly data from initial reports volumes there is surprisingly little coherence between caco3 mar time series from different pacific regions although regional patterns exist a transition from high to low caco3 mar from 23 20 ma is the only event common to the entire pacific ocean this event is found worldwide the most likely cause of lowered pelagic carbonate burial is a rising sea level trend in the early miocene the central and eastern equatorial pacific is the only region with adequate drill site coverage to study carbonate compensation depth ccd changes in detail for the entire neogene the latitude dependent decrease in caco3 production away from the equator is an important defining factor of the regional ccd which shallows away from the equatorial region examination of latitudinal transects across the equatorial region is a useful way to separate the effects of changes in carbonate production productivity from changes in bottom water chemistry dissolution upon carbonate burial

internal deformation and compaction of the makran accretionary wedge the makran accretionary wedge is one of the largest on earth a 7 km thick column of sands and quartzolithic turbidites are incorporated into this wedge in a series of deformed thrust sheets we present the results of prestack depth migration and focusing error analysis migration velocity analysis performed on a profile across the makran wedge the depth section shows the deformation style of the accreted sediments and the migration velocities allow us to estimate porosity variations in the sediments the thrust sheets show evidence of fault propagation folding with a long wavelength of deformation 12 km and secondary thrusting in the kink bands of the folds such that the central part of each thrust sheet is elevated to form an additional ridge this deformation style and the 15 steep surface slope of the first ridge suggest a high degree of consolidation porosities were calculated from the seismic migration velocities and the ratio of fluid pressure to lithostatic pressure was estimated for 5 locations along the profile rather than being undercompacted and overpressured as in most accretionary wedges the sedimentary input is normally compacted exponential porosity decay throughout almost the whole wedge however a slight increase in porosity and at depth with respect to the normal compaction curve indicates that the turbiditic sequence might be overpressured landward of the deformation front

the final miocene carbonate crash in the atlantic assessing carbonate accumulation preservation and production abstract in many palaeoceanographic studies low carbonate contents in neogene deep sea sediments are interpreted as dissolution events related to prominent shifts in intermediate and deep water circulation in the middle to late miocene such carbonate crash events were a widespread phenomenon throughout the world s ocean basins however low carbonate contents may also derive from terrigenous dilution and carbonate production changes in surface waters as shown for late miocene records of the south atlantic diester haass et al 2004 westerhold et al 2005 in order to contribute to solve this question we investigated the youngest of the miocene crash events 10 5 ma to 9 0 ma at a prominent key location of global deep water circulation i e the equatorial atlantic ceara rise depth transect odp sites 926 927 and 928 by analysing different carbonate preservation proxies like carbonate content silt grain size parameters sand content fragmentation index and mass accumulation rates we were able to quantify dissolution dilution and production and to assess previous conflicting interpretations critical evaluation of the different dissolution proxies and comparison to former studies revealed that silt preservation proxies do not entirely reproduce the depth dependant dissolution in the miocene sediments as indicated by foraminifer fragmentation in the investigated late miocene time slice three minima of carbonate accumulation were detected the predominant control on these events changed from dissolution of carbonate to lowered production the last event at 9 6 ma is accompanied by an increased preservation but reduced carbonate accumulation during this time the first prominent pulses of northern component water ncw occurred sourced most probably from the labrador sea this may be understood as a first short term precursor of long time persistent and much stronger ncw production that is characteristic for younger periods and marks the turnover to the modern type of basin to basin fractionation between the atlantic and pacific furthermore preservation trends of caribbean and ceara rise records were in phase in contrast to former hypotheses that suggested antithetical preservation during carbonate crash events the similarity of the preservation records in the studied ocean basins in conjunction with the inconsistency in timing of the cc events and proposed ncw estimates indicates that changes in the carbon cycle other than dissolution controlled the cc events prior to 9 9 ma

numerical models of the evolution of accretionary wedges and fold and thrust belts using the distinct element method a 2 d numerical model is used to investigate the evolution of accretionary wedges and fold and thrust belts the numerical method is based on the distinct element method dem unlike many continuum numerical models demallows localization to occur even after substantial amounts of deformation the method is used to study the evolution of simple accretionary wedges and thrust belts with a rigid backstop and base experiments are done with a large range of coefficients of interelement friction e and element wall friction b two modes of deformation which depend mainly on b are observed for the weak base case low b the dominant mode is frontal accretion by pop up structures at or near the toe of the wedge for the strong base case high b uplift is concentrated near the back of the wedge and is accompanied by underthrusting along a flat ramp flat or staircase thrust fault structure at intermediate values of b the wedge oscillates between the two modes of deformation during periods of frontal accretion normal faulting sometimes occurs in regions where the material has thickened considerably the transition between the two modes of deformation is found to be a strong function of b but a weak function of e a simple explanation of the experimental results is made using the principle of work minimization comparisons between the results and some accretionary wedges fold and thrust belts are also made

effective stress and pore pressure in the nankai accretionary prism off the muroto peninsula southwestern japan 1 we developed a theoretical method for predicting effective stress and pore pressure based on rock physics model we applied the method to reveal the pore pressure distribution within the nankai accretionary prism off southwestern japan and to investigate variations in pore pressure associated with evolution of the plate boundary decollement from the crack aspect ratio spectrum estimated from laboratory and well log data we calculated a theoretical relationship between acoustic velocity and mean effective stress by using differential effective medium theory by iteratively fitting the theoretically calculated velocity to the seismic velocities derived from 3d tomographic inversion we estimated in situ mean effective stress within the accretionary prism pore pressure is then the difference between the effective stress and the confining stress when we calculated pore pressure we considered compressive state of stress in the accretionary prism our results confirm that pore fluid pressure is high within the subducting sedimentary sequence below the decollement we determined a normalized pore pressure ratio of 0 4 0 7 abnormal pore pressures develop in the under thrust sequence as a result of the increase in overburden load because of the thickened overlying prism and a low permeability barrier across the decollement overpressuring within the accreted sequence is initiated at the deformation front and proceeds landward the increase in horizontal compaction within the accreted sequence may raise pore pressures within the accreted sequence and the pore pressure mean effective stress contrast at the decollement becomes smaller landward of the deformation front

a paleoceanographic model for the late campanian early maastrichtian sedimentation in the polish carpathian flysch basin based on nannofossils abstract the study is a qualitative and semi quantitative analysis of calcareous nannofossils in marlstone and calcareous mudstone beds in a 4 6 m thick section of the kropivnik fucoid marls late campanian early maastrichtian in the skole nappe of the polish outer carpathians this turbiditic succession is conspicuous by its short term alternation of marlstone rich and siliciclastic rich packages ca 1 m thick a model for paleoceanographic control on sedimentation is proposed on the basis of nannofossil assemblages in the studied section the nannofossils form assemblages of mixed synsedimentary taxa derived from diverse marine environments differing in trophy water temperature and distance from land the mixing occurred due to submarine resedimentation processes the nannofossils indicate that the siliciclastic rich packages were deposited during periods of lowered water temperature decreased calcareous nannoplankton production and oligotrophic to mesotrophic water conditions which corresponded to the sea level fall and lowstand anti estuarine circulation and downwelling due to arising shelf break front are inferred as the factors responsible for oligotrophy in contrast in the marlstone rich packages the calcareous nannofossils indicate sedimentation during periods of significantly increased calcareous nannoplankton production which corresponded to the sea level rise and highstand the increased nannoplankton production is attributed to estuarine circulation and upwelling the changes in sea level and water circulation conform to the climate changes postulated independently for the late campanian early maastrichtian

generation of metaluminous a type granites by low pressure melting of calc alkaline granitoids melting experiments on two calc alkaline rocks a tonalite and a granodiorite demonstrate that shallow dehydration melting of hornblende and biotite bearing granitoids generates metaluminous a type granitic melts profuse crystallization of calcic plagioclase orthopyroxene during low pressure p 4 kbar incongruent melting explains all of the major and trace element characteristics of a type granites e g their low al ca mg sr and eu contents and high ga al and k na ratios as well as their common association with anorthositic and mangeritic rocks the residual assemblage produced during melting of the same rocks in the deep crust p 8 kbar is dominated by clinopyroxene and as a consequence most of the chemical features diagnostic of a type granites are lost the high temperatures 900 c and low h 2 o contents 4 wt that are characteristic of a type magmas are adequately explained by the h 2 o poor nature of the granitoid source materials and require neither a granulitic nor a f rich source previous attempts to explain the origin of a type granites have focused on the nature of the source material these experiments show that pressure of melting is at least as important as source composition in engendering a type melts

geochronological and geochemical constraints on the petrogenesis of the early paleoproterozoic potassic granite in the lushan area southern margin of the north china craton abstract we conducted a geochronological and geochemical study on the paleoproterozoic potassic granites in the lushan area southern margin of the north china craton ncc to understand the tectonic regime of the ncc at 2 2 2 1 ga this rock suite formed at 2194 29 ma the rocks are rich in sio2 76 10 77 73 wt and k2o 5 94 6 90 wt with high k2o na2o contents from 7 56 wt to 8 48 wt but poor in cao 0 10 0 28 wt p2o5 0 02 0 05 wt and mgo 0 01 0 30 wt mg 1 08 27 3 indicating they experienced fractional crystallization major element compositions suggest the potassic granites share an affinity with high k calc alkaline granite even though the lushan potassic granitic rocks have high a cnk ratios 1 11 1 25 which can reach peraluminous feature the very low p2o5 contents and negative correlation of p2o5 and sio2 ruling out they are s type granites different from peralkaline a type granites the lushan potassic granites have variable zr concentrations 160 344 ppm 226 ppm on average and 10 000 ga al ratios 1 76 3 00 together with high zircon saturation temperatures tzr 826 885 c indicating they are fractionated aluminous a type granites enriched lree la yb n 9 72 81 8 negative eu anomalies and low sr y with no correlations in sr y and sr zr versus cao suggest the possible presence of ca rich plagioclase and absence of garnet in the residual magmatic zircon grains have variable ehf t values 2 4 to 7 3 with zircon two stage hf model ages tdmc varying from 2848 ma to 2306 ma mostly around ca 2 5 ga and are plotted in the evolution line of crustal felsic rock we propose that the rocks mainly formed by partial melting of ca 2 50 ga tonalitic granodioritic crust as a result of upwelling mantle derived magmas which provided thermal flux and source materials in an intra continent rifting the ca 2 2 ga magmatism suggests that intra continental rifting occurred at 2 35 1 97 ga at least in the southern margin of the ncc after its final cratonization in the late neoarchean

depositional mechanisms for upper miocene sediments in the south china sea central basin evidence from calcareous nannofossils abstract in 2014 the international ocean discovery program iodp expedition exp 349 discovered carbonate layers at sites located in the south china sea scs central basin preliminary results from shipboard investigation showed that most of these carbonate layers are calcareous nannofossil ooze and interpreted them as turbidites however more detailed studies are required to determine their depositional mechanisms and paleoenvironmental implications in this study we have analyzed caco3 content calcareous nannofossil abundance assemblage composition and preservation in carbonate and clay samples from the upper miocene sections of site u1433 in the scs central basin we confirm that these carbonate layers are of turbidity current genesis but are not typical nannofossil ooze while the clays represent regular in situ deep sea sediments the study interval spanned approximately from 10 to 5 million years before present ma calcareous nannofossil abundance and assemblage are used to reconstruct the evolution of the upper seawater structure in the scs central basin our results indicate that the seawater shifted from strongly stratified to well mixed due to enhanced monsoon this shift was accompanied by an increase in pelagic productivity at 8 ma this environmental change is critical for component variations of the carbonate turbidites since we suggest that these turbidites mainly sourced from periplatform ooze in the southern scs composed by of mixed neritic and pelagic particles the neritic and pelagic caco3 production have inversed responses to the change of seawater environment in the scs and thus cause the component variations in the carbonate turbidites of site u1433

opposing gradients of permanent strain in the aseismic zone and elastic strain across the seismogenic zone of the kodiak shelf and slope alaska an industry acquired seismic reflection line from the aleutian arc to the alaska trench was prestack depth migrated and further constrained with wide angle ocean bottom hydrophone seismic velocity data and swath mapping from the seismic reflection image individual thrust slices the backstop and the forearc basin were resolved sufficiently well to balance the section tectonic shortening and age estimates provided rates of permanent contraction in five discrete segments over 150 km during the past 3 myr about 70 of the plate convergence is measured as contraction across the 40 km wide neogene accretionary prism which composes less than 10 of the forearc permanent contraction is minimal above the 1964 alaska earthquake aftershock zone indicating dominently elastic strain since elastic strain is poorly recorded in the geological record it was approximated from the historical record of the 1964 alaska earthquake the 1964 coseismic vertical deformation displays an inverse pattern to the permanent strain consistent with the earthquake cycle concept elastic deformation is dominant beneath the shelf and upper slope rapid permanent deformation is concentrated beneath the lower slope and a transition between them occurs beneath the middle slope

miocene to pliocene development of surface and subsurface temperatures in the benguela current system 1 the initiation of the benguela upwelling has been dated to the late miocene but estimates of its sea surface temperature evolution are not available this study presents data from ocean drilling program odp site 1085 recovered from the southern cape basin samples of the middle miocene to pliocene were analyzed for alkenone based u37k sstuk and glycerol dialkyl glycerol tetraether gdgt based tex86 temptex water temperature proxies in concordance with global cooling during the miocene sstuk and temptex exhibit a decline of about 8 c and 16 c respectively the temperature trends suggest an inflow of cold antarctic waters triggered by antarctic ice sheet expansion and intensification of southern hemisphere southeasterly winds a temperature offset between both proxies developed with the onset of upwelling which can be explained by differences in habitat alkenone producing phytoplankton live in the euphotic zone and record sea surface temperatures while gdgt producing thaumarchaeota are displaced to colder subsurface waters in upwelling influenced areas and record subsurface water temperatures we suggest that variations in subsurface water temperatures were driven by advection of cold antarctic waters and thermocline adjustments that were due to changes in north atlantic deep water formation a decline in surface temperatures an increased offset between temperature proxies and an increase in primary productivity suggest the establishment of the benguela upwelling at 10 ma during the messinian salinity crisis between 7 and 5 ma surface and subsurface temperature estimates became similar likely because of a strong reduction in atlantic overturning circulation while high total organic carbon contents suggest a biogenic bloom in the pliocene the offset between the temperature estimates and the cooling trend was reestablished

tectonic features of the japan trench convergent margin off sanriku northeastern japan revealed by multichannel seismic reflection data near the japan trench convergent plate margin the seaward edge of the continental plate is deformed by subduction of the oceanic plate we report the results of a multichannel seismic survey in the northern japan trench in which this deformed zone is demarcated from the rigid continental framework by a pronounced landward dipping reflector the oceanic plate also undergoes deformation as the two plates interact in the subduction processes resulting in a progressive deformation or destruction of a horst structure along the top of the subducting oceanic crust this may eventually lead to the formation of a smooth plate boundary at the greater depth more than 45 km landward from the trench axis a smooth reflector suggesting a stable slip plane is visible along and above the oceanic crust our data indicate that the deformed zone pinches out landward 60 km from the axis at 13 km depth and the slip plane becomes less obvious there seismicity of interplate earthquakes rapidly increases landward from this location leading us to speculate that this is where coupling at the plate boundary becomes strong enough for earthquakes to occur we conclude that the updip limit of the seismogenic zone of interplate earthquakes in the study area is characterized by the tectonic feature of a pinchout of the deformed sediments which overlie the subducting oceanic crust

the middle to late miocene climatic development of southwest africa derived from the sedimentological record of odp site 1085a sediments from the odp site 1085a were studied to investigate the impacts of global cooling in the middle and late miocene on the climate in southwestern africa the size composition of the sediment was analysed emphasising the silt fraction a comparison with the modern grain size distribution and suitable transport processes made it possible to assign specific transport processes to the grain size composition three processes are considered for transport of terrigeneous silt while there was no evidence found for 1 transport by ocean currents the analyses showed signals of 2 wind transport indicating dry conditions associated with a cool climate and 3 fluvial transport that points to humid and warm conditions three climatic phases were defined the first phase from 13 8 to 11 8 myr reveals a stable humid climate in southwest africa independent of the antarctic glaciations during the second phase from 11 8 to 10 4 myr the regional climate cooled considerably but was not drier additionally the climate during this phase reacted to the antarctic glaciations this cooling trend continued during phase 3 from 10 4 to 9 0 myr with a significant increase in dust input pointing to overall drier conditions however fluvial transport still remained as the main source

accretionary prisms in subduction earthquake cycles the theory of dynamic coulomb wedge 1 we expand the theory of critically tapered coulomb wedge for accretionary prisms by considering stress changes in subduction earthquake cycles building on the coulomb plasticity of the classical theory we assume an elastic perfectly coulomb plastic rheology and derive exact stress solutions for stable and critical wedges the new theory postulates that the actively deforming most seaward part of an accretionary prism the outer wedge overlies the updip velocity strengthening part of the subduction fault and the less deformed inner wedge overlies the velocity weakening part the seismogenic zone during great earthquakes the outer wedge is pushed into a compressively critical state with an increase in basal and internal stresses and pore fluid pressure after the earthquake the outer wedge returns to a stable state the outer wedge geometry is controlled by the peak stress of the updip velocity strengthening part of the subduction fault achieved in largest earthquakes the inner wedge generally stays in the stable regime throughout earthquake cycles acting as an apparent backstop and providing a stable environment for the formation of forearc basins the new theory has important implications for the studies of the updip limit of the seismogenic zone the evolution of accretionary prisms and forearc basins activation of splay faults and tsunami generation evolution of the fluid regime and mechanics of frontal prisms at margins dominated by tectonic erosion

late archaean granites a typology based on the dharwar craton india abstract extensive field work in the eastern dharwar craton associated with petrographic and geochemical major and trace elements investigations allows four main types of late archaean granitoids to be distinguished 1 na rich granitoids of trondhjemitic tonalitic and granodioritic composition ttg that are characterised by strongly fractionated ree patterns and low hree contents and generally interpreted as slab melts generated by partial melting of metamorphosed hydrated basalt most likely in a subduction environment 2 sanukitoids which are k and mg rich monzonites and granodiorites with ttg like ree patterns associated with marked lile enrichment and considered to result from the reaction of slab melts generated in a subduction environment with and assimilation of mantle wedge peridotite 3 uncommon high hfse mg and k granites with strongly ree and lile enrichment that probably formed by partial melting of an enriched mantle source unlike in the genesis of sanukitoids in this case the slab melt is considered to be wholly consumed by reaction with mantle minerals resulting in mantle enrichment subsequent melting of this enriched mantle probably in a post subduction setting gives rise to high hfse mg and k magmas as demonstrated for the closepet granite dharwar craton the hot mantle derived magma can induce melting of continental crust and then mix with the anatectic products 4 k rich mg poor anatectic biotite granites with ree patterns that are less fractionated and show negative eu anomalies these granites result from the remelting of old basement or recently accreted plutons both with ttg compositions such anatexis can occur either in a subduction or in a post subduction environment this typology based on the well exposed dharwar craton can be extended to late archaean granitoids from all over the world about such 500 analysis were compiled and a number of discriminant diagrams based on both major and trace elements are proposed the most likely tectonic setting for the observed rock types is an accretionary orogen with accretion of continental blocks above a subduction system followed by thermal reworking of the newly accreted continental material however it has been found that archaean subduction related granitoids are significantly different from their modern counterparts implying progressive changes in the modes of magma generation at convergent margins from the archaean to the present

identifying the crystal graveyards remaining after large silicic eruptions abstract the formation of crystal poor high silica rhyolite via extraction of interstitial melt from an upper crustal mush predicts the complementary formation of large amounts of typically unerupted silicic cumulates however identification of these cumulates remains controversial one hindrance to our ability to identify them is a lack of clear predictions for complementary chemical signatures between extracted melts and their residues to address this discrepancy we present a generalized geochemical model tracking the evolution of trace elements in a magma reservoir concurrently experiencing crystallization and extraction of interstitial melt our method uses a numerical solution rather than analytical thereby allowing for various dependencies between crystallinity partition coefficients for variably compatible and or incompatible elements and melt extraction efficiency results reveal unambiguous fractionation signatures for the extracted melts while those signatures are muted for their cumulate counterparts our model is first applied to a well constrained example searchlight pluton usa and provides a good fit to geochemical data we then extrapolate our results to understanding the relationship between volcanic and plutonic silicic suites on a global scale utilizing the navdat database to identify crystal accumulation or depletion signatures for each suite we suggest that many large granitoids are indeed silicic cumulates although their crystal accumulation signature is expected to be subtle

miocene history of the benguela current and antarctic ice volumes evidence from rhythmic sedimentation and current growth across the walvis ridge deep sea drilling project sites 362 and 532 the history of the northward growth and of the fluctuations of the benguela current from the middle miocene ca 14 ma to the uppermost late miocene ca 5 ma has been reconstructed from analyses of the coarse and clay fractions and geochemical analyses of the organic matter in sediments from deep sea drilling project sites 362 and 532 in the middle miocene the benguela current had not yet reached the walvis ridge and consequently no local upwelling occurred carbonate preservation was good during this period of low productivity rhythmic changes related to antarctic ice volumes were weak leading to slightly higher organic matter concentrations and higher montmorillonite illite m i ratios in sediments deposited during times of greater continental ice montmorillonite was carried north from the orange river by the benguela current whereas illite originated from the nearby namib desert in the early late miocene the benguela current reached the walvis ridge in colder periods and led to well developed rhythmic increases in opal which was transported from near coastal upwelling areas organic matter concentration carbonate dissolution and m i ratios were also enhanced during periods of greater ice volume during the late miocene farther northward migration of the benguela current led to a change in the uppermost miocene sediments opal contents and m i ratios were high during times of low ice volume rather than in colder times during the cold periods local winds from the namib desert increased the illite supply to the walvis ridge and depressed the m i ratio in these periods the benguela current flowed farther to the north reaching the angola basin as it did in quaternary glacial periods throughout these changes sediment organic carbon content was always enhanced in high ice volume periods from near bottom downslope transport of organic matter from shelf regions during regressions and carbonate dissolution in the sediments was increased as a result of oxidation of this organic matter

geochronological and geochemical constraints on the petrogenesis of late triassic aluminous a type granites in southeast china abstract the late permian triassic granites in the south china block scb have important tectonic significance for the evolution of east asia most of the intrusive bodies in the central scb consist of s type granites here we report the first unambiguous discovery of wengshan aluminous a type granites in southwest zhejiang province southeast china shrimp and la icp ms zircon u pb analyses yielded similar ages about 224 ma for two samples from the wengshan pluton in concordance with peak time of late indosinian 234 205 ma tectonic magmatic activity in the scb the wengshan granites are peraluminous with a cnk values ranging from 1 09 to 1 24 enriched in k rb th ree and zr nb ce y have elevated feo tot mgo and ga al ratios and are depleted in mg ca mn ba sr p and ti in addition they show high zircon saturation temperatures 820 878 c their ree patterns are highly fractionated with la yb n ratios of 36 89 101 24 and strongly negative eu anomalies eu eu 0 28 0 53 in situ zircon hf isotopic analyses indicate that the wengshan granites have e hf t values ranging from 15 9 to 12 7 and two stage depleted mantle hf model ages from 2 05 to 2 26 ga sr nd isotope systematics are characterized by a high initial 87 sr 86 sr ratios of 0 7092 0 7218 and a low e nd t values of 15 0 to 12 1 with two stage depleted mantle nd model ages of 1 99 2 22 ga consistent with those of paleoproterozoic basement rocks in the area it is inferred that the wengshan granite magma formed by partial melting of paleoproterozoic metasedimentary sources in cathaysia block meanwhile the underplating of mafic magma in the lower crust may provide the heat source for the formation of the wengshan granites the data show that the wengshan granites unlike other s type granites in the scb are late triassic aluminous a type granite therefore they may indicate distinct geodynamic significance our study of the wengshan a type granites 224 ma together with jingju a type granites 215 ma in zhejiang province and gaoxi a type granites 215 ma xiaotao a type granites 222 ma tieshan 254 ma and yangfang 242 ma alkaline syenites in fujian province define important extensional events in triassic considering these available data we suggest that the late permian triassic a type granites and alkaline syenites in the coastal region of the scb probably formed under an extensional regime related to oblique subduction of the paleo pacific plate

consolidation patterns during initiation and evolution of a plate boundary decollement zone northern barbados accretionary prism borehole logs from the northern barbados accretionary prism show that the plate boundary decollement initiates in a low density radiolarian claystone with continued thrusting the decollement zone consolidates but in a patchy manner the logs calibrate a three dimensional seismic reflection image of the decollement zone and indicate which portions are of low density and enriched in fluid and which portions have consolidated the seismic image demonstrates that an underconsolidated patch of the decollement zone connects to a fluid rich conduit extending down the decollement surface fluid migration up this conduit probably supports the open pore structure in the underconsolidated patch

the late miocene onset of high productivity in the benguela current upwelling system as part of a global pattern abstract we have examined the history of the elevated primary productivity associated with the benguela current upwelling system off southwest africa using sediments from 7 5 to 4 8 ma at ocean drilling program site 1085 in the middle cape basin sedimentation rates are low until 6 9 ma low accumulation rates of benthic foraminifers and organic carbon indicate that biological productivity was also low paleoproductivity dramatically increased at 6 7 6 5 ma and was highly variable until 4 8 ma with productivity maxima during cooler periods the presence of radiolarian opal only between 5 8 and 5 2 ma suggests an interlude of silica rich intermediate water in the cape basin the onset of heightened productivity under the benguela current is mirrored by similar increases reported between 6 9 and 6 7 ma in the tropical eastern pacific the western and northern pacific and the indian ocean the similarity between the patterns at site 1085 and in the pacific and indian oceans suggests that the dramatic productivity increase off southwest africa is part of a global response to paleoceanographic changes

deformation and fluid pressure variation during initiation and evolution of the plate boundary decollement zone in the nankai accretionary prism 1 the plate boundary decollement zone in the muroto region of the nankai accretionary prism records deformation and consolidation histories that have been affected by temporal changes in fluid pressure microstructural observations and chemical analysis demonstrate that the decollement zone initiated in an interval of porous clayey sediments characterized by cementation due to intergranular bonding of authigenic clays crosscutting relations of microstructures indicate that the decollement zone records two compactive deformations the early compactive deformation involved destruction of porous cemented structure probably caused by fluid pressure fluctuation the late compactive deformation was characterized by clay particle rotation and porosity collapse along the sets of slip surfaces resulting in zones of preferred orientation of clay particles these compactive deformations led to significantly higher bulk densities within the decollement zone compared to the compaction trend of the overlying prism sediments elevated fluid pressure following compactive deformations induced an overconsolidated state within the decollement zone with fluid filled dilatant fractures bulk density abruptly decreases at the top of the underthrust sediments but there is no microstructural evidence for cementation fluids in the dilated fractures and underconsolidated underthrust sediments are potential sources for the elevated fluid pressure in and below the decollement zone resulting in mechanical decoupling of the accretionary prism from underthrust sediments the fault fluid interactions in the muroto region may be applicable to other convergent plate margins where high temperature associated with the subduction of a spreading ridge or hot young oceanic crust enhances diagenesis and cementation

pleistocene fluctuations in the northern benguela current system as revealed by coccolith assemblages late pleistocene coccolith data of odp site 1082 from the eastern south atlantic off namibia were analysed to document spatial and temporal changes in surface ocean circulation upwelling intensity and associated productivity the studied site is located on the continental slope close to prominent oceanographic features such as the coastal upwelling mixing area the openoceanic realm and the warm water regime of the angola current highest numbers of total coccoliths up to 30 10 9 coccoliths g 1 sediment were observed during relatively warm periods characterised bydecreased upwellingintensity these elevatednumbers ofcoccoliths are due to highersurface water temperatures anddocumentadomainprobablyinfluencedbylowernutrientconcentrationsinanareawhereoceanicandfilamentouswatermix in contrast low numbers of coccoliths during glacial periods probably characterise time intervals of enhanced upwelling accompanied byabroad filamentous mixing area coccoliths of the genusgephyrocapsaarethe most abundant taxain samples of site 1082 their occurrences are characterised by abrupt changes in the investigated time interval in addition calcidiscus leptoporus pseudoemilinia lacunosa and reticulofenestra asanoi contribute a common part of the assemblage coccoliths accountformorethanhalfofthecarbonate withpeakcontributionofupto 80 intheinterval560to280kawhengephyrocapsa caribbeanica is the major caco3 producer increased carbonate accumulation during this time interval point to an overall weakening of the coastal upwelling intensity accompanied by a reduction of the mixing zone influencing the site location during this event d 2004 elsevier b v all rights reserved

erosion and exhumation in accretionary orogens experimental and geological approaches 1 this study examines the role of erosion on exhumation and deformation history of accretionary orogens through experimental and geological methods fault propagation kinematics and exhumation rates in an eroded thrust wedge are analyzed through scaled sandbox simulations by testing different parameters friction rheologic layering and sediment input output ratio a model coulomb wedge is submitted to erosion under flux steady state conditions that is the volume of eroded material remains equal to the volume of newly accreted material maintaining a constant surface slope during shortening results show that the way of exhumation depends on the internal dynamics of thrust wedges and conversely on how this dynamics is modified by erosion in the eroded thrust wedges the diversity of exhumation patterns is controlled by the mode of fault propagation depending on the basal friction high or low the vertical component of exhumation is generally higher for the wedges with high basal friction than for low friction wedges the uplift of material occurs along a cluster of subvertical thrusts in the middle part of the eroded thrust wedge with low basal friction the material is exhumed along a series of inclined 20 50 thrusts in the rear of the high friction wedge the zone of maximum exhumation is generally localized in the middle part of the thrust wedge and migrates toward the backstop with continued shortening the vertical exhumation rate increases with time and the material accreted later is rapidly transferred to the main exhumation zone compared to the material accreted during the early stages the exhumation occurs at rates 2 times faster in the wedges with half the thickness than in full thickness wedges the total quantity of eroded material at the end of convergence constitutes 36 50 of initial model area for the thrust wedges with low basal friction and 20 40 for the high basal friction wedges the extent of basal underplating increases with total shortening the area of basal underplated material constitutes up to 30 and 40 of the eroded thrust wedge area for the models with low and high basal friction respectively presence of decollements in the accreted series allows underplating of thrust units developing an anticlinal stack whose growth and location is favored by erosion our results are compared to different present day active and ancient convergent orogens himalaya taiwan southern alps of new zealand and cascadia orogen allowing the different specific exhumation processes to be characterized

late quaternary nannofossil indicators of climate change in two deep sea cores associated with the leeuwin current off western australia the stratigraphic variations in nannoflora observed in the two deep sea cores recovered from the offshore western australia revealed a sequence of paleoceanographic changes for the last 250 kyr the dominance of small placolith taxa in the upper photic flora indicates continuous mild upwelling for the isotope stages 7 5 in this region the most important controlling factor for the abundance variation of florisphaera profunda in this region is considered to be the lower photic temperature rather than the stability of water column or the water turbidity the significant reduction in the abundance of f profunda combined with the stable in the northern core rs96gc21 and reduced in the southern core rs9 150 abundance of the small taxa suggest an intensified upwelling for the penultimate glaciation the sharp increase in f profunda abundance indicates a weakening of upwelling at the last interglacial climax lic offshore western australia after isotope stage 5 the existence of upwelling is not clear except in stage 2 when it was not strong enough to produce blooms of emiliania huxleyi among the subordinate taxa calcidiscus leptoporus calciosolenia murrayi neosphaera coccolithomorpha and oolithotus fragilis provide meaningful paleoceanographic signals supporting the conclusions of previous studies a weakening or cessation of leeuwin current for the glacial periods the shift of dominance from small gephyrocapsa mostly g ericsonii to small reticulofenestra mostly r parvula can be an useful datum event offshore western australia the reduced diversity of nannoflora during the interglacial periods can be explained by a lessened competition within nannoplankton community caused by particularly favorable conditions for the opportunists such as e huxleyi f profunda and small placoliths the variations in nannofossil abundance suggests an increased aeolian flux at the stages 6 and 2 offshore north west cape as well as at stage 6 offshore southwestern australia

calcareous nannofossil biostratigraphy biochronology and paleoecology at the tortonian messinian boundary of the faneromeni section crete abstract a detailed calcareous nannofossil analysis was performed on 180 samples from the astronomically dated faneromeni section to identify the most important calcareous nannofossil events across the tortonian messinian t m boundary in the mediterranean and to date and correlate them precisely to the previously observed sections here we confirm the reliability of the genus amaurolithus the successive entries of amaurolithus primus amaurolithus aff amplificus and amaurolithus delicatus have been dated at 7 430 7 388 and 7 250 ma respectively the first occurrence and the first common occurrence of reticulofenestra rotaria are further confirmed bioevents dated at 7 421 and 7 264 ma these events improve the biostratigraphic resolution in the t m boundary interval at least for the mediterranean quantitative analysis revealed marked changes in the calcareous nannofossil assemblage associated with the sapropels as previously observed in other sections of the same time interval a decrease in the total number of coccoliths but an increase in abundance of the genera rhabdosphaera and helicosphaera characterize sapropels the observed fluctuations are comparable to the previous findings in neogene mediterranean sapropel sediments and suggest that similar mechanisms were responsible for their formation during this interval the possible influence of preservation on nannofossil assemblages is discussed

evolution of the nankai trough decollement from the trench into the seismogenic zone inferences from three dimensional seismic reflection imaging we mapped the amplitude of the nankai trough subduction thrust seismic reflection from the trench into the seismogenic zone with three dimensional seismic reflection data the decollement thrust forms within the lithologically homogeneous lower shikoku basin facies along an initially nonreflective interface the reflection develops from a porosity contrast between accreted and underthrust sedimentary material because of accretionary wedge consolidation and rapid loading and delayed consolidation of the underthrust section a decollement amplitude map shows a significant decline from high amplitudes at the trench to barely detectable levels 25 30 km landward three other observations coincide with the amplitude decline 1 the decollement initially steps down to deeper stratigraphic levels 2 the wedge taper increases dramatically and 3 the thrust becomes seismogenic the amplitude decline and the coincident decollement and accretionary wedge tectonic and seismogenic behavior are attributed to the loss of fluids and potentially loss of excess fluid pressures downdip along the subduction thrust

effect of increased shear stress along a plate boundary fault on the formation of an out of sequence thrust and a break in surface slope within an accretionary wedge based on numerical simulations abstract we investigated the effect on accretionary wedge structure of increased shear stress which describes the frictional sliding resistance along a decollement arising from an increase in material friction or reduction in pore pressure to clarify the nature of the effect we performed numerical simulations using two models a stable friction model and an increased friction model the stable friction model produced a low angle smooth surface slope and an in sequence thrust whereas the increased friction model produced a break in surface slope scarp and an out of sequence thrust ost that cuts through the thrust sheet the ost formed via the connection of segments of two adjacent thrusts and its formation resulted in a change in the thickening mode of the wedge from thrust sheet rotation and back thrust activity to underplating this contrast in thickening mode between the landward high friction zone and seaward low friction zone resulted in the formation of a clear break in slope as the landward zone is steeper than the seaward zone consistent with critical taper theory the subduction of a basement slice or seamount can produce similar structures arising from an increase in resistance to basal shear sliding however the distinctive structures arising in an accretionary wedge as a result of increased shear sliding resistance include a flat basal plane and absence of slope failure sediments beneath the ost these structural features are observed in accretionary wedges of the nankai trough off muroto japan the sunda strait and the barbados ridge

accretion and erosion in subduction zones the role of fluids convergence occurs today along about 60 000 km of plate boundaries parsons 1981 in this paper we consider frontal subduction which occurs along about 40 000 km of convergent plate boundaries using the kine matic solution of de mets et al 1990 and including 0 5 km 2 yr1 of marginal basin openings otsuki 1989 we find that 2 95 km 2 per year disappear in frontal subduction at an average rate of 7 3 cm yr the upper overriding plate is significantly modified not only through the addition of a large amount of magmatic material markhinin 1968 but also through tectonic processes which result either in outgrowth of the margin through addit ion of sediments silver 1969 or in recession through frontal and subcrustal erosion which appears to be linked to sediment subduction coats 1962 gilluly 1963 the various phenomena observed must in some way be related to variations in mechanical coupling between the two plates uyeda kanamori 1979 had proposed that the transition from erosion to accretion results from increasing mechanical coupling the marianas trench was presented as typical of an crosional margin and the chile trench of an accreting margin ruff kanamori 1980 and kana mori 1986 showed that the magnitude of the subduction earthquakes increases with the convergence velocity and the inverse of the age of the subducted lithosphere this correlation was qualitatively explained by the control of age on the d ip of the platc and of convergence velocity on the

an early permian ca 280 ma silicic igneous province in the alxa block nw china a magmatic flare up triggered by a mantle plume abstract silicic igneous provinces have important implications for geodynamic processes in this study we show that the predominantly early permian ca 280 ma granitoids in the alxa block correspond to an important silicic igneous province with areal distribution in northwest nw china we present zircon u pb age and hf o isotopic and whole rock geochemical and sr nd isotopic data for the newly identified i and a type granites and high sr y granodiorites in the bayannuoergong batholith and gabbros diorites and quartz diorites in the bijiertai complex alxa block the i type monzogranites granodiorites and a type granites of the bayannuoergong batholith were probably generated by mixing between crust and mantle derived magmas partial melting of a thickened eclogitic lower crust and underplated mantle derived mafic rocks respectively the gabbros quartz diorites and diorites of the bijiertai complex were likely generated by partial melting of the lithospheric mantle differentiation from basaltic magmas and partial melting of the middle crust respectively despite pronounced geochemical and isotopic heterogeneity of these rocks new sims and la icp ms u pb zircon dating indicates that they were formed over a very short time interval at ca 280 ma thus the mantle lower crust and middle crust underwent widespread melting at this time given that these magmatic rocks have no spatial or temporal zonation and were probably formed in an extensional setting we suggest that the ca 280 ma tarim mantle plume in nw china may have triggered the magmatic flare up of the alxa silicic igneous province

high silica granites terminal porosity and crystal settling in shallow magma chambers abstract high silica 70 wt sio2 granites hsgs are important carriers of highly incompatible elements thus understanding their origin is relevant to understanding how the composition of the continental crust evolves we examined a large scale geochemical study of plutons in the peninsular ranges batholith in southern california usa to better understand the petrogenetic relationships between hsgs and the batholith using highly incompatible and compatible elements we show that hsgs represent residual liquids within a felsic 69 72 wt sio2 magmatic crystal mush at crystal fractions of 50 60 and residual liquid fractions of 40 50 trace element systematics show that separation of the hsg liquid from the crystal mush is inefficient such that no more than 70 80 of the hsg is fully extracted and the remaining greater than 20 30 remains trapped in cumulate mush we find little evidence of more efficient liquid crystal segregation which suggests that compaction induced segregation may be too slow to be important on a large scale instead the terminal porosity of 20 30 coincides with theoretical maximum packing fraction of unimodal particles settled out of suspension 0 74 which may indicate that crystal settling perhaps in the form of hindered settling drives segregation of viscous silicic melts and crystals unlike compaction settling operates on timescales of 1 10 ky fast enough to generate large volumes of hsg and complementary cumulates with trapped melt before magma chambers freeze many felsic plutons may thus be cumulates but because of trapped melt they are difficult to geochemically distinguish from plutons whose compositions fall along liquid lines of descent the approach here using a combination of highly incompatible and compatible elements provides a way of identifying and quantifying trapped melt fractions finally we show that hsgs appear to form only in the shallow crust

updip limit of the seismogenic zone beneath the accretionary prism of southwest japan an effect of diagenetic to low grade metamorphic processes and increasing effective stress off southwest japan the seaward limit of coseismic displacement or updip limit of the seismogenic zone of the 1946 mw 8 3 thrust earthquake reaches to 4 km depth and 40 km landward of the trench this limit coincides with the estimated location of the 150 c isotherm and has been linked to changes in physical properties associated with the smectite to illite clay mineral transition here we show that this limit correlates with a suite of diagenetic to low grade metamorphic processes characterized by 1 declining fluid production and decreasing fluid pressure ratio and 2 active clay carbonate and zeolite cementation and the transition to pressure solution and quartz cementation these diagenetic to low grade metamorphic changes cause the onset of velocity weakening during thrust faulting an increase in effective stress and strengthening of the hanging wall which together combine to produce recordable earthquakes

paleoproterozoic evolution of the eastern alxa block westernmost north china evidence from in situ zircon u pb dating and hf o isotopes abstract the alxa block is the westernmost part of the north china craton ncc in the past it has been considered to be part of the archean ncc however formation and evolution of this block remain poorly understood and this has hampered a broader understanding of the ncc in this study we analyzed the in situ zircon u pb age and hf o isotopes for the two oldest rock units in the eastern alxa block namely the bayanwulashan and diebusige complexes the bayanwulashan complex consists mainly of metamorphic rocks with mafic and felsic protoliths sims u pb zircon dating results indicate that the primary magmatic ages of the mafic and felsic igneous rocks are ca 2 34 ga and ca 2 32 2 30 ga respectively and both sets of rocks were overprinted by two metamorphic events at ca 1 89 ga and ca 1 79 ga geochemical and zircon hf o isotopic data suggest that the ca 2 34 ga amphibolites within the bayanwulashan complex are characteristically high in tio 2 up to 3 2 zr up to 394 ppm and ti v 30 resembling the basaltic rocks formed within continental rifts the ca 2 32 2 30 ga felsic rocks were generated by remelting of dominant meta igneous rocks that have zircon hf model ages of 2 92 2 81 ga the diebusige complex is composed of amphibolites mafic gneisses and paragneisses it was intruded by granites dated at ca 1 97 1 98 ga and subjected to high grade metamorphic events at ca 1 89 ga and ca 1 79 ga re examination of u pb ages for detrital and metamorphic zircons indicate that the depositional ages of protoliths of the diebusige paragneisses may be considered to be between ca 2 45 and 2 0 ga these u pb zircon age data do not support the existence of exposed archean rocks in the eastern alxa block although the possibility of archean rocks at deeper crustal levels cannot be ruled out entirely the eastern alxa block contrasts to the neighboring yinshan block and khondlite belt as well as the trans north china orogen in terms of the timing and evolution of magmatism and metamorphism thus we conclude that the alxa block is a separated paleoproterozoic terrane from the western block of the ncc rather than the western extension of the yinshan block or part of the khondalite belt as previously proposed

late miocene washhouse climate in europe abstract we present two eight million year long proxy records of precipitation for southwest and central europe covering the middle to late miocene 5 3 13 ma at a temporal resolution of about 60 kyr and 150 kyr respectively the estimates of precipitation are based on the ecophysiological structure of herpetological assemblages amphibians and reptiles from 13 0 ma until about 9 ma both records show a similar trend evolving from a long dry period 13 11 ma into a washhouse climate 10 2 9 8 ma characterized by global warm conditions and several times more precipitation than present the transition from washhouse to a dryer climate between 9 7 and 9 5 ma and the concomitant cooling episode appear to have triggered a severe biotic event known as the vallesian crisis which included the extinction of hominoids in western europe a second washhouse period 9 0 8 5 ma coeval with a global warm episode was unprecedentedly intense in southwest europe but less pronounced in central europe from 8 ma onward a divergence in the two precipitation records is observed with southwest europe staying wetter and central europe becoming dryer than present both precipitation records are combined into a common run off curve as a measure of the relative intensity of the hydrological cycle for moderate latitudes of continental europe the run off curve shows a remarkable positive correlation with atlantic deep water temperatures from ceara rise by lear et al 2003 which are significantly higher up to 3 c during the two washhouse periods and show no other positive excursion of comparable magnitude we discuss potential links and the role of the coeval temporary restriction of the central american seaway on ocean and atmosphere circulation

timing petrogenesis and tectonic setting of the late paleozoic gabbro granodiorite granite intrusions in the shalazhashan of northern alxa constraints on the southernmost boundary of the central asian orogenic belt abstract the late paleozoic tectonic setting and location of the southernmost boundary of the central asian orogenic belt caob with respect to the alxa block or alxa north china craton ancc are debated this paper presents new geochronological petrological geochemical and zircon hf isotopic data of the late paleozoic intrusions from the shalazhashan in northern alxa and discusses the tectonic setting and boundary between the caob and ancc using zircon u pb dating intrusions can be broadly grouped as late carboniferous granodiorites 301 ma middle permian gabbros 264 ma and granites 266 ma and late permian granodiorites monzogranites and quartz monzodiorites 254 250 ma the late carboniferous granodiorites are slightly peraluminous and calcic the remarkably high zircon hf isotopes e hf t 6 10 and characteristics of high silica adakites suggest that these granodiorites were mainly derived from hot basaltic slab melts of the subducted oceanic crust the middle permian gabbros exhibited typical cumulate textures and were derived from the partial melting of depleted mantle the middle permian granites are slightly peraluminous with high k calc alkaline and low e hf t values from 0 9 to 2 9 these granites were most likely derived from juvenile materials mixed with old crustal materials the late permian granodiorites monzogranites and quartz monzodiorites are characterized as metaluminous to slightly peraluminous with variable peacock alkali lime index values from calc alkalic to alkali calcic these rocks were mainly derived from juvenile crustal materials as evidenced by their high e hf t values 3 3 to 8 9 the juvenile sources of the above intrusions in the shalazhashan are similar to those of the granitoids from the caob but distinct from the granitoids within the alxa block these findings suggest that the shalazhashan zone belongs to the caob rather than the alxa block and that its boundary with the alxa block can be regarded as the southernmost boundary of the caob the recognition of late carboniferous typical adakite magmatism in the region provides evidence for the subduction of the oceanic crust of the caob the middle late permian magmatisms 266 250 ma display a bimodal association with high k calc alkaline features and are interpreted as forming in a post collision setting these studies by interaction of regional geology provide new constraints on the tectonic evolution of the southern caob during the late paleozoic and the location of the southernmost margin of the caob

middle to late miocene fluctuations in the incipient benguela upwelling system revealed by calcareous nannofossil assemblages odp site 1085a middle to late miocene calcareous nannofossil data of odp site 1085 from the eastern south atlantic off namibia were analysed to document spatial and temporal changes in surface ocean circulation upwelling initiation and associated productivity our data show that calcareous nannofossils constitute a significant part of the carbonate fraction throughout the investigated interval from 12 5 to 7 7 million years ma highest numbers of calcareous nannofossils up to 38 000 10 6 nannofossils g 1 sediment were observed during the intervals 9 9 to 9 7 and 8 7 to 8 0 ma these elevated numbers of calcareous nannofossils may generally be linked to the initiation of upwelling at about 10 ma in the studied region in contrast diminished numbers of calcareous nannoplankton as in the interval 9 6 to 9 0 ma probably characterise time intervals of weaker productivity resulting in a decrease of nannofossil carbonate contents in the sediments of site 1085 this decrease in nannofossil production could be one possible explanation for the major caco3 depression in between 9 6 and 9 0 ma coccoliths of the genus reticulofenestra are the most abundant taxa their occurrences are characterised by changes in the investigated time interval in addition coccolithus pelagicus calcidiscus leptoporus and umbilicosphaera spp contribute a common part of the assemblage calcareous nannofossils account for more than half of the carbonate with peak contribution up to 80 at 8 8 ma d 2005 elsevier b v all rights reserved

mechanism of decollement formation in subduction zones summary the mechanism of decollement formation was investigated through a particle based simulation model assuming homogeneity e g no weak layer or pore fluid a decollement like structure appeared as a spontaneously localized shear deformation near the bottom of the sediment when the thickness of the sediment was sufficient to balance the gravitational force and tectonic loading in contrast no such decollement like structure was formed when the sediment was too thin in this case the entire prism was deformed because of plate motion these results are consistent with various observations in real subduction zones a precise analysis of the stress state evolution during accretion reveals that the formation of a decollement like structure is controlled by the spatio temporal distribution of isotropic compression states

nannofossil evolutionary events in the mid pliocene an assessment of the degree of synchrony in the extinctions of reticulofenestra pseudoumbilicus and sphenolithus abies abstract the mid pliocene was an interval of subtle reorganisation within the nannoplankton community including the prominent and biostratigraphically important last occurrences of sphenolithus abies and reticulofenestra pseudoumbilicus the transition is part of the pliocene to recent attrition of nannofossil species that resulted from changes in the distribution of trophic resources and deep water and surface water current systems likely associated with the initiation of northern hemisphere glaciation the extinctions of sphenolithus abies and reticulofenestra pseudoumbilicus were analysed in detail at odp sites 659 662 and 926 in the equatorial and subequatorial atlantic these taxa show significantly different patterns of duration and timing of decline based on high resolution abundance records and calibration with oxygen isotope stratigraphy the initiation of abundance decline between 3 71 and 3 67 ma and the extinction of s abies between 3 56 and 3 52 ma are diachronous this extinction may have been a response to the intensification of glacial intervals at this time in contrast the last occurrence of r pseudoumbilicus at 3 81 3 82 ma appears to be a valid example of biostratigraphic although not necessarily biological synchrony in the fossil record direct environmental forcing is not attributable for the extinction of r pseudoumbilicus however indirect physical and or biological environmental stress may explain the observed patterns

overlapping sr nd hf o isotopic compositions in permian mafic enclaves and host granitoids in alxa block nw china evidence for crust mantle interaction and implications for the generation of silicic igneous provinces abstract in general the mantle provides heat and or material for the generation of the silicic igneous provinces sips the rarity of mafic microgranular enclaves mmes however hampers understanding of the mantle s role in generating sips and the process of crust mantle interaction the widespread distributed mmes in the newly reported alxa sip provide an opportunity to study these processes this study integrates in situ zircon u pb age and hf o isotope analyses whole rock geochemistry and sr nd isotope results for the mmes and host granitoids in the alxa block sims zircon u pb dating reveals that there are two generations of mmes and host granitoids the mmes in the bayannuoergong batholith were formed at ca 278 ma similar to the age 280 ma of host granitoids and the mmes and host granitoids in the yamaitu pluton were formed at ca 272 270 ma all mmes have relatively low sio 2 50 7 61 4 wt and th 0 8 2 8 ppm but relatively high mgo 2 6 4 9 wt cr 23 146 ppm and ni 6 38 ppm contents compared to the host granitoids with sio 2 63 6 77 5 wt th 5 2 41 ppm mgo 0 23 2 1 wt cr 10 38 ppm and ni 5 14 ppm all mmes have whole rock sr nd and zircon hf o isotope compositions similar to their corresponding host granitoids the 280 ma mmes have lower whole rock e nd t 13 5 and higher initial 87 sr 86 sr values 0 7095 and zircon 18 o values 6 3 compared to the e nd t 11 5 initial 87 sr 86 sr values 0 7070 and zircon 18 o values 5 6 of the 270 ma mmes the occurrences of quartz xenocrysts k feldspar megacrysts corroded feldspars and acicular apatites indicate that the mmes are the products of the mixing between mantle and crust derived magmas the striking similarities in the zircon hf o isotopic compositions in both mme host granitoid pairs indicate that the granitoids and mmes have similar sources the granitoids are proposed to be mainly sourced from magmas generated by remelting of newly formed mafic rocks which were generated by earlier basaltic magmas underplating or injected into the mid to lower crust the continuing basaltic magma underplating and remelting of earlier formed mafic rocks in the mid to lower crust indicate that the mantle provided both heat and materials for the generation of the alxa sip the scenario also suggests that basaltic magma underplating may play a more important role in the crustal growth in the alxa sip than it is generally recognized in other sips

reduced rapakivi type granites the tholeiite connection reduced rapakivi type granites are the most iron enriched and reduced i e least oxidized of the anorogenic granite association the low oxygen fugacity and chemical composition of these granites severely limit their sources in this paper we argue that reduced rapakivi type granites and their eruptive equivalents high potassium fayalite rhyolites are derived from mafic sources because tholeiitic magmas and their derivatives have the required low oxygen fugacity reduced rapakivi type granites are produced either by extreme differentiation of basaltic melts or by partial melting of underplated basalts and their differentiated equivalents they form in extensional environments where the asthenosphere is present at shallow depths we envision three stages in the origin of these rocks 1 tholeiitic melts are emplaced at the base of the crust 2 continued introduction of heat partially remelts these tholeiitic rocks and 3 the hot dry melts so produced migrate into the middle crust to produce rapakivi batholiths or erupt as rhyolites partial melting of felsic continental crust may accompany the intrusion of rapakivi type magmas thereby producing the other metaluminous and peraluminous granite compositions of the anorogenic suite

environment and climate of early human evolution evaluating the relationships between climate the environment and human traits is a key part of human origins research because changes in earth s atmosphere oceans landscapes and ecosystems over the past 10 myr shaped the selection pressures experienced by early humans in africa these relationships have been influenced by a combination of high latitude ice distributions sea surface temperatures and low latitude orbital forcing that resulted in large oscillations in vegetation and moisture availability that were modulated by local basin dynamics the importance of both climate and tectonics in shaping african landscapes means that integrated views of the ecological environmental and tectonic histories of a region are necessary in order to understand the relationships between climate and human evolution

geochemistry and zircon geochronology of the i type high k calc alkaline and s type granitoid rocks from southeastern roraima brazil orosirian collisional magmatism evidence 1 97 1 96 ga in central portion of guyana shield abstract the understanding of the geological evolution of the uatuma anaua domain in southeastern roraima central region of guyana shield is of major significance in the study of the amazonian craton this region lies between some major paleoproterozoic geological geochronological provinces tapajos parima or ventuari tapajos dominant maroni itacaiunas or transamazon northwest and central amazonian or central amazon southeast and east geological mapping of the northern area of uatuma anaua domain integrated with whole rock geochemistry data and previous and new pb evaporation and u pb zircon geochronology point out for a plutonic collisional magmatic event 1975 1968 ma represented by i type high k calc alkaline martins pereira and s type serra dourada granitoid rocks this magmatism was probably generated from crustal sources by partial melting during amalgamation of the ttg like anaua magmatic arc 2028 ma with transamazonian 2 2 2 0 ga and central amazonian older than 2 3 ga terranes local cumulatic leucogranites fills planar structures of the martins pereira granites these leucogranites show younger ages 1909 ma and several inherited zircons 2354 2134 1997 and 1959 ma suggesting origin from crustal sources

distribution of coccoliths in surface sediments of the south eastern south atlantic ocean ecology preservation and carbonate contribution abstract recent coccoliths from 52 surface sediment samples recovered from the south eastern south atlantic were examined qualitatively and quantitatively in order to assess the controlling mechanisms for their distribution patterns such as ecological and preservational factors and their role as carbonate producers total coccolith abundances range from 0 2 to 39 9 coccoliths 109 g 1 sediment four assemblages can be delineated by their coccolith content characterising the northern benguela the middle to southern benguela the walvis ridge and the deeper water distinctions are based on multivariate ordination techniques applied on the relative abundances of the most abundant taxa emiliania huxleyi calcidiscus leptoporus gephyrocapsa spp coccolithus pelagicus and subtropical to tropical species the coccolith distribution seems to be temperature and nutrient controlled co varying with the seaward extension of the upwelling filament zone in the benguela a preservation index cex based on the differential dissolution behaviour of the delicate e huxleyi and gephyrocapsa ericsonii versus the robust c leptoporus is applied in order to detect the position of the coccolith lysocline although some samples were recognised as dissolution affected the distribution of the coccoliths in the surface sediments reflects the different oceanographic surface water conditions mass estimations of the coccolith carbonate reveal coccoliths to be only minor contributors to the carbonate preserved in the surface sediments the mean computed coccolith carbonate content is 17 wt equivalent to a mean contribution of 23 to the bulk carbonate

plankton in the norwegian greenland sea from living communities to sediment assemblages an actualistic approach a synoptic study is carried out to reconstruct the development of the plankton community in the late quaternary in the norwegian greenland sea it comprises quantitative analyses of coccolithophores dinoflagellate cysts diatoms and radiolarians an actualistic approach is applied to evaluate the different fossil records of these plankton groups the preliminary results of the current investigation are reported here

controls on carbonate and terrigenous deposition in the incipient benguela upwelling system during the middle to the late miocene odp sites 1085 and 1087 this study deals with the variability of biogenic carbonate and terrigenous depositional patterns in the cape basin area of the south atlantic during the middle to the late miocene focusing on odp sites 1085 and 1087 sediments at the benguela upwelling area southwest africa the investigations are based on grain size analysis and clay mineralogy the sedimentary records at both sites provide information about climate and environmental changes the direct input of terrigenous material from southwestern africa is driven by a complex interaction of river and wind transport and ocean currents the considerable amount of dust input derived from the namib desert started to become important after 11 2 ma pointing to an increased supply of cool intermediate waters to mid latitude surface waters in the upwelling regions which coincided with the establishment of the winddriven benguela upwelling system a major drop in caco3 concentration between 10 4 and 10 1 ma is caused mainly by changes in calcareous nannoplankton production while another drop between 9 6 and 9 ma is triggered by a combination of production changes of calcareous nannoplankton and dilution in response to high shelf supply during global lowering of the sea level

carbonate crash and biogenic bloom in the late miocene evidence from odp sites 1085 1086 and 1087 in the cape basin southeast atlantic ocean 1 middle late miocene to early pliocene sedimentary sequences along the continental margin of southwest africa have changes that correspond to the carbonate crash 12 9 ma and biogenic bloom events 7 4 ma described in the equatorial pacific by farrell et al 1995 to explore the origins of these changes we analyzed the carbon and coarse fraction contents of sediments from odp sites 1085 1086 and 1087 at a time resolution of 5 to 30 kyr several major drops in caco3 concentration between 12 and 9 ma are caused by dilution from major increases in clastic input from the oranje river during global sea level regressions abundant pyrite crystals and good preservation of fish debris reflect low oxygenation of bottom pore waters regional productivity was enhanced during the time equivalent to the carbonate crash period higher benthic planktic foraminiferal ratios indicate that caco3 dissolution at site 1085 peaked between 9 to 7 ma which was after the global carbonate crash this period of enhanced dissolution suggests that site 1085 was located within a low oxygen water mass that dissolved caco3 more easily than north atlantic deep water which began to bathe this site at 7 ma at 7 to 6 ma the onset of the biogenic bloom increases and variations in total organic carbon and benthic foraminiferal accumulation rates show that paleoproductivity increased significantly above values observed during the carbonate crash period and fluctuated widely we attribute the late miocene paleoproductivity increase off southwest africa to ocean wide increases in nutrient supply and delivery1