zircon lu hf isotopic compositions of metaluminous and peralkaline a type granitic plutons of the emeishan large igneous province sw china constraints on the mantle source abstract permian a type granitoids of the emeishan large igneous province elip in the panxi region sw china are predominantly peralkaline and metaluminous in composition in situ zircon hf isotopic compositions along with whole rock trace element and nd isotopic data are used to examine the probable source characteristics of these a type granitic plutons zircons from the baima and taihe peralkaline plutons have weighted average ehf t values of 8 7 0 4 and 9 2 1 0 whereas the woshui and huangcao metaluminous plutons have differing ehf t values 8 6 0 2 and 5 8 0 3 respectively previous studies have suggested that the plutons were derived by differentiation e g fractionation or partial melting of mafic magmas from an enriched mantle source similar to oceanic island basalt with little or no contribution from the crust end t 1 3 to 3 2 zr nb 0 9 14 4 rb nb 0 1 8 4 and th ta t values of the woshui and baima plutons indicates that spatially and temporally associated peralkaline and metaluminous a type granitic rocks can originate from the same mantle source without crustal assimilation

gas hydrate reservoirs and gas migration mechanisms in the terrebonne basin gulf of mexico highlights identify new fine grained hydrate filled fracture units in the terrebonne basin identify new hydrate bearing thin sands mostly within fractured muds present detailed seismic amplitude maps of the new hydrate bearing units discuss methane migration mechanisms and hydrate formation in thin sands identify and discuss source reservoir relationships between thick muds and thin sands abstract the interactions of microbial methane generation in fine grained clay rich sediments methane migration and gas hydrate accumulation in coarse grained sand rich sediments are not yet fully understood the terrebonne basin in the northern gulf of mexico provides an ideal setting to investigate the migration of methane resulting in the formation of hydrate in thin sand units interbedded with fractured muds using 3d seismic and well log data we have identified several previously unidentified hydrate bearing units in the terrebonne basin two units are 100 m thick fine grained clay rich units where gas hydrate occurs in near vertical fractures in some locations these fine grained units lack fracture features and they contain 1 4 m thick hydrate bearing sands in addition several other thin sand units were identified that contain gas hydrate including one sand that was intersected by a well at the location of a discontinuous bottom simulating reflector using correlation of well log data to seismic data we have mapped and described these new units in detail across the extent of the available data allowing us to determine the variation of seismic amplitudes and investigate the distribution of free gas and or hydrate we present several potential source reservoir scenarios between the thick fractured mud units and thin hydrate bearing sands we observe that hydrate preferentially forms within thin sand layers rather than fractures when sands are present in larger marine mud units based on regional mapping showing the patchy lateral extent of the thin sand layers we propose that diffusive methane migration or short migration of microbially generated methane from the marine mud units led to the formation of hydrate in these thin sands as discontinuous sands would not be conducive to long range migration of methane from deeper reservoirs

intraplate extension prior to continental extrusion along the ailao shan red river shear zone left lateral movement of the ailao shan red river shear zone lends support to the hypothesis of continental extrusion resulting from the collision of india with asia our new observations from northwestern yunnan china and northwestern vietnam on different sides of the shear zone demonstrate that the sinistral offset was 600 km according to correlations of permian triassic flood basalt successions and late paleogene highly potassic mafic magmas we conclude that the shear was propagating on the south china continental margin and does not correspond to a suture between south china and indochina furthermore the highly potassic magmas were emplaced from ca 40 to 30 ma before the shear movement which was caused by the late oligocene to early miocene ca 27 22 ma extrusion activity this suggests that a late eocene to early oligocene intraplate extension possibly induced by delamination of thickened continental lithosphere took place in northwestern yunnan or eastern tibet as a response to the india asia collision this extension and sea floor spreading of the south china sea that began ca 30 ma could have accounted for the initiation of the ailao shan red river shear zone

linking basin scale and pore scale gas hydrate distribution patterns in diffusion dominated marine hydrate systems the goal of this study is to computationally determine the potential distribution patterns of diffusion driven methane hydrate accumulations in coarse grained marine sediments diffusion of dissolved methane in marine gas hydrate systems has been proposed as a potential transport mechanism through which large concentrations of hydrate can preferentially accumulate in coarse grained sediments over geologic time using one dimensional compositional reservoir simulations we examine hydrate distribution patterns at the scale of individual sand layers 1 20 m thick that are deposited between microbially active fine grained material buried through the gas hydrate stability zone ghsz we then extrapolate to two dimensional and basin scale three dimensional simulations where we model dipping sands and multilayered systems we find that properties of a sand layer including pore size distribution layer thickness dip and proximity to other layers in multilayered systems all exert control on diffusive methane fluxes toward and within a sand which in turn impact the distribution of hydrate throughout a sand unit in all of these simulations we incorporate data on physical properties and sand layer geometries from the terrebonne basin gas hydrate system in the gulf of mexico we demonstrate that diffusion can generate high hydrate saturations upward of 90 at the edges of thin sands at shallow depths within the ghsz but that it is ineffective at producing high hydrate saturations throughout thick greater than 10 m sands buried deep within the ghsz furthermore we find that hydrate in fine grained material can preserve high hydrate saturations in nearby thin sands with burial

permian rifting related fayalite syenite in the panxi region sw china abstract within the panxi rift of the permian emeishan large igneous province elip sw china a 20 km 2 quartz bearing fayalite syenitic pluton intrudes the 260 ma baima igneous complex bic the fayalite syenite is dated using shrimp zircon u pb techniques at 252 2 5 ma the panxi fayalite syenite is mainly composed of alkali feldspar ferroan augite fayalite quartz and fe ti oxides and contains 61 8 63 7 wt sio 2 5 0 7 3 wt fe 2 o 3 t and 9 9 12 1 wt total alkalis belonging to an a type variety of ferroan alkalic metaluminous granitoids the fayalite syenite displays positive eu anomalies eu eu 0 9 8 9 and is enriched in lree la yb n 6 6 11 5 the nd values and 87 sr 86 sr initial ratios of the fayalite syenite range from 1 3 to 1 9 and from 0 7035 to 0 7068 respectively the fayalite syenite and the bic had the same mantle source however the 8 myr age gap between them suggests the fayalite syenite formed by remelting of underplated mafic magmas injected into the crust during elip magmatism the ages of the fayalite syenite and bic are coincident with two permian mass extinction events and suggest that elip may have influenced both

the indosinian orogeny a perspective from sedimentary archives of north vietnam the triassic stratigraphic framework for the song da and the sam nua basins north vietnam suffers important discrepancies regarding both the depositional environments and ages of the main formations they contain using sedimentological analyses and dating foraminifer biostratigraphy and u pb dating on detrital zircon we provide an improved stratigraphic framework for both basins a striking feature in the song da basin located on the southern margin of the south china block is the diachronous deposition over a basal unconformity of terrestrial and marine deposits the sedimentary succession of the song da basin points to a foreland setting during the late early to the middle triassic which contrasts with the commonly interpreted rift setting on the northern margin of the indochina block the sam nua basin recorded the activity of a proximal magmatic arc during the late permian up to the anisian this arc resulted from the subduction of a southward dipping oceanic slab that separated the south china block from the indochina block during the middle to the late triassic the song da and sam nua basins underwent erosion that led to the formation of a major unconformity resulting from the erosion of the middle triassic indosinian mountain belt built after an ongoing continental collision between the south china and the indochina blocks later during the late triassic as syn to post orogenic foreland basins in a terrestrial setting the song da and sam nua basins experienced the deposition of very coarse detrital material representing products of the mountain belt erosion

does neoproterozoic nam co formation in northwest vietnam belong to south china or indochina abstract the nam co belt is located to the northeast of song ma fault in northwestern vietnam the nature of the precambrian basement rocks in this area and its tectonic attribute have still been controversial in this study zircon u pb hf isotopic compositions and whole rock geochemical compositions of meta sedimentary rocks from the nam co formation in this belt have been integrated to reveal the components and tectonic setting of the formation of the basement rocks in the nam co belt zircon u pb dating results show that the nam co formation is of middle late neoproterozoic age the lower nam co formation probably formed between 821 and 726 ma and the upper formation was deposited later than 726 721 ma the sedimentary rocks from the lower and upper nam co formations are characterized by numerous neoproterozoic 900 800 ma detrital materials but the upper part contains more meso to paleoproterozoic sediments than the lower part detrital zircon age distribution and hf isotope compositions of the nam co formation are similar to those of neoproterozoic sedimentary rocks in the southern yangtze block both zircon u pb hf isotopic and whole rock geochemical data suggest that the source materials of the neoproterozoic nam co formation were dominated by felsic rocks with minor mafic materials and the detrital materials were mainly derived from the jiangnan orogenic belt in the southern yangtze block with minor contribution from the panxi belt in the western yangtze block ailaoshan song hong and phan si pan belts the lower nam co formation contains more sediments derived from the panxi belt than the upper part the middle late neoproterozoic nam co formation probably was deposited in a continental extensional basin and the sedimentation environment in later period

two magma series and associated ore deposit types in the permian emeishan large igneous province sw china the late middle permian 260 ma emeishan large igneous province in sw china contains two magmatic series one comprising high ti basalts and fe rich gabbroic and syenitic intrusions the other low ti basalts and mafic ultramafic intrusions the fe rich gabbros are spatially and temporally associated with syenites each series is associated with a distinctive type of mineralization the first with giant fe ti v oxide ore deposits such as panzhihua and baima the second with ni cu pge sulfide deposits such as jinbaoshan limahe and zhubu new shrimp zircon u pb isotopic data yielded 263 3 ma for the limahe intrusion 261 2 ma for the zhubu intrusion and 262 2 ma for a syenitic intrusion these new age dates together with previously reported shrimp zircon u pb ages suggest that all these intrusions are contemporaneous with the emeishan flood basalts and formed during a major igneous event at ca 260 ma the oxide bearing intrusions have higher al2o3 feo as total iron and total alkalis na2o k2o but lower mgo than the sulfide bearing intrusions all intrusions are variably enriched in lree relative to hree the oxide bearing intrusions display positive nb and ti anomalies and in certain cases negative zr hf anomalies whereas the sulfide bearing intrusions have obvious negative nb and ti anomalies a feature of crustal contamination individual intrusions have relatively small ranges of nd t values all the intrusions however have nd t values ranging from 3 9 to 4 6 and initial 87sr 86sr ratios from 0 7039 to 0 7105 the syenites have very low mgo 2 wt but highly variable fe2o3 2 5 to 13 wt with initial 87sr 86sr ratios ranging from 0 7039 to 0 7089 magmas from both series could have derived by melting of a heterogeneous mantle plume the high ti series from a fe rich more fertile source and the low ti series from a fe poor more refractory source in addition the low ti series underwent significant crustal contamination the two magma series evolved along different paths that led to distinct mineralization styles

three dimensional velocity imaging of the kachchh seismic zone gujarat india abstract to understand the causative mechanism of the continued occurrence of earthquakes in kachchh gujarat for the last six years we estimated high resolution three dimensional vp vs and vp vs structures in the aftershock zones of the 2001 mw7 7 bhuj and 2006 mw5 6 gedi earthquakes we used 13 862 p and 13 736 s wave high quality arrival times collected from the seismograms of 2303 aftershocks recorded at 5 18 three component seismograph stations during 2001 06 seismic images revealed a marked spatial variation in the velocities from 20 to 14 in vp from 12 to 13 in vs and from 12 to 12 increase in vp vs in the 0 34 km depth range beneath the bhuj aftershock zone relatively more increase in vp than vs resulting in an increase in vp vs in the crust beneath the seismically active causative fault north wagad fault nwf zone of 2001 bhuj mainshock suggests a rigid mafic crust beneath the region they also delineate an increase of 8 in vp and 14 in vs and a decrease of 4 in vp vs in the almost vertical rupture zone of the 2006 gedi earthquake extending up to 12 km depth this high velocity body associated with the gedi mainshock is inferred to be a gabbroic intrusive the banni region and the wagad uplift are found to be associated with high velocity intrusive bodies inferred to be mafic extending from 5 to 35 km depth which might have intruded during the rifting in early jurassic 160 ma aftershock activity is mainly confined to the zones characterized by high vp high vs and low vp vs ratio which might be representing the strong competent and brittle parts of the fault zone intrusive bodies that could accumulate large strain energy for generating aftershocks for more than six years it is inferred that the crustal stress concentrations associated with the intrusive bodies are contributing significant perturbation to the crustal stress regime to generate the intraplate earthquakes in the kachchh rift zone a few patches of slow vp and vs and high vp vs between 10 to 30 km depth have also been detected on the causative 45 south dipping north wagad fault nwf for the 2001 mainshock which may be attributed to the fluid filled fractured rock matrix interestingly the 2001 bhuj mainshock hypocenter is found to be associated with such a low velocity patch thus it is inferred that the occurrence of 2001 bhuj mainshock might have facilitated by the presence of fluids at hypocentral depth

geochemical and sr nd isotopic characteristics of granitic rocks from northern vietnam abstract five major felsic igneous suites from northern vietnam with ages from mid proterozoic to early cenozoic were studied representative granitic rocks from the posen complex mid proterozoic and the dienbien complex late permian to early triassic show geochemical characteristics similar to those of calc alkaline to high k calc alkaline i type granites however the former located in the south china block has significantly higher initial nd isotopic ratios end t 0 7 to 1 5 and older nd isotopic model ages tdm 1 7 ga than the latter end t 4 7 to 9 7 tdm 1 3 1 5 ga which were emplaced south of the song ma suture and thus in the indochina block the generation of both complexes may be attributed to subduction related processes that occurred in two distinct crustal provenances with different degrees of mantle inputs on the other hand jurassic to cretaceous granitic rocks from the phusaphin complex contemporaneous rhyolites from the tule basin and late paleogene granitic rocks from the yeyensun complex all exposed in the south china block between the ailao shan red river shear zone and the song ma suture display geochemical features similar to those of a type granites with intermediate end t values 0 6 to 2 8 and younger tdm ages 0 6 1 1 ga these magmas are suggested to have been generated as a consequence of intraplate extension in the western part of the south china block yunnan and to have been transported to their present position by mid tertiary continental extrusion along the ailao shan red river shear zone related to the india asia collision overall the isotopic and model age data reported in this study indicate that in northern vietnam the most important crust formation episode took place in the proterozoic likewise repeated mantle inputs have played a role in the petrogenesis of phanerozoic granitic rocks

tectonics and crustal structures related to bhuj earthquake of january 26 2001 based on gravity and magnetic surveys constrained from seismic and seismological studies abstract gravity and magnetic data of the kachchh basin and surrounding regions have delineated major e w and nw se oriented lineaments and faults which are even extending up to plate boundaries in the north arabian sea and western boundary of the indian plate respectively the epicentral zone of bhuj earthquake and its aftershocks is located over the junction of rann of kachchh and median uplifts viz kachchh mainland and wagad uplifts which are separated by thrust faults gravity data with constraints from the results of the seismic studies along a profile suggest that the basement is uplifted towards the north along thrust faults dipping 40 60 south similarly gravity and magnetic modeling along a profile across wagad uplift suggest south dipping 50 60 basement contacts separating rocks of high susceptibility and density towards the north one of these contacts coincides with the fault plane of the bhuj earthquake as inferred from seismological studies and its projection on the surface coincides with the e w oriented north wagad thrust fault a circular gravity high in contact with the fault in northern part of the wagad uplift along with high amplitude magnetic anomaly suggests plug type mafic intrusive in this region several such gravity anomalies are observed over the island belt in the rann of kachchh indicating their association with mafic intrusions the contact of these intrusives with the country rock demarcates shallow crustal inhomogeneities which provides excellent sites for the accumulation of regional stress a regional gravity anomaly map based on the concept of isostasy presents two centers of gravity lows of 11 to 13 mgal 10 5 m s2 representing mass deficiency in the epicentral region their best fit model constrained from the receiver function analysis and seismic refraction studies suggest crustal root of 7 8 km deep crustal inhomogeneity under them for a standard density contrast of 400 kg m3 it is therefore suggested that significant amount of stress get concentrated in this region due to a buoyant crustal root b regional stress due to plate tectonic forces and c mafic intrusives as stress concentrators and the same might be responsible for the frequent and large magnitude earthquakes in this region including the bhuj earthquake of january 26 2001

geological controls on focused fluid flow through the gas hydrate stability zone on the southern hikurangi margin of new zealand evidenced from multi channel seismic data highly concentrated gas hydrate deposits are likely to be associated with geological features that promote increased fluid flux through the gas hydrate stability zone ghsz we conduct conventional seismic processing techniques and full waveform inversion methods on a multi channel seismic line that was acquired over a 125 km transect of the southern hikurangi margin off the eastern coast of new zealand s north island initial processing employed with an emphasis on preservation of true amplitude information was used to identify three sites where structures and stratal fabrics likely encourage focused fluid flow into and through the ghsz at two of the sites western porangahau trough and eastern porangahau ridge sub vertical blanking zones occur in regions of intensely deformed sedimentary layering it is interpreted that increased fluid flow occurs in these regions and that fluids may dissipate upwards and away from the deformed zone along layers that trend towards the seafloor at eastern porangahau ridge we also observe a coherent bottom simulating reflection bsr that increases markedly in intensity with proximity to the centre of the anticlinal ridge 1d full waveform inversions conducted at eight points along the bsr reveal much more pronounced low velocity zones near the centre of the ridge indicating a local increase in the flux of gas charged fluids into the anticline at another anticline western porangahau ridge a dipping high amplitude feature extends from the bsr upwards towards the seafloor within the regional ghsz 1d full waveform inversions at this site reveal that the dipping feature is characterised by a high velocity zone overlying a low velocity zone which we interpret as gas hydrates overlying free gas these results support a previous interpretation that this high amplitude feature represents a local up warping of the base of hydrate stability in response to advective heat flow from upward migrating fluids these three sites provide examples of geological frameworks that encourage prolific localised fluid flow into the hydrate system where it is likely that gas charged fluids are converting to highly concentrated hydrate deposits

a lower crust origin of some flood basalts of the emeishan large igneous province sw china abstract high seismic velocity layers within the lower crust i e 40 km of the yangtze block are interpreted as mafic underplated rocks derived from the late permian emeishan mantle plume however the region experienced a previous magmatic event during the neoproterozoic 800 ma that produced the kangdian basalts and associated mafic intrusions and therefore the lower crust seismic velocity layers may represent a mixture of two different magmatic episodes the identification of inherited neoproterozoic i e 750 to 850 ma zircons within emeishan magmatic rocks indicates either assimilation of older material during emplacement or that the rocks could be derived from a mafic neoproterozoic precursor equilibrium partial melt modeling of neoproterozoic kangdian basalts can produce compositions similar to emeishan basalt at a pressure of 1 2 gpa i e 40 km depth it is suggested that the injection of high temperature picritic magmas into the lower crust of the yangtze block could be sufficient to induce high degrees of melting of the lower crust and produce magmas that are similar in composition to some emeishan mafic rocks and or produce melts that could contaminate pristine emieshan magmas

crustally derived granites in the panzhihua region sw china implications for felsic magmatism in the emeishan large igneous province abstract in the panxi region of the late permian 260 ma emeishan large igneous province elip there is a bimodal assemblage of mafic and felsic plutonic rocks most emeishan granitic rocks were derived by differentiation of basaltic magmas i e mantle derived or by mixing between crustal melts and primary basaltic magmas i e hybrid the yingpanliangzi granitic pluton within the city of panzhihua intrudes sinian 600 ma marbles and is unlike the mantle derived or hybrid granitic rocks the shrimp zircon u pb ages of the yingpanliangzi pluton range from 259 8 ma to 882 22 ma younger ages are found on the zircon rims whereas older ages are found within the cores field relationships and petrography indicate that the yingpanliangzi pluton must be pm anomalies low end 260 ma values 3 9 to 4 4 and high i sr 0 71074 to 0 71507 consistent with a crustal origin the recognition of a crustally derived pluton along with mantle derived and mantle crust hybrid plutons within the panxi region of the elip is evidence for a complete spectrum of sources as a consequence the types of panxi granitoids can be distinguished according to their asi eu eu end t ehf t t zr c and nb ta pm values the diverse granitic magmatism during the evolution of the elip from 260 ma to 252 ma demonstrates the complexity of crustal growth associated with lips

geochemical constraints on the origin of the permian baimazhai mafic ultramafic intrusion sw china the 260 ma baimazhai mafic ultramafic intrusion is considered to be part of the emeishan large igneous province and consists of orthopyroxenite surrounded by websterite and gabbro the intrusion is variably mineralized with a massive sulfide ore body 20 vol in the core of the intrusion silicate rocks have ni cu ratios ranging from 0 3 to 46 with majority less than 7 and are rich in lree relative to hree and show nb and ta anomalies in primitive mantle normalized trace element patterns with low nb th 1 0 4 5 and nb la 0 3 1 0 ratios their nd t values range from 3 3 to 8 4 uniform pd pt 0 7 3 5 and cu pd 100 000 400 000 ratios throughout the intrusion indicate that all the sulfides in the rocks were formed in a single sulfide saturation event modeling suggests that the baimazhai rocks were formed when an mg rich magma became crustally contaminated in a deep seated staging chamber crustal contamination up to 35 drove the magma to s saturation and forced orthopyroxene opx onto the liquidus the crystal bearing magma forced out of the staging chamber was migrated by flow differentiation and consequently the denser sulfide melt and the opx crystals became centrally disposed in the flowing magma to form the baimazhai intrusion

late permian basalts in the yanghe area eastern sichuan province sw china implications for the geodynamics of the emeishan flood basalt province and permian global mass extinction abstract we report the finding of a 20 m thick sequence of massive pyroxene plagioclase phyric basalt lava flows in the yanghe area of the northeastern sichuan basin within the yangtze craton of sw china which were previously considered to be located outside the emeishan flood basalt province this basaltic sequence above the middle permian maokou formation fm is overlain by the late permian longtan fm thus the yanghe basalts should be stratigraphically correlated with the emeishan flood basalts the yanghe basalts show typical oceanic island basalt oib affinity and geochemically resemble emeishan basalts especially in the case of high ti ht basalts from the eastern domain of the emeishan flood basalt province the rocks have low age corrected 87sr 86sr t t 260 ma ratios 0 704158 0 704929 and pb isotopic ratios 206pb 204pb t 18 264 18 524 207pb 204pb t 15 543 15 58 and 208pb 204pb t 38 147 38 519 and positive end t values 3 15 to 3 61 suggesting that the lavas have not undergone any significant crustal contamination the crystallization temperature of clinopyroxene is estimated to be 1368 1420 c suggesting anomalously thermal inputs from a mantle source and a possible plume head origin the fractionation of middle rare earth elements mree to heavy ree hree suggests that these rocks were produced by small degrees of partial melting of mantle peridotite within the garnet spinel transition region the stratigraphic relationships and similar geochemical signatures with the emeishan flood basalts suggest that the yanghe basalts are part of the emeishan flood basalt province and can be considered as the northeastern limit of the emeishan flood basalt province our finding extends the diameter of the emeishan flood basalt province to 1200 1400 km covering an area of up to 7 105 km2 two times more than previously estimated the larger areal extent and giant eruption volume incorporating the sichuan basin lend support to the correlation between the end guadalupian mass extinction and the emeishan flood basalt province

rhyolites in the emeishan large igneous province sw china with implications for plume related felsic magmatism abstract in the emeishan large igneous province elip western yangtze block sw china minor amounts of silicic volcanic rocks 5 are spatially and temporally associated with flood basalts the distribution and origin of these silicic rocks are keys to understanding the processes associated with mantle plume activities in the binchuan and panzhihua areas southwestern elip there are nb ta poor and nb ta rich rhyolites nb ta poor rhyolites interlayered with basalts in the binchuan area occur in the lower part of the emeishan volcanic sequence and have a crustal origin they are characterized by negative nb ta anomalies low end t and high 87sr 86sr i values and zircon saturation temperatures 836 852 c nb ta rich rhyolites are associated with the main phase of the emeishan volcanic sequence in the binchuan and panzhihua areas and are characterized by positive nb ta anomalies oceanic island basalt oib like trace element ratios low 87sr 86sr i ratios positive end t and ehf t values and high a cnk ratios and fe s in addition they have high ga al ratios high field strength elements hfses and high zircon saturation temperatures 920 c typical of aluminous a type silicic rocks similar to the spatially distributed syenitic intrusions of the elip these nb ta rich rhyolites are best interpreted as hybrid products of crystal fractionation of high ti basaltic magmas coupled with minor crustal assimilation 6 14 on the basis of rhyolite melts and ec afc simulations the temporal and spatial distributions of these two types of rhyolite reflect various extents of thermal and mass exchange between mantle derived basaltic magma and crustal material above a mantle plume when the plume head reached the base of the western yangtze block enriched components in the lithospheric mantle were melted and the melts partially ponded near the crust mantle boundary or in the lower crust where they induced partial melting of crustal materials to form nb ta poor rhyolites during the main phase of volcanism large magma chambers and extensive plumbing systems developed due to decompressional melting of the mantle plume in this stage abundant mafic ultramafic and felsic intrusions were formed and nb ta rich rhyolites and a type granites were emplaced

the significance of pge variations with sr nd isotopes and lithophile elements in the emeishan flood basalt province from sw china to northern vietnam abstract new analyses of siderophile lithophile elements and sr nd isotopes in the permian basalts and picrites from northern vietnam the southernmost occurrence of the emeishan flood basalt province together with previously published data are used to address the question of whether any meaningful correlation between these elements and isotopes exists at a province scale the available data show that negative correlations between e nd 87 sr 86 sr i and mantle normalized nb th n are present in the basalts but not in the associated picrites this indicates that crustal contamination is negligible in the picrites but significant in some of the basalts the picrites and basalts from the entire province show negative correlations between rh ru n pt ru n pd ru n and mg number this indicates that ru behaves compatibly whereas rh pt and pd behave incompatibly during magma differentiation the incompatible behavior of rh in natural basaltic systems is also supported by the fact that pt rh n remains constant with decreasing mg number in the lavas depletions of pd and pt and to a lesser degree cu in some basaltic samples characterized by relatively low e nd and nb th n support the notion that sulfide saturation in the magmas was triggered by a combination of siliceous crustal contamination and addition of external sulfur within the entire flood basalt province only the picrites from song da northern vietnam show clear depletion in ir relative to ru these picrites are also characterized higher al 2 o 3 tio 2 and lower mantle normalized la yb 0 2 2 4 than those from elsewhere in the province possibly due to the involvement of an ir depleted fertile mantle component in magma generation at this location

magnetovariational study over a seismically active area in the deccan trap province of western india this paper presents the results of a magnetovariational study undertaken over a basalt covered region of western india which has been experiencing a swarm of seismic activity since early 1986 the most striking feature of the transient variations is the anomalous enhancement of the east west y component at stations bounding the seismically active belt from the nature of the horizontal transfer functions and the anomalous induction arrows it is inferred that this behaviour is related to the presence of a resistive body embedded in a north south oriented conductive belt consistent with an evolutionary model of deccan volcanism and other geophysical data it is suggested that the central resistive block coincident with the concentrated belt of earthquake epicentres represents either a dormant volcanic plug or a plutonic body rooted in upwarped conducting mantle to account for the phase relationship between the anomalous y variations and the normal north south x variations it is proposed that the upwarped asthenosphere with its associated high geothermal anomaly acts as a conducting path to channel induced current from the region of the cambay triple junction located towards the northwest of the array the existence of the strong induced currents in the region of the triple junction conforms with the induction arrow pattern observed at the array stations

precise age determination of mafic and felsic intrusive rocks from the permian emeishan large igneous province sw china abstract radiogenic isotopic dating using different mainly micro beam analyses techniques of rocks from the emeishan large igneous province elip has yielded a range of ages from the capitanian to the early triassic in contrast estimates based on paleontological and paleomagnetic data suggest the elip was emplaced rapidly 2 ma and that volcanism ended by 257 ma new zircon ca tims u pb ages that have superior precision compared to ages from micro beam analyses were obtained from strategic intrusive rocks of the panxi region inner zone of the elip the huangcao 258 9 0 7 ma daheishan 259 1 0 5 ma woshui 259 6 0 5 ma and cida 258 4 0 6 ma syenitic and granitic plutons and three mafic dykes 259 2 0 4 ma 259 5 0 8 ma 257 6 0 5 ma yielded a narrow range of ages between 257 ma and 260 ma the new results are consistent with the estimates from magneto biostratigraphic data and a rapid emplacement model the new wuchiapinqian ages of the huangcao and daheishan plutons suggests that magmatism may have lasted significantly less than 10 ma as previously suggested and the precise ages of the mafic dykes refute the hypothesis that emeishan high ti basalts represent the waning stages of the elip magmatism the short duration of magmatism also implies that degassing of the host sediments mostly carbonates and thus releasing of greenhouse gasses occurred within a substantially shorter interval than previously thought which significantly changes the loading of atmosphere per time unit the latter corollary potentially has severe environmental effects that may have contributed to causing the end guadalupian mass extinction

sedimentary evidence for a rapid kilometer scale crustal doming prior to the eruption of the emeishan flood basalts biostratigraphic and sedimentologic investigations in 67 sections have been carried out for the middle permian maokou formation that immediately underlies the emeishan flood basalts in southwest china the results suggest a domal crustal thinning before the emplacement of the emeishan large igneous province variably thinned carbonates in the maokou formation are capped by a subaerial unconformity which is generally manifested by karst paleotopography paleoweathering zone or locally by relict gravels and basal conglomerates provenance analysis indicates that these gravels and conglomerates were mainly derived from the uppermost maokou formation therefore the stratigraphic thinning likely resulted from differential erosion due to regional uplift iso thickness contours of the maokou formation delineate a subcircular uplifted area in accordance with the crustal doming caused by a starting mantle plume as predicted by experimental and numerical modeling the duration of this uplift is estimated to be less than 3 myr and the magnitude of uplift is greater than 1000 m the sedimentary records therefore provide independent supporting evidence for the mantle plume initiation model for the generation of the emeishan flood basalts

induction models of southern india and the effect of off shore geology abstract simple numerical tests based on thin sheet modelling are used to define the conductors most probably responsible for current channelling in the region including southern india a combination of crustal conductors located in the indo ceylon graben and underneath the comorin ridge best reproduces the observations at periods of 50 100 min in so far as channelling is concerned daytime variations do not help to define the location of conductors any better than night time data and may thus be ignored

gas hydrate growth and dissociation in narrow pore networks capillary inhibition and hysteresis phenomena marine sediments hosting gas hydrates are commonly fine grained silts muds clays with very narrow mean pore diameters 0 1 mm this has led to speculation that capillary phenomena could play an important role in controlling hydrate distribution in the seafloor and may be in part responsible for discrepancies between observed and predicted from bulk phase equilibria hydrate stability zone hsz thicknesses numerous recent laboratory studies have confirmed a close relationship between hydrate inhibition and pore size stability being reduced in narrow pores however to date the focus has been hydrate dissociation conditions in porous media with capillary controls on the equally important process of hydrate growth being largely neglected here we present experimental methane hydrate growth and dissociation conditions for synthetic mesoporous silicas over a range of pressure temperature pt conditions 273 293 k to 20 mpa and pore size distributions results demonstrate that hydrate formation and decomposition in narrow pore networks is characterized by a distinct hysteresis solid growth occurs at significantly lower temperatures or higher pressures than dissociation hysteresis takes the form of repeatable irreversible closed primary growth and dissociation pt loops within which various characteristic secondary scanning curve pathways may be followed similar behaviour has recently been observed for ice water systems in porous media and is characteristic of liquid vapour transitions in mesoporous materials the causes of such hysteresis are still not fully understood our results suggest pore blocking during hydrate growth as a primary cause

two episodes of rees mineralization at the sin quyen iocg deposit nw vietnam abstract the sin quyen iron oxide copper gold iocg deposit in northwestern vietnam contains a proven reserve of 52 8 mt ores at 14 fe 0 91 cu 0 7 rees rare earth elements and 0 44 g t au rees mineralization is hosted within ca silicate alteration zone and associated with fe cu ore bodies in the gneiss mica schist and marble of the upper sin quyen formation two mineralization episodes are identified based on the field and petrographic relations the first episode includes sodic alteration stage i 1 calcic alteration and associated ree mineralization stage i 2 and localized k fe alteration stage i 3 the second episode is main stage mineralization and contains four main stages of alteration pre ore na alteration stage ii 1 ca fe alteration and the associated fe ree mineralization stage ii 2 k fe alteration and associated cu au mineralization stage ii 3 and the final carbonate stage of quartz carbonate veins stage ii 4 crosscutting the above mentioned alteration phases and ores primary allanite and type 1 titanite tnt 1 from episode 1 yield weighted mean 206pb 238u ages of 882 3 ma and 881 8 ma 2 respectively which are interpreted as the timing of ree mineralization in episode 1 secondary allanite and type 2 titanite tnt 2 associated with iron oxide and cu au ores however have u pb ages of 840 7 ma and 838 5 ma 2 respectively these ages are hence considered to represent the timing of polymetallic iocg mineralization in episode 2 880 ma titanite grains have end t values ranging from 7 00 to 5 43 which are intermediate between the neoproterozoic felsic intrusions and ore hosting meta sedimentary rocks in the region and likely reflect interaction between magmatic fluids and ore hosting rocks in contrast 840 ma titanite have end t values ranging from 4 31 to 1 97 similar to those of the neoproterozoic felsic intrusions indicating a possible magmatic fluid source for the episode ii iocg mineralization multiple episodes of mineralization at sin quyen were likely formed during the prolonged tectonothermal events related to backarc extension during the neoproterozoic subduction along the western margin of the yangtze block

zircon u pb and hf isotope constraints on crustal melting associated with the emeishan mantle plume abstract shrimp zircon u pb dates combined with in situ hf isotopic data provide new constraints on the petrogenesis and protolith of peralkaline metaluminous and peraluminous intrusions and rhyolitic tuffs in the emeishan large igneous province with significant bearing on crustal melting associated with mantle plumes syenite and a type granitic intrusions from huili miyi and taihe in the center of this large igneous province yield u pb dates at 260 ma consistent with the ages obtained for mafic layered intrusions in the same province zircon from these rocks exhibits a wide range of initial hf isotope ratios ehf t 1 4 to 13 4 with corresponding tdm1 of 400 900 ma the highest ehf t value is only marginally lower than that of depleted mantle reservoir at 260 ma suggesting that their source is primarily juvenile crust added during emeishan volcanism with incorporation of variable amounts of neoproterozoic crust the trigger of crustal melting is most likely related to advective heating associated with magmatic underplating in contrast the 255 251 ma peraluminous granites from ailanghe and 238 ma rhyolitic tuff from binchuan have negative initial ehf values of 1 3 to 4 4 and of 7 7 to 14 respectively hf isotopic model ages and presence of inherited zircons indicate their derivation from mesoproterozoic and paleoproterozoic crust respectively given the time lag relative to the plume impact 260 ma and insignificant mantle contribution to 255 238 ma magmatism conductive heating is suggested as the trigger of crustal melting that resulted in formation of delayed felsic magmas the involvement of older crust in younger felsic magmas is consistent with upward heat transfer to the lithosphere during plume impregnation if the age of crust is inversely stratified i e changes from paleoproterozoic to mesoproterozoic to neoproterozoic to permian with increasing depth such crust may have resulted from episodic downward crustal growth during the evolution of the western yangtze craton

early paleoproterozoic magmatism in the yangtze block evidence from zircon u pb ages sr nd hf isotopes and geochemistry of ca 2 3 ga and 2 1 ga granitic rocks in the phan si pan complex north vietnam abstract our understanding of the early evolution of the yangtze block is limited by the sparsely dispersed nature of pre neoproterozoic exposures new integrated petrographic zircon u pb age and hf nd isotope analyses and whole rock geochemical data for early paleoproterozoic granites in the phan si pan complex provides new insights into the evolution of the yangtze block as well as its role in the pre nuna supercontinent la icp ms zircon u pb dating of magmatic zircons from quartz monzonite and gneissic granite yielded 207pb 206pb ages of 2306 12 ma and 2096 15 ma respectively zircons from the quartz monzonite have ehf t values ranging from 4 1 to 2 1 corresponding to tdm2 model ages of 3002 2890 ma whereas zircons in the gneissic granite have ehf t values between 0 95 and 1 72 and corresponding tdm2 model ages of 2660 2516 ma which are consistent with their whole rock nd isotope values geochemically the quartz monzonites are i type granites combined with their relatively high sr y ratios and low y concentrations as well as fractionated ree patterns with relatively high lree but low hree concentrations they were probably generated by partial melting of the thickened middle lower crust under elevated temperature geochemical and isotopic signatures suggest that the ca 2 1 ga gneissic granites are high k calc alkaline ferroan a type granites formed by partial melting of juvenile crustal source at high temperature and low pressure with little involvement of ancient crustal material the phan si pan complex has a distinct early paleoproterozoic crustal evolution history compared with the other crustal provinces of the yangtze block suggesting independent histories that were not unified until the late paleoproterozoic during the assembly of nuna moreover the magmatism and tectonic evolution of the north vietnam region is broadly similar to that of the arrowsmith orogen of the rae craton in laurentia suggesting a potential spatial linkage the geologic record of the yangtze block does not support an early paleoproterozoic shutdown of plate tectonics

petrogenesis of eocene to early oligocene granitic rocks in phan si pan uplift area northwestern vietnam geochemical implications for the cenozoic crustal evolution of the south china block abstract the phan si pan uplift area of nw vietnam is a part of the archean to paleoproterozoic yangtze block sw china that was displaced along the ailaoshan red river fault shear zone and adjacent structures i e song da zone the region is of interest as it is at the forefront of the debate regarding the extrusion of se asia during the early paleogene two cenozoic silicic plutonic complexes of contrasting composition intruded the basement rocks of the phan si pan uplift area the ban xeo granite is ferroan a type alkalic and yielded a zircon weighted mean 206pb 238u age of 49 0 9 ma the major and trace element compositions of the ban xeo granite are similar to the spatially associated late permian muong hum granites the end t values 2 5 to 1 4 87sr 86sr initial ratios 0 70793 0 70989 and 0 75341 and zircon ehf t values 1 1 to 3 0 indicate that the ban xeo granite is likely derived by differentiation of a mafic parental magma that had emii like mantle source characteristics the younger ye yen sun yys granite is magnesian alkali calcic to calc alkalic and yielded zircon u pb weighted mean ages of 35 3 0 5 to 34 1 0 4 ma the yys granites have 87sr 86sr initial ratios of 0 70636 to 0 70860 end t values from 8 7 to 2 6 and zircons ehf t values from 9 3 to 0 6 which indicate a crustal origin the yys dioritic rocks range from syeno diorite to granodiorite and have similar sr 87sr 86sr 0 70627 and end t 3 8 values as the yys granites however the zircon hf isotopes ehf t 0 3 to 3 2 of the syeno diorite are slightly more chondritic than the granites and indicates that the parental magma was likely derived from a mantle source rhyolite melts modeling results show that the yys granites were likely generated by fractionation of a parental magma similar in composition to the granodiorite furthermore the yys syeno diorites may be representative of a cumulate rock that developed during the early stages of emplacement or crystallization the parental magma of yys igneous rocks likely formed by partial melting of mafic mesoproterozoic juvenile rocks lower crust uppermost mantle by heat from the upwelling lithospheric mantle during a period of crustal relaxation the formation of the ban xeo granites and ye yen sun igneous rocks are likely related to lateral melt migration along the eastern edge of the india asia collision zone it is possible that repeated episodes of cenozoic magmatism was pivotal in the development and initiation of fault movement of the ailaoshan red river shear zone

first evidence for archean continental crust in northern vietnam and its implications for crustal and tectonic evolution in southeast asia southeast asia or in a more strict sense the indochinese continent was previously considered to be composed entirely of proterozoic to phanerozoic rocks and reliable evidence for archean crust was lacking gneisses from the cavinh complex south of the red river shear zone northern vietnam however show archean nd model ages of 3 4 3 1 ga zircon separates from the rocks yielded u pb dates of 2 8 2 5 ga the first convincing evidence for the presence of a late archean complex in southeast asia given that the red river shear zone was propagating in the south china block with a left lateral offset of 600 km the cavinh complex can be correlated with the late archean kangding complex in the western margin of the yangtze craton southwestern china the cavinh complex therefore represents one of the oldest crustal nuclei of the south china block

gas hydrate fluid flow and free gas formation of the bottom simulating reflector abstract gas hydrate in continental margins is commonly indicated by a prominent bottom simulating seismic reflector bsr that occurs a few hundred metres below the seabed the bsr marks the boundary between sediments containing gas hydrate above and free gas below most of the reflection amplitude is caused by the underlying free gas gas hydrate can occur without a bsr however and the controls on its formation are not well understood here we describe two complementary mechanisms for free gas accumulation beneath the gas hydrate stability zone ghsz the first is the well recognised hydrate recycling mechanism that generates gas from dissociating hydrate when the base of the ghsz moves upward relative to hydrate bearing sediment the second is a recently identified mechanism in which the relationship between the advection and diffusion of dissolved gas with the local solubility curve allows the liquid phase to become saturated in a thick layer beneath the ghsz when hydrate is present near its base this mechanism for gas production called the solubility curvature mechanism is possible in systems where the influence of diffusion becomes important relative to the influence of advection and where the gas water solubility decreases to a minimum several hundred metres below the ghsz we investigate a number of areas in which gas hydrate occurs to determine where gas formation is dominated by the solubility curvature mechanism and where it is dominated by hydrate recycling we show that the former is dominant in areas with low rates of upward fluid flow such as old rifted continental margins low rates of seafloor uplift and high geothermal gradient and or pressure conversely free gas formation is dominated by hydrate recycling where there are rapid rates of upward fluid flow and seabed uplift such as in subduction zone accretionary wedges using these two mechanisms to investigate the formation of free gas beneath gas hydrate in continental margins we are able to resolve a number of problems these include why sub bsr free gas zones are generally thin in convergent margins and thick in passive margins and why areas such as the gulf of mexico and mackenzie delta do not have regional bsrs finally we use our understanding of how bsrs are formed to show that regional gas hydrate is unlikely to exist in substantial quantities in passive continental margins outside areas with an observable bsr

emeishan large igneous province sw china in recent years there have been major advances in our understanding of the emeishan large igneous province lip of sw china following publication of a number of lip focused investigations of the terrain and associated rocks this paper reviews the current state of knowledge the volcanic and upper intrusive portion of the province is relatively small 0 3 106 km3 even when offset fragments eroded sections and buried portions are included in the volume calculation the most reliable radiometric age dates zircon u pb shrimp from an associated layered intrusive body several kilometers in area indicate generation at 259 ma consistent with the end guadalupian end middle permian stratigraphic age in addition several ar ar dating studies have been carried out mainly on the volcanic rocks with a number of reported dates 253 251 ma late permian but a consensus is emerging that these ages are problematic because they are in conflict with the stratigraphic data possibly due to a monitor standard miscalibration the ar ar investigations have also yielded a large number of secondary ages which are clustered at 175 142 98 and 42 ma and these are inferred to record sub regional tectonic events that affected the western yangtze block as east asia was assembled and later deformed by india s collision indentation into asia magnetostratigraphic data and field observations suggest that the bulk of the volcanic sequence formed within 1 2 my the geochemistry of the volcanic rocks and bio lithostratigraphic studies of the underlying maokou formation suggests a mantle plume generated the province the basalts can be classified into low and high ti groups with different parental magmas the low ti basalts are confined to the western part of the province and are overlain by the high ti basalts the low ti magmas formed at shallow mantle depths in the spinel garnet field transition zone 60 80 km whereas the high ti magmas formed by low degrees of partial melting within the deeper garnet stability field this observation suggests a deepening of the melting column as the lithosphere thickened due to under plating and a transition from peak basalt generation to the waning stage outstanding issues which might focus future studies are also outlined

influences of holocene sea level regional tectonics and fluvial gravity and slope currents induced sedimentation on the regional geomorphology of the continental slope off northwestern india abstract regional geomorphology derived from echosounding and bathymetric data lines of a total of 12 000 km depth to basement regional deep seated tectonic features surficial sediment distribution and holocene sedimentation rates of the continental margin off northwestern india are presented results indicate that the shelf edge over most of the region shelf break between 80 and 154 m water depth is covered with relict carbonate facies and partially buried bioherms of holocene age particularly off bombay and saurashtra regions significant changes in the depth and orientation of the shelf edge adjacent to saurashtra and the bombay high areas in relict carbonate facies are recorded at locations shelf edge depth is much more up to 150 m than the early holocene eustatic sea level still stand in this region about 100 m the upper boundary of the slope also varies considerably in depth about 1450 m in the vicinity of the indus fan having high sedimentation and 2900 m further southwards the gradient of the slope off the kachchh is steepest and some regional notches are observed at the upper slope the gentler slope off saurashtra and the bombay high area has large benches at depths of 180 230 m width 2 10 km and 650 780 m well below the holocene sea level the bombay high area has slope breaks between 400 and 600 m whereas off saurashtra steep breaks in the slope occur between 560 and 960 m depth further southwards at the foot of the slope elevations and depressions are present variations in the geomorphology of the slope off the northwestern continental margin of india are attributed to 1 late pleistocene holocene sea levels 2 regional tectonics 3 fluvial sedimentation and 4 gravity and slope currents induced sediment dynamics the locations under influences of slumping turbidity and tectonic movements with altered in situ sequence of sedimentations are delineated

crustally derived granites in dali sw china new constraints on silicic magmatism of the central emeishan large igneous province we have identified a new crustally derived granite pluton that is related to the emeishan large igneous province elip this pluton the wase pluton near dali shows two distinct shrimp zircon u pb age groups 768 and 253 ma as it has an intrusive relationship with devonian limestone the younger age is interpreted as its formation which is related to the elip event whereas the 768 ma neoproterozoic aged zircons were inherited from precambrian crustal component of the yangtze block implying the pluton has a crustally derived origin this is consistent with its peraluminous nature negative nb ta anomaly enrichment in light rare earth elements high 87sr 86sr i ratio 0 7159 0 7183 and extremely negative e nd i values 12 15 to 13 70 indicative of melts derived from upper crust materials the wase pluton intruded devonian strata lie stratigraphically below the shangcang elip sequence which is the thickest volcanic sequence 5400 m in the whole elip the uppermost level of the shangcang sequence contains laterally restricted rhyolite although the rhyolite has the same age as the wase pluton its geochemical features demonstrate a different magma origin the rhyolite displays moderate 87sr 86sr i 0 7053 slightly negative e nd i 0 18 and depletions in ba cs eu and sr implying derivation from differentiation of a mantle derived mafic magma source the coexistence of crustally and mantle derived felsic systems along with the robust development of dike swarms vent proximal volcanics and thickest flood basalts piles in dali shows that the dali area was probably where the most active emeishan magmatism had once existed

focussed fluid flow on the hikurangi margin new zealand evidence from possible local upwarping of the base of gas hydrate stability the southern hikurangi subduction margin is characterized by significant accretion with predicted high rates of fluid expulsion bottom simulating reflections bsrs are widespread on this margin predominantly occurring beneath thrust ridges we present seismic data across the porangahau ridge on the outer accretionary wedge the data show high amplitude reflections above the regional bsr level based on polarity and reflection strength we interpret these reflections as being caused by free gas we propose that the presence of gas above the regional level of bsrs indicates local upwarping of the base of gas hydrate stability caused by advective heatflow from upward migrating fluids although we cannot entirely rule out alternative processes simplified modelling of the increase of the thermal gradient associated with fluid flow suggests that funnelling of upward migrating fluids beneath low permeability slope basins into the porangahau ridge would not lead to the pronounced thermal anomaly inferred from upwarping of the base of gas hydrate stability focussing of fluid flow is predicted to take place deep in the accretionary wedge and or the underthrust sediments above the high amplitude reflections sediment reflectivity is low a lack of lateral continuity of reflections suggests that reflectivity is lost because of a destruction of sediment layering from deformation rather than gas hydrate related amplitude blanking structural permeability from fracturing of sediments during deformation may facilitate fluid expulsion on the ridge a gap in the bsr in the southern part of the study area may be caused by a loss of gas during fluid expulsion we speculate that gaps in otherwise continuous bsrs that are observed beneath some thrusts on the hikurangi margin may be characteristic of other locations experiencing focussed fluid expulsion

a model for the diffusive growth of hydrate saturation anomalies in layered sediments 1 sediment particles affect the phase behavior of gas hydrates both by increasing the surface energy where pore geometry forces hydrate crystals to attain high curvatures and through wetting interactions that cause aqueous films to coat particle surfaces these effects produce only slight changes to the gas solubility through most of the hydrate stability zone so the particle size has only a modest influence on the rate of hydrate accumulation when the sediments are homogeneous in hydrate reservoirs however discontinuous changes in sediment properties are common and such stratigraphic boundaries often coincide with hydrate anomalies these anomalies are a natural consequence of variations in subsurface sediment properties by accounting for sediment hydrate interactions i show how compositional diffusion supplies the growth of hydrate spikes in coarse grained sediments immediately adjacent to hydrate free regions hfrs in more fine grained sediments where the solubility is slightly elevated over timescales comparable with milankovitch cycles hydrate spikes are typically less than a meter in width and contain essentially all of the hydrate that would have otherwise occupied the much larger adjacent hfr if sediment heterogeneities were absent hydrate can form in the more fine grained sediments only once the spike achieves a sufficiently high saturation level often 90 of pore volume that the solubility is continuous across the stratigraphic boundary the wetting interactions that stabilize much of the residual liquid when hydrate forms an interconnected skeleton spanning many pore diameters can also partially unload sediment particle contacts and lead to the growth of segregated hydrate nodules and lenses

zircon u pb ages and hf isotopic compositions of alkaline silicic magmatic rocks in the phan si pan tu le region northern vietnam identification of a displaced western extension of the emeishan large igneous province abstract in situ zircon u pb and hf isotope analyses were carried out for alkaline silicic magmatic rocks from the phan si pan tu le region in northern vietnam to constrain their possible sources and to determine their petrogenetic relationship with the emeishan large igneous province elip sw china nine granites and nine rhyolites yield zircon 206pb 238u ages from 262 ma to 249 ma coinciding with the timing of silicic magmatism in the panxi area of the elip the zircon ehf t values 14 to 3 of these granites and rhyolites suggest a moderately depleted mantle source and overlap with those of peralkaline and metaluminous granites in the panxi area i e ehf t 14 to 4 the zircon hf isotope ratios show that the zircons probably record the original source characteristics whereas whole rock nd isotope data indicate an evidence for crustal contamination that may have occurred at lower temperatures during magma emplacement the synchroneity coupled with petrological and geochemical similarities indicate that silicic rocks from the phan si pan tu le region are cogenetic with the panxi silicic plutonic rocks and that they are likely derived by similar petrogenetic processes i e fractionation of mafic magmas or partial melting of mafic rocks therefore we propose that the phan si pan tu le region represents a displaced portion of the elip inner zone

stress field and seismicity in the indian shield effects of the collision between india and eurasia intraplate stresses and intraplate seismicity in the indian subcontinent are strongly affected by the continued convergence between india and eurasia the mean orientation of the maximum horizontal compression in the indian subcontinent is subparallel to the direction of the ridge push at the plate boundary as well as to the direction of compression expected to arise from the net resistive forces at the himalayan collision zone indicating that the intraplate stresses in the subcontinent including the shield area are caused by plate tectonic processes spatial distribution of historic and instrumentally recorded earthquakes indicate that the seismic activity is mostly confined to linear belts while the remaining large area of the shield is stable the available conventional heat flow data and other indicators of heat flow suggest hotter geotherms in the linear belts leading to amplification of stresses in the upper brittle crust many of the faults in these linear belts which happen to be 200 80 m y old are being reactivated either in a strike slip or thrust faulting mode the reactivation mechanisms have been analyzed by taking into consideration the amplification of stresses pore pressures geological history of the faults and their orientation with respect to the contemporaneous stress field the seismicity of the indian shield is explained in terms of these reactivation mechanisms

early paleoproterozoic ca 2 36 ga post collisional granitoids in yunnan sw china implications for linkage between yangtze and laurentia in the columbia supercontinent abstract the early paleoproterozoic is an essential period for understanding the early tectonic evolution of the yangtze block and its reconstruction in the columbia supercontinent whereas these issues are largely unknown due to the scarce preservation of early paleoproterozoic geological records this study presents rarely reported early paleoproterozoic granitoids from the southwestern yangtze block with integrated studies of zircon u pb ages lu hf isotopic compositions and whole rock geochemistry magmatic zircon u pb dating for two monzogranite samples yields weighted mean 207pb 206pb ages of 2359 16 ma and 2363 16 ma providing unambiguous evidence for the presence of early paleoproterozoic magmatism in the yangtze block all the samples have high a cnk values of 1 30 1 61 typical of s type granite these rocks have low total ree contents of 72 5 139 0 ppm with marked enrichment in lrees la yb n 10 73 23 83 and conspicuously negative eu anomalies eu eu 0 46 0 55 the rocks exhibit uniform primitive mantle normalized trace element patterns with enrichment in rb th u and pb and depletion in sr nb hf and ti these dated zircon grains have consistent negative ehf t values of 6 2 to 2 8 and tdm2 ages of ca 2 97 3 16 ga indicative of mesoarchean crustal materials as their magma source considering the existence of the ca 2 36 ga regional high grade tectono metamorphic event we propose that the ca 2 36 ga monzogranites were most likely generated in the early stage of post collisional extension therefore the southwestern yangtze block likely experienced a collision event during ca 2 40 2 36 ga indicative of the involvement of the yangtze block in the ca 2 5 2 3 ga arrowsmith orogeny of laurentia in addition a series of accretionary events might also have occurred in the western yangtze block before the final assembly of the columbia supercontinent implying a tight spatial connection between the yangtze block and northwestern laurentia within this global scale supercontinent

petrogenesis of late permian silicic rocks of tu le basin and phan si pan uplift nw vietnam and their association with the emeishan large igneous province abstract major and trace elements and whole rock sr nd isotope data of late permian silicic plutonic and volcanic rocks from the phan si pan tu le region in nw vietnam were collected in order to establish their petrogenetic relationship with the magmatic rocks of the song da zone nw vietnam and the alkaline silicic rocks of the panxi area of the emeishan large igneous province elip in sw china the granites and rhyolites have geochemical characteristics of anorogenic granites e g high fe and high ga al and are further subdivided based on mineralogy the phu sa phin and muong hum granites contain sodic to sodic calcic amphiboles and sodic pyroxene while the phan si pan granite does not the phu sa phin and muong hum granites occasionally show peralkaline to metaluminous compositions while the phan si pan granites and the tu le rhyolites have metaluminous to peraluminous compositions the chondrite normalized ree patterns la yb n 3 6 51 3 gd yb n 1 4 3 4 and the primitive mantle normalized spidergrams enrichment in high field strength elements are similar to those of the song da silicic rocks and the panzhihua and taihe granites in the panxi area rhyolite melts modeling suggest that the silicic magmas were likely generated by fractionation of the song da high ti basalts the end t values range from weakly negative to moderately positive values 2 2 to 2 2 suggesting the silicic rocks may be assimilated by basement rocks during magma emplacement permian magmatic rocks in the phan si pan tu le region are petrologically geochemically and geochronologically comparable to those of the inner zone of elip although permian lithospheric mantle and lower crustal structure of the phan si pan tu le region may be destroyed by a lithospheric removal in response to cenozoic india eurasia collision our results along with recent geophysical analyses in nw vietnam suggest that the upper to middle crust of phan si pan tu le region represent a remnant upper to middle crust of inner zone of elip

ocean warming and gas hydrate stability on the mid norwegian margin at the storegga slide abstract the sensitivity of oceanic gas hydrates and submarine slope stability to the combined forcing of sea level changes and bottom water perturbation is a critical issue for risk assessment in the storegga slide area on the mid norwegian margin evidence for the existence of gas hydrates both inside and outside the storegga slide complex comes from reflection seismic profiles where a bottom simulating reflection marks today s base of the gas hydrate stability zone paleo bottom water temperatures show a relatively fast increase at approximately 12 5 10 ka calendar years following the younger dryas while stable warm water conditions have prevailed since then despite sea level rise this warm water inflow caused a major reduction in the thickness of the gas hydrate stability zone along the upper slope of the mid norwegian margin modelling results indicate that critical hydrate stability conditions and consequently the maximal potential pore pressure build up occur at the location of the storegga slide headwall although the major phase of hydrate melting predates the storegga slide event dated at 8 2 ka calendar years reduced hydrate stability conditions could have facilitated or contributed to sub marine slope failure additionally the bottom simulating reflections within the slide complex seem to have nearly adjusted to new equilibrium conditions highlighting the dynamics of hydrate stability in continental margin sediments under environmental changes climate change geohazards

geologic geochemical and geophysical consequences of plume involvement in the emeishan flood basalt province prevolcanic kilometer scale lithospheric doming in the emeishan large igneous province southwest china allows us to evaluate the spatial and temporal consequences of uplift on the paleogeography geology geochemistry and geophysics of the region systematic spatial variations are observed across the domal structure in the distribution and thickness of clastic and carbonate sediments the extent of erosion thickness and chemistry of volcanic rocks and the crust mantle structure these features which are best explained by a mantle plume may be used to track older plume sites in the geologic record

microstructural evolution of gas hydrates in sedimentary matrices observed with synchrotron x ray computed tomographic microscopy the formation process of gas hydrates in sedimentary matrices is of crucial importance for the physical and transport properties of the resulting aggregates this process has never been observed in situ at submicron resolution here we report on synchrotron based microtomographic studies by which the nucleation and growth processes of gas hydrate were observed at 276 k in various sedimentary matrices such as natural quartz with and without admixtures of montmorillonite type clay or glass beads with different surface properties at varying water saturation both juvenile water and metastably gas enriched water obtained from gas hydrate decomposition was used xenon gas was employed to enhance the density contrast between gas hydrate and the fluid phases involved the nucleation sites can be easily identified and the various growth patterns are clearly established in sediments under saturated with juvenile water nucleation starts at the water gas interface resulting in an initially several micrometer thick gas hydrate film further growth proceeds to form isometric single crystals of 10 20 m size the growth of gas hydrate from gas enriched water follows a different pattern via the nucleation in the bulk of liquid producing polyhedral single crystals a striking feature in both cases is the systematic appearance of a fluid phase film of up to several micron thickness between gas hydrates and the surface of the quartz grains these microstructural findings are relevant for future efforts of quantitative rock physics modeling of gas hydrates in sedimentary matrices and explain the anomalous attenuation of seismic sonic waves

electrical conductance map for the kachchh rift basin constraint on tectonic evolution and seismotectonic implications geomagnetic field variations recorded by an array of magnetometers spread across the kachchh rift basin are reduced to a set of induction arrows as a diagnostic of lateral electrical conductivity variations a non uniform thin sheet electrical conductance model is developed to account for the salient induction patterns it indicates that the imaged conductivity anomalies can be related to the sediment filled structural lows in between the fault bounded uplifts it is suggested that sagging structural lows preserved the marine sediments deposited during the mesozoic sea transgression and later developed into first order embayment basins for the deposition of sediments in association with late eocene transgression depth integrated electrical conductance helped in mapping two depo centres along the ene wsw trending banni half graben bounded by the kachchh main fault on the south and second along the vinjan depression formed in response to the subsidence between the vigodi fault and westward extension of the katrol hill fault together with the westward bending of the median high presence of metamorphosed graphite schist clasts in shale dominated mesozoic sequence and or thin films of carbon resulting from the thermal influence of deccan activity on carbonate rich formations can account for the high electrical conductivity anomalies seen in the depo centres of thick mesozoic and tertiary sediments additionally two high conductivity zones are imaged encompassing a block defined by the 2001 bhuj earthquake and its aftershocks in agreement with gravity magnetic and seismic velocity signatures aqueous fluids released by recrystallizing magmatic bodies intruded in association with deccan trap activity account for mapped high conductivity zones high fluid pressure in such a fractured domain surrounding the intruded magmatic plugs perturb the regional stress concentrations to produce frequent and low magnitude aftershocks in the shallow section of the epicentral track of the 2001 bhuj earthquake

constraints on gas hydrate distribution and morphology in vestnesa ridge western svalbard margin using multicomponent ocean bottom seismic data source at https doi org 10 1029 2018jb016574 2019 american geophysical union all rights reserved

timing of magmatism and ultrahigh to high grade metamorphism in the kannak complex kon tum massif vietnam magmatic activity and its tectonic implications abstract detailed geochronological and petrological investigations of metamorphic and igneous rocks can significantly improve our understanding of the tectonic processes that occur in orogenic belts the kon tum massif central vietnam includes various types of magmatic and metamorphic rocks the latter including ultrahigh to high temperature units the late permian early triassic metamorphic rocks in the massif correspond to the metamorphic characteristics of a collision between the indochina and south china blocks in terms of their metamorphic evolution and timescales of deformation u pb geochronological analyses of zircon obtained from two samples of garnet granite in the massif yielded magmatic ages of 239 6 3 9 ma and 243 5 3 4 ma this permitted the reevaluation of previous th u pb electron microprobe analyses of monazite from the garnet granite and garnet orthopyroxene gneiss and previously published u pb dating these data reveal the metamorphic and magmatic history of the western part of the massif the metamorphic rocks show a thermal evolution from 260 ma to 240 ma magmatic activity started at 260 ma and ceased at 240 ma considering this in terms of the regional scale igneous activities in the kon tum massif the base of the massif already thickened by the collision was situated above underplating plume related magma such magmatic activity resulted in high to ultrahigh temperature metamorphism and crustal melting during the late permian to early triassic between 260 and 240 ma

morphotectonics and mechanics of the central makran accretionary wedge off pakistan abstract swath bathymetry images and 3 5 khz data acquired during rv sonne cruise 123 in autumn 1997 are the base for analysing the morphology and deformation styles of the huge makran accretionary wedge the continental slope off pakistan is characterised by three morphological provinces a the lower slope is built of a sequence of imbricate thrust slices which form long and narrow accretionary ridges with flanks locally as steep as 20 and between 10 and 20 on average while the regional slope is only 1 2 b the nearly flat mid slope terrace is present in the central east part of the margin narrows significantly towards the east and is absent east of 63 45 and c the upper slope up to the shelf has a regionally uniform slope as steep as about 8 and comprises a very rough topography with many gullies and canyons and evidence for mass wasting two meandering canyons crossing the entire margin down to the abyssal plain at about 63 15 63 30 can be correlated with onshore rivers they have been formed by episodic erosion through turbidity flows and are characterised by erosional and depositional portions sinuosity of the canyons is exclusively caused by the accretionary ridges functioning as obstacles for flow to be directly downhill application of coulomb rheology to the frontal part of the makran wedge implies that the mid level decollement is intrinsically extraordinary weak with a great strength contrast to the overlying sediments comparison with nankai cascadia and the western mediterranean ridge reveals new insights in the parameters controlling the shape of an accretionary wedge and the role of a mid level decollement

2 d geoelectrical model for the parnaiba basin conductivity anomaly of northeast brazil and tectonic implications abstract a magnetometer array study in the north northeast of brazil has revealed a roughly ne sw trending conductive structure in the southeastern part of the intracratonic parnaiba basin the magnetovariational response functions of this structure are numerically modelled to constrain its geometry to facilitate its geological and tectonic interpretation the 2 d numerical model that incorporates the ocean effect and can account for the spatial and period dependence of the observed response locates the source regions of enhanced conductivity in a graben structure in the basement as well as in a block confined to the central part of the basin with an embedded resistive body the anomalous electrical character of the sediments in the central part of the basin is consistent with the magnetotelluric data the graben structure in the basement is corroborated by the aeromagnetic data the formation of the graben structure is considered to be a manifestation of the extensional tectonics associated either with the brasiliano orogeny or with the jurassic cretaceous magmatic events the diabase dikes intruded in the basin in association with the jurassic cretaceous magmatic activity are shown to be accountable for the mapped resistive body entrapped in the conducting paleozoic sediments the thermal effects associated with magmatic activities are invoked to produce enhanced conductivity by the generation of carbon through the pyrolysis of hydrocarbon saturated sediments

sediment thicknesses and qs vs qp relations in the kachchh rift basin gujarat india using sp converted phases delineation of the top sedimentary structure and its qs vs qp relationship using the travel time difference of direct s and converted sp phase is key to understanding the seismic hazard of any sedimentary basin area we constructed filtered displacement waveforms from local etna episensor acceleration recordings as well as local velocity recordings of aftershocks of the 2001 bhuj earthquake recorded by the kachchh seismological network of the national geophysical research institute ngri hyderabad india during 2001 2004 stations are within 15 70km of epicenters and the resulting displacement waveforms are generally simple displaying prominent p sp and s wave pulses particle motion of p and s waves suggest near vertical raypaths consistent with preliminary depth estimates the direct s wave on the horizontal component is characterized by lower frequency content than the converted sp phase on the vertical component this difference in frequency content between s and sp phases can be explained in terms of different attenuation effects for p and s waves in the unconsolidated sediments the sp phase is generated by s to p phase conversion at the base of mesozoic sediments of the kachchh basin travel time inversion velest of 2565 p and 2380 s arrivals from 658 well located aftershocks recorded at 8 14 three component local seismic stations led to 1 d velocity models indicated very slow sediments in the upper 0 2 km depth range vp 2 92 km s and vs 0 90 km s and an increasing trend of velocities with depth at 2 40 km depth the estimated sediment thicknesses beneath 12 accelerograph and 6 seismograph sites from the estimated velocity model and the travel time difference between s and converted sp phases reaches a maximum of 1 534 0 117 km beneath bandri near the location of 2001 bhuj mainshock and attains a minimum sediment thickness of 0 858 0 104 km beneath ramvav and burudia the spectral ratios between sp and s from 159 three component accelerograms have been used to study seismic wave attenuation in the kachchh rift basin the estimated qs vs qp relations for 12 accelerograph sites vary from qs 0 184 qp at chobari to qs 0 505 qp at dudhai for stations chobari chopdwa jahawarnagar vondh and tapar the spectral ratio slopes and hence the calculated qs vs qp relations are effectively the same and the correlation coefficients are quite high 0 91 0 93 stations adhoi manfara new dudhai dudhai and sikara have similar qs vs qp relationships to each other and also have high correlation coefficients 0 78 0 87 the spectral ratios for stations anjar and ramvav are small and poorly constrained resulting in less reliable qs vs qp relations this could be due to noisy data fewer available waveforms or scattering due to velocity heterogeneities and or interface irregularities

capillary effects on hydrate stability in marine sediments 1 we study the three phase liquid gas hydrate stability of the methane hydrate system in marine sediments by considering the capillary effects on both hydrate and free gas phases the capillary pressure a measure of the pressure difference across a curved phase interface exerts a key control on the methane solubility in liquid hydrate l h and liquid gas l g systems by calculating the l h and l g solubilities as a function of water depth pressure and pore size interface curvature we show how the solubility requirements for forming both gas hydrate and free gas can be met in a three phase zone the top of the three phase zone shifts upward in sediments as the water depth increases and the mean pore size decreases the thickness of the three phase zone increases as the distribution of pore sizes widens the top of the three phase zone can overlie or underlie the bulk three phase equilibrium depth at blake ridge we predict that the three phase zone is 27 7 m thick and that the top of the three phase zone lies 13 m above the predicted bulk equilibrium depth this reconciles the observation of the bottom simulating reflector bsr at blake ridge that is shallower than the predicted bulk equilibrium depth in contrast at hydrate ridge where water depth is shallower we predict that the three phase zone is 20 4 m thick and that the top of the three phase zone lies 0 7 m below the predicted bulk equilibrium depth our model which predicts an upward shift in the top of free gas occurrence with increasing water depth pressure is compatible with worldwide observations that the bsr is systematically shifted upward relative to the bulk equilibrium depth as water depth pressure is increased

reappraisal of the jianchuan cenozoic basin stratigraphy and its implications on the se tibetan plateau evolution abstract we present a new stratigraphy of the jianchuan basin one of the largest cenozoic sedimentary basins in southeastern tibet this basin was regarded as recording sedimentation from the eocene up to the pliocene and as such has been the focus of several studies aiming at constraining the environmental tectonic and topographic evolution of the area within the shuanghe and jianchuan formations thirteen new zircon u pb ages and one biotite 40 ar 39 ar age of interbedded and cross cutting ultrapotassic magmatic rocks show that a brief magmatic event occurred from 35 7 to 34 5 ma 35 2 0 4 ma on average the uppermost formation jianchuan fm supposedly pliocene in age is related to this magmatic event and is 35 4 0 8 ma old all sedimentary formations are thus eocene in age with neither oligocene nor miocene sediments the coal bearing shuanghe formation yields a fossil of a large amynodontid typical of the upper eocene ergilian interval 37 2 to 33 9 ma sedimentation of the shuanghe formation took place in a short time interval at 35 9 0 9 ma after a large scale drainage reorganization that induced the abandonment of a large braided river system this reorganization was possibly linked with the initiation of the left lateral ailao shan red river fault and or to widespread magmatism in the jianchuan basin previous high paleoaltitude estimates for the jianchuan basin are thoroughly re evaluated and yield a value of 1200 1200 m a s l for the upper eocene

electrical imaging of deep crustal features of kutch india summary a regional magnetotelluric mt study was carried out with 55 mt soundings distributed along five traverses across the kutch mainland unit kmu on the west coast of india a region characterized by a series of successive uplifts and intervening depressions in the form of half graben bounded by master faults we obtain the deeper electrical structure of the crust beneath kutch from 2 d modelling of mt data along the 5 traverses in order to evaluate the geo electrical signatures if any of the known primary tectonic structures in this region the results show that the deeper electrical structure in the kutch region presents a mosaic of high resistive crustal blocks separated by deep rooted conductive features two such crustal conductive features spatially correlate with the known tectonic features viz the kutch mainland fault kmf and the katrol hill fault khf an impressive feature of the geo electrical sections is an additional well defined conductive feature running between jakhau and mundra located at the southern end of each of the five mt traverses and interpreted to be the electrical signature of yet another hidden fault at the southern margin of the kmu this new feature is named as jakhau mundra fault jmf it is inferred that the presence of jmf together with the kathiawar fault nkf further south located at the northern boundary of the saurashtra horst would enhance the possibility of occurrence of a thick sedimentary column in the gulf of kutch the region between the newly delineated fault jmf and the kathiawar fault nkf could thus be significant for hydrocarbon exploration

the effect of lithologic heterogeneity on diffusive methane migration in hydrate systems abstract short range diffusive gas migration has been proposed as a means of supplying methane to coarse grained hydrate bearing sediments in this scenario methane is generated microbially in fine grained sediments and moves diffusively into adjacent coarse grained sediments due to a dissolved methane concentration gradient caused by pore size contrasts previous work has shown that short range diffusion is sufficient to supply thin isolated sands 1 m thickness with enough methane to achieve high hydrate saturations 80 in systems with a high ratio of clay to sand several notable hydrate reservoirs such as those encountered at daini atsumi knoll in the nankai trough and walker ridge block 313 and green canyon block 955 in the gulf of mexico exhibit tens of meters of alternating sand silt and clay layers that are decimeters in thickness the coarse grained layers in these settings are typically filled with hydrate to saturations exceeding 80 while short range diffusion is capable of filling thin isolated sands with hydrate thicker sands within more heterogeneous systems require mechanisms other than or in addition to short range diffusion using 1 d numerical simulations we demonstrate that short range diffusive migration of microbial methane is insufficient for reaching consistently high hydrate saturations in such systems this work shows that a different mechanism for gas transport such as advective migration is necessary to achieve these observed hydrate saturations in layered systems

zircon u pb and hf isotopic constraints from eastern transhimalayan batholiths on the precollisional magmatic and tectonic evolution in southern tibet abstract before the indian collision with asia northward subduction of the neo tethyan oceanic lithosphere produced an andean type convergent margin in south asia characterized by arc magmatism starting from the early jurassic and lasting until the eocene the magmatic arc is now represented by widespread intrusive bodies or the so called transhimalayan batholiths in the lhasa terrane of southern tibet that have been divided into two main magmatic suites i e the northern plutonic belt and the southern gangdese batholith their temporal distribution concerning how exactly the magmatic suites correlate eastwards and then southeastwards around the eastern himalayan syntaxis however remains poorly constrained here we report the first combined zircon u pb and hf isotopic study of the transhimalayan batholiths from the eastern part of the lhasa terrane 95 97 5 e and 28 5 30 n se tibet zircon u pb dating results of 24 granitoids indicate that the rocks were emplaced principally in the early cretaceous 133 110 ma and subordinately in the paleocene 66 57 ma long after an older granite intrusion in the earliest jurassic 198 ma the zircon ehf t values range from 5 to 20 yielding hf crustal model ages tdmc between 0 8 and 2 4 ga that peak at 1 7 ga suggesting a major episode of crustal growth in the proterozoic and a predominantly or even exclusively crustal source for the granitoid petrogenesis these zircon u pb and hf isotopic constraints together with whole rock geochemical characteristics allow us to correlate them to s type granitoids in the northern plutonic belt rather than to the i type gangdese batholith from the central part of the southern lhasa terrane in contrast to the gangdese magmatism that formed with a significant juvenile mantle input related to the neo tethyan subduction these s type granitoids show geochemical similarities to the north american cordilleran interior batholiths that did not form directly from a subduction setting thus we attribute the petrogenesis of the widespread early cretaceous granitoids in the northern belt and eastern himalayan batholiths to a postcollisional regime due to the late jurassic early cretaceous continental collision between the lhasa and qiangtang terranes under this framework the s type magmatism was generated in response to collision induced crustal thickening which may have capabilities of causing crustal anatexis by itself in the region however continued interplay with the neo tethyan subduction zone processes such as back arc extension could also have played a crucial and long lasting role in the magma generation

three fe ti oxide ore bearing gabbro granitoid complexes in the panxi region of the permian emeishan large igneous province sw china the permian 260 ma emeishan large igneous province of sw china contains three nearly identical gabbro granitoid complexes that host giant fe ti oxide deposits the fe ti oxide deposits are within the lower portions of evolved layered gabbroic intrusions and are spatially and temporally associated with a type granitic plutons the 264 3 ma taihe layered gabbroic intrusion hosts a large magmatic fe ti oxide deposit and is coeval with the taihe peralkaline a type granitic pluton which is dated at 261 2 ma within the a type granitic pluton are microgranular enclaves which have compositions intermediate between the gabbro and host granite primitive mantle normalized incompatible element plots show corresponding reciprocal patterns between the mafic and felsic rocks the chondrite normalized ree patterns show eu anomalies changing from positive eu eu 1 5 to 5 9 in the gabbroic intrusion to negative in the enclaves eu eu 0 4 to 0 6 and granites eu eu 0 2 to 0 5 whole rock end t values of the gabbroic intrusion end t 2 5 to 3 3 are similar to those of the enclaves end t 1 0 to 2 0 and granite end t 1 5 to 1 9 whereas the zircon ehf t values of the gabbro ehf t 8 1 0 8 are indistinguishable from those of the granites ehf t 9 2 1 0 suggesting that the parent magmas for all rock types originated from the same mantle source geochemical modeling indicates that the gabbros and granites can be generated by fractional crystallization of a common parental magma similar to high ti emeishan flood basalt the compositional jump from the gabbro to the enclaves is attributed to the crystallization of fe ti oxide minerals the results of this study and other studies suggest that the magmatic conditions for example pressure composition fo2 which led to the formation of at least three fe ti oxide bearing gabbro granitoid complexes were relatively common during the development of the emeishan large igneous province

neoproterozoic granitoids from the phan si pan belt northwest vietnam implication for the tectonic linkage between northwest vietnam and the yangtze block abstract several neoproterozoic granitic intrusions including the phin ngan intrusion and some smaller ones from the sin quyen cu mine have been identified in the phan si pan belt northwest vietnam whole rock geochemical and sm nd isotopic data and zircon u pb age and hf o isotopes have been determined for these intrusions in order to constrain their ages petrogenesis and tectonic implications the phin ngan intrusion mainly composed of syenogranite was emplaced at 824 4 ma rocks from this intrusion have high sio 2 72 1 73 5 wt and k 2 o 5 14 5 52 wt with a cnk values ranging from 1 02 to 1 06 they have negative whole rock e nd t 8 4 to 5 2 and zircon e hf t values 8 1 to 5 5 and high zircon 18 o values 9 7 10 9 these geochemical features suggest that the phin ngan intrusion was derived from ancient k rich crustal sources granitic intrusions from the sin quyen mine occur as stocks or dykes intruding ores and ore hosting rocks they are composed of monzogranite and granodiorite emplaced between 736 8 and 758 6 ma they have high sio 2 68 2 76 7 wt and k 2 o na 2 o 6 93 8 54 wt and low mgo 0 19 0 87 with a cnk values ranging from 0 91 to 1 14 their whole rock e nd t values range from 6 6 to 0 mostly clustered between 6 6 and 4 3 their zircon e hf t values vary from 6 7 to 11 1 mostly lower than 2 0 such geochemical data indicate that these intrusions were produced mainly through partial melting of ancient crustal materials coupled with subordinate involvement of mantle juvenile crust derived components the phin ngan intrusion and intrusions from the sin quyen mine are sub alkaline in nature and have arc like trace element compositions compiled with other broadly synchronous igneous rocks with arc like affinities in the region the neoproterozoic magmatism in the phan si pan belt was generated in a subduction related setting the neoproterozoic intrusions in the phan si pan belt are geochronologically and geochemically correlated with the neoproterozoic subduction related igneous rocks along the western yangtze block thus the phan si pan belt represents the southern most part of the giant arc system of the western yangtze block

regional tectonic framework structure and evolution of the western marginal basins of india abstract the kutch saurashtra cambay and narmada basins are pericontinental rift basins in the western margin of the indian craton these basins were formed by rifting along precambrian tectonic trends interplay of three major precambrian tectonic trends of western india dharwar nnw sse aravalli delhi ne sw and satpura ene wsw controlled the tectonic style of the basins the geological history of the basins indicates that these basins were formed by sequential reactivation of primordial faults the kutch basin opened up first in the early jurassic rifting was initiated in late triassic along the delhi trend followed by the cambay basin in the early cretaceous along the dharwar trend and the narmada basin in late cretaceous time along the satpura trend the evolution of the basins took place in four stages these stages are synchronous with the important events in the evolution of the indian sub continent its breakup from gondwanaland in the late triassic early jurassic its northward drifting during the jurassic cretaceous and collision with the asian continent in the early tertiary the most important tectonic events occurred in late cretaceous time the present style of the continental margins of india evolved during early tertiary time the saurashtra arch the extension of the aravalli range across the western continental shelf subsided along the eastern margin fault of the cambay basin during the early cretaceous it formed an extensive depositional platform continuous with the kutch shelf for the accumulation of thick deltaic sediments a part of the saurashtra arch was uplifted as a horst during the main tectonic phase in the late cretaceous the present high thermal regime of the cambay bombay high region is suggestive of a renewed rifting phase

integrated interpretation for sub basalt imaging in saurashtra basin india a major part of the estimated global reserves of hydrocarbons occurs in sediments overlain by thick basaltic sequences which makes imaging structures in the basalt covered older sediments a key exploration challenge in many frontier areas e g in the west african continental margins the atlantic margin of northwest europe and a number of indian basins both offshore and on land saurashtra peninsula in the western part of india figure 1 with thick mesozoic sediments overlain by thick basalt of the deccan trap extruded during late cretaceous early paleocene is one such indian prospect however it is virtually unexplored because of the limitations of conventional p wave seismic in imaging the structures in the sub basalt sedimentary sequences and the intrabasalt features

permian peralkaline peraluminous and metaluminous a type granites in the panxi district sw china their relationship to the emeishan mantle plume abstract numerous permian granitic intrusions plutons in the panxi region sw china are spatially and temporally associated with the emeishan large igneous province elip granites from the 260 ma panzhihua and taihe plutons are peralkaline asi 0 76 1 12 na k al 0 78 1 2 and have high fe values 0 92 0 99 with negative eu anomalies eu eu 0 22 0 98 contrasted with the peralkaline plutons syenites from the 260 ma woshui pluton are metaluminous asi 0 84 1 0 na k al 0 91 1 01 and have moderately high fe 0 79 0 81 and positive eu anomalies eu eu 0 82 1 91 both the peralkaline and metaluminous rocks have mantle isotopic signatures end t 1 3 3 2 and were derived from the elip mantle plume source the peralkaline rocks were likely derived by fractional crystallization of the temporally spatially and chemically associated layered mafic intrusions which host giant magmatic fe ti v oxide deposits the metaluminous granitic rocks although spatially related to the peralkaline granites and layered gabbros were likely generated by partial melting of underplated mafic rocks granites from the 251 ma ailanghe pluton are peraluminous asi 0 97 1 29 and have high fe 0 82 0 91 values and negative eu anomalies eu eu 0 10 0 37 their crustal isotopic signature end t 5 7 to 6 7 indicates possible derivation from melting of yangtze block basement rocks the ailanghe pluton is contemporaneous with other a type granites in the region supporting the view that elip magmatism was sporadic for at least 9 million years the coexistence of three chemically exclusive a type granites in the elip suggests that a variety of a type granites can be generated in the same tectonic setting

the 2001 bhuj earthquake tomographic evidence for fluids at the hypocenter and its implications for rupture nucleation 1 the january 26 2001 bhuj earthquake mw 7 6 is one of the most catastrophic indian earthquakes we have investigated the 3 d seismic velocity and poisson s ratio structures of the bhuj source area to understand the probable cause of triggering the earthquake we used 1948 p and 1865 s wave high quality arrival times from 331 aftershocks recorded at a temporary seismic network significant variations up to 5 in velocity and 10 in poisson s ratio are revealed in the aftershock area the mainshock is located in a distinctive zone characterized by high vp low vs and high poisson s ratio in the depth range of 20 to 30 km and extending 15 to 30 km laterally this feature is very similar to that of the 1995 kobe earthquake zhao et al 1996 the anomaly may be due to a fluid filled fractured rock matrix which might have contributed to the initiation of the bhuj earthquake

burial driven methane recycling in marine gas hydrate systems abstract natural gas hydrate may be buried with sediments until it is no longer stable at a given pressure and temperature resulting in conversion of hydrate into free gas this gas may migrate upward and recycle back into the hydrate stability zone to form hydrate as of yet however no quantitative description of the methane recycling process has been developed using multiphase flow simulations to model burial driven gas hydrate recycling in this study we present a series of 1d multiphase transport simulations to investigate the methane recycling process in detail by invoking the effects of capillary phenomena on hydrate and gas formation in pores of varying size we find that a free gas phase can migrate a significant distance above the bulk base of hydrate stability since the top of the free gas occurrence is often identified as the base of the hydrate stability zone from seismic data our results demonstrate that not only could this assumption mischaracterize a hydrate system but that under recycling conditions the highest hydrate saturations can occur beneath the top of the free gas occurrence we show that the presence of pore size distributions requires a replacement zone through which hydrate saturations progressively decrease with depth and are replaced with free gas this replacement zone works to buffer against significant gas buildup that could lead to fracturing of overlying sediments this work provides a framework for simulating flow and transport of methane within the 3 phase stability zone from a mass conservation perspective

methane recycling between hydrate and critically pressured stratigraphic traps offshore mauritania three dimensional seismic reflection data from offshore mauritania reveal a base hydrate reflection beneath which are 56 stratigraphically trapped gas accumulations 0 08 gt methane only 2 are sealed by the base of the hydrate 0 004 gt methane and therefore constitute the free gas zone some of the stratigraphically trapped gas accumulations are critically pressured there are also 360 gas chimneys beneath the gas hydrate but not outside the region covered by it they are evidence for repetitious leaking from the accumulations only 12 of the chimneys terminate at a contemporaneous seabed showing that methane venting was not significant instead upward resetting of the base hydrate triggered gas dissociation and caused the traps to become charged and critically pressured vertical leaking to intermediate shallower stratigraphic traps or to the gas hydrate itself where it was reincorporated occurred once the seal capacity was exceeded most 95 of the free gas is stored stratigraphically and re migrates in this way and this trapping mechanism may be typical of fine grained passive margin continental slopes only 5 of the gas is within the free gas zone the role of recycling of methane between the hydrate and stratigraphic traps has not previously been recognized and represents a store for this greenhouse gas that is not susceptible to changes in ambient conditions

2 9 2 36 and 1 96 ga zircons in orthogneiss south of the red river shear zone in viet nam evidence from shrimp u pb dating and tectonothermal implications abstract orthogneissic rocks coexisting with migmatites and containing small amphibolite lenses are exposed in the center of the metamorphic belt which runs parallel to the day nui con voi red river shear zone in northern viet nam the orthogneiss complex has given some radiogenic dates of early proterozoic and late archean which are the oldest ages ever registered for the southeast asian continent zircon grains separated from three samples of the orthogneiss complex have been dated to establish the protolith age and the timing of high grade tectonothermal events in the complex sixty five shrimp u th pb analyses of these zircons define three age groups of 2 84 2 91 2 36 and 1 96 ga the age groups correspond to three periods of zircon generation the oldest 2 9 ga cores indicate a minimum age for the protolith of the orthogneiss complex two younger generations including 2 36 ga outer cores and 1 96 ga rims probably grew during later high grade tectono metamorphic events which were previously suggested by k ar and 40 ar 39 ar cooling ages of 2 0 ga for synkinematic hornblendes an early thermal history of the orthogneiss complex has been constrained including a primary magma crystallization stage starting at 2 9 ga followed by two early proterozoic 2 36 and 1 96 ga high grade tectonothermal events the ca 2 9 ga protolith age of the orthogneiss complex documented in this study provides new convincing evidence for the presence of archean rocks in indochina and clearly indicates that the crustal evolution of northern viet nam started as early as late archean time

imaging methane hydrates growth dynamics in porous media using synchrotron x ray computed microtomography commercial scale methane ch4 extraction from natural hydrate deposits remains a challenge due to among other factors a poor understanding of hydrate host sediment interactions under low temperature and high pressure conditions that are conducive to their existence we report the use of synchrotron x ray computed microtomography cmt to image for the first time time resolved pore scale methane ch4 hydrate growth from an aqueous solution containing 5 wt barium chloride bacl2 and pressurized ch4 hosted in glass beads all contained in an aluminum cell with an effective volume of 3 5 ml multiple two dimensional 2 d cross sectional images show ch4 hydrates with 7 5 m resolution distributed in patches throughout the system without dependence on distance from the cell walls the time resolved three dimensional 3 d images constructed from the 2 d slices exhibited pore filling hydrate formation from dissolved ch4 gas similar to natural ch4 hydrates si in the marine environment furthermore the 3 d images show that the aqueous phase was the wetting phase of the glass beads i e the host and the formed hydrate were separated by an aqueous layer these results provide some fundamental understanding of the nucleation phenomenon of gas hydrate formation at the pore scale pore filling ch4 hydrate growth is likely to result in a reduced bulk modulus and thus could affect seafloor stability during the reverse phenomenon i e dissociation of natural hydrate deposits

water permeability in hydrate bearing sediments a pore scale study permeability is a critical parameter governing methane flux and fluid flow in hydrate bearing sediments however limited valid data are available due to experimental challenges here we investigate the relationship between apparent water permeability k and hydrate saturation sh accounting for hydrate pore scale growth habit and meso scale heterogeneity results from capillary tube models rely on cross sectional tube shapes and hydrate pore habits thus are appropriate only for sediments with uniform hydrate distribution and known hydrate pore character given our pore network modeling results showing that accumulating hydrate in sediments decreases sediment porosity and increases hydraulic tortuosity we propose a modified kozeny carman model to characterize water permeability in hydrate bearing sediments this model agrees well with experimental results and can be easily implemented in reservoir simulators with no empirical variables other than sh results are also relevant to flow through other natural sediments that undergo diagenesis salt precipitation or bio clogging

the connection between natural gas hydrate and bottom simulating reflectors bottom simulating reflectors bsrs on marine seismic data are commonly used to identify the presence of natural gas hydrate in marine sediments although the exact relationship between gas hydrate and bsrs is undefined to clarify this relationship we compile a data set of probable gas hydrate occurrence as appraised from well logs of 788 industry wells in the northern gulf of mexico we combine the well log data set with a data set of bsr distribution in the same area identified from 3 d seismic data we find that a bsr increases the chances of finding gas hydrate by 2 6 times as opposed to drilling outside a bsr and that the wells within a bsr also contain thicker and higher resistivity hydrate accumulations even so over half of the wells drilled through bsrs have no detectable gas hydrate accumulations and gas hydrate occurrences and bsrs do not coincide in most cases

sedimentation controls on methane hydrate dynamics across glacial interglacial stages an example from international ocean discovery program site u1517 hikurangi margin

formation of gas hydrate from dissolved gas in natural porous media abstract migration of methane bearing fluids from the depth is commonly proposed to explain the formation of marine gas hydrate this formation mechanism is viable only if the gas hydrate can nucleate and grow directly from an aqueous solution while phase equilibrium calculations suggest that the gas hydrate should crystallize from the dissolved gas barriers associated with nucleation may impede or even prevent hydrate formation at present experimental support for the phase equilibrium calculations is insufficient to confirm whether the hydrate can form in the sea floor from the dissolved gas here we report on a set of experiments in which the hydrate is crystallized from dissolved co 2 in natural porous media the experiments demonstrate that the hydrate crystals can nucleate in the absence of free gas providing important experimental justification for models of hydrate formation that depend on fluid migration our results are also relevant for other formation models that depend on in situ methane generation from biogenic sources experimental evidence for hydrate formation at low gas concentrations implies less stringent requirements on the biogenic sources of methane compared with some previous estimates