climatic control on the late permian main dolomite ca2 deposition in northern margin of the southern permian basin and implications to its internal cyclicity

clay minerals in late glacial and holocene sediments of the northern and southern aegean sea different source areas oceanography and climate regimes influenced the clay mineral assemblages and grain size distribution of two sediment cores from the north and south aegean sea during the last glacial and the holocene in the north aegean sea clay mineral composition is mainly controlled by sea level evolution melting of southeastern european glaciers and establishment of the connection between the black sea and aegean sea the long term development of clay mineral assemblages in the south aegean sea reflects changes in the nile discharge and african dust input at this site the establishment of pluvial conditions in the nile catchment during the early to middle holocene resulted in a substantial rise in smectite illite ratios in the late holocene stepwise aridification of the southern borderlands caused an increase in windblown sediment material and a decrease in nile suspended material the clay mineral records exhibit periodic millennial scale fluctuations in the north aegean sea the changes are centred at a period of 1 3 1 8 ka and can be attributed to short term climate and weathering changes in the northern borderlands the changes in the south aegean sea are centred at periods of 3 2 4 3 1 9 2 4 and 1 3 1 7 ka reflecting short term changes in wind strength and northeast african hydrology 2007 elsevier b v all rights reserved

erosional history of the eastern tibetan plateau since 190 kyr ago clay mineralogical and geochemical investigations from the southwestern south china sea a high resolution study of clay mineralogy and major element geochemistry has been conducted on high sedimentation rate cores md01 2393 and md97 2150 collected off the mekong river mouth in the southwestern south china sea in order to reconstruct the erosional and weathering history of the mekong basin the chronology is based upon planktonic foraminiferal oxygen isotope records combined with carbonate stratigraphy clay minerals and major element results suggest that the mekong river is the major sedimentary source over the past 190 kyr for both cores illite and chlorite were derived mainly from the eastern tibetan plateau kaolinite was derived mainly from active erosion of inherited clays from reworked sediments in the middle part of the mekong basin smectites originated mainly through chemical weathering of parent aluminosilicate and ferromagnesian silicate under warm and humid conditions in the middle to lower parts of the mekong basin smectites illite chlorite and smectites kaolinite ratios coupled to k2o sio2 and al2o3 sio2 ratios allow us to reconstruct a history of chemical weathering versus physical erosion good correlations between those ratios suggest monsoon controlled weathering and erosion over the eastern tibetan plateau and the mekong basin variations with time scales of 0 3 0 6 kyr for the last 70 kyr and 1 2 kyr for 70 190 kyr ago in the chemical weathering erosional history are strongly related to the east asian monsoon evolution which is itself related to the summer solar radiation calculated for the northern hemisphere d 2004 elsevier b v all rights reserved

vegetation evolution based hydrological climate history since lgm in southern south china sea abstract the reconstruction of vegetation evolution and climate changes at low latitudes since the last glacial maximum lgm remains incomplete in this study we reconstructed the palynological history of vegetation changes since the lgm using a sediment core collected from the slope in the southern south china sea scs the chronology of the core was based on eight ams14c dates and 85 stratigraphic samples were palynologically analyzed our results denote that the vegetation reconstructed during the lgm in the southern scs was characterized by a relatively high percentage of herb pollen and a modest decrease in the number of fern spores suggesting a temperature decline and humidity change in this region further the preservation of a rich diversity of pollen from the tropical lowland rainforest indicated that the rainforests might have covered large areas of the exposed sunda shelf during the lgm the dynamic concentration of the pinus pollen in the core which is a proxy for the east asian winter monsoon eawm satisfactorily matched the changes in the atlantic meridional overturning circulation and australian indonesian summer monsoon especially during the heinrich stadial 1 this suggests that the climate at the high latitudes of the northern hemisphere can drive the intertropical convergence zone itcz southwards based on the enhanced eawm the southward shifting itcz may change the hydrological climate of the equator and the southern hemisphere the pteridophyte spores which are good regional precipitation indicators were positively correlated with the warm pool convection and inversely related with the eawm possibly indicating the considerable impact of the high latitude climate in the northern hemisphere on precipitation in the tropical regions our results indicate that during the last deglaciation the tropical hydrological changes were relatively moderate when compared with the drastic change observed at high latitudes and were attributed to the regulation of the el nino southern oscillation like state our sequence of pollen and spores from the southern scs can be used to observe the detailed dynamics of climatic evolution in tropical regions the results of this study are essential to understand the complicated climate processes and the relation between high and low latitudes

geochemistry of sediments of the peninsular rivers of the ganga basin and its implication to weathering sedimentary processes and provenance abstract geochemistry of the himalayan rivers has been a major point of contention in sedimentary geochemistry because of its global impact on ocean chemistry and climate among the himalayan rivers the ganga and its himalayan catchment have been studied extensively however the peninsular rivers constituting a significant part of the ganga basin have not received much attention heretofore in the present geochemical study of the peninsular river s sediments we have found that the initial chemical weathering of rocks in the catchment produces texturally mineralogically and geochemically diverse sediments and fluvial processes further segregate them into the geochemically contrasting coarse channel and fine grained suspended sediments the overbank sediments possess an intermediate texture and geochemistry but more towards the suspended sediments the sediment geochemistry data indicates that the peninsular river s sediments show an increase in the weathering intensity from the west banas river to the east son river along the climate gradient i e from the arid semiarid to the sub tropical condition the higher chemical index of alteration cia values of the peninsular river s sediments than the yamuna and ganga river sediments reflect the control of weatherable lithology in the tectonically stable low relief and subtropical climate condition the yamuna and ganga river sediments with lower cia values show more physical weathering in the tectonically active himalaya the suspended and overbank sediments of the yamuna and ganga rivers show an increase in their cia values after the confluence of the peninsular rivers indicating the contribution of more weathered materials by the peninsular rivers it has been found that there is a geochemical split between the channel and suspended sediments of the peninsular rivers for sharing different sources i e the dominant contribution of felsic sources to the channel and mafic sources to the suspended sediments the weathering geochemistry trace element systematics and ree patterns suggest that the dominant sources of the channel sediments of the peninsular rivers are the felsic crystallines and sedimentary lithologies of the aravalli range bundelkhand and chhotanagpur granite and gneisses and vindhyan sandstones whereas the mafic lithologies such as the deccan traps and mafic components of the felsic lithologies appear to be the major sources of the suspended sediments within the range of intermediate composition the overbank sediment chemistry reflects more contribution of the mafic sources except for the ken which shows a higher contribution of the felsic sources the geochemical split between the channel and suspended sediments of the peninsular river s sediments for the different provenances indicates differential weathering of the deccan basalts bundelkhand crystalline and vindhyan sedimentary rocks and also the hydrodynamic control of the peninsular rivers during erosion transport and deposition however the himalayan rivers do not show such contrast between the channel and suspended sediments because of the higher physical weathering and sediment mixing in the high gradient topography of the himalayan catchment

2 d tomographic imaging across the north american mid continent rift system summary 2 d traveltime tomography was used to model the seismic velocity structure of the mid continent rift system underneath lake superior the data set came from line a of the 1986 great lakes international multidisciplinary program on crustal evolution glimpce experiment in line data analysed in this study were recorded on four land stations and on five ocean bottom seismometers shot spacing on the 240 km long line was approximately 333m both direct and refraction arrival times were used to image the upper crust before expanding the process to cover greater distances and depths crustal thickness was determined using reflection pmp data from the moho several known near surface structures were resolved an extensive high velocity zone 7 0 7 2kms 1 under the central basin of the lake was found to be composed of a plume like structure rising to a depth of approximately 10km this plume originates from a broader based approximately 50km mantle bulge and probably indicates an area of crustal weakness along the rift a smaller less complex intrusion appears under the northern basin of the lake moho depths increase from about 36 km under the southern flank to about 55 km under the central basin decreasing again to about 45 km in the north there is some evidence in the pmp seismic record that the moho under the central basin is a disrupted transition zone the velocity model presented here corroborates this a stability test applied to the tomography velocity model along with comparisons to other line a models indicate that the result is a reliable and a reasonable representation of the mid continent rift system under lake superior these findings support the theory that this complex structure is part of a failed keweenawan 1100 ma tectonic rifting event

provenance shallow to deep diagenesis and chemical mass balance in supermature arenites and pelites ordovician simpson group oklahoma and kansas u s a abstract this study considering original detrital compositions and diagenetic alterations relates sandstone and shale mineral and chemical composition to provenance and mass balance between the sedimentary assemblage and its provenance mature quartz rich sandstones and siltstones with clay rich shales in cores at depths of 60 m 5 2 km and burial temperatures of 150 c from over 50 wells were utilized in this work these provide the material for analysis of compositions and alterations in coastal and shallow marine clastic sedimentary rocks of the middle ordovician simpson group in oklahoma and kansas u s a mineral and geochemical data were obtained by optical petrography x ray diffraction scanning electron microscopy electron microprobe and whole rock x ray fluorescence and icp ms analyses results of the study show significant progressive diagenetic changes with increased burial depth and temperature mineralogically and chemically less altered rocks are comparable to modern quartz rich sands and kaolinitic muds in siliciclastic sediments developed during humid tropical weathering in crystalline basement shield provenance in africa and south america by analogy to modern tropical cases muds composed of major kaolinite with minor smectite and illite at deposition were diagenetically transformed to illite rich rocks by possible microbial action at surface to shallow 20 in sandstone buried 3 km during deep burial high temperature 110 c calcite cement from deep basin sources was precipitated in sandstones geochemical mass balance estimates physical and chemical evidence suggest quartz cement was sourced from within clastic rocks of the simpson group with minor external contribution of sio2 na is in low abundance due to loss in solution during weathering in the provenance rare earth elements ree have typical abundances and distributions in shales with enrichment in light ree littoral environment siltstones and matrix rich sandstones display enrichment of heavy ree in phosphatic brachiopod shell fragments

magnetic signature of river sediments drained into the southern and eastern part of the south china sea malay peninsula sumatra borneo luzon and taiwan abstract magnetic properties of 22 river samples collected in the malay peninsula sumatra borneo luzon and taiwan have been investigated in order to magnetically characterize the sediments drained and deposited into the south china sea the geological formations as well as the present climatic conditions are different from one region to another laboratory analyses include low field magnetic susceptibility anhysteretic arm and isothermal irm remanent magnetizations acquisition and decay back field acquisition thermal demagnetization of three axes irm hysteresis cycles and low temperature magnetic measurements the magnetic properties indicate that the sediments are a mixture of hematite magnetite and pyrrhotite in different proportions depending on the region combined with results previously reported for the three main asian rivers pearl red and mekong rivers the new data indicate that in general hematite rich sediments are delivered to the southern basin of the south china sea while the northern basin is fed with magnetite and pyrrhotite rich sediments in addition to this general picture some variability is observed at smaller geographic scales indeed the magnetic assemblages are closely related to the geology of the various catchments while clay minerals previously reported for the same samples are more representative of the climatic conditions under which the parent rocks have evolved within each catchment the magnetic fraction now well characterized in the main river sediments drained into the south china sea can be used as a tracer for changes in precipitation on land and in oceanic water mass transport and exchange

geophysical investigations and crustal structure of the north american midcontinent rift system abstract geophysical investigations have had a prominent role in studying the 1100 ma midcontinent rift system because of limited surface exposures and deep drilling of the rift rocks gravity and magnetic anomaly data delineate the rift system its central volcanic filled graben and overlying and adjacent post rift sedimentary basins and the major faults of the system over its 2000 km length in central north america crustal seismic studies indicate a high velocity crust that is thickened to as much as 55 km along the rift deeply penetrating seismic reflection data obtained across the rift show that the crust was thinned to less than one half of its original thickness during rifting and the present crust has been thickened by addition of up to a total of 30 km of mantle derived volcanic rocks within grabens and clastic sediments deposited in overlying sag basins as well as by intrusions into the crust and crustal underplating these data also confirm the occurrence of a post rift regional compressional event that locally caused broad folding of the rift rocks and reactivation of the graben normal faults as high angle reverse faults with vertical throws of several kilometers geophysical data indicate that the rifting event had a profound effect on the entire vertical range of the crust and that structures and thus the tectonic evolution are similar along the entire length of the rift

geochemical records of taiwan sourced sediments in the south china sea linked to holocene climate changes we present a multi proxy geochemical analysis of core kng5 retrieved from the northern south china sea slope spanning the last 13 0 ka bp to assess the response of weathering and erosion processes in the southwestern taiwan river basin to climate change a provenance analysis indicates that the core sediments were primarily derived from the southwestern taiwan river and can be used to reconstruct the erosion and or weathering history of this river basin the chemical index of alteration the plagioclase index of alteration rb sr al ti principle component 1 and k al combined with terrigenous mass accumulation rates and the proportion of clay exhibit sharp excursions from 12 0 to 5 5 ka peaking at 10 0 ka which suggests strong chemical weathering and physical erosion during the early mid holocene the period of the maximum summer monsoon abrupt falling weathering trends during 5 2 and 3 5 ka bp correlate with a weaker monsoon after 3 5 ka bp a stable lower weathering trend to the present day was evident indicating the deposition of less weathered sediments in the slope when the monsoon rainfall was weakening a good correlation between our proxy records of chemical weathering and variations of the monsoon intensity in south china emphasizes that variation in monsoon intensity is most likely the primary drive of the intensity of weathering and erosion in southwestern taiwan river sediments over the last 13 0 ka bp finally we concluded that geochemical parameters can be used as reliable proxies for climate variation on a millennial centennial time scale in such areas as taiwan where weathering limited regimes exist c 2015 elsevier b v all rights reserved

geochemistry of river borne clays entering the east china sea indicates two contrasting types of weathering and sediment transport processes the east china sea is characterized by wide continental shelf receiving a huge input of terrigenous matter from both large rivers and mountainous rivers which makes it an ideal natural laboratory for studying sediment source to sink transport processes this paper presents mineralogical and geochemical data of the clays and bulk sediments from the rivers entering the east china sea aiming to investigate the general driving mechanism of silicate weathering and sediment transport processes in east asian continental margin two types of river systems tectonically stable continental rivers and tectonically active mountainous rivers coexist in east asia as the direct weathering products clays can better reflect the silicate weathering regimes within the two river systems provenance rock types are not the dominant factor causing silicate weathering intensity difference existed in the east asian rivers the silicate weathering intensity of tectonically stable river basins is primarily driven by monsoon climate and the sediment transfer is relatively slow because of natural trapping process and increasing damming effect the geochemistry of these river borne sediments can thus indicate paleo weathering intensities in east asian continent in contrast silicate weathering intensity in tectonically active mountainous rivers is greatly limited by strong physical erosion despite the high temperature and highest monsoon rainfall the factors controlling silicate weathering in tectonically active catchments are complex and thus it should be prudent to use river sediment records to decipher paleoclimate change these two different silicate weathering regimes and sediment transport processes are manifestations of the landscape evolution and overall dominate the sedimentation in asian continental margin

world wide delivery of river sediment to the oceans new data and new estimates from old data show that rivers with large sediment loads annual discharges greater than about 15 times 10 6 tons contribute about 7 times 10 9 tons of suspended sediment to the ocean yearly extrapolating available data for all drainage basins the total suspended sediment delivered by all rivers to the oceans is about 13 5 times 10 9 tons annually bedload and flood discharges may account for an additional 1 2 times 10 9 tons about 70 of this total is derived from southern asia and the larger islands in the pacific and indian oceans where sediment yields are much greater than for other drainage basins

geochemistry and geodynamic implications of the mesoproterozoic english bay granite rhyolite complex northwestern ontario the mesoproterozoic english bay complex consists of a granite rhyolite assemblage outcropping on the shores of lake nipigon in western superior province canada it intrudes neoarchean rocks and is disconformably overlain by a rift infracratonic basin sedimentary succession recording subsidence following a heating event the granites and rhyolites are characterized by light rare earth element lree enrichment la smn 2 8 5 1 and only weakly fractionated heavy ree hree gd ybn 1 1 1 6 the felsic igneous rocks are high k enriched in zr nb y and ree satisfying all the criteria for an a type suite trace element geochemistry particularly the absence of any negative nb anomalies indicates this melt did not originate in a suprasubduction zone setting unlike the st francois mountain complex to the south the english bay complex may record the northern portion of a mesoproterozoic plume track a plume that possibly led to earlier igneous activity and infracratonic basin formation to the north and would later interact with a suprasubduction zone margin to the south

collision induced rifting the grenville orogeny and the keweenawan rift of north america abstract we propose that the keweenawan rift formed as a result of convergence related to the synchronous grenville orogeny this proposal is based on 1 1 isotopic ages in the keweenawan rift and the grenville province are similar according to the best u pb and rb sr dates the grenville orogeny existed from 1150 to 1100 ma and keweenawan rifting occurred 1120 1100 ma 2 2 a similar modern orogenic belt and rift relationship in the middle east serves as an actualistic model the red sea gulf of aden rift system has formed as a continuous series of coalesced pull apart basins and is opening parallel to the orogenic front of the zagros and bitlis mountains rifting began in the early miocene while the collision began in the eocene suggesting that rifting is related to the collision 3 3 many other ancient rift collision pairs have a similar age relationship and geometrical configuration based on comparisons of these tectonic regimes we have derived a model whereby these rifts formed as a result of strike slip faults forming in either the foreland or the hinterland of orogenic zones

the influence of the sunda strait opening on paleoenvironmental changes in the eastern indian ocean abstract with sea level rise in the early holocene the warm and low salinity sea water from the java sea was transported into the eastern indian ocean after the opening of the sunda strait however the impact of this process on sediment provenance and paleoproductivity is rarely known in this study we analyzed the grain size sr nd pb isotopic compositions and biogenic elements of core cj01 185 1538 m water depth in the eastern indian ocean off the sunda strait our new results reveal a large volcanic eruption in the west of java at 2 1 ka and the krakatau eruption at 1883 ce on the basis of the pb isotopic compositions 87 sr 86 sr ratios and e nd values we infer that sediments from core cj01 185 were derived primarily from java island there is a distinct change in sr nd pb isotopic compositions of the sediments from the last glacial period to the holocene indicating less radiogenic nd isotopes from the java sea into the eastern indian ocean after the opening of the sunda strait at 10 ka moreover the sedimentation rate increased significantly from 6 5 cm kyr 1 during the last glacial period to 20 cm kyr 1 in the holocene the input of additional terrigenous nutrients from the java sea induced paleoproductivity with higher toc and tn concentrations after the opening of the sunda strait our results thus suggest that the paleoproductivity was mainly influenced by terrigenous materials input in the holocene other than southeast monsoon or upwelling in the last glacial period

clay minerals sr nd isotopes and provenance of sediments in the northwestern south china sea abstract the clay mineralogy and sr nd isotopic composition of sediments collected in the northwestern south china sea scs were investigated to understand sediment provenance and changes in the scs since the late pleistocene our results show that the clay mineral assemblages of the surface sediments are characterized by high amounts of illite and smectite and low amounts of kaolinite and chlorite the 87sr 86sr ratios of the surface sediments vary from 0 709617 to 0 734416 and end values range from 11 6 to 8 3 the clay minerals and sr nd isotope records suggest that sediments in the northwestern scs are derived from multiple sources the sediments deposited in the eastern shelf offshore of hainan originate predominantly from hainan island and south china whereas sediments in the western shelf offshore of hainan are derived mainly from indochina and hainan island however sediment source and transport mechanisms in the northwestern slope of the scs are more complicated our results suggest that input from the west may be the major sediment source but regions in the east taiwan and luzon also contribute part of the terrigenous materials downcore variations of sr nd isotopic composition in core sediments show clear glacial interglacial fluctuations higher 87sr 86sr ratios occur during glacial period indicating that the input of terrigenous materials from old continental rocks becomes one of the important sediment sources during these periods the exposure of continental shelf and southward migration of the coastline during the glacial sea level lowstand led to an increase of terrigenous supply to the northwestern slope of the scs the paleo red river and indochina are also potentially important contributors

geochemical evidence for initiation of the modern mekong delta in the southwestern south china sea after 8 ma abstract sedimentary records in the southwestern south china sea reflect the evolving erosion and drainage systems that have operated in southeast asia during the neogene analyses of the chemistry and clay mineral composition of sediments from international ocean discovery program iodp site u1433 allow us to examine these processes over the last 17 ma sediment older than 8 ma was deposited relatively slowly sr and nd isotopes indicate a variable provenance with sequences of less and more altered material accompanied by strong changes in the proportion of smectite sediment flux was probably from indochina as well as from a more primitive volcanic source most likely the palawan ophiolite and or luzon sediments younger than 8 ma show a more stable sr and nd isotope character indicating sources close to those seen in the modern mekong river although with some influx from smaller rivers draining the indochina margin especially from 4 to 8 ma our data are consistent with seismic estimates for an onset to the mekong in its present location after 8 ma following an avulsion from the gulf of thailand

terrigenous supplies variability over the past 22 000 yr in the southern south china sea slope relation to sea level and monsoon rainfall changes changing weathering intensity sediment transport and provenance variations over the past 22 0 lea bp have been investigated by high resolution clay mineralogy grain size and stable oxygen isotopes of planktonic foraminifera records along core cg2 recovered from the continental slope of the sunda shelf southern south china sea our results indicated that the reworking of older sediments outcropping on the sunda shelf exerted a great influence on the sediment supply during the last glacial and most of the last deglacial modulated by sea level and monsoon rainfall changes during the last 9 01 a bp relative increased kaolinite and heavier delta o 18 seawater values might reflect the higher influence of the tropical indonesian islands sources due to the reopen of southern straits implying the formation of modern oceanic circulation and depositional patterns high sediment fluxes in core cg2 during heinrich stadial 1 might be a synthetic result of the intensified monsoon rainfall originated from the southward shift of the intertropical convergence zone and the proximal location of the study core before the flooding of the sunda shelf fluctuations in smectite illite chlorite ratios correlated well with monsoon intensity and periods of strong monsoon rainfall lighter delta o 18 seawater values were associated with an intensification of erosion of pre existing more weathered materials on the sunda shelf finally we concluded that sediment composition and mineralogy in the southern south china sea slope were controlled by varying degrees of reworking on the sunda shelf as well as climatically modulated sediment supply from the mekong river and southern tropical islands over the last 22 0 ka bp c 2015 elsevier ltd all rights reserved

assessing effective provenance methods for fluvial sediment in the south china sea sediment is delivered by the rivers of se asia to the south china sea where it provides an archive of continental environmental conditions since the eocene interpreting this archive is complicated because sediment may be derived from a number of unique sources and the rivers themselves have experienced headwater capture that also affects their composition a number of methods exist to constrain provenance but not all work well in this area anthropogenic impacts most notably agriculture mean that the modern rivers contain more weathered materials than they did up until about 3000 years ago the rivers have also changed their bulk chemistry and clay mineralogy in response to climate change so that these proxies as well as sr isotopes are generally unreliable provenance indicators nd isotopes resolve influx from luzon but many other sources in se asia have similar values and clear resolution of end members can be difficult instead thermochronology methods are best suited especially apatite fission track which shows more diversity in the sources than either u pb zircon or ar ar muscovite dating nonetheless even fission track is best used as part of a multiproxy approach if a robust quantitative budget is desired

the mesoproterozoic midcontinent rift system lake superior region usa abstract exposures in the lake superior region and associated geophysical evidence show that a 2000 km long rift system developed within the north american craton 1109 1087 ma the age span of most of the volcanic rocks this system is characterized by immense volumes of mafic igneous rocks mostly subaerial plateau basalts generated in two major pulses largely by a hot mantle plume a new ocean basin was nearly formed before rifting ceased perhaps due to the remote effect of the grenville continental collision to the east broad sagging subsidence combined with a system of axial half grabens separated along the length of the rift by accommodation zones provided conditions for the accumulation of as much as 20 km of volcanic rocks and as much as 10 km of post rift clastic sediments both along the rift axis and in basins flanking a central post volcanic horst pre rift mature quartzose sandstones imply little or no uplift prior to the onset of rift volcanism early post rift red bed sediments consist almost entirely of intrabasinally derived volcanic sediment deposited in alluvial fan to fluvial settings the exception is one gray to black carbon bearing lacustrine unit this early sedimentation phase was followed by broad crustal sagging and deposition of progressively more mature red bed fluvial sediments with an extra basinal provenance

climatic control of sediment transport from the himalayas to the proximal ne bengal fan during the last glacial interglacial cycle clay mineralogy siliciclastic grain size major elements 87sr 86sr and end analyses of deep sea sediments cored in the north eastern bay of bengal are used to reconstruct evolution of detrital sources and sediment transport to the proximal part of the bengal deep sea fan during the last climatic cycle end values 13 3 to 9 7 and 87sr 86sr ratios 0 721 0 733 indicate a mixture of sediments originating from the ganges brahmaputra rivers and the indo burman ranges interglacial marine isotopic stages mis 5 and 1 are associated with a higher contribution of sediments from the ganges brahmaputra river system than is the case for glacial mis 6 4 3 and 2 siliciclasitic grain size combined with si al and si fe ratios indicate coarser glacial sediments with numerous turbidite layers glacial turbidite layers display similar clay mineralogical compositions to hemipelagic sediments only few of turbidite layers mis 6 4 and 2 are slightly unradiogenic end 13 3 suggesting a higher contribution of ganges brahmaputra river sediments independently of changes in the sedimentary sources the smectite illite chlorite ratio of cores located on the ne bengal fan indicates higher inputs of primary minerals illite and chlorite from the highlands of the river basins relief during glacial mis 6 4 3 and 2 and an increased contribution of pedogenic minerals smectite and kaolinite during interglacial mis 5 and 1 maximum smectite illite chlorite ratios during the warm sub stages of mis 5 suggest an intensification of summer monsoon rainfall associated with higher rates of physical erosion of the indo gangetic flood plain and or dominant summer hydrological conditions transporting a higher proportion of sediments deriving from the ganges brahmaputra rivers to the ne bengal fan in addition a higher production of smectite in soils of the indo gangetic flood plain during periods of intensification of monsoon rainfall cannot be excluded

climate changes of the northeastern tibetan plateau since the late glaciation inferred from clay mineralogy of sediments in kuhai lake a high resolution study of clay mineralogy has been carried out on a sediment core recovered from kuhai lake in order to investigate climatic and environmental changes in the northeastern tibetan plateau since the late glacial period clay mineralogy reveals significant changes in sedimentary sources and hydrolysis conditions in the kuhai lake catchment area before 14 9 cal ka bp the predominance of illite and relatively high amount of kaolinite derived mainly from the reworking of sedimentary rocks imply unstable cold arid conditions and significant glacier cover that prevented from weathering of metamorphic and mafic rocks in the high relief of the catchment between 14 9 and 5 4 cal ka bp an increase in smectite and chlorite contents associated with relatively low values of the illite chemistry index indicate that clay minerals derived from the weathering of metamorphic and volcanic rocks in the highlands of the catchment area such modifications were triggered by a melting of glaciers in the lake s catchment during the bolling allerod interstadial followed by an intensification of monsoon precipitations in the early holocene around 10 0 cal ka bp both processes are responsible for significant physical erosion in the highlands together with an efficient transport of smectite and chlorite from areas of high relief to the lake after 5 4 cal ka bp lower smectite and chlorite proportions associated with a predominance of al rich illite and kaolinite indicate a return to relatively dry conditions and a lowering of physical erosion in the drainage basin comparison between clay mineralogy and pollen records indicates that variations in weathering intensity in the kuhai lake catchment area and changes in its surrounding vegetation are synchronous demonstrating the large potential of clay mineralogical analysis to trace climate change of the tibetan plateau

prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations abstract the exposed crust consists mainly of plagioclase 35 quartz 20 k feldspar 11 volcanic glass 12 biotite 8 and muscovite 5 quartz is a resistate thus feldspars and glass represent approximately 75 percent of the labile minerals the weathering characteristics of these constituents are summarized in the context of thermodynamic mass balance and kinetic considerations experimentally determined release rate constants were used to predict the proportions of ca na and k released by feldspars of plutonic rocks granites to gabbros to weathering solutions the chemical weathering trends of the weathered residues calculated from the kinetic data conform closely to the initial trends observed in some recent weathering profiles demonstrating the accuracy of the predictions since the weathering of feldspars is controlled by processes that should not change through geological time the relative release rates of ca na and k from the feldspars of granitic rocks can be calculated for future and past episodes of continental weathering experimentally determined release rate constants are not available for a wide range of volcanic glass compositions but the limited data indicate that compositional trends are predictable in weathering profiles developed on volcanic rocks the kinetic data available for rhyolitic glasses accurately predict the initial weathering trends observed in a recent rhyolite weathering profile

reviewing the configuration and extent of the midcontinent rift system abstract uncertainty exists in the configuration and extent of the midcontinent rift system mrs because of deficiencies in geophysical data and limited information from outcrops and basement drill holes additional ambiguity is caused by misunderstanding the definition of continental rifts six principal problematic regions in mapping the mrs are described gravity and magnetic data supported by drill hole and seismic reflection data show that the eastern lake superior rift segment of the mrs continues south from lake superior and connects to a much narrower rift in northern lake michigan the eastern margin of this transition is ill defined because of the lack of definitive anomalies and supporting seismic and drill hole data but is interpreted to occur near the u s canada border the rift segment in southeastern michigan intersects the grenville front and likely continues eastwards in modified form to near the boundary with canada cross cutting gravity and magnetic signatures may reflect grenvillian overthrusts near the terminus of the mrs in michigan the proposed southerly extensions of both branches of the rift system into oklahoma and ohio are based primarily on positive gravity anomalies but neither postulated extension appears to be associated with rifted troughs rather the gravity anomalies of the western branch are related to intrusive mafic rocks and those of the eastern branch are most likely related to deep crustal metamorphic rocks thrust into juxtaposition with less dense crust by grenville orogenesis recent paleomagnetic investigations in conjunction with high resolution radiometric dating imply that the mrs developed during the rapid southward movement of laurentia during a quiescent period along its eastern continental margin massive magmatic activity accompanying the rifting was likely due to rising mantle material that was displaced by subducted lithosphere along the southern margin the heated crust was made more ductile fostering rifting due to extensional stresses the nipigon embayment remains as a possible candidate for an early third branch of the mrs but current evidence is insufficient to include the fort wayne rift as part of the mrs future studies of the mrs would be well served by new age dating and high resolution seismic studies of the lithosphere

an investigation of superior shoal central lake superior with a manned submersible a johnson sea link submersible was used to examine the geology of superior shoal in central lake superior here glacially scoured vertical cliffs some more than 100 m high are formed of 1 1 ga middle keweenawan basaltic lava flows displaying ophitic interiors and red amygdaloidal tops flat lying sandstones lithologically similar to the upper keweenawan bayfield jacobsville sequences occur to the north of the volcanic rocks these are inferred to have been downthrown along an eastward extension of the isle royale fault a major boundary fault of the midcontinent rift the volcanic rocks are normally magnetized supporting lithological evidence that they correlate with the middle keweenawan sequence on isle royale paleomagnetic data suggest that the volcanics have a complex structure possibly involving drag folding along the isle royale fault

weathering and global denudation a negative correlation between sediment yield and weathering history as measured by the chemical alteration cia of the suspended sediment is observed for many of the world s major rivers and other regions of denudation the weathering history is a first order control on the sediment yield of such areas termed equilibrium denudation regions for other areas data scatter with either apparent increases or decreases of sediment yield for given cia values these areas are termed nonequilibrium denudation regions low sediment yields can be attributed to moderated erosion either natural or human induced and or the incorporation of unweathered glacial debris accelerated erosion resulting in high sediment yield is primarily human induced and results from cultivation and other land use each of these effects has a profound influence on global sediment discharge from the continents pre human suspended sediment discharge from the continents is estimated to be 12 6 times 10 15 g yr or about 0 6 the

primary curvature in the mid continent rift paleomagnetism of the portage lake volcanics northern michigan usa abstract rocks of the north american mid continent rift mcr in the keweenaw peninsula of northern michigan display a change in structural trend from east to west varying in strike from 100 in the east to 35 farther west before returning to a more east west trend of 80 near wisconsin in general curvature can be described either as of primary origin meaning that the arc developed in its present curved state or as of secondary origin when the arc formed from an initially straighter geometry a powerful tool in evaluating the origin and degree of curvature in any deformed belt is paleomagnetism where coincident change in declination and strike is evidence for secondary curvature several paleomagnetic studies have been completed on rift related sedimentary rocks as well as lava flows from the mcr in the lake superior region paleomagnetic directions for the sedimentary rocks vary much more in declination than in inclination possibly reflecting a vertical axis rotation if secondary rotation has affected the sedimentary rocks then underlying volcanic rocks should also demonstrate a similar rotation thirty one sites were collected from the portage lake volcanics in the most highly curved part of the keweenaw peninsula in the upper peninsula of michigan sites were chosen to maximize the variation in structural trend thermal demagnetization results showed two components in samples from most sites a lower temperature a component 580 c both components were tested for primary remanence using a conglomerate test the a component carried by magnetite passes the conglomerate test at a 95 confidence level and is therefore considered a primary remanence the b component carried by hematite fails at the 95 level and is considered a secondary magnetization most importantly declination of the primary a magnetization between sites shows no correlation with strike demonstrating that vertical axis rotations cannot explain the curvature of the mid continent rift we conclude that curvature in the lake superior region is a primary feature likely reflecting a pre existing zone of weakness that was exploited during rifting

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

heat flow in the nipigon arm of the keweenawan rift northwestern ontario canada 1 in the archean superior province the nipigon embayment in the area of lake nipigon north of lake superior is covered by midproterozoic sediments intruded by keweenawan diabase sills it has been interpreted as a failed arm of the ca 1100 ma keweenawan rift six new heat flow values in this area show that the region of low heat flow associated with the keweenawan rift in lake superior extends northwards along the western margin of the nipigon embayment the average heat flow in the nipigon area 39 5 mwm 2 is only slightly lower than

sea level induced variations in clay mineral composition in the southwestern south china sea over the past 17 000 yr variations in clay mineral composition of sediment cores from the margin and continental slope of the sunda shelf southern south china sea se asia covering the past 17 000 yr reflect changing influences of sediment sources together with clay mineral partitioning processes in shallow waters we identify the deglacial sea level rise as the principal factor driving these changes during the late glacial high values of kaolinite are interpreted to reflect a higher contribution of clays from soils that have formed on the exposed sunda shelf and in the southern archipelagos of indonesia at this time core sites were located in close proximity to the mouths of the sunda shelf palaeo drainage systems on the emerged shelf sundaland the progressive landward displacement of the coastline and breakdown of these vast drainage systems during deglaciation led to a decrease in influence of the kaolinite rich southern sources when the coastline had retreated closely to its present day position in mid holocene times the former dominance of southern sources was replaced by a stronger influence of illite rich sources e g borneo the overriding control of sea level changes on the clay mineral distribution patterns precludes a definite climatic interpretation of clay mineral data in terms of climatic monsoonal changes in such highly dynamic sedimentary environment

accumulation of fossil fuels and metallic minerals in active and ancient rift lakes a study of active and ancient rift systems around the world suggests that accumulations of fossil fuels and metallic minerals are related to the interactions of processes that form rift valleys with those that take place in and around rift lakes the deposition of the precursors of petroleum gas oil shale coal phosphate barite cu pb zn sulfides and uranium begins with erosion of uplifted areas and the consequent input of abundant nutrients and solute loads into swamps and tectonic lakes hot springs and volcanism add other nutrients and solutes the resulting high biological productivity creates oxidized reduced interfaces and anoxic and h2s rich bottom waters which preserves metal bearing organic tissues and horizons in the depositional phases the fine grained lake deposits are in contact with coarse grained beach delta river talus and alluvial fan deposits earthquake induced turbidites also are common coarse grained deposits of rift lakes postdepositional processes in rifts include high heat flow and a resulting concentration of the organic and metallic components that were dispersed throughout the lakebeds postdepositional faulting brings organic and metal rich sourcebeds in contact with coarse grained host and reservoir rocks a suite of potentially economic deposits is therefore a characteristic of rift valleys

source to sink transport processes of fluvial sediments in the south china sea the south china sea offers an excellent case for studying source to sink transport processes of fluvial sediments among the global marginal seas this study synthesizes existing clay mineralogical and geochemical data from similar to 1500 samples from the seafloor and surrounding rivers deepwater mooring observation results and high resolution glacial cyclic clay mineralogy records from six high quality sediment cores source to sink sediment transport from the river mouth to the continental shelf and then to the abyssal basin is investigated at two time scales modern process and late quaternary glacial cycles the results firstly show the high diversity of clay mineralogical and geochemical compositions in riverbed surface sediments surrounding the south china sea e g dominant illite and chlorite in taiwan overwhelming majority of smectite in luzon dominant kaolinite in south china and similar amounts of the four clay mineral species in red and mekong river systems the formation of these fluvial sediments through chemical weathering in surrounding drainage systems is controlled principally by the east asian monsoon climate with warm temperature and high precipitation and subordinately by tectonic activity and specific lithological character the basin wide distribution of clay mineral assemblages combined with neodymium and strontium isotopic compositions reflects strong provenance control and differential settling effects the differential settling of kaolinite in slightly saline and proximal regions relative to smectite in more saline and distal regions is well demonstrated in the south china sea through combining clay mineralogical distributions on the seafloor with observed oceanic current systems the modern transport pathways can be well established e g in the northern south china sea illite and chlorite from taiwan are mainly carried by contour currents with the strong influence of mesoscale eddies high resolution sediment dynamic analysis for the late quaternary reveals different sediment transport patterns in the northern western and southern south china sea when the land sea configuration dramatically changed during glacial conditions in the north the terrigenous dispersal is mainly controlled by provenance supply and oceanic current transport in the west the clay mineral assemblage reflects variations of the prevailing surface current influenced by monsoon winds in the south the clay mineral input indicates intensive chemical weathering during interglacial periods and strengthened physical erosion during glacial periods the transport of terrigenous sediments since the last glaciation is quantitatively reconstructed through studying two deepwater sediment cores located in the northern south china sea the relative contributions from three provenances south china luzon and taiwan highlight their distinct variability over the last 28 ka implying that the southward shift of the inter tropical convergence zone itcz at 16 ka bp caused an increased sediment contribution from south china the intensified influence of the kuroshio current intrusion controls more efficient westward transport of luzon sediments and the stronger deepwater current transports taiwan sourced sediments further westward from the last glaciation to the holocene c 2015 elsevier b v all rights reserved

moderate chemical weathering of subtropical taiwan constraints from solid phase geochemistry of sediments and sedimentary rocks abstract the well known earthquake and storm triggered extremely high physical weathering rate in taiwan is consistent with present geochemical studies of sediments from different subenvironments offshore coastal river and lake and sedimentary rocks of different geological ages indicating a moderate chemical weathering condition major and trace element concentrations normalized to the average upper crust of yangtze craton show that the sediments and the average composition of sedimentary rocks of taiwan are depleted in ca mg na and sr enriched in rb and zr and unchanged with respect to k indicating their moderately altered nature the mean chemical index of alteration cia 71 75 and plagioclase index of alteration pia 81 86 values of coastal and offshore sediments reveal the sediments derivation from sedimentary rocks by moderate silicate chemical weathering processes the mean cia value 62 of sedimentary rocks of taiwan is similar to that for chinese sediment 61 further confirming

a possible southeastern extension of the midcontinent rift system located in ohio in 1988 a stratigraphic test core drilled in southwestern ohio penetrated a previously unknown structure and sedimentologic unit core analyses disclosed a sequence of lithic arenite and siltstone of probable pre phanerozoic age deposited within an alluvial fluvial environment a seismic reflection profile across the core site shows a sequence of strong horizontal paleozoic reflectors unconformably overlying eastward dipping layered units of poor reflectivity beyond the core hole total depth a regime of excellent reflectors is seen the pre mount simon upper cambrian sedimentary sequence encountered has been defined as the type section of the middle run formation point counter analyses of typical middle run formation samples indicate that this sequence is similar in composition to middle proterozoic units associated with the midcontinent rift system in the lake superior region the newly discovered structure in ohio is also similar to half graben structures composing the lake superior basin portion of the midcontinent rift on the basis of similarities in lithology stratigraphy structure and proximity to regional gravity anomalies the middle run formation and its apparent half graben basin is proposed as evidence for the probable extension of the midcontinent rift system southward from its normally accepted termination in southeastern michigan into southwestern ohio

tectonic setting of the midcontinent rift system abstract the continental lithosphere which hosts the midcontinent rift mcr system comprises a crustal assemblage ranging in age from 3 6 to 1 5 ga the most voluminous igneous activity along the mcr occurred within late archean 2 7 ga crust presumably due to magmatism associated with a rising mantle plume two major arms of the rift extend to the southeast and the southwest where rifting magmatism and sedimentation were confined to a 2000 km long rift that cuts through the various crustal provinces ranging in age from 3 6 to 1 5 ga tensional reactivation of pre existing crustal discontinuities may have influenced the location of parts of the mcr however direct cause and effect are difficult to distinguish for want of sufficient exposures the most active phases of magmatism and rifting occurred between 1109 and 1087 ma a u pb age of 1097 ma for the southern end of the mcr shows that magmatic activity was synchronous throughout its length there is evidence of rifting and magmatism prior to 1109 ma but there is little evidence of magmatism after 1087 ma rifting may have effectively ceased by that time rifting may have ceased as a result of compressional forces being exerted on the foreland during the grenville collisional event spanning 1030 970 ma and compressional reactivation of normal faults may have resulted in partial inversion of the mcr chemical data and u pb and sm nd isotopic data for igneous rocks along the rift are similar to those for mcr rocks in the lake superior area which are best explained in terms of a mantle plume model however since the distal arms of the mcr were developed in proterozoic lithosphere and did not involve nearly as much magmatism as in the lake superior area mantle plume sources may not have been as important in formation of the distal magmas data from the buried arms are also consistent with the mafic magmas being derived by decompression melting of proterozoic lithosphere during rifting

geochemistry of the mesoproterozoic intrusive rocks of the nipigon embayment northwestern ontario evaluating the earliest phases of rift development the intrusive rocks of the nipigon embayment comprise a series of four mafic to ultramafic intrusions and a number of laterally extensive diabase sills that are among the oldest expression of the

weathering processes in the ganges brahmaputra basin and the riverine alkalinity budget abstract we present river chemistry data for a network of rivers draining the western and central nepal himalaya our sampling locations cover the system from the sources of rivers in tibet to the gangetic plain water samples were collected throughout the year including the monsoon season for rivers in nepal and for the ganges and brahmaputra in bangladesh rivers draining the north himalaya are characterized by low discharge under a cold and arid climate main stream waters have 13 c dic near 0 and high so42 so42 hco3 ratios xso4 with values around 40 eq and high cl ca is the dominant cation ca2 cations 55 to 75 eq after correction of sodium by chloride dissolved sulfate is produced in waters from the tethian sedimentary series whereas chlorine is related to thermal waters dissolved sulfate is primarily derived from sulfide oxidation rather than evaporite dissolution 13 c dic values of up to 3 9 show that metamorphic co2 is an important weathering agent rivers of the north himalayan basins have about 50 of their alkalinity derived from carbonate dissolution 20 from biogenic activity and 30 from metamorphic co2 on the south flank of the himalaya rivers are more depleted in 13 c and have on average lower xso4 most rivers have 13 c dic and xso4 values compatible with a simple mixing between soil co2 and sulfate derived from sulfide oxidation during the monsoon discharge increases by a factor 20 but the total dissolved concentration is only slightly reduced xso4 and 13 c decrease while ca cations increases implying enhanced dissolution of pedogenic calcite as a whole the g b riverine flux of alkalinity derived from silicate weathering is around 2 7 1011 mol year modest at the global scale sulphuric acid controls 20 30 of the weathering reaction in the brahmaputra and 6 to 9 in the ganges na and k balance 60 to 65 of the silicate derived alkalinity flux and the long term co2 consumption by ganges and brahmaputra is near 6 4 1010 mol year the flux of metamorphic co2 converted to alkalinity via weathering reactions is ca 1 1010 mol year

enhanced silicate weathering of tropical shelf sediments exposed during glacial lowstands a sink for atmospheric co2 atmospheric co2 and global climate are closely coupled since 800 ka co2 concentrations have been up to 50 higher during interglacial compared to glacial periods because of its dependence on temperature humidity and erosion rates chemical weathering of exposed silicate minerals was suggested to have dampened these cyclic variations of atmospheric composition cooler and drier conditions and lower non glacial erosion rates suppressed in situ chemical weathering rates during glacial periods however using systematic variations in major element geochemistry sr nd isotopes and clay mineral records from ocean drilling program sites 1143 and 1144 in the south china sea spanning the last 1 1 ma we show that sediment deposited during glacial periods was more weathered than sediment delivered during interglacials we attribute this to subaerial exposure and weathering of unconsolidated shelf sediments during glacial sealevel lowstands our estimates suggest that enhanced silicate weathering of tropical shelf sediments exposed during glacial lowstands can account for 9 of the carbon dioxide removed from the atmosphere during the glacial and thus represent a significant part of the observed glacial interglacial variation of 80 ppmv as a result if similar magnitudes can be identified in other tropical shelf slope systems the effects of increased sediment exposure and subsequent silicate weathering during lowstands could have potentially enhanced the drawdown of atmospheric co2 during cold stages of the quaternary this in turn would have caused an intensification of glacial cycles

reprint of comparison of modern fluid distribution pressure and flow in sediments associated with anticlines growing in deepwater brunei and continental environments iran abstract differences in fluids origin creation of overpressure and migration are compared for end member neogene fold and thrust environments the deepwater region offshore brunei shale detachment and the onshore arid central basin of iran salt detachment variations in overpressure mechanism arise from a the availability of water trapped in pore space during early burial deepwater marine environment vs arid continental environment and b the depth temperature at which mechanical compaction becomes a secondary effect and chemical processes start to dominate overpressure development chemical reactions associated with smectite rich mud rocks in iran occur shallow 1900 m smectite to illite transformation causing load transfer related moderate overpressures whereas mechanical compaction and inflationary overpressures dominate smectite poor mud rocks offshore brunei the basal detachment in deepwater brunei generally lies below temperatures of about 150 c where chemical processes and metagenesis are inferred to drive overpressure development overall the deepwater brunei system is very water rich and multiple opportunities for overpressure generation and fluid leakage have occurred throughout the growth of the anticlines the result is a wide variety of fluid migration pathways and structures from deep to shallow levels particularly mud dykes sills laccoliths volcanoes and pipes fluid escape pipes crestal normal faults thrust faults and widespread inflationary type overpressure in the central basin the near surface environment is water limited mechanical and chemical compaction led to moderate overpressure development above the upper red formation evaporites only below thick early miocene evaporites have near lithostatic overpressures developed in carbonates and marls affected by a wide range of overpressure mechanisms fluid leakage episodes across the evaporites have either been very few or absent in most areas locations where leakage can episodically occur e g detaching thrusts deep normal faults salt welds are sparse however in both iran and brunei crestal normal faults play an important role in the transmission of fluids in the upper regions of folds

chemical weathering under mid to cool temperate and monsoon controlled climate a study on water geochemistry of the songhuajiang river system northeast china abstract for the first time the river water geochemistry of the songhuajiang basin the third largest river system in china has been studied in order to understand chemical weathering and associated co2 consumption rate in a mid to cool temperate and monsoon controlled climatic zone the major ion compositions of the river waters are characterized by the dominance of ca2 and hco 3 accounting for 46 and 74 of major cations and anions respectively the average total dissolved solids tds 116 mg l 1 and total cation concentration tz 1388 eq l 1 of the river waters are similar to those of global major rivers the chemical weathering rates of carbonate silicate and evaporites in the whole songhuajiang basin are estimated to be approximately 5 15 2 23 and 0 40 t km 2 a 1 respectively the total rock weathering rate for the whole songhuajiang basin is approximately 7 78 t km 2 a 1 which is at the lower end of the spectrum for global major rivers and is comparable with that of the amur and the congo zaire river the estimated co2 consumption rates for the whole songhuajiang basin are 53 4 103 mol km 2 a 1 and 66 6 103 mol km 2 a 1 by carbonate and silicate weathering respectively as a sub basin the 2nd songhuajiang has the highest 18 9 t km 2 a 1 while the nenjiang river basin has the lowest total rock weathering rate 5 03 t km 2 a 1 which indicates important controls of regional climate and lithology compared to the large rivers of china total rock weathering rates increase from north to south supporting the idea that the climate acts as major control on global chemical weathering the co2 consumption rate by silicate weathering within the whole songhuajiang river basin though under mid to cool temperate climate is in the same order of magnitude as that of huanghe 82 4 103 mol km 2 a 1 and is not much lower than that of changjiang 112 103 mol km 2 a 1 which suggests that the role of songhuajiang river weathering in long term climate change cannot be neglected compared to those of the large rivers of china

mesoproterozoic porphyry copper mineralization at mamainse point ontario canada in the context of midcontinent rift metallogeny abstract the jogran copper prospect in southern ontario canada is located on the northeastern shoulder of the north american midcontinent rift system where it is centered on two small closely spaced quartz monzonite porphyry intrusions emplaced into neoarchean mafic metavolcanic rocks notwithstanding an atypical alteration zoning pattern the distinctive alteration styles and veinlet types confirm its porphyry copper affiliation two samples of molybdenite from chalcopyrite bearing quartz veinlets in the two centers returned re os ages of 1101 5 and 1094 5 ma showing that the mineralization took place during the main stage of extension and magmatic activity that gave rise to the midcontinent rift and associated large igneous province lip the nearby copper bearing tribag breccia pipes and coppercorp veins have been shown previously to also be intrusion related and likely formed penecontemporaneously with jogran the jogran and nearby copper mineralization overlapped temporally with generation of low grade magmatic copper nickel pge deposits in the duluth intrusive complex but predated the strata bound native copper and chalcocite deposits in the keweenaw peninsula which are believed to have formed from basinal fluids during rift inversion although such magmatic and strata bound copper deposits typify intracontinental rift metallogeny porphyry copper deposits are normally confined to convergent margin magmatic arcs thereby making the rift lip setting of jogran exceptional and possibly unique

distribution and constraining factors of planktonic and benthic foraminifers in bottom sediments of the southern south china sea abstract a total of 93 surface sediment samples were collected to analyze the environmental factors constraining the foraminiferal distribution pattern in the southern south china sea scs species distributions and stable isotopic compositions were combined to reveal the controlling factors such as depth salinity substrate runoff currents and cold seep activities water depth is the dominant factor controlling both assemblage composition and 18o of benthic foraminiferal tests the 1000 m isobath separates the sites into two clusters cluster a and b which are dominated by deep water species and shallow water species respectively the sites in the deep water zone cluster a are characterized by lower absolute abundances species richness and shannon index values a measure of diversity and higher proportions of planktonic foraminifers compared with the sites in the shallow water zone cluster b increasing proportions of agglutinated tests with depth and rapidly decreasing proportions of planktonic foraminifera in the nansha trough basin provide evidence of calcium dissolution occurring at a depth corresponding with previous reports oxygen stable isotopes 18ob of benthic foraminifera become more positive with depth only up to 1000 m and remain constant beyond differences in the proportion of agglutinated and porcelaneous tests in the shallow water zone suggest that terrestrial runoff from nearby river systems mekong river and northern borneo rivers and seasonal surface currents scs southern cyclonic gyre and scs southern anticyclonic gyre jointly influence the distribution patterns of foraminifera in the study area enrichment of planktonic 18o is a response to cold waters brought by the scs southern cyclonic gyre during winter an upwelling current winter natuna off shelf current containing higher amounts of organic matter nutrients contributes to the depleted 13c of planktonic foraminifera and to the abnormal abundance of foraminifera at the sites within its area of influence the dominance of the foraminifer melonis barleeanus at sites belonging to subcluster a1 and the stable isotope compositions of benthic foraminifera 18o 0 13c

a gravity study of the coldwell complex northwestern ontario and its petrological significance gravity studies of the coldwell alkaline complex show that the complex is associated with a positive gravity anomaly the anomaly indicates that a significant volume of dense material occurs below the current levels of exposure of principally felsic rocks a new high resolution gravity profile used to model the infrastructure of the complex shows that the felsic rocks form a 3 5 km thick layer over a differentiated basic intrusion this intrusion is interpreted to consist of a layer of gabbro 3 5 km in thickness underlain by 3 km of peridotite and or pyroxenite intrusion of the basic magma was associated with keeweenawan continental rifting the presence of large volumes of basic magma supports the hypothesis that some of the coldwell felsic rocks were derived by differentiation of a basic parent

clay mineralogical and geochemical constraints on late pleistocene weathering processes of the qaidam basin northern tibetan plateau abstract at the qarhan salt lake qsl on the central eastern qaidam basin northern tibetan plateau quaternary lacustrine sediments have a thickness of over 3000 m and mainly composed of organic rich clay and silty clay with some silt halite and halite in this study a 102 m long sediment core isl1a was obtained from the qsl combining with ams 14c and 230th dating clay minerals and major element concentrations of isl1a were used to reconstruct the weathering process and trend of the qsl since late pleistocene the results reveal that the clay mineral from

climatic and tectonic controls on chemical weathering in tropical southeast asia malay peninsula borneo and sumatra abstract clay mineralogy and major element geochemistry were investigated on 58 surface sediment samples collected in 27 rivers draining malay peninsula borneo and sumatra to assess the chemical weathering process and its controlling factors in tropical southeast asia the clay mineral assemblages show high kaolinite contents in malay peninsula average 80 sumatra 58 78 and south borneo 41 55 high illite contents in north borneo 47 77 and moderate smectite contents in sumatra 6 29 intensive chemical weathering is identified in all three investigated regions from both clay mineralogical and geochemical bulk and clay fraction results regardless of their various lithological and tectonic settings relatively the chemical weathering intensity increases gradually from north borneo to south borneo and further to malay peninsula and sumatra our results suggest that the east asian australian monsoon climatic condition with constant warm temperature and abundant precipitation throughout the year is the principal forcing factor on the chemical weathering and the tectonic activity including mountain uplift earthquakes and volcanic eruptions and their specific lithological characters is only a subordinate factor on the intensity of chemical weathering in tropical southeast asia

clay mineralogy of the riverine sediments of hainan island south china sea implications for weathering and provenance abstract clay mineralogy of 54 fluvial samples collected from 20 major rivers on hainan island are investigated in order to determine compositional changes of clay minerals and to assess the weathering processes the clay mineral assemblages consist dominantly of kaolinite 31 66 with a lesser abundance of chlorite 22 44 and illite 4 33 and a trace amount of smectite 0 15 fluvial sediments from the east and northwest of hainan island are characterized by a higher kaolinite content and illite chemical index and poorer illite crystallinity than those from southwest hainan only minor smectite mean of 7 occurs in the sediments from west hainan smectite is total lacking in east hainan compared with the adjacent basins hainan island is characterized by moderate to intensive chemical weathering with strong hydrolysis our results suggest that rainfall is the principal factor controlling the intensity of chemical weathering on hainan island with more intense chemical weathering occurring in eastern and northwestern hainan another practical implication of this study is that it provides a missing end member hainan island in the provenance discrimination study focused on the northern south china sea scs hainan fine grained sediments likely play an important role in providing clay minerals to the northern scs carried by the south china sea warm current scswc during the summer

on the slippage of deep drogued drifters deep drogued drifters are in use to measure the near surface geostrophic currents an attempt is made to study the slippage of these drifters due to wind and ekman currents the results are based on a data set from the unstratified north sea obtained in winter 1991 1992 where the currents were decomposed into ekman currents and barotropic currents the influence of these ekman currents on the drift performance of drifters drogued below the mixed layer in the barotropic current is determined by using quadratic drag laws in 90 of all cases 1540 data points the combined effect of wind drag and ekman currents on buoy and 100 m tether produces a slippage of less than 2 cm s drifters drogued within the mixed layer show less slippage due to the reduced drag on the tether but they are primarily designed to measure the actual near surface currents which are strongly dependent on the wind conditions it is concluded that deep drogued drifters are a reliable device to study weakly baroclinic geostrophic currents

clay mineral distribution in surface sediments of the south atlantic sources transport and relation to oceanography abstract surface samples mostly from abyssal sediments of the south atlantic from parts of the equatorial atlantic and of the antarctic ocean were investigated for clay content and clay mineral composition maps of relative clay mineral content were compiled which improve previous maps by showing more details especially at high latitudes large scale relations regarding the origin and transport paths of detrital clay are revealed high smectite concentrations are observed in abyssal regions primarily derived from southernmost south america and from minor sources in southwest africa near submarine volcanoes of the antarctic ocean south sandwich bouvet island smectite contents exhibit distinct maxima which is ascribed to the weathering of altered basalts and volcanic glasses the illite distribution can be subdivided into five major zones including two maxima revealing both south african and antarctic sources a particularly high amount of mg and fe rich illites are observed close to east antarctica they are derived from biotite bearing crystalline rocks and transported to the west by the east antarctic coastal current chiorite and well crystallized dioctaedral illite are typical minerals enriched within the subantarctic and polarfrontal zone but of minor importance off east antarctica kaolinite dominates the clay mineral assemblage at low latitudes where the continental source rocks west africa brazil are mainly affected by intensive chemical weathering surprisingly a slight increase of kaolinite is observed in the enderby basin and near the filchner ronne ice shelf the investigated area can be subdivided into ten large scale clay facies zones with characteristic possible source regions and transport paths clay mineral assemblages of the largest part of the south atlantic especially of the western basins are dominated by chlorite and illite derived from the antarctic peninsula and southernmost south america and supported by advection within the circumantarctic deep water flow in contrast the east antarctic provinces are relatively small assemblages of the eastern basins north of 30 s are strongly influenced by african sources controlled by weathering regimes on land and by a complex interaction of wind river and deep ocean transport the strong gradient in clay mineral composition at the brazilian slope indicate a relatively low contribution of tropically derived assemblages to the western basins

variations in the reflectivity of the moho transition zone beneath the midcontinent rift system of north america results from true amplitude analysis of glimpce data true amplitude processing of the great lakes international multidisciplinary program on crustal evolution seismic reflection data from the midcontinent rift system of north america shows large differences in the reflectivity of the moho transition zone beneath the axial rift beneath the rift flanks and outside of the rift the moho reflection from the axial rift has a discontinuous diffractive character marginally stronger several decibels than an otherwise transparent lower crust and upper mantle beneath the axial rift moho is interpreted to be a synrift igneous feature beneath the rift flanks the reflectivity of the moho transition is generally well developed with two identifiable boundaries although in places it is weakly reflective to nonreflective similar to moho outside the rift the two boundaries are interpreted as the base of essentially intact although stretched prerift archean crust upper boundary and new synrift moho 1 2 s 6 7 km deeper lower boundary beneath the rift flanks the layered reflection moho transition results from the preexisting crustal composition and fabric modified by synrift igneous processes and extensional tectonic metamorphic processes the geologic evidence for extensive basaltic magmatism in the rift is the basis for interpreting the moho signature as a keweenawan structure that has been preserved for 1 1 b y extension and magmatism appear to enhance reflectivity in the lower crust and moho transition zone only where stretching factors are moderate rift flanks and not where they are extreme axial rift this leads to the prediction that the reflectivity across analogous volcanic passive continental margins should be greatest beneath the moderately stretched continental shelves and should decrease towards the ocean continent boundary

paleomagnetism and upb geochronology of volcanic rocks from michipicoten island lake superior canada precise calibration of the keweenawan polar wander track abstract paleomagnetic poles have been determined for normally magnetized samples of late proterozoic volcanic rocks from 23 sites on michipicoten island lake superior canada the results are in general accord with an earlier less precise study by one of the authors and indicate a small polar shift between rocks of the mamainse point formation 186 e 38 n and the michipicoten island formation 175 e 26 n a quartz feldspar porphyry intruding the lower mamainse point formation but unconformably overlain by the michipicoten island formation was sampled for u pb zircon dating the zircons define an age of emplacement of 1086 5 1 3 3 0 ma this age and the pole positions are consistent with and supplement ages previously determined for the keweenawan paleomagnetic polar wander curve these data define an apparent polar wander rate of 14 cm a 1 this age combined with previously published u pb ages indicates a time span of 22 ma for most keweenawan igneous activity in addition the time of the keweenawan paleomagnetic reversal is constrained to be between 1086 5 1 3 3 0 and 1097 6 3 6 ma the age of the upper part of the reversely magnetized osler group the zircon population contained an inherited component from a source 1403 50 ma old this suggests that the porphyry magma was at least partly derived by melting of an anorogenic pluton emplaced during the mid proterozoic

crustal structure of the central graben of the midcontinent rift beneath lake superior abstract travel times and amplitudes of refracted and reflected arrivals observed during glimpce great lakes international multidisciplinary program on crustal evolution from the midcontinent rift in lake superior have been modeled using two dimensional ray tracing techniques forward modeling was used to iteratively refine an initial velocity model that was constructed from results of tau sum analysis of the travel time data at each station combined with information from a coincident common midpoint cmp reflection profile when converted to time the resulting model agrees quite well with the cmp reflection profile within the central graben of the midcontinent rift approximately 2 km of sedimentary rock 2 8 4 6 km s overlie an 8 km thick sequence of volcanics and interflow sediments 5 0 6 5 km s within the central rift graben observed on the reflection data beneath this sequence is a 6 8 km thick sequence of 6 6 7 0 km s material interpreted to represent metamorphosed volcanics the velocity of the material at the base of the rift graben is not well constrained approx 7 0 7 2 km s but probably comprises an additional 10 12 km of metavolcanic rocks and intrusions that extend to the base of the graben as imaged on the coincident seismic reflection profile the central graben is ranked by reverse faults the isle royal and keweenawan faults that probably represent normal faults associated with graben formation that were reactivated during the late precambrian a 50 100 ms time advance anomaly associated with the isle royal fault is observed at every station this anomaly has been modeled as shallow high velocity blocks located directly beneath the fault the blocks correlate well with the walls of a steep sided bathymetric trough and are believed to represent highly indurated upper keweenawan sediments resulting from hydrothermal alteration a marked decrease in the apparent velocity and amplitude of the first arrivals is observed on reversed sections at offsets exceeding 100 km and has been modeled as pre rift continental crustal rocks with a velocity of about 6 5 km s adjacent to the 7 0 km s material in the graben calculation of the gravity response of the seismic model demonstrates that the gravity high centered over the rift can be entirely attributed to high density rocks occupying the central half graben imaged on the cmp profile wide angle reflections from about 15 30 km depth beneath the flanks of the graben indicate the presence of velocity discontinuities that may represent rift related detachment surfaces and or pre rift structures within the pre rift crust additional wide angle reflections define the top and bottom of high velocity lower crustal layer the high velocity lower crustal layer is thickest beneath the rift graben and is interpreted to represent magmatic material underplated to the crust during rifting as discussed by trehu et al 1991 total crustal thickness beneath the lake varies from about 55 km beneath the graben to about 35 km beneath the southern shore and 40 km beneath the northern shore

palaeopedogenesis of red palaeosols in yunnan plateau southwestern china pedogenical geochemical and mineralogical evidences and palaeoenvironmental implication abstract red palaeosol as an important archive of ancient pedoenvironments effectively reflects the palaeoenvironmental change and tectonic uplift history of the plateau four red palaeosol profiles were collected at 2200 to 2400 m elevations from the yunnan plateau yp to investigate the pedogenic features chemical weathering process and mineral assemblages of soils pedological geochemical and mineralogical techniques were used to understand the palaeopedogenic palaeoenvironmental change and landscape evolution of the plateau pedologically the red palaeosols were characterized by the dark red color a hue of 5yr or redder strong acidity ph 50 g kg 1 and clay 50 contents the red palaeosols were enriched with fe 2 o 3 al 2 o 3 and tio 2 with very low contents of cao mgo na 2 o and k 2 o and slightly low contents of si 2 o and mno chemical weathering indices such as cia cia al 2 o 3 al 2 o 3 cao na 2 o k 2 o 100 sa sa sio 2 al 2 o 3 saf saf sio 2 al 2 o 3 fe 2 o 3 and a cn k diagram al 2 o 3 cao na 2 o k 2 o indicated that the red palaeosols had experienced strong chemical weathering processes scanning electron microscopy analyses revealed that quartz grains of the red palaeosols were characterized by a number of deep dissolution pits and cracks on their surfaces the clay minerals of the red palaeosols were composed of kaolinite gibbsite hematite and vermiculite in the order of abundance pedogenical geochemical and clay mineralogical evidences revealed that the red palaeosols were developed by palaeopedogenic processes under tropical climatic environments the presence of highly weathered soils at an altitude of 2200 2400 m indicated the influence of tectonic uplift on the soil vertical distribution it is estimated that the red palaeosols had uplifted about 1600 2000 m since their initial formation palaeopedogenesis of red palaeosols provides new insight into the palaeoclimatic and palaeoenvironmental reconstruct and tectonic movement of yp

evolution of weathering patterns in the indo burman ranges over the last 280 kyr effects of sediment provenance on 87sr 86sr ratios tracer a high resolution study of mineralogy and major element geochemistry combined with sr and nd isotopes has been conducted on high sedimentation rate cores collected off the irrawaddy river mouth in the andaman sea and the bay of bengal to reconstruct the erosional and weathering history of the irrawaddy river basin in both cores nd 0 values imply that both glacial and interglacial sediments share a common crustal source the irrawaddy river strong glacial interglacial cycles are recorded by 87sr 86sr interglacial periods yield values between 0 713 and 0 717 whereas glacial periods show higher values between 0 717 and 0 719 variations of the pedogenic clays smectite and kaolinite to primary mineral feldspar quartz illite and chlorite ratios show strong precessional cycles suggesting a control by past changes in the summer monsoon intensity each increase in pedogenic clays content is also associated with a net loss of labile elements na k and ca from the detrital minerals under chemical weathering wet periods of summer monsoon reinforcement correspond to an increase in weathering of the irrawaddy plain soils and a decrease of 87sr 86sr ratio plotting 87sr 86sr versus 87rb 86sr gives a pseudo isochrons interpreted as a mixing line representing the strength of chemical weathering during glacial stages enhanced physical erosion induced by glacier scour and frost action in the highland of the irrawaddy river basins produced high volumes of unaltered rb rich minerals the low sea level of glacial times constricted the river to the main channel in the lower reaches and permitted an efficient transport of unaltered rb rich minerals with high radiogenic sr composition from the high relief of the indo burman ranges and the tibetan plateau to the indian ocean

hydrocarbon source rock evaluation of middle proterozoic solor church formation north american mid continent rift system rice county minnesota hydrocarbon source rock evaluation of the middle proterozoic solor church formation keweenawan supergroup as sampled in the lonsdale 65 1 well rice county minnesota shows that 1 the rocks are organic matter lean 24 of 25 samples have less than 0 8 organic carbon 2 the organic matter is thermally post mature probably near the transition between the wet gas phase of catagenesis and metagenesis dry gas zone and 3 the rocks have minimal potential for producing additional hydrocarbons genetic potential 0 30 mg hc g rock although no direct evidence exists from which to determine maximum burial depths the observed thermal maturity of the organic matter requires significantly greater burial depths a higher geothermal gradient or both it is likely t least on the saint croix horst that thermal maturation of the organic matter in the solor church took place relatively early and that any hydrocarbons generated during this early phase were probably lost prior to deposition of the overlying middle proterozoic fond du lac formation keweenawan supergroup

formation and fate of sedimentary depocentres on southeast asia s sunda shelf over the past sea level cycle and biogeographic implications abstract sea level variations are the major factor controlling sedimentation as well as the biogeographic patterns at continental margins over late quaternary times fluctuations on millennial time scales produce locally complex deposits in coasts and on shelves associated with short term influence on species development this article reviews the sedimentary and biogeographic history of the tropical siliciclastic sunda shelf as an end member of continental shelves regarding extreme width an enormous sediment supply and highest biodiversity in response to rapid sea level fluctuations we describe particular depositional segments as part of a genetic succession of zones from land to the deep sea based on literature data field observations and calculation of hydro isostatic adjustment effects on changing relative sea level these segments are characterized by individual sedimentary processes and deposits and by a specific potential for material storage and re mobilization long term regressive intervals led to overall sigmoidal promoting extremely thick and wide succeeding units in contrast rapid lateral shifts of defined depocentres over long distances took place in response to short term sea level fluctuations fully isolated small scale sediment bodies formed when sea level changed at exceptionally high rates as a result of the high availability of organic rich sediments mangrove and freshwater peats formed frequently over late quaternary times the appearance of thick massive and widespread peats is mainly linked to time intervals of a sea level rise at slow rates whilst organic matter appears much more dispersely in the sediments during episodes of rapidly changing sea level the preservation potential of the regressive units is generally high due to highest initial sediment supply stabilizing soil formation during exposure and rapid subsidence preservation of depositional elements from other periods is more exceptional and either restricted to local morphological depressions or to episodes of rapid sea level change besides complex channel incision an overall lowering of the sediment surface related to erosion as deep as 20 m or more over wide areas took place mainly during sea level lowering the final export of shelf material is documented by enormous mass wasting packages on the associated continental slope from a palaeogeographic perspective the rapid formation or disappearance of special habitat zones such as mangrove fringes and extended mud flats led to species establishment or truncation in distribution in addition the opening or closure of ocean passages as narrow bridges allowing limited species crossing or as fully colonized corridors had severe impact on eco fragmentation and the expansion or contraction of species independent of such particular conditions sea level changes have been too rapid over the past climatic cycle to allow full regeneration and mature development of coast related ecosystems

natural controls of fluvial denudation rates in major world drainage basins we present a new compilation of estimates of modern rates of mechanical and chemical denudation for externally drained basins exceeding 5 105 km2 in area these estimates are based on sediment and solute load data selected in order to represent natural rates as far as possible chemical denudation rates have been calculated by deducting the nondenudational component of solute load mechanical denudation rates range from 1 mm kyr 1 for the st lawrence and dnepr basins to 670 mm kyr 1 for the brahmaputra basin chemical denudation rates vary from 1 mm kyr 1 kolyma niger nile and rio grande basins to 27 mm kyr 1 chiang jiang basin the kolyma basin has the lowest 4 mm kyr 1 and the brahmaputra basin the highest overall rate of denudation 688 mm kyr 1 relationships between denudation rates and a range of morphometric hydrologic and climatic variables are investigated through correlation and regression analysis morphometric variables such as mean local relief are accurately calculated for large basins for the first time by using the national geophysical data center 10 minute topographic database variables expressing basin relief characteristics and runoff are found to be most strongly associated with both mechanical and chemical denudation rates with more than 60 of the variance in total denudation being accounted for by basin relief ratio and runoff basin area runoff variability and mean temperature however are only weakly associated with rates of denudation although direct comparisons cannot be made it appears that rates of basin denudation derived from present day mass flux estimates are not overall significantly different from estimates of long term rates based on sediment volume and thermochronologic data it therefore appears that the key factors identified as controlling denudation rates here are also applicable to the geological time spans relevant to the interaction between tectonic and denudational processes

chemical weathering in luzon philippines from clay mineralogy and major element geochemistry of river sediments clay mineralogy and major element geochemistry of 35 surface sediment samples collected in 21 major to moderate rivers of luzon philippines are used to evaluate the present chemical weathering process the clay mineral assemblage consists mainly of smectite average 86 with minor kaolinite 9 and chlorite 5 and very scarce illite 1 and does not show strong island wide differences the major element results of both bulk and clay fraction sediments indicate that the formation of clay minerals is accompanied by leaching of ca and na first and of fe and mn thereafter during the chemical weathering process a low moderate chemical weathering degree of bulk sediments and a moderate intensive degree of clay fraction sediments are obtained in luzon rivers based on proxies of chemical index of alteration cia and smectite crystallinity it is suggested that the majority of andesitic basaltic volcanic and sedimentary rocks along with the tectonically active geological setting and sub tropical east asian monsoon climate are responsible for the predominance of smectite in the clay mineral assemblage

neogene arc continent collision in sabah northern borneo malaysia abstract the neogene evolution of sabah can be interpreted in terms of an arc continent collision initiated during early middle miocene time and followed by intraplate shortening the latter being active in this area at the present day the volcanic arc of late oligocene middle miocene age is imbricated with melanges and thrust northwestward on a polyphase deformed turbiditic sequence of late cretaceous middle miocene age crocker range intraplate shortening is seen in the thrusting and folding offshore sabah along the palawan north borneo trench and also by broad open folds and strike slip faulting in the previously sutured terranes this collision can either be interpreted as the final stage of southward subduction of the hypothetical proto south china sea or the result of back thrusting of the sulu volcanic arc resulting from the subduction of the celebes sea toward the north

climatic and tectonic controls on weathering in south china and indochina peninsula clay mineralogical and geochemical investigations from the pearl red and mekong drainage basins 1 results of clay mineralogy major element geochemistry and sr and nd isotopes in 93 argillaceous samples collected from drainage basins of the pearl red and mekong rivers reveal different degrees of chemical weathering in southeast asia despite similar climate conditions across these regions the kaolinite illite ratio illite chemistry index and illite crystallinity can be used as indicators of chemical weathering intensity these mineralogical proxies combined with the k2o na2o cao molar ratio chemical index of alteration cia and weathering trends observed from major element results indicate intensive silicate weathering in the pearl river basin moderate to intensive in the mekong river basin and moderate in the red river basin although a significant modification of epsilon nd 0 values in our riverine sediments during chemical weathering and transport is unlikely sr 87 sr 86 ratios are controlled by various states of chemical weathering of high sr minerals such as plagioclase rich in na and ca with a linear decrease trend from the pearl mekong to red river basins our results suggest that it is not the warm climate with heavy monsoon precipitation but tectonics playing the most significant role in controlling weathering and erosion processes in south china and indochina peninsula strong physical erosion caused by tectonic activities and river incision along the eastern margin of the tibetan plateau and along the red river fault system is responsible for high contents of primary minerals in the lowlands of red and mekong river basins

detrital fine grained sediment contribution from taiwan to the northern south china sea and its relation to regional ocean circulation article i nfo results of clay mineralogy in 140 samples collected in major rivers and lakes in southwestern taiwan and on the seafloor off taiwan in the northeastern south china sea scs combined with clay mineral records of the pearl river drainage basin rivers in luzon and the south china shelf and slope are used to semi quantitatively evaluate the detrital fine grained sediment contribution of taiwan to the northern scs the clay mineral assemblage of the taiwan sourced sediments consists dominantly of illite average 56 and chlorite 41 with very scarce kaolinite and smectite their respective distribution from the rivers and lakes to the seafloor off taiwan does not show obvious basin wide differences linear correlations of illite chemistry index with illite crystallinity and of illite crystallinity with kaolinite present two end members of provenances the pearl river and taiwan for the south china shelf and slope assuming that kaolinite in the northern scs is provided completely from the pearl river the contribution of taiwan in clay minerals is evaluated as 29 to the south china shelf and 23 to the south china slope respectively accordingly the contribution of the pearl river to the south china shelf and slope is 52 and 31 respectively the luzon arc accounts for the rest of clay mineral components for the northern scs mainly by providing smectite the bashi strait crossed branches of the southward deep north pacific deep water and the northward surface kuroshio current in the western pacific may transport taiwan sourced suspended sediments westwards to the northern scs

an integrated study of geochemistry and mineralogy of the upper tukau formation borneo island east malaysia sediment provenance depositional setting and tectonic implications abstract an integrated study using bulk chemical composition mineralogy and mineral chemistry of sedimentary rocks from the tukau formation of borneo island sarawak malaysia is presented in order to understand the depositional and tectonic settings during the neogene sedimentary rocks are chemically classified as shale wacke arkose litharenite and quartz arenite and consist of quartz illite feldspar rutile and anatase zircon tourmaline chromite and monazite all of them are highly matured and were derived from a moderate to intensively weathered source bulk and mineral chemistries suggest that these rocks were recycled from sedimentary to metasedimentary source regions with some input from granitoids and mafic ultramafic rocks the chondrite normalized ree signature indicates the presence of felsic rocks in the source region zircon geochronology shows that the samples were of cretaceous and triassic age comparable ages of zircon from the tukau formation sedimentary rocks granitoids of the schwaner mountains southern borneo and tin belt of the malaysia peninsular suggest that the principal provenance for the rajang group were further uplifted and eroded during the neogene additionally presence of chromian spinels and their chemistry indicate a minor influence of mafic and ultramafic rocks present in the rajang group from a tectonic standpoint the tukau formation sedimentary rocks were deposited in a passive margin with passive collisional and rift settings our key geochemical observation on tectonic setting is comparable to the regional geological setting of northwestern borneo as described in the literature

paleomagnetism structure and longitudinal correlation of middle precambrian dykes from northwestern ontario and minnesota a primary magnetization about 2 1 ga old is well preserved in the kenora kabetogama dyke swarm in minnesota and ontario its direction the mean for 12 dykes incorporating results from 30 paleomagnetic sites is 115 9 55 6 yielding a paleomagnetic pole position of latitude 42 7 s longitude 4 2 e dm 7 3 dp 5 2 the primary nature of the magnetization can be demonstrated using geochemistry to correlate individual dykes over distances that can exceed 300 km whereas the remanence direction varies between dykes it remains constant along a dyke sufficient proof that it was formed during initial cooling of the intrusion a weak longitudinal increase in hydrous alteration mgo content and degree of magnetic overprinting is observed as the swarm is followed to the south and may reflect increasing proximity to a precambrian continental margin that underwent rifting sedimentation and deformation during the time interval 2 2 1 6 ga bp although the cause of the longitudinal changes in terms of ma

staged fine grained sediment supply from the himalayas to the bengal fan in response to climate change over the past 50 000 years abstract the bengal fan as the largest submarine fan in the world receives a large amount of sediments discharged from the himalayas through the ganges brahmaputra g b river system however previous studies of this system seldom focused on the role of channels which are widely developed in the bengal fan in sediment transportation over the last glacial cycle here we discuss the coupled sedimentary archive in two gravity cores along the active channel in the bengal fan our findings are based on grain size clay minerals and sr and nd isotope compositions of these two sediment cores end member modeling of grain size data reveals that the intermediate end member represents the flux of distal fluvial particles from the g b river system the fine end member denotes regional sediment supply of weathered volcanic materials especially from the eastern indian peninsula and the coarse end member probably reflects nearby terrigenous input aeolian input and or volcanic glass sediment provenance analysis based on clay minerals and sr and nd isotopes confirmed sediment supply from the g b river system which was characterized by high illite percentage high 87sr 86sr ratios and low end values especially during five stages 50 45 ka 42 37 ka 31 28 5 ka 24 20 ka and 14 9 ka during these stages heavy fine grained sediment supply from the g b river system was discharged into the study area under the influence of climate change rather than sea level fluctuation additionally emphasizing the importance of channels in the sediment transport process moreover we find a significant alteration of sediment sources at 26 ka which may be related to conversion of the deep water currents in the bay of bengal

transpressive tectonism in the eastern penokean orogen canada consequences for proterozoic crustal kinematics and continental fragmentation abstract results of a structural analysis of archean basement huronian cover and various igneous rocks in the sudbury area ontario and regional tectonometamorphic correlations suggest that structures in the eastern penokean orogen formed dominantly by dextral transpression bulk horizontal shortening started with the ca 2 4 to 2 2 ga blezardian orogeny and was accompanied by peak metamorphism and local granitoid magmatism during the 1 89 to 1 83 ga penokean orogeny horizontal dextral shear parallel to the curved axis of the eastern penokean orogen gained in importance and is most likely related to docking of proterozoic magmatic arc terranes to the superior province margin this tectonic activity led to distributed heterogeneous deformation of archean basement rocks and exhumation of deep crustal rocks in the sudbury area and the kapuskasing structural zone but may have failed to accrete paleoproterozoic terranes in the lake huron area the lack of evidence for horizontal extension following blezardian and penokean contraction suggests that crustal fragmentation of the archean supercontinent started at 2 45 ga finally the great lakes tectonic zone and the murray fault master dislocations in the penokean orogen may have acted as crustal weakness zones which influenced the orientation and development of 1 1 ga midcontinent rift segments

weathering of the ganga alluvial plain northern india implications from fluvial geochemistry of the gomati river abstract in the northern part of the indian sub continent the ganga alluvial plain gap feeds its weathering products to the ganga brahmaputra river system one of the world s largest fluvial systems the authors present a geochemical study of the gap weathering products transported by the gomati river the ganga river tributary to understand weathering processes of an alluvial plain in a humid sub tropical climate a total of 28 sediment samples were collected during the monsoon season and were analysed by x ray fluorescence spectrometry for 25 major and trace elements bulk chemistry of the channel flood and suspended sediments mostly consists 90 80 and 75 respectively of three elements al si and fe major element concentrations normalised with respect to upper continental crust ucc show strong depletion of highly mobile elements na ca and enrichment of immobile elements ti si silica enrichment in the sand fraction is probably caused by chemical weathering of feldspar mineral sorting during fluvial transportation acts as the single important factor that controls the geochemistry of these weathering products and also strongly influences major and trace element distribution in the individual sediment samples trace element ba cr cu nb ni pb v and zn concentrations were strongly correlated with major element si al fe mn and k concentrations indicating that the abundance of trace elements is controlled by the same processes that control the major element distribution in these sediments the gap weathering products were geochemically distinguished as arkose to litharenite in rock classification chemical mobility normalised with respect to tio 2 in ucc indicates that si na zr ba and sr mainly derived from feldspar muscovite and biotite are lost during weathering iron and zn remained immobile during weathering and were strongly adsorbed by phyllosilicates and concentrated in fine grained sediment fractions the chemical index of alteration indicates that the gap has experienced chemical weathering of incipient to moderate intensity the gap weathering products also demonstrated a progressive incomplete alteration in the alluvial sequence made up of the himalayan derived sediments a model has been proposed to better understand weathering processes and products of the gap in temporary storage of 50 ka in a humid sub tropical climate

p wave teleseismic traveltime tomography of the north american midcontinent the remains of the 1 1 ga midcontinent rift mcr lie in the middle of the tectonically stable portion of north america previous and ongoing studies have imaged strong heterogeneity associated with the mcr in the crust but have not imaged such within the mantle it is unclear whether this is due to the absence of rift related mantle structures or these studies had insufficient resolution to image them to address this issue we measured 46 374 teleseismic p wave delay times from seismograms recorded by the usarray transportable array superior province rifting earthscope experiment and surrounding permanent stations we included these and 54 866 delay times from prior studies in our tomographic inversion we find that high velocity anomalies are widespread in our study area but there are also prominent low velocity anomalies two of these are coincident with high bouguer gravity anomalies associated with the mcr in iowa and the minnesota wisconsin border at 50 to 150 km depth extensive resolution testing shows that these anomalies could be the result of downward vertical smearing of relatively low velocities from rift related material that underplated the crust although we cannot exclude that the subcrustal mantle lithosphere beneath the mcr is anomalously enriched hydrated or warm other anomalies occur at syntaxes of the penokean orogen one with the superior province and marshfield terrane in southern minnesota and another with the yavapai and mazatzal terranes both at 100 to 250 km depth in the midmantle we image two linear high velocity anomalies interpreted as subducted fragments of the farallon and kula plates

evolution and variability of the asian monsoon system state of the art and outstanding issues the asian monsoon is comprised of the indian and east asian subsystems these two components are linked to one another in varying degrees by regions of strong sensible heating indo asian landmass and strong latent heat export the western pacific warm pool and the southern subtropical indian ocean variability within the indian and east asian subsystems interactions among them and the extent to which they interact with other climate phenomena e g enso are current topics of modern and paleoclimate research this work provides an overview of past and current paleomonsoon research on tectonic to interannual time scales with a primary focus on marine sediment records while past and current work has contributed greatly to our understanding of paleomonsoon variability at all time scales additional efforts are required to make further progress in two critical areas 1 additional efforts are needed in terms of proxy development and evaluation requiring concerted efforts at long term sediment trap deployments in key monsoon influenced regions as well as development of adequate and widely available core top databases these are necessary to assess the impact of modern oceanographic and seafloor processes on potential monsoon proxies 2 additional effort is also needed in acquiring a sufficient geographic distribution of downcore records to assess linkages among the subsystems and their role in the context of extratropical climate change these records should have high sedimentation rates including varved sections this requires substantial survey support to identify the most appropriate coring and drilling targets c 2004 elsevier ltd all rights reserved

holocene climatic optimum in southwest africa evidence from the marine clay mineral record abstract sediment composition grain size and clay mineral record of a high resolution sediment core from the continental slope off namibia was investigated to gain information on the deposition of terrigenous matter in this part of the southwest african continental margin during the last 18 k y the depositional processes involved are fluvial input by the kunene river and eolian input from the namib and kalahari deserts each supplying characteristic mineral suites during low sea level erosion of the exposed shelf yields additional material the amount of eolian or fluvial matter depends on the strength of the transport process which is a function of aridity or humidity of the source area thus allowing paleoclimatic interpretations arid conditions prevailed during a low sea level from 18 to 15 ka and unconsolidated shelf sediment was mobilized and supplied to the slope by short distance transport by southerly winds a dramatic increase in the accumulation of terrigenous sediment is recorded from 15 to 10 ka without major changes in sediment composition which is attributed to increased runoff of the kunene river and fluvial erosion of shelf sediments this period coincides with a strengthening of the monsoonal system during a precessional minimum which is observed in numerous sites in central africa and indicates an intensified influence of the monsoon at the kunene headwaters a distinct shift in clay mineralogy towards river derived material marks a second period of increased river runoff during high sea level from 9 to 5 ka indicating maximum humidity in the source area from 6 to 5 ka this corresponds to the holocene climatic optimum observed in the arid belts of northern africa the present balance between fluvial and eolian input was reached approximately at 4 ka

the midcontinent rift system one of the most prominent features on gravity and aeromagnetic maps of the united states is a series of major generally linear anomalies that extend from central kansas to lake superior and then turn southward into central michigan figures 1 2 this system of anomalies clearly reflects major geological features and as such it must represent a very significant episode in the history of the north american continental lithosphere the geologic features associated with these anomalies particularly axial basins filled with basalt and immature clastic rocks along with evidence of crustal extension indicate that the episode was probably one of incipient rifting