discriminating fluid source regions in orogenic gold deposits using b isotopes abstract the genesis of orogenic gold deposits is commonly linked to hydrothermal ore fluids derived from metamorphic devolatilization reactions however there is considerable debate as to the ultimate source of these fluids and the metals they transport tourmaline is a common gangue mineral in orogenic gold deposits it is stable over a very wide p t range demonstrates limited volume diffusion of major and trace elements and is the main host of b in most rock types we have used texturally resolved b isotope analysis by secondary ion mass spectrometry sims to identify multiple fluid sources within a single orogenic gold ore district the loulo mining district in mali west africa hosts several large orogenic gold ore bodies with complex fluid chemistry associated with widespread pre ore na and multi stage b metasomatism the gara deposit as well as several smaller satellites formed through partial mixing between a dilute aqueous carbonic fluid and a hypersaline brine hydrothermal tourmaline occurs as a pre ore phase in the matrix of tourmalinite units which host mineralization in several ore bodies clasts of these tourmalinites occur in mineralized breccias disseminated hydrothermal and vein hosted tourmaline occur in textural sites which suggest growth during and after ore formation tourmalines show a large range in 11 b values from 3 5 to 19 8 which record a change in fluid source between paragenetic stages of tourmaline growth pre mineralization tourmaline crystals show heavy 11 b values 8 19 8 and high x site occupancy na ca 0 69 1 apfu suggesting a marine evaporite source for hydrothermal fluids syn mineralization and replacement phases show lighter 11 b values 3 5 to 15 1 and lower x site occupancy 0 62 0 88 apfu suggesting a subsequent influx of more dilute fluids derived from devolatilization of marine carbonates and clastic metasediments the large overlapping range in isotopic compositions and a skew toward the opposing population in the 11 b data for both tourmaline groups reflects continual tourmaline growth throughout mineralization which records the process of fluid mixing a peak in 11 b values at 8 largely controlled by tourmalines of syn to post ore timing represents a mixture of the two isotopically distinct fluids this paper demonstrates that b isotopes in tourmaline can be instrumental in interpreting complex and dynamic hydrothermal systems the importance of b as an integral constituent of orogenic ore forming fluids and as a gangue phase in orogenic gold deposits makes b isotope analysis a powerful tool for testing the level of source region variability in these fluids and by extension that of metal sources

multistage evolution of the neoarchean ca 2 7 ga igarape cinzento gt 46 iron oxide copper gold deposit cinzento shear zone carajas province brazil

aftsolve a program for multi kinetic modeling of apatite fission track data aftsolve is a computer program for deriving thermal history information from apatite fission track data it implements a new fission track annealing model that takes into account the known kinetic variability among different apatite species to fully utilize this model a fission track worker must obtain data that can be used to infer the kinetic characteristics of each apatite grain from which a measurement was taken such data can consist of etch figure lengths or chemical composition the benefit of this overall approach is that it allows useful information to be derived from previously unusable analyses extends the practical range of geological temperatures constrained by fission track analyses and increases overall confidence in model predictions aftsolve also incorporates the effects of fission track orientation relative to the apatite crystallographic c axis variation in initial track length and the biasing effect of 2 5 2 cf irradiation for enhancing confined horizontal track length detection aftsolve is written for windows operating systems and has a graphical interface that allows interactive input of thermal histories and real time generation of estimates for fission track length distributions and ages for up to six simultaneously modeled kinetic populations it also includes procedures for estimating the range of time temperature histories that are statistically consistent with a data set and constraints entered by the user

controls on erosional retreat of the uplifted rift flanks at the gulf of suez and northern red sea the gulf of suez and red sea rifts are currently bordered by large asymmetric uplifts that are undergoing erosion we find that the amount and timing of erosion vary systematically along the strike of the margin and have been controlled by variations in the prerift stratigraphy the prerift strata are composed of cliff forming eocene cretaceous carbonates overlying the easily eroded cretacous cambrian nubian sandstone this lithologic succession promotes scarp retreat of the sedimentary section followed by dissection of the underlying basement the prerift section thins from over 2000 rn at the northern end of the rift to less than 400 rn at its junction with the red sea thus at the northern part of the gulf of suez the nubian sandstone is minimallyexposed and the carbonates form a scarp at the rift border fault farther south undercutting of the carbonates by erosion of the sandstone has resulted in scarp retreat the escarpment cuts diagonally away from the border fault and is over 100 km inland from the border fault at the southernmost gulf of suez the amount of retreat varies inversely with the sediment thickness exposure and erosion of basement are initiated by the retreat of the escarp ment and the depth of erosion as indicated by fission track ages increases with distance from the escarpment these observations are explained by a model in which erosion along the gulf of suez is initiated as rift flank uplift becomes sufficiently large to expose the friable sandstones undercutting of the escarpment and exhumation of basement has been propagating northward and westward for at least 20 m y the average rate of scarp retreat has been 6 km m y and the along strike propagation of the erosion has been 12 km m y the diachronous erosion of the the rift flanks at the gulf of suez highlights the importance of distinguishing between the timing of uplift and of erosion both thermochronometric and stratigraphic data primarily indicate the timing of erosion which may differ significantly from the timing of the uplift that initiates it they must be interpreted carefully to avoid erroneous conclusions about rift tectonics

integrated provenance and detrital thermochronology studies on the andrill and 2a drill core late oligocene early miocene exhumation of the transantarctic mountains southern victoria land antarctica terra nova 22 361 368 2010 abstract detrital apatite fission track data combined with provenance analysis of gravel fraction detritus are used in this study to investigate the neogene exhumation of the west antarctic rift system samples have been collected from the upper 1000 m of the andrill and 2a drill core which documents the miocene history of the victoria land basin most samples have apatites with fission track ages between 23 and 36 ma and gravel clasts derived from the erebus volcanic province and the transantarctic mountains exposed along the southern victoria land coast these results indicate the existence of a so far undetected oligocene early miocene exhumation phase in the region between the blue koettlitz and skelton mulock glaciers suggesting the importance of fluctuation of local glaciers in the royal society range and or expansions into the mcmurdo sound of an ice sheet mainly sourced by larger outlet glaciers located further south

a detrital apatite fission track study of the ciros 2 sedimentary record tracing ice pathways in the ross sea area over the last 5 ma

fluvial origin of the valley system in northern victoria land antarctica from quantitative geomorphic analysis a network of sinuous valleys with typical dendritic pattern characterizes northern victoria land nvl in antarctica subparallel to parallel and angular to rectangular patterns are also present quantitative geomorphic analysis of the valley network has been carried out utilizing gis spatial analysis while drainage densities and drainage frequencies show low values segments of the nvl valley network are substantially well organized as indicated by bifurcation ratio r b and direct bifurcation ratio r bd parameters all basins faithfully adhere to horton s laws of drainage network composition quantitative geomorphic analysis suggests that the valley system can be ascribed to fl uvial origin and that consequently a morphoclimatic system completely different from that of present day must have driven its carving the resulting data provide indications about the origin of the valley network and this invaluable information can be used for the reconstruction of earlier phases of glacial history and climatic and tectonic evolution of this signifi cant antarctic region fluvial erosion enhanced the denudation of the transantarctic mountains from at least 55 ma to at least the eocene oligocene boundary ca 34 ma fluvial basins adapted to the tectonic structure following the main regional fault systems a well developed alpine topography postdates the fl uvial morphology temperate glaciers were responsible for denudation until the late miocene parasitic glaciers presently mantle the previously sculpted topography present day glacial erosion is negligible and denudation has been exceedingly slow since 7 5 ma

tectonic and climatic signals from apatite detrital fission track analysis of the cape roberts project core records south victoria land antarctica abstract the cenozoic tectonic reorganization of the west antarctic rift system in the ross sea region occurred concurrently with a major change in the global climate system and a global reorganization of plate motions this region thus provides the opportunity to study in detail a range of geological issues dealing with tectonic and climate feedbacks during the late eocene early oligocene greenhouse icehouse transition at high latitudes with the aim to decipher tectonic vs climatic forcing in the ice sheet drainage evolution a detrital apatite fission track study was carried out throughout the cenozoic drill cored stratigraphic succession of the cape roberts project mcmurdo sound victoria land basin apatite fission track ages of detrital samples with depositional ages between 34 ma and 17 ma were decomposed into statistically significant age populations three age peaks were detected reflecting different bedrock provenance areas i a young peak p1 between 34 ma and 26 ma recording the signal of a source area exhumed with a constant denudation rate but absent in the proximal continental area and ii two peaks older than 40 ma that are instead compatible with thermochronological data available from the onshore bedrock p1 peak testifies a late oligocene early miocene denudation event and traces a southern provenance of a fraction of the sediments filling the cape roberts basin we propose that this denudation event might have been triggered by renewed n s transtensional shearing along the western shoulder of victoria land basin controlling glacial flow pattern and the associated sediment transport

the case for dynamic subsidence of the u s east coast since the eocene the dynamic subsidence of the united states east coast is addressed using the discrepancy between regional and global estimates of sea level elevation of paleoshorelines and adjoint models of mantle convection that assimilate plate motions and seismic tomography the positions of eocene and miocene paleoshorelines are lower than predicted by global sea levels suggesting at least 50 m and possibly as much as 200 m of subsidence since the end of the eocene dynamic models predict subsidence of the east coast since the end of eocene although the exact magnitude is uncertain this subsidence has been occurring during an overall global sea level fall with the eustatic change being larger than the dynamic subsidence this results in a regional sea level fall in the absence of land subsidence dynamic subsidence is consistent with the difference between eustasy and regional sea level at the new jersey coastal plain

long term rates of denudation in t he dry valleys transantarctic mountains southern victoria land antarctica bases on in situ produced cosmogenic 21ne abstract concentrations of cosmogenic 21 ne 21 ne c measured in quartz have been used to estimate long term rates of denudation for contrasting landscape components in the dry valleys area of the transantarctic mountains southern victoria land antarctica samples of beacon supergroup sandstones and granitic basement were collected from two contrasting landscape elements low relief high elevation surfaces and rectilinear slopes to assess variations in rates of denudation with topographic position the sample sites for rectilinear slopes were selected because of proximity to 40 ar 39 ar dated lavas and ash avalanche deposits all 21 ne 20 ne ratios are significantly greater than the atmospheric value and concentrations of 21 ne c were calculated from the measured 21 ne values assuming an atmospheric composition for the trapped component apparent exposure ages calculated from the concentrations of 21 ne c assuming no denudation since exposure range from 3 78 4 66 ma for samples from the high elevation plateau surface and 0 61 2 48 ma for samples from the rectilinear slopes exposure ages for 3 he c were 2 to 42 times lower than those derived from the abundances of 21 ne c because of preferential diffusive loss of 3 he c from the quartz lattice concentrations of 3 he c were therefore not used in the calculation of rates of denudation we interpret the abundances of 21 ne c as reflecting variations in the rates of denudation rather than exposure age in view of independent evidence for prolonged exposure 15 ma of bedrock surfaces at the sample sites calculated maximum rates of denudation range from 0 26 1 02 m ma 1 for the rectilinear slopes down to only 0 133 0 164 m ma 1 for the high elevation surface sites these rates are comparable to other estimates of denudation rates for the dry valleys derived from analyses of cosmogenic isotopes but are around two orders of magnitude lower than the long term mean rate over the past 50 ma estimated from fission track thermochronology combined with the preservation of volcanic deposits dating back to the mid miocene these 21 ne c data demonstrate that only minimal modification of the landscape has occurred in the dry valleys over at least the past 15 ma this significant conclusion supports the view that the east antarctic ice sheet has been essentially stable over this period rather than experiencing major fluctuations as late as the pliocene as has previously been suggested

the significance of scarp retreat for cenozoic landform evolution on the colorado plateau u s a since the beginning of the cenozoic period several hundreds of metres of the sedimentary cover have been removed from the colorado plateau palaeoclimatic considerations show that the colorado plateau has been dominated by dry climates throughout the cenozoic with the possible exception of the early palaeocene today in the still prevailing arid climate which strongly accentuates differences in rock resistance the relief shows a structurally controlled cuesta scarp topography in the slightly deformed strata of alternating resistance in examining whether the denudational efficiency of scarp retreat was sufficient to account for the wide erosional gaps in the sedimentary cover rates of scarp retreat were determined by using the information of dated volcanic material and by applying a new method which calculates the amount of retreat from the width of beheaded valleys of known age rates of retreat range from 0 5 to 6 7 km my 1 the results show that the rates of retreat are controlled by the thickness and resistance of the caprocks a model of cenozoic scarp retreat demonstrates that the rates of recession calculated for the scarps in upper cretaceous rocks were sufficient to bring them into their present positions from the centre of the monument uplift on the central colorado plateau the late eocene positions of the cliffs in the early tertiary formations give an indication of their maximal extent after the upper cretaceous sediments had been removed from the uplifts erosion cut through successively older rocks and activated scarps in stratigraphically lower positions scarp retreat can operate simultaneously and independently at different levels which enabled this erosional mechanism to remove great proportions of the sedimentary cover of the colorado plateau during the cenozoic

thermochronology of lower cretaceous source rocks in the idaho wyoming thrust belt lower cretaceous organic rich source rocks that are thermally mature to postmature crop out on the absaroka darby and prospect plates in linear belts that run parallel to the trace of the thrusts in the idaho wyoming portion of the idaho wyoming utah thrust belt although the common assumption is that burial by thrust plates and the synorogenic sediments derived from them have been responsible for thermal maturation of the organic rich strata commercial amounts of hydrocarbons have not been found in structural traps in this portion of the thrust belt in a companion paper burtner and nigrini demonstrated that gravity driven fluid flow in the idaho wyoming portion of the thrust belt was responsible for moving large amounts of heat from the depths of the early cretaceous foreland basin eastward toward the stable platform in this paper we demonstrate through the application of organic maturation indicators and a new refinement of the apatite fission track technique that this process heated lower cretaceous organic rich source rocks to temperatures sufficient to generate hydrocarbons hydrocarbon generation and migration occurred prior to the development of the thrusts that are often assumed to have played a major role in the generation and entrapment of hydrocarbons in this portion of the thrust belt

geochronology and geochemistry of porphyritic intrusions in the duolong porphyry and epithermal cu au district central tibet implications for the genesis and exploration of porphyry copper deposits abstract the duolong district in central tibet hosts a number of porphyry as well as high sulfidation epithermal copper gold deposits and prospects associated with voluminous calc alkaline volcanism and plutonism in this study we present new geochronological geochemical isotopic and mineralogical data for both economically mineralized and barren porphyritic intrusions from the duobuza and naruo porphyry cu au deposits zircon u pb analyses suggest the emplacement of economically mineralized granodiorite porphyry and barren granodiorite porphyry at naruo deposit took place at 119 8 1 4 ma and 117 2 0 5 ma respectively four molybdenite samples from the naruo deposit yield an isochron re os age of 119 5 3 2 ma indicating mineralization occurred synchronously with the emplacement of the early granodiorite porphyry at duobuza deposit the barren quartz diorite porphyry intruded at 119 5 0 7 ma and two economically mineralized intrusions intruded at 118 5 1 2 ma granodiorite porphyry and 117 5 1 2 ma quartz diorite porphyry respectively petrographic investigations and geochemical data indicate that all of the porphyritic intrusions were oxidized water rich and subduction related calc alkaline magmas zircons from the porphyritic intrusions have a wide range in the ehf 0 11 1 indicating that they were sourced from mixing of mantle derived mafic and crust derived felsic melts moreover the variation of trace element content of plagioclase phenocrysts indicates that the magma chambers were recharged by mafic magmas comparison of the composition of amphibole phenocrysts indicates the porphyry copper gold mineralization at duolong was generated in magma chambers at low crystallization temperatures and pressures 754 to 791 c 59 m to 73 mpa n 8 and under highly oxidizing conditions nno 2 2 to 2 7 n 8 in contrast barren intrusions were sourced from the magma chambers with higher crystallization temperatures and pressures 816 to 892 c 111 to 232 mpa n 22 that were less oxidizing nno 0 6 to 1 6 n 22 the requirement for a thermal contrast is supported by the declining of ti content in magnetite crystals in barren intrusions 12 550 to 34 200 ppm versus those from economically mineralized intrusions 600 to 3400 ppm moreover the v content in magnetite crystals from economically mineralized intrusions 990 to 2510 ppm is lower than those recorded from barren intrusions 2610 to 3510 ppm which might reflect the variation in oxidation state of the magma the calculated water solubility of the magma forming the economically mineralized intrusions 3 2 3 7 wt is lower than that of magma forming the barren intrusions 4 6 6 4 wt based on the chemical physical characteristics of economically mineralized magma our study suggests that the development of porphyry cu au mineralization at duolong was initiated by shallow level emplacement of a magma that crystallized at lower temperatures and pressures experimental studies show that copper and water solubilities in silicate melts decrease with falling temperatures and pressures indicating metals and ore forming fluids are more likely to be released from a magma reservoir emplaced at shallow crustal levels we propose the magnetite might be a convenient exploration tool in the search for porphyry copper mineralization because the variations in ti and v content of mineral concentrates and rock samples are indicative of barren versus mineralized intrusions

post orogenic evolution of the dabie shan eastern china from u th he and fission track thermochronology the dabie shan of eastern china is a 200 kilometers wide mountain range with nearly 2 kilometers of relief and is an archetype of deep ultrahigh pressure metamorphic rock exhumation despite its regional and petrologic importance little is known about the low temperature and post orogenic evolution of the dabie shan here we present apatite and zircon u th he ahe and zhe respectively and apatite fission track aft cooling ages from the dabie shan that constrain the patterns and history of exhumation over the last 115 myr on the scale of the whole orogen zhe and ahe ages are inversely correlated with mean elevation and are systematically younger in the core of the range these cooling ages were converted to exhumation rates assuming steady state erosion and accounting for topographic effects these results indicate that since the eocene flanks of the range have eroded at rates as low as 0 02 km myr while the range core has eroded at about 0 06 km myr even in the core of the range these recent exhumation rates are at least 10 to 20 times slower than those estimated for the initial stages of exhumation in the triassic jurassic in a 1 4 kilometer vertical transect in the core of the range all ages are positively correlated with elevation with zhe ages increasing from 76 to 112 ma aft from 44 to 70 ma and ahe from 24 to 43 ma we present a simple model for topographic correction of thermochronometric ages in vertical transects using the admittance ratio ratio of isotherm relief to topographic relief applied to the ahe age elevation relationship this yields tertiary exhumation rates of 0 05 to 0 07 km myr in the core of the dabie shan in good agreement with regional exhumation rate patterns finally age elevation relationships for all three chronometers in the vertical transect are consistent with a constant exhumation rate of 0 06 0 01 km myr since the creta ceous with a possible modest increase in exhumation rates as high as 0 2 km myr between 80 to 40 ma these data show no evidence for significant variations in exhumation rates over the last 115 myr as might be expected for decay of old topography or tectonic reactivation of old structures

cenozoic landscape evolution of the convoy range to mackay glacier area transantarctic mountains onshore to offshore synthesis on the basis of geomorphic mapping we reconstruct the landscape evolution of the convoy range to mackay glacier area of the rifted margin of the transantarctic mountains and compare the land record with that obtained from offshore marine sedimentary rocks in the ross sea three landform assemblages reflect 1 fluvial planation and dissection 2 local glaciation under temperate conditions and 3 overriding by the east antarctic ice sheet overall landscape evolution is typical of a passive continental margin contrasts in morphology between the convoy range and the adjacent dry valleys and royal society mountains reflect the varying location of the initial drainage divide in relationship to the rifted coast in a wider synthesis we draw the following conclusions for the 260 km long mcmurdo sector of the transantarctic mountains denudation since rifting at ca 55 ma has removed a wedge 4 7 km thick at the coast declining inland to 1 km most denudation occurred in the eocene from planation and incision of river valleys near the coast a subsequent pulse of denudation most rapid at 34 31 ma and declining until ca 17 ma coincided with further crustal extension and a change from cool temperate to polar climate during the same interval there was a progressive decrease in landscape modification by warm based glaciers and or rivers between 14 8 and 13 6 ma the maximum overriding antarctic ice sheet flowed northeastward across the mountains leaving meltwater features crossing high altitude saddles and areally scoured bedrock near the coast since 13 6 ma the landscape has seen little change under a hyperarid polar climate either by local glaciers or subaerially fragile middle miocene surficial deposits still survive there has been tectonic stability since the middle miocene

use of packrat middens to determine rates of cliff retreat in the eastern grand canyon arizona packrat midden data can be used to calculate rates of cliff retreat by relating midden age to the distance between cliff face and midden regression analysis using 14 radiocarbon dated packrat deposits from the mississippian redwall limestone in the eastern grand canyon suggests that the redwall has been retreating at an average rate of 0 45 m 103 14c yr this rate of cliff retreat which is comparable to other cliff retreat rates reported from arid environments implies that the colorado river cut through the redwall limestone in the vicinity of horseshoe mesa about 3 7 m y b p

the age and origin of the labyrinth western dry valleys antarctica evidence for extensive middle miocene subglacial floods and freshwater discharge to the southern ocean a 50 km long network of bedrock channels and scoured terrain occupies the ice free portion of a major trough that crosses the transantarctic mountains in southern victoria land the channels collectively termed the labyrinth emerge from beneath the margin of the east antarctic ice sheet wright upper glacier and are incised into a 300 m thick sill of ferrar dolerite at the head of wright valley upper and intermediate elevation erosion surfaces of the labyrinth exhibit striations and molding characteristic of glacial erosion channels and canyons on the lower surface are as much as 600 m wide and 250 m deep have longitudinal profiles with many reverse gradients and contain potholes 35 m deep at tributary junctions these characteristics are most consistent with incision from fast flowing subglacial meltwater estimated discharge is on the order of 1 6 2 2 10 6 m 3 s 1 our 40 ar 39 ar analyses of volcanic tephra from the labyrinth show that the chan nels are relict that major channel incision predates 12 4 ma and that the last major subglacial flood occurred sometime between 14 4 ma and 12 4 ma the most plausible origin for the labyrinth is erosion associated with episodic drainage of subglacial lakes in east antarctica one compelling possibility is that discharge of large volumes of sub glacial meltwater to the southern ocean and to the ross sea in particular may have coincided with and contributed to oscillations in regional and or global climate during the middle miocene

remagnetizations in late permian and early triassic rocks from southern africa and their implications for pangea reconstructions a paleomagnetic study of late paleozoic and early mesozoic sedimentary rocks from southern africa suggests wide spread remagnetization of these rocks samples of the mofdiahogolo formation in botswana and of the lower beaufort group in south africa were treated using thermal alternating field and chemical demagnetization the mofdiahogolo redbeds show a univectoral decay of the remanence revealing a characteristic direction of

a marine and terrestrial sirius group succession middle beardmore glacier queen alexandra range transantarctic mountains antarctica a succession of sirius group glacigene sediments which crop out along the western margins of the beardmore valley between cherry icefall and hewson glacier below the cloudmaker is designated as the stratotype of the cloudmaker formation this new formation overlies a multiply glaciated pavement dominion erosion surface cut into the precambrian goldie formation and is disconformably overlain by the meyer desert formation sirius group the cloudmaker formation comprises bedded and massive diamictons bedded sands and silts and laminated clays assemblages of foraminifera occur throughout the cloudmaker formation and indicate that these basal sirius group sediments were deposited in brackish glacial marine environments the general absence of diatoms suggest these marine waters were ice covered similar marine assemblages are also present in basal sirius group sediments at oliver bluffs dominion range recycled marine diatom assemblages in the sirius group at the latter locality indicate that the host sediments have an age of 3 8 ma pliocene the cloudmaker formation is placed in the pliocene although a latest miocene age for the basal sediments cannot be ruled out stratigraphie sedimentologic and paleontologic evidence suggests that beardmore valley was occupied by a fjord and tidewater glacier system that extended at least 165 km through the transantarctic mountains from the southwestern ross sea the stratigraphy of the cloudmaker formation consists of a succession of members separated by disconformities it is hypothesised that these strata were deposited by a dynamic valley glacier system that underwent a history of glacier advance and grounding alternating with glacier retreat and flotation over a marine water column a combination of fjord basin sediment filling and sea level oscillations may also have influenced the pattern of glacier ice advance and retreat within beardmore paleofjord the marine cloudmaker formation is overlain by the terrestrial diamicton dominated meyer desert formation at oliver bluffs the meyer desert formation diamictons are interbedded with fluvial and lacustrine sediments successions that contain in situ vascular plant fossils principally the southern beech nothofagus mosses and beetle remains a magellanic type flora and fauna occupied the coastal margins of the beardmore paleofjord the vertical transition from the basal marine cloudmaker formation to terrestrial meyer desert formation provides a sea level datum that can be used to assess the extent of post sirius group tectonic uplift uplift rates at the cloudmaker section 90 km inland from the transantarctic mountain front or rift shoulder margin in the queen alexandra mountain block are determined to be 429 or 350 m myr this assumes a total uplift of 1331 m for the uppermost marine sediments of the cloudmaker formation and maximum diatom based ages for the sirius group of 3 1 ma or 3 8 ma gross similarities in stratigraphy and interpreted paleoenvironments are apparent between the cloudmaker succession beardmore paleofjord and the upper miocene pliocene successions at the mouth of taylor paleofjord 800 km to the north contrasting present day elevational settings for these two widely separated marine successions indicates the post sirius rate of tectonic uplift for the transantarctic mountains has been significantly greater in the queen alexandra block

minimal pliocene pleistocene uplift of the dry valleys sector of the transantarctic mountains a key parameter in ice sheet reconstructions we propose that there has been little or no pliocene pleistocene uplift of the dry valleys sector of the transantarctic mountains on the basis of isotopic dating and mapping of in situ cinder cone deposits on the walls of taylor valley twenty seven 40ar 39ar incremental heating analyses on whole rock samples from subaerially erupted olivine basanite volcanic outcrops of known elevations define 14 eruptions ranging in age from 3 89 to 1 50 ma because taylor valley opens directly onto the ross sea these results show that any surface uplift during the past 2 57 m y was 300 m our conclusion of minimal uplift contradicts previous models of dramatic uplift up to 3000 m since 3 ma and associated hypotheses that uplift caused climatic cooling and growth of a polar east antarctic ice sheet

evidence of rapid cenozoic uplift of the shoulder escarpment of the cenozoic west antarctic rift system and a speculation on possible climate forcing the cenzoic west antarctic rift system characterized by cenozoic bimodal alkalic volcanic rocks extends over a largely ice covered area from the ross sea nearly to the bellingshausen sea it is bounded on one side by a spectacular 4 to 5 km high rift shoulder scarp maximum bedrock relief 5 to 7 km from northern victoria land queen maud mountains to the ellsworth whitmore horlick mountains jurassic tholeiites crop out with the late cenozoic volcanic rocks along the section of the transantarctic mountains from northern victoria land to the horlick mountains the cenozoic rift shoulder diverges here from the jurassic tholeiite trend and the tholeiites are exposed discontinuously along the lower elevation 1 2 km section of the transantarctic mountains to the weddell sea various lines of evidence no one of which is independently conclusive lead the authors as others have also suggested to interpret the following the transantarctic mountains part of the rift shoulder and probably the entire shoulder has been rising since about 60 ma at episodic rates of approximately 1 km m y most recently since mid pliocene time rather than continuously at the mean rate of 100m m y uplift rates vary along the scarp which is cut by transverse faults the authors speculate that this uplift may have climatically forcedmore the advance of the antarctic ice sheet since the most recent warm period they suggest a possible synergistic relation between episodic tectonism mountain uplift and volcanism in the cenozoic west antarctic rift system and waxing and waning of the antarctic ice sheet beginning about earliest oligocene time less

cenozoic noncoaxial transtension along the western shoulder of the ross sea antarctica and the emplacement of mcmurdo dyke arrays field data on cenozoic faults and the mcmurdo dyke arrays in the reeves glacier mawson glacier area victoria land antarctica allow us to support noncoaxial transtensional tectonics along the n s trending western shoulder of the ross sea dyke injection within a crustal scale right lateral strike slip shear zone is testified by magma filled tension gash like arrangements within some master fault zones and by the left stepping arrangements of dykes in the intrafault zones the noncoaxiality of deformation is shown by the re activation of many dyke walls as right lateral strike slip faults this suggests an increase in the strike slip component over time along the western shoulder of the ross sea our data support the relevance of transtensional to strike slip tectonics for triggering melting and controlling the geometry and modes of magma emplacement

tectonic and denudational history of the rennick graben north victoria land implications for the evolution of rifting between east and west antarctica 1 the tectonic depression of the rennick graben northern victoria land antarctica is a key element for deciphering the cenozoic separation history between east and west antarctica and evolution of the west antarctic rift system the geometry kinematics and timing of the boundary faults of the rennick graben are presented on the basis of a detailed structural study of the major fault segments combined with apatite fission track data the structural architecture of the rennick graben is controlled by distributed and partitioned right lateral strike slip faulting fission track data indicate that the last significant denudation stage of the rennick graben shoulders started from eocene times with increasing cooling and denudation since 40 50 ma a direct structural connection is then established between cenozoic denudation and the activation of the dextral strike slip regime in north victoria land during the cenozoic in this framework the rennick graben is interpreted as a localized tectonic depression generated as a displacement accommodation area of the regionally sized dextral strike slip shearing the results of this study 1 invalidate the commonly assumed link between mesozoic ross sea rifting and opening of the rennick graben 2 argue against orthogonal rifting models for the cenozoic evolution of the ross sea region and 3 provide constraints to the modalities and timing of the separation between the east antarctic craton and the west antarctica extensional province

geochronology of pediment surfaces in southern peru implications for quaternary deformation of the andean forearc abstract the geomorphology of the andean forearc has historically been viewed as an old remnant of a late miocene planar landscape with no significant active structures accommodating quaternary deformation however the well preserved sequence of planation surfaces and strath terraces developed within the forearc of southern peru provide evidence of recent uplift along the western margin previously the abandonment of these surfaces was attributed to uplift of late miocene age however abrupt changes in topography and drainage incision within the pediment surfaces indicate recent deformation we use in situ produced 10be to determine the exposure ages of these abandoned surfaces in order to derive the spatial and temporal extent of this tectonic activity our new results in contrast to previous work yield pediment surface ages of 119 ka 203 ka 278 ka 549 ka and 1003 ka indicating a youthful morphology additionally these surfaces are affected by steeply dipping active faults producing localized deformation which can be quantified incision rates based on the exposure ages of abandoned strath surfaces are on the order of 0 04 0 3 mm yr the data set presented here not only suggests significant active deformation within the forearc but also highlights a sharp contrast between the style of deformation observed on the eastern and western margins of the andes while the general notion is that active deformation is localized in the subandean fold and thrust belt our data support an emerging view where active deformation is occurring in the western margin as well

cenozoic range front faulting and development of the transantarctic mountains near cape surprise antarctica thermochronologic and geomorphologic constraints 1 the transantarctic mountains tam define the western flank of the west antarctic rift system cape surprise near the shackleton glacier in the central tam is the only location along the range s 3500 km length where upper paleozoic beacon supergroup strata are down faulted to near sea level previous studies have inferred a range front master normal fault accommodates extension and rock uplift across the tam front in this region the history of rock uplift is debated suggested as early as mesozoic typically cenozoic or even pliocene or younger structural observations apatite fission track aft thermochronology and geomorphologic mapping undertaken within the tam front east of the shackleton glacier indicate extension faulting and denudation was mostly late eocene late oligocene and likely into the early miocene an exhumed aft partial annealing zone is found at the coast and traced 50 km inland the base of that exhumed partial annealing zone indicates denudation accelerated at 40 ma near the coast and at 30 35 ma on the inland side of the tam front vertically offset aft isochrones across the tam front reveal a step faulted architecture rather than a single master fault the cumulative vertical offset of the 55 ma isochrone is 2 3 2 7 km compared to 2 4 2 6 km offset of beacon strata indicating that all significant normal faulting is cenozoic and not related to mesozoic extension within the west antarctic rift system denudation from 26 ma to 14 ma produced a locally preserved erosion surface within the tam front erosion surface remnants indicate the tam front has undergone minimal internal deformation and only 290 790 m of surface uplift along offshore faults since the middle miocene

microscale data to macroscale processes a review of microcharacterization applied to mineral systems microanalysis can provide rapid quantitative characterization of mineral systems that complements the field and core scale observations traditionally made in ore deposits we review recent innovations in microanalytical procedures and their application to studies of ore deposits case studies are presented examining how microanalysis can provide constraints on macroscopic processes within mineral systems synchrotron x ray fluorescence shows centimetre scale chemical variations associated with proximity to mineralization in samples from sunrise dam gold mine western australia pseudomorphs of igneous plagioclase and chemically driven recrystallization interpreted from electron backscatter diffraction suggest that the system was dominated by fluid driven brecciation with very little shearing both the fluid chemistry and fluid pressure evolved during a protracted sequence of vein formation and alteration accompanying gold mineralization a second case study of sulphide mineralogy at the mt keith nickel sulphide deposit western australia demonstrates how x ray computed tomography combined with trace element mapping can constrain the chemistry and dynamics of magmatic systems large scale interaction between silicate and sulphide melts shown by homogenous palladium enrichment in pentlandite leads to a large proportion of globular ores with a high nickel content increasing use of microanalysis in ore deposit geology is resulting in the constant reassessment of established models for ore genesis though a combination of micro and macroscale datasets

neogene glacial record from the sirius group of the shackleton glacier region central transantarctic mountains antarctica neogene glacigenic strata collectively referred to as the sirius group are widely distributed throughout the transantarctic mountains the group is particularly well exposed near the head of shackleton glacier 858109 to 858409s on roberts massif and at bennett platform these deposits are critical for examining the nature of former ice flow from the east antarctic ice sheet into the ross embayment the sirius group rests on a glacially grooved and striated pavement named herein the shackleton erosion surface the sub sirius group surface on roberts massif was of low relief and glaciogenic sediment was deposited on it as a sheet of uniform thickness at bennett platform stratigraphic sections attain a thickness of 110 m thick and are subdivided into the shackleton glacier formation maximum thickness 98 m and the overlying bennett platform formation 44 m separated by an unconformity a third older lithified diamictite containing wood fragments occurs as clasts within these formations and as boulders in the modern lateral moraines of shackleton glacier the dominant facies massive diamict is interpreted primarily as lodgement till other facies indicate glaciofluvial and glaciolacustrine deposition facies associations suggest deposition either by sliding temperate or by polythermal glaciers under much warmer conditions than those of today widespread large and small scale faulting has affected the sirius group and underlying rocks to the extent that inland exposures are over 500 m higher than those to the north over a distance of 30 km thus at the time of deposition of the sirius group the mountains were probably lower and the ice sheet was much thinner

long term and short term global cenozoic sea level estimates backstripping analysis of three continuously cored well dated boreholes from the new jersey coastal plain ocean drilling program odp leg 150x indicates a long term 10 8 n10 7 yr eustatic fall of 100 m since 55 ma early eocene and suggests short term 0 5n3 m y eustatic falls of less than 70 m eustatic estimates are calculated from residuals between the decompacted unloaded and paleodepth corrected records and tectonic subsidence assuming a cooling lithospheric plate because the residuals are similar among the three sites we interpret them as an approximation of the eustatic signal

variability in geochemical provenance and weathering history of sirius group strata transantarctic mountains implications for antarctic glacial history abstract the major element and trace element geochemistry of 57 diamicts and muds of the sirius group is evaluated to determine the provenance and stratigraphic relations of geographically widespread outcrops in the transantarctic mountains the origin and age of the sirius group are heavily debated because of the presence of marine diatoms inferred to indicate a reduced east antarctic ice sheet as recent as the pliocene a key question is whether glaciers transported sediments and the enclosed diatoms from the east antarctic interior to the transantarctic mountains major element and trace element ratios indicate that the greater part of the sirius group sediments have an east antarctic provenance and that a few high elevation deposits are locally derived the degree of chemical weathering of the materials in the sirius group expressed as the chemical index of alteration cia has a wide range 41 70 on the basis of geochemical provenance weathering intensity and morphostratigraphic position the sirius group can be subdivided into three subgroups which probably represent multiple glacial phases 1 sediments with a local provenance from high elevation outcrops on spurs and mountain summits 2000 m which contain abundant weathered materials 2 sediments in high elevated outcrops 2000 m with intermediate cias originating from an east antarctic ice sheet which was overriding parts of the transantarctic mountains and 3 sediments recovered within or on the margins of the present glacial troughs at 1500 1800 m with low cias and an east antarctic provenance comparison with the weathering history inferred from drilling records in the nearby victoria land basin reveals that glaciation in the transantarctic mountains probably commenced with small ice caps and alpine glaciers prior to the eocene oligocene transition and from the oligocene onward continued with multiple phases of continental glaciation these results and interpretations contribute considerably to the understanding of the landscape evolution of the transantarctic mountains the sirius group debate and the history of east antarctic glaciation

from the roots to the roof an integrated model for the neoarchean carajas iocg system brazil abstract the carajas mineral province located in northern brazil comprises the oldest iron oxide copper gold mineralizing system on earth and is one of the world s most significant cluster of large tonnage iocg deposits based on a critical assessment of the literature a new integrated model for the carajas neoarchean iocg deposits the model considers the hydrothermal alteration footprint and the rock textures the geometry of the mineralization and the ore assemblage and the isotopic and geochronological signature the majority of the deposits form in the hypozonal domain pervasive and well developed sodic magnesian and calcic iron hydrothermal alteration halos form through magmatic derived hydrothermal fluids the occurrence of hydrothermal orthopyroxene marialite in the ni rich gt 34 and salobo deposits extend further the p t boundaries to temperatures of up to 700 c and 5 kbars thus co is proposedrresponding to the root zone of the system epizonal deposits conversely are characterized by low temperature potassic biotite phlogopite propylitic and chloritic hydrolytic hydrothermal assemblages towards shallower crustal levels although a mantle contribution to the hydrothermal fluids is recognized the o b stable isotopic signature reveals the participation of surface derived basinal brines in the ore forming process the occurrence of polymetallic ooids in the alemao orebody igarape bahia deposit hosting identical hydrothermal alteration mineralogy to the chalcopyrite breccias of other iocg deposits in the carajas mineral province suggests that the mineralizing fluids may have reached the paleo surface of the carajas basin in the neoarchean representing the roof of the iocg system conversely the ore mineralogy is consistent throughout the vertical scheme magnetite is the only iron oxide formed and precipitate from iron rich hydrothermal alteration accompanying the calcic alteration similarly chalcopyrite represents the cu sulfide there is no transition to more oxidized assemblages towards shallower zones supporting that the overall system was relatively reduced since more oxidizing phases hematite bornite are not observed as part of the neoarchean iocg system thus the neoarchean carajas iocg system represents a crustal scale continuum of a vertical paleo hydrothermal system formed coevally to the voluminous bimodal magmatism and the formation of the world class banded iron formation hosted iron ore

analysis of antarctic glacigenic sediment provenance through geochemical and petrologic applications abstract the number of provenance studies of glacigenic sediments in antarctica has increased dramatically over the past decade providing an enhanced understanding of ice sheet history and dynamics along with the broader geologic history such data have been used to assess glacial erosion patterns at the catchment scale flow path reconstructions over a wide range of scales and ice sheet fluctuations indicated by iceberg rafted debris in circumantarctic glacial marine sediments it is notable that even though most of the bedrock of the continent is ice covered and inaccessible provenance data can provide such valuable information about antarctic ice and can even be used to infer buried rock types along with their geo and thermochronologic history glacigenic sediments provide a broader array of provenance analysis opportunities than any other sediment type because of their wide range of grain sizes and in this paper we review methods and examples from all size fractions that have been applied to the antarctic glacigenic sedimentary record interpretations of these records must take careful consideration of the choice of analytical methods uneven patterns of erosion and spatial variability in sediment transport and rock types which all may lead to a preferential identification of different elements of sources in the provenance analyses because of this we advocate a multi proxy approach and highlight studies that demonstrate the value of selecting complementary provenance methods

site formation at kudu koppie a late earlier and middle stone age site in northern limpopo province south africa in the mapungubwe national park near the confluence of the shashe and limpopo rivers along south africa s northernmost borders with botswana and zimbabwe the site of kudu koppie is characterized by three lithologically and archaeologically distinct stone age units from bottom to the top these units are 1 the lower kudu koppie unit lkku which includes large tools such as handaxes picks and cleavers characteristic of a late earlier stone age phase 2 the middle kudu koppie unit mkku which contains bifacially retouched points characteristic of a middle stone age msa industry and 3 the upper kudu koppie unit ukku which has sporadic segments and other tools characteristic of the later stone age lsa a refitting and nodule analysis which matches lithic pieces based on microscopic similarities in colour texture and other visible characteristics demonstrates that site formation processes have caused some vertical displacement of material within but not between the lkku and mkku within the national park the kudu koppie sandstone outcrop is unusual in that it has an overhanging structure which undoubtedly contributed to the initial formation and eventual preservation of the archaeological deposits this paper presents several lines of evidence indicating that stratigraphic integrity at kudu koppie has been preserved and that the three horizons are in primary or near primary context

influence of rock strength properties on escarpment retreat across passive continental margins we examine the influence of bedrock strength properties imparted by jointing and fracturing on the retreat of erosional escarpments across passive continental margins two types of escarpment are identified and focus is placed on the gorge head knickpoint form of escarpment where upland streams drain across the escarpment along the macleay river system of eastern australia major knickpoints associated with escarpment retreat are currently located about 200 km upstream from the river mouth despite differences in bedrock lithology and order of magnitude differences in drainage areas upstream of the gorge heads implying an average escarpment retreat rate of 2 km m y since the margin formed 85 m y ago field observations from apsley river gorge a macleay river tributary indicate that bedrock jointing profoundly affects the evolution of the gorge headwall and sidewalls where dominant jointing dips into hillslopes failure occurs by block and column toppling conversely where jointing dips out of hillslopes mass wasting occurs by joint parallel rock sliding allowing slopes to remain steep as material is shed from the gorge sidewalls the toppling slopes develop lower overall gradients the gorge becomes asymmetric in cross section and the channel remains nearer to the steeper sidewall affected by joint parallel sliding coupling between fluvial and hillslope processes is essential to maintain headward propagation of the knickpoint gorge head and to facilitate mass wasting from the gorge sidewalls peak stream discharge across the gorge head must exceed a threshold sufficient to transport debris away from the gorge head to keep the knickpoint rock face free of talus to incise the stream channel and to erode the toes of the sidewalls if these conditions on fluvial processes are met then hillslope processes including rockslope failure mechanisms are the rate controlling processes for knickpoint propagation and therefore determine the rate of escarpment retreat across the passive margin

petrogenesis of the rambler rhyolite formation controls on the ming vms deposit and geodynamic implications for the taconic seaway newfoundland appalachians canada the ming cu zn ag au volcanogenic massive sulfide vms deposit is hosted by the upper cambrian lower ordovician rambler rhyolite formation which consists of a folded northeast plunging felsic dome complex developed in the uppermost segment of the obducted supra subduction baie verte oceanic tract in the north central newfoundland appalachians the deposit is overlain by interstratified mafic volcanic flows and volcaniclastic rocks of the snooks arm group the upper 1 km of the rambler rhyolite formation consists of a coherent felsic lithofacies at its base overlain by a quartz bearing volcaniclastic dominated lithofacies units 1 2 and 1 3 geochemically these rocks are light rare earth element lree and large ion lithophile element lile enriched la yb pn 4 20 th 2 58 4 05 ppm and high field strength element hfse depleted y 5 64 6 59 ppm rhyodacite with calk alkalic affinities th yb 2 zr y 7 the last pulse of felsic volcanism unit 1 3 is intimately associated with the massive sulfide lenses and has fi and fii type rhyolite signatures high la yb ratios commonly attributed to deep 30 km crustal melting the end t values of the rhyolites range from 2 5 to 1 3 indicating the rocks were influenced by crustal material a 10 percent batch partial melt of a garnet amphibole metamorphosed normal mid oceanic ridge basalt n morb and island arc tholeiite iat crustal material can generate a melt similar to the felsic rocks hosting the ming deposit however the iat reproduces much better the absolute abundances of lile th sr hfse nb zr y and middle rare earth elements mree of the felsic rocks it is interpreted that the felsic melt came from melting of a subducted slab with iat affinity thus explaining the depth of melt generation and fi fii signatures of the felsic rocks moreover the slab derived siliceous melt may also have contributed magmatophile elements for example ag au se te sn sb hg into the hydrothermal system once it had reached near surface depths and may explain the epithermal suite element enrichment in the ming deposit this study shows that strongly fractionated felsic volcanic rocks associated with boninites such as those associated with the ming vms deposit can be important hosts for precious metal enriched vms and may be as prospective as tholeiitic felsic volcanic sequences for example fiii fiv type rhyolite low la yb ratios the base of the snooks arm group is comprised of spatially restricted sulfide bearing mafic breccia overlain by a regionally extensive sedimentary sequence end t 3 1 to 5 5 which is in turn overlain by interstratified high mg basalt end t 1 6 th enriched back arc basin basalt babb enriched mid oceanic ridge basalt e morb end t 6 6 and lree enriched low ti tholeiitic tuffs end t 0 5 four generations of mafic to intermediate dikes and sills cross cut the rambler rhyolite formation and share similar geochemical characteristics to the extrusive rocks of the snooks arm group suggesting comagmatic relationships the combination of variations in nb yb th yb and end t values within and between units suggest melts derived from depleted to enriched mantle material with melts synchronously generated by both slab metasomatized mantle wedge and upwelling back arc asthenosphere

bedrock platforms within the ross embayment west antarctica hypotheses for ice sheet history wave erosion cenozoic extension and thermal subsidence ice penetrating radar mostly airborne and marine seismic surveys have revealed plateaus and terraces about 100 350 m below sea level beneath parts of the ross embayment including the west antarctic ice sheet the ross ice shelf and the eastern ross sea these surfaces cover many thousands of square kilometers and are separated by bedrock troughs occupied by the west antarctic ice streams prominent plateaus are under edward vii peninsula siple dome and roosevelt island marine seismic data and gravity data over the buried plateaus support an interpretation that they were caused by erosion into basement the flat and level nature of the surfaces that were formed by erosion are near the same depth over large distances and fringe the buried rugged bedrock topography of marie byrd land supports an interpretation of marine rather than glacial erosion marine seismic reflection profiles over one of the plateau remnants show it as an angular unconformity cut into gently dipping sediments of late oligocene age draped with thin flat lying sediments implying that the plateau is early miocene or younger the younger limit for plateau erosion follows from the interpretation of the mechanism erosion must predate permanent ice along the coast associated with the growth of the west antarctic ice sheet probably prior to 10 ma and possibly prior to 14 ma the plateaus along the siple coast with depths around 350 meters do not rebound to close to sea level and wave base for models of removing past and present ice load this discrepancy can be explained if ross embayment lithosphere has been cooling and subsiding since significant extension in eocene to oligocene time our interpretation requires crustal subsidence in the ross sea which in turn implies higher bedrock elevations in the past within this region higher standing topography presented an opportunity for the accumulation of local ice sheets and caused sea level fluctuation prior to late cenozoic time

surface uplift uplift of rocks and exhumation of rocks uplift of the surface of mountain belts requires forces that are comparable in magnitude to those associated with plate motion and therefore determination of rates of surface uplift could provide important information on the dynamics of mountain ranges rates of uplift of the surfaces of mountain ranges have not however been quantified sufficiently well that they provide useful constraints on those processes many reports off surface uplift in mountain ranges are based on mistaking exhumation of rocks or uplift of rocks for surface uplift and provide no information whatsoever on the rates of surface uplift

ross sea mylonites and the timing of intracontinental extension within the west antarctic rift system there are few direct constraints on the timing and style of faulting in the ross sea sector of the west antarctic rift system although cretaceous plate reconstructions indicate that ross sea extension between east and west antarctica occurred prior to breakup of the gondwana margin ca 80 ma mylonitic gneisses dredged from the eastern ross sea indicate shear zone deformation considerably earlier at 98 95 ma strain analysis of fab rics indicates 85 100 extension overprinting brittle structures record translation of shear zone gneisses into the upper crust samples yield sensitive high resolution ion microprobe u pb zircon ages of 102 97 ma correlated to byrd coast granite onshore and concordant 40 ar 39 ar biotite and k feldspar ages of 98 95 ma indicating that granites were mylonitized soon after emplacement and cooled rapidly apatite fission track data corroborate this rapid cooling event and reveal a second rapid cooling event ca 80 ma evidence for contemporaneous deformation and a similar thermal evolution at deep sea drilling project site 270 on the ross sea central high and for a migmatite dome on land attests to the regional extent of intracontinental extension extension occurred at a time of complex microplate interactions along the cretaceous active gondwana margin sug gesting that distributed deformation in the overriding antarctic plate may be related to plate boundary dynamics

cenozoic erosion of the transantarctic mountains a source to sink thermochronological study abstract the formation of the transantarctic mountains tam is strictly related to the evolution of the west antarctic rift system but the timing of their exhumation is still not fully assessed in this work we provide new apatite fission track data collected on the region between the royal society range and the britannia range cooling ages are late eocene oligocene in the center of the region but they get older both northwards and southwards we infer that exhumation was strictly controlled by tam parallel fault strands that were active after the oligocene the royal society range and the britannia range represent transition zones corresponding to transverse structures probably inherited from early basement crustal discontinuities and reactivated as transfer regions during rift propagation the exhumation of the investigated region has been then modeled and predicted thermochronological ages have been compared with detrital data from the miocene sedimentary succession drilled in the victoria land basin results indicate that this sector of the tam is the most probable candidate for the source of sediments and that during the neogene 3 km but up to 5 km of rocks was exhumed

landscape evolution of the dry valleys transantarctic mountains tectonic implications there are different views about the amount and timing of surface uplift in the transantarctic mountains and the geophysical mechanisms involved our new interpretation of the landscape evolution and tectonic history of the dry valleys area of the transantarctic mountains is based on geomorphic mapping of an area of 10 000 km2 the landforms are dated mainly by their association with volcanic ashes and glaciomarine deposits and this permits a reconstruction of the stages and timing of landscape evolution following a lowering of base level about 55 m y ago there was a phase of rapid denudation associated with planation and escarpment retreat probably under semiarid conditions eventually downcutting by rivers aided in places by glaciers graded valleys to near present sea level the main valleys were flooded by the sea in the miocene during a phase of subsidence before experiencing a final stage of modest upwarping near the coast there has been remarkably little landform change under the stable cold polar conditions of the last 15 m y it is difficult to explain the sirius group deposits which occur at high elevations in the area if they are pliocene in age overall denudation may have removed a wedge of rock with a thickness of over 4 km at the coast declining to 1 km at a point 75 km inland which is in good agreement with the results of existing apatite fission track analyses it is suggested that denudation reflects the differences in base level caused by high elevation at the time of extension due to underplating and the subsequent role of thermal uplift and flexural isostasy most crustal uplift 2 4 km is inferred to have occurred in the early cenozoic with 400 m of subsidence in the miocene followed by 300 m of uplift in the pliocene

late neogene sirius group strata in reedy valley antarctica a multiple resolution record of climate ice sheet and sea level events late neogene sirius group strata from tillite spur and quartz hills in the reedy glacier area antarctica demonstrate the variability in sirius group facies and contrasts sirius group strata deposited at high and low paleo elevation respectively the tillite spur and quartz hills formations pliocene are formally defined here the tillite spur formation type section crops out on the edge of the wisconsin plateau overlooking tillite spur it comprises 32 m of alternating coarse gray conglomerate and muddy olive brown diamictites the quartz hills formation type section crops out above the western margin of reedy glacier in a pre existing cirque towards the southern end of the quartz hills it comprises c 100 m of alternating massive diamictites and rhythmically interbedded sandstone and laminated mudstones which were deposited close to sea level and subsequently rapidly uplifted 500 m myr 1 to their present elevation at c 1500 m three orders of paleoclimatic variability are recorded in the sirius group strata from reedy valley 1 recycled marine microfloras in glacial diamictites indicate intervals of marine incursion into the antarctic cratonic interior co occurring with reductions in the east antarctic ice sheet 2 an advancing and retreating paleo reedy glacier deposited a glacial interglacial sequence alternating on a 10 100 kyr scale 3 centimeter and millimeter stratification in strata of the quartz hills formation record annual kyr scale variability

evidence of a full west antarctic ice sheet back to the early oligocene insight from double dating of detrital apatites in ross sea sediments the west antarctic ice sheet is the most unstable component of the antarctic cryosphere its fluctuations are well documented since the pliocene but its behaviour over the last 35 ma is more controversial particularly during periods of past high global pco2 values similar to those predicted in future global climate scenarios here we present new u pb dating of detrital apatite grains previously dated by the fission track method from cape roberts project oligocene to pliocene marine sediments in the ross sea two past ice flow patterns were identified one formed by outlet glaciers sourcing short travelled apatites and one northerly directed bringing far travelled apatite grains the latter provides the first robust physical evidence for the presence and repeated expansion of an oligocene west antarctic ice sheet

early jurassic pillow lavas and palynomorphs in the karoo of eastern botswana volcanic rocks in conformable contact with fossiliferous sedimentary strata apparently present excellent possibilities for the calibration of the stratigraphical record by radiometric age determination unfortunately many continental sedimentary sequences are poor in zone fossils for example correlation of the desert sediments in the upper karoo sequence of southern africa relies on broad lithostratigraphical divisions1 although vertebrate fossils can also be used2 in some areas however precise dating of the onset of karoo vulcanicity is crucial to the description of gondwanaland here we note a highly exceptional area in eastern botswana where the top of the largely aeolian cave sandstone and the base of the overlying stormberg lavas can be dated by the combined use of palaeontological and radiometric methods fluvio lacustrine sediments within the lowest stormberg lavas are conformably overlain by basalts in places pillow lavas with a probable k ar minimum age of 181 myr early jurassic palynomorphs classopollis intrareticulatus suggest that the maximum age of these sediments is in the upper sinemurian between 195 and 200 myr

constraints on age erosion and uplift of neogene glacial deposits in the transantarctic mountains determined from in situ cosmogenic 10be and 26al 10 be and 26 al data from sandstone boulders in three neogene glacial deposits in the mcmurdo sound dry valleys region of southern victoria land antarctica indicate minimum exposure ages of approx 3 ma and maximum long term erosion rates of approx 5 12 cm m y supporting the suggestion that polar desert conditions have persisted in the dry valleys since at least late pliocene time variation of cosmogenic nuclide production rate with altitude also allows constraints on past uplift rates model calculations employing 10 be data indicate little or no uplift in the dry valleys region in the past 3 m y precluding rapid 1 km m y late pliocene uplift previously suggested for some parts of the transantarctic mountains

interpretation of u th he single grain ages from slowly cooled crustal terranes a case study from the transantarctic mountains of southern victoria land low temperature thermochronologic techniques e g apatite fission track aft thermochronology and u th he dating constrain near surface t t paths and are often applied to uplift denudation and landscape evolution studies samples collected in vertical profiles from granitic walls on either side of the ferrar glacier southern victoria land antarctica were analyzed using aft thermochronology and apatite u th he dating to further constrain the lowest temperature thermal history of this portion of the transantarctic mountains aft central ages vary systematically with elevation and together with track length information define a multi stage cooling denudation history in the cretaceous and early tertiary apatite u th he single grain age variation with elevation is not as systematic with considerable intra sample age variation although many complicating factors e g u and th rich micro inclusions fluid inclusions variation in crystal size particle ejection correction zonation and particle ejection correction implantation of he into a crystal or impediment of he diffusion out of a crystal and 147sm derived particles may contribute to age dispersion we found that variation in single grain ages correlated with cooling rate samples that cooled relatively quickly have less variation in single grain ages whereas samples that cooled relatively slowly 3 c m y or resided within an u th he partial retention zone heprz prior to more rapid cooling have a comparatively greater variation in ages decay of u and th via particle emission creates a 4he concentration profile dependent upon the initial parent u th within a crystal variation of single grain ages for samples with non homogeneous u th distributions will be enhanced with long residence time in the partial retention zone i e slow cooling because of the relative importance of loss via volume diffusion and loss via particle ejection with respect to the u th zonation and the grain boundary correction of ages for particle ejection ft correction factor typically assumes uniform u and th distribution within the crystal and when applied to a population of crystals with different u and th distributions will enhance the variation in ages most complicating factors listed above for apatite u th he ages result in ages that are too old we propose that if considerable variation in u th he single grain ages exists that a weighted mean age is determined once outlier single crystal ages are excluded using the criterion of chauvenet or a similar approach we suggest that the true age or most representative age for that age population lies between the minimum u th he age and the weighted mean age we apply this approach coupled with composite age profiles to better constrain the t t history of the profiles along the ferrar glacier significant intra sample variation in single crystal apatite u th he ages and other minerals dated by the u th he method should be expected especially when the cooling rate is slow the variation of u th he single crystal ages is therefore another parameter that can be used to constrain low temperature thermal histories