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

structural and geochronological constraints on the tectonic evolution of the dulong song chay tectonic dome in yunnan province sw china abstract the dulong song chay tectonic dome lies on the border of china se yunnan province and northern vietnam and consists of two tectonic and lithologic units a core complex and a cover sequence separated by an extensional detachment fault these two units are overlain unconformably by late triassic strata the core complex is composed of gneiss schist and amphibolite shrimp zircon u pb dating results for the orthogneiss yield an age of 799 10 ma which is considered to be the crystallization age of its igneous protolith formed in an arc related environment a granitic intrusion within the core complex occurred with an age of 436 402 ma which probably formed during partial closure of paleotethys within the core complex metamorphic grades change sharply from upper greenschist low amphibolite facies in the core to low greenschist facies in the cover sequence there are two arrays of foliation within the core complex detachment fault and the cover sequence s 1 and s 2 the pervasive s 1 is the axial plane of intrafolial s 0 folds d 1 deformation related to this foliation is characterized by extensional structures the strata were structurally thinned or selectively removed along the detachment faults indicating exhumation of the dulong song chay tectonic dome the major extension occurred at 237 ma determined by shrimp zircon u pb and 39 ar 40 ar isotopic dating techniques regionally simultaneous tectonic extension was associated with pre indosinian collision between the south china and indochina blocks the s 2 foliation appears as the axial plane of nw striking s 1 buckling folds formed during a compressional regime of d 2 d 2 is associated with collision between the south china and indochina blocks along the jinshajiang ailao shan suture zone and represents the indosinian deformation the dulong granites intruded the dulong song chay dome at 144 2 140 2 and 116 10 ma based on 39 ar 40 ar measurement on muscovite and biotite the dome was later overprinted by a conjugate strike slip fault and related thrust fault which formed a vortex structure contemporaneously with late cenozoic sinistral movement on the ailao shan red river fault

the evolution of gravel bed channels after dam removal case study of the anaconda and union city dam removals the anaconda and union city dams on the naugatuck river in connecticut were removed in february and october 1999 a detailed study of the sites prior to removal was undertaken including sediment testing and predictions of upstream channel formation post dam removal the 3 35 m high timber crib rock fill spillway of the anaconda dam partially breached during a storm prior to the dam s scheduled removal allowing a portion of the impounded sediment to move down through the river system this event changed the removal plans and the remainder of the spillway was removed under an emergency order in the course of 4 days the union city dam a 2 44 m high timber crib rock fill dam capped with concrete and stone was removed on schedule a portion of the impounded sediment was removed by mechanical means during the deconstruction of the structure the evolution of the two upstream channels post project provided unique challenges and valuable insights as to what kind of channel transition can be expected in gravel bed river systems after a low head dam has been removed this paper describes the initial engineering analysis and design the subsequent removal of the two dams and compares observations on the transition of the upstream channels following dam removal to the initial engineering predictions and other models the relatively steep gravel bed channels evolved in a predictable manner except where anthropogenic barriers sanitary sewer rock weir interrupted

channel adjustments following two dam removals in wisconsin 1 we examined channel response following the removal of low head dams on two lowgradient fine to coarse grained rivers in southern wisconsin following removal channels eroded large quantities of fine sediment resulting in deposition 3 5 km downstream at one site baraboo river upstream changes were rapid and included bed degradation minimal bank erosion and sediment deposition on channel margins and new floodplain sand was transported through the former impoundment and temporarily deposited downstream at the second site koshkonong river head cut migration governed channel adjustments a deep narrow channel formed downstream of the headcut with negligible changes upstream of the head cut fluvial changes were summarized in a conceptual channel evolution model that highlighted 1 similarities between adjustments associated with dam removal and other events that lower channel base level and 2 the role of reservoir sediment characteristics particle size cohesion in controlling the rates and mechanisms of sediment movement and channel adjustment index terms 1815 hydrology erosion and sedimentation 1824 hydrology geomorphology 1625 1857 hydrology reservoirs surface keywords dams dam removal channel evolution model

understanding sedimentation in the song hong yinggehai basin south china sea the cenozoic song hong yinggehai basin in the south china sea contains a large volume of sediment that has been used in previous studies together with regional geomorphology to argue for the existence of a large palaeodrainage system that connected eastern tibet with the south china sea to test this and to understand the significance of sediment volumes deposited in the song hong yinggehai basin this study compared erosion histories of source regions with sediment volumes deposited during the two main stages in basin evolution spanning active rifting and subsidence 30 15 5 ma and postrift sedimentation 15 5 ma to present the study of basin provenance by detrital zircon u pb dating revealed hainan was an important and continuous source of sediment and a bedrock thermochronological study quantified its overall contribution to basin sedimentation comparison between the accumulated mass of basin sediment and volumes of eroded bedrock calculated from apatite thermochronometry across the modern red river drainage in northern vietnam as well as hainan island accounted for the bulk of sediment deposited since 30 ma consequently if an expanded paleodrainage ever existed it must have predated the oligocene

baleen whale distribution and seasonal occurrence revealed by an ocean bottom seismometer network in the western indian ocean abstract from the acoustic data acquired by the rhum rum reunion hotspot and upper mantle reunions unterer mantel ocean bottom seismometer obs network between october 2012 and november 2013 this study revealed baleen whale occurrence in the western indian ocean io low frequency songs from three species antarctic blue whales pygmy blue whales and fin whales as well as p calls or spot calls from an unknown species were recorded on the dataset the wide arrangement of the obs network 2000 km 2000 km provided valuable information to draw seasonal patterns of occurrence and distribution all over the area these species occurred sympatrically in the western io at least during austral autumn months emphasizing the importance of this region for these populations this data set helped to refine the knowledge on their spatio temporal distribution and complete the picture built by previous studies a tighter sub network of 8 obss deployed on the south west indian ridge provided ideal inter sensor spacing for whale tracking we demonstrated the capability of such array of detecting and tracking the three different whale species up to 50 km and for several hours as a result and to understand the effect of acoustic wave propagation songs from the tracking were described at a close and remote distance of the sensor this work could also help to understand the local behavior of these species during austral autumn months in this area of the western indian ocean

a model of channel response in disturbed alluvial channels dredging and straightening of alluvial channels between 1959 and 1978 in west tennessee caused a series of morphologic changes along modified reaches and tributary streams degradation occurred for 10 to 15 years at sites upstream of the area of maximum disturbance and lowered bed levels by as much as 6 1 m following degradation reaches upstream of the area of maximum disturbance experienced a secondary aggradation phase in response to excessive incision and gradient reduction aggradation downstream of the area of maximum disturbance reached 0 12 m per year with the greatest rates occurring near the stream mouths the adjustment of channel geometry and phases of channel evolution are characterized by six process oriented stages of morphologic development premodified constructed degradation threshold aggradation and restabilization down cutting and toe removal during the degradation stage causes bank failure by mass wasting when the critical height and angle of the bank material is exceeded threshold stage channel widening continues through the aggradation stage as the slough line develops as an initial site of lower bank stability the bank profile develops three dynamic elements 1 vertical face 70 to 90 2 upper bank 25 to 50 and 3 slough line 20 to 25 alternate channel bars form during the restabilization stage and represent incipient meandering of the channel

tectonics and denudation adjacent to the xianshuihe fault eastern tibetan plateau constraints from fission track thermochronology the xianshuihe xiaojiang fault system extending from eastern tibet to central yunnan china is a major left shear structural boundary accommodating the clockwise rotation of crustal rocks between the eastern himalayan syntaxis and the south china block zircon and apatite fission track ft data are reported for 111 samples of basement collected from both sides of the northern part of this fault and spanning 300 km across the eastern margin of the tibetan plateau where its mean elevation drops from 3500 to 1500 m above sea level the zircon ft ages define two fossil partial annealing zones at different elevations one fossilised at circa 130 ma and the other at circa 21 ma as a result of cooling probably via regional denudation the apatite ft ages are mostly less than 25 ma but a few granitoids higher up on the plateau retain late cretaceous apparent ages within the apatite ft data with neogene ages there may be several partial annealing zones with mixture modeling identifying age components suggestive of discrete cooling phases at circa 22 7 and 2 ma the first order pattern of the minimum amount of neogene denudation for a section across the plateau margin immediately northeast of xianshuihe fault suggests that a relatively uniform 4 6 km has been eroded from the inner part of the plateau margin increasing to 7 10 km at kangding where the oldest rocks precambrian are exposed and then decreasing markedly into sichuan basin these new ft data combined with published ft data suggest that the present extent of the tibetan plateau was defined during the early miocene

geochemical evolution of oligocene middle miocene sediments in the deep water area of the pearl river mouth basin northern south china sea abstract the deep water area of the pearl river mouth basin in the south china sea has received much scientific attention since the ocean drilling program odp leg 184 in 1999 due to its potential economic prospects and distinct tectono sedimentary evolutionary processes in this study we present the composition of major and trace elements from two newly sampled deep water boreholes by6 and lw3 in the baiyun sag of the southern pearl river mouth basin the geochemical evolution in the oligocene middle miocene as well as potential controlling factors are investigated based on a comparative study with previous data from odp site 1148 and borehole py33 the chemical index of alteration cia and a cn k plot reveal that the observed weathering trends are not compatible for the four discussed boreholes sedimentary sorting is primarily observed in borehole py33 where data trend away from the a apex to the feldspar join in the a cn k plot and show a spread of al 2 o 3 sio 2 and nb zr ratios compared to chemical weathering and hydrodynamic sorting provenance has a greater impact on sediment composition of the deep water area from the north the pearl river was the primary sediment supply however a positive eu anomaly and the provenance discrimination diagrams i e la th versus th yb and zr co versus th co reveal the mafic nature of borehole by6 sediments in the zhuhai lower zhujiang 32 0 18 5 ma and upper hanjiang 13 8 10 5 ma formations these compositions are unusual and differ from the well defined felsic sources in the majority of the baiyun sag these discrepancies are likely related to multistage magmatism the sediments at site 1148 are characterized by slightly enriched heavy rare earth elements and relatively high zr co ratios which could possibly be caused by zircon enrichment from local sources

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

understanding mesozoic accretion in southeast asia significance of triassic thermotectonism indosinian orogeny in vietnam results from a zircon u pb study of the metamorphic basement of vietnam reveal that a large part of the continental crust was affected by a short lived episode of ductile deformation and high temperature metamorphism between 258 6 ma and 243 5 ma although coincident with final stages of north south china collision qinling orogenesis the thermotectonism in vietnam was caused by accretion of sibumasu to indochina south china this accretion event indosinian orogeny has regional significance because it contributed to the final stages of north south china collision an aspect not explicitly recognized in qinling orogenic models

the delimitation between the mature and juvenile crustal provinces in se asia insights from detrital zircon u pb and hf isotopic data for the salween drainage myanmar abstract to understand the geological control and tectonic forcing on the development of a major river system in se asia new detrital zircon u pb and hf isotopic data from the salween drainage myanmar is reported three samples with a range of deposition ages corresponding to different tectonic periods of the sibumasu terrane reveal a dramatic change in the drainage system differing from the detrital compositions of modern sediments from the salween river mouth a late triassic sandstone sample shows zircon age distribution implying that the salween drainage may have been connected at that time to the northeastern part of the songpan ganzi terrane and western part of the south china block u pb ages and hf t isotopic data of detrital zircons from other major rivers in se asia including the yangtze red and mekong rivers in the east and the irrawaddy river and the yalu tsangpo in the west are further synthesized a marked discrepancy in zircons younger than jurassic ages is identified similar to results from the salween samples these young zircons from the eastern rivers are overwhelmed by an old recycled crustal component exhibiting negative hf t values in contrast those from the western rivers are dominated by juvenile components that are characterized by high and positive hf t values the salween drainage therefore lies in the frontier with its western outline broadly depicting the boundary between mature and juvenile crustal provinces in se asia

direct dating of left lateral deformation along the red river shear zone china and vietnam 1 exposures of high grade midcrustal rocks within the red river shear zone rrsz which separates the indochina and south china blocks exhibit clear evidence of left lateral ductile deformation assuming that the south china sea represents a pull apart basin formed at the southeastern termination of the rrsz it has been argued that seafloor magnetic anomalies constrain the timing of sinistral slip accommodated by the rrsz between 32 and 17 ma at a rate of 4 cm yr while 40ar 39ar thermochronometry indicates that left lateral slip occurred along the rrsz between 25 and 17 ma the timing of earlier high temperature deformation has not been directly constrained in situ th pb ion microprobe dating of monazite inclusions in garnets allows direct assessment of the timing of amphibolite grade metamorphism and synchronous left lateral shearing results from northern segments of the rrsz in yunnan china indicate that synkinematic garnet growth occurred between 34 and 21 ma and are the first to document late oligocene metamorphism and left lateral shearing data from the southern rrsz within vietnam are complicated by tertiary overprinting of rocks that experienced amphibolite facies metamorphism during the indosinian orogeny 220 ma the period during which sinistral deformation is now constrained to have occurred along the rrsz i e 34 17 ma is essentially coincident with spreading of the south china seafloor 32 17 ma this temporal and kinematic link between left lateral shearing along the rrsz and opening of the south china sea supports the view that indochina was extruded from asia as a block along lithospheric scale strike slip faults

the impact of single natural vegetation elements on flow characteristics in this experimental study measurements were conducted to explore the impacts of different forms of individual natural vegetative elements within the flow domain on velocity and turbulence characteristics all the experiments were performed in a flume measuring 26 m in length 0 98 m in width and 0 85 m in depth and real tree saplings were utilized to represent the vegetation element in order to analyse this commonly observed nature phenomenon in floodplains trees with wide trunks were classified into three groups on the basis of their volume versus height relation throughout the velocity measurements three acoustic doppler velocimeters were employed time averaged velocity streamwise and vertical turbulence intensities and turbulent kinetic energy parameters were examined additionally a formulation that gives the velocity profile at a certain distance downstream of vegetation was introduced and the validity of the proposed formulation was checked with experimental data it is seen that despite their porous structures the presence of vegetation considerably disturbs the flow field and dissipates a remarkable amount of energy by turbulence copyright 2008 john wiley sons ltd

the 20 hz signals of finback whales balaenoptera physalus the 20 hz signals of finback whales balaenoptera physalus were analyzed from more than 25 years of recordings at a variety of geographic locations on near surface hydrophones close to whales and on deep hydrophone systems these signals were composed of 1 s pulses of sinusoidal waveform with downward sweeping frequency from approximately 23 to 18 hz at variable source levels up to 186 db re 1 pa at 1 m usually with slightly lower levels for the pulses at the beginning and end of sequences these 20 hz pulses were produced in signal bouts separated by more than 2 h lasting as long as 32 5 h bouts were composed of regularly repeated pulses at intervals of 7 26 s typically either at one nominal pulse rate or at two alternating doublet pulse intervals signal bouts were interrupted by rests of 1 20 min at roughly 15 min intervals and by irregular gaps lasting between 20 and 120 min the distribution of these signals throughout the year and their temporal sequence were analyzed from the cont

late miocene provenance change on the eastern margin of the yinggehai song hong basin south china sea evidence from u pb dating and hf isotope analyses of detrital zircons abstract understanding the provenance of the reservoir in a sedimentary basin is of great importance in hydrocarbon exploration and production the yinggehai song hong basin is one of the most important cenozoic petroliferous basins in the south china sea to better understand the provenance characteristics of the upper miocene huangliu formation tortonian messinian on the eastern margin of the basin and test the influence of uplift of the tibetan plateau in the south china sea 172 single zircons from two sandstone samples in the lingtou gas field were dated by u pb chronometer and 54 of these grains were spot analyzed for hf isotopes the results indicate that the upper member of the huangliu formation in this gas field shows two major peaks of u pb ages at ca 250 ma and ca 432 ma along with four subordinate peaks at ca 757 ma ca 1926 ma ca 2529 ma and ca 2775 ma the initial hf isotope ratios ehf t of these zircons are negative to positive for each age group suggesting that existing crustal materials were mixing with new mantle melts in some magmatic episodes in contrast the lower member of the huangliu formation show two major age peaks at ca 98 ma and 248 ma with a subordinate peak at ca 1453 ma the ehf t values are concentrated between 11 6 and 3 4 revealing that they were derived from older crust comparing with the source characteristics of tectonic units surrounding the basin the source of the upper member in the gas field was mainly from the southern yangtze block whereas the lower member was derived from the hainan uplift together with the change of climate and sedimentation rate after 10 ma on the northern margin of the south china sea we attribute the provenance change to the uplift of the tibetan plateau at the same time

recent channel adjustments in alluvial rivers of tuscany central italy drastic channel adjustments have affected the main alluvial rivers of tuscany central italy during the 20th century bed level adjustments were identified both by comparing available topographic longitudinal profiles of different years and through field observations changes in channel width were investigated by comparing available aerial photographs 1954 and 1993 98 bed incision represents the dominant type of vertical adjustment and is generalized along all the fluvial systems investigated the arno river system is the most affected by bed level lowering up to 9 m whereas lower incision generally less than 2 m is observed along the rivers of the southern part of the region human disturbances appear to be the dominant factors of adjustments the main phase of vertical change occurred during the period 1945 80 in concomitance with the phase of maximum sediment mining activity at the regional scale the second dominant type of adjustment that involved most of the rivers in the region consists of a narrowing of the active channel based on measurements of channel width conducted on aerial photographs 38 of the reaches analysed experienced a narrowing greater than 50 of the initial channel width the largest values of channel narrowing were observed along initially braided or sinuous with alternate bars morphologies in the southern portion of the region a regional scheme of channel adjustments is derived based on initial channel morphology and on the amounts of incision and narrowing different styles of channel adjustments are described rivers that were originally sinuous with alternate bars to braided generally became adjusted by a moderate incision and a moderate to intense narrowing in contrast sinuous meandering channels mainly adjusted vertically with a minor amount of narrowing copyright 2003 john wiley sons ltd

the sedimentary and tectonic evolution of the yinggehai song hong basin and the southern hainan margin south china sea implications for tibetan uplift and monsoon intensification 1 the yinggehai song hong basin is one of the world s largest pull apart basins lying along the trace of the red river fault zone in the south china sea south of hainan island this basin crosscuts the rifted margin of the northern south china sea in this paper we present for the first time a regional compilation of multichannel seismic reflection data from both the strike slip and rifted margins the basins started to open after similar to 45 ma especially after similar to 34 ma the yinggehai basin was folded and inverted in the middle miocene after 21 ma in the north and 14 ma in the south before rapidly subsiding again after similar to 5 ma because of continued tectonism this subsidence has caused shale diapirism especially driven by associated sedimentation in the late pliocene 2 6 2 0 ma extension along the adjacent south hainan margin shows preferential lower crustal extension suggestive of lower crustal flow increasing toward the continent ocean transition during breakup sediment supply is reconstructed to peak in the middle miocene then falls between 14 and 10 3 ma to reach a low in the late miocene however rates rose again in the pliocene pleistocene the red river sediment budget is incompatible with climate models that propose stronger monsoon rains starting at 8 ma or with large scale river capture away from the red river after similar to 10 ma both lines of evidence point to major uplift in the red river drainage being middle miocene or older the recent preindustrial red river carried much more sediment than the average pleistocene accumulation rate indicating modest sediment buffering onshore at least in recent geologic time

geochemical and nd isotope composition of detrital sediments on the north margin of the south china sea provenance and tectonic implications a major re organization of regional drainages in eastern tibet and southwestern china took place in the cenozoic as deformation from the growing himalayas and tibetan plateau affected an increasingly wider area the effects of these changes on the regional sediment routing systems is not well constrained this study examines the geochemical and nd signatures of sedimentary rocks from the ying qiong and nanxiong basins on the northern margin of the south china sea to constrain and identify any significant changes in sediment source upper cretaceous to lower eocene sedimentary rocks in the nanxiong basin show higher th sc la sc th cr and th co ratios and lower eu eu ratios than paas post archaean australian shale which indicates that palaeozoic sedimentary rocks of the south china block were the main basin sediment source in contrast oligocene to pleistocene sedimentary rocks of the ying qiong basin show an abrupt change in these trace element ratios between 16ae3 and 10ae4 ma indicating a mid miocene shift in provenance end values from the ying qiong basin range 11ae 1t o 2ae1 reinforce this with pre 13ae8 ma sedimentary rocks having average end of 5ae6 range 2ae 1t o 7ae4 and post 13ae8 ma sedimentary rocks having average end of 9ae3 range 8ae 7t o 11ae1 during the oligocene the centre of rifting transferred south and basins on the north margin of the south china sea experienced rapid subsidence further uplift and erosion then exposed mesozoic and cenozoic granites that supplied large amounts of granitic detritus especially to the ying qiong basin then a change occurred at ca 13 ma resulting in less input from local sources i e the fault blocks formed by mesozoic cenozoic tectonics and magmatism to an increasing contribution of older continental material mostly from indochina to the west of the south

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

the structure and formation of diapirs in the yinggehai song hong basin south china sea abstract the occurrence of shale diapirs in the yinggehai song hong ygh sh basin is well documented as is their association with big petroleum fields in order to better understand how and why the diapirs form we performed a detailed geophysical analysis using a new regional compilation of high resolution two and three dimensional seismic reflection data as well as drilling data that cover the diapirs in ygh sh basin as many as 18 diapirs were identified and are arranged in six n s striking vertical en echelon zones on seismic reflection sections gas chimney structures diapiric faults and palaeo craters are genetically linked with the process of diapirism here we use geophysical and geological observations to propose a three stage model for diapirism initiation emplacement and collapse during these three stages different diapiric structure styles are formed which we describe in detail these include buried diapirs piercing diapirs and collapsed diapirs we link the diapirism to activity on the offshore continuation of the red river fault as shown on our high resolution seismic reflection data which is also related to a high paleogeothermal gradient caused by crustal thinning we also recognize the role of loading by the very large volume of sediment eroded from the edges of the tibetan plateau and delivered by the red river to the basin

cenozoic plate reconstruction of southeast asia abstract the india eurasia collision alone set up a series of chain reactions and caused the formation and destruction of sedimentary basins within the domain of the collision belt changes in the rate and angle of convergence between the india and eurasia plates reflect different stages of tectonic development in southeast asia for example extrusion of the indochina block induced the consumption of the pre existing proto south china sea along northeastern kalimantan and led to the eventual opening of the south china sea along the south china margin subsequent motions of the sino burma thailand malay peninsula sumatra and kalimantan blocks have produced a succession of basins stretching from north sumatra to central thailand and on to the natuna area we present reconstructions of the southeast asia region at 60 ma 50 ma 40 ma 30 ma 20 ma 15 ma 10 ma and 5 ma it is clear from the reconstructions that the impact between greater india and southeast asia took place in the northwestern part of southeast asia the duration for the impact was probably from the middle eocene to early miocene this timing is in good agreement with the dating of the main red river fault motion wu et al 1989 scharer et al 1990 since the impact between greater india and southeast asia was basically west of the burma block there is no reason to assume that the sumatra malay peninsula and kalimantan should extrude to the southeast first along the left laterally displaced mae ping and three pagodas fault zones as suggested by peltzer and tapponnier 1988 if the opening of the central thailand basins the gulf of thailand and the malay basin are taken into consideration a dextral megashear zone is required to compensate the relative motion between indochina and the malay peninsula this dextral megashear zone might even extend into western kalimantan and serve as a boundary between the indochina block and kalimantan

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

permo triassic granitoids in the northern part of the truong son belt nw vietnam geochronology geochemistry and tectonic implications abstract the northern segment of the truong son belt of northwestern vietnam is composed of three granitoid complexes chieng khuong dien bien and song ma respectively our new la icp ms u pb dating results of magmatic zircons from five granitoid samples in the dien bien and the chieng khuong complexes reveal the existence of three age populations of magma emplacement and crystallization from 276 to 202 ma the new data show that the quartz diorite of the chieng khuong complex is dated to be 271 ma however the previously named dien bien complex exposed at different localities has various ages from 281 to 202 ma of magmatic crystallization the chieng khuong quartz diorite exhibits high mg value 60 7 and enriched sr nd isotopic features i sr 0 713822 end t 11 34 with subduction related geochemical features e g depletion in nb ta and ti and enrichment in rb and la the dien bien gabbroic diorite at muong lay is characterized by relatively depleted isotopic data i sr 0 705050 end t 0 09 and extensive depletion in nb and ta by contrast the dien bien granodiorite and quartz monzonite at dien bien are high k calc alkaline to calc alkaline with high mg values 45 5 to 54 4 and they display enriched sr nd isotopic data i sr 0 714912 to 0 716430 end t 9 58 to 8 85 and post collisional geochemical characteristics e g enrichment in total alkali lile and lree and depletion in eu sr p ti and nb under the framework of regional tectonic evolution the permo triassic magmatism along the northern truong son belt is thus grouped into an early stage from ca 280 to 250 ma and a late stage from 229 to 202 ma the early permian early triassic arc magmatism 280 270 ma and 250 245 ma which is widely distributed in the truong son belt provides reliable information on the continuous subduction of the song ma oceanic plate beneath the indochina block furthermore the magmatism of the dien bien complex at dien bien was formed during post collisional extension at ca 229 to 202 ma diachronous suturing of the eastern tethyan evolution is characterized by the differences in the ages of subduction post collisional evolution along the ailaoshan and the song ma belts

palaeozoic and mesozoic tectonic evolution and palaeogeography of east asian crustal fragments the korean peninsula in context east and southeast asia comprises a complex assembly of allochthonous continental lithospheric crustal fragments terranes together with volcanic arcs and other terranes of oceanic and accretionary complex origins located at the zone of convergence between the eurasian indoaustralian and pacific plates the former wide separation of asian terranes is indicated by contrasting faunas and floras developed on adjacent terranes due to their prior geographic separation different palaeoclimates and biogeographic isolation the boundaries between asian terranes are marked by major geological discontinuities suture zones that represent former ocean basins that once separated them in some cases the ocean basins have been completely destroyed and terrane boundaries are marked by major fault zones in other cases remnants of the ocean basins and of subduction accretion complexes remain and provide valuable information on the tectonic history of the terranes the oceans that once separated them and timings of amalgamation and accretion the various allochthonous crustal fragments of east asia have been brought into close juxtaposition by geological convergent plate tectonic processes the gondwana derived east asia crustal fragments successively rifted and separated from the margin of eastern gondwana as three elongate continental slivers in the devonian early permian and late triassic late jurassic as these three continental slivers separated from gondwana three successive ocean basins the palaeo tethys meso tethys and cenotethys opened between these and gondwana asian terranes progressively sutured to one another during the palaeozoic to cenozoic south china and indochina probably amalgamated in the early carboniferous but alternative scenarios with collision in the permo triassic have been suggested the tarim terrane accreted to eurasia in the early permian the sibumasu and qiangtang terranes collided and sutured with simao indochina east malaya in the early middle triassic and the west sumatra terrane was transported westwards to a position outboard of sibumasu during this collisional process the permo triassic also saw the progressive collision between south and north china with possible extension of this collision being recognised in the korean peninsula culminating in the late triassic north china did not finally weld to asia until the late jurassic the lhasa and west burma terranes accreted to eurasia in the late jurassic early cretaceous and proto east and southeast asia had formed palaeogeographic reconstructions illustrating the evolution and assembly of asian crustal fragments during the phanerozoic are presented d 2005 international association for gondwana research published by elsevier b v all rights reserved

origin and assembly of the tethyside orogenic collage at the expense of gondwana land abstract the tethysides are a superorogenic complex flanking the eurasian continent to the south and consisting of the cimmerides and alpides products of palaeo and neo tethys respectively we here review their evolution mainly on the basis of new maps showing the distribution of sutures magmatic rocks certain palaeobiogeographically and palaeoclimatologically significant taxa and facies and fragments of pan african 900 450 ma orogenic system forming the basement of many tethyside blocks these are supplemented by palaeomagnetic data reported in the literature a fundamental tenet of this paper is that major sutures which contain ophiolite fragments represent tectonic sections between continental blocks where oceanic crust has been subducted palaeo tethys came into existence largely in late carboniferous time coevally it began to be consumed by both internal and peripheral subduction zones which continued into the permian some of these had been inherited from pre tethyan times in the later permian rifting subparallel with the northern margin of gondwana land began between the zagros and malaysia separating a cimmerian continent from n gondwana land and thus heralding the opening of neo tethys and other smaller oceans that were back arc basins of palaeo tethys this rifting possibly also extended farther west into crete and mainland greece however the north china block yangtze block huanan block the eastern moity of the qangtang block north tibet and annamia all originally pieces of the end proterozoic early palaeozoic gondwana land had already separated from it in pre late carboniferous times possibly during the devonian all of these blocks and the cimmerian continent were characterized by cathaysian floral elements in late palaeozoic time palaeomagnetic and palaeontological data showing the original gondwana land affinity of these continental blocks are supplemented by correlating late proterozoic early palaeozoic pan african sutures orogenic belts and sedimentary basin fragments across tethyside sutures late permian foraminiferal provinces are related to this palaeogeographical interpretation by triassic times most cimmeride subduction zones were already in existence the cimmerian continent accelerated its separation from gondwana land and locally in the late permian began disintegrating internally along the waser rushan pshart banggong co nu jiang mandalay ocean by late triassic time all of the chinese blocks except lhasa and annamia had collided with each other and with laurasia the resulting enormous orogenic collage had a soft cushion between itself and laurasia in the form of the enormous accretionary complex of the songpan ganzi this connection enabled laurasian land vertebrates to reach south east asia by late triassic time in late triassic to middle jurassic times most major cimmeride collisions were completed widespread aridity in central asia occurred in late jurassic time probably in the rain shadow of the newly formed cimmeride mountain wall neo tethyan subduction systems formed along the s margin of the cimmerides or within neo tethyan oceanic lithosphere during the jurassic most if not all were north or east dipping they continued the northerly migration of the tethyside blocks evolution of the tethysides influenced the distribution of marine and terrestrial organisms and affected sea level changes and patterns of atmospheric circulation during much of the mesozoic and cainozoic it is likely to have reflected the surface expression of a persistent trend in the large scale convective circulation in the mantle that continuously transported material northward into the tethyan domain

the generation and expulsion of gases in ya131 gas field south china sea implication of laboratory pyrolysis results abstract laboratory pyrolysis methods were used to determine the source and thermal maturity of the gas accumulation in the ya131 gas field south china sea the mudstone source rock samples used in the pyrolysis experiments were selected from the yacheng and meishan formations in the ying qiong basin the total amount of gases generated and expelled from the yacheng formation mudstone is larger than that from the meishan formation mudstone suggesting that the gas generation potential of the yacheng formation is higher than that of the meishan formation the similarity between the gases from the ya131 field and the gases produced at the higher pyrolysis temperatures used in the maturation study in terms of wetness the amounts of c 1 c 2 c 3 and the percentages of c 1 c 2 c 4 and c 5 c 7 implies that the gases of the ya131 field are high maturity gases however there are distinct differences between the normalised molecular compositions of the c 6 and c 7 species of the gases from the ya131 field and those of the pyrolysis gases produced in the laboratory comparison of the stable carbon isotope composition of the n alkanes pristane pr and phytane ph in the saturated hydrocarbon fraction of the condensate from the ya131 field with that of rock extracts from the yacheng meishan ying huang formations indicates no definite correlation between the condensate and the extracts these observations suggest that there may be additional source rocks for the ya131 field existing within the deeper part of the basin

provenance of sediments during subduction of palaeotethys detrital zircon ages and olistolith analysis in palaeozoic sediments from chios island greece abstract detrital zircon geochronology and analysis of fossiliferous olistoliths from chios island greece are used here to constrain terrane accretion processes and the provenance of crustal sources for sediments during the subduction of a branch of the palaeotethys ocean u pb ages obtained by ion microprobe shrimp ii analyses of detrital zircons from a carboniferous greywacke belonging to the tectonostratigraphic lower unit of chios gave major age groups of 2150 1890 ma 640 540 ma 505 475 ma and 365 322 ma detrital zircons from a permian triassic sandstone yielded prominent age clusters of 2200 1840 ma 1100 910 ma 625 560 ma and 385 370 ma the lack of zircon ages between 1 8 and 1 1 ga in both samples coupled with the occurrence of ca 2 ga old zircons imply a northern gondwana nw africa source the conodont fauna recovered from an orthoceras bearing limestone lens from the carboniferous succession of the lower unit indicates a late silurian age the fauna is typical of the ludfordian polygnathoides siluricus conodont zone the material has a conodont colour alteration index cai of about 1 2 indicating very low grade thermal alteration of less than 100 c the closest localities with similar conodont bearing limestones are to be found in the balkan region and in the istanbul zone of northern turkey the occurrence of orthoceras limestone can be used as an indicator of palaeosource reconstruction our new zircon ages in conjunction with provenance analysis of silurian to lower carboniferous olistoliths strongly suggest that the clastic succession of chios received its detritus from basement rocks of the sakarya microcontinent in western turkey and time and facies equivalents of palaeozoic units from the istanbul zone in northern turkey and the balkan region due to subduction of a branch of palaeotethys close to the southern active margin of eurasia in late palaeozoic times the multidisciplinary approach of this study underlines the importance of terrane accretion during stepwise closure of palaeotethys

finite strain calculations of continental deformation 2 comparison with the india asia collision zone numerical experiments on a thin viscous sheet model for deformation of continental lithosphere subjected to an indenting boundary condition yield distributions of crustal thickness of stress and strain rate and of latitudinal displacements that may be compared with observations in the india asia collision zone a simple indenting boundary condition applied to initially laterally homogeneous sheets obeying a power law rheology produces results that are in broad agreement with the observations provided that the power law exponent is three or greater and the sheet can support vertically integrated stress differences of 2 1013 5 1012 n m 1 in regions in front of the indenter under these conditions the calculated deformation shows accommodation of convergence primarily by crustal thickening to produce a plateau in front of the indenter palaeomagnetic data from india and tibet and the observed distribution of topography suggest that much of the post eocene convergence of india with asia has been taken up by deformation within asia that involved crustal thickening the principal difference between calculation and observation is the absence from the calculated strain rate fields of east west extension of the plateau in front of the indenting boundary the calculations show that once such a plateau is formed the buoyancy force associated with the crustal thickness contrast inhibits further thickening and the plateau strains at less than half the rate of its immediate surroundings seismically determined regional strain rates exhibit a similar distribution with the tibetan plateau straining at about one quarter the rate of the tien shan and ningxia gansu regions calculated principal compressive stress orientations and regional strain rates agree with the seismically determined quantities in the mongolia baikal tien shan tibet and ningxia gansu regions of asia to within the uncertainty of the latter the vertically integrated stresses that are calculated for the viscous sheet are comparable with those that can be supported by a theologically stratified continental lithosphere obeying laboratory determined flow laws we suggest that the thin viscous sheet model described in this paper and its companion gives a simple and physically plausible description of the observed deformation in central asia in this description the predominant mechanism of accommodation of continental convergence is diffuse crustal thickening with shear on vertical planes playing a subsidiary role once large crustal thickness contrasts have been established

hinterland setting and composition of an oligocene deep rift lake sequence gulf of tonkin vietnam implications for petroleum source rock deposition abstract most se asian oil accumulations are sourced from paleogene lacustrine mudstones even so the mechanisms controlling formation of these source rocks are not fully understood bach long vi island located in the northwestern south china sea offers a unique opportunity to study these mechanisms a continuous 800 m thick organic rich and highly oil prone lacustrine section was investigated in outcrop and in a continuously cored section the oligocene deep lake succession formed in the bach long vi graben that connects the beibuwan basin with the yinggehai song hong basin petrography detrital zircon u th pb geochronology and heavy mineral assemblages of sandstone intervals document a locally derived sediment source isolated from the red river orientations of ripple cross lamination in gravity flow sands indicate axial sediment transport along the graben combined with the local structural configuration this suggests that sediments were derived from the qixi high through a relay ramp located about 10 km south of the island rare earth and major elements detrital zircon ages sandstone petrography and garnet composition suggest a dominantly ordovician silurian aged felsic and metasedimentary hinterland the limited sediment supply from the small hinterland allowed tectonic subsidence to keep pace with graben infill this promoted the establishment of a long lived deep lake favorable for source rock generation in addition heavy mineral and geochemical data document an unusually high content of detrital phosphate minerals in the investigated section indicative of a phosphate rich hinterland tropical leaching resulted in an extraordinary high phosphorus input into the lake that catalyzed a high organic productivity and promoted the generation of highly prolific oil prone source rocks the study documents that hinterland size and composition play a primary role in lacustrine source rock deposition comparable petroleum source rocks are likely to exist in other ancient rift basins surrounded by small phosphate rich hinterlands sheltered from significant sediment input

late cenozoic evolution of the eastern margin of the tibetan plateau inferences from 40ar 39ar and u th he thermochronology high topography in central asia is perhaps the most fundamental expression of the cenozoic indo asian collision yet an understanding of the timing and rates of development of the tibetan plateau remains elusive here we investigate the cenozoic thermal histories of rocks along the eastern margin of the plateau adjacent to the sichuan basin in an effort to determine when the steep topographic escarpment that characterizes this margin developed temperature time paths inferred from 40 ar 39 ar thermochronology of biotite multiple diffusion domain modeling of alkali feldspar 40 ar release spectra and u th he thermochronology of zircon and apatite imply that rocks at the present day topographic front of the plateau underwent slow cooling 30 50 c m y coincident with exhumation from inferred depths of 8 10 km at denudation rates of 1 2 mm yr samples from the interior of the plateau continued to cool relatively slowly during the same time period 3 c m y suggesting limited exhumation 1 2 km however these samples record a slight increase in cooling rate from 1 to 3 c m y at some time during the middle tertiary the tectonic significance of this change remains uncertain regardless late cenozoic denudation in this region appears to have been markedly heterogeneous with the highest rates of exhumation focused at the topographic front of the plateau margin we infer that the onset of rapid cooling at the plateau margin reflects the erosional response to the development of regionally significant topographic gradients between the plateau and the stable sichuan basin and thus marks the onset of deformation related to the development of the tibetan plateau in this region the present margin of the plateau adjacent to and north of the sichuan basin is apparently no older than the late miocene or early pliocene 5 12 ma

decoupling of erosion and precipitation in the himalayas the hypothesis that abrupt spatial gradients in erosion can cause high strain rates in active orogens has been supported by numerical models that couple erosional processes with lithospheric deformation via gravitational feedbacks1 2 3 most such models invoke a stream power rule in which either increased discharge or steeper channel slopes cause higher erosion rates spatial variations in precipitation and slopes are therefore predicted to correlate with gradients in both erosion rates and crustal strain here we combine observations from a meteorological network across the greater himalaya nepal along with estimates of erosion rates at geologic timescales greater than 100 000 yr from low temperature thermochronometry across a zone of about 20 km length spanning the himalayan crest and encompassing a more than fivefold difference in monsoon precipitation significant spatial variations in geologic erosion rates are not detectable decreased rainfall is not balanced by steeper channels instead additional factors that influence river incision rates such as channel width and sediment concentrations must compensate for decreasing precipitation overall spatially constant erosion is a response to uniform upward tectonic transport of greater himalayan rock above a crustal ramp

dongfang 1 1 gas field in the mud diapir belt of the yinggehai basin south china sea abstract the dongfang 1 1 df1 1 mud diapir field is the largest gas field discovered in the yinggehai basin up to now with a proven gas reserve of 99 68 10 9 m 3 the gas pay zones are within the pliocene fine sandstones and siltstones of the littoral and sand bar facies with normal pressure regimes the gases include variable amounts of co 2 and n 2 in addition to hydrocarbon gases the hydrocarbon gases with the exception of biogenic methane and nitrogen originated from the marine source rocks within the overpressured lower to middle miocene strata while co 2 is most likely the product of thermal decomposition of the tertiary calcareous mudstones and pre tertiary carbonates through relatively long distance vertical migration the field has received at least four phases of natural gas charging occurring relatively late most likely during the post pliocene himalayan orogeny diapiric faults act as the main conduit for upward movement of fluids released from the deep high pressure system with abnormally high pressures providing the significant driving force for gas migration because of the episodic nature of the deep high pressured fluid release and the diapir fault activity this shallow diapir gas field is characterized by multiple gas sources and episodic fluid accumulation

zircon u pb ages and ree composition constraints on the provenance of the continental slope parallel submarine fan western qiongdongnan basin northern margin of the south china sea abstract the ledong submarine fan in the western qiongdongnan basin runs parallel to the continental slope with an area of over 10 000 km2 and maximum thickness of greater than 2000 m using bulk rare earth element ree compositions of the sediments and detrital zircon u pb age data we analysed the provenance of the ledong submarine fan and its regional tectonic implications the zircon grains in the ledong submarine fan show a wide range of shapes from prismatic crystals to oval grains with mostly rounded corners indicating the long transport distance of these zircon grains prior to deposition the zircon ages have a wide range from 27 to 3006 ma with five major age peaks at ca 27 ma 88 ma 256 ma 415 ma and 785 ma along with a number of subordinate age peaks at ca 1024 ma 1376 ma 1879 ma and 2374 ma these age distribution patterns are similar to those from the red river at yen bai the total ree content range is between 87 72 and 210 38 mg g in the ledong submarine fan the chondrite normalized ree patterns show light ree lree enrichment and weakly negative eu anomalies which are similar to those in the red river at hanoi the evidence from zircon u pb ages and ree geochemistry indicates that the provenance of the ledong submarine fan is the red river system the development of the ledong submarine fan indicates that a rapid subsidence occurred in the western qiongdongnan basin during the late miocene and the strike slip reversal of the red river fault from sinistral to dextral motion was the key factor that induced gravity flow to form the ledong submarine fan

effects of stronach dam removal on fluvial geomorphology in the pine river michigan united states abstract although dam removal has been increasingly used as an option in dam management and as a river restoration tool few studies provide detailed quantitative assessment of the geomorphological response of rivers to dam removal in this study we document the response of the pine river michigan to the gradual removal of stronach dam in 1996 prior to the initiation of removal 31 permanent cross sectional transects were established in the 10 km study area these transects were surveyed annually during the course of the removal 1996 2003 and for the three years following removal 2004 2006 dam removal resulted in progressive headcutting of sediments in the former impoundment extending upstream 3 89 km of the dam over the course of the 10 years since dam removal was initiated a net total of 92 000 m 3 of sediment erosion occurred the majority of sediments stored in the former reservoir remained in place with only 12 of the estimated reservoir sediment fill being eroded approximately 14 of the net erosion was deposited within the stream channel 1 km downstream of the dam location with the remainder being transported further downstream or deposited in the floodplain sediment fill incision resulted in a narrower and deeper channel upstream with higher mean water velocity and somewhat coarser substrates downstream deposition resulted in a wider and shallower channel with little change in substrate size composition counter intuitively water velocity also increased downstream because of the increased slope that developed prior to removal bedforms in the former impoundment were dominated by runs but are showing signs of restoration toward reference conditions continuing changes in river geomorphology are evident even three years following removal and are likely to occur for years to come

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

linking the indochina block and gondwana during the early paleozoic evidence from u pb ages and hf isotopes of detrital zircons abstract to constrain the paleoposition of indochina within gondwana during the early paleozoic we performed in situ u pb and hf isotope analyses on detrital zircons from three river sediment samples in the truong son belt of the indochina block the age distributions yield dominant neoarchean 2 5 ga mesoproterozoic 1 7 1 4 ga grenvillian 0 95 ga and pan african 0 65 0 5 ga age groups and minor paleo to meso archean zircons hf isotope compositions of zircons for each age group exhibit large ranges of ehf t suggesting that the zircon host rocks have diverse sources the oldest hf model ages for zircons of neoarchean grenvillian and pan african age group yield 3 7 ga or older while those of mesoproterozoic age group show 3 3 ga the remarkable similarity of age distribution and hf isotope compositions among detrital zircons of indochina and those of tethyan himalaya western cathaysia and qiangtang suggests that indochina was located outboard of qiangtang and south of south china in the indian margin of gondwana during the early paleozoic our results are consistent with the paleontological correlations of east gondwana margin during the early paleozoic

sensitivity of east asian monsoon climate to the uplift of the tibetan plateau abstract using the cola center for ocean land atmosphere studies general circulation model a series of numerical experiments were conducted to examine the impact of uplift of the tibetan plateau on the evolution of the monsoon climate in east asia an attempt was made to isolate the effect of the plateau uplift from the changes in other forcing mechanisms we examined the spatial and temporal variation patterns of a variety of climatic variables and indices including monsoon intensity sea level pressure surface and upper air winds temperature at different levels precipitation air humidity and soil moisture our results suggest that the evolution of the east asia monsoon may be more sensitive to the uplift of the tibetan plateau than that of the south asia monsoon moreover the effect of the plateau uplift on the east asia winter monsoon is more significant than that on the summer monsoon in the northern part of east asia north china the formation of the monsoon climate is marked mainly by the establishment of the northerly winds in winter or the winter monsoon system corresponding to the uplift of the plateau when the height reaches approximately 50 of its current elevation on the other hand the establishment of the monsoon climate in the southern part of east asia the yangtze river valley and the area south of it may be much earlier than that in the northern part

evaluating the evolution of the red river system based on in situ u pb dating and hf isotope analysis of zircons 1 the red river originates from sw china and se tibet and has a total length 1000 km in this study we present new u pb dating and hf isotopic analysis of zircon grains from both modern and paleoriver sands in order to constrain the provenance of the modern river and to decipher drainage evolution through time our data show that the yangtze and songpan garze blocks are the most important sources for sediment although this material is mostly reworked via younger sedimentary rocks in the upper reaches of the red river sands in the da river and to a lesser extent the lo river have zircon ages indicating that they are minor contributors to the net flow consistent with rock uplift which is strongest in the upper reaches of the red river rather than precipitation being the primary control on erosion sediments eroded from the metamorphic rocks along the red river fault zone appear to have made a greater contribution during the miocene zircon ages suggest that the red river flowed north of the day nui con voi in the middle late miocene the red river appears to have had a largely stable provenance since at least the late miocene upper miocene sedimentary rocks ne of the red river indicate the presence of a separate large river in the late miocene hf isotope data indicate that the irrawaddy river was never part of the red river system although we do not exclude the salween mekong and yangtze rivers from having been part of the red river any connection must have been pre middle miocene

20 hz pulses and other vocalizations of fin whales balaenoptera physalus in the gulf of california mexico low frequency vocalizations were recorded from fin whales b a l a e n o p t e r a p h y s a l u s in the gulf of california mexico during three cruises in march 1985 recorded 20 hz pulses were in sequences of regular 9 s interpulse intervals in august 1987 nearly all were in sequences of doublets with alternating 5 and 18 s interpulse intervals no 20 hz pulse sequences of any kind were detected in february 1987 the typical pulse modulated from 42 to 20 hz and its median duration was 0 7 s 1985 data most other fin whale sounds were also short tonal pulses averaging 82 56 and 68 hz respectively for the three cruises 89 were modulated in frequency mostly downward compared to atlantic and pacific ocean regions gulf of california 20 hz pulses were unique in terms of frequency modulation interpulse sound levels and temporal patterns fin whales in the gulf may represent a regional stock revealed by their soundcharacteristics a phenomenon previously shown for humpback whales birds and fish regional differences in fin whale sounds were found in comparisons of atlantic and pacific locations

tracking blue whales in the eastern tropical pacific with an ocean bottom seismometer and hydrophone array low frequency northeastern pacific blue whale calls were recorded near the northern east pacific rise 9 degrees n latitude on 25 ocean bottom mounted hydrophones and three component seismometers during a 5 day period november 22 26 1997 call types a b c and d were identified the most common pattern being approximately 130 135 s repetitions of the ab sequence that for any individual whale persisted for hours up to eight individual blue whales were recorded near enough to the instruments to determine their locations and were tracked call by call using the b components of the calls and a bayesian inversion procedure for four of these eight whales the entire call sequences and swim tracks were determined for 20 26 h periods the other whales were tracked for much shorter periods the eight whales moved into the area during a period of airgun activity conducted by the academic seismic ship r v maurice ewing the authors examined the whales locations and call characteristics with respect to the periods of airgun activity although the data do not permit a thorough investigation of behavioral responses no correlation in vocalization or movement with airgun activity was observed

late cenozoic uplift of southeastern tibet the age of surface uplift in southeastern tibet is currently unknown but the initiation of major river incision can be used as a proxy for the timing of initial uplift the topographically high eastern plateau and gently dipping southeastern plateau margin are mantled by an elevated low relief relict landscape that formed at a time of slow erosion at low elevation and low tectonic uplift rates prior to uplift of the eastern tibetan plateau thermochronology from deep river gorges that are cut into the relict landscape shows slow cooling between ca 100 and ca 10 20 ma and a change to rapid cooling after ca 13 ma with initiation of rapid river incision at 0 25 0 5 mm yr between 9 and 13 ma a rapid increase in mean elevation of eastern tibet beginning at this time supports tectonic climate models that correlate the lateral eastern expansion of high topography in tibet with the late miocene intensification of the indian and east asian monsoons

the red river fault zone in the yinggehai basin south china sea abstract the red river fault a major strike slip fault of onshore southeast asia is suggested to be closely related to the formation of the south china sea and uplift of the tibetan plateau yet published documentation of the offshore segment of the red river fault is sparse high resolution seismic reflection and borehole data obtained by the petroleum industry along the northwestern margin of the south china sea permit mapping of the offshore segment of the red river fault in the yinggehai basin as well as demonstration of the timing of its shear movement and slip reversal two boundary faults and two basin center faults are mapped three successive deformation phases for the offshore red river fault zone include sinistral movement from 30 to16 ma slip reversal between 16 and 5 5 ma and dextral movement with low rates after 5 5 ma although no robust piecing points were observed for the sinistral movement period the horizontal displacement of dextral movement is estimated to be tens of kilometers this study supports a two stage model for the evolution of the india asia collisional orogen before the middle miocene sinistral movement of the red river fault likely was linked to spreading of the south china sea consistent with the concept of continental extrusion after the middle miocene distributed shortening better explains a low rate of dextral accommodation of the red river fault in response to the continuing india asia collision

spatial distribution and geochemistry of the nearshore gas seepages and their implications to natural gas migration in the yinggehai basin offshore south china sea abstract about 120 gas seepage vents were documented along the west and southwest coast of the hainan island south china sea in water depths usually less than 50 m the principal seepage areas include the lingtou promontory the yinggehai rivulet mouth yazhou bay the nanshan promontory and the tianya promontory they occur along three major zones reflecting the control by faults and lateral conduits within the basement it is estimated that the total gas emission from these seepage vents is 294 956 m 3 year the seepage gases are characterized by a high ch 4 content 76 heavy 13 c 1 values 38 to 33 and high c 1 c 1 5 ratios 0 95 1 0 resembling the thermogenic gases from the diapiric gas fields of the yinggehai basin hydrocarbon source correlation shows that the hydrocarbons in the sediments from seepage areas can be correlated with the deeply buried miocene source rocks and sandstone reservoirs in the central depression the 2d basin modeling results based on a section from the source rock center to the gas seepage sites indicate that the gas bearing fluids migrated from the source rocks upward through faults or weak zones encompassed by shale diapirism or in up dip direction along the sandstone rich strata of huangliu formation to arrive to seabed and form the nearshore gas seepages it is suggested that the seepage gases are sourced from the miocene source rocks in the central depression of the yinggehai basin this migration model implies that the eastern slope zone between the gas source area of the central depression and the seepage zone is also favorable place for gas accumulation

overpressuring mechanisms in the yinggehai basin south china sea yinggehai basin is an elongate cenozoic rift basin on the northwestern margin of the south china sea continental shelf its thick 17 km basin fill is characterized by high geothermal gradient and high overpressure overpressure associated with nonequilibrium compaction mainly occurs at depths more than 2800 m at the basin center and more than 4000 m at the basin margin because the shallow buried neogene and quaternary strata lack effective seals this regional overpressure distribution however is disrupted at basin center where high overpressure occurs in permeable formations at a depth as shallow as 1400 m on top of a series of deep seated faults and fractures we studied the processes and mechanisms of overpressuring via numerical modeling that couples basin filling sediment compaction and thermal and pressure fields to approach the origin of the shallow high overpressure model results indicated that an increase of fluid volume due to natural gas generation by organic cracking is not large enough to generate the overpressure because of the limited amount of organic matter the shallow overpressure has probably been generated allogenically deep open faults have served as vertical hydraulic conduits and channeled the deep high pressure into shallow permeable formations

tracking fin whales in the northeast pacific ocean with a seafloor seismic network ocean bottom seismometer obs networks represent a tool of opportunity to study fin and blue whales a small obs network on the juan de fuca ridge in the northeast pacific ocean in 2 3 km of water recorded an extensive data set of 20 hz fin whale calls an automated method has been developed to identify arrival times based on instantaneous frequency and amplitude and to locate calls using a grid search even in the presence of a few bad arrival times when only one whale is calling near the network tracks can generally be obtained up to distances of 15 km from the network when the calls from multiple whales overlap user supervision is required to identify tracks the absolute and relative amplitudes of arrivals and their three component particle motions provide additional constraints on call location but are not useful for extending the distance to which calls can be located the double difference method inverts for changes in relative call locations using differences in residuals for pairs of nearby calls recorded on a common station the method significantly reduces the unsystematic component of the location error especially when inconsistencies in arrival time observations are minimized by cross correlation

geochemistry and episodic accumulation of natural gases from the ledong gas field in the yinggehai basin offshore south china sea abstract the ledong gas field consisting of three gas pools in a shale diapir structure zone is the largest gas discovery in the yinggehai basin the gases produced from the pliocene and quaternary marine sandstone reservoirs show a considerable variation in chemical composition with 5 4 88 ch 4 0 93 co 2 and 1 23 7 n 2 the co 2 enriched gases often display heavier methane 13 c values than those with low co 2 contents the 15 n values of the gases range from 8 to 2 and the n 2 content correlates negatively with the co 2 content the high geothermal gradient associated with a relatively great burial depth in this area has led to the generation of hydrocarbon and nitrogen gases from the lower middle miocene source rocks and the formation of abundant co 2 from the tertiary calcareous shales and pre tertiary carbonates the compositional heterogeneities and stable carbon isotope data of the produced gases indicate that the formation of the ld221 gas field is attributed to three phases of gas migration initially biogenic gas followed by thermogenic hydrocarbon gas and then co 2 rich gas the filling processes occurred within a short period approximately from 1 2 to 0 1 ma based on the results of the kinetics modeling geophysical and geochemical data show that the diapiric faults that cut through miocene sediments act as the main pathways for upward gas migration from the deep overpressured system into the shallow normal pressure reservoirs and that the deep overpressure is the main driving force for vertical and lateral migration of the gases this gas migration pattern implies that the transitional pressure zone around the shale diapir structures was on the pathway of upward migrating gases and is also a favorable place for gas accumulation the proposed multiple sources and multiple phases of gas migration and accumulation model for the ledong gas field potentially provide useful information for the future exploration efforts in this area

la icpms zircons u pb dating of permo triassic and cretaceous magmatisms in northern vietnam geodynamical implications abstract in northeastern vietnam the major tectonic episode responsible for nappes emplacement is triassic these allochtonous structures are intruded by granitic melts two post tectonic massifs showing no sign of deformation have been dated by the la icpms zircon u pb techniques dating reveals a multiphase history with zircon cores showing evidence of proterozoic magmatism the emplacement of the phia bioc granite intrusive in allochtonous units is 248 245 ma an age which assesses a younger limit for the major nappes tectonic this tectonic could be synchronous of the tectonometamorphic strike slip faulting events 250 245 ma defined in the truong son belt as the indosinian orogen the phia bioc intrusion is probably linked with the intra plate magmatism of the emeishan large igneous province or with magmatism associated with the paleotethys closure the age of the phia oac granite intrusion in displaced units is much younger at 87 3 1 2 ma this granite is probably linked to the magmatic activity produced during the paleo pacific plate subduction under the se asia continental plate during the mesozoic although the cenozoic red river fault system is close to these two plutons this last thermotectonic episode has not been strong enough to disturb the u pb system zircons rims do not show any tertiary magmatic or metamorphic overprint

geochronology and isotope analysis of the late paleozoic to mesozoic granitoids from northeastern vietnam and implications for the evolution of the south china block abstract in northeastern vietnam late paleozoic and permo triassic granitic plutons are widespread but their tectonic significance is controversial in order to understand the regional magmatism and crustal evolution processes of the south china block scb this study reports integrated in situ u pb hf o and sr nd isotope analyses of granitic rocks from five plutons in northeastern vietnam zircon sims u pb ages of six granitic samples cluster around in two groups 255 228 ma and 90 ma bulk rock end t ranges from 11 to 9 7 suggesting that continental crust materials were involved in their granitic genesis in situ zircon hf o isotopic measurements for the granitic samples yield a mixing trend between the mantle and supracrustal derived melts it is suggested that the granitic rocks were formed by re melting of the continental crust these new data are compared with the paleozoic and mesozoic granitic rocks of south china we argue that northeastern vietnam belongs to the south china block though still speculated an ophiolitic suture between ne vietnam and south china so called babu ophiolite appears unlikely the late paleozoic to mesozoic magmatism in the research area provides new insights for the magmatic evolution of the south china block

geochemistry and origins of natural gases in the yinggehai and qiongdongnan basins offshore south china sea abstract four gas fields and a number of gas bearing structures have been discovered in the yinggehai and qiongdongnan basins offshore south china sea chemical and isotopic data indicate the presence of at least three genetic groups of gases in these basins biogenic gas thermogenic gas and gases with mixed origin thermogenic gases produced from the yacheng field are characterized by relatively high contents of benzene and toluene relatively low 13c values of toluene and high abundance of bicadinanes and oleanane in the associated condensates showing good correlation with the coal bearing sequence of the yacheng formation in the qiongdongnan basin in contrast the gases from the df1 1 and ld gas fields contain high amounts of n2 and co2 low to moderate amounts of benzene and toluene with relatively high 13c values of toluene these characteristics correlate well with the miocene neritic shales in the yinggehai basin analyses on potential source rock samples indicate excellent gas source potential for both the coal bearing sequence in the yacheng formation and the miocene neritic shales containing type iii ii2 kerogens as the result of recent rapid subsidence and sedimentation high temperature and overpressure systems are well developed in the yinggehai and qiongdongnan basins the rapid heating resulted in advanced maturation of organic matter deposited in normally pressured and overpressured strata whereas the strongly overpressured systems led to retarded organic matter maturation postponing the time when the source rocks reached peak gas generation results of 1 d modeling indicate that the coal bearing yacheng formation in the yacheng sag reached peak gas generation during mid miocene pliocene time after the deposition of upper oligocene to lower miocene reservoir rocks the lower mid miocene marine shales reached their peak gas generation stage during the pliocene quaternary in the central yinggehai basin supplying abundant gases to charge the pliocene reservoirs more importantly focused episodic gas migration significantly increased the hydrocarbon expulsion efficiencies of source rocks and thus provided favorable conditions for the accumulation of large amounts of gases in the diapiric structures in a very short geological time the gas source relationship for the yacheng field suggests dominantly short distance gas migration and thus strong source facies controls on the geographic distribution of gas fields within the qiongdongnan basin

provenance of upper miocene to quaternary sediments in the yinggehai song hong basin south china sea evidence from detrital zircon u pb ages abstract the yinggehai song hong y sh basin located on the northwestern shelf of the south china sea is one of the world s largest pull apart basins in order to identify the provenance of the upper miocene to quaternary sediments of the basin ten sediment samples were collected from drill cores in the dongfang gas field u pb dating of detrital zircon grains extracted from the ten samples using a laser ablation inductively coupled mass spectrometer la icp ms yields ages ranging from the archean to cenozoic 2898 46 ma to 26 1 ma the zircon u pb age spectra of the upper miocene to quaternary show a similar pattern with major peaks clustered at ca 32 ma ca 99 ma ca 160 ma ca 247 ma ca 422 ma and three broad age groups 600 1000 ma 1700 2100 ma and 2200 2800 ma suggestive of a wide range of igneous rock ages in the source areas the detrital zircon ages provide diagnostic criteria for identification of the various source areas and reveal that the detritus was derived from multiple sources comparison of the results with the rock types and their ages surrounding the potential source areas indicates that the clastic material was derived from three dominant age sources 1 yangtze craton 2 hainan island and 3 indochina block the yangtze craton was a major and continuous source area contribution to the basin comparing the age characteristics and its source areas since 10 ma the largely stable provenance also suggests that if the red river capture ever existed it should have happened before the late miocene

zircon u pb and hf isotope constraints from the ailao shan red river shear zone on the tectonic and crustal evolution of southwestern china abstract the ailao shan red river asrr shear zone one of the most prominent geologic strike slip shear zones in southeast asia consists of high grade metamorphic complexes that provide a rare opportunity to sample the mid crustal rocks along the western margin of the yangtze block of south china here we report combined in situ analyses of zircon u pb and lu hf isotopes of eight gneisses from the diancang shan and ailao shan segments of the asrr shear zone our zircon u pb data indicate that the rocks contain abundant magmatic zircons ranging in age from 1785 to 25 ma with peak ages at ca 770 350 and 240 ma suggesting three major periods of protolith formation the 350 ma zircons observed only in an orthogneiss from the diancang shan show uniform initial hf isotopic ratios marked by high and positive e hf t values from 16 to 10 this is in contrast with the other two zircon populations that are more common occurring in both the diancang shan and ailao shan and overall delineate very heterogeneous hf isotopic compositions with e hf t values ranging from 15 to 16 many zircons reveal distinctive core rim age variations zircon rims formed between ca 34 and 26 ma show significant variations in th u ratios 5 0 01 and e hf t values 14 to 10 that suggest complicated magmatic and metamorphic zircon overgrowth during the oligocene the presence of both metamorphic and magmatic overgrowths on zircons suggests that the metamorphism reached upper amphibolite facies corresponding to mid crustal level p t conditions as also suggested by structural and petrologic data our data furthermore suggest that the asrr gneisses were not produced solely by shear heating during the tertiary strike slip faulting but are uplifted mid crustal basement rocks that formed essentially during two major stages of magmatism represented by the neoproterozoic kangding complex and late permian emeishan large igneous province as a result of the india asia collision these basement rocks underwent regional magmatic and metamorphic overprinting in the oligocene that based on our relevant work searle et al 2010 geosphere 6 1 23 predated the initial left lateral movement along the asrr shear zone the 350 ma protolith which requires a dominant depleted mantle input in the petrogenesis cannot be linked to any major magmatic events in south china but may be interpreted as part of the paleotethys remnants that were later incorporated into the asrr shear zone

natural streams and the legacy of water powered mills gravel bedded streams are thought to have a characteristic meandering form bordered by a self formed fine grained floodplain this ideal guides a multibillion dollar stream restoration industry we have mapped and dated many of the deposits along mid atlantic streams that formed the basis for this widely accepted model these data as well as historical maps and records show instead that before european settlement the streams were small anabranching channels within extensive vegetated wetlands that accumulated little sediment but stored substantial organic carbon subsequently 1 to 5 meters of slackwater sedimentation behind tens of thousands of 17th to 19th century milldams buried the presettlement wetlands with fine sediment these findings show that most floodplains along mid atlantic streams are actually fill terraces and historically incised channels are not natural archetypes for meandering streams

development of the east asian monsoon mineralogical and sedimentologic records in the northern south china sea since 20 ma we here reconstruct the past change of the east asian monsoon since 20 ma using samples from ocean drilling program odp site 1146 in the northern south china sea based on a multi proxy approach including a monomineralic quartz isolation procedure identification of clay minerals by x ray diffraction xrd and grain size analysis of isolated terrigenous materials terrigenous supply to odp site 1146 was dominated by changes in the strength of multiple sources and transport processes grain size data modeled by an end member modeling algorithm indicate that eolian dust from the and asian inland and fluvial input have contributed on average 20 and 80 of total terrigenous material to odp site 1146 respectively specifically about 40 53 of the total quartz feldspar and only 6 11 of the total clay is related to eolian supply at the study site detailed analysis of the sedimentary environment and clay minerals combined with previous studies shows that smectite originates mainly from luzon kaolinite from the pearl river and illite and chlorite from the pearl river taiwan and or the yangtze river the proportion and mass accumulation rate mar of the coarsest end member em1 interpreted as eolian dust ratios of illite chlorite smectite quartz feldspar and mean grain size of terrigenous materials at odp site 1146 were adopted as proxies for east asian monsoon evolution the consistent variation of these independent proxies since 20 ma shows three profound shifts in the intensity of east asian winter monsoon relative to summer monsoon as well as aridity of the asian continent occurred at similar to 15 ma similar to 8 ma and the youngest at about 3 ma in comparison the summer monsoon intensified contemporaneously with the winter monsoon at 3 ma the phased uplift of the himalaya tibetan plateau may have played a significant role in strengthening the asian monsoon at similar to 15 ma 8 ma and 3 ma c 2007 elsevier b v all rights reserved

sounds from airguns and fin whales recorded in the mid atlantic ocean 1999 2009 between 1999 and 2009 autonomous hydrophones were deployed to monitor seismic activity from 16 n to 50 n along the mid atlantic ridge these data were examined for airgun sounds produced during offshore surveys for oil and gas deposits as well as the 20 hz pulse sounds from fin whales which may be masked by airgun noise an automatic detection algorithm was used to identify airgun sound patterns and fin whale calling levels were summarized via long term spectral analysis both airgun and fin whale sounds were recorded at all sites fin whale calling rates were higher at sites north of 32 n increased during the late summer and fall months at all sites and peaked during the winter months a time when airgun noise was often prevalent seismic survey vessels were acoustically located off the coasts of three major areas newfoundland northeast brazil and senegal and mauritania in west africa in some cases airgun sounds were recorded almost 4000 km from the survey vessel in areas that are likely occupied by fin whales and at some locations airgun sounds were recorded more than 80 days month for more than 12 consecutive months

importance of hydrology on channel evolution following dam removal case study and conceptual model abstracta slow draining reservoir on the u s east coast was monitored to identify the processes governing channel evolution upstream of a dam removal channel evolution was documented through cross section surveys sediment size analysis discharge measurements and visual assessments of vegetative growth the reservoir drained slowly allowing for an analysis of channel evolution and identification of the morphometric parameters defining the path and time required for a channel to reach dynamic equilibrium channel evolution was a multidirectional process and evolving channel reaches actively migrated laterally while alternating between aggradation and degradation channel formation was dominated primarily by the hydrologic regime at the time of dam removal and secondarily by the ability of vegetation to establish and stabilize the channel form the importance of seasonal site hydrology over the minimum time required for channel evolution and the process through which the channel evolves indicates the

geochemical and sr nd isotopic constraints from the kontum massif central vietnam on the crustal evolution of the indochina block abstract the kontum massif central vietnam consists mainly of high grade amphibolite to granulite facies metamorphic rocks and represents the largest basement exposure core complex of the indochina block to explore the crustal evolution of indochina sr and nd isotopic and geochemical data for various rock types from the massif are reported the basement rocks show a wide range of present day end values from 22 gneiss to 15 amphibolite yielding depleted mantle model ages tdm from 1 2 to 2 4 ga along with an exceptionally old tdm of 2 7 ga for a granulite these data indicate that crustal formation in the indochina block took place principally during the paleoproterozoic and mesoproterozoic and do not support the conventional notion that the kontum core complex is composed of archean rocks geochemical data indicate that the gneisses and schists have heterogeneous compositions characterized by a calc alkaline nature whereas most of the amphibolites are tholeiitic basalts with intraplate magmatic signatures therefore the former may be interpreted as products from pre existing proterozoic crustal materials and the latter as resulting from the paleozoic rifting event that disintegrated the indochina block from gondwanaland during its accretion with other se asian continental blocks in permo triassic time the indochina core complex was subjected to the indosinian orogeny characterized by a high temperature granulite facies metamorphism in the lower crust with associated charnockite magmatism and subsequent regional exhumation

neoproterozoic bimodal magmatism in the cathaysia block of south china and its tectonic significance shrimp u pb zircon age geochemical and sm nd isotopic results are reported for the mamianshan volcanic rocks in the cathaysia block of southeastern south china the mamianshan volcanic rocks are bimodal in composition and are dominantly transitional to mildly alkaline basalts and subordinate alkaline rhyolite with an eruption age of 818 9 ma the basaltic samples are characterized by lree enriched and humped trace element patterns similar to many alkali basalts in continental rifts variablend t values between 3 33 and 4 35 indicate that the primary magma of these basalts was derived from an oib like mantle source and underwent fractional crystallization plus crustal contamination the rhyolitic rocks are highly enriched in th ta nb ree zr hf and y and depleted in sr p eu and ti sharing affinity to a 1 type granites combined with their slightly positive nd t values 0 22 to 0 92 the mamianshan felsic rocks were most likely generated by partial melting of the regional paleoproterozoic mayuan amphibolites the mamianshan bimodal volcanic rocks in the cathaysia block are coeval with the widespread intraplate magmatism around the yangtze block our results support the idea that a coherent south china craton was formed during the ca 1 0 ga sibao orogeny and it subsequently underwent extensive continental rifting related to mantle plume or superplume activities beneath rodinia since ca 825 ma 2004 elsevier b v all rights reserved

varietal heavy mineral analysis of sediment provenance lower cretaceous scotian basin eastern canada abstract the origin of a major increase in sand supply in the early cretaceous of the scotian basin offshore eastern canada has been investigated by analysis of heavy mineral varieties the chemistry of detrital minerals from the scotian basin was compared with that of minerals from potential source rocks sampled from older sedimentary successions river sands and glacial tills on land heavy mineral varieties were established based on 1000 high quality electron microprobe emp analyses and used to classify heavy mineral types by mineral liberation analysis mla which uses a rapid scanning electron microscope particle analyser heavy mineral varieties were compared between the cretaceous quartz arenites and subarkoses and samples from potential source areas different chemical varieties of the same mineral group have different grain size distributions so that their relative modal abundances may be influenced by hydraulic sorting but modal abundance of different varieties of tourmaline garnet and spinel provides useful information widely used provenance sensitive mineral ratios are difficult to interpret because of the abundance of polycyclic sources comparisons of the shape parameters of the individual mineral types are also useful for provenance identification aspect ratio in elongate grains such as tourmaline and zircon is a measure of polycyclic reworking but is influenced by the initial shapes of minerals from different source rocks uniformity of varietal heavy minerals over a large area and stratigraphic range and the covariance of spinel and zircon with quite different crystalline sources indicate predominance of polycyclic sources to the central scotian basin probably principally from carboniferous and lower paleozoic sandstones in part sourced from western newfoundland ophiolites heavy mineral varieties in the west scotian basin can be closely matched to sources in the adjacent meguma terrane of the appalachians all the data show that the sands of the scotian basin were derived from reworking of crystalline basement of the appalachian orogen and its paleozoic sandstone cover uplifted during rifting of iberia from the grand banks there is no evidence of a significant supply of sand to the scotian basin from a proto st lawrence river

shrimp zircon u pb dating and hf isotope analyses of the muniushan monzogranite guocheng jiaobei terrane china implications for the tectonic evolution of the jiao liao ji belt north china craton abstract the paleoproterozoic granitoid rocks in the jiaobei terrane which lies in the southern part of the jiao liao ji belt jljb in china provide valuable information on the tectonic and evolutionary history of the north china craton ncc shrimp u pb dating of zircons from the muniushan monzogranite in the guocheng gold deposit area as well as in situ hf isotope analyses of the zircons were used to constrain the ages of emplacement and metamorphism and to understand the petrogenesis of the granite the geochronological data show emplacement ages of ca 2 1 ga and metamorphic ages of ca 1 85 ga the magmatic cores of the zircons have ehf t values of 0 51 to 4 00 average 1 98 and depleted mantle two stage model ages tdm2 of 2444 2703 ma average 2563 ma the rocks show the same trends of crustal evolution as ca 2 5 ga tonalite trondhjemite granodiorite ttg rocks in the jiaobei terrane indicating that they were formed by the remelting of those ttg rocks the muniushan monzogranite is part of a linear belt of ca 2 2 2 0 ga paleoproterozoic granitoids within the jljb the granitoids are a types and are accompanied by coeval mafic intrusions and bimodal volcanic rocks thus indicating widespread extension along the jljb in the period 2 2 2 0 ga the ca 1 85 ga metamorphism was related to the collision of the longgang and nangrim blocks the paleoproterozoic evolution of the ncc constrains the crustal architecture and the source of mesozoic granitoids in the jiaobei sulu region inherited zircons of paleoproterozoic age are widespread in the mesozoic granitoids in addition paleoproterozoic ages have been reported from lower crustal granulite xenoliths in the sulu orogen and the southeastern margin of the ncc mirrors the evolution of the jljb it is probable therefore that ncc lower crust underlies the sulu orogen and that the ncc was the main source for the mesozoic granitoids in the jiaobei sulu region this view is consistent with the appearance of inherited zircons of archean and paleoproterozoic age on both sides of the orogen normal 18o values of magmatic zircons and paleoproterozoic to archean two stage hf model ages for the granitoids

long and complex thermal history of the song chay metamorphic dome northern vietnam by multi system geochronology multi system geochronology was used to unravel the long and complex thermal history of the song chay range northern vietnam a high grade granitic and metamorphic dome in the vicinity of the cenozoic ailao shan red river fault zone it was considered to be proterozoic south china basement but its geological history was basically unknown scattered field observations suggest three episodes of high temperature deformation firstly at the time of granite emplacement secondly a decollement with top to the north shear and thirdly anticlinal doming of the foliations formed during the two first stages p t estimates suggest that metamorphism coeval with the second deformation phase culminated at 580 ca nd 4 5 kbar 16 km depth multi system geochronology is applied to a two micas granite sample slightly deformed within the decollement u pb dating of zircon yields an age of 428 5m a 2s interpreted as the time of granite crystallization within the south china caledonian belt rb sr on white micas and biotite yields ages of 206 10 ma and 176 5 3 ma respectively 2s whereas 39ar 40 ar ages of the same minerals are 210 9a nd 190 8m a 2s these ages suggest an upper triassic episode of rapid cooling interpreted as due to doming a few million years after the end of movement on the decollement the k feldspar irregular 39ar 40ar age spectrum can to the first order be explained by a cooling history with two episodes of rapid cooling one at 140 ma and a second around 41 to 25 ma apatite fission tracks central age 33 6 3 6 ma 1s confirms a tertiary rapid cooling event interpreted as the final exhumation of the song chay dome 2000 elsevier science b v all rights reserved

tracking fin whale calls offshore the galicia margin north east atlantic ocean data recorded during a temporary deployment of ocean bottom seismometers obss are used in this study to monitor the presence of fin whales around the array in the summer of 2003 ten obss were placed 250 km from the nw coast of iberia in the galicia margin ne atlantic ocean for a period of one month the recorded data set provided a large variety of signals including fin whale vocalizations identified by their specific acoustic signature the use of a dense array of seafloor receivers allowed investigation into the locations and tracks of the signal generating whales using a seismological hypocentral location code individual pulses of different sequences have been chosen to study such tracks problems related to the correct identification of pulses discrimination between direct and multiple arrivals and the presence of more than one individual have been considered prior to location fin calls were concentrated in the last two weeks of the deployment and the locations were spread around the area covered by the array these results illustrate that besides its classical seismological aim deployment of semipermanent seafloor seismic arrays can also provide valuable data for marine mammal behavior studies