a multi model analysis of the role of the ocean on the african and indian monsoon during the mid holocene we investigate the role of the ocean feedback on the climate in response to insolation forcing during the mid holocene 6 000 year bp using results from seven coupled ocean atmosphere general circulation models we examine how the dipole in late summer sea surface temperature sst anomalies in the tropical atlantic increases the length of the african monsoon how this dipole structure is created and maintained and how the late summer sst warming in the northwest indian ocean affects the monsoon retreat in this sector similar mechanisms are found in all of the models including a strong wind evaporation feedback and changes in the mixed layer depth that enhance the insolation forcing as well as increased ekman transport in the atlantic that sharpens the atlantic dipole pattern we also consider changes in interannual variability over west africa and the indian ocean the teleconnection between variations in sst and sahelian precipitation favor a larger impact of the atlantic dipole mode in this region in the indian ocean the strengthening of the indian dipole structure in autumn has a damping effect on the indian dipole mode at the interannual time scale

melt generation during rifting in the north sea in a forthcoming paper mckenzie and o nions1 invert rare earth element concentrations in mid ocean ridge basalts to calculate the variation of melt fraction with depth in the asthenospheric mantle beneath ocean ridges they find that melting persists to greater depths about 80 km than would be predicted by parameterizations of melting experiments2 about 50 km this result has important implications for predicting the volume and composition of melt produced from asthenosphere of normal potential temperature during continental rifting the north sea rift is a good location to test models for melt generation because the physical conditions of rifting are well constrained3 8 using the distribution of melt fraction with depth derived in ref 1 we calculate the volume and composition of melt produced from the asthenosphere in the most stretched part of the north sea the forties region when this melt is mixed in approximately equal proportions with an alkaline melt similar to that produced from metasomatized lithosphere by very small amounts of stretching on the rift flanks it accounts for the volume and the elemental and isotopic composition of the basalts in the most extended region

the influence of changing orbital parameters and surface boundary conditions on climate simulations for the past 18 000 years abstract general circulation model experiments at 3000 year intervals for the past 18 000 years were made to estimate the magnitude timing and pattern of the climatic response to prescribed changes of orbital parameters date of perihelion axial tilt eccentricity and glacial age lower boundary conditions ice sheets land albedo sea ice and sea surface temperature the experiments used the community climate model ccm of the national center for atmospheric research ncar the response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial age boundary conditions the continental interior of eurasia was 2 4 k warmer in summer and summer monsoon precipitation of north africa south asia was increased by 10 20 between 12 000 and 6000 yr bp before present when perihelion occurred during northern summer rather than in winter as now and the earth s axial tilt was larger than now southern hemisphe

age relations among oslo rift magmatic rocks implications for tectonic and magmatic modelling abstract this paper presents a compilation of 67 published and unpublished radiometric age determinations by the rb sr method on magmatic rocks in the continental oslo rift in southeast norway the data include most major rock types and structural units in the subaerial part of the rift the implications of these data with respect to the tectonomagmatic evolution of the rift and processes in the mantle and crust are discussed the oslo rift was active for about 60 million years the earliest recorded magmatic activity was the intrusion of basaltic and syenitic sills in the pre rift lower paleozoic sediments at about 300 ma extrusion of plateau lavas of basaltic and intermediate composition rhomb porphyries started about 295 ma and lasted to about 275 ma with largest magma volumes and highest extrusion rate from 295 to 285 ma in the vestfold graben segment gs one flow per 250 000 years and 0 30 km3 per 103 years the main faulting activity and graben formation occurred during this period the plateau volcanism was succeeded by a stage of bimodal central volcanoes most of which later underwent caldera collapses ring dykes and central intrusions in these calderas give ages in the range 268 266 ma in the vestfold and 266 243 ma in the akershus gs large composite intrusive complexes were emplaced in the periods 27 268 and 273 241 ma in the vestfold and akershus gs respectively a rate of propagation of the rift from ssw towards the nne has been estimated to 1 2 cm y 1 on the basis of 87 sr 86 sr initial ratios and type of magmatic products we estimate magmas to have been emplaced into the crust only during the first 20 25 ma of the magmatic period some of the magmatism appears to be initiated by movement along precambrian fault zones which were reactivated during the oslo rifting event

a 13 000 year climate record from western tibet although the tibetan plateau is important in influencing the atmospheric circulation of the northern hemisphere1 3 there are only a few continuous palaeoclimate records available and these are limited to the plateau s northeastern margin4 6 here we present a 13 000 yr record from sumxi co western tibet constructed from both lake core and shoreline studies which shows that conditions in the early middle holocene were warmer and wetter than at present these results confirm model predictions of an intensified monsoon over the region at 9 000 yr bp owing to an orbitally induced increase in summer insolation7 8 we also find evidence for warm humid pulses at 12 500 and 10 000 yr bp in phase with the steps of the last deglaciation and for a return to cold dry conditions at 11 10 000 yr bp none of which can be explained by orbital variations the existence of the cold episode confirms that the cooling associated with the younger dry as event occurred in continental china6 9 and provides further evidence of the global nature of this event10

quantitative reconstructions of annual rainfall in africa 6000 years ago model data comparison precipitation are based on 85 pollen sites 14 c dated at 6000 500 years b p and distributed over the whole of africa to improve the reliability of the pollen based climate reconstruction two methods are used the modern analogues technique mat and the plant functional types pft methods we then conduct a model data comparison for five distinct regions allowing an evaluation of model outputs the sahara sahel the eastern sahara western equatorial africa east africa and madagascar the pollen inferred reconstructions are compared with 21 mid holocene simulations yielded by atmospheric general circulation models agcms and coupled ocean atmosphere vegetation models oavgcms the large scale feature of the hydrological changes is shown to be well captured by most of the models data show that during the mid holocene the sahara was considerably wetter than today 200 to 700 mm yr the results reinforce the conclusion that the agcms significantly underestimate this precipitation increase in the sahara whereas the oavgcm simulations are in accordance with the data our results show that vegetation and ocean feedbacks do not have a strong impact in the intertropical zone and that models fail to properly reproduce the climatic conditions in east africa and madagascar the model data comparison also suggests that the lengthening of the dry season during boreal winter in the west equatorial region is a robust feature although the pollen based reconstruction shows no change or only slight drying there

the oslo rift its evolution on the basis of geological and geophysical observations abstract the formation and evolution of the oslo rift have puzzled geoscientists for more than a century we have reexamined this problem in view of recent detailed geophysical information bearing on crust and lithosphere structures in the general rift area for example the rift contours closely correlate with a moho elevation of 3 5 km the crustal thinning partly predates the rifting and apparently the same is the case for lithospheric deformations cambro silurian sediments are found mainly within the present day rift and this implies rifting contemporaneous with uplift the southern rift segment the skagerrak graben appears to be somewhat different from the northern segment the oslo graben with relatively less magmatism and larger subsidence crustal laminae are found inside and especially outside the rift area including sub moho dipping reflectors whose origin is attributed to magmatic intrusions and or stretching the strength of the lithosphere increases northwards with increasing thicknesses of both the brittle crust and the lithosphere a variety of continental rifting mechanisms has been examined in view of the observational data at hand basically a passive rifting mechanism is presumed with dominant east west tensional stresses albeit we are unable to be specific on driving forces the model presented in this paper reflects concepts introduced by dunbar and sawyer 1988 and neugebauer 1983 on preweakening of the crust and lithosphere and diapirism as a transport mechanism the essential elements of the model are 1 1 the area in question was preferential for rifting in view of previous crust and lithosphere deformations from late precambrian to silurian times 2 2 lithosphere stretching is needed for creating favorable conditions for diapirism 3 3 the shield strength increases rapidly towards the northern part of the rift preventing further migration

finite duration extension and decompression melting abstract decompression melting of the upwelling mantle during extension of continental lithosphere provides a viable mechanism for the magmatism observed in many rifted sedimentary basins and passive volcanic margins a model has been developed in which in addition to the adiabatic temperature decrease syntectonic conductive cooling melt transport and emplacement at crustal levels are accounted for the heat equation including latent heat is solved numerically and the incremental melt fraction is traced to simulate dynamic melting the emplaced magma is considered an integral part of the continental crust we find that relative to an instantaneous rifting model with only adiabatic cooling there is a more than 10 decrease in the melt thickness even when the duration of rifting is as short as 1 m y whether extension takes place with constant horizontal velocity or strain rate has important implications for the amount of melt produced consequently pressure release melting does not only depend on the amount of stretching and duration of the rift episode but also on how extension develops through time

mid holocene east asian summer climate as simulated by the pmip2 models abstract in the present study datasets derived from twelve coupled ocean atmosphere general circulation models oagcms in the paleoclimate modelling intercomparison project phase two were used to analyze the east asian summer climate during the mid holocene about 6000 calendar years ago on the whole the oagcms reproduced warmer and wetter summer climate conditions in east asia during the mid holocene the multi model ensemble showed that in east asia the regionally averaged summer surface air temperature sat increased by 0 89 c summer precipitation was 5 8 higher and an obviously strengthened southerly wind corresponded to a strong summer monsoon in the mid holocene when compared to preindustrial levels the data model comparison in china reveals a good agreement between the oagcms results and the reconstructed changes in the summer sat in east china during the mid holocene in north china the simulated sat anomalies are 0 5 c lower overall than reconstruction in contrast the oagcms fail to capture the strongest warming in the southern qinghai tibetan plateau on the other hand the simulated precipitation agrees well with proxy data except for in the central parts of china where the simulated summer precipitation disagrees in sign with reconstruction in addition there is a large spread among the simulations particularly over and around the qinghai tibetan plateau and inter model discrepancies are larger for precipitation than for sat as a whole

testing model simulations of the thermocline depth variability in the tropical atlantic from 1982 through 1984 abstract the multivariate model toning procedure of frankignoul et al has been extended to the general time series case thus allowing to test ocean model ability at simulating the interannual variability the method aims at distinguishing between model inadequacies and data uncertainties model performances are assessed from a misfit evaluated in a space of strongly reduced dimension with basis vectors issued from a double application of common principal component analysis the testing procedure has been used to investigate the ability of cane s linear multimode model at simulating the evolution of the 20 c isotherm depth in the equatorial atlantic during the 1982 1984 focal sequal experiment using monte carlo techniques and five different drag laws 25 equally plausible wind stress fields were constructed to represent the wind stress uncertainties consistently with the sample means and variances of the original ship measurements even during this well sampled period the forcing uncertainties were sub

mid holocene nao a pmip2 model intercomparison 1 the mid holocene 6000 years before present north atlantic oscillation nao from nine models in the paleoclimate modeling intercomparison project phase 2 is studied primarily through principal component analysis of winter time north atlantic sea level pressure slp modeled mid holocene nao and mean slp show small changes compared to pre industrial control runs with a shift in mean state towards a more positive nao regime for three of the models modeled nao variability shows little change with a small increase for some models in the fraction of time spent in the nao negative phase during the mid holocene proxy based reconstructions of the nao indicate a more positive nao regime compared to present day during the mid holocene we hypothesise that there was a small nao like shift in mean state during the mid holocene

the role of low angle normal faulting and isostatic response in the evolution of the suez rift egypt abstract a new geometric model for the suez rift suggests that isostatic uplift of an underlying low angle normal fault system accompanying early block faulting localized subsequent deep seated extension this in turn created a superimposed late stage graben system offset to the east from an earlier locus of extension initial extension in the oligocene and earliest miocene drove minor rotation on wide tilt blocks over an eastward dipping low angle normal fault system and formed a series of wide half grabens with increasing extension more closely spaced faults formed smaller rotating tilt blocks fragmenting the larger earlier half grabens and creating numerous sub basins in response to this brittle thinning of the upper crust isostatic uplift of the lower crust arched the underlying low angle fault surface limiting gravity driven tilt block movement in the west by flattening the low angle fault surface and creating a broad gulf parallel high in the fault surface and moho under the eastern portions of the rift this process diminished faulting in the west and localized continuing deep seated extension under the crest of the arch possibly through induced mantle convection through going faults thus initiated cut the old low angle fault system and bound a system of down dropped grabens offset to the east of the most extended portions of the older tilt block array in the red sea immediately to the south this process has continued with the increased separation allowing the formation of mantle derived oceanic crust this model suggests that early movement over low angle detachments and accompanying isostatic response may play an important role in localizing deformation within rifts and are probably critical elements in the early formation of passive margins

some comments on two layer extensional models for the evolution of sedimentary basins simple extensional models that involve listric faulting in the brittle upper crust and plastic flow in the lower lithosphere have been shown to account for the subsidence history of various continental sedimentary basins and continental shelves in this paper we present structural evidence that extension in the continental lithosphere during rifting is not uniform we review evidence that the rifting process in major intracratonic rift basins is spasmodic rather than continuous that extension and subsidence within the rift during a tectonic phase is generally accompanied by uplift and erosion of the rift flanks that subcrustal heat input during rifting occurs over an area much broader than the rift proper and that there may be broad regional uplift before and or during rifting to explain the uplift of the flanks during rifting and their postrift subsidence we invoke two mechanisms subcrustal thinning by extension below the flanks and tilted major normal faults in the crust between graben and flanks the uplift and erosion of the flanks due to tilted faults probably do not significantly affect estimates of extension from subsidence analysis but uplift due to both sources can in certain cases be incorrectly attributed to regional uplift or doming we present a simplified analysis of the two layer extensional model for the elementary case in which extension is instantaneous and the crust is thinned by a different amount from the subcrustal lithosphere we provide a simple graphical method of data analysis to obtain the parameters that describe thinning of the crust and lithosphere during extension from the subsidence history application of this method to the central part of the pattani trough in the intracratonic gulf of thailand shows that extension and thinning of the lithosphere of a factor of 2 since late eocene can account for much of the subsidence history and the present day heat flow our analysis suggests that thinning of the crust in this area is 20 greater than thinning of the whole lithosphere assuming that none of the lithosphere is removed from the region of extension during rifting we show by simple geometric examples that a process of nonuniform extension implies that much greater thicknesses of sediment can be deposited in the center of a young basin than in the case of uniform extension of both crust and subcrustal lithosphere such an extensional process produces significant uplift of the flanks of the rift and as a result of erosion of the uplifted areas the effective cross sectional area of the basin can be increased by as much as 10 20 depending on the rate of erosion compared to the area that would have been created by a process of uniform extension finally we show that it may ultimately be possible to compute erosion rates from seismic cross sections of basins

long term weakening of the east asian summer and winter monsoons during the mid to late holocene recorded by aeolian deposits at the eastern edge of the mu us desert abstract the east asian summer monsoon easm and east asian winter monsoon eawm are major drivers of environmental conditions in east asia however due to the lack of high resolution eawm records the phase relationship between the easm and eawm during the holocene epoch is still debated here we use magnetic and grain size measurements from a sequence of aeolian sediments from the eastern edge of the mu us desert to track the history of the easm and eawm from 7 5 to 2 5 ka the results show that both the easm and eawm exhibit a similar long term weakening trend during this interval in view of the stable glacial boundary conditions and the significantly decreased rates of atmospheric co 2 rise since the mid holocene we suggest that orbitally induced insolation was the major cause of the weakening of both the easm and eawm decreasing summer insolation and increasing winter insolation reduced the thermal contrast between the ocean and the asian continent in both summer and winter time resulting in the weakening of both monsoon systems during the mid to late holocene

strong coherence between solar variability and the monsoon in oman between 9 and 6 kyr ago variations in the amount of solar radiation reaching the earth are thought to influence climate but the extent of this influence on timescales of millennia to decades is unclear a number of climate records show correlations between solar cycles and climate1 but the absolute changes in solar intensity over the range of decades to millennia are small2 and the influence of solar flux on climate is not well established the formation of stalagmites in northern oman has recorded past northward shifts of the intertropical convergence zone3 whose northward migration stops near the southern shoreline of arabia in the present climate4 here we present a high resolution record of oxygen isotope variations for the period from 9 6 to 6 1 kyr before present in a th u dated stalagmite from oman the 18o record from the stalagmite which serves as a proxy for variations in the tropical circulation and monsoon rainfall allows us to make a direct comparison of the 18o record with the 14c record from tree rings5 which largely reflects changes in solar activity6 7 the excellent correlation between the two records suggests that one of the primary controls on centennial to decadal scale changes in tropical rainfall and monsoon intensity during this time are variations in solar radiation

paleozoic paleo cover of the southern part of the fennoscandian shield fission track constraints abstract granitic intrusions older than 1200 ma from an area west of lake vanern south sweden show fission track ages of 680 50 ma 2 late precambrian for sphene and 220 10 ma late triassic for apatite the samples were taken at the present post glacial erosion surface which is close to the sub cambrian peneplain the c 680 ma sphene fission track ages are thought to be related to the uplift of the area which led to the establishment of this erosion surface at c 600 ma ago the much younger apatite fission track ages and the confined spontaneous track length distributions along with regional stratigraphic constraints suggest that the basement was subsequently buried under 3 4 km of paleozoic supracrustals this induced in the basement a rise in temperature which obliterated the older apatite fission tracks but not those in sphene which has a higher closure temperature at the beginning of the mesozoic the area was uplifted again the cover rocks were removed and the basement rocks cooled slowly below the temperature range of fission track annealing in apatite

a dynamical model of lithosphere extension and sedimentary basin formation convection in the upper mantle can give rise to normal stresses on the base of the continental lithosphere that may cause localized elevation of its surface and extensional deviatoric stresses this possibility is investigated assuming homogeneous biaxial strain in a continental lithosphere whose strength is governed by a combination of brittle failure and power law creep in the crust and by brittle power law or dorn law behavior in the mantle for a given geotherm and for rheologies based on current laboratory data the lithosphere responds to convection induced uplift in one of three ways depending primarily on the magnitude of the driving stresses these regimes are as follows 1 when the driving stress exceeds a threshold value the resulting strain rates are high enough greater than about 3 10 16 s 1 that the lithosphere strains almost isothermally its effective rheology is strain thinning and it undergoes accelerating extension the magnitude of the dynamic uplift required to produce the threshold value of the driving stress depends on the geotherm characterized here by the moho temperature and varies from about 1 km for a moho temperature of 700 c to around 3 km for a moho temperature of 550 c an uncertainty of at least 100 c is attached to these temperatures owing to the inaccuracies in extrapolating laboratory data to geological strain rates 2 at stress levels due to less than about half of the critical uplift extensional strain is negligible 3 at intermediate stress levels moderate uplift produces extension that is bounded by strength increases due to cooling of the ductile portion of the lithosphere during extension

geodynamics of rifting and implications for hydrocarbon habitat abstract rifting activity is probably mainly governed by frictional forces exerted on the base of the lithosphere by the convecting sub lithospheric upper mantle combined with deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system and by far filed stresses controlling the interaction of lithospheric plates moving relative to each other mantle plumes rising from the deep mantle do not appear to be a major driving force of rifting therefore it is questioned whether a distinction between active and passive rifts is still justified lithospheric extension triggers by decompression partial melting of the lower lithosphere and upper asthenosphere magmatic processes contribute materially to thinning of the subcrustal lithosphere and probably also of the lower crust this could explain the frequently observed discrepancy between upper crustal extension by faulting and lower crustal attenuation as defined by refraction and deep reflection seismic data the duration of the rifting stage of palaeorifts and passive margins is highly variable depending on plate interaction crustal separation can be achieved after as little as 7 ma and as much as 280 ma of rifting activity the availability of crustal discontinuities does not appear to play a significant role in the time required for crustal separation to be achieved the structural style of rifts is controlled by the physical state of the lithosphere the mode and amount of its extension and the lithological composition of pre and syn rift sediments subsidence of rift basins is controlled by the balance between isostatic adjustment of the crust to lithospheric stretching and uplift induced by mechanical and thermal attenuation of the subcrustal lithosphere post rift basin subsidence is mainly governed by the decay of rift induced thermal anomalies densification of the lower crust and intra plate stresses provide overprinting effects in view of this it is unlikely that quantitative analyses of post rift basin subsidence give a direct measure of lithospheric extension tectonically active rifts palaeorifts and passive margin basins contain major hydrocarbon provinces their hydrocarbon charge can rely exclusively on pre rift syn rift or post rift sedimentary sequences or a combination thereof maturation of source rocks can be achieved during the syn and or post rift stage of basin evolution during rifting conductive and convective heat transfer accounts for elevated geothermal gradients these play an important role in the maturation of pre and syn rift source rocks thermal gradients decrease asymptotically during the post rift stage

compositional variations among gabbroic intrusions in the oslo rift magmatic activity associated with the formation of the oslo rift south norway in permo carboniferous time includes formation of a series of gabbroic intrusions of minor volume the rock types in these intrusions range from pyroxenite and gabbro to monzonite and anorthosite the mafic units bear evidence of the crystallization order ol cpx opx plag whereas the more evolved rock types have crystallized olivine and plagioclase before clinopyroxene amphibole occasionlly represents an early phase major and trace element data imply that removal of clinopyroxene has dominated the early crystallization history of all these rocks this is also true for rocks which give petrographic evidence of crystallization of cpx after plag rb sr isotope data define a whole rock age of 265 11 for the snaukollen and a mineral age of 266 5 ma for the dignes intrusion whole rock samples from the other intrusions however do not define isochrons but cover a range of 87sr86sr ratios from 0 70395 to 0 71187 it is concluded that the gabbroic oslo rift rocks are produced through a two stage crystallization history combined with crustal contamination 1 1 mafic magmas ascending from some melt region in the mantle were retained in the lower crust where they crystallized olivine and clinopyroxene 2 2 at different stages of evolution residual magmas rose to their present position in the upper crust depending on composition the crystallization sequence there was ol cpx plag or ol plag cpx 3 3 during the final stage of evolution the magmas were to different degrees contaminated by precambrian amphibolite facies and cambro silurian carbonaceous country rock early crystallization of amphibole combined with interstitial calcite in some rocks and highly variable fe3 contents in clinopyroxene imply significant variations in the volatile phase of the magmas which gave rise to the gabbroic rocks these differences may have been introduced through the contamination however the possibility of primary compositional contrasts cannot be excluded

an evaluation of the simulated water balance of eurasia and northern africa at 6000 y bp using lake status data changes in the water balance of eurasia and northern africa in response to insolation forcing at 6000 y bp simulated by five atmospheric general circulation models have been compared with observations of changes in lake status all of the simulations show enhancement of the asian summer monsoon and of the high pressure cells over the pacific and central asia and the middle east causing wetter conditions in northern india and southern china and drier conditions along the chinese coast and west of the monsoon core all of the models show enhancement of the african monsoon causing wetter conditions in the zone between ca 10 20 n four of the models show conditions wetter than present in southern europe and drier than present in northern europe three of the models show conditions similar to present in the mid latitude continental interior while the remaining models show conditions somewhat drier than present the extent and location of each of the simulated changes varies between the models as does the mechanism producing these changes the lake data confirm some features of the simulations but indicate discrepancies between observed and simulated climates for example the data show 1 conditions wetter than present in central asia from india to northern china and mongolia indicating that the simulated asian monsoon expansion is too small 2 conditions wetter than present between ca 10 30 n in africa indicating that the simulated african monsoon expansion is too small 3 that northern europe was drier but the area of significantly drier conditions was more localized around the baltic than shown in the simulations 4 that southern europe was wetter than present apparently consistent with the simulations but pollen data suggest that this reflects an increase in summer rainfall whereas the models show winter precipitation and 5 that the mid latitude continental interior was generally wetter than present

holocene climate change in the western part of taihu lake region east china abstract the taihu lake basin is one of the most sensitive areas to climate change in the east asian summer monsoon easm influenced region there are significant differences regarding the interpretation of climate history for this area during the holocene due to a lack of high resolution paleoclimate records to better understand this scientific issue here we provide a new record of holocene climate change from the western part of taihu lake region east china a 4 m depth profile from gaochun has provided a 13 200 year record of climate change holocene climate change was reconstructed for the western part of taihu lake area based on various proxy records including pollen charcoal degree of humification doh and loss on ignition loi from the gaochun site the results indicate that climate during the late pleistocene 13 200 10 700 cal yr b p and early holocene 10 700 8000 cal yr b p was warm and humid alternated with several cold and dry intervals the late pleistocene and early holocene was a transitional period from cold dry to warm wet climates the middle holocene 8000 4400 cal yr b p was still warm and humid with a relatively stable climate the period between 8000 and 5300 cal yr b p was the holocene climate optimum hco during the late holocene 4400 0 cal yr b p the climate was cool but still humid in addition climate change at the western part of taihu lake during the late pleistocene and holocene was unstable containing several short term climate events such as younger dryas yd 8 2 ka and 4 2 ka events these abruptly occurred climate events were indicated by various paleoclimatic proxy indicators and can be compared with related records from other regions our findings suggest that the easm strength was strongest during the middle holocene between 8000 and 5300 cal yr b p which can be depicted as the period of hco

decoupled warming and monsoon precipitation in east asia over the last deglaciation our understanding of the continental climate development in east asia is mainly based on loess paleosol sequences and summer monsoon precipitation reconstructions based on oxygen isotopes delta o 18 of stalagmites from several chinese caves based on these records it is thought that east asian summer monsoon easm precipitation generally follows northern hemisphere nh summer insolation however not much is known about the magnitude and timing of deglacial warming on the east asian continent in this study we reconstruct continental air temperatures for central china covering the last 34 000 yr based on the distribution of fossil branched tetraether membrane lipids of soil bacteria in a loess paleosol sequence from the mangshan loess plateau the results indicate that air temperature varied in phase with nh summer insolation and that the onset of deglacial warming at similar to 19 kyr bp is parallel in timing with other continental records from e g antarctica southern africa and south america the air temperature increased from is c at the onset of the warming to a maximum of 27 degrees c in the early holocene similar to 12 kyr bp in agreement with the temperature increase inferred from e g pollen and phytolith data and permafrost limits in central china comparison of the tetraether membrane lipid derived temperature record with loess paleosol proxy records and stalagmite 5180 records shows that the strengthening of easm precipitation lagged that of deglacial warming by ca 3 kyr moreover intense soil formation in the loess deposits caused by substantial increases in summer monsoon precipitation only started around 12 kyr bp ca 7 kyr lag our results thus show that the intensification of easm precipitation unambiguously lagged deglacial warming and nh summer insolation and may contribute to a better understanding of the mechanisms controlling ice age terminations

a new regional mid holocene palaeoprecipitation signal of the asian summer monsoon the dongge cave speleothem 18o record which lies in the pearl river basin china has been interpreted as recording a regional decline in asian summer monsoon precipitation over the last 6 5 ka the same overall trend is seen in the bulk sedimentary organic 13corg record from a core in the pearl river estuary however the two records differ in detail and the regional nature of the dongge palaeoprecipitation signal has therefore been questioned our study re evaluates both records by constructing for the same estuarine core biomarker and compound specific 13c records which have better constrained terrestrial and marine end members than 13corg providing additional insights into the evolution of the asian summer monsoon the branched isoprenoidal tetraether bit index reflects the ratio of soil versus marine organic matter the bit record from the estuarine core co varies with the dongge cave 18o record suggesting the two share a common control which is likely to be driven by regional climate by contrast the sterols n alcohols and n fatty acid ratios show the same overall trend as dongge but parallel the 13corg record s variability between 6 5 and 2 ka indicating a partial decoupling between soil and land plant organic matter fluxes in the pearl river basin there is clear divergence between the biomarker and 13corg records from 2 ka to present analysis of the leaf wax 13c suggests that this results from an abrupt change in vegetation probably resulting from local anthropogenic cultivation two thousand years ago the basin scale of these estuarine records equates to up to 15 grid cells in typical earth system models used for simulating global climate this permits comparison of palaeoclimate model intercomparison project simulations of the mid holocene with spatially equivalent data relating to the summer asian monsoon for the first time

mass estimates of cumulates and residues after anatexis in the oslo graben the magmatic province of the permian oslo rift southeast norway is dominated by rocks of monzonitic to granitic composition most of the monzonitic rocks are believed to have formed by fractional crystallization from mantle derived magmas whereas melts produced by anatexis in the lower crust are important to the formation of the syenitic and granitic rocks these processes have left dense cumulates and dense residues after melting in the crust the surplus masses of cumulates and residues at depth in the crust can be estimated by using petrological and geological information on the rocks exposed today quantitative estimates for the oslo graben are comparable to corresponding data derived from geophysical crustal models the petrological geological estimates suggest excess masses of 7 10 1010 kg per linear meter assuming prismatic bodies along the rift axis in the southern and 5 7 1010 kg m in the northern part of the oslo region these estimates are dependent upon the volume proportion of trapped liquid in the cumulates and the ratio of cumulates to residues after anatectic melting in the crust the amount of trapped liquid appears to be low less than 25 the results agree well with estimates by wessel and husebye 1986 of excess mass in the crust based on gravimetric data which gave 5 7 1010 kg m this agreement testifies to the validity of making mass estimates on the basis of petrological and geological data the data bear on the composition and evolution of the upper lithosphere in the graben area some cumulates may reside in the subgraben mantle

crustal reflectivity in the skagerrak area abstract reflectors within the crystalline crust are often observed in deep seismic reflection profiling surveys the lower crust in extensional areas is generally credited with an abundance of reflectors the deep seismic reflection data 16 s twt from the m v mobil search cruise in skagerrak show a reflective lower crust and a relatively transparent upper crust in most of the area reflectivity seems to be less inside the oslo rift and also beneath the sediment covered areas reflectivity maxima are found near the moho and at depths of 10 20 km the latter is taken to coincide with the transition between the brittle upper and ductile lower crust the distribution of crustal reflectors in skagerrak and their possible relationships with seismic velocities earthquake depth distribution and major tectonic elements such as the fennoscandian border zone the oslo rift system and the shield environment are discussed hypotheses on the formation of the crustal reflectors are also briefly reviewed

continental rifting at pre existing lithospheric weaknesses continental rifting is the poorly understood thermomechanical process by which continents break up and new ocean basins form two dominant styles of continental rifting are recognized1 crevice style rifts are characterized by initial graben formation local uplift of the graben shoulders and little or no volcanism arch volcanic rifts exhibit an initial doming of the crust extensive volcanism and late stage graben formation the prevailing expla nation for the two styles is that crevice rifts form in response to regional horizontal stresses principally derived from the inter action of the lithospheric plates along their edges whereas arch volcanic rifts are thought to be the result of mantle convection currents which upwell directly beneath the rift axis2 we propose an alternative working model of continental rifting in which both styles initiate in response to regional horizontal stresses the different surface expressions of the two styles are explained in terms of differences in the mode of failure at different kinds of pre existing weaknesses in the continental lithosphere rather than differences in the nature of the driving forces involved

flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension we suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound this mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes and magmatic thickening of the crust our hypothesis is based on two critical concepts first the lithosphere retains finite mechanical strength or flexural rigidity during extension second isostatic rebound uplift of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation we develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks we first investigate the isostatic consequences of finite simple slip on an initially planar dipping normal fault cutting the entire lithosphere when the lithosphere retains flexural rigidity during extension the topography resulting from this model resembles a half graben and the footwall rift flank is flexurally uplifted this simple normal faulting model explains free air gravity anomalies and topography observed at rift flanks in oceanic lithosphere such as broken ridge in the eastern indian ocean the caroline ridges sorol trough in the western equatorial pacific and the coriolis trough behind the new hebrides island arc we then investigate a general kinematic model for lithospheric extension where simple slip on a surface of arbitrary shape is accompanied by pure shear extension in the upper and lower plates when the simple slip component is not zero or the distribution of pure shear in the upper and lower plates is not identical the surface of slip can be regarded as a detachment by simplification our general model accounts for pure shear extension of the lithosphere that is uniform with depth in this case detachments have no meaning in the geologic sense however the kinematics of depth independent pure shear may nevertheless be described in terms of a surface which we term a kinematic reference surface at some depth in the lithosphere we speculate that the depth of this surface may be rheologically controlled the magnitude of rift flank uplift by flexure depends critically on the depth of this reference surface in contrast if local isostasy is assumed when the lithosphere undergoes a given amount of depth independent pure shear the resulting topography will be the same regardless of how the kinematics of that extension are formulated the basin and rift flank topography and free air gravity anomaly over young continental rifts such as the rhine graben can be satisfied using our general extensional model with a small amount 5 km of extension along a listric shaped detachment soling into the crust mantle boundary because the flexural rebound mechanism explains the observed topography and gravity anomaly over both oceanic and continental extensional domains we suggest that rheological differences between the two lithospheric types may not be important in their overall response to extension

last glacial maximum and holocene climate in ccsm3 abstract the climate sensitivity of the community climate system model version 3 ccsm3 is studied for two past climate forcings the last glacial maximum lgm and the mid holocene the lgm approximately 21 000 yr ago is a glacial period with large changes in the greenhouse gases sea level and ice sheets the mid holocene approximately 6000 yr ago occurred during the current interglacial with primary changes in the seasonal solar irradiance the lgm ccsm3 simulation has a global cooling of 4 5 c compared to preindustrial pi conditions with amplification of this cooling at high latitudes and over the continental ice sheets present at lgm tropical sea surface temperature sst cools by 1 7 c and tropical land temperature cools by 2 6 c on average simulations with the ccsm3 slab ocean model suggest that about half of the global cooling is explained by the reduced lgm concentration of atmospheric co2 50 of present day concentrations there is an increase in the antarctic circumpolar current an

reconstruction of climate and vegetation changes of lake bayanchagan inner mongolia holocene variability of the east asian monsoon a high resolution pollen and pediastrum record spanning 12 500 yr is presented for lake bayanchagan 115 21 e 41 65 n and 1355 m a s l southern inner mongolia individual pollen taxa pt mat and the pft affinity scores pft mat were used for quantitative climatic reconstruction from pollen and algal data both techniques indicate that a cold and dry climate similar to that of today prevailed before 10 500 cal yr b p the wettest climate occurred between 10 500 and 6500 cal yr b p at which time annual precipitation was up to 30 60 higher than today the early holocene increases in temperature and precipitation occurred simultaneously but mid holocene cooling started at approximately 8000 cal yr b p 1500 yr earlier than the drying vegetation reconstruction was based on the objective assignment of pollen taxa to the plant functional type the results suggest that this region was dominated by steppe vegetation throughout the holocene except for the period 9200 to 6700 cal yr b p when forest patches were relatively common inner mongolia is situated at the limit of the present east asian monsoon and patterns of vegetation and climate changes in that region during the holocene probably reflect fluctuations in the monsoon s response to solar insolation variations the early to middle holocene monsoon undoubtedly extended to more northern latitudes than at present

mid holocene east asian summer monsoon strengthening insights from paleoclimate modeling intercomparison project pmip simulations abstract the east asian summer june july august monsoon easm is typically thought to have been stronger during interglacial periods based on spatially sparse proxy data on a large scale however whether this view is true and if so its underlying dynamic mechanisms remain unclear using all pertinent experiments within the paleoclimate modeling intercomparison project pmip here we present an analysis of the easm during the mid holocene 6000 years ago supporting the paleodata the mid holocene easm as measured by regionally averaged meridional wind at 850 hpa became stronger than the baseline period in 27 out of 28 pmip models with a demonstrable ability to simulate the modern easm climatology on average the easm strengthened by 32 across all the models and by a larger magnitude in 23 coupled models 35 than in five atmospheric models 20 it is proposed that an enhanced land sea thermal contrast and hence sea level pressure gradient between the east asian continent and adjacent oceans as a result of orbital forcing was responsible for the easm strengthening during the mid holocene

cataclastic rocks along the nesodden fault oslo region norway a reactivated precambrian shear zone abstract the nesodden fault segment nfs is the northern segment of the oslofjorden fault of which separates the precambrian rocks in the east from palaeozoic rocks in the west the nfs is characterized by two groups of fault rocks group 1 comprises a series of crush breccias to ultracataclasites containing material derived from the precambrian basement these fault rocks were formed at a depth greater than 5 km by attrition brecciation and distributed crush brecciation at some time between the sveconorwegian orogeny 950 ma and the deposition of the cambrian alum shale 540 505 ma group 2 comprises five crush breccias with a similar quartz matrix containing fragments of older rocks and permian intrusive rocks they were formed at a depth of less than 3 5 km by the interaction of cataclasis and hydrothermal percolation attrition and implosion brecciation during the formation of the permo carbonifereous oslo rift 300 230 ma evidence of repeated hydrothermal fluid flow and fluidization has been recognized in the youngest breccias the seven different fault rocks emphasize the importance of fault reactivation and show that old fault zones commonly act as planes of weakness along which repeated deformation takes place and act as pathways for hydrothermal fluids

the oslo rift a review the tectonomagmatic history of the oslo rift may be subdivided into 5 main periods stage 1 300 ma development of a shallow depression stage 2 300 295 ma roughly simultaneous onset of widespread basalt volcanism b1 and vertical movement along nnw sse to n s trending faults stage 3 295 275 ma main rifting period accompanied by volcanism dominated by rhomb porphyry lavas latites stage 4 275 240 ma change of magmatic style from basaltic shield volcanism to central volcanoes of mixed compositions many of which went through caldera collapses intrusion of composite batholiths of intermediate to granitic compositions stage 5 240 ma dike intrusions may have continued into triassic time both the crust and total lithosphere under the rift are thinned relative to the precambrian basement on both sides a 12 km thick high density and high velocity body occupies the lower crust along the rift axis and extends under the eastern and western rift shoulder petrological and geochemical data imply that this massive layer represents dense cumulates and gabbroic rocks formed from mantle derived magmas in deep crustal magma chambers two mantle sources one nearly undepleted age corrected nd of about 1 and sr of 10 to 15 the other mildly depleted nd of about 4 sr of 10 contributed melts to the oslo rift magmatism the two sources are interpreted as different parts of a heterogeneous upper mantle which before the rifting event belonged to the subcontinental lithosphere on their way to the surface the mantle derived magmas were retained in deep crustal magma chambers where they underwent extensive fractional crystallization some also suffered moderate contamination anatectic melts and melts formed by mixing between mantle derived and anatectic melts gave rise to syenitic and granitic intrusions

tracing the magmatic hydrothermal transition in regional low strain zones the role of magma dynamics in strain localization at pluton roof implications for intrusion related gold deposits structural controls are critical during magmatic to hydrothermal transition in the formation of intrusion related gold deposits they may explain why only some parts of intrusions are mineralized and why only very few intrusions host large deposits moreover most of these gold deposits postdate peak regional metamorphism and were formed in zones of relatively low strain indeed the efficiency of structural gold traps is highest along favourably oriented intrusion host rock contacts where mechanical instability maintains high permeability in the cracked thermal aureole this may be reinforced by melt injections from an underlying root zone we present a structural analysis of the deformation features of a granite pluton roof large scale dykes and a network of gold veins are intensively developed in this roof recording a succession of mechanical instabilities our gravity survey underlines the presence of a pluton feeder zone located just beneath the mineralized network it is argued that interferences between regional stress and melt injection in the feeder zone favoured the development of the network by strain located close to the granite roof this stresses the role of mechanical instabilities triggered by the combined effects of regional stress and melt dynamics in determining the location and size of this type of gold deposit

skagerrak graben the offshore continuation of the oslo graben abstract new seismic mapping of skagerrak reveals eight structural provinces in particular the skagerrak graben appears to be the continuation of the oslo graben the skagerrak graben is defined by two major late paleozoic faults trending northeast southwest connecting with the faults bounding the oslo graben extensive rotated block faulting is observed within the graben and a thickness of more than 2 s twt of lower paleozoic sediments has been preserved along the boundary faults sediments of probable late paleozoic age have been deposited as erosional products from the fault scarps and a central uplifted area a substantial part of these sediments may have a volcaniclastic origin the skagerrak graben continues to the north as a province of igneous rocks exposed at the sea floor with precambrian basement on either side the skagerrak graben is flanked by small half grabens containing probable early paleozoic sediments indicating that a wider area was involved in the rift process the formation of the skagerrak graben is related to the dextral strike slip movements along the sorgenfrei tornquist zone in late paleozoic time the kimmerian extensional movements along this zone indicate however a decoupling of the skagerrak graben at this time only minor inversion in the southwestern part of the skagerrak graben reflects the major inversion tectonism along the sorgenfrei tornquist zone in late cretaceous early tertiary times

tropical climate instability the last glacial cycle from a qinghai tibetan ice core an ice core record from the guliya ice cap on the qinghai tibetan plateau provides evidence of regional climatic conditions over the last glacial cycle 36 cl data suggest that the deepest 20 meters of the core may be more than 500 000 years old the 18 o change across termination i is 5 4 per mil similar to that in the huascaran peru and polar ice cores three guliya interstadials stages 3 5a and 5c are marked by increases in 18 o values similar to that of the holocene and eemian 124 000 years ago the similarity of this pattern to that of ch 4 records from polar ice cores indicates that global ch 4 levels and the tropical hydrological cycle are linked the late glacial stage record contains numerous 200 year oscillations in 18 o values and in dust nh 4 and no 3 levels

evidence for a thick oceanic crust adjacent to the norwegian margin the oceanic crust created during this first few million years of accretion in the norwegian greenland sea lies at an unusually shallow depth for its age has a smooth upper surface and in many places the results of multichannel seismic reflection profiling reveal that its upper layers comprise a remarkable sequence of arcuate seaward dipping reflectors these have been attributed to lava flows generated during a brief period of subaerial seafloor spreading we describe the results of inversions of digitally recorded sonobuoy measurements and two ship expanded spread profiles collected over the oceanic crust adjacent to the norwegian passive margin we find that the crust of the deep lofoten basin is indistinguishable from normal oceanic crust in thickness and structure closer to the margin we observe up to a four times expansion in thickness of layers with velocities equal to those of oceanic layer 2 while the layer 3 region retains approximately the same thickness the area over which the seaward dipping reflectors can be observed on reflection profiles corresponds to the region of greatest expansion in layer 2 thickness in the very oldest crust immediately adjacent to an escarpment that probably marks the continent ocean boundary we see evidence for a low velocity zone overlying an indistinct reflector that may mark the dyke lava interface in the thick crust comparing the structure of the thick crust to that of eastern iceland we find a strong resemblance especially in the expansion in thickness of material with layer 2 velocities these results support the suggestion that during the earliest stages of spreading extrusive volcanism at the ridge crest was unusually voluminous building a thick pile of lavas erupted from a subaerial spreading center

the dry holocene megathermal in inner mongolia the paleoclimate since 14 kyr bp 14c age was reconstructed based on a 16 22 m long sediment core collected from lake yanhaizi a saline lake located near the northern limit of the east asian summer monsoon in inner mongolia coarse sediments were deposited there during a shrinkage phase of the lake when sand dunes reactivated these sediments have low organic carbon contents but high maturity indices indicating that they were deposited in an arid environment by contrast based on high organic contents and low maturity indices fine sediments were deposited during periods of high lake stand in a humid environment it was in general dry between 8 0 and 4 3 kyr bp the above dry and wet phases are consistent with those recovered from the arid semiarid transition zone elsewhere but are unlike the widely perceived humid holocene megathermal reported in east china and the newly reconstructed record in the alpine retreat lake in taiwan the discrepancy may be due to a relative insensitivity to humidity changes in these two areas since they have both been under the total influence of the summer monsoon on the other hand much enhanced evaporation over higher monsoon precipitation at lake yanhaizi reduces the effective humidity in the warm climate near the northern boundary of the summer monsoon this also accounts for the fact the high temperature holocene megathermal as revealed in the okinawa trough and the northern south china sea is correlated to the dry phases at lake yanhaizi conversely the 4 2 kyr bp coldest period in the holocene corresponds to a wet phase at lake yanhaizi

response of the african monsoon to orbital forcing and ocean feedbacks in the middle holocene simulations with a climate model that asynchronously couples the atmosphere and the ocean showed that the increased amplitude of the seasonal cycle of insolation in the northern hemisphere 6000 years ago could have increased tropical atlantic sea surface temperatures in late summer the simulated increase in sea surface temperature and associated changes in atmospheric circulation enhanced the summer monsoon precipitation of northern africa by more than 25 percent compared with the middle holocene simulation with prescribed modern sea surface temperatures and provided better agreement with paleorecords of enhanced monsoons

the verification of millennial scale monsoon water vapor transport channel in northwest china summary long term changes of the asian summer monsoon water vapor transport play a pivotal role in the variability of monsoon precipitation paleo climate simulations have shown that there is an important monsoon vapor transport channel in western china previous studies mostly focused on the correlation between monsoon precipitation and intensity little research has been done on the verification of the water vapor channel compared with speleothem and lacustrine systems the hydrological cycle of land surface sediments is more directly related to the monsoon water vapor in this study we used carbonate 18 o and organic matter 13 c of the surface eolian sediments from the piedmont of the northern qilian mountains to verify the monsoon water vapor on the holocene millennial scale two surface sedimentary sections were selected to study paleo monsoon water vapor transport proxy data including carbonate 18 o and organic matter 13 c of surface eolian sediments as well as total organic matter and carbonate content were obtained from the two eolian sections we also synthesized transient simulations of the ccsm3 and the kiel climate models the pmip 3 0 project and trace isotopic simulations were also compared with the reconstructed monsoon water vapor transport our findings indicate that the strength of the holocene asian summer monsoon is consistent with the water vapor transport in western china that has significant impacts to long term monsoon precipitation in northern china this study verifies a significant millennial scale correlation between the monsoon strength and monsoon water vapor transport intensity along the eastern qinghai tibet plateau

holocene vegetation and climate dynamics of ne china based on the pollen record from sihailongwan maar lake abstract high resolution palynological analysis on annually laminated sediments of sihailongwan maar lake shl provides new insights into the holocene vegetation and climate dynamics of ne china the robust chronology of the presented record is based on varve counting and ams radiocarbon dates from terrestrial plant macro remains in addition to the qualitative interpretation of the pollen data we provide quantitative reconstructions of vegetation and climate based on the method of biomization and weighted averaging partial least squares regression wa pls technique respectively power spectra were computed to investigate the frequency domain distribution of proxy signals and potential natural periodicities pollen assemblages pollen derived biome scores and climate variables as well as the cyclicity pattern indicate that ne china experienced significant changes in temperature and moisture conditions during the holocene within the earliest phase of the holocene a large scale reorganization of vegetation occurred reflecting the reconstructed shift towards higher temperatures and precipitation values and the initial holocene strengthening and northward expansion of the east asian summer monsoon easm afterwards summer temperatures remain at a high level whereas the reconstructed precipitation shows an increasing trend until approximately 4000 cal yr bp since 3500 cal yr bp temperature and precipitation values decline indicating moderate cooling and weakening of the easm a distinct periodicity of 550 600 years and evidence of a mid holocene transition from a temperature triggered to a predominantly moisture triggered climate regime are derived from the power spectra analysis the results obtained from shl are largely consistent with other palaeoenvironmental records from ne china substantiating the regional nature of the reconstructed vegetation and climate patterns however the reconstructed climate changes contrast with the moisture evolution recorded in s china and the mid latitude semi arid regions of n china whereas a clear insolation related trend of monsoon intensity over the holocene is lacking from the shl record variations in the coupled atmosphere pacific ocean system can largely explain the reconstructed changes in ne china

early mesozoic rift basins of eastern north america and their gravity anomalies the role of detachments during extension two end member models of extension involving detachments have been developed one model incorporates a fault that soles at mid crustal level overlying a broad region of pure shear in the lower crust the second referred to as the simple shear model includes a detachment continuing through the entire crust and terminating in a region of concentrated extension in the lower crust both models predict basins with no localized thermal effect with the inclusion of flexural isostasy both models predict footwall uplift whose amplitude and wavelength are controlled by the detachment geometry and the lithospheric strength a gravity anomaly over the hanging wall block distinguishes the simple shear model from the intracrustal detachment model the early mesozoic basins of the eastern north america believed to have formed as the result of the normal slip reactivation of a paleozoic thrust system as the atlantic opened are associated with distinctive hanging wall gravity highs these gravity highs the basin geometry the lack of a thermal subsidence phase in the rift basins and the presence of a highly extended and heated region to the east suggest that the simple shear model may be applied the simple shear model fits the outer hanging wall anomaly and permits a region of lower crustal extension to be mapped these basins contain an abundance of basalt flows and diabase sills despite the lack of evidence for regional heating or thermal subsidence implying that a source external to the basin must exist for this magmatic material the detachment fault may facilitate the movement of the molten mafic material into the rift basins from an offshore region of greater heating and extension the addition of 2 km of mafic material along the model detachment accounts for the observed inner gravity high and the lack of a negative gravity anomaly across these basins the match of this modified simple shear model to the observed gravity suggests that the region of greater extension seaward of the hinge zone is the source for the widespread dikes and sills within the basins and the coastal plain and helps explain the geochemical homogeneity of these intrusives and extrusives along 2000 km of the eastern seaboard

origin of seaward dipping reflectors in oceanic crust off the norwegian margin by subaerial sea floor spreading a remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the norwegian continental margin it comprises a seaward dipping complex of reflectors in the form of a wedge we suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean basin genesis

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

constraints on extension of continental lithosphere the extension of the continental lithosphere is considered in terms of the deformation of a thin viscous sheet with a temperature dependent rheology which is governed primarily by the strength of the upper mantle the lithosphere is subjected either to a constant strain rate or to a strain rate decreasing with time corresponding to the stretching of a region of fixed volume with a constant velocity boundary condition the progress of the extension is influenced by the decrease in strength of the lithosphere owing to its attenuation and by an increase in strength resulting from cooling the relative importance of these two effects depends on two parameters a dimensionless strain rate and the total strain for a given strain rate the extending lithosphere goes through two phases as the strain increases in the first the effect of thinning the lithosphere is predominant and the average strength of the lithosphere remains close to and perhaps slightly below its original strength in the second the effect of cooling of the lithosphere leads to very rapid increase in strength as a function of strain resulting presumably in an effective limit on the degree of stretching that the lithosphere experiences for the case of strain rate decreasing with time when the initial strain rates are of order 10 15s 1 this maximum extension is between 100 and 150 for a strain rate of 10 l4 s 1 the maximum lies around 400 to 500 at higher strain rates than this very large extension may occur before cooling significantly affects the strength of the lithosphere this model accords well with the constraints available from the continental margins of the north atlantic

intercomparison of simulated global vegetation distributions in response to 6 kyr bp orbital forcing abstract the response of ten atmospheric general circulation models to orbital forcing at 6 kyr bp has been investigated using the biome model which predicts equilibrium vegetation distribution as a diagnostic several common features emerge a reduced tropical rain forest as a consequence of increased aridity in the equatorial zone b expansion of moisture demanding vegetation in the old world subtropics as a consequence of the expansion of the afro asian monsoon c an increase in warm grass shrub in the northern hemisphere continental interiors in response to warming and enhanced aridity and d a northward shift in the tundra forest boundary in response to a warmer growing season at high northern latitudes these broadscale features are consistent from model to model but there are differences in their expression at a regional scale vegetation changes associated with monsoon enhancement and high latitude summer warming are consistent with palaeoenvironmental observations but the simulated sh

timing and spatial distribution of mid holocene drying over northern china response to a southeastward retreat of the east asian monsoon 1 to determine the timing and spatial distribution of mid holocene drying over northern china the mineralogical and oxygen isotopic composition of authigenic carbonate from a closed lake at bayanchagan southern inner mongolia were measured further analysis and synthesis of the spatial geological data were performed results from lake bayanchagan show a significant drying at 6000 calendar years cal years b p indicated by dolomite precipitation and a striking rise in 18o values the synthesis of spatial data reveals a zonal distribution for timing of drying over northern china in the mid holocene which began at 9000 7000 cal years b p in deserts of north central china at 7000 5500 cal years b p drying extended into the desert steppe transitional zone and at 4500 cal years b p into northeastern and south central china this pattern indicates that the east asian summer monsoon has significantly retreated southeastward since the mid holocene which may be related to orbitally induced northern hemisphere insolation changes a retreat of 400 550 km is inferred for the front of the summer monsoon from 6500 to 4500 cal years b p

a stretching model for the oslo rift abstract data on the tectonic and magmatic activity crustal thickness seismic velocity and gravity have been integrated to constrain a stretching model for the late carboniferous early permian oslo rift the crustal structure and numerical modelling of temperature and melt generation during rifting suggest that the oslo rift in skagerrak developed in response to crustal stretching with a factor between 1 4 and 1 6 however upper crustal extension by faulting accounts for a stretching factor of only 1 1 1 2 thermal expansion of the lithosphere due to an increased asthenospheric potential temperature the intrusion of melted mantle material into the lower crust and probably a lowering of the erosional base by a fall in sea level all contributed to the erosion of 2 4 km of lower palaeozoic pre rift sediments over large areas the lateral extent of the thermal anomaly reached far beyond the oslo and skagerrak grabens

a holocene climatic record from arid northwestern china abstract the history of climatic changes during the interval 8500 3000 cal yr b p has been reconstructed from stratigraphical and chronological studies and the results of total organic carbon toc total inorganic carbon tic element composition pollen and stable isotope analyses of a section along the hongshui river in the southern tengger desert nw china the record suggests that from 8450 bottom of the studied section to 7500 yr b p the climate was characterized by instability from 7500 to 5070 yr b p the climatic conditions improved and can be divided into two parts a warm humid spell between 7290 and 6380 yr b p during which the average temperature was 3 4 c higher than that of today and a warm dry spell lasting from 5950 to 5720 yr b p the climate deteriorated between 6380 and 5950 yr b p from 5720 to 5070 yr b p the temperature decreased but humidity increased an abrupt temperature drop occurred between 5340 and 5290 yr b p that indicated the decline of the warmer and humid mid holocene climate from 5070 yr b p onward the climate oscillated significantly and there were three large temperature decreases coinciding with high mountain glacier advances between 5070 4670 yr b p 4300 3740 yr b p and 3410 3230 yr b p top of the section respectively the climatic fluctuations recorded in the southern tengger desert appear in phase with climatic changes recognized in the tibetan plateau suggesting that the period between 7290 and 6380 yr b p was the most warm humid spell one extremely dry event occurred at ca 3000 yr b p and subsequently the fluvial lacustrine depositional process terminated and wind action prevailed in the area both of these features can be attributed to the rapid strengthening and weakening of the summer monsoon circulation which are closely connected with global changes

the role of tectonic grain size reduction in the rheological stratification of the lithosphere with the aid of published constitutive flow laws for common crustal and upper mantle rock types it is possible to construct curves showing variation of rock strength with depth in the continental lithosphere assuming a particular crustal stratification of rock types and a particular geothermal gradient such exercises lead to the conclusion that marked changes in strength can occur at major petrological interfaces such as the moho discontinuity which are therefore likely to be associated with a tendency for major sub horizontal detachments to form constitutive flow laws used for such purposes are usually assumed to represent steady state flow at constant rock microstructure

an oslo graben experiment shooting at sea and recording on land abstract during the eugeno s field work in 1984 a large number of airgun shots was fired along a line between larvik norway and lysekil sweden the signals were recorded on land at larvik using a stationary v shaped land array with 48 recording channels the airguns were fired every 300 m along the profile with an effective recording distance range of 30 150 km but data quality deteriorated rapidly beyond 80 km a hybrid variant of slant stacking was used in record analysis with the robust semblance technique for estimating velocity spectra when the various p traveltime branches were properly located in the p plane the inversion of the corresponding kinematic parameters to a 1 d velocity depth model was minor the main results are a presumed 5 km thick upper weathered layer with a velocity of 5 6 km s 1 a 5 13 km thick intermediate layer with a velocity of 6 3 km s 1 and a 13 31 km thick lower crustal layer with a velocity of 6 8 km s 1 the weathered layer is exceptionally thick and the intermediate layer seems to typify the transition between upper and lower crust in a typical oslo graben tectonic setting

thermal and isostatic consequences of simple shear extension of the continental lithosphere abstract quantitative results are presented for a model involving large scale simple shear of continental lithosphere the locally compensated finite element model includes the effects of sedimentation radiogenic heat production in the crust and sediments and finite rates of extension flexural effects during firting are examined using a simple shear model with a uniform initial elastic thickness the locally compensated model predicts that a breakup unconformity develops on the upper plate when extension occurs at typical plate tectonic velocities subsidence occurs as the upper and lower lithospheric plates separate extension creates the potential for large accumulations of sediment 20 km most of which is syn rift if sedimentation keeps pace with subsidence the effect of flexure is to modify the shape of the detachment surface and the uplift patterns on the flanks of the upper and lower plate sedimentary basins a model which incorporates flexure predicts significant uplift of the lower plate as it is unloaded by the upper plate during extension the wernicke simple shear model can be distinguished from its opposite end member the mckenzie pure shear model by the predicted asymmetry in thermal evolution higher predicted heat flux within the upper plate uplift subsidence histories and stratigraphic development of the upper and lower plates

evaluation of coupled ocean atmosphere simulations of the mid holocene using palaeovegetation data from the northern hemisphere extratropics we have used the biome4 biogeography biochemistry model and comparison with palaeovegetation data to evaluate the response of six ocean atmosphere general circulation models to mid holocene changes in orbital forcing in the mid to high latitudes of the northern hemisphere all the models produce a a northward shift of the northern limit of boreal forest in response to simulated summer warming in high latitudes the northward shift is markedly asymmetric with larger shifts in eurasia than in north america b an expansion of xerophytic vegetation in mid continental north america and eurasia in response to increased temperatures during the growing season c a northward expansion of temperate forests in eastern north america in response to simulated winter warming the northward shift of the northern limit of boreal forest and the northward expansion of temperate forests in north america are supported by palaeovegetation data the expansion of xerophytic vegetation in mid continental north america is consistent with palaeodata although the extent may be over estimated the simulated expansion of xerophytic vegetation in eurasia is not supported by the data analysis of an asynchronous coupling of one model to an equilibrium vegetation model suggests vegetation feedback exacerbates this mid continental drying and produces conditions more unlike the observations not all features of the simulations are robust some models produce winter warming over europe while others produce winter cooling as a result some models show a northward shift of temperate forests consistent with though less marked than the expansion shown by data and others produce a reduction in temperate forests elucidation of the cause of such differences is a focus of the current phase of the palaeoclimate modelling intercomparison project

spreading rate dependence of three dimensional structure in oceanic spreading centres segmentation by transform faults and other types of along axis discontinuity1 is a well described but poorly explained characteristic of oceanic spreading centres here we use numerical experiments to explore the dynamics of mantle flow and melting beneath a mid ocean ridge buoyant upwelling driven by compositional density variations resulting from the extraction of the melt that forms the ocean crust exhibits a spreading rate dependent transition between two dimensional and three dimensional upwelling structures for low spreading rates and mantle viscosities an initial two dimensional structure transforms into a three dimensional one at high spreading rates an initially two dimensional structure remains two dimensional these results suggest that the origin of spreading centre segmentation may be different at fast and slow spreading rates

generating melt during lithospheric extension pure shear vs simple shear the uniform stretching model has been used to calculate the volume and composition of melt generated by adiabatic decompression during extension of the continental lithosphere the consequences of this approach for melt generation by litbospheric simple shear were investigated the results show that given an initially planar detachment fault it is extremely difficult to generate melt from the asthenosphere under either normal or elevated asthenospheric potential temperatures our conclusion is independent of the initial dip of the detachment fault and also holds when the initial dip in the lithospheric mantle is double that in the crust in the north sea lithospheric simple shear fails to account for the existence and location of magmatism these conclusions may also apply more generally to passive margin development

skagerrak evolution derived from tectonic subsidence abstract stratigraphic data gathered from 32 points in the continental skagerrak basin have been analyzed by basin modelling methods non tectonic effects of sediment loading were removed by backstripping giving the observed tectonic subsidence this was compared to synthetic subsidence curves constructed on the basis of a combined lithospheric extension and metamorphism model where gabbroic magma which was emplaced during the permian is transformed to granulits eclogite facies the overall picture is that prior to 230 ma subsidence was roughly similar north and south of the fennoscandian border zone fbz after this time the northern area sagged due to the still preserved lithospheric thermal anomaly and continued metamorphism whereas the southern area was also affected by extension a physical explanation for this contrast in evolution is given by considering the relative lithospheric strength history of the areas the northern area becoming progressively stronger with time relative to the southern area as crustal thicknesses are reduced total post 570 ma extension is approximately 1 4 40 north of the fbz and 1 5 50 south of it north of the fbz the average thickness of the metamorphic layer is between 3 and 7 km whereas south of the zone a thickness of 1 2 km is estimated this difference may reflect variations in the degree of permian magmatism the temperature history of the sediments and the degree of organic maturation has been calculated by the tti method the youngest sediments to have experienced temperatures high enough for significant oil generation are estimated to be of bathonian age the present model heat flow of about 57 mw m 2 is in agreement with observations lateral displacements along the fbz are of the order of 10 20 km and have consequently only played a minor role in the development of the skagerrak basin a thermal lithosphere thickness of about 125 km is inferred

tomographical mapping of the lithosphere and asthenosphere beneath southern scandinavia and adjacent areas abstract in this paper we report on a seismic tomographical survey of the lithosphere asthenosphere beneath southern scandinavia and adjacent areas the ach inversion technique has been used and the observational data in the form of p wave travel time residuals have been obtained from seismogram records from networks in denmark southern norway and southern sweden the volume under investigation subdivided into four levels layers of 7 7 knots blocks each for estimation of velocity perturbations covers the area 54 62 n and 7 20 e and depth range 0 600 km major findings are as follows level 1 0 100 km pronounced velocity lows beneath southern jutland zealand the variscan foredeep and the sveco norwegian province of southeastern norway 1500 ma old the svecofennides of southeastern sweden exhibit relatively high velocities while the sveco norwegian province of southwestern sweden 1700 ma old is characterized by moderately high velocities the transition between velocity highs and lows coincides with the tornquist teisseyre line ttl or fennoscundian border zone level 2 100 300 km velocity anomalies are less pronounced at this level although velocity lows are found beneath the northern jutland zealand area and slightly northwestwards of the oslo rift the only velocity high is found beneath the southeasternmost part of sweden the ttl does not appear to have a clear counterpart in the velocity anomaly pattern in this depth range although a clearly unambiguous answer here would require far more extensive observational data a research goal of the future level 3 300 500 km and level 4 500 600 km velocity anomalies are weak which is not unexpected for the upper mantle beneath shield areas