reassessment of net surface mass balance in antarctica abstract recent in situ measurements of surface mass balance and improved calculation techniques are used to produce an updated assessment of net surface mass balance over antarctica a new elevation model of antarctica derived from ers 1 satellite altimetry supplemented with conventional data was used to delineate the ice flow drainage basins across antarctica the areas of these basins were calculated using the recent digital descriptions of coastlines and grounding lines the delineation of drainage basins was achieved using an automatic procedure which gave similar results to earlier hand drawn catchment basins more than 1800 published and unpublished in situ measurements of net surface mass balance from antarctica were collated and then interpolated a net surface mass balance map was derived from passive microwave satellite data being employed as a forcing field to control the interpolation of the sparse in situ observations basinwide integrals of net surface mass balance were calculated using t

glaciers and ice sheets mapping orbiter concept 1 we describe a concept for a spaceborne radar system designed to measure the surface and basal topography of terrestrial ice sheets and to determine the physical properties of the glacier bed our primary objective is to develop this new technology for obtaining spaceborne estimates of the thickness of the polar ice sheets with an ultimate goal of providing essential information to modelers estimating the mass balance of the polar ice sheets and estimating the response of ice sheets to changing climate our new technology concept employs vhf and p band interferometric radars using a novel clutter rejection technique for measuring surface and bottom topographies of polar ice sheets from aircraft and spacecraft our approach will enable us to reduce signal contamination from surface clutter measure the topography of the glacier bed at better than 1 km intervals with an accuracy of 20 m and paint a picture of variations in bed characteristics the technology will also have applications for planetary exploration including studies of the martian ice caps and the icy moons of the outer solar system through the concept developed here we believe that we can image the base and map the three dimensional basal topography beneath an ice sheet at up to 5 km depth

inferring ice flow directions from single ice sheet surface images using the radon transform we present a new method for extracting the direction of surface flow for ice sheets based on the detection of flow induced features that are visible in satellite imagery the orientation of linear features is determined using a radon transform and only requires a single image the technique is demonstrated by applying it to the radarsat mosaic of antarctica over the lambert glacier amery ice shelf region of east antarctica comparisons with both existing flow direction fields and traced streamlines over the same area provide an evaluation of the method we also illustrate its application to landsat 7 imagery

thermochronologic record of pluton emplacement deformation and exhumation at courtright shear zone central sierra nevada california thermochronology and thermobarometry of two plutons and a ductile shear zone at their contact in the central sierra nevada california as defined over the range 700 to 300 c by 40 ar 39 ar dating of hornblende and biotite and u pb dating of zircon indicate that ductile shear coincided closely with dramatically increased cooling and possible decompression of both plutons deformation occurred as the younger 90 ma mount givens pluton was emplaced against the older 102 ma dinkey creek pluton which was softened by conductive heating during extension these events coincided with significant regional exhumation possibly a result of crustal inflation recorded by a major pulse of rapid sediment accumulation in contemporaneous fore arc strata of the great valley sequence

helium chronometry of apatite and titanite using nd yag laser heating we developed a technique for extracting 4he from apatite and titanite using laser heating that permits measurement of u and th and hence u th he ages on a single sample aliquot unlike direct laser heating quantitative he extraction and u th retention can be achieved when samples are indirectly laser heated to temperatures below their melting point resulting he ages are consistent with those obtained from conventional furnace heating of larger samples the major advantages of this technique include faster analysis and far lower more reproducible he blanks that permit single crystal analyses single crystal dating opens a variety of new avenues for u th he chronometry

a discussion on the lead isotope geochemistry of galenas from the bambui group minas gerais brazil lead isotope data on galenas from the bambui group minas gerais have been assembled and discussed the isotope data when plotted on the single stage model iii lead isotope development curve of cumming and richards 1975 yield 207 206 ages of 650 50 m y and a linear array of data points with negative model ages which intercept the growth curve at 650 60 m y and 1850 150 m y the radiometric dates coincide with the two major orogenic episodes the brazilian and the transamazonian the data are interpreted in terms of a two stage model where the source of lead for galenas formed during brazilian orogeny is the basement rocks of transamazonian age the calculated th u value for the source rock is 2 9 0 3 the geographical distribution of lead isotope ratios shows a zonation in vazante paracatu region an area containing large deposits of zinc the implication of the distribution on the ore formation is discussed

geology petrography and mineral chemistry of the vazante non sulfide and ambrosia and fagundes sulfide rich carbonate hosted zn pb deposits minas gerais brazil the vazante paracatu region represents the most important zn district known in brazil and includes the vazante hypogene non sulfide zn deposit composed mainly of willemite zn2sio4 and sphalerite rich carbonate hosted zn pb deposits fagundes is a stratabound deposit characterized by strong silicification dolomitization and a fe rich carbonate alteration halo the main ore is represented by rhythmically banded colloform and zoned pyrite sphalerite and galena related to wall rock dissolution and sulfide infilling which probably occurs late during the burial diagenesis later veins and breccia ore types reflect epigenetic mobilization related to brittle ductile structures the ambrosia mineralization is mainly fault controlled and related to brecciated dolomite which is tectonically imbricated with black shales and slates typical features include host rock recrystallization minor silicification baroque dolomite and ankerite formation pyrite marcasite sphalerite and minor galena occur in brecciated comb veins and veinlets which overprint stylolites and tectonic fractures suggesting an epigenetic origin for the ore the vazante deposit differs from all other deposits of the district due to the presence of willemitic ore which is composed of willemite dolomite siderite quartz hematite zn rich chlorite barite franklinite and zincite the willemitic ore occurs tectonically imbricate to small sulfide ore bodies which comprises sphalerite and galena metabasites and hydrothermalized dolomites within the vazante shear zone the relationships between willemite formation and the development of mylonitic structures suggest that willemitic mineralization and deformation are synchronous episodes closely related to the vazante shear zone low zn cd ratios in sphalerite from vazante 64 to 98 and fagundes colloform 96 to 244 and zoned sphalerite 89 to 305 could reflect the regional role of mineralizing fluids with similar low zn cd ratios and low contents of reduced sulfur sred high ge up to 2200 ppm and the low fe cu mn and ag contents in late diagenetic fagundes sphalerite might suggest that this metal bearing fluid could be derived from the underlying basin fill which comprise clastic sediments and organic matter rich pelitic sequences systematic relationships among sphalerite composition and paragenetic evolution of the fagundes and ambrosia deposits suggest that progressive fluid mixing processes were important for the genesis of the sulfide rich deposits in the district these mixing processes possibly involved the hot metal bearing fluid with low contents of reduced sulfur and moderate temperature highly saline fluids which could represent an important sulfur supply the predominance of the highly saline brines in later epigenetic mineralization episodes might be related to episodic migration of hydrothermal fluids mainly derived from reduced shale sequences during the brasiliano compressive events the small variation in chemical and sulfur isotopic composition of the vazante sphalerite could imply that the high temperature metal bearing fluid with low zn cd ratios could represent a minor reservoir of reduced sulfur which permitted only subordinate sphalerite precipitation in the vazante deposit the lack of reduced shale sequences above the vazante deposit could represent a limiting factor for h2s supply additionally overall mixture between this hot sulfur deficient metal bearing fluid and meteoric fluids channeled to the vazante shear zone favor the high fo2 s2 conditions responsible for the stability of the vazante willemitic assemblage

organized flow from the south pole to the filchner ronne ice shelf an assessment of balance velocities in interior east antarctica using radio echo sounding data 1 ice flow through central antarctica has the potential to transmit accumulation changes from deep interior east antarctica rapidly to the shelf but it is poorly constrained owing to a dearth of ice velocity observations we use parameters derived from airborne radio echo sounding res data to examine the onset areal extent and englacial conditions of an organized flow network tributaries feeding an ice stream draining from the south pole to the filchner ronne ice shelf we classified res flight tracks covering the region according to whether englacial stratigraphy was disrupted i e internal layers diverged significantly from the surface and bed echoes or undisrupted i e internal layers closely parallel surface and basal topography and we calculated subglacial roughness along basal reflectors where satellite measured surface ice flow speeds are available covering 39 of the study region regions of fast and tributary flow correspond with res flight tracks that exhibit more disrupted internal layers and smoother subglacial topography than their counterparts in regions of slow flow this suggests that disrupted internal layering and smooth subglacial topography identified from res profiles can be treated as indicators of past or present enhanced flow tributaries where neither satellite nor ground based ice flow measurements are available we therefore use these res derived parameters to assess the balance flux modeled steady state flow regime between the south pole and filchner ronne ice shelf the res analysis confirms that an organized flow network drains a wide region around the south pole into the filchner ronne ice shelf however the spatial extent of this network as delineated by the res data diverges from that predicted by currently available balance flux models

dating topography of the sierra nevada california using apatite u th he ages the upward motion of rock masses relative to the earth s surface has been documented for most of the main mountain belts using thermochronological and petrological techniques more fundamental to the physical processes of mountain building however is the motion of the earth s surface itself which remains elusive here we describe a technique for estimating the age of topographic relief by mapping the low temperature thermal structure imparted by river incision using the ages of apatites determined from their uranium thorium and helium contents the technique exploits horizontal variations in temperature in the shallow crust that result from range normal river drainages because cooling beneath ancient river valleys occurs earlier than beneath intervening ridges our results from the sierra nevada california indicate that two of the modern transverse drainages the kings and the san joaquin had developed deep canyons by the late cretaceous period suggesting that the high topography of the range is 50 60 million years older than generally thought

mass budget of the grounded ice in the lambert glacier amery ice shelf system we used remote sensing and in situ measurements of surface accumulation rate ice surface velocity thickness and elevation to evaluate the mass budgets of grounded ice flow regimes that form the lambert glacier amery ice shelf system three distinct drainage regimes are considered the western and eastern margins of the ice shelf and the southern grounding line at the major outlet glacier confluence which can be identified with drainage zones 9 11 and 10 respectively of giovinetto and zwally 2000 our findings show the entire grounded portion of the basin is approximately in balance with a mass budget of 4 2 9 8gta 1 drainages 9 10 and 11 are within balance to the level of our measurement uncertainty with mass budgets of 2 5 2 8 gta 1 2 6 7 8 gta 1 and 0 9 2 3 gta 1 respectively the region upstream of the australian lambert glacier basin lgb traverse has a net mass budget of 4 4 6 3 gt a 1 while the downstream region has 8 9 9 9 gt a 1 these results indicate that glacier drainages 9 10 and 11 upstream and downstream of the australian lgb traverse are in balance to within our measurement error

post 10 ma uplift and exhumation of the northern coast mountains british columbia apatite u th he ages reveal three distinct periods in the exhumation history of the northern coast mountains 54 n a well developed helium partial retention zone indicates little or no exhumation between ca 30 and 10 ma beginning at 10 ma and extending to at least 4 ma the range underwent steady but slow exhumation of 0 22 mm yr after which the exhumation rate increased by at least 70 an 85 km long he age traverse across the orogen at sea level shows that vertical offsets on post 10 ma faults are minor furthermore the sea level he ages 14 2 8 ma inversely correlate with local mean elevation along the traverse these data suggest that the coast mountains first appeared as a significant topographic feature only within the past few million years this history is not consistent with suggestions that uplift of the range resulted from formation of the queen charlotte basin in early middle miocene time we speculate that intense erosion by alpine and continental glaciation after 2 5 ma triggered isostatic uplift and creation of the modern topography of the coast mountains

antarctic ice sheet balance velocities from merged point and vector data we estimate antarctic ice flow balance velocities which are the average speeds that ice must flow downslope through a volume assuming that there are equal amounts of ice entering and leaving the ice volume we use the osu ohio state university digital elevation model of antarctica recent ice accumulation rate data and the bedmap ice thickness data compilations for antarctica to characterize the physical properties of the ice sheet that are included in the balance velocity calculation we adapt a flux algorithm from the hydrology literature that enables us to calculate the flux distribution from any cell in any order flux from one cell to its neighbors is partitioned as a simple function of surface slope direction digitized flow stripe directions from satellite images minimize errors in flow direction where surface slopes are low or complex we estimate errors in balance velocity arising from errors in the data and show semi quantitatively how properties of the algorithm bias the balance velocity result we find a favorable comparison between our model and observed velocity data as well as the balance velocity patterns reported by other researchers

uranium lead isotopic ages from the sierra nevada batholith california this study provides new information on the timing and distribution of mesozoic magmatic events in the sierra nevada batholithic complex chiefly between 36 and 37 n latitude u pb ages have been determined for 133 zircon and 7 sphene separates from 82 samples of granitoid rocks granitoid rocks in this area range in age from 217 to 80 m y triassic intrusions are restricted to the east side of the batholith jurassic plutons occur south of the triassic plutons east of the sierra nevada as isolated masses within the cretaceous batholith and in the western foothills of the range cretaceous plutons form a continuous belt along the axis of the batholith and occur as isolated masses east of the sierra nevada no granitic intrusions were emplaced for 37 m y east of the sierra nevada following the end of jurassic plutonism however following emplacement of the eastern jurassic granitoids regional extension produced a fracture system at least 350 km long into which the dominantly mafic calc alkalic independence dike swarm was intruded 148 m y ago the dike fractures probably represents a period of regional crustal extension caused by a redistribution of the regional stress pattern accompanying the nevadan orogeny intrusion of cretaceous granitic plutons began in large volume about 120 m y ago in the western sierra nevada and migrated steadily eastward for 40 m y at a rate of 2 7 mm y this slow and constant migration indicates remarkably uniform conditions of subduction with perhaps downward migration of parent magma generation or a slight flattening of the subduction zone such steady conditions could be necessary for the production of large batholithic complexes such as the sierra nevada the abrupt termination of plutonism 80 m y ago may have resulted from an increased rate of convergence of the american and eastern pacific plates and dramatic flattening of the subduction zone u pb ages of the giant forest alaskite sequence in sequoia national park are all in the range 99 3 m y indicating a relatively short period of emplacement and cooling for this nested group of plutons u pb ages of a mafic inclusion and its host granodiorite indicate that both were derived from a common source or that the mafic inclusion was totally equilibrated with the granodioritic magma comparison of isotopic ages determined by different methods such as zircon u pb sphene u pb hornblende k ar and biotite k ar suggests that zircon u pb ages generally approximate the emplacement age of a pluton however some plutons probably contain inherited or entrained old zircons and the zircons of some samples are disturbed by younger thermal and metamorphic events the ages reported here are consistent with u pb age determinations previously made on granitic rocks to the north stern et al 1981 the age distribution of granitic belts determined here is in general agreement with those established by k ar dating evernden and kistler 1970 but does not differentiate the five epochs of plutonism determined in their study

sediment hosted lead zinc deposits of the neoproterozoic bambui group and correlative sequences sao francisco craton brazil a review and a possible metallogenic evolution model abstract the proterozoic sediment hosted zn pb sulfide and non sulfide deposits of the sao francisco craton brazil are partially syn diagenetic and epigenetic and were probably formed during extensional events the majority of the deposits occur within shallow water dolomites the pb isotopic data of sulfides are relatively homogeneous for individual deposits and plot above the upper crust evolution curve of the plumbotectonic model some of the deposits are characterized by highly radiogenic lead 206 pb 204 pb 21 originating from the highly radioactive crust of the sao francisco craton pb and s isotopic data suggest the sources of metal and sulfur for the deposits to be the basement rocks and seawater sulfates in the sediments respectively the relatively high temperatures of formation 100 to 250 c and moderate salinity 3 to 20 nacl equiv of the primary fluid inclusions in the sphalerite crystals suggest the participation of basinal mineralizing fluids in ore formation the steep paleo geothermal gradient generated by the radioactively enriched basement rocks probably assisted in heating up the circulating mineralizing fluids

fluid evolution in the southern part of the proterozoic vazante group brazil implications for exploration of sedimentary hosted base metal deposits abstract the proterozoic vazante group carbonate rocks were submitted to multiple stages of fluid circulation from diagenesis to orogenesis as documented by detailed mineralogical fluid inclusion and isotopic studies of the upper morro do pinheiro and lower pamplona members from the serra do poco verde formation in the southern part of the paleo basin these units are the main hosts for the hypogene structurally controlled zinc silicate deposits in the vazante zinc district including the vazante mine which is considered to be the largest willemitic zn 2 sio 4 deposit in the world with estimated total resources of 40 60 mt at 20 zn five hydrothermal alteration types were identified in the southern extension of the vazante group i early stage alteration comprises dolomite substitution bands and nodules associated with moderate salinity h 2 o nacl cacl 2 fluids with temperatures around 90 c interpreted as late diagenetic the calculated c and o isotopic compositions of the fluids suggest meteoric and or connate origin and interaction with organic carbon ii pre ore stage alteration is evidenced by dog tooth dolomite and quartz with minor fe oxi hydroxides which fills dissolution voids formed by h 2 o nacl cacl 2 mgcl 2 fluids at temperatures around 100 to 150 c the c isotopic data also indicate interaction with organic carbon the two first stages are poor in ore related elements iii ore stage encompasses four phases of mineral precipitation the first is distal and characterized by red stained dolostones due to disseminated hematite and red dolomite the second is the main phase of the ore stage composed of massive red dolomite massive hematite and willemite with enrichment mainly in fe 2 o 3 sio 2 ag as cd cu ni pb se and zn the third phase comprises white dolomite hematite and traces of willemite with enrichment in mno cd ni and pb the calculated c and o isotopic compositions of the fluids at 180 c associated with pervasive alteration yielded values heavier than the diagenetic stage and lower than the host rock the fourth phase corresponds to zn chlorite and quartz which are associated with h 2 o nacl fluids of variable salinities and distinct temperatures 90 140 c and 170 190 c iv pyrite bearing vein characterized by pyrite sphalerite white dolomite fine hematite and late quartz with c and o isotopic and fluid inclusion compositions that are similar to the three phases of dolomite of the ore stage these data suggest a co genetic relationship of these two alteration styles v the late sulfide stage characterized by rare galena bearing stringers with sphalerite chalcocite greenockite covellite and white dolomite that cross cut the main ore stage phases the ore related fluid compositions associated with the silicate zinc mineralization in the southern part of the vazante group are also similar to the fluids reported in previous work for the sulfide zinc lead deposits in the northern part indicating favorable conditions for metal transport during the brasiliano orogeny sulfide ore would have precipitated in zones where sulfur was available and silicate zinc in structures where mineralizing fluids interacted with evolved meteoric water this finding implies that carbonate sequences in other districts with sulfide lead zinc deposits may also host hypogene silicate zinc deposits

accumulation variability and mass budgets of the lambert glacier amery ice shelf system east antarctica at high elevations the temporal and spatial variability of the annual accumulation rate and the mass budgets of five sub basins of the lambert glacier amery ice shelf system las east antarctica at high elevations are assessed using a variety of datasets derived from field measurements and modeling the annual temporal variations of the accumulation rate for four cores from the west and east sides of the las are around 34 decadal fluctuation of the accumulation from the dt001 firn core drops to 10 and the 30 year fluctuation to 5 which is assumed to contain the information about the regional and long term trend in accumulation the 15 point running mean of the annual accumulation rate derived from stake measurements can remove most of the high frequency spatial variation so as to better represent the local accumulation model simulations show that the spatial variability of erosion deposition of snow by the wind has a noticeable impact on the surface mass balance at the higher parts of the las mass budget estimates at high elevation sub basins of the las suggest drainage 9 has a negative imbalance of 0 7 0 4 gt a 1 lambert and mellor glaciers have a positive imbalance of 3 9 2 1 and 2 1 2 4 gt a 1 respectively and fisher glacier and drainage 11 are approximately in balance the higher elevation region as a whole has a positive mass imbalance of 4 4 6 3 gt a 1 which is consistent with the most recent radar altimetry assessment that shows an overall thickening over this region

erosion and tectonics at the margins of continental plateaus the topography across the eastern margin of the central andean plateau north of 18 s beni region bears a strong resemblance to the topography of the southern margin of the tibetan plateau nepal himalaya with both regions featuring a steep frontal slope and high peaks at the plateau edge in contrast the topography of the eastern margin of the central andean plateau south of 18 s pilcomayo region tapers toward the foreland more gently and has no line of high peaks at the margin both the himalayan and the beni regions have been the sites for large amounts of denudation and in both regions geologic evidence suggests that erosion has been sufficiently vigorous for the physiographic plateau margin to have retreated toward the plateau interior during the neogene we hypothesize that the steep frontal slope and high peaks of the beni region and himalayan front largely reflect the high orographic precipitation and high erosion rates occurring in these regions and that the more gentle topography of the semiarid pilcomayo region reflects a tectonic landform only slightly modified by erosion we propose that orographic precipitation impinging on a plateau margin will generally tend to drop moisture low on the slope eroding back the plateau while enhancing or maintaining the steep long wavelength slope a numerical model coupling orographic precipitation erosion and tectonic uplift demonstrates the plausibility of this hypothesis the erosional efflux in both the beni and nepal himalaya have been considerable and simple mass balance calculations for the himalaya suggest that during the neogene the erosional mass efflux has generally outpaced the tectonic mass influx this contrasts with the apparent prior domination of tectonic influx and may reflect a decrease in the rate of tectonic addition during the same period and or increased late cenozoic erosion rates

a new analytical method for estimating antarctic ice flow in the 1960s from historical optical satellite imagery ice flow velocity is used to estimate ice mass changes in glaciers and is a significant indicator of the stability of the antarctica ice sheet in global change studies the existing regional antarctica ice flow speed maps are usually derived from radar or optical satellite observations of modern satellites since the 1970s this paper presents a new analytical photogrammetric method for estimating antarctica ice flow velocity fields by using film based stereo argon photographs collected in the 1960s the key of the proposed innovative method is a parallax decomposition that separates the effect of the terrain relief from the ice flow motion an innovative implementation strategy is developed by using a framework that involves key techniques of hierarchical stereo image matching ice flow direction determination parallax decomposition and ice flow speed estimation this method is applied in the rayner glacier in eastern antarctica by using two sets of argon images with a two month interval in 1963 the produced digital terrain model and speed map achieved a ground position accuracy of 61 m and a speed accuracy of 70 m text a 1 a comparison with recent products from 2000 to 2010 shows no significant topographic changes in the study area furthermore the speed around the grounding line remained at the same level while the speed in the ice shelf front decreased by 73 m text a 1 the ice shelf front advanced by approximately 7 km over more than 40 years overall the observation results indicate favorable conditions for the stability of the rayner glacier ice shelf system

the evolution of surface flow stripes and stratigraphic folds within kamb ice stream why don t they match flow stripes seen in satellite imagery of ice streams and ice shelves are caused by surface undulations with kilometer scale spacing and meter scale relief and generally indicate current or recent fast ice flow on a similar scale folding of internal ice stratigraphy depicted in cross flow ice penetrating radar profiles is also a common occurrence in ice streams suggesting a possible relationship between the two sets of features we have traced surface flow stripes in radarsat and modis imagery on kamb ice stream west antarctica from the onset of streaming flow into the near stagnant trunk we compare the morphology and evolution of the surface flow stripes to the folds seen in the internal stratigraphy in cross ice stream radar profiles we find essentially no correspondence in the observed locations or spacings between the radar internal layer folds at depths greater than 100 m and the flow stripes on the surface the gap in the radar data and the surface mappings in the top 100 m of firn prevents a precise depiction of how the flow stripes and fold patterns at depth diverge we explore hypotheses about how flow stripes and internal stratigraphic folds can originate and evolve differently as ice flows downstream we suggest that flow stripes are subject to surface processes that can modify their morphology independently of the internal stratigraphy leading to changes in the pattern of flow stripes relative to the internal layers below

the carbonate hosted mvt morro agudo zn pb deposit central brazil abstract morro agudo is the largest known mississippi valley type deposit mvt in brazil with original estimated reserves of 18 3 mt 5 08 zn and 1 75 pb of which 3 92 mt 4 zn and 1 45 pb remained in 2017 in spite of having been mined for more than 30 years a comprehensive study of morro agudo metallogenic features was still lacking in the international literature this deposit is hosted within the 250 km long vazante sequence a n s trending mesoproterozoic neoproterozoic carbonate dominant sedimentary sequence thrusted over neoproterozoic rocks during the brasiliano orogeny sulfide bodies in morro agudo are restricted to the morro do calcario formation as a 1300 m long ore envelope averaging 9 m thick comprising stacked up stratabound orebodies that thin out down dip and are limited to the ne by the main fault five orebodies are described a breccia hosted orebody ghi b sphalerite rich doloarenite orebody jk c galena rich doloarenite orebody l d lower replaced stratiform orebody m e upper replaced stratiform orebody n comparison of these orebodies reveals mineral zonation patterns and variations in zn pb grade consistent with sulfide textural relationships these relationships indicate five mineralization styles that provide spatial and timing constraints for morro agudo a stratiform sulfides b massive ore c laminated ore d breccia hosted ore e late stage fracture filling ore carbon and oxygen stable isotope data for each mineralization style shows a pattern of mixing of signatures of mineralizing fluids and barren host rocks and supports our interpretation that these mineralization styles occurred in more than one stage of ore formation the ore was later offset by a set of strike slip normal faults associated with compression related to the brasiliano orogeny of which the main fault falha principal is the most important because it limits the deposit to the ne previous interpretation of upper orebodies as part of a sedex system is revisited in the present work in our model data used to support a syngenetic origin for morro agudo such as synsedimentary structures pb isotopes in sulfides and fluid inclusion data indicate a younger barren generation of sulfides deposited long before zn pb mineralization during sedimentation of the morro do calcario formation in the mesoproterozoic these early sulfides were preserved in cherts and as massive pyrite beds that were later overprinted by sphalerite and galena overprinting thus mixed meso and neoproterozoic pb isotopic sources and led to a long standing argument over whether the morro agudo ores were syngenetic or epigenetic this interpretation is also in agreement with the current classification of the vazante sequence as containing imbricated neoproterozoic and mesoproterozoic sedimentary rocks based on our work of sulfide textural relationships stable isotopes whole rock geochemistry and previously published data we argue that morro agudo ores formed by at least two stages of percolation of mildly hot hydrothermal fluid in the neoproterozoic the primary reason for sulfide precipitation is interpreted to be mixing of mildly hot metal bearing basinal brines and seawater connate waters

the 1500m south pole ice core recovering a 40 ka environmental record supported by the us national science foundation a new 1500 m approximately 40 ka old ice core will be recovered from south pole during the 2014 15 and 2015 16 austral summer seasons using the new us intermediate depth drill the combination of low temperatures relatively high accumulation rates and low impurity concentrations at south pole will yield detailed records of ice chemistry and trace atmospheric gases the south pole ice core will provide a climate history record of a unique area of the east antarctic plateau that is partly influenced by weather systems that cross the west antarctic ice sheet the ice at south pole flows at approximately 10m a exp 1 and the south pole ice core site is a significant distance from an ice divide therefore ice recovered at depth originated progressively farther upstream of the coring site new ground penetrating radar collected over the drill site location shows no anthropogenic influence over the past approximately 50 years or upper 15 m depth age scale modeling results show consistent and plausible annual layer thicknesses and accumulation rate histories indicating that no significant stratigraphic disturbances exist in the upper 1500m near the ice core drill site

dynamic thinning of glaciers on the southern antarctic peninsula growing evidence has demonstrated the importance of ice shelf buttressing on the inland grounded ice especially if it is resting on bedrock below sea level much of the southern antarctic peninsula satisfies this condition and also possesses a bed slope that deepens inland such ice sheet geometry is potentially unstable we use satellite altimetry and gravity observations to show that a major portion of the region has since 2009 destabilized ice mass loss of the marine terminating glaciers has rapidly accelerated from close to balance in the 2000s to a sustained rate of 56 8 gigatons per year constituting a major fraction of antarctica s contribution to rising sea level the widespread simultaneous nature of the acceleration in the absence of a persistent atmospheric forcing points to an oceanic driving mechanism

cenozoic thermal evolution of the central sierra nevada california from uth he thermochronometry apatite u th he cooling ages are reported for igneous apatite samples from the central sierra nevada and compared to published apatite fission track ages and track length data from the same mineral separates helium ages are youngest at low elevations and increase systematically toward higher elevations ranging from 43 to 84 ma at yosemite valley 32 to 74 ma at kings river canyon and 23 to 75 ma at mt whitney helium ages from high elevation samples are generally concordant with corresponding fission track ages while lower elevation helium ages are substantially younger cooling histories inferred from present laboratory derived fission track annealing and helium diffusion models do not match well suggesting that either helium diffusion rates or fission track annealing rates are miscalibrated at temperatures below about 60 c for geologic exposure periods unlike the fission track results the helium data do not indicate a very low geothermal gradient in the sierra nevada during early to middle tertiary time

global correlation of the vazante group sao francisco basin brazil re os and u pb radiometric age constraints abstract the vazante group consists of precambrian carbonate dominated platform deposits that extend along more than 300 km in the external zone of the brasilia fold belt of the sao francisco basin in east central brazil the sequence is about 4 8 km thick and contains a preserved glaciomarine diamictite unit containing dropstone at the top and a lower diamictite unit at the bottom previous c and sr isotope profiles suggested the correlation of the upper diamictite unit with the sturtian glacial event ca 750 643 ma however new re os isotope data from the shales associated with the upper diamictites yield radiometric age estimates between 993 46 and 1100 77 ma u pb measurements on a suite of clear euhedral zircon crystals that were separated from the same shales associated with the upper diamictite and from the arkosic sandstone above the lower diamictite yield ages as young as 988 15 and 1000 25 ma respectively based on the re os and u pb ages the best age estimate of the vazante group is constrained to be 1000 1100 ma and thus the two diamictite units are not correlative with the sturtian glaciation s but most likely are records of glacial events that occurred during the late mesoproterozoic

dynamic distributed drainage implied by the flow evolution of the 1996 1998 adventure trench subglacial lake discharge abstract the transport of subglacial water beneath the east antarctic ice sheet is an enigmatic and difficult to observe process which may affect the flow of the overlying ice and mixing of the oceans in the sub ice shelf cavities and ultimately global climate periodic outbursts are a critical mechanism in this process recent analysis of satellite data has inferred a subglacial hydraulic discharge totaling 2 km 3 traveling some 260 km along the ice bed interface of the adventure subglacial trench between 1996 and 1998 wingham et al 2006 rapid discharge connects antarctic subglacial lakes nature 440 1033 1036 using radar echo sounding data from the adventure subglacial trench region in conjunction with the previously reported satellite observations along with some basic modeling we calculate a mass budget and infer a flow mechanism for the 1996 1998 event the volume released from the source lake exceeded the volume received by the destination lakes by 1 1 km 3 this discrepancy indicates that some water must have escaped downstream from the lowest destination lake from 1997 onward the downstream release of water from the destination lakes continued until at least 2003 several years after the 1998 cessation of surface subsidence at the source lake by 2003 a total of 1 5 km 3 or nearly 75 of the water released by the source lake had traveled downstream from the destination lakes the temporal evolution of discharge from the outlet can be simulated with the classic ice walled semicircular channel model if and only if the retreat of the source lake shoreline is taken into account further downstream the ice bedrock geometry along the inferred flow path downstream includes many sections where thermal erosion of the overlying ice would not be sustainable along these reaches mechanical lifting of the ice roof and or erosion of a sedimentary substrate by a broad shallow water system would be most effective means of sustaining the discharge a distributed system is also consistent with the 3 month delay between water release at the source lake and water arrival at the destination lake observations of intermittent flat bright bed reflections in radar data acquired along the flow path are consistent with the presence of a broad shallow water system ultimately the presence of large subglacial lakes along the flow path of the 1996 1998 adventure subglacial trench flow path delayed the arrival of water to points downstream by approximately 12 months

development of an antarctic digital elevation model by integrating cartographic and remotely sensed data a geographic information system based approach we present a high resolution digital elevation model dem of the antarctic it was created in a geographic information system gis environment by integrating the best available topographic data from a variety of sources extensive gis based error detection and correction operations ensured that our dem is free of gross errors the carefully designed interpolation algorithms for different types of source data and incorporation of surface morphologic information preserved and enhanced the fine surface structures present in the source data the effective control of adverse edge effects and the use of the hermite blending weight function in data merging minimized the discontinuities between different types of data leading to a seamless and topographically consistent dem throughout the antarctic this new dem provides exceptional topographical details and represents a substantial improvement in horizontal resolution and vertical accuracy over the earlier continental scale renditions particularly in mountainous and coastal regions it has a horizontal resolution of 200 m over the rugged mountains 400 m in the coastal regions and approximately 5 km in the interior the vertical accuracy of the dem is estimated at about 100 130 m over the rugged mountainous area better than 2 m for the ice shelves better than 15 m for the interior ice sheet and about 35 m for the steeper ice sheet perimeter the antarctic dem can be obtained from the authors

mass balance of the lambert glacier amery ice shelf system east antarctica a comparison of computed balance fluxes and measured fluxes we combine european remote sensing satellite ers 1 radar altimeter surface elevations fricker and others 2000 with six different accumulation distributions to compute balance fluxes for the lambert glacier amery ice shelf drainage system these interpolated balance fluxes are compared with fluxes derived from in situ measurements of ice thickness and velocity at 73 stations of the lambert glacier basin traverse and at 11 stations further downstream to assess the system s state of balance for the upstream line we obtain a range of imbalance estimates from 23 8 to 19 9 of the observed flux reflecting the sensitivity to the accumulation distributions for some of the accumulation distributions the imbalance estimates vary significantly between different parts of the line imbalance estimates for the downstream line range from 17 7 to 70 2 with four of the estimates exceeding 30 again reflecting the sensitivity of the result to input accumulation and strongly suggesting that the mass balance of the region between the two lines is positive our results confirm the importance of accurate estimates of accumulation in ice sheet mass balance studies furthermore they suggest that it is not possible to accurately determine the state of balance of large antarctic drainage basins on the basis of currently available accumulation distributions

provenance of the vazante group new u pb sm nd lu hf isotopic data and implications for the tectonic evolution of the neoproterozoic brasilia belt abstract the vazante group show varied u pb provenance patterns along the basin zircon ages range from 936 to 3409 ma but paleo and mesoproterozoic terrains constitute the main sources of the original sediments the youngest population 930 ma establishes the maximum depositional age of the group sm nd t dm data show the predominance of paleoproterozoic ages 1 90 2 08 ga and also indicate some input from younger sources in rocks of the lapa formation 1 67 to 2 0 ga in the upper part of the group whereas rocks of the serra do garrote formation present the oldest model ages 2 03 to 2 76 ga hf isotopic compositions of the detrital zircons indicate that they were derived mainly from recycled paleoproterozoic crust with a minor mesoproteroic juvenile component terranes within the sao francisco craton represent the main sources of detrital sediments of this group and reinforce the interpretation that it may be a passive margin sequence developed along the western margin of the original continent however the origin of mesoproterozoic grains remains uncertain slightly younger sm nd model ages in the lapa formation however are not entirely consistent with derivation solely from the craton and may indicate contribution from younger sources such as the neoproterozoic goias magmatic arc

inference of velocity pattern from isochronous layers in firn using an inverse method this study investigates the suitability of a kinematic approach to nd the velocity eld from dated internal layer architecture in rn internal layers are isochrones and the depositional age of a layer particle is treated as a tracer the forward problem uses two dimensional steady state advection of age and conservation of mass to predict layer ar chitecture dieren t combinations of constraints on horizontal or vertical velocity properties are added d the inverse problem can be formulated as the solution of underdetermined and overdetermined systems of equations e the systems are solved using singular value decomposition allowing analysis of the singular value spectrum model d resolution e and data resolution d solutions of the inverse problem e are evaluated by comparing the velocity eld solutions with synthetic input velocity data compared to conventional accumulation estimates d the new approach takes lateral advection into account e enabling improved separation of spatial and temporal variations in accumulation d two glaciological applications are presented the determination of the migration velocity of a spa tially non stationary accumulation pattern and reconstruction of past accumulation and its stationarity over time e

lithospheric gravitational potential energy and past orogenesis implications for conditions of initial basin and range and laramide deformation gravitational body forces i e buoyancy forces have come to be seen as critical to the evolution of orogens nevertheless constraining the role of body forces in specific geologic scenarios is made difficult by the substantial number of poorly constrained physical parameters needed to fully relate forces to deformation by separating the calculation of buoyancy forces from the calculation of the resulting deformation models based on relatively simple descriptions of the lithosphere can yield geologically useful constraints among these are the importance of paleoelevation in driving syn and postcontractional extension and in localizing contractional strain although such phenomena have been considered in more complex models of continental deformation the simpler analysis presented here clearly establishes first order limits on lithospheric structures and paleoelevations consistent with buoyancy driven deformation in the early cenozoic great basin of the western united states we show that the low elevations inferred in much of the geologic literature are inconsistent with a body force origin for observed extensional tectonism east of the colorado plateau localization of laramide deformation coincides with pre laramide subsidence of the western interior seaway this subsidence prestressed the lithosphere making the southern rocky mountains the weak link in responding to regional compressional stress

magmatic arc asymmetry and distribution of anomalous plutonic belts in the batholiths of california effects of assimilation crustal thickness and depth of crystallization in order to better understand geologic fac tors controlling pronounced regional variations in whole rock chemistry mineralogy and mineral chemistry in the batholiths of california we calculate the magmatic intensive variables f hf f h 2 o f hf f hcl and f o 2 regional scale west to east increases in f oh in mafic silicates corresponding to the systematic i wc to i mc and i sc progression reflect orders of magnitude increase in f hf f h 2 o attending pluton crystallization low f hf f h 2 o of formation of western i wc types is consistent with their derivation from low fluorine source rocks in subducted oceanic slabs or the upper mantle in contrast higher f hf f h 2 o of crystallization of i mc and i sc types to the east implies the involvement of 1 progressively greater amounts of continental crustal source material such as biotite bearing metamorphic rocks their unweathered sedimentary derivatives retaining f rich mafic minerals or their fusion products and or 2 source materials which become more f rich toward the continental interior the regional distribution of i mc and i sc types suggests that the precambrian craton of western north america or derivative sediments may extend farther north in california and be morphologically more complex than previously thought from seemingly out of place occurrences of i wc plutons on the eastern slopes of the sierra nevada batholith we infer the existence of regions where precambrian basement was thin or absent in mesozoic time which prevented extensive cratonal contamination of subducted slab or upper mantle derived magmas new methods for estimating t f o 2 relations in the magmas demonstrate that i scr granites crystallize at oxygen fugacities as much as five orders of magnitude lower than those of i wc i mc and i sc types under conditions at or below the maximum stability limit of graphite in equilibrium with a c o h s gas phase the local scale formation of i scr granites in plutonic belts within specific wall rock terranes containing highly reducing sediments or metasediments may occur by contamination of i types with graphitic pelite or in some cases by the direct fusion of this reducing pelitic wall rock the spatial distribution of i scr granite provinces therefore is controlled simply by wall rock lithology amphibole geobarometry demonstrates a general west to east decrease in crystallization pressure across the sierra nevada batholith in contrast the peninsular ranges batholith displays a west to east crystallization pressure increase the bulk of the california batholiths crystallized at pressures less than 4 5 kb and depths less than about 15 19 km in the southern sierra nevada batholith the san gabriels and the eastern peninsular ranges however plutons crystallized at pressures exceeding 6 kb at deep crustal levels 23 km reconstruction of the pre erosion top of the batholith shows that in an east west cross section the central sierra nevada batholith was a horizontal tabular body with an aspect ratio of at least five to one

reconstructing the paleotopography of mountain belts from the isotopic composition of authigenic minerals the paleorelief of mountain belts can be estimated from the 18 o value of authigenic minerals development of relief during mountain building often creates lee side rain shadows in which precipitation is depleted in 18 o and d the magnitude of this rain shadow effect is strongly correlated to relief a compilation of 18 o data from surface waters throughout the globe shows a linear relationship between net elevation change and 18 o r 2 0 79 through the use of this relationship we investigated the timing and magnitude of elevation change in the southern alps of new zealand and the sierra nevada of california the 18 o values of kaolinites from new zealand show an 6 decrease in the early pliocene that corresponds to an 2 km elevation change in the southern alps the 18 o of smectites from the sierra nevada show little change since 16 ma suggesting that these mountains have been a long standing topographic feature

the world s lead zinc mineral resources scarcity data issues and opportunities abstract lead and zinc keep humanity powered and sheltered yet a comprehensive understanding of pb zn resources in known mineral deposits has been lacking leading to uncertainty over when we might expect the supply of these metals to face potential constraints addressing this we compile an extensive database of the world s known pb zn mineral deposits and provide in depth analyses of their contained resources ore grades economic value by products and geological settings our data indicate that at least 226 1 mt pb and 610 3 mt zn are present within 851 individual mineral deposits and mine waste projects from 67 countries and one in international waters at an average grade of 0 44 pb and 1 20 zn the identified resources are dominantly present within sediment hosted pb zn deposits 490 6 mt pb zn cu which contain the equivalent of vms skarn porphyry epithermal and mixed sediment hosted deposits combined and 49 of these resources are reported in australia russia peru and canada alone the reported pb zn resources appear to be sufficient to meet global demand for both pb and zn until 2050 although this estimate is most certainly a minimum as our case studies indicate a prevailing trend of deposits cumulatively producing well beyond their reported resources over time indeed despite increasing historical production of pb and zn estimated reserves and resources have also increased and this is expected to continue we also present an analysis and review of additional aspects affecting the future sustainability of pb zn resources including an account of the history of pb zn mining case studies and trends in reporting classifications of the dominant pb zn mineral deposit types analysis of reported by product companion metals review of tailings resource potential and case studies on the numerous challenges in environmental management historically faced for pb zn mining these analyses alongside our comprehensive resource data indicate that the future supply of pb and zn is likely to be governed by prevailing economic social and environmental factors much more so than sheer resource constraints

widespread complex flow in the interior of the antarctic ice sheet it has been suggested that as much as 90 of the discharge from the antarctic ice sheet is drained through a small number of fast moving ice streams and outlet glaciers fed by relatively stable and inactive catchment areas here evidence obtained from balance velocity estimates suggests that each major drainage basin is fed by complex systems of tributaries that penetrate up to 1000 kilometers from the grounding line into the interior of the ice sheet this finding has important consequences for the modeled or estimated dynamic response time of past and present ice sheets to climate forcing

lead isotope constraints on the genesis of pb zn deposits in the neoproterozoic vazante group minas gerais brazil abstract the neoproterozoic vazante group at the western border of the sao francisco craton brazil hosts the largest zn pb district in south america several authors have classified this mineral district as mississippi valley type mvt based on the intimate association with carbonates and the epigenetic character of most ore bodies in this paper we present 47 new lead isotope data from four deposits located along the 300 km n s vazante paracatu unai linear trend pb isotope ratios indicate sources with relatively high u pb and th pb ratios considering the 206pb 204pb and 208pb 204pb ratios as indicative parameters for the source we suggest an upper crustal source for the metals the small variation on the pb isotope ratios compared to those observed in the classical mvt deposits and other geological fluid inclusion and sulphur isotopic data indicates a metallogenic event of long duration it was characterized by focused circulation of hydrothermal fluids carrying metals from the basement rocks and from the sedimentary pile the data obtained are more compatible with an evolution model similar to that of irish type deposits the existence of three pb isotopic populations could be the result of regional differences in composition of the source rocks and in the fluid rock interaction since the mineralization is a long term process

balance velocities and measured properties of the antarctic ice sheet from a new compilation of gridded data for modelling this paper presents a new compilation of gridded data sets for three dimensional modeling of the antarctic ice sheet these are for surface elevation icethickness bedrock elevation and accumulation rate as interpolated on a 281 x 281 mesh with 20 km spacing and encompass all of the ice sheet and thesurrounding continental shelf data sources include the bamber digital elevation model from ers 1 radar altimeter data a redigitization of available ice thicknessdata the giovinetto accumulation data recent ice thickness data from british and german expeditions as well as accumulation data from german and norwegianexpeditions in particular new data were incorporated for the filchner ronne schelfeis and for dronning maud land arising from the epica pre site survey special attention was devoted to carefully match the various data sources both among themselves as across the grounding line and below the ice shelves toenable ice sheet expansion and retreat in dynamic situations as an application the balance flow is calculated over the entire ice sheet using a two dimensionalfinite difference scheme and compared with a previous assessment this brought to light the existence of ice streaming features extending well inland of the icesheet a detailed zoom over dronning maud land exhibits the general flow characteristics of interest for locating a future deep drilling site as a by product anupdated value of 26 4 x 106 km3 was obtained for the total volume of the ice sheet and ice shelves or equivalent to 61 1 m of global sea level rise after removalof the ice sheet and subsequent oceanic invasion and isostatic rebound the total accumulation over the grounded ice sheet including the antarctic peninsula is1924 gta 1 or between 5 and 20 higher than earlier estimates including all of the ice shelves the value is 2344 gta 1

digital elevation model networks demon a model of flow over hillslopes for computation of contributing and dispersal areas current algorithms for computing contributing areas from a rectangular grid digital elevation model dem use the flow routing model of o callaghan and mark 1984 which has two major restrictions 1 flow which originates over a two dimensional pixel is treated as a point source nondimensional and is projected downslope by a line one dimensional moore and grayson 1991 and 2 the flow direction in each pixel is restricted to eight possibilities we show that large errors in the computed contributing areas result for any terrain topography divergent convergent or planar we present a new model called digital elevation model networks demon which avoids the above problems by representing flow in two dimensions and directed by aspect demon allows computation of both contributing and dispersal areas demon offers the main advantage of contour based models e g moore et al 1988 the representation of varying flow width over nonplanar topography while having the convenience of using rectangular grid dems

balance velocities of the greenland ice sheet we present a map of balance velocities for the greenland ice sheet the resolution of the underlying dem which was derived primarily from radar altimetery data yields far greater detail than earlier balance velocity estimates for greenland the velocity contours reveal in striking detail the location of an ice stream in northeastern greenland which was only recently discovered using satellite imagery enhanced flow associated with all of the major outlets is clearly visible although small errors in the source data result in less accurate estimates of the absolute flow speeds nevertheless the balance map is useful for ice sheet modelling mass balance studies and field planning

extensional fault systems in sedimentary basins a review of analogue model studies abstract scaled analogue models have provided graphic new insights into the progressive development of extensional fault systems this paper reviews recent results of an on going analogue modelling programme and compares the structural styles in the models with examples of natural extensional fault systems four fundamental model styles scaled to simulate deformation in brittle sedimentary rocks in the upper 10 km of the crust are reviewed extension above a basal detachment which undergoes stretching over a limited are produces an asymmetric rift graben bounded by planar faults and in which the internal deformation is accommodated by rotational domino faults these experimental results are directly comparable to structures found in intracontinental rifts extension above a basal detechment which undergoes extension under the whole model and which has no constraints at the ends produces domino style fault arrays in the pre rift sequence and listric growth faults in the syn rift sequence these results compare with highly extended rift basins and terranes such as the basin and range province usa extension above a simple listric fault produces a characteristic hangingwall roll over and crestal collapse graben system at high values of extension superposed crestal collapse grabens occur and a fan of listric growth faults is developed these structures bear striking similarities to growth faults in progradational delta systems ramp flat listric extensional fault systems using sand mica models produce a characteristic family of structures with an upper roll over crestal collapse system a ramp syncline and reverse fault fold zone and a lower roll over crestal collapse system an example of a ramp flat fault system from the gulf coast usa is presented as a comparison with the analogue model results preliminary results of three dimensional extensional model experiments are presented to illustrate the complexities in such fault systems in all of the experimental models it is found that the geometry of the underlying detachment system exerts fundamental control on structures produced in the hangingwall the analogue models illustrated in this review provide valid geometric and kinematic templates for the progressive evolution of extensional fault systems in sedimentary basins

paleo geomorphology of the sierra nevada california from u th he ages in apatite new apatite u th he ages from the central sierra nevada california place limits on the morphology and evolution of longitudinal profiles of major transverse river drainages developed in the late cretaceous helium ages from a new orogen parallel constant elevation sample transect are relatively uniform 60 ma and not correlated with topography unlike those from a similar lower elevation transect approx 15 km to the west we interpret the marked difference in the two orogen parallel profiles to reflect a headward decrease in long wavelength transverse relief at the time of cooling consistent with either a concave up stream gradient typical of those observed near the headwaters of modern sierran trunk streams or a convex gradient like those found along slope breaks of eroding plateau edges from the san joaquin drainage we obtained a new helium age versus elevation profile which has a comparable slope to those previously reported for the yosemite valley kings canyon and mt whitney areas this new profile yields slightly older ages at a given elevation as expected from its position adjacent to the largest major drainage the age versus elevation profiles imply that the mean denudation rate of the region in the cenozoic was about 0 04 to 0 05 mm yr we show that long wavelength 70 km relief inferred from longitudinal helium age variations is a strong function of erosion rate such that even relatively subdued relief is detectable with helium age profiling provided that denudation rates were low using the rate implied by sierran age versus elevation profiles we infer long wavelength relief of 1500 500 m in the late cretaceous by analogy with modern orogenic plateaus this value of long wavelength relief suggests a cretaceous interior cordilleran plateau lay at an elevation of at least 3000 m