rift propagation detachment faulting and associated magmatism in bransfield strait antarctic peninsula multichannel seismic data from bransfield strait a young 4 ma marginal basin located at the nw tip of the antarctic peninsula reveal a complex array of rifting styles short of clearly defined seafloor spreading the strait is undergoing basinwide extension while enriched mid ocean ridge basalts have been dredged from topographic highs in the ne part of the basin seismic evidence does not support the formation of oceanic crust there acoustic signatures of volcanic crust at known axial deep highs extend laterally subseafloor their distribution and style support propagation of volcanism and associated extension from ne to sw active extensional structures along both margins control a strong physiographic asymmetry the south shetland islands margin is composed of a few closely spaced large offset normal faults the gentler antarctic peninsula margin exhibits broader distributed extension to the ne normal fault polarity reversal structures nfprs dominate listric faults sole into nw dipping low angle reflectors interpreted as detachments to the sw larger nfprs and convex up geometries up to 20 km across suggest broad uplift rather than detachment based extension such uplift may precedes seafloor volcanism and rapid associated deflation along detachments nw se trending accommodation zones also segment the basin along strike bransfield strait extension may result both from plate boundary tectonics at the south shetland trench and at the evolving antarctic scotia plate boundary extension was probably initiated by onset of subduction rollback at the trench as a slab window opened from the sw augmented extension in the ne part of the basin may be a response to left lateral transtension associated with reorganization of the triple junction at the shackleton fracture zone south scotia ridge intersection

leakage of clogged channels that partially penetrate surficial aquifers a theoretical formulation describing leakage from clogged channels that partially penetrate surficial aquifers is reported in this paper leakage characteristics are parameterized by a reach transmissivity which is defined as the volume flow rate out of the channel per unit length of the channel per unit drawdown an expression is developed to relate the reach transmissivity to the transmissivity of the formation mean channel width distance of drawdown measurement from the channel centerline ratio of drawdowns on both sides of the channel and local reach transmissivity associated with clogging this theoretical expression and associated limitations are verified using a fine scale numerical model the proposed formulation was field tested at two channels the measurements of reach transmissivity were found to vary linearly with the drawdown ratio as predicted by the theory furthermore the aquifer transmissivity obtained by fitting the theoretical formulation to the measurements is in close agreem

tectonic evolution of the west scotia sea joint inversion of isochron and flowline data from the flanks of the extinct west scotia ridge spreading center yields five reconstruction rotations for times between the inception of spreading prior to chron c8 26 5 ma and extinction around chron c3a 6 65 9 ma when they are placed in a regional plate circuit the rotations predict plate motions consistent with known tectonic events at the margins of the scotia sea oligocene extension in powell basin miocene convergence in tierra del fuego and at the north scotia ridge and miocene transpression at the shackleton fracture zone the inversion results are consistent with a spreading history involving only two plates at rates similar to those between the enclosing south america and antarctica plates after chron c5c 16 7 ma but that were faster beforehand the spreading rate drop accompanies inception of the east scotia ridge back arc spreading center which may therefore have assumed the role of the west scotia ridge in accommodating eastwards motion of the trench at the eastern boundary of the scotia sea this interpretation is most easily incorporated into a model in which the basins in the central parts of the scotia sea had already formed by chron c8 contrary to some widely accepted interpretations and which has significant implications for paleoceanography and paleobiogeography

granulites in the tongbai area qinling belt china geochemistry petrology single zircon geochronology and implications for the tectonic evolution of eastern asia the tongbai area of the eastern qinling belt in china includes granulite grade metamorphic assemblages qinling complex which were previously regarded as archean to early proterozoic in age and belonging to the southern margin of the north china plate craton our petrological and geochemical data characterize these rocks as two pyroxene granulites and garnet granulites which formed at temperatures of 757 840 c and pressures of about 9 5 kbar and are now found as xenoliths in granodioritic gneisses the protoliths of these rocks were granodiorites and tholeiitic basalt or gabbro the 207pb 206pb ratios derived from evaporation of single zircons yield ages of 470 20 and 470 14 ma respectively for the basic granulites which we interpret to reflect the time of protolith emplacement these are intruded by a 435 14 ma granodioritic gneiss post dating granulite formation a metaquartzite sample contains detrital zircons as old as 2555 8 ma two samples of granitoid gneiss from the tongbai complex s of the qinling granulites have single zircon 207pb 206pb evaporation ages of 776 8 and 746 10 ma respectively and document late proterozoic igneous activity we suggest that the qinling granulites document an important and hitherto unknown phase of early silurian crustal thickening following subduction and continental collision and that both the qinling and tongbai complexes were part of the southern margin of the north china craton prior to this event and record late proterozoic igneous activity

joint geophysical and petrological models for the lithosphere structure of the antarctic peninsula continental margin summary the antarctic peninsula ap is a composite magmatic arc terrane formed at the pacific margin of gondwana through the late mesozoic and cenozoic subduction has stopped progressively from southwest to northeast as a result of a series of ridge trench collisions subduction may be active today in the northern part of the ap adjacent to the south shetland islands the subduction system is confined by the shackleton and hero fracture zones the magmatic arc of the ap continental margin is marked by high amplitude gravity and magnetic anomaly belts reaching highest amplitudes in the region of the south shetland islands and trench the sources for these anomalies are highly magnetic and dense batholiths of mafic bulk composition which were intruded in the cretaceous due to partial melting of upper mantle and lower crustal rocks 2 d gravity and magnetic models provide new insights into crustal and upper mantle structure of the active and passive margin segments of the northern ap our models incorporate seismic refraction constraints and physical property data this enables us to better constrain both moho geometry and petrological interpretations in the crust and upper mantle model along the dss 12 profile crosses the ap margin near the anvers island and shows typical features of a passive continental margin the second model along the dss 17 profile extends from the drake passage through the south shetland trench islands system and bransfield strait to the ap and indicates an active continental margin linked to slow subduction and on going continental rifting in the backarc region continental rifting beneath the bransfield strait is associated with an upward of hot upper mantle rocks and with extensive magmatic underplating

evolution of stream aquifer hydrologic connectedness during pumping experiment the evolution of the hydrologic relationship between streams and aquifers resulting from groundwater pumping or drainage near a stream is of great interest to the research community and to water management decision makers a stream aquifer ditch sandbox was built to be used to characterize the hydrologic connectedness of the stream aquifer responding to declines of the water table over a vertical profile presumably as a result of increased pumping near the stream we called the position of water table over the vertical profile the water table curve six hydrologic cases were chosen to investigate the behaviors of the water table curve all cases are roughly based on existing and possible real hydrogeological conditions in nature emphasis was placed on the analysis of the dynamic processes of evolution in the stream aquifer system where the stream aquifer changes from a fully connected condition to a disconnected condition the experimental results for the six cases were shown by differences in hydraulic head contours pressure head distributions and the shapes of the water table curve over a vertical profile the thickness of the inverted saturated zone and the induced recharge of groundwater from the stream factors that strongly affect the evolution of stream groundwater relationships are discussed including changes in stream stages the declines of the water table curves stream geometries streambed clogging and aquifer heterogeneity then a new concept of a disconnected stream with aquifer is given according to the evolution of stream groundwater relationships

estimating streambed parameters for a disconnected river evaluation of stream aquifer interaction and water balance for a catchment often requires specific information on streambed parameters such as streambed hydraulic conductivity seepage flux across the streambed and so on this paper describes a simple inexpensive instrument that is used to measure these streambed parameters under the condition of a stream disconnected from groundwater our method includes a seepage cylinder for simulation of river water depth the proposed method was applied to estimate the vertical hydraulic conductivity of a streambed and the changes in vertical seepage rate from stream to groundwater with varied stream water depth in the manasi river of xinjiang uygur autonomous region china the vertical hydraulic conductivities of the streambed determined from 12 sites along the manasi river vary from 1 01 to 29 m day where the stream disconnects from the groundwater the experimental results suggest that there are two kinds of relations between the vertical seepage rate and the simulated stream water depth one is a linear relation between the two variables with low reynolds numbers less than 10 the other is a nonlinear relation exponential relation between the two variables with larger reynolds numbers greater than 10 this second relationship is quite different from the traditional model that usually calculates the vertical seepage rate from stream to groundwater under the condition of disconnection using a linear relation darcy s law our results suggest that a linear relation can only be used for a limited range of river water depth this method gives a convenient tool for rapidly estimating the streambed hydraulic conductivity and the changes in the vertical seepage rate across streambed with varied stream water depths for the case of a stream disconnected from groundwater copyright 2013 john wiley sons ltd

the early cenozoic tectonic history of the southeast pacific abstract new marine geophysical data collected in the southeast pacific enable us to define better the early cenozoic tectonic history of the area and to interpret previously unexplained bathymetric features in terms of plate tectonic processes we have identified a major re organization in plate boundaries at anomaly 21 time when a large fragment was broken off the pacific plate and joined to the antarctic plate this interpretation is based on three pieces of evidence 1 a major ese trending feature the humboldt fracture zone marks the southern boundary of the farallon antarctic anomalies identified by weissel et al 1 2 a north south striking trough the hudson trough which represents the scar left by the pacific antarctic ridge when it rifted across old pacific crust 3 a sequence of north south striking magnetic lineations south of the humboldt fracture zone and east of the hudson trough is correlated to anomalies 27 to 22 and identified with pacific aluk spreading

experimental and numerical assessment of transient stream aquifer exchange during disconnection understanding the state of connection processes of stream aquifer systems is of great interest for water resources management particularly in semi arid regions and where groundwater is extracted in the vicinity of a river bank here we present a combined experimental numerical study to explain physical processes involved in disconnected stream aquifer systems a stream aquifer sand box was built to measure the infiltration rate through the stream bed during aquifer drainage the pressures in the saturated zone of the aquifer and the infiltration rate were measured in order to quantify the fluid flow in this system the transient transitional stage between connected and disconnected flow regimes which was obtained experimentally is characterised by a maximum infiltration rate across the stream bed before a decrease towards a constant value this behaviour is analysed by means of transient numerical simulations using relevant hydrodynamic parameters the importance of the drainage kinematics and unsaturated zone parameters for the temporal variation of the infiltration rate is demonstrated the possible occurrence of a maximum infiltration rate value during the transitional stage is characterised into a general view of the stream aquifer disconnection with direct implications for pumping near a stream

effects of ridge crest trench interaction on antarctic phoenix spreading forces on a young subducting plate precise measurements of spreading rates on marine magnetic profiles collected to the west of the antarctic peninsula have enabled some consideration of the forces governing plate motion since antarctic phoenix motion has been controlled by the local rather than global force balance over the last 35 m y the total effective driving force per unit length of trench is calculated to have ranged between 2 6 and 3 6 1012 n m which is much less than is commonly thought necessary to support subduction conventional calculations may overestimate slab pull for old slabs because they neglect the effect of extensional disruption in limiting the contribution to the balance of forces at the trench the low estimate of driving forces obtained here implies that resistive forces are also smaller than is generally assumed driving forces show a strong correlation with observed spreading rates which indicates that resistive forces were largely velocity dependent fluid migration up the subduction zone may elevate temperatures in and around the shear zone reducing resistive forces below the levels required by purely conductive models changes in convergence rate may affect the depths of both the brittle ductile deformation boundary and the basalt eclogite phase change causing a negative feedback which would appear as a velocity dependent resistive force the different driving forces acting on the ne and sw parts of the phoenix plate as a consequence of older oceanic lithosphere at the trench in the ne caused antarctic phoenix spreading to take place about a near pole to the sw since 21 m y ago and ultimately resulted in disruption of the phoenix plate about 9 m y ago spreading rates decreased abruptly about 6 m y ago probably because of e w compression across the long transform faults bounding the phoenix plate however spreading on the last three segments of the antarctic phoenix ridge continued at least until 4 m y ago either spreading stopped progressively from sw to ne or the final stage took place about a very near pole to the sw a magnetic quiet zone extends up to 95 km from the margin between the tula fracture zone and the north anvers fracture zone and is thought to indicate that the ridge crest became buried by terrigenous sediment prior to collision the absence of a magnetic quiet zone associated with the most recent ridge crest trench collisions suggests a change in sedimentary regime during the late miocene anomalously fast apparent spreading rates between 23 and 21 m y ago are thought to indicate an error in this part of the magnetic reversal time scale

seismicity and tectonics of the south shetland islands and bransfield strait from a regional broadband seismograph deployment 1 we investigate the tectonics of the south shetland trench and bransfield strait by performing a detailed study of local seismicity during 1997 1999 we deployed seven land seismometers and 14 ocean bottom seismometers in the south shetland island region the data we obtained indicate a high level of local seismicity mb 2 5 and we accurately located 150 earthquakes many of the earthquakes occur at locations and depths indicative of ongoing subduction in the south shetland trench a focal mechanism for the largest event in the forearc indicates shallow angle thrusting the maximum depth of seismicity is 65 km but the majority of the events are shallower than 30 km these seismic results are consistent with recent magnetic gps and multichannel seismic reflection data that suggest continued subduction at a very slow rate the south shetland trench thus represents an extreme end member of hot subduction resulting from slow convergence of young lithosphere and the absence of intermediate depth earthquakes is consistent with thermal assimilation of the slab at shallow depths we have located many earthquakes associated with volcanism and rifting in bransfield strait a swarm of events near a submarine volcano suggests current magmatic activity a normal faulting focal mechanism in the northeastern part of the strait gives evidence of extension earthquakes associated with rifting in the northeastern portion of the strait are clustered along well established rifts but the seismicity is much more diffuse to the southwest this observation is consistent with other evidence that extension has propagated from northeast to southwest

autopsy on a dead spreading center the phoenix ridge drake passage antarctica new bathymetric and magnetic anomaly data from the phoenix ridge antarctica show that extinction of all three remaining segments occurred at the time of magnetic chron c2a 3 3 0 2 ma synchronous with a ridge trench collision south of the hero fracture zone this implies that the ultimate cause of extinction was a change in plate boundary forces occasioned by this collision spreading rates slowed abruptly at the time of chron c4 7 8 0 3 ma probably as a result of extinction of the west scotia ridge which would have led to an increase in slip rate and transpressional stress across the shackleton fracture zone spectacular high relief ridges flanking the extinct spreading center mapped for the first time using multibeam swath bathymetry are interpreted as a consequence of a reduction in spreading rate involving a temporary magma oversupply immediately prior to extinction

tectonics and palaeoceanographic evolution recorded by contourite features in southern drake passage antarctica abstract the sedimentary record in the vicinity of the triple junction at the southern drake passage is analyzed in order to decode the palaeoceanographic evolution and the influence of tectonic events the break up of the last connection between south america and antarctica led to the circulation of important oceanographic bottom flows including the antarctic circumpolar current acc and the weddell sea deep water wsdw the shackleton fracture zone sfz a ridge crossing the central drake passage has been proposed as a major barrier that constrained the free circulation of bottom flows in the area but whose timing and importance is poorly established also the south scotia ridge ssr a prominent relief composing the southern part of the scotia arc has controlled oceanographic exchanges between the weddell and scotia seas as bottom flows from the weddell sea to the scotia sea have been conducted across narrow gateways along the ssr on the basis of a network of multichannel seismic profiles we interpret the uplift dynamics of the sfz in the southern drake passage and its influence on the evolution of the bottom current circulation and by extension on contourite processes six main seismic units identified and correlated between the scotia and the former phoenix plates depict a south west directed tilting of the deposits above a mid miocene reflector reflector c on the phoenix plate the regional correlation of the main reflections and contourite features indicates that the deepest fraction of the acc the lower circumpolar deep water lcdw as the lower part of the circumpolar deep water cdw flowed freely from the latest eocene to the middle miocene and that these palaeo flows have been active until the present in the abyssal plain of the central western scotia sea sfz uplift was initiated at the latest during the middle miocene about 12 ma when the sfz began to be an effective barrier to bottom flows in the southern drake passage the ridge forced the acc and the polar front to shift northward contributing to the thermal isolation of antarctica and more polar conditions the northward displacement of the lcdw and the opening of passages along the ssr favored the insertion of wsdw flows along the southern part of the drake passage westward into the pacific ocean and northward into the abyssal plain of the southwestern scotia sea based on the morphology and evolution of the main erosional features we also calculate a ratio between two volumetric flow rates related to the two major branches of the wsdw in the southwestern scotia sea area the highest ratio is found for the age of reflector b and is probably related with the strongest incursions of the wsdw in the area as well as with sfz uplift this work demonstrates the common occurrence of large depositional and erosional features in deep marine environments related to bottom current activity and their important implications on decoding palaeoceanographic climatic and tectonic events

fluid flow during exhumation of deeply subducted continental crust zircon u pb age and o isotope studies of a quartz vein within ultrahigh pressure eclogite quartz veins in high pressure to ultrahigh pressure metamorphic rocks witness channelized fluid flow that transports both mass and heat during collisional orogenesis this flow can occur in the direction of changing temperature pressure during subduction or exhumation shrimp u pb dating of zircon from a kyanite quartz vein within ultrahigh pressure eclogite in the dabie continental collision orogen yields two age groups at 212 7 and 181 13 ma which are similar to two groups of la icpms age at 210 4 and 180 5 ma for the same sample these ages are significantly younger than zircon u pb ages of 224 2 ma from the host eclogite thus the two age groups from the vein date two episodes of fluid flow involving zircon growth the first due to decompression dehydration during exhumation and the second due to heating dehydration in response to a cryptic thermal event after continental collision laser fluorination o isotope analyses gave similar 18o values for minerals from both vein and eclogite indicating that the vein forming fluid was internally derived synchronous cooling between the vein and eclogite is suggested by almost the same quartz mineral fractionation values with regularly decreasing temperatures that are in concordance with rates of o diffusion in the minerals while the quartz veining was caused by decompression dehydration at 700 650 c in a transition from ultrahigh pressure to high pressure eclogite facies retrogression the postcollisional fluid flow was retriggered by heating dehydration at 500 c without corresponding metamorphism in either case the kyanite quartz vein formed later than the peak ultrahigh pressure metamorphic event at the middle triassic pointing to focused fluid flow during exhumation rather than subduction the growth of metamorphic zircon in the eclogite appears to have depended on fluid availability so that their occurrence is a type of geohygrometer besides geochronological applicability to dating of metamorphic events in orogenic cycles

different crustal sources for au rich and au poor ores of the grasberg cu au porphyry deposit abstract the grasberg is a porphyry copper deposit that is crosscut by a second stage mineralization which is greatly enriched in gold sulfides from the porphyry type event yield a 2 9 0 3 ma re os isochron that agrees with published geochronology the initial 187os 188os ratio of the isochron is 0 56 0 02 and implies a significant crustal component for the source of os and by inference the other base metals the samples from the crosscutting secondary event do not form an isochron but form a mixing line with the older porphyry style mineralization as one of the end members and a very radiogenic end member possibly shale the initial 187os 188os ratios from samples of the second event range from 0 81 to 1 26 and correlate with gold content the most radiogenic samples have the highest gold the re os isotope data indicate different crustal sources for the ore forming elements at the grasberg cu au deposit and support a model in which gold is derived from sedimentary protoliths that may have been pre enriched by sedimentary processes the gold derived from these sources is concentrated by magmatic hydrothermal systems the requirement of a sedimentary source for gold to produce hydrothermal gold deposits has been debated for decades and this study provides strong support for the model

modeling groundwater with ocean and river interaction we develop and implement the groundwater model saturated unsaturated flow and transport in 3d suft3d to integrate water quantity quality data and simulations with models of other hydrologic cycle components namely rivers and the ocean this work was done as part of the sea air land modeling operational network salmon project supported by the ibm international foundation through its environmental research program the first research steps presented here address the simulation of typical hydrologic conditions to demonstrate suft3d s effectiveness and accuracy the theory behind the modeling of seawater intrusion and groundwater river interaction is summarized along with the numerical methods and characteristics of suft3d the code was applied to different increasingly complex scenarios confined to unconfined conditions local to regional scale homogeneous to increasing heterogeneity two to three dimensional of particular interest were the impacts of different boundary conditions and influence of river interactions on seawater intrusion results are illustrated discussed and compared when possible to those in the literature simulating groundwater exchange between both the river and the ocean has provided interesting results that better depict the dynamics of flow and transport in coastal zone groundwater systems different models can be developed to simulate water flow and quality at large scales in both time and space each part of the hydrological cycle i e ocean surface water groundwater and atmosphere is simulated using models adapted to each particular component usually each model is designed for only one part of the water cycle and interactions with other parts are taken into account by using prescribed input output flux at the boundaries this is commonly considered as satisfactory so that groundwater river and ocean models are usually con nected only by these prescribed boundary conditions how ever the actual water and contaminant fluxes between the models should be simulated more realistically without pre scribing any flux integrated water quantity and quality prob lems in coastal zones were investigated as part of the sea air land modeling operational network salmon project sup ported by the ibm international foundation this research focuses on introducing natural junctions between the differ ent models to adequately compute exchange flux as of 1995 three models ocean beckers 1991 river smitz et al 1997 and groundwater dassargues 1994 had been developed in different departments of the university of liege now these are being connected to form an integrated model able to describe water and contaminant flux over the entire system at regional scales including input from marine river groundwa ter and atmospheric hydrologic cycle components connecting the different models is done through a specific interface called a junction designed to manage data exchange

source side shear wave splitting and upper mantle flow in the chile ridge subduction region the actively spreading chile ridge has been subducting beneath patagonian chile since the middle miocene after subduction continued separation of the faster nazca plate from the slow antarctic plate has opened up a gap a slab window between the subducted oceanic lithospheres beneath south america we examined the form of the asthenospheric mantle flow in the vicinity of this slab window using s waves from six isolated unusual 2007 earthquakes that occurred in the generally low seismicity region just north of the ridge subduction region the s waves from these earthquakes were recorded at distant seismic stations but were split into fast and slow orthogonally polarized waves at upper mantle depths during their passage through the slab window and environs we isolated the directions of fast split shear waves near the slab window by correcting for upper mantle seismic anisotropy at the distant stations the results show that the generally trench parallel upper mantle flow beneath the nazca plate rotates to an ene trend in the neighborhood of the slab gap consistent with upper mantle flow from west to east through the slab window

tectonic development of the north patagonian andes and their related miocene foreland basin 41 30 43 s 1 the northern patagonian andes have been constructed through multiple mechanisms that range from tectonic inversion of extensional structures of early to middle jurassic age in the main andes to oligocene in the precordilleran region these have acted during two distinctive orogenic stages first in late early cretaceous and later in miocene times late oligocene extension separates these two contractional periods and is recorded by half grabens developed in the retroarc region the last contractional stage coexists with an eastward foreland expansion of the late miocene arc whose roots are presently exposed as minor granitic stocks and volcanic piles subordinately in the main andes east of the present arc as a consequence of this orogenic stage a foreland basin has developed having progressed from 18 ma in the main north patagonian andes where the mountain front was flooded by a marine transgression corresponding to the base of the nirihuau formation to 11 ma in the foreland area cannibalization of this foreland basin occurred initially in the hinterland and then progressed to the foreland zone blind structures formed a broken foreland at the frontal zone inferred from growth strata geometries during pliocene to quaternary times most of the contractional deformation was dissipated in the orogenic wedge at the time when the arc front retracted to its present position

southeast pacific tectonic evolution from early oligocene to present plate tectonic reconstructions of the nazca antarctic and pacific plates are presented from late oligocene to present these reconstructions document major plate boundary reorganizations in the southeast pacific at dirons 6c 24 ma 6 o 20 ma and 5a 12 ma and a smaller reorganization at chron 3 o 5 ma during the chron 6 o reorganization it appears that a ridge propagated into crust north of the northernmost pacific antarctic ridge between the chiloe fracture zone fz of the chile ridge and agassiz fz of the pacific nazca ridge which resulted in a northward jump of the pacific antarctic nazca pac ant naz mid ocean triple junction during the chron 5a reorganization the chile ridge propagated northward from the valdivia fz system to the challenger fz through lithosphere formed roughly 5 myr earlier at the pacific nazca ridge during this reorganization a short lived microplate the friday microplate existed at the pac ant naz triple junction the pac ant naz triple junction jumped northward 500 km as a result of this reorganization from a location along the valdivia fz to a location along the challenger fz the chron 5a reorganization also included a change in spreading direction of the chile and pacific antarctic ridges the reorganization at chron 3 o initiated the formation of the juan fernandez and easter microplates along the east pacific rise the manner of plate boundary reorganization at chron 6 o and chron 5a and possibly today at the juan fernandez microplate included a sequence of rift propagation transfer of lithosphere from one plate to another microplate formation and microplate abandonment and resulted in northward migration of the pac ant naz triple junction the associated microplate differs from previously studied microplates in that there is no failed ridge

isotopic ages and palaeomagnetism of selected magmatic rocks from king george island antarctic peninsula abstract a range of mafic to intermediate extrusive and intrusive igneous rocks from the central part of king george island were sampled for isotopic and palaeomagnetic studies single grain u pb dating of zircon from basalts to trachytes from the upper part of the section gave eocene ages 53 0 0 7 to 47 8 0 5 ma consistent with a whole rock 40 ar 39 ar age of 52 7 0 6 ma these ages correspond to the first stages of the opening of the drake passage a late cretaceous 40 ar 39 ar age 75 4 0 9 ma was obtained only from one basalt sample from the bottom part of the section uchatka point formation these new age determinations change substantially the existing stratigraphic chart of king george island palaeomagnetic poles show a marked departure from the east antarctic apparent polar wander path reflecting anticlockwise rotation of the rocks after the early eocene the mean age of zircon grains from a basaltic lava flow overlying the tillite at herve cove 48 9 0 7 ma and the normal polarity magnetization of surrounding basalts allow correlation of the tillite with the upper part of the c22 polarity chron 49 4 48 6 ma the consistency in age crystal morphology and u and th contents strongly suggests that the zircon records the magmatic event however derivation of the zircon from an older magma and a slightly younger age for the tillite cannot be totally excluded

tectonic reconstructions for paleobathymetry in drake passage abstract a minimum complexity tectonic reconstruction based on published and new basin opening models depicts how the scotia sea grew by cenozoic plate divergence dismembering a jurassic sheared margin of gondwana part of the jurassic early cretaceous ocean that accreted to this margin forms the core of the central scotia plate the arc plate above a trench at the eastern end of the scotia sea which migrated east away from the antarctic and south american plates a sequence of extensional basins opened on the western edge of the central scotia plate at 50 30 ma decoupled from the south american plate to the northwest by slow motion on a long transform fault succeeding the basins seafloor spreading started around 30 ma on the west scotia ridge which propagated northwards in the 23 17 ma period and ceased to operate at 6 ma the circuits of plate motions inside and outside the scotia arc are joined via rotations that describe antarctic central scotia plate motion in powell basin until 20 ma and along the south scotia ridge thereafter the modelled relative motion at the northern edge of the scotia sea is thus constrained only by the plate circuit but nonetheless resembles that known coarsely from the geological record of tierra del fuego a paleobathymetric interpretation of nine time slices in the model shows drake passage developing as an intermediate depth oceanographic gateway at 50 30 ma with deep flow possible afterwards initially this deep flow would have been made tortuous by numerous intermediate and shallow barriers a frontal pattern resembling that in the modern scotia sea would have awaited the clearance of significant barriers by continuing seafloor spreading in the scotia sea at 18 5 ma at shag rocks passage and after 10 ma southeast of south georgia

an ephemeral stream aquifer interaction model the boundary integral equation method is applied to the interaction between an ephemeral stream and an unconfined aquifer through a semipervious streambed the resulting two dimensional vertical slice model describes groundwater flow when the stream and aquifer are hydraulically disconnected or connected as well as the transition between these two states it incorporates the green ampt equation with the hydraulically disconnected boundary conditions the model was successfully calibrated using stream discharge and bore hydrograph data from the little para river recharge study south australia it predicts with sufficient accuracy the inflow and outflow from the stream to the aquifer during fluctuating water levels in the stream

late cretaceous to early tertiary subduction history of the antarctic peninsula quantitative estimates of the rates and azimuths of phoenix plate convergence with the antarctic peninsula have been derived from plate rotation calculations for two periods in the late cretaceous and early tertiary 83 0 67 7 and 61 1 53 4 ma published marine magnetic anomaly identifications and flow lines interpreted from gravity anomaly maps were used in simple spherical geometry calculations to derive phoenix pacific stage rotations these were combined with published pacific antarctic rotation data to determine contemporaneous phoenix antarctic stage rotations the results indicate a significant change in azimuths of relative motion between the late cretaceous and early tertiary late cretaceous and perhaps earlier oblique subduction probably caused migration of fore arc slivers along the margin resulting in variations in width of the accretionary prism comparison between synthetic magnetic profiles and a 900 km long magnetic profile across ocean floor produced at the antarctic phoenix ridge during the early tertiary establishes the time of a major decrease in spreading rate and hence also in convergence rate as chron c23r 52 3 ma the associated change in subduction dynamics may have caused the dextral transtensional deformation observed in the george vi sound region and initiated uplift of blueschist facies rocks now exposed on smith island the calculated convergence history does not provide a simple explanation for the occurrence of high mg andesite lavas on alexander island which were erupted more than 20 ma before the antarctic phoenix ridge arrived at the margin the existence of these lavas implies either earlier subduction of another ridge or slab break up beneath alexander island

revised history of early tertiary plate motion in the south west pacific reanalysis of early tertiary magnetic anomalies on the pacific plate south of the campbell plateau indicates that in early tertiary time there was a previously unrecognized triple junction of the pacific the antarctic and a third plate now beneath the bellingshausen sea corresponding revised parameters for pacific antarctica motion do not require early tertiary displacements within antarctica or new zealand but suggest a change in direction of the pacific plate over the hotspots of approximately the same sense and timing as the hawaiian emperor bend

southern patagonian plateau basalts and deformation backarc testimony of ridge collisions abstract the distribution and volume of tertiary patagonian plateau basalts and the evolution of the patagonian fold and thrust belt between 46 and 49 s appear to be closely tied with ridge trench interactions along the continental margin to the west eocene plateau basalts south of 43 s are associated with a gap in the eocene arc and can be related to a proposed collision of the aluk farallon ridge against the trench at this time eruptions of late miocene to recent plateau basalts between 46 and 49 s can be related to time transgressive slab windows in the underlying mantle generated by the collision of segments of the chile ridge at 10 14 ma and 6 ma the principal deformation in the patagonian fold and thrust belt predates the eruption of the basalts and also appears to be related with late miocene ridge collision comparison of plateau basalts related to the collision of the three distinct ridge segments suggests that the volume of basalt erupted increases with the length of the collided ridge segment almost all eocene to recent plateau basalts between 46 and 49 s have oib like ocean island basalt trace element signatures showing that the mantle source region changed little with time plateau basalts with more arc like signatures appear to be contaminated by continental crust relative melting percentages inferred from the trace element chemistry of the plateau basalts correlate with volumes of erupted basalt implying spatial and temporal changes in temperatures in the mantle source that can be correlated with slab windows

life of the bellingshausen plate the onset of independent bellingshausen plate movement in the south pacific is estimated at just after chron c33o this event closely follows a change in the offset sense on the transform arm of the late cretaceous pacific phoenix antarctic triple junction and accompanies the creation and or lengthening by different mechanisms of long offsets on the pacific antarctic ridge at the udintsev tharp heezen and v transforms as the triple junction stepped away from the ridge at around chron c27 the bellingshausen plate ceased to move independently and the pacific antarctic ridge inherited the set of long offset transforms shortly afterwards the gravity fabric on the flanks of much of the ridge displays evidence for a spreading rate decrease possibly related to transpression on the long offset transforms we speculate on a link between these events and regional tectonic changes around chron c27

geophysics of the pitman fracture zone and pacific antarctic plate motions during the cenozoic multibeam bathymetry and magnetometer data from the pitman fracture zone fz permit construction of a plate motion history for the south pacific over the past 65 million years reconstructions show that motion between the antarctic and bellingshausen plates was smaller than previously hypothesized and ended earlier at chron c27 61 million years ago the fixed hot spot hypothesis and published paleomagnetic data require additional motion elsewhere during the early tertiary either between east antarctica and west antarctica or between the north and south pacific a plate reorganization at chron c27 initiated the pitman fz and may have been responsible for the other right stepping fracture zones along the ridge an abrupt 8 clockwise rotation in the abyssal hill fabric along the pitman flowline near the young end of chron c3a 5 9 million years ago dates the major change in pacific antarctic relative motion in the late neogene

structure and evolution of cenozoic arc magmatism on the antarctic peninsula a high resolution aeromagnetic perspective structure and evolution of cenozoic arc magmatism on the antarctic peninsula a high resolution aeromagnetic perspective

geomorphic signature of antarctic submarine gullies implications for continental slope processes five quantitatively distinct gully types are identified on the antarctic continental margin from swath bathymetric data of over 1100 individual features the gullies differ in terms of length width depth width depth ratio cross sectional shape branching order sinuosity and spatial density quantitative analysis suggests that antarctic gully morphology varies with local slope character i e slope geometry gradient regional factors i e location of cross shelf troughs trough mouth fans subglacial meltwater production rates drainage basin size sediment yield and ice sheet history in keeping with interpretations of previous researchers most gullies are probably formed by hyperpycnal flows of sediment laden subglacial meltwater released from beneath ice sheets grounded at the continental shelf edge during glacial maxima the limited down slope extent of gullies on the western antarctic peninsula is explained by the steep gradient and slope geometry at the mouth of marguerite trough which cause flows to accelerate and entrain seawater more quickly resulting in a reduction of the negative buoyancy effect of the sediment load due to pressure gradients at the ice sheet bed caused by variations in ice thickness inside and outside palaeo ice stream troughs subglacial meltwater flow was generally focussed towards trough margins this has resulted in gullies with larger cross sectional areas and higher sinuosities at the trough margins a unique style of gullying is observed off one part of the western antarctic peninsula corresponding to an area in which the ice sheet grounding line is not thought to have reached the shelf edge during the last glacial maximum we interpret the features in this area as the cumulative result of slope processes that operated over a long period of time in the absence of hyperpycnal meltwater flows

molybdenite re os and albite 40ar 39ar dating of cu au mo and magnetite porphyry systems in the yangtze river valley and metallogenic implications abstract the area of the middle lower yangtze river valley eastern china extending from wuhan hubei province to western zhenjiang jiangsu province hosts an important belt of cu au mo and fe deposits there are two styles of mineralization i e skarn porphyry stratabound cu au mo fe deposits and magnetite porphyry deposits in several nne trending cretaceous fault bound volcanic basins the origin of both deposit systems is much debated we dated 11 molybdenite samples from five skarn porphyry cu au mo deposits and 5 molybdenite samples from the datuanshan stratabound cu au mo deposit by icp ms re os isotope analysis nine samples from the same set were additionally analyzed by ntims on re os results from the two methods are almost identical the re os model ages of 16 molybdenite samples range from 134 7 2 3 to 143 7 1 6 ma 2 the model ages of the five samples from the datuanshan stratabound deposit vary from 138 0 3 2 to 140 8 2 0 ma with a mean of 139 3 2 6 ma their isochron age is 139 1 2 7 ma with an initial os ratio of 0 7 8 1 mswd 0 29 these data indicate that the porphyry skarn systems and the stratabound deposits have the same age and suggest an origin within the same metallogenic system albite 40 ar 39 ar dating of the magnetite porphyry deposits indicates that they formed at 123 to 125 ma i e 10 20 ma later both mineralization styles characterize transitional geodynamic regimes i e the period around 140 ma when the main ns trending compressional regime changed to an ew trending lithospheric extensional regime and the period of 125 115 ma of dramatic ew trending lithospheric extension

rifting of old oceanic lithosphere geophysical data from five regions in the pacific and indian oceans reveal that long distance 400 km spreading center jumps have occurred in the past the present day seafloor morphology is used to develop a scenario for a spreading center jump the major events are 1 thinning and weakening of the lithosphere at the future rifting site 2 rifting of the weakened lithosphere during rifting the crack is filled from above by normal faulting and wedge subsidence viscous upwelling fills the crack from below 3 spreading at the rift site results in a ridge bounded by two troughs spreading ceases at the dying spreading center resulting in a deep central graben surrounded by flexural ridges periods of slow spreading at both spreading centers produce rough topography 4 ageing and cooling that produce a general deepening of the abandoned spreading ridge and also reduce the thermal contrast across the fossil rifting site the new spreading center develops into a normal spreading rift the major topographic expressions apparent in the seafloor today are the deep trough of the abandoned spreading center and the proximal and distal troughs which formed when the emerging spreading center bisected the fossil rifting site the proximal trough nearer the new spreading ridge and the distal trough farther from the new ridge are first order topographic features 100 1000 km long and 300 km wide resembling fracture zones with which they are often confused they share with fracture zones the characteristic of bringing together fragments of lithosphere of different ages but unlike fracture zones they are generally parallel to magnetic lineations

estimating the spatial extent of unsaturated zones in heterogeneous river aquifer systems the presence of unsaturated zones at the river aquifer interface has large implications on numerous hydraulic and chemical processes however the hydrological and geological controls that influence the development of unsaturated zones have so far only been analyzed with simplified conceptualizations of flow processes or homogeneous conceptualizations of the hydraulic conductivity in either the aquifer or the riverbed we systematically investigated the influence of heterogeneous structures in both the riverbed and the aquifer on the development of unsaturated zones a stochastic 1 d criterion that takes both riverbed and aquifer heterogeneity into account was developed using a monte carlo sampling technique the approach allows the reliable estimation of the upper bound of the spatial extent of unsaturated areas underneath a riverbed through systematic numerical modelling experiments we furthermore show that horizontal capillary forces can reduce the spatial extent of unsaturated zones under clogged areas this analysis shows how the spatial structure of clogging layers and aquifers influence the propensity for unsaturated zones to develop in riverbeds where clogged areas are made up of many small spatially disconnected patches with a diameter in the order of 1 m unsaturated areas are less likely to develop compared to riverbeds where large clogged areas exist adjacent to unclogged areas a combination of the stochastic 1 d criterion with an analysis of the spatial structure of the clogging layers and the potential for resaturation can help develop an appropriate conceptual model and inform the choice of a suitable numerical simulator for river aquifer systems

antarctic tectonics constraints from an ers 1 satellite marine gravity field a high resolution gravity field of poorly charted and ice covered ocean near west antarctica from the ross sea east to the weddell sea has been derived with the use of satellite altimetry including ers 1 geodetic phase wave form data this gravity field reveals regional tectonic fabric such as gravity lineations which are the expression of fracture zones left by early 65 to 83 million years ago pacific antarctic sea floor spreading that separated the campbell plateau and new zealand continent from west antarctica these lineations constrain plate motion history and confirm the hypothesis that antarctica behaved as two distinct plates separated from each other by an extensional bellingshausen plate boundary active in the amundsen sea before about 61 million years ago

late cenozoic tectonics of the southern chile trench marine geophysical data from off shore of southern chile are used to define the interaction between the chile ridge and chile trench during late cenozoic time we identify three distinct ridge trench collision events between 14 and 10 ma a 700 km long nearly continuous section of the chile ridge was sub ducted between 55 s and 48 s shorter sections of the ridge offset by large transform faults were subducted at 6 and 3 ma between 48 s and 47 s at the present day triple junction the subduction of the ridge has a strong influence on the chile trench in this region the landward trench slope has undergone a recent episode of subduction driven tectonic erosion the trench slope is narrower and steeper than along other sectors of the margin and the trench axis has migrated shoreward evidence exists for late neogene and quaternary uplift and plutonism on the adjacent continental margin south of 48 s where collision took place between 10 and 14 ma the effects of collision are much less pronounced in particular the landward trench slope does not appear to have been subjected to extensive tectonic erosion we conclude that the configuration of the spreading centers and transform faults on the chile rise is the chief factor controlling ridge trench tectonic interaction tectonic erosion of the landward trench slope and tectonic activity in the adjacent continental margin are much greater when short sections of ridge offset by large transform faults are subducted a major sub marine channel leads along the trench axis south ward from the triple junction this channel cuts across the outer trench rise and carries sediment westward to the mornington abyssal plain the paleogene tectonic history of the southern chile trench includes a southward migrating ridge trench collision involving the farallon aluk spreading center in eocene time

an early miocene ridge crest trench collision on the south scotia ridge near 36 w abstract preliminary interpretation of marine geophysical data and geochemical analyses of dredged rocks has led to the identification of a ridge crest trench collision zone southeast of the south orkney islands the existence of a series of such collision zones along the south scotia ridge had earlier been hypothesised to explain changes in scotia sea evolution the collision zone takes the form of a double ridge with dissecting trough aligned ne sw and about 200 km long oceanic magnetic lineations in the northern weddell sea young northwestward towards the zone with the youngest identified as anomaly 6 20 ma reaching the central trough volcanics dredged from jane bank the northwestern ridge are chemically very similar to the more siliceous of the low k tholeiite series which characterises the presently active south sandwich arc jane bank is interpreted as an in situ remnant of the arc and upper fore arc of an intra oceanic arc produced by subduction of south american oceanic lithosphere like the present south sandwich islands part of the upper fore arc and all of any accretionary wedge appear to have been subducted unlike ocean floor at collision zones off chile and the antarctic peninsula the seabed in the trough and on the ridge to the southeast does not conform to the normal oceanic age depth relationship the estimated collision time 20 ma is close to the time of onset of n s extension in the central scotia sea the limited data set described here however does not establish the synchroneity of the collision over the entire 200 km of jane bank so an assessment of the extent of any causal relationship between the two events must await the analysis of a more extensive survey

tectonic implications from re os dating of mesozoic molybdenum deposits in the east qinling dabie orogenic belt abstract the east qinling dabie molybdenum belt is part of a larger east west trending metallogenic belt in eastern china most of the molybdenum deposits occur as porphyry or porphyry skarn type but there are also some vein type deposits following systematic re os dating of molybdenite from 13 deposits and comparisons with two previously dated deposits we have recognized that the molybdenum mineralization in the east qinling dabie belt was developed during hydrothermal activity linked to magmatism and the emplacement of granitoid stocks three pulses of granitoid magmatism and mo mineralization are recognized corresponding to significant tectonic events in the east qinling dabie belt vein type deposits dated at 233 221 ma were formed in detachment fractures indicating localized extension within the collisional setting of the north china and yangtze cratons i type and transitional i and s type granites and related mineralization dated at 148 138 ma may have formed part of a continental magmatic arc with widespread magmatism and back arc extension caused by subduction of the izanagi or paleopacific plate beneath the eurasian continent in a wnw ese direction in the late jurassic early cretaceous both s type and transitional s and i type granite associated porphyry molybdenum deposits dated at 131 112 ma are part of an extensive mineralization event throughout east china that can be ascribed to regional large scale lithospheric thinning delamination and thermal erosion

modelling stream aquifer seepage in an alluvial aquifer an improved loosing stream package for modflow abstract seepage from a stream which partially penetrates an unconfined alluvial aquifer is studied for the case when the water table falls below the streambed level inadequacies are identified in current modelling approaches to this situation a simple and improved method of incorporating such seepage into groundwater models is presented this considers the effect on seepage flow of suction in the unsaturated part of the aquifer below a disconnected stream and allows for the variation of seepage with water table fluctuations the suggested technique is incorporated into the saturated code modflow and is tested by comparing its predictions with those of a widely used variably saturated model swms 2d simulating water flow and solute transport in two dimensional variably saturated media comparisons are made of both seepage flows and local mounding of the water table the suggested technique compares very well with the results of variably saturated model simulations most currently used approaches are shown to underestimate the seepage and associated local water table mounding sometimes substantially the proposed method is simple easy to implement and requires only a small amount of additional data about the aquifer hydraulic properties

opening history of powell basin antarctic peninsula the opening of powell basin was part of the regional response to n55 w relative plate motion of south america away from antarctica which led to the formation of drake passage during the eocene and oligocene restoration of microplates around the basin using gridded magnetic anomalies from its margins illustrates the pre break up continuity of the pacific margin anomaly magnetic high associated with a mesozoic arc batholith newly compiled magnetic anomaly data over the powell basin show subdued linear seafloor spreading type anomalies these are used together with marginal and regional geology to constrain the opening history of the basin magnetic reversal modelling suggests that slow spreading in powell basin probably occurred between 29 7 ma and 21 8 ma following rifting of mesozoic continental crust with associated break up volcanism a simple two phase model for the rotation of the south orkney microcontinent away from the antarctic peninsula accounts for the pattern of magnetic reversals recorded in powell basin and for the structure of its margins

importance of stream infiltration data for modelling surface water groundwater interactions summary an infiltration experiment at the transect scale was performed in an ephemeral stream at the time it was dry resulting in direct estimates of stream infiltration groundwater heads measured in a transect of piezometers orthogonal to the stream highlighted the development of a groundwater mound displaced several meters into the streambank the experiment was modelled using the two dimensional integrated surface subsurface model hydrogeosphere and calibrated with pilot points using different combinations of groundwater heads and infiltration volume data modelling results showed that despite the fact that groundwater heads were all below the streambed and that an unsaturated zone was present beneath the streambed the stream and the aquifer become laterally connected within several hours of flow onset through a high permeability preferential path in the streambank although the model can be calibrated to produce a good match to measured groundwater heads accurate predictions of stream infiltration can be made only if the heads used for model calibration capture the groundwater mound the use of infiltration flux or volume during model calibration instead provides the best calibration results and thus estimates of stream infiltration during natural flow events given that heterogeneity of streambed and streambank sediments leads to the possibility that the groundwater mound developed during flow events will be poorly characterised or perhaps missed altogether incorporating infiltration flux or volume data into the model calibration increases the likelihood of obtaining accurate estimates of stream infiltration during natural flow events

abbot ice shelf structure of the amundsen sea continental margin and the southern boundary of the bellingshausen plate seaward of west antarctica inversion of nasa operation icebridge airborne gravity over the abbot ice shelf in west antarctica for subice bathymetry defines an extensional terrain made up of east west trending rift basins formed during the early stages of antarctica zealandia rifting extension is minor as rifting jumped north of thurston island early in the rifting process the amundsen sea embayment continental shelf west of the rifted terrain is underlain by a deeper more extensive sedimentary basin also formed during rifting between antarctica and zealandia a well defined boundary zone separates the mildly extended abbot extensional terrain from the deeper amundsen embayment shelf basin the shelf basin has an extension factor of 1 5 1 7 with 80 100 km of extension occurring across an area now 250 km wide following this extension rifting centered north of the present shelf edge and proceeded to continental rupture since then the amundsen embayment continental shelf appears to have been tectonically quiescent and shaped by subsidence sedimentation and the advance and retreat of the west antarctic ice sheet the bellingshausen plate was located seaward of the amundsen sea margin prior to incorporation into the antarctic plate at about 62 ma during the latter part of its independent existence bellingshausen plate motion had a clockwise rotational component relative to antarctica producing convergence across the north south trending bellingshausen gravity anomaly structure at 94 w and compressive deformation on the continental slope between 94 w and 102 w farther west the relative motion was extensional along an east west trending zone occupied by the marie byrd seamounts key points abbot ice shelf is underlain by e w rift basins created at 90 ma amundsen shelf shaped by subsidence sedimentation and passage of the ice sheet bellingshausen plate boundary is located near the base of continental slope and rise

calc alkaline and alkaline miocene and calc alkaline recent volcanism in the southernmost patagonian cordillera chile abstract in the southernmost patagonian cordillera south of lat 54 45 previously unrecorded calc alkaline dacites and andesites and adjacent alkali basalts were dated at 21 and 18 ma k ar respectively in the same area post glacial low k calc alkaline andesites identified at isla cook represent the southernmost recent volcanism recorded so far in south america about 400 km south of monte burney previously known as the southernmost recent volcano the association of contemporaneous calc alkaline and alkali volcanics suggests a subduction related environment for the miocene volcanism and the recent calc alkaline volcanics indicate that currently there is a small component of subduction of the antarctic plate beneath the south american part of the scotia plate

seismic noise tomography in the chile ridge subduction region summary we used cross correlation of ambient seismic noise recorded in the chile triple junction ctj region to estimate interstation surface wave time domain green s functions and then inverted traveltimes to obtain crustal surface wave velocity models interstation distances within the chile ridge subduction project crsp temporary seismic network ranged from 40 to 100 km we selected 365 d and cross correlated and stacked 24 hr of vertical component data at 38 stations pairs resulting in nominally 703 traveltimes along assumed straight interstation paths velocities in 2 d cells of 30 km 30 km were calculated using a linear least squares inversion of the rayleigh wave group velocity traveltimes furthermore we performed a rayleigh wave group velocity dispersion analysis to estimate the sensitivity of different period waves at depth and to calculate a 3 d shear velocity model of the patagonian crust the process was applied to cross correlation pairs determined in two period bands 5 10 s corresponding to shallow crustal velocities down to approximately 10 km depth and 10 20 s for velocities down to around 20 km our results show that cell velocities correlate well with known geological features we find high crustal velocities where the patagonian batholith outcrops or is likely present at depth and low velocities correlate with the active volcanic arc of the southern volcanic zone and the subducted chile ridge in taitao peninsula where thermal activity of hot springs is present high velocities in the mountainous portions of the southeastern study area appear to correlate with outcropping older metamorphic units low velocity in the east correlate with sequences of volcaniclastic deposits

field assessment of surface water groundwater connectivity in a semi arid river basin murray darling australia in semi arid and arid river basins understanding the connectivity between rivers and alluvial aquifers is one of the key challenges for the management of groundwater resources the type of connection present gaining losing connected transitional and losing disconnected was assessed at 12 sites along six murray darling basin river reaches the assessments were made by measuring the hydraulic head in the riparian zone near the rivers to evaluate if the water tables intersected the riverbeds and by measuring fluid pressure in the riverbeds the rationale for the latter was that will always be greater than or equal to zero under connected conditions either losing or gaining and always lesser than or equal to zero under losing disconnected conditions a mixture of losing disconnected losing connected and gaining conditions was found among the 12 sites the losing disconnected sites all had a riverbed with a lower hydraulic conductivity than the underlying aquifer usually in the form of a silty clay or clay unit 0 5 2 m in thickness the riparian water tables were 6 to 25 m below riverbed level at the losing disconnected sites but never lower than 1 m below riverbed level at the losing connected ones the contrast in water table depth between connected and disconnected sites was attributed to the conditions at the time of the study when a severe regional drought had generated a widespread decline in regional water tables this decline was apparently compensated near losing connected rivers by increased infiltration rates while the decline could not be compensated at the losing disconnected rivers because the infiltration rates were already maximal there copyright 2012 john wiley sons ltd

cordilleran slab windows the geometry and geologic implications of subducted spreading ridges are topics that have bedeviled earth scientists ever since the recognition of plate tectonics as a consequence of subduction of the kula farallon and east pacific rises slab windows formed and migrated beneath the north american cordillera the probable shape and extent of these windows which represent the asthenosphere filled gaps between two separating subducting oceanic plates are depicted from the late cretaceous to the present possible effects of the existence and migration of slab windows on the cordillera at various times include cessation of arc volcanism and replacement by rift or plate edge volcanism lithospheric uplift attenuation and extension and increased intensity of compressional tectonism eocene extensional tectonism and alkaline magmatism in southern british columbia and the northwestern united states were facilitated by slab window development

uplift rates from a new high density gps network in palmer land indicate significant late holocene ice loss in the southwestern weddell sea the measurement of ongoing ice mass loss and associated melt water contribution to sea level change from regions such as west antarctica is dependent on a combination of remote sensing methods a key method the measurement of changes in earth s gravity via the grace satellite mission requires a potentially large correction to account for the isostatic response of the solid earth to ice load changes since the last glacial maximum in this study we combine glacial isostatic adjustment modelling with a new gps dataset of solid earth deformation for the southern antarctic peninsula to test the current understanding of ice history in this region a sufficiently complete history of past ice load change is required for glacial isostatic adjustment models to accurately predict the spatial variation of ongoing solid earth deformation once the independently constrained effects of present day ice mass loss have been accounted for comparisons between the gps data and glacial isostatic adjustment model predictions reveal a substantial misfit the misfit is localized on the southwestern weddell sea where current ice models under predict uplift rates by approximately 2 mm yr 1 this under prediction suggests that either the retreat of the ice sheet grounding line in this region occurred significantly later in the holocene than currently assumed or that the region previously hosted more ice than currently assumed this finding demonstrates the need for further fieldwork to obtain direct constraints on the timing of holocene grounding line retreat in the southwestern weddell sea and that grace estimates of ice sheet mass balance will be unreliable in this region until this is resolved

u pb zircon ages of granitoid rocks in the north dabie complex eastern dabie shan china the north dabie complex ndc comprises a high temperature metamorphic suite dominated by amphibolite facies granitoid orthogneiss intruded by voluminous granites u pb zircon dating on these rocks yields the following new results 1 two of the granitoid orthogneiss samples have early cretaceous protolith emplacement ages of 133 7 2 3 ma and 134 0 2 8 ma 2 an undeformed granodiorite intrusive into granitoid gneiss yields an emplacement age at 125 6 0 3 ma these early cretaceous ages entirely post date late triassic early jurassic continental subduction and exhumation evident in the neighboring ultrahigh and high pressure metamorphic terrains to the south in the dabie shan hence they do not support previous tectonic models that infer paleozoic or triassic protolith emplacement ages for these granitoid orthogneisses 3 a mylonitic granite from the northern boundary of the ndc yields a concordant age of 756 6 0 8 ma which is interpreted as magma emplacement age the concordance of this late

mantle flow and dynamic topography associated with slab window opening insights from laboratory models abstract we present dynamically self consistent mantle scale laboratory models that have been conducted to improve our understanding of the influence of slab window opening on subduction dynamics mantle flow and associated dynamic topography over geological time scales the adopted setup consists of a two layer linearly viscous system simulating the subduction of a fixed plate of silicone lithosphere under negative buoyancy in a viscous layer of glucose syrup mantle our experimental setting is also characterized by a constant width rectangular window located at the center of a laterally confined slab modeling the case of the interaction of a trench parallel spreading ridge with a wide subduction zone we found that the opening of a slab window does not produce consistent changes of the geometry and the kinematics of the slab on the contrary slab induced mantle circulation quantified both in the vertical and horizontal sections using the feature tracking image analysis technique is strongly modified in particular rollback subduction and the opening of the slab window generate a complex mantle circulation pattern characterized by the presence of poloidal and toroidal components with the importance of each evolving according to kinematic stages mantle coming from the oceanic domain floods through the slab window indenting the supra slab mantle zone and producing its deformation without any mixing between mantle portions the opening of the slab window and the upwelling of sub slab mantle produce a regional scale non isostatic topographic uplift of the overriding plate that would correspond to values ranging between ca 1 and 5 km in nature assuming that our modeling results can be representative of the natural behavior of subduction zones we compared them to the tectonics and volcanism of the patagonian subduction zone we found that the anomalous backarc volcanism that has been developing since the middle miocene could result from the lateral flow of sub slab mantle and that part of the patagonian uplift could be dynamically supported

tectonics of the qinling central china tectonostratigraphy geochronology and deformation history abstract the qinling orogen preserves a record of late mid proterozoic to cenozoic tectonism in central china high pressure metamorphism and ophiolite emplacement songshugou ophiolite assembled the yangtze craton including the lower qinling unit into rodinia during the 1 0 ga grenvillian orogeny the lower qinling unit then rifted from the yangtze craton at 0 7 ga subsequent intra oceanic arc formation at 470 490 ma was followed by accretion of the lower qinling unit first to the intra oceanic arc and then to the sino korea craton subduction then imprinted a 400 ma andean type magmatic arc onto all units north of the northern liuling unit oblique subduction created silurian devonian wnw trending sinistral transpressive wrench zones e g lo nan shang dan and late permian early triassic subduction reactivated them in dextral transpression lo nan shang xiang shang dan and subducted the northern edge of the yangtze craton exhumation of the cratonal edge formed the wudang metamorphic core complex during dominantly pure shear crustal extension at 230 235 ma post collisional south directed shortening continued through the early jurassic cretaceous reactivation of the qinling orogen started with nw se sinistral transtension coeval with large scale early cretaceous crustal extension and sinistral transtension in the northern dabie shan it presumably resulted from the combined effects of the siberia mongolia sino korean and lhasa west burma qiangtang indochina collisions and pacific subduction regional dextral wrenching was active within a ne sw extensional regime between 60 and 100 ma an early cretaceous andean type continental magmatic arc with widespread early cretaceous magmatism and back arc extension was overprinted by shortening related to the collision of yangtze indochina block with the west philippines block strike slip and normal faults associated with eocene half graben basins record paleogene nne ssw contraction and wnw ese extension the neogene is characterized by normal faults and nne trending sub horizontal extension pleistocene quaternary nw se extension and ne sw contraction comprises sinistral strike slip faults and is part of the nw se extension imposed across eastern asia by the india asia collision

trench proximal volcanism following ridge crest trench collision along the antarctic peninsula on the basis of multichannel seismic reflection magnetics and bathymetry data a pleistocene holocene 0 1ma volcanic seamount has been identified at a location very close to the shelf edge off the west coast of the antarctic peninsula the seamount overlies the subducted trace of a transform fracture zone which divides segments of the margin along which collision took place 3 1 and 6 0 ma ago new trace element and isotopic data from dredged samples demonstrate that the seamount appears to be predominantly formed of relatively primitive undersaturated alkali basalt the basalts have geochemical signatures that are indistinguishable from ocean island basalts and some continental alkali basalts e g th ta 1 0 1 5 rb nb 0 5 87sr 86sr 0 70265 0 7028 143nd 144nd approximately 0 5129 and are totally lacking in any evidence for interaction with subduction enriched mantle geochemical similarities with other more trench distal postsubduction alkalic basalts along the antarctic peninsula are striking strongly implying that all the postsubduction basalts were derived from a chemically similar asthenospheric source region the basalts were most likely to have been generated as a result of the formation of a slab window beneath the antarctic peninsula following ridge crest trench collision subduction component free subslab asthenosphere upwelled into the incipient void left by the continued sinking of the leading plate following collision and decompressional melting resulted this type of trench proximal volcanism following ridge crest trench collision differs from that in other locations where calc alkaline volcanism persisted or ophiolite obduction occurred

evaluation of infiltration from losing disconnected rivers using a geophysical characterisation of the riverbed and a simplified infiltration model summary despite their significance to alluvial aquifer water balances there are few field derived estimates of infiltration from losing disconnected rivers infiltration was estimated over a 2 km section of billabong creek murray darling basin australia using a combination of field measurements and modelling techniques a method was developed whereby in river and riverbank electrical resistivity surveys were used to map the spatial coverage and thickness of the riverbed clay layer thought to be the key control on infiltration in this environment inverted resistivity measurements were used to generate a surrogate for a spatial map of clay layer thickness with constraint provided by coring the riverbed at selected high and low resistivity locations to independently determine the thickness of the clay the survey showed that the clay layer was continuous across the study reach but varied in thickness 1 m to 4 m a simple infiltration model was developed for infiltration through a clay layer and shown to be accurate under steady state conditions when compared to estimates obtained using a variable saturation numerical model for idealised riverbed cross sections infiltration rates across the study reach were estimated to range between 1700 and 7800 m 3 km 1 year 1 with an average of 3400 m 3 km 1 year 1 a sensitivity analysis showed that infiltration rates were most sensitive to clay hydraulic conductivity not clay layer thickness however it is anticipated that when applied at a larger scale 10 100 km infiltration rates will be more sensitive to the presence or absence of a clay layer in the riverbed the proposed methodology can provide independent estimates of infiltration in losing disconnected rivers at a scale suitable for the calibration of regional groundwater models

p wave velocity structure beneath the northern antarctic peninsula evidence of a steeply subducting slab and a deep rooted low velocity anomaly beneath the central bransfield basin summary upper mantle structure between 100 and 300 km depth below the northern antarctic peninsula is imaged by modelling p wave traveltime residuals from teleseismic events recorded on the king sejong station ksj the argentinean italian stations juba and espz an iris gsn station pmsa and the seismic experiment in patagonia and antarctica sepa broad band stations for measuring traveltime residuals we applied a multichannel cross correlation method and inverted for upper mantle structure using vandecar s method the new 3 d velocity model reveals a subducted slab with a 70 dip angle at 100 300 km depth and a strong low velocity anomaly confined below the se flank of the central bransfield basin the low velocity is attributed to a thermal anomaly in the mantle that could be as large as 350 560 k and which is associated with high heat flow and volcanism in the central bransfield basin the low velocity zone imaged below the se flank of the central bransfield basin does not extend under the northern bransfield basin suggesting that the rifting process in that area likely involves different geodynamic processes

impact of lateral flow on the transition from connected to disconnected stream aquifer systems abstract understanding the mechanisms by which stream water infiltrates through streambeds to recharge groundwater systems is essential to sustainable management of scarce water resources in arid and semi arid areas an inverted water table iwt can develop under a stream in response to the desaturation between the stream and underlying aquifer as the system changes from a connected to disconnected status however previous studies have suggested that the iwt can only occur at the bottom of a low permeability streambed in which only the vertical flow between the stream and groundwater during disconnection was assumed in the present study numerical simulations revealed that the lateral flow induced by capillarity or heterogeneity also plays an essential role on interactions between streams and aquifers three pathways were identified for the transition from connection to disconnection in homogenous systems notably the lowest point of an iwt can develop not only at the bottom of the streambed but also within the streambed or the aquifer in response to the initial desaturation at above or below the interface between the streambed and aquifer ibsa respectively a sensitivity analysis indicated that in wide streams the lowest point of an iwt only occurs at the bottom of the streambed however for a stream half width of 1 m above a 6 m thick sandy loam streambed the lowest point occurs in the streambed as stream depth is less than 0 5 m this critical stream depth increases with streambed thickness and decreases with stream width thus in narrow streams the lowest point can also develop in a thick streambed under a shallow stream in narrow streams the lowest point also forms in the aquifer if the ratio of the hydraulic conductivity of the streambed to that of the aquifer is greater than the ratio of the streambed thickness to the sum of the stream depth and the streambed thickness correspondingly the streambed is thin but relatively permeable and the stream is deep furthermore in heterogeneous streambed systems two or three pathways can simultaneously occur and various parts of the iwt occur at distinct positions relative to the ibsa this challenges the commonly held assumption that streambed under a disconnected stream is always fully saturated and limits the methods which introduce a negative or atmospheric pressure value at the ibsa to calculate seepage rate or assess stream aquifer connectivity

re os isotopic dating of molybdenites in the xiaoliugou w mo deposit in the northern qilian mountains and its geological significance abstract the xiaoliugou is a large scale w mo deposit newly explored in the northern qilian mountains caledonian fold belt northwestern china it is a vein and skarn type deposit genetically associated with the xiaoliugou caledonian granodiorite stock re os isotopic dating for the molybdenite in the deposit yields an isochron age of 462 13 ma 2 and model ages of 436 to 496 ma these results suggest that w mo mineralization occurred before the collision of plates and mineralized substances mainly originated from the crust

animated tectonic reconstruction of the southern pacific and alkaline volcanism at its convergent margins since eocene times abstract an animated reconstruction shows south pacific plate kinematics in the reference frame of west antarctica between 55 ma and the present day the ocean floor in the region formed due to seafloor spreading between the antarctic pacific phoenix and nazca plates a plate formed by fragmentation of the farallon plate early in oligocene times the pacific antarctic ridge remained fairly stable throughout this time migrating relatively northwestwards by various mechanisms behind the rapidly moving pacific plate the nazca and phoenix plates also moved quickly but relatively towards the east or southeast and were subducted in these directions beneath the south american and antarctic plates segments of spreading centres forming at the trailing edges of the nazca and phoenix plates periodically collided with these subduction zones resulting in the total destruction of the nazca phoenix spreading centre and the partial destruction of the nazca antarctica spreading centre the chile ridge and antarctic phoenix ridge which ceased to operate shortly before its northeasternmost three segments could collide with the antarctic margin following collision of segments of the chile ridge parts of the antarctic plate underwent subduction at the chile trench after these collisions slab windows should have formed beneath both the south american and antarctic convergent margins and the animation shows occurrences of alkaline volcanism that have been or can newly be related to them further occurrences of alkali basalts at the margins of the powell basin and more speculatively james ross island can be related to the formation of a slab window beneath them following the collision of segments of the south america antarctica spreading centre in the northwest weddell sea

neogene patagonian plateau lavas continental magmas associated with ridge collision at the chile triple junction extensive neogene patagonian plateau lavas 46 5 to 49 5 s southeast of the modern chile triple junction can be related to opening of asthenospheric slab windows associated with collisions of chile rise segments with the chile trench at 12 ma and 6 ma support comes from 26 new total fusion whole rock 40ar 39ar ages and geochemical data from back arc plateau lavas in most localities plateau lava sequences consist of voluminous tholeiitic main plateau flows overlain by less voluminous 2 to 5 million year younger alkalic postplateau flows northeast of where the ridge collided at 12 ma most lavas are syncollisional or postcollisional in age with eruptions of both sequences migrating northeastward at 50 to 70 km ma plateau lavas have ages from 12 to 7 ma in the western back arc and from 5 to 2 ma farther to the northeast trace element and isotopic data indicate main plateau lavas formed as larger percentage melts of a garnet bearing oceanic island basalt oib like mantle than postplateau lavas the highest percentage melts erupted in the western and central plateaus in a migrating slab window model main plateau lavas can be explained as melts that formed as upwelling subslab asthenosphere which flowed around the trailing edge of the descending nazca plate and then interacted with subduction altered asthenospheric wedge and continental lithosphere alkaline postplateau lavas can be explained as melts generated by weaker upwelling of subslab asthenosphere through the open slab window thermal problems of high pressure melt generation of anhydrous mantle can be explained by volatiles h2o and co2 introduced by the subduction process into slab window source region s an oib like rather than a mid ocean ridge basalt morb like source region and the lack of magmatism northeast of where ridge collision occurred at 13 to 14 ma can be explained by entrainment of weak plume s or regional variations in an ambient oib like asthenosphere

arctic and antarctic submarine gullies a comparison of high latitude continental margins abstract submarine gullies are common features of high latitude continental slopes and over the last decade have been shown to play a key role in continental margin evolution submarine erosion downslope sediment transport slope deposits and the architecture of petroleum reservoirs however the processes that form these gullies the timescales over which they develop and the environmental controls influencing their morphology remain poorly constrained we present the first systematic and comparative analysis between arctic and antarctic gullies with the aim of identifying differences in slope character from which we infer differences in processes operating in these environments quantitative analysis of multibeam echosounder data along 2441 km of the continental shelf and upper slope and morphometric signatures of over 1450 gullies show that six geomorphically distinct gully types exist on high latitude continental margins we identify distinct differences between arctic and antarctic gully morphologies in the arctic data sets deep relief 30 m gully incision depth at 50 m below the shelf edge and shelf incising gullies are lacking these differences have implications for the timescales over which the gullies were formed and for the magnitude of the flows that formed them we consider two hypotheses for these differences 1 some antarctic gullies developed through several glacial cycles and 2 larger antarctic gullies were formed since the last glacial maximum as a result of erosive flows i e sediment laden subglacial meltwater being more abundant on parts of the antarctic margin over longer timescales a second difference is that unique gully signatures are observed on arctic and on antarctic margins environmental controls such as the oceanographic regime and geotechnical differences may lead to particular styles of gully erosion observed on arctic and antarctic margins

modeling the effects of unsaturated stratified sediments on groundwater recharge from intermittent streams abstract unsaturated stratified sediments beneath intermittent stream channels affect groundwater recharge from these streams using four different cases of sediment stratification we simulate transient variably saturated flow in a two dimensional 2 d vertical cross section between the stream and the underlying water table these cases include homogeneous sediments low permeability streambed sediments narrow low permeability lenses and extensive low permeability layers the permeability of the sediments in these cases greatly affects the timing and rate of channel loss and groundwater recharge flow patterns and the style of stream water table connection are controlled by the location and geometry of low permeability sediments in cases with homogeneous sediments and narrow low permeability lenses stream water table connection occurs by a saturated column advancing from above in cases with low permeability streambed sediments and extensive low permeability layers connection occurs by a water table mound building from below the style of stream water table connection suggests simplified physically based interaction models that may be appropriate for these settings we compared channel loss and groundwater recharge computed using two simplified models a darcian seepage equation and the green ampt infiltration equation with the results from our 2 d simulations simplified models using parameters from the 2 d simulations appear to perform well in cases with homogeneous and low permeability streambed sediments in cases with low permeability lenses or layers the simplified models require calibrated parameters to perform well

analysis of james ross island volcanic complex and sedimentary basin based on high resolution aeromagnetic data abstract high resolution aeromagnetic surveys provide a geophysical tool to help image the subsurface structure of volcanoes and their tectonic framework here we interpret high resolution aeromagnetic data and models for james ross archipelago and surrounding regions to provide a new perspective on neogene magmatism emplaced along the eastern margin of the antarctic peninsula based on the analysis and modelling of magnetic anomalies we were able to image the subglacial extent of miocene to recent alkaline rocks of the james ross island volcanic group and map tectonic structures that appear to have exerted important controls on neogene magmatism high amplitude linear anomalies detected over mount haddington stratovolcano were modelled as caused by subvertical feeder bodies extending to a depth of at least 3 km these feeder bodies may have been emplaced along a n s oriented volcano tectonic rift zone we also identified several effusive subglacial centres and imaged two concentric magnetic arcs which we related to neogene volcanic and subvolcanic lineaments likely controlled by mid cretaceous strike slip fault belts and associated deformation zones the regional magnetic quiet zone that encompasses james ross island is caused by the thick sedimentary infill of the larsen basin and a low amplitude magnetic high within the basin is inferred to reflect a basement push up structure associated with strike slip faulting along the eastern edge of the antarctic peninsula in the offshore regions magnetic anomalies southwest of tabarin peninsula and west of vega island may reflect recent volcanic structures that have yet to emerge from the sea floor

tectonic evolution of the antarctic phoenix plate system since 15 ma abstract joint inversion of magnetic isochron and fracture zone data from the extinct antarctic phoenix spreading system in sw drake passage yields seven new finite reconstruction poles the inversion results are very well constrained for such a short length of plate boundary although this is partly because the finite poles are located close to the reconstructed region the optimum use of fracture zone identifications from satellite derived free air gravity data is also important as the stability of stage poles throughout the short intervals in the model affirms the model results describe a well organised spreading system since magnetic anomaly chron c5ad 15 ma in which the phoenix plate rotated about stage poles nearby to the southwest stage pole locations are broadly consistent with a hypothesis of pivoting subduction as the driving force of phoenix plate movement and there is some evidence in the progression of stage poles for late stage movement of the subduction pivot in response to the changing azimuth of the subduction zone at which the phoenix plate was being consumed the model kinematics alone provide no unequivocal support for previous interpretations of disruption of the subducted part of the phoenix plate the very latest stages of spreading saw falling spreading rates between magnetic anomaly chrons c4 8 1 ma and c2a 3 3 ma when the antarctic phoenix ridge became extinct this is consistent with an increase in shear stress across the plate bounding shackleton fracture zone due to a plate reorganisation in the neighbouring scotia sea following the cessation of spreading on the west scotia ridge

the chile ridge a tectonic framework a new chile ridge tectonic framework is developed based on satellite altimetry data shipboard geophysical data and primarily 38 500 km of magnetic data gathered on a joint u s chilean aeromagnetic survey eighteen active transforms with fossil fracture zones fzs including two complex systems the chile fz and valdivia fz systems have been mapped between the northern end of the antarctic nazca plate boundary chile ridge at 35 s and the chile margin triple junction at 47 s chile ridge spreading rates from 23 ma to present have been determined and show slowdowns in spreading rates that correspond to times of ridge trench collisions the valdivia fz system previously mapped as two fzs with an uncharted seismically active region between them is now recognized to be a multiple offset fz system composed of six fzs separated by short ridge segments 22 to 27 km long at chron 5a 12 ma the chile ridge propagated from the valdivia fz system northward into the nazca plate through crust formed 5 myr earlier at the pacific nazca ridge evidence for this propagation event includes the friday and crusoe troughs located at discontinuities in the magnetic anomaly sequence and interpreted as pseudofaults this propagation event led to the formation of the friday microplate which resulted in the transferal of crust from the nazca plate to the antarctic plate and in a 500 km northward stepwise migration of the pacific antarctic nazca triple junction rift propagation microplate formation microplate extinction and stepwise triple junction migration are found to occur during large scale plate motion changes and plate boundary changes in the southeast pacific

a numerical modelling and neural network approach to estimate the impact of groundwater abstractions on river flows summary evaluation of the impacts of groundwater abstractions on surface water systems is a necessary task in integrated water resources management a range of hydrological hydrogeological and geomorphological factors influence the complex processes of interaction between groundwater and rivers this paper presents an approach which uses numerical modeling of generic river aquifer systems to represent the interaction processes and neural networks to capture the impacts of the different controlling factors the generic models describe hydrogeological settings representing most river aquifer systems in england and wales high diffusivity e g chalk and low diffusivity e g triassic sandstone aquifers with flow to rivers mediated by alluvial gravels the same aquifers where they are in direct connection with the river and shallow alluvial aquifers which are disconnected from regional aquifers numerical model simulations using the shetran integrated catchment modeling system provided outputs including time series and spatial variations in river flow depletion and spatially distributed groundwater levels artificial neural network models were trained using input parameters describing the controlling factors and the outputs from the numerical model simulations providing an efficient tool for representing the impacts of groundwater abstractions across a wide range of conditions there are very few field data sets of accurately quantified river flow depletion as a result of groundwater abstraction under controlled conditions one such data set from an experimental study carried out in 1967 on the winterbourne stream in the lambourne catchment over a chalk aquifer was used successfully to test the modeling tool this modeling approach provides a general methodology for rapid simulations of complex hydrogeological systems which preserves the physical consistency between multiple and diverse model outputs