stratigraphic evolution of the late holocene ganges brahmaputra lower delta plain sediment cores from the ganges brahmaputra delta in bangladesh were examined for sedimentological character clay mineralogy elemental trends c n s and 14c geochronology to develop a model for the sedimentary sequence resulting from lower delta plain progradation in the late holocene a widespread facies succession from muddy sand to interbedded mud records progradation of shoal island complexes and the transition from subtidal to intertidal conditions mangrove vegetated islands and peninsulas represent the final phase of progradation a mottled mud that is deposited by penetration of turbid coastal water into the mangroves during high water events organic matter preservation is generally low 1 toc in most of these well drained deposits that are characterized by a permeable silt dominated granulometry clay mineralogy in the cores records the relative influence of smectite and kaolinite rich ganges sediments and illite and chlorite rich brahmaputra material the lower delta plain west of the modern river mouths was deposited as a ganges dominated delta in three phases since 5000 cal years bp with brahmaputra influence confined to the meghna estuary area and to the supratidal section of western delta deposits evolution of the lower delta plain in the late holocene was influenced by regional subsidence patterns in the tectonically active bengal basin which controlled distributary channel avulsion and migration and the creation of accommodation space

detrital zircon u pb dating of low grade metamorphic rocks in the sulu uhp belt evidence for overthrusting of the north china craton onto the south china craton during continental subduction the shiqiao pingshang low grade metasedimentary rocks sporadically crop out in the sulu ultrahigh pressure uhp belt in east central china major and trace element data indicate that they were deposited in a passive margin basin probably located at the southern margin of the north china craton sensitive high resolution ion microprobe shrimp u pb dating of detrital zircons from a quartzite sd53 from pingshang and a quartz schist sd54 from shiqiao records ages ranging from 2800 to 1900 ma three age populations are defined at 2660 2500 2350 2150 and 2100 1900 ma with peak ages at 2522 2212 and 2020 ma respectively muscovite ar ar dating of the quartz schist sd54 yields a plateau age of 265 9 1 2 ma these data support the view that the shiqiao pingshang low grade metasedimentary rocks originated from the north china craton and underwent deformation in the permian during subduction of the south china craton yangtze block beneath the north china craton this requires that the low grade metasedimentary rocks were overthrust for several tens of kilometres onto the south china craton explaining why source materials from the north china craton are common in triassic to jurassic sedimentary basins adjacent to the dabie sulu orogenic belt

ultrahigh pressure metamorphism in the magnesite aragonite stability field evidence from two impure marbles from the dabie sulu uhpm belt two magnesite bearing impure dolomitic marbles from the dabie sulu ultrahigh pressure uhp region have been investigated to clarify if they had actually attained p t conditions outside the dolomite stability field limited by the reaction dolomite aragonite magnesite and to test their potential for recording u hp conditions in both cases the silicate mineral assemblage records conditions around the terminal amphibole breakdown reaction amphibole aragonite quartz clinopyrox ene talc which is a good geobarometer between at least 2 0 and 2 6 gpa at higher pressures the terminal breakdown of talc to clinopyroxene coesite is the last p t milestone below 4 gpa recordable by the silicate assemblage but evidence of former coesite is ambiguous the dolomite dissociation curve becomes strongly divariant in fe bearing marbles and may thus be attainable during cold subduction near the 5 ck m 1 geotherm at least one of the samples from xinyan village near taihu dabie shan preserved relicts of both magnesite and aragonite and most likely attained conditions within the aragonite magnesite stability field for the second sample from sanqingge village in the sulu terrane no certain evidence has been found in this study impure dolomitic marbles have considerable potential to preserve ultra high pressure relicts particularly massive impure marbles that have not been pervasively infiltrated by fluids during exhumation are considered to preserve high pressure relicts better than metapelites paragneisses and orthogneisses and perhaps almost as well as eclogites or metaperidotites the inconspicuous mineral assemblage clinopyroxene talc or quartz after former coesite may in fact record uhp conditions

ultrahigh pressure texture inheritance during retrogression evidence from magnetofabrics in eclogites and ultramafic rocks chinese continental scientific drilling project abstract in order to contribute to a better understanding of exhumation related retrogression processes ultrahigh pressure uhp mafic and ultramafic rocks from the chinese continental scientific drilling ccsd in the maobei eclogite body of the sulu ultrahigh pressure metamorphic belt in eastern china were studied for their magnetofabrics variably retrogressed eclogites and serpentinized ultramafic rocks were retrieved from the depth interval of 100 to 1000 m of the borehole a vein network of irregular shaped veins with a retrograde metamorphic assemblage cuts across the eclogite foliation at both low and high angles sem imaging of the eclogites documents that magnetite associated with retrograde pargasitic amphiboles developed around shape preferred garnet sem imaging of the serpentinized ultramafic rocks documents that magnetite rims grew around shape preferred garnet and that magnetite formed within a mesh texture related to serpentinization syn serpentinization magnetite growth increased bulk susceptibilities but reduced anisotropy maximum susceptibility axes from both eclogites and serpentinized garnet peridotites trend n s i e parallel with the stretching lineation as defined by olivine and omphacite grains this implies that the magnetic fabric mimics the uhp fabric and survived retrogression and that the fabric is inherited from the uhp stage as a consequence retrogression was not associated with substantial ductile deformation and the mafic ultramafic maobei body behaved as a rigid body within a ductile deforming quartzo feldspathic matrix during exhumation internal strain is restricted to brittle fracturing associated fluid circulation and vein formation facilitating retrograde reactions in the mafic ultramafic rocks

subaqueous delta of the ganges brahmaputra river system abstract the ganges brahmaputra is among the world s three largest river systems in terms of sediment load but until now no high resolution seismic data have been obtained to document the nature of the sediment deposit seaward of the rivers mouths the other two amazon huanghe discharge into energetic coastal environments and form subaqueous deltas with characteristic clinoform stratigraphy high resolution seismic reflection profiles of the bengal shelf reveal similar stratigraphy topset beds dip gently 0 036 and diverge offshore more steeply dipping foreset beds 0 190 converge farther seaward and relatively thin gently dipping bottomset beds 0 022 extend across the outer shelf overlying an erosional surface presumed to be of late pleistocene age sediment accumulation rates are highest in the foreset region 5 cm year and reduced in the bottomset region

the submarine delta of the ganges brahmaputra cyclone dominated sedimentation patterns the ganges brahmaputra belongs to the world s largest rivers in terms of sediment discharge but little is known yet about the processes controlling sediment distribution on the shelf and the documentation of these processes in large and small scale sedimentary structures for this study the submarine delta of the river system was investigated by high resolution seismic reflection profiles and piston and gravity coring logged physical properties of the cores were combined with visual core description grain size analysis and radiography sedimentation rates were estimated by 210pb and 137cs gamma spectrometry and cross checked with evaluation of seismic profiles the data reveal that the broad delta topset 20 m waterdepth is formed by a thick layer of sand silt highest sedimentation rates about 10 cm a 1 occur at the centre of clinoform delta foreset beds where about 20 of the total riverine sediment load is deposited and thus cause an annual progradation of the subaqueous delta front by about 15 m the foreset beds consist of graded sand silt layers which are thought to be deposited by sediment laden flows generated during the surge of tropical cyclones tempestites interbedded with silty clay layers the head of a canyon the swatch of no ground that deeply incises into the shelf forms a sediment trap along the westward cyclonic transport path where sedimentation rate increases to about 50 cm a 1 episodically earthquakes or storms destabilize especially the rapidly accumulated foreset beds in the eastern delta creating up to 8 m thick mass flows on the outer shelf in water depth beyond 80 m no holocene sediment is deposited

microfabrics of uhp metamorphic granites in the dora maira massif western alps no evidence of deformation at great depth in the dora maira massif western alps essentially undeformed ultrahigh pressure uhp metamorphic granites brossasco granite are embedded in and locally grade into granite gneisses or augengneisses and mylonites in this study the quartz microfabrics of the undeformed granites are compared against the augengneisses and mylonites in a representative number of samples from several locations in the undeformed granites the fine grained quartz aggregates that formed from coesite upon decompression are characterized by a foam structure and random crystallographic orientation in the deformed granites the quartz microstructures and the crystallographic preferred orientation cpo indicate deformation by dislocation creep most of the deformation of the granites if not all must have happened at a late stage during exhumation after transformation of coesite to quartz at greenschist facies conditions in the middle crust the deformed granites provide no evidence of deformation during subduction at u hp metamorphic conditions and in the earlier stages of exhumation the diameter of internally undeformed slices of continental crust subducted to and exhumed from about 100 km can exceed that of the presently exposed brossasco granite i e it can be on the kilometre scale

acoustic facies and depositional processes in the upper submarine canyon swatch of no ground bay of bengal abstract the swatch of no ground song is a canyon that incises the bengal shelf deeply over more than 100 km length landward of the shelf edge and 100 km west of the ganges brahmaputra meghna river mouth it acts as conduit for the fluvially discharged sediment that bypasses the shelf and reaches the deep sea fan what transport deposition and mass wasting processes contribute to the sediment export and what sediment structures and main pathways are associated with these processes is studied by closely spaced bathymetric hydrosweep swath sounder and parasound sediment echosounder profiles over the upper song an analogue side scan sonar profile completes the acoustic data set from the parasound sediment echosounder data three main acoustic facies types related to undisturbed normal deposition mass wasting and erosive transport are defined and mapped with respect to the hydrosweep bathymetry of the song generally nearly all facies types show an unusually high sediment echosounder signal penetration of 80 100 m indicating that unconsolidated high porosity sediments occur on the canyon floor flanks and banks additionally the acoustic facies distribution map yields that about half of the canyon floor is covered by undisturbed well stratified fine grained muddy and coarse grained graded beds they are deposited from a suspension plume transported to the canyon either during fine weather conditions by westward oriented tidal currents or by storm events that mobilize sediments on the inner shelf curved or inclined faults or growth faults are found at all steep flanks indicating that active mass wasting processes occur along all channels and gullies on both canyon flanks such mass wasting processes are probably initiated by severe storms earthquakes high sediment load or excess pore pressure and lead to slides slumps or mudflows they stop in regions with low inclination angle and form acoustically transparent deposits of various extents on the channel and gully floors and on the southwestern canyon thalweg current marks identified on the canyon floor by high backscatter amplitudes in the side scan sonar images indicate that low density turbidity currents episodically flow down the canyon they are generated in an area of very high sedimentation rates close to the canyon head by high energy conditions like severe storms or earthquakes additionally a small gravity driven flow along the main channel s obviously continuously erode the steeper northeastern branches and inhibit sediment deposition in the more gentle southwestern channel below 450 500 m water depth together with larger scale mudflows and low density turbidity currents this gravity driven flow contributes to an active growth of the channel levee system in the deep sea fan

early mesozoic unroofing pattern of the dabie mountains china constraints from the u pb detrital zircon geochronology and si in white mica analysis of synorogenic sediments in the jianghan basin abstract this paper presents the results of an integrated u pb detrital zircon geochronology and si in white mica analysis for synorogenic sediments in the jianghan basin to the south of the dabie orogen the results provide an improved understanding of the provenance of these sediments and the unroofing pattern of the early mesozoic dabie mountain si contents of detrital white micas range from 3 09 to 3 34 atoms pfu for the upper triassic sandstones whereas 3 06 to 3 59 atoms pfu for the lower and middle jurassic sandstones the majority of detrital white micas in the lower jurassic sandstones is phengitic and originated exclusively from the dabie high to ultrahigh pressure rocks the u pb dating results of the detrital zircons for seven samples suggest that these synorogenic sediments have a significant change of provenance from late triassic to early and middle jurassic for the upper triassic sandstone the u pb age clusters of these zircons are characterized by 420 450 ma 750 820 ma 1050 1200 ma and 2500 ma with minor luliangian 1700 2000 ma components in contrast the zircon ages of the jurassic sandstones are dominated by the luliangian 1700 2000 ma ages with only minor caledonian 420 450 ma and greenville 1050 1200 ma ages in combination with other available geological data it can be concluded that the dabie hp uhp rocks might initially be exposed to the surface at the beginning of early jurassic 190 ma the jiangnan terrain also named jiangnan old continental in chinese to the south of the jianghan basin provided the predominant supply of upper triassic sediments whereas the paleoproterozoic yangtze crustal materials overlying the present dabie complex at the time were the important provenance of the jurassic sediments in the jianghan basin

suspended sediment response to semidiurnal and fortnightly tidal variations in a mesotidal estuary columbia river u s a abstract a turbidity maximum is the dominant feature of the suspended sediment concentration field in the columbia river estuary u s a in this estuary the turbidity maximum is an unsteady feature which changes on semidiurnal fortnightly and seasonal time scales in response to the semidiurnal tides the turbidity maximum is advected landward and seaward and may have an excursion of approximately 20 km in response to the fortnightly tidal variations the turbidity maximum degenerates or decreases in concentration during neap tides and regenerates or increases in concentration during spring tides finally in response to seasonal freshwater discharge variations suspended sediment concentrations in the turbidity maximum increase and its general location is pushed seaward during high discharge conditions and conversely concentrations decrease and its general location is farther landward during low discharge conditions the magnitude of suspended sediment concentration variations for these three time scales is similar therefore any consideration of estuarine dynamics and the suspended sediment concentration field in the columbia river estuary should include the semidiurnal and fortnightly time scales as well as the seasonal ones because the columbia is a mesotidal estuary and fortnightly tidal variations are not extremely large it is expected that these time scales and associated processes may be important in many other estuarine systems

the composition of suspended matter from ganges brahmaputra sediment dispersal system during low sediment transport season abstract suspended matter supplied from the ganges brahmaputra river system was sampled in waters of the adjacent shelf area in times of low suspended matter load november december 1997 the absolute amount of suspended matter ranges between 500 mg l near the river mouth and chemical and mineralogical analyses of the suspended matter show changes in its composition with distance from the source compositional changes are based on grain size partitioning quartz and heavy minerals are precipitating sooner leaving the suspended matter from the outer shelf depleted in quartz ree zr hf nb ta w u th y and ca the other part of the compositional changes is based on the reaction of suspended matter with seawater manganese hydroxide is producing coatings on suspended matter grains and adsorbing elements out of the seawater as a consequence enrichment of mn zn ni cr co li cs as sb pb tl and cd can be observed in the suspended matter from the outer shelf arsenic is accumulating on the inner shelf by a factor of 2 chemical partitioning in sequential leaching experiments shows that arsenic does not follow the enrichment mechanism of manganese and other transition metals more than 90 of the total arsenic is bound to nearly insoluble phases of the suspended matter e g crystalline iron oxides residual minerals for comparison with suspended matter adjacent surface sediments were sampled and analysed in this sediment the dominating phase is the one which settled down during the times of high suspended load with more quartz and heavy minerals and consequent chemical composition the enrichment of manganese oxide and adsorbed transition metals gets lost because of anaerobic conditions in the bottom near seawater they dissolve from the sediment and get recycled in seawater arsenic is found in nearly insoluble parts of the sediment as it is the case in the suspended matter

texture and mineralogy of sediments from the ganges brahmaputra meghna river system in the bengal basin bangladesh and their environmental implications the bengal basin bangladesh represents one of the most densely populated recent floodplains of the world the sediment flux through the basin is one of the highest on a global scale a significant portion of this sediment load find its sink in the basin itself because of its lower elevation and frequent flooding the textural mineralogical and chemical nature of the sediments thus have an important bearing on the environmental quality of the basin as well as for the bay of bengal

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

timing of collision of the sino korean and yangtse cratons u pb zircon dating of coesite bearing eclogites the suture zone between the sino korean and yangtse cratons in central china is marked by a series of fault bounded belts composed of crustal protoliths in the dabie mountains some of these blocks contain coesite and diamond indicating subduction to minimum depths of 120 km as a result of continental collision the blocks range from the structurally lowest ultrahigh pressure metamorphic belt and decrease in metamorphic grade upsection to the structurally highest low greenschist facies belt furthermore the faults mark large contrasts in metamorphic grade these features form a structure that is similar to metamorphic core complexes indicating extension as part of the exhumation process timing of the continental collision has previously been inferred as late triassic from regional geologic relations or caledonian from 40 ar 39 ar dating of the adjacent and possibly related qinling metamorphic belt u pb dating of zircon from ultra high pressure eclogites yields a metamorphic age of 209 2 ma this age coincides with later stages of collision and is therefore a minimum estimate of the time of collision

paleo heat flow evolution of the tabei uplift in tarim basin northwest china abstract the paleo heat flow evolution of the tabei uplift in the tarim basin is investigated based on burial and thermal history reconstruction of 14 wells and using basin modeling numerous geological parameters such as temperature data and missing sediment thickness by erosion were used in the modeling the basin model was calibrated using 460 measured vitrinite reflectance vr o and vitrinite like maceral reflectance vlmr o values to constrain the validity of the maturity model the heat flow history of the tabei uplift tarim basin shows the following characteristics 1 the highest paleo heat flow was predicted to have occurred in the early ordovician as 65 5 mw m 2 and gradually decreased to 55 5 mw m 2 during the late carboniferous 2 a thermal kick was modeled to have occurred in the permian as suggested by an abrupt rise in the heat flow 3 the heat flow gradually decreased since the triassic 4 the present day heat flow was predicted to be as low as 38 mw m 2 this heat flow history honors the geologic and tectonic evolution history of the tabei uplift and is suggested as the best case heat flow model

the coastal depositional system of a small mountainous river a perspective from grain size distributions abstract tseng wen river is a small mountainous river on a wave dominated coast of taiwan the spatial distribution patterns of 17 individual grain sizes offshore near the river mouth show strong morpho textural relationships with the bathymetry which also vary considerably according to the grain size the influence of seven hypothetical factors that could affect the observed grain size patterns were examined individually by using a methodology that combines filtering concept and a multivariate analysis technique eof these factors include the sediments discharged by the river offshore palimpsest sediments sediments eroded from a submarine sand ridge and the remnant river delta the effect of wave sorting and the northward and southward littoral drift the study results indicate that the tseng wen river is the most important source for modern sediments in the study area the reworking of offshore palimpsest sediments is also an important sediment source the agitation by waves is an important sorting mechanism in the study area and the net sediment transport in the study area is northward the findings based on grain sizes are also corroborated by short term observations of the near bed sediment dynamics and transport during the flood season of the tseng wen river

mineral and fluid inclusions in zircon of uhp metamorphic rocks from the ccsd main drill hole a record of metamorphism and fluid activity abstract the chinese continental scientific drilling ccsd main drill hole 0 3000 m in donghai southern sulu orogen consists of eclogite paragneiss orthogneiss schist and garnet peridotite detailed investigations of raman cathodoluminescence and microprobe analyses show that zircons from most eclogites gneisses and schists have oscillatory zoned magmatic cores with low pressure mineral inclusions of qtz pl kf and ap and a metamorphic rim with relatively uniform luminescence and eclogite facies mineral inclusions of grt omp phn coe and rt the chemical compositions of the uhp metamorphic mineral inclusions in zircon are similar to those from the matrix of the host rocks similar uhp metamorphic p t conditions of about 770 c and 32 kbar were estimated from coexisting minerals in zircon and in the matrix these observations suggest that all investigated lithologies experienced a joint in situ uhp metamorphism during continental deep subduction in rare cases magmatic cores of zircon contain coesite and omphacite inclusions and show patchy and irregular luminescence implying that the cores have been largely altered possibly by fluid mineral interaction during uhp metamorphism abundant h 2 o co 2 h 2 o or co 2 dominated fluid inclusions with low to medium salinities occur isolated or clustered in the magmatic cores of some zircons coexisting with low p mineral inclusions these fluid inclusions should have been trapped during magmatic crystallization and thus as primary only few h 2 o and or co 2 dominated fluid inclusions were found to occur together with uhp mineral inclusions in zircons of metamorphic origin indicating that uhp metamorphism occurred under relatively dry conditions the diversity in fluid inclusion populations in uhp rocks from different depths suggests a closed fluid system without large scale fluid migration during subduction and exhumation

grain scale deformation in ultra high pressure metamorphic rocks an indicator of rapid phase transformation abstract conspicuous grain scale deformation is observed in some ultra high pressure uhp metamorphic rocks of the dora maira massif western alps although no significant strain is discernible on the mesoscopic scale in a jadeite kyanite quartz rock some of the jadeite crystals reveal 100 deformation twins indicating local differential stress levels above 0 3 gpa many kyanite crystals show marked kink or deformation bands with a slip system 100 001 in contrast the adjacent coarse grained quartz matrix grain size ca 0 2 mm which has formed from coesite during exhumation from 100 km depth reveals a foam structure the quartz grain boundary configuration is controlled by interfacial free energy the grains are optically strain free and there is no crystallographic preferred orientation preservation of this foam microstructure which indicates grain growth during low stress annealing precludes that deformation of the jadeite and kyanite crystals is a result of a late stage low temperature overprint the orientation distribution of jadeite and kyanite with and without twins or deformation bands respectively has been investigated with a combination of universal stage and ebsd techniques on the scale of a thin section there is no preferred orientation of twinned jadeite and bent kyanite crystals and undeformed crystals respectively thus the orientation of the inferred local shortening direction is random this precludes deformation driven by a homogeneous far field tectonic stress but suggests an internally controlled stress field which is highly inhomogeneous on the scale of a few grain diameters laboratory experiments show that the coesite to quartz transformation proceeds within hours after decompression from 3 0 to 2 7 gpa at 800 c the microstructures of incompletely transformed samples indicate that the quartz growing at the expense of coesite undergoes crystal plastic deformation and recrystallizes with a very fine grain size during transformation in this case the deformation of quartz is attributed to the volumetric strain v 10 inherent in the coesite quartz transformation which causes a highly inhomogeneous stress field inside the sample related to the progress of the transformation we propose that a similar process has taken place in the polyphase natural rock during exhumation with the transient stresses causing mechanical twinning of jadeite and bending or kinking of kyanite when the transformation had gone to completion grain growth obliterated the microstructures of the quartz matrix while the deformed jadeite and kyanite crystals preserved the record of inhomogeneous deformation at high temperatures the peak differential stresses locally exceeded 0 3 gpa which indicates very high strain rates and a correspondingly rapid transformation of coesite to quartz comparable to the laboratory results

high fluxes of radium and barium from the mouth of the ganges brahmaputra river during low river discharge suggest a large groundwater source the annual flux of sediment from the ganges brahmaputra river is among the highest in the world the desorption of226ra and ba from this sediment produces a major point source of226ra and ba input to the ocean highest226ra and ba fluxes are expected to occur during high river flow june september when most of the sediment is discharged surprisingly during low discharge of the ganges brahmaputra river fluxes of226ra and ba to the northern bay of bengal are comparable to expected river derived fluxes during peak river discharge a large non riverine source of ra and ba is required to explain the high fluxes during low river discharge i suggest that this source is submarine discharge of groundwater containing high concentrations of226ra and ba total annual fluxes of226ra and ba to the ocean from this system are significantly greater than estimated previously

provenance and depositional variability of the thin mud facies in the lower ganges brahmaputra delta west bengal sundarbans india the sundarbans is one of the largest coastal wetland sites in the world and covers an area of approximately one million hectares of the western delta of the ganges and brahmaputra g b rivers located across bangladesh and india since the late holocene the western delta has not been directly fluvially sourced due to the ganges shift to the east present day bangladesh the depositional facies thin mud facies of the late holocene abandoned western region the sundarbans is derived from dominant estuary tidal dynamics however the provenance of the associated tmf sedimentation in this far western zone indian sundarbans per se is as yet equivocal in this study sediment cores from the indian sundarbans saptamukhi thakuran estuary were closely examined for grain size distributions gsds mineralogy through x ray diffraction xrd and geochemistry with x ray fluorescence xrf the tmf in the west bengal sundarbans has been determined to show intensively weathered terrestrial sediment derived principally from the ganges alluvial plain gap there is a predominance of quartz mica and clay minerals with quartz interpreted as a product of low relief tropical weathering sourced via the g b rivers draining the himalayas lithofacies interpreted through gsd analysis of the tmf is indicative of a muddy tidal flat environment with aggradation and a general fining up trend between the adjacent estuaries the sediment provenance indicates a continuing g b sediment source which moves westward along the bay of bengal from the active delta front and is then reworked over the far western abandoned delta by tidal estuarine forcing

time and temperature in petroleum formation application of lopatin s method to petroleum exploration n v lopatin in the soviet union has developed a method for taking both time and temperature into account as factors in thermal maturation of kerogen lopatin s time temperature index of maturity tti values correlate with the thermal regimes corresponding to generation and preservation of hydrocarbons because such information is potentially of great interest for oil exploration a calibration and evaluation have been made of lopatin s method within the limitations of the data presently available the following statements can be made 1 the rate of the chemical reactions involved in thermal maturation of organic material appears to double with every 10 c 18 f rise in temperature 2 threshold values of lopatin s time temperature index of maturity tti are 15 onset of oil generation 75 peak oil generation 160 end oil generation 500 40 oil preservation deadline 1 000 50 oil preservation deadline 1 500 wet gas preservation deadline 65 000 dry gas preservation deadline 3 tti values calculated from lopatin reconstructions consistently agree with other maturation parameters commonly used by petroleum geochemists potential applications of lopatin s method for oil exploration include timing of oil generation calculation of volume of hydrocarbons generated within a basin and determination of economic deadlines

composition and pre metamorphic geodynamic setting of the ultrahigh pressure metabasic rocks from dabie shan e china as part of the yangtze plate segments of the dabie shan terrane of central china underwent ultra high pressure metamorphism during triassic subduction we studied the geochemistry of the abundant eclogites to evaluate the nature of the protoliths and their geodynamic setting although some previous geochemical work exists the analyses and interpretation herein are based on a new subdivision of the ultra high pressure sequence into basement and cover units changpu and ganghe unit revealing new and important results in addition eclogites of the so called hp unit south of the uhp units were studied whereas the large ion lithophile elements indicate postmagmatic metasomatic changes of some samples the high field strength elements and the rare earth elements display original magmatic trends the geochemical characteristics of the eclogites of the ultra high pressure areas display a strong dependence on their structural and geographic position the eclogites of the basement and the changpu unit indicate melt intrusion and extrusion in a continental rift system i e during extensional tectonics in contrast the ganghe unit is characterized by a pronounced chemical homogeneity the composition of the eclogites indicates generation from a mantle source highly influenced by slab derived fluids those of the hp unit show similar characteristics magmatism of the ganghe and hp unit probably occurred in a continental arc setting a similar age for both units geographically and or tectonically separated is possible the geodynamic interpretation based on the geochemistry of the four units points to a neoproterozoic scenario in which the protoliths of the hp and the cover units could have been of similar age and deposited in one evolving geological system a rift related larger scale basin might have formed e g a continental back arc basin behind a magmatic arc after or simultaneous to sedimentation and magmatism in the magmatic arc alternatively magmatism occurred in independent geodynamic settings distinct in time and space the units were juxtaposed during exhumation after subduction to varying depths

determination of paleoheat flux from vitrinite reflectance data a method is presented for determining paleoheat flux from the reflectance of vitrinite collected from wells the maturation of vitrinite is modeled using a time temperature integral that assumes pseudo first order kinetics in testing the numerical algorithm it was necessary to introduce a critical temperature of 295 k below which vitrinite does not mature this result is consistent with the latest findings of independent researchers the degree of consistency obtained from a number of wells is not perfect much of the inconsistency can be attributed to large uncertainties in the vitrinite reflectance readings to test the method heat flow history was determined for the north sea basin and the national petroleum reserve of alaska quantitative results were obtained detailing the increase and or decrease of heat flux with time and were consistent with the qualitative behavior inferred for paleoheat flow from current geologic models

high temperature geochronology constraints on the tectonic history and architecture of the ultrahigh pressure dabie sulu orogen 1 new u pb zircon and th pb monazite ages are presented from the giant sulu ultrahigh pressure uhp terrane combined with sm nd ages rb sr ages inclusion relationships and geologic relationships they help define the timing of peak recrystallization the timing of subsequent amphibolite facies metamorphism and the architecture of the dabie sulu suture zone between the collided sino korean and yangtze cratons the data indicate a 15 myr record of uhp recrystallization the first clearly documented for a giant uhp terrane this requires that continental subduction in the dabie sulu orogen involved multiple uhp tectonic or recrystallization events a 244 236 ma precursor uhp event seen only in the dabie shan was followed by a second 230 220 ma main uhp event which was itself terminated by a 220 205 ma amphibolite facies overprint older eclogite facies events seen in the qinling segment of this orogenic belt raise the possibility that these rocks have undergone u hp metamorphism three or four times but at present there is no geochronological evidence in the dabie sulu area to support this the subduction of the lower yangtze plate did not proceed in a simple fashion the ages of inherited zircon cores demonstrate that a ribbon continent of yangtze affinity escaped subduction and became wedged against the sino korean plate hanging wall

shelf sedimentation off the ganges brahmaputra river system evidence for sediment bypassing to the bengal fan the nature of shelf sedimentation seaward of the world9s largest sediment dispersal system is examined by using sedimentological and geochronological techniques on a unique suite of sediment cores and grab samples sediments from the ganges brahmaputra river system are currently accumulating on the shelf in water depths of less than about 80 m forming a clinoform like deposit similar to subaqueous deltas found off other major river systems the highest sediment accumulation rates on the shelf occur near the head of the swatch of no ground a major submarine canyon that indents the shelf west of the present river mouths this observation together with textural data suggests that river sediments are transported seaward and westward and that the swatch of no ground is currently a major conduit for the transport of sediments from the bengal shelf

holocene and modern sediment budgets for the ganges brahmaputra river system evidence for highstand dispersal to flood plain shelf and deep sea depocenters the partitioning of fluvial sediment load across continental margins is an important control on strata formation and sequence development however few quantitative sediment budgets that encompass entire dispersal systems exist for the ganges brahmaputra river system sediment discharge is estimated to be 10 9 t yr at gauging stations 300 km inland of the coast but little has been known of the downstream fate of this material geochronological geophysical and stratigraphic investigations of the lowland flood plain delta plain and shelf help to delineate the extent of holocene fill and allow calculation of a first order sediment budget results reveal that 1500 10 9 m 3 of sediment fill has been sequestered within the flood plain and delta plain since ca 7000 yr b p or about one third of the annual discharge the remaining load appears to be apportioned between the prograding subaqueous delta 1970 10 9 m 3 and transport to the deep sea bengal fan via a nearshore canyon modern 100 yr budget estimates based on short term accretion rates indicate a similar dispersal pattern and show that contemporaneous deposition continues within these disparate depocenters the roughly equal partitioning of sediment among flood plain shelf and deep sea settings reflects the respective influence of an inland tectonic basin a wide shelf and a deeply incised canyon system the findings also support new sequence stratigraphic models for these settings and indicate the important insight that modern river deltas can provide for ancient margin systems furthermore results affirm that values of riverine sediment flux to the oceans may be considerably overestimated by not accounting for loss to the flood plains downstream of the gauging stations

thermal history of alberta deep basin comparative study of fluid inclusion and vitrinite reflectance data the thermal history of the alberta deep basin the gas saturated westernmost part of the western canadian sedimentary basin has been studied by analyzing aqueous and hydrocarbon fluid inclusions in diagenetic cements and comparing the results with coal maturity data results indicate that most of the diagenetic quartz druse precipitated from low salinity fluids 2 3 wt at a temperature of 170 195 c calcite cement precipitated after quartz druse during uplift and cooling at temperatures of 108 169 c inclusions rich in larger hydrocarbon molecules occur in cements that precipitated at 150 c whereas methane rich inclusions occur in cements that precipitated at 165 c or at lower temperatures after significant uplift and cooling of the b sin maximum burial temperatures tmb calculated from vitrinite reflectance data using the lopatin waples tti ro calibration are significantly lower 150 c than the temperature at which quartz druse precipitated in the same area 190 c an equation directly relating tmb and ro proposed by barker and pawlewicz in 1986 yields maximum burial temperatures 200 210 c only slightly higher than the fluid temperatures the results from this study suggest that hot fluids migrated updip along permeable conglomerates and bedding plane fractures to produce unexpectedly high geothermal gradients 38 c km in the western part of the study area the apparent restriction of hot fluids to the western part of the study area sugge ts that convective heat transfer by fluids may have operated on a scale of only tens of kilometers in lower cretaceous rocks of the alberta deep basin

suspended sediment movement in the estuary of the ganges brahmaputra meghna river system abstract topographical flow volume and suspended sediment concentration ssc measurements are presented in the large area of a turbidity maximum in the estuary of the ganges brahmaputra meghna river system in bangladesh the zone of turbidity maximum is indicated by the satellite imagery taken during the low river discharge period topographical and flow volume measurements the aspect ratio and the flow ratio indicate that the system is divided into flood and ebb dominated channels suspended sediment movement in the estuary appears to be associated with the horizontal stratification of flow and with the residual movement induced by the asymmetry of the tidal wave the flow ratio at different sections indicates that net import of suspended sediment takes place through flood channels while the net export occurs through ebb channels despite this difference in the direction of net movement of sediments measurements show that the ssc both in the flood channel and in the ebb channel is dependent on the tidal range the spring tidal ssc is about twice that of the neap tide and locations with higher tidal range show higher magnitudes of ssc the response of sediments to the varying energy environment of tidal range and tidal current however is not instantaneous the ssc shows a time lag in its variation with the varying energy condition the amplification and asymmetry of the tidal wave in the flood channels and the slack water asymmetry measured in a flood dominant location indicate that a possible landward residual transport of suspended sediment due to this factor is more pronounced through flood channels

rheology of the plate interface dissolution precipitation creep in high pressure metamorphic rocks abstract subduction zone models invoke deformation to be concentrated along the plate interface in a region of particularly low temperature geophysical observations do not provide constraints on temperature stress and deformation patterns with desired resolution in contrast the record of high pressure metamorphic rocks exhumed from subduction zones provides details on p t history deformation mechanisms and stress state albeit not readily correlated with the former dynamic situation on larger scale here we review available information on dissolution precipitation creep dpc in high pressure metamorphic rocks which if representative for subduction zones in general can pose constraints on conditions rheology and flow patterns along the plate interface the key observations and conclusions are that 1 deformation is typically highly inhomogeneous and localized into shear zones 2 stresses are generally too low to drive crystal plastic deformation 3 microfabrics suggest dissolution precipitation creep to be the predominant deformation mechanism 4 an aqueous fluid at quasi lithostatic pressure is available throughout allowing for tensile fracturing and crack healing or sealing 5 low stress combined with high strain rates required for localized deformation at typical subduction rates implies low viscosity and 6 contribution of shear heating to the thermal budget of subduction zones should be moderate the dominant deformation mechanism dpc is reviewed in some detail including experimental and theoretical approaches various examples of dpc in high pressure metamorphic rocks are illustrated emphasizing the role of interphase boundaries as sites of dissolution rheology governed by dpc is proposed to control interplate coupling and development of a subduction channel with return flow being a likely candidate for rapid exhumation of high pressure metamorphic rocks

preservation of primary volcanic textures in the ultrahigh pressure terrain of dabie shan delicate primary volcanic features such as ash layers volcaniclastic breccias and pillow lavas with amygdaloidal rims have been identified in a terrigenous volcanosedimentary sequence forming part of the ultrahigh pressure unit of the dabie shan china the presence of coesite relics in crosscutting dikes confirms an ultrahigh pressure evolution for the entire sequence evaluation of the probable reaction history for zeolite compositions as supposed for the amygdaloidal rims reveals a scarcity of reactions at very high pressures such special conditions of low reactivity and fluid conserving reactions combined with a lack of directed stress have allowed the remarkable preservation of these primary volcanic features despite deep subduction

oolitic beach barriers of the last glacial sea level lowstand at the outer bengal shelf abstract during rv sonne cruise 93 the bengal shelf was surveyed using sediment echosounding side scan sonar and sediment coring techniques a sequence of shallow ridges was discovered each between 100 m and 260 m wide and extending for kilometres parallel to the 120 130 m isobaths the ridges mark the top of a barely covered regressive sediment sequence and consist of oolitic limestone of late pleistocene age they are interpreted as contouring fossil shorelines of sea level lowstands of the last glacial maximum their lithology indicates that fine grained sediment supplied by the ganges brahmaputra river system did not pollute the nearshore environment during glacial times this argues for sediment bypassing of the bengal shelf and a direct supply to the bengal deep sea fan

permo triassic collision subduction zone metamorphism and tectonic exhumation along the east asian continental margin abstractconvergent plate motion at 320 210 ma generated the tongbai dabie sulu east central china imjingang gyeonggi central korea renge suo southwestern japan sikhote alin orogen along the paleo pacific edge of cratonal asia this amalgamated belt reflects collision between the sino korean and yangtze cratons on the sw portion and accretion of outboard oceanic arcs sialic fragments against the ne margin subducted proterozoic paleozoic continental and oceanic crustal complexes underwent high and ultrahigh pressure metamorphism at low to moderate temperatures tectonic slices of sialic crust episodically disengaged from the downgoing plate and driven by buoyancy ascended rapidly to midcrustal levels from depths exceeding 90 200 km after continental collision in east central china plus or minus korea and from 30 50 km after arrival of far traveled oceanic terranes in sw japan and the russian far east on achieving neutral buoyancy and stalling out at 10 20 km depth later doming gravitation

zircon u pb age and o isotope evidence for neoproterozoic low 18o magmatism during supercontinental rifting in south china implications for the snowball earth event identification of neoproterozoic low 18 o igneous rocks is a key to demonstrate that high t water rock interaction and low 18 o magmatism in rift tectonic zones can serve as an efficient interface to transport heat and material from the earth s interior to exterior at the onset of the snowball earth event low 18 o values of 10 9 to 4 4 permil characteristic of meteoric origin were observed on an areal extent over 30 000 km2 for mid neoproterozoic metaigneous zircons from the ultrahigh pressure eclogite facies metamorphic belt of triassic ages along the dabie sulu orogenic belt china although the zircon o isotope system could be reset to acquire its low 18 o values by exchange with matrix minerals during wet eclogite facies metamorphism the abnormal 18 o depletion is confirmed by occurrence of neoproterozoic low 18 o zircons in low grade metaigneous rocks in the same belt cathodoluminscence imaging of zircons from low grade metagranitoid and metavolcanic rocks shows preservation of the internal structures of oscillatory zoning typical of magmatic origin hornblende from two granites of low 18 o zircons gave ar ar plateau ages of 747 6 and 776 12 ma ruling out that phanerozoic metamorphism effected mineral o isotope values two groups of u pb ages were measured at 756 5 ma and 782 5 ma respectively for the low 18 o zircons this indicates that low 18 o zircons crystallized during two episodes of low 18 o magmatism in response to tectonic evolution from supercontinental rifting at 780 ma to breakup at 750 ma the second episode of low 18 o magmatism is contemporaneous with the continental kaigas iceage a plate rift model is advanced to account for the tectonic connection between the low 18 o magmatism the continental glaciation and the supercontinental rifting during the cryogenian period high t meteoric water rock interaction occurred prior to low 18 o magmatism in rift tectonic zones and caldera collapse was responsible for melting of hydrothermally altered low 18 o rocks during rifting of the south china block from the rodinia supercontinent

monsoon sedimentation on the abandoned tide influenced ganges brahmaputra delta plain abstract annual sediment delivery by the ganges and brahmaputra rivers to the bengal margin has kept pace with sea level rise since the mid holocene sustaining subaerial growth of the delta however the sundarbans region of the tidal delta is disconnected from major distributary sources of sediment and is often thought to be sediment starved eroding and susceptible to the meter of sea level rise predicted for the 21st century despite these assumptions direct sedimentation measurements on the tidal delta plain reveal widespread mean annualized accretion rates of 1 1 cm yr 1 although heterogeneous depositional patterns indicate that topography and internal creek networks influence local sediment distribution short lived radioisotope inventories 7be t1 2 53 3 days measured on the freshly accumulated sediments indicate that about of the mass deposited on the lower delta was sourced directly from the seasonal flood pulse of the river the remaining is derived from older 1 yr reworked sediments net sedimentation on this part of the delta traps 10 of annual ganges brahmaputra sediment load with accretion rates roughly equivalent to the mean regional rate of relative sea level rise rslr of 1 0 cm yr 1 if these sedimentation rates are representative of longer term trends and subsidence rates remain stable over the next century the lower delta plain may continue to maintain its elevation and stability despite documented mangrove retreat around its seaward edges

suspended sediment distribution and residual transport in the coastal ocean off the ganges brahmaputra river mouth abstract an examination of suspended sediment dispersal in the coastal ocean off the ganges brahmaputra river system using shipboard and satellite data reveal distinct patterns of sediment distribution and sediment motion which help explain the dynamics of the world s largest sediment dispersal system during the low discharge period suspended sediment concentration distribution is mainly a function of tidal energy and the sediments are primarily transported as a suspension population satellite and sea bed data indicate that transport of river sediments is largely restricted to an area landward of about the 20 isobath during the low discharge period a simplistic estimate of residual transport and its separation into tidal and non tidal components indicate that non tidal transport contributes about 47 of the total transport and has transport directions mostly southwestward during the northeast monsoon the tidal residual transport which contributes about 53 of the total transport however does not show any particular direction for most of the stations although some landward stations show shoreward transport although few water column measurements were made during the high discharge period data from this and previous studies suggest that the dominant transport direction is to the southwest for most of the year

neoproterozoic granitoid did not record ultrahigh pressure metamorphism from the southern dabieshan of china abstract it has been often debated whether all granitic gneisses associated with coesite bearing eclogites in southern dabieshan china have also been subjected to ultrahigh pressure uhp metamorphism we show here that a metagranitoid adjacent to the bixiling eclogite ultramafic complex has preserved primary granitic textures and an igneous mineral assemblage of documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape mathrm biotite mathrm plagioclase mathrm microcline mathrm quartz mathrm allanite pm mathrm amphi

crustal structure of the eastern dabie shan interpreted from deep reflection and shallow tomographic data abstract a 20 km long seismic line characterises the crustal reflection pattern of the easternmost dabie shan the archetypal ultrahigh pressure uhp orogen of eastern china the weak to non reflective upper crust 5 s two way travel time twt 15 km depth is interpreted to comprise uhp rocks thrust over lithologically similar but non uhp crust the tectonic contact although not imaged as a distinct reflector is probably outlined by the rather abrupt change to diffuse but strong reflectivity within the mid to lower crust thus the seismic pattern of the upper crust implies that mafic oceanic crust does not constitute a significant proportion the middle to lower crust 5 10 s twt 15 33 km depth probably represents cratonal yangtze basement unaffected by the uhp metamorphism the prominent lowermost reflectors 10 12 s twt 33 40 km depth are interpreted to trace the moho excluding the presence of a crustal root inherited from the uhp orogeny a tomographic p wave velocity model for the uppermost crust

u pb zircon ages constrain the architecture of the ultrahigh pressure qinling dabie orogen china new shrimp and tims zircon ages 40ar 39ar ages and eclogite locations contribute significantly to our understanding of the ultrahigh pressure dabie shan 1 the geographic extent of the yangtze craton that was subducted to ultrahigh pressure extends to the northern edge of the dabie shan 2 the northern half of the dabie shan is a magmatic complex intruded over a 10 myr interval between 137 and 126 ma that accommodated 100 n s stretching of the pre existing collisional architecture 3 granitic orthogneisses and enclosing ultrahigh pressure paragneisses have indistinguishable zircon populations the population of triassic zircon ages ranges from 219 to 245 ma leading us to question the prevailing assumption that 219 ma zircons formed at ultrahigh pressure and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation

basement cover sequences within the uhp unit of the dabie shan field data define two lithologically distinct basement cover sequences within the ultrahigh pressure metamorphic uhpm unit of the dabie shan eastern china one of the cover units the changpu unit comprises calc arenitic metasediments in stratigraphic contact with a basement consisting of gneisses of the yangtze craton the second cover unit the ganghe unit consists of felsic intermediate metavolcanics and clastic metasediments basement exposure of the ganghe unit is not known fold axes in the ganghe unit are oblique to those of the changpu unit which are parallel to those of the yangtze gneisses preservation of primary textures in some volcanic rocks and tectonic separation from the yangtze gneisses by a greenschist facies mylonite support an interpretation of the ganghe unit as a low strain domain protolith associations in the ganghe and changpu units are compatible with deposition in a rift setting and along a passive continental margin respectively a u pb single zircon age of 761 33 ma for volcanoclastic rocks of the ganghe unit demonstrates a neoproterozoic deposition age concordant with inferred rifting at that time eclogite facies parageneses in the gneisses and both cover units along with p t data demonstrate regional uhpm in the dabie shan

order from chaos a field based estimate on bulk rheology of tectonic melanges formed in subduction zones abstract many tectonic hp lt melanges exhibit block in matrix structures in contrast to the pervasively deformed matrix the blocks of various lithologies and pt histories usually appear undeformed if we accept that an hp lt melange reflects persistent high strain deformation in a deep 30 km subduction channel the record of these rock associations can provide insight into stress state rheology and flow patterns along a plate interface the bulk rheology of a block in matrix structure is controlled by a the dominant deformation mechanism of the matrix b the volume fraction of blocks engulfed in that matrix and c the geometry of the blocks the microstructures of the matrix indicate distributed deformation by dissolution precipitation creep dpc while crystal plastic deformation is subordinate dpc is grain size sensitive controlled by the type of interfaces and requires an aqueous pore fluid at quasi lithostatic pressure the rheology of a rock body undergoing deformation by dpc is believed to be newtonian viscous matrix viscosities are estimated to be on the order of 10 19 pa s or less here we explore the effect of rigid blocks embedded in a weak matrix on the bulk rheology using stereology we determine the block size distributions as well as the total block volume fractions for block in matrix structures from several regions the volume fractions are found to range from 2 to 70 with typical values between 5 and 50 using published mixing laws we model the subduction channel as a suspension of rigid spheres with a linear viscous matrix and predict that for the typical values of 5 to 50 the viscosity increase is less than one order of magnitude consequently the influence of blocks on the bulk viscosity of a subduction channel is minor instead bulk rheology is primarily controlled by the deformation behaviour of the matrix material

borosilicate and phengite bearing veins from the grenville province of labrador evidence for rapid uplift quartzo feldspathic veins emplaced within a migmatite terrane near wilson lake in the grenville province of central labrador record a metamorphic event not evident in the host rocks the discordant veins are undeformed and have undisturbed primary igneous hydrothermal textures most of the veins contain euhedral kyanite as well as aggregates of kyanite k feldspar phlogopite and minor dumortierite which are likely pseudomorphs after primary phengite the reconstructed phengite compositions range from 3 1 to 3 2 si per 11 oxygen formula unit the pseudomorph assemblage is interpreted as the product of phengite quartz melting under h2o undersaturated conditions which brackets p t conditions of formation to about 9 16 kbar and 775 875 c a parallel vein that is likely of the same generation contains the borosilicate phases dumortierite prismatine and grandidierite but no kyanite the borosilicate assemblages constrain the p t conditions of vein crystallization to 10 kbar and c 750 850 c vein emplacement is constrained to t 875 c at the same pressures which is well within the kyanite zone because the host rocks and veins must have experienced the same p t history following vein emplacement the presence of unreacted sillimanite in the host migmatites implies insufficient time for host rock equilibration slow reaction rates because of anhydrous conditions are not a likely explanation given the abundance of biotite and hornblende in the host rocks the ductility implied by the breakdown of a hydrous phase phengite and the production of an h2o undersaturated melt in the veins contrasts with the apparently brittle behaviour of the host rocks the absence of deformation since the time of vein emplacement even at temperatures above 750 c suggests that the deep crust in this part of labrador had a very short residence time under conditions of the kyanite zone rapid decompression from those conditions is consistent with quartz phengite melting and accounts for the relatively brittle behaviour of the terrane as it was uplifted

radical concentration of kerogen as a maturation parameter a good correlation was found between spin concentration and burial depth for kerogen extracted with benzene methanol followed by pyridine or dioxane differences in slopes of the regression lines of spin concentration vs burial depth are dependent upon the geothermal gradient of the studied areas free radical concentration might be a good maturation parameter for kerogens of the same type

tidal influence on suspended sediment distribution and dispersal in the northern andaman sea and gulf of martaban abstract surface and water column profiles of suspended matter collected during april may 2002 and satellite images were used to study factors influencing suspended sediment concentrations sscs and dispersal in the northern andaman sea and gulf of martaban one of the largest highly turbid areas of the world s oceans perennial high ssc in the gulf of martaban is due to a combination of factors including resuspension of sediments by strong tidal currents shallow bathymetry and seasonal sediment influx from rivers from satellite images it was observed that in the central portion of the gulf of martaban the turbidity front oscillates about 150 km in phase with spring neap tidal cycles and the area covered by the turbid zone ssc 15 mg l 1 increases from less than 15 000 km2 during neap tide to more than 45 000 km2 during spring tide the sediment discharged by the ayeyarwady river is transported mainly eastward along the coast into the gulf of martaban occasionally during the winter monsoon period sediment plumes are seen heading westward into the bay of bengal turbidity profiles show that bottom nepheloid layers are actively transporting some of the sediments into the deep andaman sea via the martaban canyon

thermobarometry of phengite bearing eclogites in the dabie mountains of central china pressure temperature conditions for formation of the peak metamorphic mineral assemblages in phengite bearing eclogites from dabieshan have been assessed through a consideration of fe2 mg2 partitioning between garnet omphacite and garnet phengite pairs and of the reaction equilibrium celadonite pyrope grossular muscovite diopside which incorporates an evaluation of the extent of the strongly pressure dependent inverse tschermak s molecule substitution in the phengites for the latter equilibrium the calibration and recommended activity composition models indicated by waters martin 1993 have been employed and importantly yield results consistent with petrographic evidence for the stability at peak conditions of coesite in certain samples and quartz in others confirmation that in some phengite eclogite samples peak silicate mineral assemblages have equilibrated at confining pressures sufficient for the stability of coesite and in some cases even diamond rather negates previous suggestions that coesite may have been stabilized in only very localized possibly just intracrystalline domains inherent difficulties in the evaluation of peak metamorphic temperatures from fe2 mg2 partitioning between mineral phases due to uncertainties over fe3 fe2 ratios in the minerals especially omphacites and to re equilibration during extensive retrograde overprinting in some samples are also assessed and discussed our results indicate the existence in south central dabieshan of phengite eclogites with markedly different equilibration conditions within two structurally distinct tectonometamorphic terranes thus our data do not support earlier contentions that south central dabieshan comprises a structurally coherent continental crust terrane with a regional p t gradient signalling previous deepest level subduction in the north instead we recognize the central dabie ultra high pressure coesite eclogite bearing terrane to be structurally overlain by a southern dabie high pressure quartz eclogite bearing terrane at a major southerly dipping shear zone along which late orogenic extensional collapse appears to have eliminated at least 20 km of crustal section

the use of free radicals of organic matter to determine paleogeothermal gradient abstract free radicals in 60 kerogen samples from 5 wells in the qiongdongnan tarim and liaohe basins in china were analyzed by electron spin resonance esr for use in geothermometry based on esr data for 44 kerogen samples from the tertiary qiongdongnan basin a regression curve was shown to fit free radical densities n g with burial depth of samples for use in geothermal gradient estimation using this curve the highest geothermal gradients experienced by carboniferous permian strata well tmc1 in the tarim basin and oligocene eocene strata well lr16 in the liaohe basin were estimated concurrently five samples from liaohe basin were examined by apatite fission track analysis to assess the reliability of the esr kerogen method for geothermal gradient determination results of esr analyses match those of vitrinite reflectance and apatite fission track analysis indicating that maximum geothermal gradients obtained from the esr curve are reliable this paper presents a useful esr kerogen method for estimating paleogeothermal gradients which is especially useful where vitrinite and apatite are not present

sediment transport in the shelf canyon swatch of no ground bay of bengal the swatch of no ground is a shelf canyon that deeply incises the bengal shelf near the ganges brahmaputra river mouth cuts the foreset beds of the subaqueous river delta and acts as temporary depocenter between river mouth and bengal fan sedimentation rates in the swatch of no ground are highest near the canyon head at 50 cm a 1 decreasing to 15 cm a 1 in 600 m water depth the canyon deposits consist of intercalated fine silt clay and coarse silt sand grained deposits in seismic profiles small scale sedimentary structures and parallel bedded layers reveal that sediment in the canyon is mainly deposited from suspension during fair weather conditions tidal currents are the dominant mechanism that transports plumes of suspended river load towards the canyon forming fine grained silt clay layers during storm conditions sediment is resuspended on the inner shelf and subaqueous delta east of the canyon storm generated bottom currents transport the resuspended sediment alongshelf to the canyon where the particles are trapped and form graded coarse silt sand layers channels and gullies in the swatch of no ground indicate active gravity driven currents with an erosional character in the upper canyon and non depositional character below 450 m suggesting that persistent or ephemeral currents presently export sediment to the bengal fan numerous slumps and slides observed in the canyon show small scale acoustically transparent layers that indicate sediment liquefaction presumably initiated along the steep canyon margin these mudflows move downslope and halt where the canyon gradient decreases one widespread acoustically transparent layer with an age of approximately 140 160 years bp can be traced throughout much of the canyon and is probably the result of large scale sediment remobilization triggered by a catastrophic event like an earthquake this may represent decennial to centennial scale events that remove large quantities of sediment and prevent the canyon from rapid infilling

plastic flow of coesite eclogite in a deep continent subduction regime microstructures deformation mechanisms and rheologic implications abstract ultrahigh pressure uhp metamorphic rocks are continental crust that once has been subducted to mantle depth and subsequently exhumed to the surface in collisional orogenic belts despite the long distance transport of the uhp rocks in a deep subduction channel and the fast strain rate required for rapid exhumation little evidence for uhp deformation is found in the exhumed uhp terrains so far identified some uhp rocks have even preserved primary igneous and volcanic structures these observations have led to the postulation that the stress level is low a few mpa in the deep subduction zones deformation microstructures of some eclogite mylonites from a ductile shear complex in the sulu uhp metamorphic belt eastern china provide for the first time convincing evidence that dislocation creep is the predominant deformation mechanism for the major constituent minerals of the uhp rocks in a highly strain localized shear zone at the uhp conditions omphacite phengite and kyanite are the main strain accommodating phases and are dynamically recrystallized through progressive subgrain rotation and grain boundary migration coesite is either fractured with irregular extinction in the moderately deformed eclogite or pseudomorphed by polycrystalline quartz aggregates that have a foam structure garnet does not show optically discernable plastic deformation p t conditions for the deformation are estimated to be p 3 3 gpa and t 624 c using garnet omphacite kyanite phengite coesite quartz thermobarometry suggesting that deeply subducted continental crust remains cold at the upper mantle depths large part of the deeply subducted crust remains relatively undeformed strain is strongly localized into narrow shear zones stress level in the deep subduction channel is extrapolated on the order of tens of mpa much higher than it was previously thought

dating of subduction and differential exhumation of uhp rocks from the central dabie complex e china constraints from microfabrics rb sr and u pb isotope systems abstract the correlation of deformation fabrics and metamorphic reactions with geochronologic data of uhp metamorphic rocks demonstrate that the multistage subduction and exhumation history of the central dabie complex requires rapid subduction and rapid initial exhumation moreover these data show that volume diffusion is not the major resetting mechanism of radiogenic isotope systems thus our age data do not simply reflect a thermal cooling history in the investigated section the maximum age for uhp is given by the 244 3 ma 2 u pb age of a pre uhp titanite phenocryst that survived uhp metamorphism and subsequent tectonometamorphic events a minimum age for uhp is set by the 238 1 ma 2 238 u 206 pb mineral isochron age of titanite and cogenetic epidote these minerals formed from local partial melts during ascent and their age suggests fast exhumation and emplacement in the middle crust in the period of ca 238 218 ma the uhp terrain records ht metamorphism local partial melting and extensive pervasive strain below the eclogite jd grt stability field exhumation was polyphase with a first phase of fast exhumation succeeded by episodes of ht metamorphism and concomitant deformation at deep mid crustal level between 238 and 218 ma slow exhumation related to the final emplacement of tectonic units along greenschist facies shear zones did not cease before ca 209 204 ma the resetting and homogenization of radiogenic isotope systems were aided by dissolution precipitation creep which was the dominant deformation mechanism in quartz feldspar rocks in combination with fluid influx

discussion of management of fluid mud in estuaries bays and lakes i present state of understanding on character and behavior by w h mcanally c friedrichs d hamilton e hayter p shrestha h rodriguez a sheremet and a teeter

nature of solute loads in the rivers of the bengal drainage basin bangladesh the bengal drainage basin is geologically one of the youngest and tectonically most active denudation regimes of the world and encompasses the total lower reaches of the ganges brahmaputra meghna gbm drainage basin the gbm river system contributes around 4 5 of the total annual global freshwater flux to the oceans the solute load of the gbm river system is dominated by the carbonate weathering products of the transport limited denudation regime however in the meghna basin which drains a mountainous region silicate weathering is slightly more predominant and the solute load tends to be more influenced by the atmospheric contribution the river system represents about 5 152 106 t yr 1 of the annual global chemical flux to the world s oceans the chemical denudation rate of the gbm system in the bengal basin is one of the world s highest 79 114 t km 2 yr 1 suggesting intensive weathering and erosion in the drainage basin both in bangladesh as well as in the hinterlands of india and china

deformation enhanced hp uhp metamorphic transformation of two pyroxene granite in ductile shear zones at yangkou sulu uhp metamorphic belt china abstract we report that high strain deformation in ductile shear zones greatly enhanced high to ultrahigh pressure hp uhp metamorphic transformation of a two pyroxene granite in deep continental subduction zone integrated structural and petrologic study of rocks from the two pyroxene granite block and shear zones in the block in the uhp shear complex at yangkou sulu uhp metamorphic belt reveals that the undeformed granite has preserved granitic texture and primary mineral assemblage of hypersthene augite plagioclase k feldspar quartz and biotite metamorphic transformation of the granite is limited to grain boundaries due to low fluid activity and low reaction kinetics in contrast rocks from the shear zones have a mylonitic texture and most of the primary minerals are transformed to hp uhp phase minerals dislocation creep accommodated by dynamic recrystallization results in significant grain size reduction of the primary minerals in the mylonite which increases reaction surface area and enhances metamorphic transformation fluid released by biotite transformation to hp uhp minerals leads to diffusion mass transfer that greatly accelerates the metamorphic process in the shear zone deformation conditions are estimated ca 546 c and pressure 2 5 gpa

hydraulic interpretation of grain size distributions using a physical model for bedload transport abstract grain size distributions of bedload sediment are modelled by considering the time variation of sediment transport in a unidirectional turbulent flow a frequency distribution of instantaneous bed shear stress is divided into a large number of shear stress ranges with mean values of oi each oi is capable of transporting a range of sediment sizes as bedload the maximum size is just at its entrainment threshold the minimum size is just too large to be suspended rates of bedload transport are calculated for each size range using a theoretical bedload function and the proportion of time that these rates occur is obtained by integrating the frequency distribut on between the boundaries of each shear stress range the relative weights of sediment transported in particular size ranges determined for all oi values constitutes the grain size distribution obtained from bedload sampling at a fixed point on the bed cumulative grain size curves have been simulated using the model in order to demonstrate the effects of the various model parameters e g mean bed shear stress viscosity all curves show a distinct break in slope which represents the maximum grain size transportable in suspension when the shear stress attains its maximum fluctuating value examination of many natural bedload distributions shows close qualitative and quantitative agreement with the model and reasons for discrepancies can be fully explained typical grain size distributions of deposited sediments can be obtained by mixing of different flow related bedload distributions in various proportions thus although deposited sediments may primarily result from bedload transport during a steady flood flow flow unsteadiness also plays a part the effects of some deposition directly from suspended load can also be surmised

ages of ultrahigh pressure metamorphism and protolith orthogneisses from the eastern dabie shan u pb zircon geochronology the dabie shan contains two rare geologic features one the occurrence of ultra high pressure metamorphic rocks peak p 2 7 gpa uhp in the east central part of this belt and two some of these uhp rocks are characterized by anomalously 18o depleted oxygen isotopic compositions geochronological data are presented that constrain the age of the uhp metamorphism the ages of protoliths and potentially the age of hydrothermal alteration believed to be responsible for the anomalous oxygen isotopic compositions the age of uhp metamorphism in the eastern dabie shan is dated at 218 5 1 7 ma using u pb in zircon separated from host gneisses this age is identical within uncertainty with a less precise age using u pb in zircon separated from uhp eclogite 225 5 3 6 ma and with previously published ages from uhp eclogites of the dabie shan map relationships strongly support an in situ as opposed to exotic orgin of the uhp eclogites in this region the identical ages of uhp metamorphism in the gneisses and eclogites provides additional support for this interpretation the age of the protolith gneisses is dated as 772 5 9 5 ma based on the upper intercept of the zircons which accords well with other dates of granitic gneisses in the qinling belt the association of mafic and felsic magmatism is interpreted to reflect rifting along the northern margin of the yangtze block at this time these upper intercept ages may date the hydrothermal circulation of 18o depleted meteoric water which gave rise to the anomalously 18o depleted uhp protoliths of the dabie shan and sulu region the coincidence of these protolith ages with the early sinian glaciation of south china is consonant with the suggestion that the highly anomalous oxygen isotopic compositions in this region date from this time the upper intercept age of the eclogite is imprecisely determined at 447 82 79 ma but is younger than the host gneisses and is not characterized by anomalous oxygen isotopic compositions

shelf to canyon connections transport related morphology and mass balance at the shallow headed rapidly aggrading swatch of no ground bay of bengal abstract the swatch of no ground song canyon in the bay of bengal is a shelf incising submarine canyon that is actively aggrading in its upper reaches despite regular gravity driven transport and mass wasting although the canyon lies 150 km downdrift of its main sediment source the ganges brahmaputra meghna gbm river mouth high sedimentation rates 5 50 cm year 1 are sustained by both progradation of the subaqueous delta into the canyon head and the conveyance of shelf generated hyperpycnal flows to the canyon floor this rapid accretion appears to be largely balanced by mass failures triggered by regularly occurring storms and less frequently by major earthquakes here we use high resolution sub bottom sonar data to elucidate dominant sediment dispersal pathways and their transport related morphology at the canyon head these include 1 a laterally prograding clinoform that intersects the canyon head at water depths of 20 120 m 2 several shelf incising bypass gullies that originate in 50 m the clinoform deposits reflect westward progradation of the gbm subaqueous delta into the upper canyon where its axis normal orientation leaves it dissected by cross cutting gullies and mass failures the morphology and acoustic stratigraphy of the gullies coupled with strong bed shear and high suspended sediment concentrations on the inner shelf suggest that these features are sustained by the regular conveyance of gravity driven fluid muds that are formed in shallow water

comparisons of measured and predicted sediment transport rates in tidal currents abstract this paper describes bedload transport measurements in a large scale sedimentary system on the british continental shelf steady flow sediment transport theories are applied to oscillatory tidal currents and various equations are compared with measured transport rates from tracer dispersion experiments the effects of waves on both measured and predicted rates are briefly considered and the predicted directions of sediment movement compared with those inferred from sedimentological data

on palaeozoic mesozoic brittle normal faults along the sw barents sea margin fault processes and implications for basement permeability and margin evolution palaeozoic mesozoic brittle normal faults onshore along the sw barents sea passive margin off northern norway give valuable insight into fault and fluid flow processes from the lower brittle crust microstructural evidence suggests that late permian early triassic faulting took place during multiple phases with initial fault movement at minimum p t conditions of c 300 c and c 240 mpa c 10 km depth followed by later fault movement at minimum p t conditions of c 275 c and c 220 mpa c 8 5 km depth the study shows that pore pressures locally reached lithostatic levels 240 mpa during faulting and that faulting came to a halt during early deep stages of rifting along the margin fault permeability has been controlled by healing and precipitation processes through time which have sealed off the core zone and eventually the damage zones after faulting a minimum average exhumation rate of c 40 m ma 1 since the late permian is estimated it implies that the debated late cenozoic uplift of the margin may be explained by increased erosion rates in the coastal regions owing to climate detoriation which caused subsequent isostatic recalibration and uplift of the marginal crust the studied faults may be used as analogues of basement involved fault complexes offshore revealing details about the offshore nature of faulting including past and present basement and fault zone permeability