magma chamber and crustal channel flow structures in the tengchong volcano area from 3 d mt inversion at the intracontinental block boundary southeast of the tibetan plateau

prolonged residence times for the youngest rhyolites associated with long valley caldera 230th 238u ion microprobe dating of young zircons abstract we describe a new ion microprobe method for dating magmatic zircon growth that is based on in situ measurement of the magnitude of238u 230th disequilibrium our results support independent inferences that zircon can remain suspended for long periods 100 ka in the convecting potions of the magma from which it crystallizes because the crystallization ages date when the magma cooled to its zircon saturation temperature even when the zircons have long magmatic residence times 238u 230th zircon dating can be used to constrain the thermochemical evolution of silicic magmas 238u 230th ages have been determined for individual zircons from rhyolites associated with the long valley magmatic system of eastern california the samples are from deer mountain an 115 3 ka low silica moat rhyolite and from the coarsely porphyritic low silica rhyolite of south deadman dome one of the 0 6 ka inyo domes previous investigations have suggested that the two lavas were derived from the same magma reservoir a few of the zircon model ages calculated with respect to the isotopic characteristics of the whole rocks are within error of that for eruption of the deer mountain rhyolite however the majority of zircons from both lavas cluster around an age of 230 ka this common interval of zircon nucleation and growth for petrologically similar lavas suggests that the younger inyo dome lava may have tapped the same magma body from which the deer mountain rhyolite erupted more than 100 ka before on the other hand most of the zircon model ages are younger than previous episodes of silicic volcanism in the long valley caldera suggesting that the rhyolites may have been generated during development of a silicic upper crustal magma chamber in the western portion of long valley caldera zircon saturation temperatures for the rhyolites studied 795 810 c are the same as those obtained from coexisting fe ti oxides 809 4 c showing that the magma cooled to

implications of pre eruptive magmatic histories of zircons for u pb geochronology of silicic extrusions zircons in silicic magmas begin to crystallize 10 s to 100 s of thousands of years ka prior to their eruption u pb and u th dating for zircons from 32 young extrusions indicate a range of mean pre eruption ages from 33 to 247 ka with a median of 92 ka explosive large volume widespread eruptions show a narrower range and a younger median of 70 ka these results indicate that the difference between initial zircon growth and eruption age measured by other e g 40ar 39ar techniques is in excess of the achievable analytical uncertainty for eruptions 30 million years ma and likely for eruptions of up to 300 ma protracted pre eruption zircon growth is well established by an increasing number of studies on magma process time scales but this source of eruption age bias has been largely ignored in chronostratigraphic applications of high precision u pb data until intrinsic properties of zircons or their host deposits enable a priori estimates of pre eruption age bias it is prudent to expand the uncertainties attending all u pb zircon ages interpreted as deposition ages in e g chronostratigraphic studies by 0 200 ka

mid tertiary 25 21 ma lamprophyres in nw mexico derived from subduction modified subcontinental lithospheric mantle in an extensional backarc environment following steepening of the benioff zone abstract the mid tertiary lamprophyre dike swarm 8 km 2 5 km in size from hermosillo sonora nw mexico has calc alkaline characteristics and includes nnw striking amphibole phyric spessartite 85 of the swarm and nne striking phlogopite phyric kersantite the 40 ar 39 ar geochronology of amphibole and phlogopite gives overlapping plateau ages ranging from 25 to 21 ma although all the lamprophyres are enriched in incompatible elements and display negative nb ta and ti anomalies on the primitive mantle normalized plots kersantite has higher k na la yb p ti and incompatible trace elements e g zr compared to spessartite the lamprophyres have radiogenic sr and nd isotopic signatures 87 sr 86 sr 0 7057 0 7065 and e nd 1 to 2 3 suggesting derivation from the subcontinental lithospheric mantle that was previously modified by subduction related fluids this mantle is similar to that beneath the southern grenvillian orogen which has younger t dm ages than the 1 6 1 7 ga t dm ages of the caborca block the lamprophyric magmas were generated at various mantle depths at the southwestern edge of north america intrusion of the lamprophyres was synchronous with extension that produced normal faults and core complexes with wsw vergence extension occurred immediately following steepening of the benioff zone during which the magmatic arc migrated from east to west of hermosillo and the lamprophyres were intruded just behind the contemporaneous arc

relationship between the morphological evolution of the river mouth bar and fluvial input in the modaomen estuary river mouth bars commonly exist in estuaries and are important for flood control estuarine ecology and the navigation of estuarine areas the modaomen estuary is a river dominated estuary in the pearl river delta and has a typical river mouth bar here hydrologic and bathymetric datasets that span 2000 2011 are used to analyze the relationship between the fluvial input of river discharge and sediment load and the evolution of the river mouth bar the results show that the eastern and western channels have notably extended seaward since 2000 the evolution rates exhibit distinct asymmetry with the seaward extension rate of the eastern channel being approximately 1 6 times greater than that of the western channel the upstream bar face and bar back of the center mouth bar have been eroded and intensive human activities have significantly affected the evolution of the center mouth bar the center mouth bar s apex has consistently moved seaward from 2000 to 2011 river discharge especially during extreme river floods rather than sediment load has played a more vital role in the development of the bifurcation channels and the center mouth bar an empirical relationship between the bar apex displacement rates and dominant discharge is obtained based on the observations indicating that greater river discharge and longer duration will move the bar apex further seaward the bar apex s displacement during the extreme river floods of 2005 6 and 2008 6 was 133 6 m and 99 4 m respectively

insight into the origin of the tengchong intraplate volcano and seismotectonics in southwest china from local and teleseismic data received 18 june 2008 revised 22 february 2009 accepted 10 march 2009 published 13 may 2009 1 a high resolution tomographic image of the crust and upper mantle under yunnan province in southwest china was determined by using a large number of teleseismic data measured precisely from digital seismograms as well as local earthquake arrival times recorded by the dense yunnan seismic network the grid spacing of the tomographic model is 70 km 78 km in the horizontal directions and 4 70 km in depth our resulting model shows a clear low velocity low v 0 3 with respect to the average one dimensional velocity model column extending from the surface down to about 400 km depth under the active tengchong volcano and some high velocity high v anomalies existing in the mantle transition zone furthermore the low vanomaly extends horizontally toward the northeast at 250 to 400 km depth combining our tomographic results with geochemical and geological investigations we infer that the tengchong volcano is a rift related volcano caused by the subduction and dehydration of the indian plate as well as corner flow in the mantle wedge though the subducted slab is a continental plate our results show that the upwelling flow under tengchong originates at 400 km depth a prominent low v anomaly along the red river fault zone extends down to the upper mantle 100 km depth reflecting that the red river fault zone may have cut through the crust to the upper mantle the two dayao earthquakes m 6 1 and m 6 2 in 2003 occurred on the margin of high v anomalies and are underlain by a prominent low v anomaly in the lower crust and upper mantle suggesting that the dayao earthquakes may be associated with fluids released from the dehydration of the subducted indian slab these results are significantly improved compared with previous studies and provide new seismic constraints on the dynamic processes of the india asia collision

effect of post magmatic processes on magnetic fabric of teschenite association rocks of the outer western carpathians abstract volcanic and subvolcanic alkaline rocks of the teschenite association of the outer western carpathians are characterized by intensive late magmatic post magmatic and also hydrothermal changes that modified their composition not only in rock forming minerals but also in accessory magnetic minerals magnetite titanomagnetite maghemite as indicated by magnetic susceptibility ranging from 10 5 to 10 1 si units the magnetic fabric of the most sills and dykes investigated is very roughly conformable to the shapes of the bodies corresponding to the most common type of magnetic fabric in dykes and sills which no doubt originates through magma flow however the fabric shows large variation much larger than expected for volcanic magnetic fabric which can be ascribed to late magmatic or post magmatic alterations of magnetic minerals diminishing the magnetic fabric homogeneity even though the teschenite association rocks occur in the sole part of the silesian thrust sheet their magnetic fabric and that of host sedimentary rocks show only very weak indications of ductile deformation consequently the silesian thrust sheet responded to the tectonic movements creating thrust sheet structure by faulting and thrusting of the deformed zones and not by ductile deformation of the bulk of the thrust sheet

isotopic and geochemical evidence for the origin of plio pleistocene volcanic rocks near the indo eurasian collisional margin at tengchong china abstract in yunnan province china regional extensional stresses associated with the collision between india and asia have formed a series of downfaulted north south trending basins near tengchong in extreme western yunnan close to the burma border one of these basins is characterized by volcanism which began in the pliocene 7m y and has continued to historic times the tengchong volcanic suite ranges in composition from basalt to dacite with most lavas being potassium rich five chemical groups can be recognized based on major element trace element and isotopic compositions these groups are broadly consistent with previous groupings made on the basis of geologic and age data the chemical and isotopic data show that the main series most voluminous lavas were derived by partial melting of a metasomatized and heterogeneous mantle source with crustal and possibly seawater components probably related to prior subduction beneath asia an andesite dacite series with similarities to tibetan calc alkaline lavas shows clear indications of assimilated crust and crystal fractionation there is no evidence in these rocks for the presence of a strongly depleted subcontinental mantle such as has been recognized elsewhere 5 21

significant effect of sediment cohesion on delta morphology delta morphology is thought to be controlled by factors such as river discharge tides and waves numerical modelling shows that sediment cohesion also strongly influences the development of a delta s characteristics the morphologies of the world s deltas are thought to be determined by river discharge tidal range and wave action 1 more recently sea level rise 2 3 and human engineering4 have been shown to shape delta evolution the effects of factors such as sediment type and the overall amount of sediment carried by rivers are considered secondary4 5 6 in particular the role of sediment cohesion which is controlled by sediment size and type of vegetation is unclear here we use a numerical flow and transport model7 8 9 10 to show that sediment cohesiveness also strongly influences the morphology of deltas we find that holding all other factors constant highly cohesive sediments form bird s foot deltas with rugose shorelines and highly complex floodplains whereas less cohesive sediments result in fan like deltas with smooth shorelines and flat floodplains in our simulations sediment cohesiveness also controls the number of channels that form within the deltas and the average angle of bifurcation of those channels as vegetation generally acts as a cohesive agent we suggest that deltas that formed before the expansion of land plants in the devonian period should show fan like characteristics a finding consistent with the limited data from the sedimentological record11

young cumulate complex beneath veniaminof caldera aleutian arc dated by zircon in erupted plutonic blocks mount veniaminof volcano alaska peninsula provides an opportunity to relate quaternary volcanic rocks to a coeval intrusive complex veniaminof erupted tholeiitic basalt through dacite in the past 260 k y gabbro diorite and miarolitic granodiorite blocks ejected 3700 14 c yr b p in the most recent caldera forming eruption are fragments of a shallow intrusive complex of cumulate mush and segregated vapor saturated residual melts sensitive high resolution ion microprobe shrimp analyses define 238 u 230 th isochron ages of 17 6 2 7 ka 5 11 10 ka and 10 2 4 0 ka 2 for zircon in two granodiorites and a diorite respectively sparse zircons from two gabbros give 238 u 230 th model ages of 36 8 ka and 26 7 ka zircons from granodiorite and diorite crystallized in the presence of late magmatic aqueous fluid although historic eruptions have been weakly explosive strombolian fountaining and small lava effusions the young ages of plutonic blocks as well as late holocene dacite pumice are evidence that the intrusive complex remains active and that evolved magmas can segregate at shallow levels to fuel explosive eruptions

observation of very large and steep internal waves of elevation near the massachusetts coast 1 we report on near bottom waves of elevation with amplitude nearly half the 25 m water column very steep and propagating into a sheared turbulent wave guide the waves contained trapped cores transporting parcels of water shoreward these large waves depart strongly from weakly nonlinear and weakly nonhydrostatic conditions and challenge established paradigms they can also represent an important factor in the across shore transport of plankton and contaminants

internal tide generation at the continental shelf modeled using a modal decomposition two dimensional results abstract stratified flow over topography is studied with oceanic applications in mind a model is developed for a fluid with arbitrary vertical stratification and a free surface flowing over three dimensional topography of arbitrary size and steepness with background rotation in the linear hydrostatic regime the model uses an expansion of the flow fields in terms of a set of basis functions which efficiently capture the vertical dependence of the flow the horizontal structure may then be found by solving a set of coupled partial differential equations in two horizontal directions and time subject to simple boundary conditions in some cases these equations may be solved analytically but in general simple numerical procedures are required using this formulation the internal tide generated by a time periodic barotropic tidal flow over a continental shelf and slope is calculated in various idealized configurations the topography and fluid motion are taken to be independent of one coordinate di

river discharge influences on particulate organic carbon age structure in the mississippi atchafalaya river system 1 applying ramped pyrolysis radiocarbon analysis to suspended river sediments we generate radiocarbon 14c age spectra for particulate organic carbon poc from the lower mississippi atchafalaya river system mars to better understand a major river system s role in carbon transport ramped pyrolysis 14c analysis generates age distributions of bulk carbon based on thermochemical stability of different organic components our results indicate higher proportions of older material in the poc during higher discharge ages increase throughout the high discharge age spectra indicating that no single component of the poc is responsible for the overall age increases observed instead older material is contributed across the poc age spectrum and unrelated to increased bedload suspension in this comparison of 2 spring discharges less than half of the poc transported during higher discharge is less than 1000 14c years in age constraining of the role of the mars as a flux of atmospheric co2 toward longer term sedimentary sinks in the mississippi delta and the gulf of mexico the results suggest that delta building processes benefit disproportionately from high discharge events carrying larger amounts of sediment because these events involve both a higher proportion of millennially aged carbon from floodplain exchange of poc and a potentially higher proportion of petrogenic carbon 30 530 increase overall an internally consistent picture of po14c age distributions from a major river system emerges as differences in space and time are small compared to the range of ages of poc sources in such a large basin

three dimensional nonhydrostatic numerical simulation of nonlinear internal wave generation and propagation in the south china sea 1 we present the results of three dimensional nonhydrostatic simulations of internal tides and waves in the south china sea scs using the suntans model model results accurately predict the observed wave arrival times at two mooring locations in the scs internal wave amplitudes are underpredicted which causes underprediction of internal wave speeds due to a lack of amplitude dispersion we show that the well known a and b waves arise from the steepening of semidiurnal internal tides that are generated due to strong barotropic flow over ridges in the luzon strait a wave generation is stronger in the southern portion of the luzon strait because diurnal internal tidal beams augment the amplitude of the semidiurnal a waves b wave generation is stronger in the northern portion where the distance between the eastern and western ridges is approximately equal to one internal tidal wavelength and leads to semidiurnal internal tidal resonance the orientation of the ridges produces large a waves that propagate into the northern portion of the western scs basin and stronger b waves that propagate into the southern portion when traced back in time along linear characteristics a waves consistently line up close to peak ebb eastward barotropic currents while b waves consistently line up with peak flood westward barotropic currents this reinforces the notion that the lee wave mechanism and associated hydraulic or nonlinear effects are weak as demonstrated by a simple linear model relating the amplitude of the simulated waves to the excursion parameter at the ridges

a nonlinear model of the shoaling and refraction of interfacial solitary waves in the ocean part i development of the model and investigations of the shoaling effect abstract oceanic internal waves are frequently observed to be of large amplitude and to exhibit significant wave front curvature as seen in synthetic aperture radar images the aim of this paper is to present a refraction model in two horizontal dimensions including moderate amplitude effects to model these common properties of internal waves the model incorporates second order nonlinearity and uses a ray method to allow arbitrary refraction for these preliminary investigations with the model a two layer ocean is assumed with an upper layer thickness of 50 m and a density difference across the layers of 0 5 kg m 3 an important aspect affecting the refraction of nonlinear waves is shoaling as the phase speed of a nonlinear wave depends on its amplitude in part i of this paper the development of the model is described and the shoaling phenomenon is investigated in detail the shoaling of interfacial internal solitary waves over both slowly varying and rapidly varying bathymetry is studied a wedg

very large subaqueous sand dunes on the upper continental slope in the south china sea generated by episodic shoaling deep water internal solitary waves abstract very large subaqueous sand dunes were discovered on the upper continental slope of the northern south china sea the dunes were observed along a single 40 km long transect southeast of 21 93 n 117 53 e on the upper continental slope in water depths of 160 m to 600 m the sand dunes are composed of fine to medium sand with amplitudes exceeding 16 m and crest to crest wavelengths exceeding 350 m the dunes apparent formation mechanism is the world s largest observed internal solitary waves which generate from tidal forcing on the luzon ridge on the east side of the south china sea propagate west across the deep basin with amplitudes regularly exceeding 100 m and dissipate extremely large amounts of energy via turbulent interaction with the continental slope suspending and redistributing the bottom sediment while subaqueous dunes are found in many locations throughout the world s oceans and coastal zones these particular dunes appear to be unique for two principal reasons their location on the upper continental slope away from the influence of shallow water tidal forcing deep basin bottom currents and topographically amplified canyon flows and their distinctive formation mechanism approximately 60 episodic extremely energetic large amplitude events each lunar cycle

remelting in caldera and rift environments and the genesis of hot recycled rhyolites abstract large and small volume rhyolites are generated in calderas and rift zones inheriting older and isotopically diverse crystal populations from their volcanic predecessors low 18o values in many of these rhyolites suggest that they were derived from the remelting of solid hydrothermally altered by meteoric water protoliths that were once close to the surface but become buried by caldera collapse or rifting these rhyolites persist for millions of years in these environments with little evidence of coeval basalts we present a series of numerical experiments on convective melting of roof rocks by the underplated by near liquidus to superheated silicic melts generated at the base of the chamber by basaltic intrusions in shallow crustal conditions we used a range of temperatures and compositions an appropriate phase diagram with a defined extended eutectic zone appropriate for these environments varied sill thickness viscosity of the boundary layer and considered hydrothermal and lower boundary heat losses the goal was to estimate melting rates and mechanisms define conditions that are required for efficient and rapid remelting in the upper crust quantitatively describe novel details of the dynamics of convecting melting and compare it to the earlier parametric and numerical treatments of roof melting by underplating resolution of numerical experiments allowed us to track mixed thermal and two phase plume like convection in silicic magma with a bulk viscosity of 104 5 105 5 pa s the following results were obtained 1 remarkably fast melting magma generation rates of many meters per year 2 intrinsic inhomogeneities in the roof accelerates convection and melting rates via rapid gravitational settling of refractory blocks and exposing detachment scars to the melting front 3 due to rapid melting hydrothermal heat loss through the roof and conductive heat dissipation through the bottom are less important on melting timescales 4 convective melting is capable of digesting cold roof rocks with high assimilation degrees which are primarily controlled by sill thickness and roof rock temperature thin 10 m sills are able to digest 40 of the initially hot roof rock t 650 c roof rock but 100 m sills achieve the same level of bulk digestion with t 400 c roof rocks the proposed model can explain the origin of hot above 800 850 c crystal poor recycled rhyolites in calderas and rift zones it can also explain the generation of large supervolcanic rhyolite volumes through remelting of their erupted and subvolcanic predecessors on rapid timescales dictated by their zoned and disequilibrium crystalline cargo

large amplitude internal tides solitary waves and turbulence in the central bay of biscay microstructure and fine scale measurements collected in the central bay of biscay during the mouton experiment are analyzed to investigate the dynamics of internal waves and associated mixing large amplitude internal tides its that excite internal solitary waves isws in the thermocline are observed its are dominated by modes 3 and 4 while isws projected on mode 1 that is trapped in the thermocline therein its generate a persistent narrow shear band which is strongly correlated with the enhanced dissipation rate in the thermocline this strong dissipation rate is further reinforced in the presence of isws dissipation rates during the period without isws largely agree with the mackinnon gregg scaling proposed for internal wavefields dominated by a low frequency mode while they show poor agreement with the gregg henyey parameterization valid for internal wavefields close to the garrett munk model the agreement with the mackinnon gregg scaling is consistent with the fact that turbulent mixing here is driven by the low frequency internal tidal shear

tectonic controls on the late miocene holocene volcanic eruptions of the tengchong volcanic field along the southeastern margin of the tibetan plateau abstract the tengchong volcanic field located along the southeastern margin of the tibetan plateau experienced multiple eruption stages since the late miocene including time intervals of 5 5 4 0 ma 3 9 0 9 ma 0 8 0 01 ma and younger than 0 01 ma these eruption stages produced different volcanic rocks principally basaltic and basaltic andesite series at the same time or prior to volcanic eruptions in the tengchong volcanic field ne nne trending rift basins and ns striking normal faults formed during the late miocene pliocene in addition rapid exhumation of the neighboring mountains occurred at 6 5 ma constrained by apatite fission track dating and its thermochronological modeling at present the state of stress in the tengchong volcanic field and its surroundings is nne ne compression and wnw nw extension based on seismic foci mechanisms petrologic and geochemical data indicate that the source of the tengchong volcanic rocks belongs to an intracontinental tectonic setting but not a subduction or collision zone between indian and eurasian plates since the late miocene the dextral strike slip motion of the sagaing fault induced e w extension the sagaing dextral strike slip motion might disturb the lower crust upper mantle of the tengchong block resulting in the partial melting of the upper mantle which in turn induced volcanic eruptions characterized by mature island arc features

depositional dynamics in a river diversion receiving basin the case of the west bay mississippi river diversion abstract river deltas are a globally distributed class of sedimentary environment that are highly productive ecologically diverse and serve as centers for population and commerce many deltas are also in a state of environmental degradation and the mississippi river delta mrd stands out as a particularly iconic example plans to restore the mrd call for partially diverting the mississippi river which should reinitiate natural deltaic land building processes while the basic physical underpinnings of river diversions are relatively straightforward there exists a considerable controversy over whether diversions can and do deliver enough sediment to the coastal zone to build sub aerial land on restoration dependent time scales this controversy was addressed through a study of crevasse splay dynamics at the west bay mississippi river diversion the largest diversion in the mrd that was specifically constructed for coastal restoration we found that most sediments were distributed over a 13 5 km area with the maximum deposition occurring at the seaward end of this field these results indicate substantial sediment deposition downstream of project boundaries and run counter to simple sedimentary models which predict that maximum sediment deposition should occur closest to the riverbank despite this most sediments appear to be retained in the nearshore zone suggesting that the sediment retention efficiency was at the higher end of the 30 70 range suggested by some sediment budgets

the multi decadal morphodynamic changes of the mouth bar in a mixed fluvial tidal estuarine channel abstract inner channel mouth bars imb are common and vital geomorphological structures in estuaries that can efficiently promote the progradation of a fluvial delta however these significant structures face serious interferences by natural and human forces this study mainly focuses on imb deposited in the mixed fluvial tidal dominated south passage sp of the changjiang estuary between 1959 and 2018 to reveal how multidecadal morphodynamic variability in a mouth bar responds to natural forcing and human interferences the results show that the volume of the imb in the sp increased from 1128 106 m3 in 1959 to 1636 106 m3 in 1989 and then decreased to 462 106 m3 in 2018 meanwhile the evolution of the imb could be divided into five stages the growing phase 1959 to 1979 which showed a v shape the partial adjustment phase 1999 2003 which showed a crest dent shape the stable phase which showed a flat shape 2004 2010 the partial adjustment phase 2011 2017 when jiangyanan shola inserted into the sp and finally evolved into a flat shape in 2018 fluvial water discharge and suspended sediment discharge ssd did not control the variation in the mouth bar enso events were normally responsible for not only the periodic variations in the landward slope and the water depth of the crest of the imb but also occasional extreme changes in the imb moreover local sandbar insertion sediment from the seaside induced by strong tidal power and intensified engineering projects resulted in the continual shrinkage of the mouth bar by depositing more sediment at the mouth bar our work implies that understanding and governing this imb will bring additional economic benefits to this fast developing society but the protection of this delicate estuarine geomorphology system should still receive great attention

pathways and processes associated with the transport of groundwater in deltaic systems summary rivers are considered to be the primary means driving hydrological and geochemical fluxes between the continents and the ocean however it is unclear how well surface water fluxes represent total fluxes or whether more diffuse subterranean fluxes of river water to the coastal ocean occur this question is important in light of research demonstrating that submarine groundwater discharge sgd is important for geochemical and hydrological fluxes here we examine the pathways and potential magnitude of the role that sgd plays in the mississippi river delta mrd the largest delta in north america we present multiple independent lines of evidence demonstrating a hydrological connection between the mississippi river mr and the mrd evidence includes hydrological budgets demonstrating downstream water losses from the mr which are unexplained and are of the same magnitude as water sources to adjacent coastal bays of the mrd well data indicating a correlation between groundwater height and the stage of the mr and excess 222 rn inventories exceeding that expected from in situ production implying an advective i e groundwater source to coastal bays sgd likely flows from the mr to its delta via paleochannels and other buried sand bodies seismic data indicates that such features are common whereas resistivity data suggest the intrusion of low salinity water to coastal bays adjacent to the river these results may be applicable to other deltas worldwide as many of the world largest rivers have deltas with numerous abandoned distributaries that could act as conduits for groundwater

observational and numerical particle tracking to examine sediment dynamics in a mississippi river delta diversion abstract river diversions may serve as useful restoration tools along coastal deltas experiencing land loss due to high rates of relative sea level rise and the disruption of natural sediment supply diversions mitigate land loss by serving as new sediment sources for land building areas in basins proximal to river channels however because of the paucity of active diversions little is known about how diversion receiving basins evacuate or retain the sediment required to build new land this study uses observational and numerical particle tracking to investigate the behavior of riverine sand and silt as it enters and passes through the west bay diversion receiving basin located on the lowermost mississippi river delta usa fluorescent sediment tracer was deployed and tracked within the bed sediment over a five month period to identify locations of sediment deposition in the receiving basin and nearby river channel a computational fluid dynamics model with a lagrangian sediment transport module was employed to predict selective pathways for riverine flow and sand and silt particles through the receiving basin observations of the fluorescent tracer provides snapshots of the integrated sediment response to the full range of drivers in the natural system the numerical model results offer a continuous map of sediment advection vectors through the receiving basin in response to river generated currents together these methods provide insight into local and basin wide values of sediment retention as influenced by grain size transport time and basin morphology results show that after two weeks of low mississippi river discharge basin silt retention was approximately 60 but was reduced to 4 at the conclusion of the study riverine sand retention was approximately near 100 at two weeks and 40 over the study period modeled sediment storage was predicted to be greatest at the margins of the primary basin transport pathway this matched the observed dynamics of the silt tracer but did not match the behavior of the sand tracer the degree to which the observational measurements deviate from the model predictions may indicate the relative influence of physical processes other than the mean riverine generated currents such as tides wind generated currents and waves

hydrodynamic controls on sediment retention in an emerging diversion fed delta abstract the morphodynamics of river dominated deltas are largely controlled by the supply and retention of sediment within deltaic wetlands and the rate of relative sea level rise yet sediment budgets for deltas are often poorly constrained in the mississippi river delta a system rapidly losing land due to natural and anthropogenic causes restoration efforts seek to build new land through the use of river diversions at the davis pond freshwater diversion a new crevasse splay has emerged since construction was completed in 2002 here we use beryllium 7 activity in sediment cores and usgs measurements of discharge and turbidity to calculate seasonal sediment input deposition and retention within the vegetated davis pond receiving basin in winter spring 2015 which included an experimental period of high discharge through the diversion davis pond received 106 800 metric tons of sediment 44 of which was retained within the basin during this time mean flow velocity was 0 21 m s 1 and mean turbidity was 56 formazin nephelometric units fnu in summer fall 2015 the davis pond basin received 35 900 metric tons of sediment 81 of which was retained mean flow velocity in summer fall was 0 10 m s 1 and mean turbidity was 55 fnu the increase in sediment retention from winter spring 2015 to summer fall 2015 may be due in part to the corresponding drop in water flow velocity which allowed more sediment to settle out of suspension although high water discharge increases sediment input and deposition increased turbulence associated with higher current velocity appears to increase sediment throughput and thereby decrease the sediment trapping efficiency sediment retention in davis pond is on the high end of the range seen in deltaic wetlands perhaps due to the enclosed geometry of the receiving basin future diversion design and operation should target moderate water discharge and flow velocities in order to jointly maximize sediment deposition and retention and provide optimal conditions for delta growth

drowning of the mississippi delta due to insufficient sediment supply and global sea level rise global sea level rise reduced sediment supply and subsidence threaten the stability of the mississippi delta calculations of riverine sediment load and storage indicate that 10 000 13 500 km2 of the delta could be submerged by ad 2100

a new lower cretaceous hydrocarbon seep locality from the czech carpathians and its fauna abstract a new lower cretaceous hydrocarbon seep locality at baska in the czech carpathians is described the seep carbonate and associated mollusk fauna have been encountered in deep water claystones and siltstones of the upper barremian upper part of the hradist formation associated with pillow lavas of so called teschenite association the investigated carbonate displays petrographic characters typical of seep sediments clotted micrite isopachous rim cements and layered pyrite aggregations the carbon isotopic composition 20 0 to 23 5 vs v pdb most likely indicates anaerobic oxidation of thermogenic methane the fossils recovered from this locality are rather poorly preserved but hokkaidoconchid gastropods and lucinid bivalves in association with trochid and zygopleurid gastropods are identified this finding confirms the existence of seep faunas in the western carpathians which was anticipated based on the presence of seep mollusks and brachiopods in old monographs that cover lower cretaceous fossils of this region the fossils illustrated in the monograph of ascher 1906 are here briefly re evaluated and possible seep members are indicated

continuity and episodicity in the early alpine tectonic evolution of the western carpathians how large scale processes are expressed by the orogenic architecture and rock record data

u pb zircon geochronology of silicic tuffs from the timber mountain oasis valley caldera complex nevada rapid generation of large volume magmas by shallow level remelting large volumes of silicic magma were produced on a very short timescale in the nested caldera complex of the sw nevada volcanic field swnvf voluminous ash flows erupted in two paired events topopah spring ts 1 200 km3 12 8 ma tiva canyon tc 1 000 km3 12 7 ma and rainier mesa rm 1 200 km3 11 6 ma ammonia tanks at 900 km3 11 45 ma all cited ages are previously published 40ar 39ar sanidine ages within each pair eruptions are separated by only 0 1 0 15 my and produced tuffs with contrasting isotopic values these events represent nearly complete evacuation of sheet like magma chambers formed in the extensional basin and range environment we present ion microprobe ages from zircons in the zoned ash flow sheets of ts tc rm and at in conjunction with 18o values of zircons and other phenocrysts which differ dramatically among subsequently erupted units bulk zircons in the low 18o at cycle were earlier determined to exhibit 1 5 core to rim oxygen isotope zoning and high spatial resolution zircon analyses by ion microprobe reveal the presence of older grains that are zoned by 0 5 2 5 the following u pb isochron ages were calculated after correcting for the initial u pb disequilibria at zircon rims 11 7 0 2 ma cores 12 0 0 1 ma pre at rhyolite lava 12 0 0 3 ma rm 12 4 0 3 tc 13 2 0 15 ma ts 13 5 0 2 average zircon crystallization ages calculated from weighted regression or cumulative averaging are older than the ar ar stratigraphy but preserve the comparably short time gaps within each of two major eruption cycles ts tc rm at notably every sample yields average zircon ages that are 0 70 0 35 ma older than the respective ar ar eruption ages the th u ratio of swnvf zircons are 0 4 4 7 higher than typically found in igneous zircons which correlates with elevated th u of the whole rocks 5 16 high th u could be explained if uranium was preferentially removed by hydrothermal solutions or is retained in the protolith during partial melting for low 18o at cycle magmas rim ages from unpolished zircons overlap within analytical uncertainties with the 40ar 39ar eruption age compared to core ages that are on average 0 2 0 3 my older than even the age of the preceding caldera forming eruption of rm tuff this age difference the core to rim oxygen isotope zoning in at zircons and disequilibrium quartz zircon and melt zircon isotopic fractionations suggest that at magma recycled older zircons derived from the rm and older eruptive cycles these results suggest that the low 18o at magmas were generated by melting a hydrothermally altered protolith from the same nested complex that erupted high 18o magmas of the rm cycle only 0 15 my prior to the eruption of the at the largest volume low 18o magma presently known

a nonlinear model of the shoaling and refraction of interfacial solitary waves in the ocean part ii oblique refraction across a continental slope and propagation over a seamount abstract the refraction of internal solitary waves can be due to variations in water depth nonlinearity stratification or shear this paper addresses the effects of the first two of these effects part i of this paper investigated how the amplitude and phase speed of an interfacial solitary wave varied as the wave propagated across a one dimensional slope important differences between the predictions of korteweg de vries kdv and extended kdv ekdv theory were found in particular the ekdv limited the amplitude growth seen in kdv as the wave passed into shallow water leading to an associated strong retardation in speed in this second part of the paper these results are applied to two dimensional bathymetry the propagation of an oblique planar wave across a wedge shaped slope is first studied whereas the linear internal wave theory predicts small refraction of the wave front the nonlinear ekdv model shows that the refraction of large amplitude internal waves is significantly greater this is exp

oblique wave wave interactions of nonlinear near surface internal waves in the strait of georgia 1 although nonlinear interactions that occur when two large internal waves collide at oblique angles are often observed in the natural world quantitative and theoretical aspects of these interactions are only poorly understood the available analyses are generally theoretical or the result of limited numerical experimentation with few if any quantitative field measurements here we describe four cases of internal wave interactions two of which involve the fundamentally non steady generation of mach stems at the site of the interaction observed in the strait of georgia canada details of the interactions are quantified using time sequences of photogrammetrically rectified oblique images obtained from a circling aircraft while simultaneous observations are made of water column properties from a surface vessel the imaging technique allows us to estimate the speed and direction of each wave as well as details of phase shifts induced by interactions an existing small amplitude theory is applied but is found to overestimate the likelihood of mach interactions at large amplitude

dynamics of the tengchong volcanic region in the southeastern tibetan plateau a numerical study abstract the constraints of surface heat flow and lithospheric structure were used to investigate the dynamics of the tengchong volcanic region based on thermal mechanical modeling three different boundary conditions representing three types of tectonic movements were modeled to discuss the thermal state and lithospheric deformation the compressive model shows that the subduction of the indian plate causes the upwelling of asthenosphere beneath the tengchong block in this process upwelling of the asthenosphere causes thinning of the lithosphere beneath the tengchong block however the surface heat flow of this model is not consistent with the observations the regional extension model shows that under the extension the asthenosphere upwells along the preexisting weak suture zone in the southeastern tibetan plateau the thickness of lithosphere is the thinnest along the suture zone and the surface heat flow is the highest however the extension time was found to generally need 10 15 myr to conform to the surface heat flow however this is not consistent with accepted geochronological data when a boundary condition from compression to extension is imposed both the surface heat flow and the lithospheric structure fit well with the observations the results show that the subduction of the indian plate leads to the upwelling of the asthenosphere which causes a thinned lithosphere beneath the tengchong block since the late miocene 8 5 ma the extension or extrusion has been predominant the thermal erosion of the lithosphere caused by upwelling of the asthenosphere renders the lithosphere even thinner along the bangong nujiang suture zone the upwelling of the asthenosphere provides an upward driving force and high temperature environment for the tengchong volcanic region these findings suggest that the origin of the tengchong volcanic region is caused by both the subduction of the indian plate and regional extension across the bangong nujiang suture zone

evaluation of sediment diversion design attributes and their impact on the capture efficiency abstractmany riverine systems have been disconnected from their receiving basins by flood protection levees and other engineered systems reconnecting these alluvial rivers with their receiving basins is a viable option to nourish and sustain existing coastal wetland systems as well as to build new land this sediment nourishment can be accomplished through direct dredging and placement or through sediment diversions efficient design of sediment diversions is important to maximize the land building potential this study s objective is to quantitatively identify key design attributes of sediment diversions influencing their ability to capture sediment the outfall channel alignment angle intake invert elevation and diversion size are hypothesized as key parameters the analysis is limited to sediment grain sizes larger than 63 m and has been performed using a validated three dimensional numerical model a time integrated sediment water ratio was used as an indicator to reflect efficiency of sedime

a nonlinear model of internal tide transformation on the australian north west shelf a numerical solution to the generalized korteweg de vries k dv equation including horizontal variability and dissipation is used to model the evolution of an initially sinusoidal long internal wave representing an internal tide the model shows the development of the waveform to the formation of shocks and solitons as it propagates shoreward over the continental slope and shelf the model is run using observed hydrographic conditions from the australian north west shelf and results are compared to current meter and thermistor observations from the shelf break region it is found from observations that the coefficient of nonlinearity in the k dv equation changes sign from negative in deep water to positive in shallow water and this plays a major role in determining the form of the internal tide transformation on the shelf there is strong temporal variability in the nonlinear coefficient due to both background shear flow and the large amplitude of the internal tide which distorts the density profile over a wave period both the model and observations show the formation of an initial shock on the leading face of the internal tide in shallow water the change in sign of the coefficient of nonlinearity causes the shock to evolve into a tail of short period sinusoidal waves after further propagation a second shock forms on the back face of the wave followed by a packet of solitons the inclusion of bottom friction in the model is investigated along with the dependance on initial wave amplitude and variability in the coefficients of nonlinearity and dispersion friction is found to be important in limiting the amplitudes of the evolving waves

mineral composition in cognate inclusions in late miocene early pliocene potassic lamprophyres with affinities to lamproites from the denizli region western anatolia turkey implications for uppermost mantle processes in a back arc setting abstract mineral assemblages of both cognate inclusions and host lamprophyres with lamproitic affinity in kocapinar district denizli western anatolia consist of phlogopitic mica diopsidic clinopyroxene k feldspar apatite opaque and carbonates calcite dolomite two distinct types of cognate inclusions have been defined in host lamprophyres clinopyroxene rich cci and clinopyroxene phlogopite rich cpci whole rock compositions of inclusions show a near primitive nature with high mgo 7 56 15 1 wt cr 195 2270 ppm ni 213 335 ppm contents and a potassic character k2o 2 0 2 8 wt na2o 0 4 2 2 wt data imply that the inclusions crystallized from magmas formed by melting of phlogopite bearing pyroxenites in a peridotitic mantle source however the presence of reverse zoning in clinopyroxenes with salitic fe rich green cores in host lamprophyres and cpci inclusions suggest that the denizli lavas represent mixtures of distinct probably ultrapotassic and alkali basaltic magmas estimated geobarometric constraints inferred from clinopyroxene compositions in cci and cpci inclusions indicate moderate pressures of pyroxene crystallization ranging between 1 7 and 2 2 gpa and corresponding to 53 70 km depths under low pressure magma fractionation results reveal that i the origin of denizli lamprophyres with transitional between arc type and intra plate type geochemical signatures is consistent with a shallow level mantle petrogenesis rather than a deep seated origin related to mantle convection ii the source was a highly refractory and metasomatized peridotitic mantle present at the base of the lower crust iii metasomatic agents that affected the mantle lithosphere beneath denizli region are distinct from those beneath other western anatolia orogenic centers and iv the transitional character of kocapinar denizli lamprophyric rocks were probably formed as a result of either underplating or contamination of asthenospheric magma at the base of the mantle lithosphere or assimilation of delaminated continental edge lithospheric mantle via ascending asthenosphere denizli lamprophyres are interpreted to have formed during the formation of a basin graben structure in the latest miocene early pliocene just after late miocene exhumation of menderes massif

diagenetic source of fluids causing the hydrothermal alteration of teschenites in the silesian unit outer western carpathians czech republic petroleum bearing vein mineralization from the střibrnik site calcite pyrite veinlets containing a small amount of quartz chlorite sphalerite and marcasite occur in teschenite which is situated within the lower cretaceous flysch siliciclastics of the silesian unit fluid inclusion study demonstrated the presence of a heterogeneous mixture of a low salinity aqueous solution and mature methane rich petroleum during precipitation of calcite and quartz the 13c values of methane and dissolved carbonate 34s values of pyrite and 18o values of parent hydrothermal solutions are all consistent with the external source of the fluid phase derived from diagenetically modified flysch sediments the vein formation occurred during an increase in both temperature from 80 c to 220 c and pressure 100 700 bar the observed discrepancy between the maximum fluid temperature and the regional burial temperature together with fluctuations in petroleum density and significant variations in chemical and isotopic compositions of fluids are interpreted in terms of episodic tectonically induced generation of the fluid phase related to deeper burial the observed ree patterns of hydrothermal calcite and pyrite can be attributed to increased concentrations of organic ligands in the fluid phase and or involvement of sorption desorption processes

contrasting punctuated zircon growth in two syn erupted rhyolite magmas from tarawera volcano insights to crystal diversity in magmatic systems abstract two mineralogically and chemically distinct rhyolite magmas t1 and t3 were syn erupted from the same conduit system during the 21 9 ka basalt intrusion triggered okareka eruption from tarawera volcano new zealand high spatial resolution u th disequilibrium dating of zircon crystals at the 3 5 m scale reveals a protracted yet discontinuous zircon crystallization history within the magmatic system both magma types contain zircon whose interiors predate the eruption by up to 200 ka the dominant age peak in the t1 magma is 30 ka with subordinate peaks at 45 75 and 100 ka whereas the t3 magma has a dominant zircon interior age peak at 90 ka with smaller modes at 35 and 150 ka these patterns are consistent with isolated pockets of crystallization throughout the evolution of the system crystal rim analyses yield ages ranging from within error of the eruption age to at least 90 ka prior to eruption highlighting that zircon crystallization frequently stalled long before the eruption continuous depth profiling from crystal rims inward demonstrates protracted growth histories for individual crystals up to 100 ka that were punctuated by asynchronous hiatuses of up to 30 ka in duration disparate zircon growth histories can result from localized thermal perturbations caused by mafic intrusions into a silicic reservoir the crystal age heterogeneity at hand sample scale requires considerable crystal transport and mixing we propose that crystal mixing was achieved through buoyancy instabilities caused by mafic magma flow through crystal mush a terminal pre eruptive rejuvenation event was capable of mobilizing voluminous melts that erupted but was too short

the cascade of tidal energy from low to high modes on a continental slope abstractthe linear transfer of tidal energy from large to small scales is quantified for small tidal excursion over a near critical continental slope a theoretical framework for low wavenumber energy transfer is derived from flat bottom vertical modes and evaluated with observations from the oregon continental slope to better understand the observations local tidal dynamics are modeled with a superposition of two idealized numerical simulations one forced by local surface tide velocities and the other by an obliquely incident internal tide generated at the mendocino escarpment 315 km southwest of the study site the simulations reproduce many aspects of the observed internal tide and verify the modal energy balances observed transfer of tidal energy into high mode internal tides is quantitatively consistent with observed turbulent kinetic energy tke dissipation locally generated and incident simulated internal tides are superposed with varying phase shifts to mimic the effects of the temporally

uranium series accessory crystal dating of magmatic processes complex and protracted crystallization histories over geologic timescales are recorded in accessory minerals e g zircon allanite although magmatic crystallization was traditionally assumed to occur essentially instantaneously for the purposes of interpreting mineral geochronometers with low absolute time resolution for ancient samples it emerged relatively recently that magmatic crystallization can occur over extended durations this discovery arose from applying high spatial resolution accessory mineral dating techniques for uranium series isotopes to young volcanic and cognate plutonic rocks the emerging pattern from these studies is that individual crystals and crystal populations record crystallization episodes lasting from 1 000 to many hundreds of thousands of years accessory mineral dating of volcanic rocks and cognate plutonic xenoliths opens new research avenues for crystal age fingerprinting that correlates pyroclastic deposits lavas and plutonic rocks by using characteristic age dis

balancing aggradation and progradation on a vegetated delta the importance of fluctuating discharge in depositional systems vegetation is an important component of constructional landscapes as plants enhance deposition and provide organic sediment that can increase aggradation rates to combat land loss we conducted two sets of laboratory experiments using alfalfa medicago sativa to determine the effects of plants on channel organization and large scale delta dynamics in the first set we found that rapid vegetation colonization enhanced deposition but inhibited channelization via increased form drag that reduced the shear stress available for sediment entrainment and transport a second set of experiments used discharge fluctuations between flood and base flow or interflood interfloods were critical for reworking the topset via channel incision and lateral migration to create channel relief and prevent rapid plant colonization these low flow periods also greatly reduced the topset slope in the absence of vegetation by removing topset sediment and delivering it to the shoreline floods decreased relief by filling channels with sediment resulting in periods of rapid progradation and enhanced aggradation over the topset surface which was amplified by vegetation the combination of discharge fluctuations and vegetation thus provided a balance of vertical aggradation and lateral progradation we conclude that plants can inhibit channelization in depositional systems and that discharge fluctuations encourage channel network organization to naturally balance against aggradation thus variations in discharge are an important aspect of understanding the ecomorphodynamics of aggrading surfaces and modeling vegetated deltaic systems and the combined influences of plants and discharge variations can act to balance vertical and lateral delta growth

nd isotope composition of seep carbonates towards a new approach for constraining subseafloor fluid circulation at hydrocarbon seeps abstract the carbonates forming at deep sea hydrocarbon seeps provide an archive for the source and migration pathways of the seeping fluids interpretation of the archived isotopic and elemental signatures is however not straightforward because of mixing between the signals of fluids and ambient seawater limited understanding of subseafloor fluid circulation and effects of diagenetic alteration the nd isotope system is among the most widely used tracers of circulation patterns in modern and past oceans but to date nd isotopes have been rarely used in investigations of fluid migration pathways at hydrocarbon seeps to test the sensitivity of nd isotope signals of seep carbonates to record past interactions between the seeping fluids and 143nd enriched volcanic derived mineral components we performed nd sr and stable isotope analyses of the lower cretaceous barremian seep carbonates found in baska czech carpathians these seep deposits are directly underlain by a complex of mafic igneous rocks which the methane charged fluids must have passed through on their way to the seafloor the end t values measured in the seep carbonates are significantly more radiogenic than the inferred signal of contemporaneous local seawater and shifted towards the composition typical of the underlying basaltoids this is in agreement with the 87sr 86sr ratios which are markedly lower that the coeval seawater signature consistent co variance trends are observed among the measured end t 87sr 86sr and 13c values attesting increasing role of the volcanic derived fluid component with increasing contribution of methane derived carbon the results of the present approach reveal the potential of nd isotopes to fingerprint exotic fluid end members present in plumbing systems of both fossil and modern methane discharges the method can be of particular use in studies of seeps associated with plate margins where 143nd enriched igneous materials are commonly encountered in the basement or in the sediment column

apparent paleomagnetic rotations reveal pliocene holocene internal deformation of the tengchong block southeastern tibetan plateau abstract we present new paleomagnetic data from early pliocene to holocene volcanic rocks of the tengchong volcanic field and mangbang basin of the tengchong lhasa block yunnan along the eastern margin of the india asia collision zone two remanent magnetization components occur a higher coercive one carried by ti rich titanomagnetite and magnetite and likely of primary origin and a lower coercive one residing in magnetite and representing a more recent overprint the pleistocene holocene rocks of the tengchong volcanic field yield an in situ overall mean direction of d i 352 5 25 4 indicating a southeastward tilt we attribute the tilting to block rotation above listric normal faults that cut the tengchong volcanic field the in situ overall mean direction derived from pliocene volcanic rocks of the mangbang basin is d i 330 5 54 2 we explain this apparent counterclockwise rotation by northeastward tilting due to half graben formation during normal faulting along the nuangdeng fault zone along the eastern basin margin

in situ u pb ages of zircons from the bishop tuff no evidence for long crystal residence times ion microprobe u pb isotope analyses of zircons from fallout and intercalated ash flow deposits of the early most chemically evolved bishop tuff east central california yield a weighted mean age of 823 11 ka mean square of weighted deviates mswd 0 8 n 22 a similar mean age of 839 36 ka mswd 0 6 n 15 obtained on unpolished zircon rims suggests that the difference between these ages and that of 760 ka for the bishop tuff obtained by recent ar isotope determinations may be largely analytical in origin considerations based only on the effect of 230 th 234 u disequilibria on the u pb ages yield an apparent maximum mean crystal residence time for zircon of 100 k y the zircon residence times are significantly shorter than those inferred from previous estimates for the timing of phenocryst growth and suggest that once evolved the early bishop tuff rhyolite was not stored for long before erupting

sediment infilling and wetland formation dynamics in an active crevasse splay of the mississippi river delta abstract crevasse splay environments provide a mesocosm for evaluating wetland formation and maintenance processes on a decadal time scale site elevation water levels vertical accretion elevation change shallow subsidence and plant biomass were measured at five habitats along an elevation gradient to evaluate wetland formation and development in brant pass splay an active crevasse splay of the balize delta of the mississippi river the processes of vertical development vertical accretion elevation change and shallow subsidence were measured with the surface elevation table marker horizon method there were three distinct stages to the accrual of elevation capital and wetland formation in the splay sediment infilling vegetative colonization and development of a mature wetland community accretion elevation gain and shallow subsidence all decreased by an order of magnitude from the open water lowest elevation to the forest highest elevation habitats vegetative colonization occurred within the first growing season following emergence of the mud surface an explosively high rate of below ground production quickly stabilized the loosely consolidated sub aerial sediments after emergent vegetation colonization vertical development slowed and maintenance of marsh elevation was driven both by sediment trapping by the vegetation and accumulation of plant organic matter in the soil continued vertical development and survival of the marsh then depended on the health and productivity of the plant community the process of delta wetland formation is both complex and nonlinear determining the dynamics of wetland formation will help in understanding the processes driving the past building of the delta and in developing models for restoring degraded wetlands in the mississippi river delta and other deltas around the world

facies development of atchafalaya delta louisiana a modern bayhead delta early studies of atchafalaya delta focused on large scale characteristics of accretion in atchafalaya bay this study concentrates on sedimentation processes and stratigraphic as well as plan view facies development in a rapidly prograding bayhead delta since initiation of subaqueous delta growth early 1950s the bay has become shallower with accumulation of progressively coarser grained sediment stratigraphic relationships indicate a buildup from the pre 1950 bay bottom through prodelta distal bar distributary mouth bar levee back bar algal flat and channel fill deposits at the present stage of delta development two distinct processes of subaerial growth are recognizable initial growth relates to channel extension bifurcation and development of sinuous overbank channels a later more mature response of lobe fusion and upstream lobe growth becomes dominant as progradation through bifurcation diminishes in older areas of the delta atchafalaya delta growth to 1982 serves as a well documented sedimentary example for understanding bayhead deltas which commonly form in protected environments

timescales of crustal magma reservoir processes insights from u series crystal ages the dynamic processes operating within crustal magma reservoirs control many aspects of the chemical composition of erupted magmas and crystals in volcanic rocks provide a temporally constrained archive of these changing environments in this review i compile 238 u 230 th ages of accessory phases and 238 u 230 th 226 ra ages of bulk mineral separates of major phases these data document that crystals in individual samples can have ages spanning most of the history of a volcanic centre age populations for accessory phases show protracted pre eruptive crystal residence times but few crystals predate magmatic activity at a given centre these data have been interpreted in the context of residence times of the host magmas or timescales of the storage of crystals within a largely crystalline portion of the reservoir system in contrast less than half of the bulk separate 238 u 230 th 226 ra ages for major phases are more than 10 kyr older than the eruption many of these apparently conflicting observations of ages of major and accessory phases can be reconciled within the context of a model where a crystal mush was remobilized during processes leading to eruption overall the compiled data show that crystals contain rich archives of magmatic processes in crustal reservoirs especially when combined with other crystal scale geochemical data

ion microprobe dating of zircons from active dayingshan volcano tengchong se tibetan plateau time scales and nature of magma chamber storage abstract the rare occurrences of active volcanoes on the tibetan plateau provide a unique opportunity to investigate time scales of magma chamber processes for post collisional potassic volcanism this research utilizes high spatial resolution u th disequilibrium dating methods to date zircons from the active dayingshan volcano tengchong se tibetan plateau which erupted last during the holocene zircon crystal ages can be used to provide constraints on the magma chamber storage time measured zircon interiors yielded an isochron age of 87 5 6 5 ka while zircon outermost rims yielded an isochron age of 58 13 ka the calculated zircon crystal storage time for the most recent eruption is 48 ka at least two separate late pleistocene zircon crystallization events have occurred where the older age population represents antecrysts remobilized from an earlier magmatic episode but protracted crystallization cannot be ruled out regardless the similarity in zircon age populations between dayingshan volcano and maanshan volcano 13 km apart in the tengchong volcanic field suggests a synchronous thermal history of the magma reservoir possibly in an extensive interconnected magma chamber beneath the tengchong volcanic field

monazite th pb age depth profiling the significant capabilities of the ion microprobe for thermochronometric investigations of geologic materials remain largely unexploited whereas sup 208 pb sup 232 th spot analysis allows approximately 10 mm scale imaging of pb loss profiles or overgrowths in sectioned monazite grains the spatial resolution offered by depth profiling into the surface region of natural crystals is more than two orders of magnitude higher the authors document here the ability of the high resolution ion microprobe to detect sup 208 pb sup 232 th age differences of 1 m y with better than 0 05 micro m depth resolution in the outer micron of tertiary monazites from the hanging wall of the himalayan main central thrust age gradients on this scale are inaccessible to ion microprobe spot analysis or conventional thermal ionization mass spectrometry interpretation of the near surface sup 208 pb distributions with available monazite pb diffusion data illustrates the potential of the approach for recovering continuous high temperature thermal history information not previously available

mississippi delta subsidence primarily caused by compaction of holocene strata coastal subsidence causes sea level rise shoreline erosion and wetland loss which poses a threat to coastal populations this is especially evident in the mississippi delta in the southern united states which was devastated by hurricane katrina in 2005 the loss of protective wetlands is considered a critical factor in the extensive flood damage the causes of subsidence in coastal louisiana attributed to factors as diverse as shallow compaction and deep crustal processes remain controversial current estimates of subsidence rates vary by several orders of magnitude here we use a series of radiocarbon dated sediment cores from the mississippi delta to analyse late holocene deposits and assess compaction rates we find that millennial scale compaction rates primarily associated with peat can reach 5mm per year values that exceed recent model predictions locally and on timescales of decades to centuries rates are likely to be 10 mm or more per year we conclude that compaction of holocene strata contributes significantly to the exceptionally high rates of relative sea level rise and coastal wetland loss in the mississippi delta and is likely to cause subsidence in other organic rich and often densely populated coastal plains

seismic structure of the tengchong volcanic area southwest china from local earthquake tomography article i nfo weperformedalocalearthquaketomographytoimagethecrustofthetengchongvolcanicareainsouthwestern china using p wave arrival data from a temporary network and permanent stations the objective is to deter mine the magma sources of the volcanic area and the seismic structure of two m 7 earthquakes in the longling area south of tengchong the result reveals a correlation between the velocity structures and the volcanic activ ities from late miocene to holocene a prominent low velocity zone is observed beneath the central volcanic area that is cored at 10 20 km depths and extends down to 30 km depth with a lateral extent of 20 30 km we infer that this unusual low velocity zone represents a magma source of the heat flows for the volcanic erup tion during pleistocene and holocene close to it a high velocity zone appears beneath the eastern volcanic area and itislikelyto reflect solidifiedmagma intrusionsand high densityremnants withinthe cooled volcanicchan nel prior topleistocene in the m 7 earthquake area theuppercrustalstructureshows evidentvariationsacross the longling fault and the nu river fault bounded by the two fault zones the gaoligong metamorphic belt and the baoshan block are imaged by high velocities that are consistent with their high strength in the structures on the contrary low velocities are imaged in the area between the longling fault and the nu river fault implying a decreaseof the strength probably caused byigneous intrusions itwas thelateral variation intherheologic struc ture that led to the uneven stress accumulations across these fault zones and created tectonic conditions for the seismic ruptures of the two m 7 earthquakes in the longling area

effects of topography and earth s rotation on the oblique interaction of internal solitary like waves in the andaman sea high resolution three dimensional mitgcm simulations and an extended miles theory are used to investigate the oblique interaction of internal solitary like waves isws in the andaman sea oblique solitary wave interaction is well studied in various fields of physics however this process in the ocean has seldom been investigated despite the observations of its surface signature by remote sensing and potential 4 fold increase of isw amplitude by the development of a mach stem this study confirms the development of stem like isws under realistic oceanic conditions the modeled interaction is found to be approximately consistent with the miles theory extended to unequal incident wave amplitudes and small but finite incident angles although topography does not appear to affect the process in the deep part of the andaman sea substantial topographic amplitude variation along the wave trough needs to be adjusted before comparisons with the theory earth s rotation has a small effect on isw propagation due to the low latitude 9 n but it does slow down the growth of stem like isws potential observation of the process by the next generation wide swath satellite altimeter is discussed

u series ages of young volcanoes from the southeastern tibetan plateau holocene eruptions and magma evolution timescales abstract the tengchong volcanic field in southwestern china represents rare quaternary volcanic eruptions and associated active geothermal activities on the southeastern margin of the tibetan plateau the reported eruption ages for the 3 youngest tengchong volcanoes heikongshan dayingshan maanshan are highly variable 3 0 482 ka because of the challenge to accurately and precisely date young volcanic rocks by radiometric methods here we use high resolution 238u 230th 226ra disequilibria to constrain the eruption ages and magma evolution timescales all 3 young volcanoes exhibit whole rock 226ra 230th disequilibrium ranging from 1 27 to 1 71 indicating eruption ages of 230th ages of zircons from heikongshan dayingshan and maanshan zircon age populations are 54 6 13 ka and 94 9 4 1 ka for heikongshan 59 6 5 8 and 90 1 8 4 for dayingshan and 53 8 8 7 ka and 76 5 11 ka for maanshan magma residence times for all 3 volcanoes are 50 kyr for a shallow stage and 85 kyr for a deep stage in a two stage magma chamber system below tengchong the similar zircon age distributions for the 3 volcanoes from tengchong indicate simultaneous magmatism at 90 ka and 55 ka and imply interconnected nature of these volcanoes

timing of the late cretaceous ignimbrite flare up at the eastern margin of the eurasian plate new zircon u pb ages from the aioi arima koto region of sw japan abstract the kinki district in the inner zone of southwest japan is characterized by late cretaceous volcanic rocks of the aioi and arima groups and the koto rhyolites from west to east these rocks are dominated by pyroclastic flow deposits related to caldera forming events here we present new laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb ages for the aioi group four isolated caldera filling deposits the lower middle and upper parts of the arima group and one intrusive rock the kashihara quartz gabbro and the lower and upper parts of the koto rhyolites including the oldest volcanic unit within these rhyolites these analyses yielded ages of ca 86 82 ma for the aioi group ca 83 81 ma for the arima group 78 6 ma for the kashihara quartz gabbro and ca 74 73 ma for the koto rhyolites the aioi group represents a cluster of calderas and the ages obtained for individual units in this group differ from the ages of adjacent units the arima group and the koto rhyolites both consist of pyroclastic flow deposits associated with caldera forming events and the ages of these rocks are all the same within error this suggests that the aioi group represents a series of individual caldera forming eruptions that are distinct from the arima group and the koto rhyolites which formed during a single stage of caldera formation the u pb ages presented here indicate that the late cretaceous caldera forming eruptions in the study area occurred at intervals of 1 myr and represent individual events that lasted for

mechanics of river mouth bar formation implications for the morphodynamics of delta distributary networks 1 in this paper we use observational data and numerical modeling to present a new explanation for the formation of river dominated delta networks observational data from deltas throughout the world show that distributary channel widths depths and lengths decrease nonlinearly with successive bifurcations trends in width and depth are an outcome of hydraulic geometry scaling the trend in channel length is a consequence of delta growth analyses of serial maps show that the positions of bifurcations are the fossilized locations of river mouth bars also called middle ground and distributary mouth bar in front of old delta channel mouths therefore the trend in channel length can be explained through the mechanics of river mouth bar formation and evolution we use delft3d a coupled hydrodynamic and morphodynamic model to simulate the process of river mouth bar formation within an expanding turbulent jet in front of distributary channel mouths our results describe in detail the formation and evolution of a river mouth bar system and demonstrate that the distance to the river mouth bar is proportional to jet momentum flux and inversely proportional to grain size therefore channel length decreases down delta because with each successive bifurcation the jet momentum flux decreases these results can be used to predict the future evolution of river dominated deltas and can be used to aid in hydrocarbon exploration of these depositional environments

breaking of internal kelvin waves shoaling on a slope in stratified flow breaking of internal waves over slopes induces resuspension of bottom sediments and transport of mass when internal waves shoal and break flow dynamics and mass transport diff

a review of sediment diversion in the mississippi river deltaic plain abstract one of the proposed methods for restoring the disappearing mississippi delta is sediment diversion which uses channels and structures to divert water and sediment from the mississippi and atchafalaya rivers into adjacent basins this study presents a comprehensive review of geological and physical aspects of sediment dynamics in the mississippi river deltaic plain mrdp with special reference to diversion studies over the past two decades we synthesize these studies present the current understanding of sediment diversions in the context of sediment dynamics identify multiple key knowledge gaps and make recommendations for future studies to maximize net land building in the mrdp management strategies should be focused on a enhancing river sediment delivery both mud and sand b increasing sediment retention in receiving basins and c minimizing erosion in bays and estuaries compared with extensive studies of land building there have been relatively fewer studies of erosional processes a heterogeneous coastal geological framework cohesive sediment erodibility and subsidence together play complicated yet critical roles in future sediment dynamics in bays and estuaries of the mrdp sediment retention rates are highly sensitive to spatial and temporal scales types of sediments and delivery season sediment diversions to seaward receiving basins provide more surge protection but tend to have lower sediment retention due to active coastal processes structures and devices that improve sediment retention trap sediments dissipate waves and build living shorelines should be explored and cost to benefit analysis is needed long term planning should consider more landward diversions strategic community relocation and nonlinear response of the complex sedimentary system of the mrdp

prototypical solitons in the south china sea 1 surface signatures associated with non linear internal waves are often seen in satellite images of the western south china sea scs slope and shelf observation in the deep sea to the east are rare here we report on the evolution of an energetic packet as it propagated through the deep central basin of the scs toward the western slope and shelf the waves have amplitudes estimated at 170 m half widths of 3 km and phase speeds of 2 9 0 1 m s faster than the mode 1 linear phase speed of 2 6 m s the shape and observed phase speed were consistent with the korteweg devries kdv model over the 65 km path that they were tracked the intrinsic velocity shear of the waves is small compared to pre existing shears and the waves exhibit weak turbulence the kdv fit and a satellite derived estimate of horizontal wave extent imply a westward energy flux of 4 5 gw for each crest

topographic effect on oblique internal wave wave interactions based on a variable coefficient kadomtsev petviashvili kp equation the topographic effect on the wave interactions between two oblique internal solitary waves is investigated in the absence of rotation and background shear the model set up featuring idealised shoaling topography and continuous stratification is motivated by the large expanse of continental shelf in the south china sea when the bottom is flat the evolution of an initial wave consisting of two branches of internal solitary waves can be categorised into six patterns depending on the respective amplitudes and the oblique angles measured counterclockwise from the transverse axis using theoretical multi soliton solutions of the constant coefficient kp equation we select three observed patterns and examine each of them in detail both analytically and numerically the effect of shoaling topography leads to a complicated structure of the leading waves and the emergence of two types of trailing wave trains further the case when the along crest width is short compared with the transverse domain of interest is examined and it is found that although the topographic effect can still modulate the wave field the spreading effect in the transverse direction is dominant

geochemistry of the neoproterozoic tilemsi belt of iforas mali sahara a crustal section of an oceanic island arc abstract the pre pan african neoproterozoic tilemsi belt of the western iforas mali sahara comprises a predominantly bimodal mafic and felsic volcanic assemblage associated with numerous intrusions mainly of mafic composition the volcanics include island arc tholeiites and the calc alkali rocks rhyolites dacites and andesites the volcanic domain shows similarities to recent oceanic island arc systems as well as to archaean greenstone belts the tilemsi magmatic sequence represents an example of a section through the middle and upper levels of a neoproterozoic intra oceanic island arc crust that is composed nearly exclusively of igneous material the middle crustal section is mainly of mafic composition while the upper crustal level contains more silica rich plutonic volcanic and volcaniclastic assemblages the mafic intrusions include a large proportion of cumulates indicating that fractional crystallization played an important role during the evolution of the oceanic arc suites compositionally the arc changed through time from an immature to a relatively developed system the rocks became more silica and incompatible element rich with higher sr isotopic ratios this trend reflects the increasing growth and maturity of the arc

mitigating land loss in coastal louisiana by controlled diversion of mississippi river sand the bonnet carre spillway diverts floodwaters from the mississippi river to lake pontchartrain and was opened for 42 days during the 2011 flood according to measurements of the newly deposited sediments at least 31 46 of the river s sand load was diverted into the spillway at this time suggesting that such diversions can help mitigate coastal wetland loss

ndsr isotope and ree geochemistry of alkali basalts from the massif central france abstract nd and sr isotopic compositions as well as trace element concentrations have been determined on a suite of alkali basalts from the massif central in france samples show a typical enrichment in incompatible elements in particular the ree patterns exhibit a strong fractionation characterized by a la yb n ratio of about 20 the ybn content is about 10 times chondrite the 143 nd 144 nd ratios exhibit a range from 0 512775 to 0 512989 values quite comparable to those from oceanic island basalts the 87 sr 86 sr ratios vary between 0 70338 and 0 70458 and are anti correlated with the nd isotopic ratio the isotopic and the trace element in particular ree data have been used in order to quantitatively model the genesis of the alkali basalts among the several types of models tested here the most likely one appears to be the model of mantle metasomatism a semi quantitative approach shows that the source of alkali basalts from the massif central was metasomatized prior to melting in such a model the basalts could be produced by rather high degrees of partial melting such as 10 or 15 of the metasomatically enriched mantle

age of a late quaternary 13 6 cal kyr bp eruption and associated marker tephra of the tengchong volcanoes yunnan province southwestern china abstract the tengchong volcanoes in western yunnan province are some of the youngest volcanoes in china and determining their eruption ages would further our understanding of their genesis and historical activity in this study the age of a late quaternary eruption of dayingshan volcano was obtained by dating organic rich sediments that envelope a 5 cm thick visible tephra layer found in the yubi peat section compositionally the purified glass shards extracted from the layer by density separation are calc alkaline trachyandesite and trachyte trachydacite which is consistent with the composition of the dayingshan lava an age depth model of the yubi section was constructed from accelerator mass spectrometry ams 14c dating of peat samples and poisson models of deposition p sequence functions using oxcal with the intcal 13 database this yielded an age for the tephra layer of 13 642 84 cal yr bp a nearly identical tephra layer occurs in the qinghai crater lake sediments the yubi tephra is now a reliable age and stratigraphic marker layer useful for regional paleoclimate and tephrochronology research

hydrodynamic and geomorphic controls on mouth bar evolution 1 while river deltas are one of the major repositories for sediments and carbon on earth there exists a paucity of field data on the formation of distributary mouth bars one of their key features here we present results from an experiment that tested a model of mouth bar development using hydroacoustic optical sedimentary and geochemical tools on a mouth bar in a crevasse splay near the mouth of the mississippi river our results validate an existing model for mouth bar development which we extend to explain mouth bar stratigraphy we propose that changes across a hydrological cycle are important for mouth bar development resulting in a stratigraphy that has alternating fine and coarse grain sediments results also indicate that sand is carried up to 6 km from the main stem of the mississippi river despite repeated channel bifurcations which has important implications for our interpretation of the rock record understanding of coastal sedimentary systems and the restoration of large deltas

internal wave detection using the moderate resolution imaging spectroradiometer modis 1 the moderate resolution imaging spectroradiometer modis with its combined attributes of fine spatial resolution large swath area and near daily global coverage has for the first time allowed sunglint the near specular reflectance pattern of sunlight off the ocean surface to be used to analyze and survey high frequency nonlinear internal solitary wave occurrences on a near global scale the sunglint area within the modis swath is large enough to cover an entire basin and it s spatial resolution is fine enough to identify internal wave signatures whose wavelength is greater than a few hundred meters this paper describes the use of modis for high frequency nonlinear internal wave detection presents the results of a survey that detected 3581 internal wave occurrences in modis imagery over the period between august 2002 through may 2004 and discusses ways modis imagery can be used to improve the study of internal waves

middle triassic volcanic rocks in the northern qiangtang central tibet geochronology petrogenesis and tectonic implications abstract although voluminous magmatism occurred during the triassic in the qiangtang terrane the petrogenesis of the volcanic rocks and the associated tectonic scenarios remain mysterious this study focuses on new identified primitive volcanic rocks from the yanshiping area northern qiangtang subterrane the whole rock major trace elemental sr nd isotopic data and zircon u pb age of volcanic rocks are reported in this paper in order to understand their petrogenesis and tectonic setting the studied volcanic rocks can be grouped into two types i e nb enriched basalts and basaltic andesites nebs in the low sequence and arc basalts in the upper sequence zircon u pb dating using la icp ms techniques yields the concordant age with a weighted mean 206 pb 238 u age of ca 242 241 ma for the nebs and ca 240 ma for the arc basalts the distinct geochemical and isotopic characteristics of whole rock and varying th u ree components of zircon grains from the volcanic rocks suggest that the nebs were derived from partial melting of relatively enriched mantle wedge that have been metasomatised by slab related melts and that the arc basalts originated from partial melting of mixing of mantle wedge and depleted asthenospheric mantle in response to the slab breakoff our new geochemical and geochronological results in combination with regional studies imply that the middle triassic magmatism was generated in the tectonic setting of northward subduction of the paleo tethys or bangong nujiang ocean subduction related processes that include melting of the slab and the slab breakoff during the permian middle triassic made an important contribution to crustal growth in the qiangtang terrane whereas vertical crustal growth associated with collision related setting during the late triassic is nonsignificant

tectonics and paleomagnetic rotation pattern of yunnan 24 n 25 n china gaoligong fault shear versus megablock drift

timing and conditions of formation of granitoid clasts erupted in recent pyroclastic deposits from tarawera volcano new zealand abstract granitoid clasts in pyroclastic deposits of the 0 7 ka kaharoa eruption from the intra caldera tarawera volcano of the okataina volcanic centre ovc new zealand provide an insight to the plutonic processes beneath one of the most productive quaternary rhyolite centers on earth sims u th data for 79 granitoid zircon crystals from six clasts reveal a unimodal age spectrum yielding a weighted average model age of 208 4 ka mswd 1 2 n 72 the remaining crystals are at secular equilibrium and u pb analyses indicate that a few of these outliers date back to 750 ka a period significantly longer than the known volcanic record at ovc probably 550 ka in contrast zircon crystallization in co erupted pumice and lava of the 0 7 ka kaharoa event and that of the three preceding rhyolite eruptions occurred mostly during 0 50 ka reflecting a separate magmatic history brittle deformation features incipient alteration and low 18o whole rock compositions 3 are consistent with a shallow solid intrusion that has interacted with meteoric hydrothermal fluids however 18o sims analyses of zircons 5 4 0 2 n 11 are consistent with magmatic compositions and thus meteoric interaction occurred post emplacement the kaharoa granitoids contrast with those ejected in the 40 60 ka caldera forming rotoiti event that were partly molten and display zircon age spectra indistinguishable from those for co erupted pumices suggesting the latter were derived from contemporaneous crystal mush the 0 7 ka kaharoa case shows that zones of the magmatic system remained solid despite frequent and voluminous magma production that included caldera forming events such long lived sub solidus zones in magma systems would act as barriers to melt connectivity and interaction with country rock but are also a potential source for antecrysts xenocrysts in subsequent eruptions this exemplifies how plutonic and volcanic evolution can diverge even in close proximity

seismic structure beneath the tengchong volcanic area southwest china from receiver function analysis abstract tengchong is a young volcanic area on the collision boundary between the indian and euro asian plates of the southeastern tibetan margin holocene volcanoes are concentrated in the tengchong basin where they align an n s trending string like cluster to study the magma activity and its relation with the volcanoes we deployed a passive seismic observation across the volcanic area in northern tengchong using tele seismic data and receiver function technique we determined the s wave velocity structure beneath nine temporary stations results show that the tengchong basin is underlain by prominent low velocity zones that are associated with the magma chambers of the volcanoes in the north a small and less pronounced magma chamber lies beneath two crater lakes with a depth range of 9 16 km and a lateral width of

short term evolution of po della pila delta lobe from time lapse high resolution multibeam bathymetry 2013 2016 abstract over the last few years 2013 2016 repeated high resolution multibeam surveys were carried out at the recent most delta lobe of the po river in correspondence to po della pila the collected multibeam bathymetry together with backscatter data seabed samples and high resolution seismic profiles provided insights on the short term morphological and sedimentological evolution of this extremely dynamic submarine portion of the delta a high variety of geomorphological features and depositional bodies were observed from the mouth bar to the prodelta slope such as for example the alongshore and transverse bars formed under the effect of marine currents gravitational instability phenomena and collapse depressions driven by fluid expulsion concurrently the analysis of the seabed reflectivity and sediment samples allowed the identification of two main dominant types of seafloor sediment corresponding to sandy and muddy seabed and the mapping of their variable distribution in the study area the comparison of time lapsed high resolution dems showed that the main changes occurred on the northern side of the prodelta slope where a new lobe shaped fine sediment deposit built up to 4 5 m adding roughly a volume of 1 16 mm3 of new sediments at the same time in the prodelta slope transverse depositional bars showed a clear migration toward the south of the system and local subsidence phenomena up to 1 5 m between 7 and 10 m water depth are observed

cretaceous alkaline lamprophyres from northeastern czech republic geochemistry and petrogenesis the northeast part of the czech republic moravia and the adjoining part of poland host a 100 km long and 15 to 25 km wide belt containing numerous isolated bodies mainly sills of lamprophyre of lower cretaceous age the lamprophyres range from mafic melanocratic to evolved feldspar rich mesocratic variants mineralogically they are characterized by compositionally zoned kaersutite phenocrysts biotite and high al ti salitic to diopsidic clinopyroxene the lamprophyres are typically alkaline as shown by high contents of p2o5 tio2 alkalies and incompatible trace elements such as light ree zr nb y ba and sr and by high ti v 50 and chondrite normalized la yb n 8 25 ratios they resemble alkali basalts basanites and nephelinites major element composition and trace element patterns and nd sr isotopic values end ca 5 5 to 6 6 and esr ca 9 5 to 24 0 indicate that the lamprophyric magma was derived from a mantle source that was compositionally similar to the source of ocean island basalts with himu affinities and some continental extension related alkali basaltic suites the lamprophyres do not show any subduction imprint they were generated in the garnet stability field by a variable degree of melting evolved lamprophyres were formed by clinopyroxene dominated fractional crystallization of mafic lamprophyric magma the lamprophyres are interpreted to have been emplaced along conduits formed during the formation of a basin graben structure in the early cretaceous

wetland loss and the subdelta life cycle abstract the rapid deterioration of marsh habitat observed during recent years in the modern mississippi river delta is a consequence at least in part of the natural life cycle of subdeltas with life spans typically less than 200 years subdeltas or bay fill deposits are scaled down versions of major delta lobes yet provide through pulses of sediment nearly all the subaerial land in an active delta using maps charts and aerial photographs curves were constructed for rates of change in land area sediment volume and linear progradation in the four subdeltas that have formed on the modern mississippi river delta since the first accurate survey in 1838 results indicate that each subdelta 1 lasted for approximately 115 175 years 2 included both periods of growth and deterioration 3 was initiated by a crevasse or break in the natural levee system 4 showed linear advancement and volumetric growth during subaerial deterioration and 5 displayed a new pulse of subaerial growth during the high discharge decade of the 1970s contrary to popular accounts demise of the mississippi river delta through deterioration of its subdeltas is not a result of the construction of artificial levees upstream or discharge of sediment off the continental shelf edge rather it is attributable to a substantial decrease and fining of sediments being transported downstream to depositional sites within a delta that has developed through natural processes a complex and inefficient channel network for delivering these sediments

petrogenesis of the natkusiak continental basalts victoria island northwest territories canada the natkusiak flows 1100 m thick sills and dykes of victoria island northwest territories are part of an extensive late hadrynian magmatic event in the northern canadian shield the flows are subaerially erupted plateau basalts and together with associated doleritic sills and dykes display many geochemical features typical of continental tholeiites in addition to fractional crystallization the rocks were affected by crustal contamination but to a smaller degree than usually encountered in continental tholeiites elsewhere the basalts were derived from a depleted upper mantle source very similar to that of n type mid ocean ridge basalts although it is suggested that the melting residue contained small amounts of garnet 2

an evaluation of subsidence rates and sea level variability in the northern gulf of mexico 1 while subsidence is widely recognized as a driver of geomorphic change in the northern gulf of mexico gom there is considerable disagreement over the rates of subsidence and the interpreted variability in these rates which leads to controversies over the impacts of subsidence on surface land area change here we present a new method to calculate subsidence rates from the tide gauge record that is based on an understanding of the meteorological drivers of inter annual sea level change in grand isle la and galveston tx we explicitly show that temporal patterns of subsidence are closely linked to subsurface fluid withdrawal and coastal land loss and suggest changes in withdrawal rates can both increase and decrease rates of subsidence and wetland loss our results also imply that the volume of sediment needed to rebuild gom wetlands may currently fall within the low end of some restoration scenarios

magnetotelluric images of deep crustal structure of the rehai geothermal field near tengchong southern china broadband 0 004 4096 s magnetotelluric mt soundings have been applied to the determination of the deep structure across the rehai geothermal field in a quaternary volcanic area near the indo eurasian collisional margin tensorial analysis of the data show evidence of weak to strong 3 d effects but for approximate 2 d imaging we obtained dual mode mt responses for an assumed strike direction coincident with the trend of the regional scale faults and with the principal impedance azimuth at long periods the data were subsequently inverted using different approaches the rapid relaxation inversion models are comparable to the sections constructed from depth converted invariant impedance phase data the results from full domain 2 d conjugate gradient inversion with different initial models are concordant and evoke a picture of a dome like structure consisting of a conductive 50 1000 m cap which is about 5 6 km thick in the central part of the known geothermal field and thickens outwards to about 15 20 km the anomalous structure rests on a mid crustal zone of 20 30 m resistivity extending down to about 25 km depth where there appears to be a moderately resistive 30 m substratum the mt images are shown to be in accord with published geological isotopic and geochemical results that suggested the presence of a magma body underneath the area of study

palaeomagnetism of the teschenitic rocks lower cretaceous in the outer western carpathians of poland constraints for tectonic rotations in the silesian unit summary results of palaeomagnetic investigations of the lower cretaceous teschenitic rocks in the silesian unit of the outer western carpathians in poland bring evidence for pre folding magnetization of these rocks the mixed polarity component reveals inclinations between 56 and 69 which might be either of cretaceous or tertiary age apparently positive results of fold and contact tests in some localities and presence of pyrhotite in the contact aureole suggest that magnetization is primary although a neogene or earlier remagnetization cannot be totally excluded since inclination only test between localities gives syn folding results higher palaeoinclinations 66 69 correlate with a younger variety of teschenitic rocks dated for 122 120 ma while lower inclinations 56 60 with an older variety 138 133 ma this would support relatively high palaeolatitudes for the southern margin of the eurasian plate in the late part of the early cretaceous and relatively quick northward drift of the plate in this epoch together with the silesian basin at its southern margin declinations are similar to the cretaceous tertiary palaeodeclinations of stable europe in the eastern part of the studied area but rotated ca 14 70 counter clockwise in the western part this indicates together with older results from czech and slovakian sectors of the silesian unit a change in the rotation pattern from counter clockwise to clockwise at the meridian of 19 e the rotations took place before the final collision of the outer carpathians nappe stack with the european foreland