an oligocene palynoflora from the middle shoalhaven catchment n s w and the tertiary evolution of flora and climate in the southeast australian highlands abstract well preserved spores and pollen from a number of middle oligocene sedimentary formations in the middle shoalhaven catchment n s w are correlated with the upper nothofagidites asperus lower proteacidites tuberculatus zones of the gippsland basin victoria further confirmation of this correlation comes from three sources a the relative abundances of pollen component groups reveal the nothofagidites proteacidites and myrtaceidites groups within the shoalhaven assemblage to be typical of late eocene to middle oligocene sequences b comparisons with the ratios of phyllocladidites mawsonii to gymnosperms araucariacites to gymnosperms n flemingii to other nothofagidites and myrtaceae to nothofagidites in the murray basin indicate at least an upper n asperus age and c strong geomorphic and stratigraphic evidence reported elsewhere shows that the spore and pollen bearing sediments were deposited in a basalt dammed lake between 29 32 ma the mid oligocene shoalhaven assemblage fills a significant gap in the spore pollen record of the southeast australian highlands and as a consequence there is now a continuous record of development of flora and climate for this region for nearly the entire tertiary average annual temperature and thermal seasonality appear to have been the two most significant climatic parameters to have varied throughout the southeastern highlands from the palaeocene to the miocene after which progressively drier conditions ensued

constraints on early cenozoic underplating driven uplift and denudation of western scotland from low temperature thermochronometry apatite u th he and fission track data from profiles in western scotland constrain the timing and magnitude of denudation during the early cenozoic when the north atlantic region was the site of intense magmatic activity related to the proto icelandic plume apatite helium ages vary from 77 8 to 265 27 ma 2 at sgorr dhonuill ballachulish and from 104 10 ma to 166 17 ma at clisham outer hebrides at both locations apatite fission track aft ages are older than the corresponding he ages at clisham they vary from 189 28 ma to 242 26 ma and from 186 6 ma to 257 12 ma at sgorr dhonuill apatite he ages increase linearly with elevation suggesting that the cooling rate remained constant in the late mesozoic however the apatite he age profile requires a period of rapid cooling after 100 ma apatite he ages predicted from the aft derived thermal histories are indistinguishable from measured he ages for a rapid cooling event of 1 to 10 myr duration between 61 and 47 ma at sgorr dhonuill and 65 to 49 ma at clisham the combined apatite ft and he derived thermal histories constrain the early cenozoic geothermal gradient at 39 9 c km at sgorr dhonuill and 19 6 c km at clisham amounts of denudation related to the rapid cooling event vary from 1330 230 m at sgorr dhonuill to 2250 750 m at clisham in agreement with models that predict greater amounts of denudation where magmatic underplating is thicker however the direct correlation between underplating driven surface uplift and denudation may only be apparent and a more complex link between spatial variation of surface uplift and denudation is suggested the integration of results from multiple low temperature thermochronometers combined with inverse and forward modelling provides a convincing and quantitative method to deduce onshore erosional histories and provides critical information about the spatial distribution of erosion that cannot be derived from the offshore record

depositional architecture of growth fault related wave dominated shelf edge deltas of the oligocene frio formation in corpus christi bay texas abstract growth faults within the frio formation define six subbasins on the south texas shelf and add to the complexity of the sediment dispersal along the shelf margin these growth faults influenced sediment pathways controlled sediment partition and provided increased accommodation for deltaic depocenters rollover and thickening of sediments occur on the downthrown side of growth faults with offsets of at least 150 m and up to 750 m growth strata are indicated by intervals that thicken landward by several tens of meters from anticline crests individual 4th order 100 500 ky regressive cycles expand about five to seven times across growth faults expansion ratios as great as 10 are observed the shelf edge is associated with the largest displacement expansion ratios and thickness of prograding deltaic sequences sedimentary structures and trace fossil associations identified in cores from nueces encinal and red fish bay subbasins indicate deposition on a wave dominated deltaic shoreline the cores exhibit highly bioturbated muddy sandstones alternating with decimeter thick clean sandstones with hummocky cross stratification alternation of fair weather wave deposits with sandy storm beds in two successive fault zones suggests that all subbasins developed under similar conditions during growth fault development high sediment supply conditions favored accumulation of thicker sediments on the downthrown sides of the faults wave dominance of deltas on the outer shelves in the growth compartments suggest that building of the upper slope rather lowers slope margin and decreases the likelihood of the presence of sandy deep water fans correlation of about 700 well logs integrated with seismic data provided the areal distribution and relative timing of sediment filling of the growth faulted compartments to unravel the evolution of higher order fourth and fifth sequences affected by these growth faults core data were used to recognize depositional environments to calibrate lithology to well logs and to characterize seismic amplitude anomalies within this shoreface succession the clean sandy storm beds will make attractive hydrocarbon reservoirs especially on the crest of rollover anticlines in downthrown compartments these sandstone bodies pinch out seaward into open marine shelf mudstones and are sealed by overlying shelf mudstones deposited during transgressive episodes

approaches to identifying reservoir heterogeneity in barrier strandplain reservoirs and the opportunities for increased oil recovery an example from the prolific oil producing jackson yegua trend south texas abstract the jackson yegua barrier strandplain sandstone play is a prolific oil producing trend in south texas although it is characterized by low recovery efficiencies low oil gravity weak solution gas drive complex stratigraphy and structural complications contribute to low recoveries this is despite very high average porosity 31 and permeability 600 md there is potential for substantial reserve growth by targeting the remaining mobile oil resource seventy six west is a typical field of the jackson yegua play detailed reservoir characterization of this field indicates a highly complex reservoir architecture consisting of a mosaic of strike oriented barrier core shoreface and inner shelf shoal sandstones and mudstone rich back barrier lagoonal deposits that are cross cut by dip oriented tidal inlet fills and fluvial channels reservoir continuity is further disrupted by small scale 10 100 ft 3 30 5m normal faults and dip reversals production trends are controlled by the complex reservoir sandstone geometry and faults and eight discrete compartments are interpreted from the reservoir architecture structure and trends in oil and water production this internal reservoir heterogeneity provides potential for bypassed mobile oil in untapped compartments and partially swept zones in waterflood areas and thus defines targets for infill drilling and highlights the opportunity to optimize waterflood design a new field discovery was made and a potential field extension also defined the case study at seventy six west is applicable to all jackson yegua fields and will ultimately help to increase oil recovery furthermore the approaches outlined here are not unique to barrier strandplain reservoirs such as the jackson yegua play and have application to all siliciclastic reservoirs where low recovery efficiencies indicate the likelihood of bypassed mobile oil

contourite drifts on early passive margins as an indicator of established lithospheric breakup the albian cenomanian breakup sequence bs offshore northwest iberia is mapped described and characterised for the first time in terms of its seismic and depositional facies the interpreted dataset comprises a large grid of regional 2d seismic reflection profiles complemented by industry and odp dsdp borehole data within the bs are observed distinct seismic facies that reflect the presence of a black shales and fine grained turbidites b mass transport deposits mtds and coarse grained turbidites and c contourite drifts borehole data show that these depositional systems developed as mixed carbonate siliciclastic sediments proximally and as organic carbon rich mudstones black shales distally on the northwest iberia margin mtds and turbidites tend to occur on the continental slope frequently in association with large scale olistostromes distally these change into interbedded fine grained turbidites and black shales showing widespread evidence of deep water current activity towards the top of the bs current activity is expressed by intra bs erosional surfaces and sediment drifts the results in this paper are important as they demonstrate that contourite drifts are ubiquitous features in the study area after aptian albian lithospheric breakup therefore we interpret the recognition of contourite drifts in northwest iberia as having significant palaeogeographic implications contourite drifts materialise the onset of important deep water circulation marking the establishment of oceanic gateways between two fully separated continental margins as a corollary we postulate the generation of deep water geostrophic currents to have had significant impact on north atlantic climate and ocean circulation during the albian cenomanian with the record of such impacts being preserved in the contourite drifts analysed in this work

geologic factors controlling co2 storage capacity and permanence case studies based on experience with heterogeneity in oil and gas reservoirs applied to co2 storage a variety of structural and stratigraphic factors control geological heterogeneity inferred to influence both sequestration capacity and effectiveness as well as seal capacity structural heterogeneity factors include faults folds and fracture intensity stratigraphic heterogeneity is primarily controlled by the geometry of depositional facies and sandbody continuity which controls permeability structure the permeability structure in turn has implications for co2 injectivity and near term migration pathways whereas the long term sequestration capacity can be inferred from the production history examples of gulf coast oil and gas reservoirs with differing styles of stratigraphic heterogeneity demonstrate the impact of facies variability on fluid flow and co2 sequestration potential beach and barrier island deposits in west ranch field in southeast texas are homogeneous and continuous in contrast seeligson and stratton fields in south texas examples of major heterogeneity in fluvial systems are composed of discontinuous channel fill sandstones confined to narrow sinuous belts these heterogeneous deposits contain limited compartments for potential co2 storage although co2 sequestration effectiveness may be enhanced by the high number of intraformational shale beds these field examples demonstrate that areas for co2 storage can be optimized by assessing sites for enhanced oil and gas recovery in mature hydrocarbon provinces

early tertiary palaeogeography landform evolution and palaeoclimates of the southern monaro n s w australia the age of the southern highlands of australia has long been controversial recent field investigations around bombala in southern new south wales have shed some light on this as well as on the early tertiary palaeoclimates in southern australia late palaeocene basalts were extruded on a landscape with a relief of at least 400 m and possibly as much as 800 m showing that the region had a substantial relief by the beginning of the tertiary fluvial and lacustrine sediments deposited on the earlier or prebasaltic landscape have been preserved by the basalt and contain palynofloras and fossil wood as do sediments deposited between basalt flows in the lower parts of the basalt pile the palynomorphs are representative of the lygistepollenites balmei zone in the upper late paleocene ca 58 60 ma the pollen and fossil wood indicate the region was covered by cool temperate rainforest podocarps and araucarian conifers were significant components of these forests in addition to nothofagus ferns were prominent in some localities the fossil wood is dominated by gymnosperms comparison of the palynofloras with bioclimatic data of a number of descendant extant species has shown that most belong to the mesotherm response category with mean annual temperatures in the range 14 20 c there is little evidence that frosts were significant data from tree rings support this and show that climates were markedly seasonal the annual rings are very thin suggesting cool climates their uniformity shows that conditions varied little from year to year

solving the production diffusion equation for finite diffusion domains of various shapes part 2 application to cases with alpha ejection and non homogeneous distribution of the source abstract in this paper numerical methods derived in part i chem geol 2002 are expanded to accommodate the effects of emission and zonation of u and th as relevant for u th he thermochronology expressions for the required coefficients are derived for spheres finite and infinite cylinders and rectangular blocks for parent nuclide distributions which are either uniform or have stepwise zoning we show that the effects of emission and zonation on the he retention in a nonspherical crystal of modest aspect ratio can be approximated by a sphere of identical surface to volume ratio s v if the zonation is transposed on to the sphere the currently usual correction method is to perform first a diffusive calculation without considering emission and zonation and to multiply afterward with a correction factor that has been derived originally for nondiffusive calculations our results demonstrate that more accurate calculations yield different results and that the differences are often too large to be ignored in practice especially for thermal histories that include a protracted residence in the partial retention zone the difference between the usual correction and our correction can be as high as 20 for homogeneous sources for zoned crystals the difference can be even larger

sedimentological characterization of the sego sandstone nw colorado usa a new scheme to recognize ancient flood tidal delta deposits and implications for reservoir potential marginal marine deposits are typically excellent hydrocarbon reservoirs around the world certain depositional environments may provide natural selective physical processes that produce better reservoir rocks than others depending on associated energy hydraulic regime etc with this paper we present new detailed sedimentological petrographic and grain size data of ancient flood tidal delta deposits that provide means to characterize this environment in the ancient as well as to determine reservoir characteristics in this study we compare grain size grain and cement composition and the ratio of pore space to cement from thin sections between tidal shoreface and flood tidal delta facies of the sego sandstone in northwest colorado the intent of this comparison is to provide an evaluation of different physical processes and the degree of diagenesis related to different environments to better understand the relative controls on reservoir characteristics we find that flood tidal deltas show a statistically larger average grain size than either shoreface or other tidal facies also flood tidal deltas have a smaller ratio of pore space to cement shoreface facies have a larger ratio of pore space to cement overall than that of flood tidal delta or other tidal facies we propose that this may be due to chemistry and higher weathering rates in the backbay environment in contrast to the shoreface environment where higher mechanical reworking may lead to selective removal of chemically unstable minerals to test this hypothesis we conducted statistical analyses of these petrophysical and grain size characteristics our results show that flood tidal deltas are characterized by clean upper fine grained cross bedded sandstone that is well sorted and subrounded to rounded with a majority of landward paleocurrent directions and a presence of tidal bundles mud drapes and reactivation surfaces

using pyroxene microphenocrysts to determine cosmogenic 3 he concentrations in old volcanic rocks an example of landscape development in central gran canaria abstract determinations of cosmogenic 3he exposure ages and erosion rates in volcanic rocks older than a few hundred thousand years are complicated by the presence of radiogenic he in addition to the magmatic and cosmogenic he in phenocryst minerals however by analysing microphenocrysts that crystallised on or immediately prior to eruption that have not trapped magmatic he the three component problem can be eliminated and accurate determinations of cosmogenic 3he made in this study we perform three experiments using pyroxene microphenocrysts in basaltic clasts in the pliocene ayacata formation breccias gran canaria that demonstrate they are free of magmatic he exposure ages and erosion rates calculated from the cosmogenic 3he concentrations are combined with a geomorphological study to produce a tentative interpretation of landscape evolution in the mountainous interior of gran canaria long term steady state erosion rates of 14 24 mm ka 1 are recorded from bedrock erosional surfaces on a high plateau headwall retreat rates for a major drainage system of 1 6 m ka 1 have been constrained from the ca 225 ka exposure age of a boulder emplaced on slopes beneath the headwall strath terraces and boulders in a small canyon system yield much younger exposure ages of 47 43 ka

the denudation history of the onshore continental margin of se brazil inferred from apatite fission track data a suite of 68 new apatite fission track analysis results from the onshore rift margin in southeast brazil are presented some of the results reflect resetting related to the eruption of the parana continental flood basalt 130 ma and four others are directly related to late cretaceous magmatism 80 ma the majority of the results were obtained from precambrian basement rocks and sediments from the parana basin the fission track ages broadly increase inland from 60 90 ma on the coastal plain to 300 ma in the continental hinterland the track length distributions show a characteristic trend being negatively skewed at the coast and showing increasing bimodality inland except for the oldest samples which are more unimodal these trends do not reflect rift related heating and resetting but are the result of protracted denudation since the opening of the south atlantic the results are consistent with more than 3 km of exhumation on the coastal plain but little more than 1 km in the hinterland the topographic morphology in the northern region of the study area is complex and is clearly influenced by structure and bedrock lithology for the region further south model calculations of the isostatic response to the denudation suggest that isostatic rebound makes a minor contribution to the long wavelength topography unless the flexural rigidity is more than 1025 n m in this case the initial elevation of the rift margin would need to be 2 5 km in order to accommodate the amount of denudation inferred from the fission track data however it is possible that the effective flexural rigidity varies across the margin becoming greater in the continental interior alternatively the broad scale wavelength topography may be the result of magmatic underplating related to the parana continental flood basalt event if so estimates of the volume of magma generated would need to be revised upward by as much as a factor of 5

pattern and tempo of great escarpment erosion extensive geological geophysical structural and geochronological data suggest that the locations of great escarpments bordering passive margins are exceptionally stable and are probably determined by crustal structure together the data do not support the established paradigm of ongoing significant and parallel escarpment retreat rather thermochronologic data and sedimentary sequences in ocean basins suggest that initial tectonically controlled rift escarpments undergo rapid and significant erosion only during the earliest stages of seafloor spreading development of stable passive margin escarpments follows this period of intense erosion escarpments increase in sinuosity as embayments retreat more rapidly than interfluves measurements of 24 escarpments suggest that sinuosity and the rate at which it increases depends upon the location of maximum uplift the geometry of the preescarpment drainage system and margin age all data suggest that the location of passive margin escarpments does not change significantly over time

long term evolution of denudational escarpments in southeastern brazil article i nfo topographic relief in southeastern brazil consists of a sequence of stepped surfaces that formed after the frag mentation of gondwana during the cretaceous tertiary and quaternary tectonic pulses this region is drained by four major rivers within four major river basins with interfluves that contain denudational escarpments fault escarpments and mountain ranges this study presents an analysis of the long term evolution of two denudational escarpments the cristiano otoni and the sao geraldo steps which divide the river basins of the sao francisco doce and paraiba do sul rivers in southeastern brazil denudation rates were obtained through the measurement of mean concentrations of in situ produced cosmogenic 10

mesozoic and cenozoic tectonic history of the central chinese tian shan reactivated tectonic structures and active deformation 1 the present day topography of the tian shan range is considered to result from crustal shortening related to the ongoing india asia collision that started in the early tertiary in this study we report evidence for several episodes of localized tectonic activity which occurred prior to that major orogenic event apatite fission track analysis and u th he dating on apatite and zircon indicate that inherited paleozoic structures were reactivated in the late paleozoic early mesozoic during a cimmerian orogenic episode and also in the late cretaceous paleogene around 65 60 ma these reactivations could have resulted from the accretion of the kohistan dras arc or lithospheric extension in the siberia mongolia zone activity resumed in the late mesozoic prior to the major tertiary orogenic phase finally the ongoing deformation which again reactivates inherited tectonic structures tends to propagate inside the endoreic basins that were preserved in the range leading to their progressive closure this study demonstrates the importance of inherited structures in localizing the first increments of the deformation before it propagates into yet undeformed areas

mesozoic exhumation history and palaeolandscape of the iberian massif in eastern galicia from apatite fission track and u th he data apatite fission track aft and u th he ahe data combined with time temperature inverse modelling reveal the cooling and exhumation history of the iberian massif in eastern galicia since the mesozoic the continuous cooling at various rates correlates with variation of tectonic boundary conditions in the adjacent continental margins the data provide constraints on the 107 timescale longevity of a relict paleolandscape aft ages range from 68 to 174 ma with mean track lengths of 10 7 2 6 to 12 6 1 8 m and ahe ages range from 73 to 147 ma fastest exhumation 0 25 km ma occurred during the late jurassic to early cretaceous main episode of rifting in the adjacent western and northern margins exhumation rates have decreased since then and have been approximately one order of magnitude lower across inland galicia the aft data are consistent with early cretaceous movement on post variscan ne trending faults this is coeval with an extensional episode offshore the ahe data in this region indicate less than 1 7 km of denudation in the last 100 ma this low exhumation suggests the attainment of a mature landscape during late cretaceous post rift tectonic stability whose remains are still preserved the low and steady rate of denudation prevailed across inland galicia despite minor n s shortening in the northern margin since 45 ma ago in north galicia rock uplift in response to nw strike slip faulting since early oligocene to early miocene has caused insufficient exhumation 3 km to remove the mesozoic cooling signal recorded by the aft data

controls on post mid cretaceous landscape evolution in the southeastern highlands of australia insights from numerical surface process models the tectonic and geomorphic evolution of the southeastern australian highlands has been a subject of controversy among geophysicists geologists and geomorphologists for several decades we employ a numerical surface process model that includes long range fluvial transport hillslope diffusion and landsliding in order to quantitatively assess the tectonic lithological and structural controls on the landscape evolution and denudation history of se australia we constrain fluvial erosion parameters for our model by fitting river profiles rockfall and landsliding are the most important processes controlling the concentration of erosion in river valleys we therefore model hillslope evolution using a very low diffusion coefficient and a threshold slope for landsliding the initial prebreakup topography of the highlands provides a fundamental control on their subsequent evolution together with the style and rate of fluvial incision secondary controls are exerted by lateral variations in lithology and local tectonic uplift along faults our modeling results indicate that 1 the observed highlands morphology requires that the drainage divide was established at its present location prior to opening of the tasman sea whether the divide is inherited from paleozoic orogeny or resulted from mesozoic syn rift uplift cannot be established however 2 in contrast to traditional interpretations escarpment retreat does not appear to be the fundamental process eroding the highlands and 3 the observed barbed river drainage may be imprinted as a result of lithological variation it does not necessarily evolve through capture the driving mechanism that is most consistent with the inferred uplift pattern of the southeastern highlands appears to be magmatic underplating

calibration of ga1550 biotite standard for k ar and 40ar 39ar dating abstract the mineral separate ga1550 biotite has become an international standard for k ar and 40 ar 39 ar dating studies although it was prepared as an intralaboratory standard at anu to monitor tracer depletion from a gas pipette it is one of a small number of samples that has been calibrated against 38 ar tracers some of which had been mixed with known amounts of atmospheric argon so that a so called primary calibration has been performed by measuring ga1550 biotite against additional tracers from the same batch we have determined the radiogenic argon content of this sample as 1 342 0 007 10 9 mol g and together with the measured k content of 7 645 0 050 weight percent we derive a best estimate for the k ar age as 98 5 0 5 ma where the error is derived from averaging the ages determined relative to the 38 ar tracer

post gondwana geomorphic evolution of southwestern africa implications for the controls on landscape development from observations and numerical experiments the relationship between morphology and surficial geology is used to quantify the denudation that has occurred across southwestern africa since the fragmentation of gondwana during the early mesozoic two main points emerge significant denudation of the order of kilometers is widespread except in the kalahari region of the continental interior the denudation is systematically distributed so that the continental exterior catchment draining directly to the cape basin is denuded to a greater depth than the interior catchment inland of the great escarpment the analysis also implies that the majority of the denudation occurred before the beginning of the cenozoic for both the exterior and interior catchments existing models of landscape development are reviewed and implications of the denudation chronology are incorporated into a revised conceptual model this revision implies that the primary effect of rifting on the subsequent landscape evolution is that it generates two distinct drainage regimes a marginal upwarp or rift flank uplift separates rejuvenated rivers that drain into the subsiding rift from rivers in the continental interior that are deflected but not rejuvenated the two catchments evolve independently unless they are integrated by breaching of the marginal upwarp if this occurs the exterior baselevel is communicated to the interior catchment that is denuded accordingly denudation rates generally decrease as the margin evolves and this decrease is reinforced by the exposure of substrate that is resistant to denudation and or a change to a more arid climate the observations do not reveal a particular style of smaller scale landscape evolution such as escarpment retreat that is responsible for the differential denudation across the region it is proposed that numerical model experiments which reflect the observational insights at the large scale may identify the smaller scale controls on escarpment development if the model and natural systems are analogous four numerical experiments are presented in which the roles of antecedent topography resistant substrate climate change and lowering the baselevel of the interior catchment are investigated for an initially high elevation margin bordered by an escarpment the model results suggest several styles of landscape evolution that are compatible with the observations escarpments may retreat in a regular manner but they also degrade and are destroyed only to reform at the drainage divide between exterior and interior catchments

equating the basal unconformity with the palaeoplain a model for passive margins abstract the palaeoplain is the terrestrial continental surface little changed from the landsurface that existed before continental breakup the basal unconformity occurs offshore separating older continental rocks from younger post rift sediments this paper considers the palaeoplain and the basal unconformity to be the same surface it presents a hypothesis for continental passive margins that integrates the geomorphology near the continental boundary with the offshore geomorphology by recognising the identity of the palaeoplain with the basal unconformity the hypothesis links the terrestrial story to the offshore story many details that were separated in earlier models come together in a single model continental rifting starts with a rift valley stage at this stage the pre breakup landsurface the palaeoplain at the edge of the continent is downwarped on the seaward side post rift sediments accumulate on the submerged palaeoplain which thus becomes the basal unconformity this model explains the parallel structures of continental divide present coast line and continental edge it also explains the creation of the basal unconformity in a short time and the comparative volumes of onshore erosion and offshore sedimentation onshore the model explains the modifications of drainage and the great escarpments that are characteristic of passive margins it also explains oldlands observed near the coast

co2 source sink matching in the lower 48 united states with examples from the texas gulf coast and permian basin documenting geographic distribution and spatial linkages between co2 sources and potential sinks in areas with significant levels of co2 emissions is important when considering carbon management strategies such as geologic sequestration or enhanced oil recovery eor for example the us gulf coast overlies a thick succession 6 000 m 20 000 ft of highly porous and permeable sandstone formations separated by thick regionally extensive shale aquitards the gulf coast and permian basin also have a large potential for eor in which co2 injected into suitable oil reservoirs could be followed by long term storage of co2 in nonproductive formations below reservoir intervals for example 6 billion barrels bbbl of oil from 182 large reservoirs is technically recoverable in the permian basin as a result of miscible co2 flooding the gulf coast also contains an additional 4 5 bbbl of oil that could be produced by using miscible co2 although the co2 pipeline infrastructure is well developed in the permian basin east texas and the texas gulf coast may have a greater long term potential for deep permanent storage of co2 because of thick brine bearing formations near both major subsurface and point sources of co2

cenozoic landscape development in the blue mountains se australia lithological and tectonic controls on rifted margin morphology abstract cenozoic landscape development on the southeastern australian rifted margin as recorded by mid tertiary basalt flows that preserve ancient landforms is generally considered to be very slow eocene miocene basalts of the southeastern australian highlands flowed down paleovalleys indicating that landscape dissection was already well under way at the time of their eruption within the deeply incised blue mountains however miocene basalts cap relatively flat hilltops suggesting that most incision postdates their emplacement we have studied the controls on these apparent lateral variations in cenozoic landscape development using both field observations and numerical models we have mapped the blue mountains basalts in detail to reconstruct the miocene landscape and to quantify both the amount of subbasalt relief and postbasalt incision rates new geochemical and k ar geochronological data indicate that most of the blue mountains basalts were derived from a common magma source and were erupted i

soil production on a retreating escarpment in southeastern australia the functional dependence of bedrock conversion to soil on the overlying soil depth the soil production function has been widely recognized as essential to understanding landscape evolution but was quantified only recently here we report soil production rates for the first time at the base of a retreating escarpment on the soil mantled hilly slopes in the upper bega valley southeastern australia concentrations of 10 be and 26 al in bedrock from the base of the soil column show that soil production rates decline exponentially with increasing soil depth these data define a soil production function with a maximum soil production rate of 53 m m y under no soil mantle and a minimum of 7 m m y under 100 cm of soil thus constraining landscape evolution rates subsequent to escarpment retreat the form of this function is supported by an inverse linear relationship between topographic curvature and soil depth that also suggests that simple creep does not adequately characterize the hillslope processes spatial variation of soil production shows a landscape out of dynamic equilibrium possibly in response to the propagation of the escarpment through the field area within the past few million years in addition we present a method that tests the assumption of locally constant soil depth and lowering rates using concentrations of 10 be and 26 al on the surfaces of emergent tors

reservoir facies architecture in a micro tidal barrier system frio formation texas gulf coast abstract barrier bar sand bodies are a complex mosaic of barrier core shoreface inlet fill tidal delta and back barrier facies in addition sandbody stratigraphy and internal depositional architecture are determined by the progradational aggradational or transgressive origin of the barrier complex the frio barrier strandplain system of the middle texas gulf coast has produced more than 3 billion bbl of oil examination of the greta glasscock and 41 a sands in west ranch field illustrates the variability of barrier reservoirs each reservoir is a mosaic of variably interconnected compartments having sheet tab pod or channel geometries conventional facies analysis isolith and log pattern mapping and limited core examination combined with semiquantitative delineation of hydrocarbon saturation distribution using resistivity logs defined the facies components of each reservoir the 41 a sand consists of juxtaposed progradational barrier core inlet fill and flood tidal delta units the glasscock sand is largely a transgressive barrier flat and washoverfan deposit the greta sand is a complex of aggradational barrier core and inlet fill facies productive attributes of each reservoir are influenced by its facies architecture and attendant relative permeabilities natural water drive is ineffective in the volumetrically restricted transgressive glasscock reservoir permeability distribution in the 41 a reservoir is facies defined erratic injection response irregular oil water contact advance and variable water oil ratios observed during the productive history of individual reservoirs document localized facies effects on fluid flow spatial variation of the gas oil ratio may also reflect facies distribution end of article last page 478

apatite u th he age constraints on the development of the great escarpment on the southeastern australian passive margin the southeast australian margin like other high elevation passive margins is characterised by a steep escarpment that separates a dissected coastal plain from a low relief inland plateau quantitative constraints on the generation of escarpments can be provided by apatite u th he ages here we use a coast perpendicular traverse across the coastal lowlands escarpment and plateau to test the three prevailing models of se australian escarpment formation namely retreat into a downwarped rift shoulder escarpment retreat and down wearing on high elevation rift shoulder with flexural rebound apatites from the coastal plain have he ages of between 87 and 112 ma suggesting that the coastal lowlands developed very rapidly after rifting and continental break up at 85 100 ma the he age data are inconsistent with the erosion of a downwarped rift margin and cannot be explained by a constant post break up rate of lateral escarpment retreat across the coastal plain or by constant down wearing the data require either rapid escarpment retreat or rapid in place excavation of the escarpment soon after break up in response to rifting and the lowering of base levels on the margin of the new continent at break up followed by a period of landscape stability and low erosion combined with the existing apatite fission track record the he data are consistent with erosion of 3 4 km within a maximum of 28 myr of break up at a minimum vertical erosion rate of 130 m myr 1 along the coast the rapid denudation period across the coastal plain in this region took less than 48 myr from the coast to the escarpment base which corresponds to an average vertical erosion rate of 45 m myr 1 this is equivalent to a mean escarpment retreat rate of 5 10 km myr 1 apatite he ages from the plateau 183 247 ma indicate that the highlands remained stable throughout continental break up experiencing average erosion rates of less than 10 m myr 1 since the late palaeozoic early mesozoic

modeling of the temperature sensitivity of the apatite u th he thermochronometer apatite u th he apparent ages will generally reflect residence for extended periods at temperatures where helium is neither quantitatively retained nor lost by diffusion to characterize the response of apatite he ages to thermal histories involving partial he retention we explored solutions to the he production diffusion equation under thermally static conditions the analytical solution to this equation coupled with published diffusivity data demonstrates that the zone of partial he retention extends from about 40 c to 85 c this zone lies at temperatures 35 c cooler than the analogous fission track partial annealing zone he ages within the partial retention zone ultimately achieve a balance between he production and loss yielding a steady state age both the ultimate age and the time it takes to achieve this age are temperature dependent for example an apatite held at 75 c equilibrates to an age of 2 ma after 17 myr regardless of whether equilibrium is approached from a higher or a lower initial he age for representative dynamic thermal histories we evaluated apatite he ages using a numerical solution to the ingrowth diffusion equation the results illustrate the sensitivity of he ages to various geologic histories and are useful for understanding he age elevation relationships and for testing time temperature paths derived from apatite fission track length distributions in addition although he diffuses rapidly from apatite at shallow crustal temperatures modeling of ambient temperature fluctuations indicates that he ages are nearly unaffected by surficial processes

applications of low temperature thermochronometry to quantification of recent exhumation in mountain belts exhumation is a primary phenomenon used to characterize geomorphic and tectonic histories the motion of rock with respect to the earth s surface is produced by erosion and tectonic denudation unroofing by erosion redistributes crustal mass and is a major accommodator of shortening in orogenic belts brandon et al 1998 zeitler et al 2001 willett et al 2003 tectonic denudation typically associated with crustal extension brings rocks to the surface at rapid rates from significant crustal depths hodges et al 1998 hacker et al 2003 vanderhaeghe et al 2003 quantifying exhumation using thermochronology is thus an important element in investigations of long term crustal scale geomorphic and tectonic processes the thermochronometers most appropriate for estimating exhumation related to landscape or neotectonic history are apatite u th he and fission track dating given their low closure temperatures and the typical geothermal gradient of the continents these cooling ages generally record exhumation from 1 4 km depths farley 2002 ehlers and farley 2003 naeser 1979 gleadow et al 1986 the accuracy of the exhumation history inferred from these data is limited by how well the spatially and temporally variant geothermal gradient is known or modeled mancktelow and grasemann 1997 ehlers and farley 2003 beyond these limitations thermochronology is often hindered by what the rocks in a given study area record cooling ages may be too old and predate the tectonic or erosional history of interest or topography accessibility bedrock lithology and exposure may hinder collection of a robust sample set in many studies experimental design is determined by logistics available funding and time and prior analytical training of the investigator thus some studies may not obtain the optimal results to address a given question although many papers deal with technical aspects of thermochronometry the purpose of this paper is to explore

influence of crystal size on apatite u th he thermochronology an example from the bighorn mountains wyoming near surface tectonic and geomorphic processes involve cooling of rocks through low temperatures 50 200 c because rates of helium diffusion in apatite titanite and zircon are sensitive to temperature variations in this range uranium thorium helium thermochronometry u th he dating is well suited to establishing the timing and rates of these processes in the geologic record however because fractional loss of he is controlled by crystal size such that larger crystals retain a larger fraction of radiogenic he u th he ages must vary not only with thermal history but also with crystal size here we present crystal size correlated he ages from co existing apatites from the bighorn mountains wyoming that range from 100 to 350 ma these correlations are a sensitive indicator of the rock s thermal history in a temperature range below the system s nominal closure temperature t c 70 c for apatite and are consistent with a thermal history involving residence in the upper 2 3 km of crust since the precambrian with maximum temperatures of 65 80 c just prior to laramide orogenic exhumation the influence of crystal size on he ages will be most apparent in rocks where temperatures have been in the range of partial he retention for long periods of time 30 70 c for 10 7 years in such cases accurate interpretation of u th he ages must incorporate the effect of crystal size and this method may provide insights to thermal histories of rocks in previously inaccessible low temperature ranges

slow rates of rock surface erosion and sediment production across the namib desert and escarpment southern africa slow erosion has characterized the namib desert the namibian escarpment and the adjacent namibian highlands over the pleistocene paired analyses n 66 of in situ produced 10be and 26al in quartz bearing samples of bedrock primarily from inselbergs of sediment from dry river and stream channels and of clasts from desert surfaces reveal large inventories of these cosmogenic nuclides indicating significant landscape stability over at least the past million years bedrock samples n 47 collected in three transects from the coast across the escarpment and into the highlands show no spatial pattern in elevation normalized nuclide abundance despite a difference in mean annual precipitation map between sample sites at the coast map 400 mm yr 1 average model erosion rates inland of the escarpment 3 2 1 5 n 9 are indistinguishable from average rates seaward of the escarpment 3 6 1 9 n 38 indicating that rock on the pedimented coastal plain is eroding at the same rate as rock in the highlands sediment samples n 3 from small streams suggest that the landscape as a whole is eroding more rapidly than the bedrock outcrops and that a basin in the steep escarpment zone is eroding several times faster 16 m my 1 than either a basin in the highlands 5 m my 1 or a basin in the coastal plain 8 m my 1 data from large rivers n 4 constrain erosion rates averaged over 105 yrs and 104 to 105 km2 between 3 and 9 m my 1 small quartz clasts n 12 collected from four desert surfaces record extraordinarily long variable and in some cases complex exposure histories simple 10be model ages are as high as 1 8 my some minimum total histories considering both 10be and 26al and including both burial and exposure exceed 2 7 my as a group the namibian cosmogenic data do not support the model of significant and on going escarpment retreat the similarity of erosion rates calculated from 10be analysis of fluvial sediments and longer term 107 yr average mass removal rates estimated by others using fission track analysis of rock suggests that namibian erosion rates have reached a steady state and are changing little over time at outcrop scales the concordance of 10be and 26al in most bedrock samples suggests that the model of steady uniform bedrock erosion is valid there is no indication of intermittent burial shedding of thick rock slabs or stripping of previous cover at an intermediate scale a transect of bedrock samples north of gobabeb demonstrates that the northern boundary of the massive namib sand sea has been steady and unshifting similarly low cosmogenically estimated erosion rates across west and central namibia suggest that the landscape is in geomorphic steady state its overall appearance changing only slowly through time

late cenozoic flexural deformation of the middle u s atlantic passive margin despite the century long recognition of regional epeirogeny along the middle atlantic passive margin relatively few studies have focused on understanding postrift uplift mechanisms here we demonstrate that epeirogenic uplift of the central appalachian piedmont and subsidence of the salisbury embayment represent first order flexural isostatic processes driven by continental denudation and offshore deposition our results show that regional epeirogenic processes present on all atlantic type passive margins are best resolved by specific stratigraphic and geomorphic relationships rather than topography a simple one dimensional geodynamic model constrained by well dated baltimore canyon trough coastal plain and lower susquehanna river piedmont stratigraphy simulates flexural deformation of the u s atlantic margin the model represents the passive margin lithosphere as a uniformly thick elastic plate without horizontal compressive stresses that deforms flexurally under the stress of strike averaged vertically applied line loads model results illustrate a complex interaction among margin stratigraphy and geomorphology the isostatic response to denudational and depositional processes and the modulating influence of exogenic forces such as eustasy the current elevation with respect to modern sea level of fluvial terraces and correlative coastal plain deposits or unconformities is successfully predicted through the synthesis of paleotopography eustatic change and margin flexure results suggest that the middle u s atlantic margin landward of east coast magnetic anomaly is underlain by lithosphere with an average elastic thickness of 40 km flexural rigidity d 4 1023 n m the margin experiences an average long term denudation rate of approximately 10 m m y and the piedmont has been flexurally upwarped between 35 and 130 meters in the last 15 m y long term isostatic continental uplift resulting from denudation and basin subsidence resulting from sediment loading are accommodated primarily by a convex up flexural hinge physiographically represented by the fall zone our results elucidate an inherent danger in using topography alone to constrain late stage passive margin deformation mechanisms only through careful synthesis of field stratigraphic and geomorphic elements such as fluvial terraces coastal plain deposits and offshore stratigraphy can age control be extended from the offshore depositional setting to the erosionally dominated continent this study demonstrates that despite a relatively subdued topography the middle u s atlantic margin experiences progressive flexural isostatic deformation similar to that proposed for high relief margins characterized by great escarpments thus margin topographic diversity remains a function of other factors such as lithospheric composition and or structure supracrustal stratigraphy and structure degree of drainage integration drainage divide migration and climate

numerical modeling of spatially variable precipitation and passive margin escarpment evolution abstract passive margin escarpments are long lasting landforms which can remain steep and high over tens of millions of years precipitation variability has seldom been considered as a contributor to passive margin escarpment evolution however large and distinct variability in precipitation rates with elevation is observed in mountainous landscapes we assess the potential impacts of spatially variable precipitation on passive margin escarpment evolution with a numerical model that couples a simple elevation dependent precipitation rule with the cascade surface processes model and a simple flexural uplift model modeled topography is more sensitive to the elevation of the precipitation maximum than to the effective elastic thickness of the lithosphere if precipitation is maximized at the escarpment crest the escarpment decreases in elevation slope and proximity to the coast more rapidly than if precipitation is focused at low elevations low elevation precipitation maxima contribute to increasing escarpment slope over time and preservation of a high elevation escarpment using tropical rainfall measuring mission trmm precipitation radar data we establish precipitation elevation relationships at high elevation passive margins across the tropics and document the occurrence of patterns similar to those included in our model despite uncertainty as to the robustness of precipitation patterns and the simplified tectonic and geomorphic framework of our model we conclude that spatial variability in precipitation is a potentially significant contributor to passive margin escarpment evolution

uplift and erosion of the san bernardino mountains associated with transpression along the san andreas fault california as constrained by radiogenic helium thermochronometry apatite helium thermochronometry provides new constraints on the tectonic history of a recently uplifted crystalline mass adjacent to the san andreas fault by documenting aspects of the low temperature 40 100 c thermal history of the tectonic blocks of the san bernardino mountains in southern california we have placed new constraints on the magnitude and timing of uplift old helium ages 64 21 ma from the large big bear plateau predate the recent uplift of the range and show that only several kilometers of exhumation has taken place since the late cretaceous period these ages imply that the surface of the plateau may have been exposed in the late miocene and was uplifted only 1 km above the mojave desert in the last few myr by thrusting on the north and south a similar range in helium ages 56 14 ma from the higher san gorgonio block to the south suggests that its crest was once contiguous with that of the big bear block and that its greater elevation represents a localized uplift that the big bear plateau did not experience the structure of the san gorgonio block appears to be a gentle antiform based on the geometry of helium isochrons and geologic constraints young ages 0 7 1 6 ma from crustal slices within the san andreas fault zone indicate uplift of a greater magnitude than blocks to the north these smaller blocks probably experienced 3 4 km of uplift at rates 1 5 mm yr in the past few myr and would stand 2 5 km higher than the big bear plateau if erosion had not occurred the greater uplift of tectonic blocks adjacent to and within the san andreas fault zone is more likely the result of oblique displacement along high angle faults than motion along the thrust fault that bounds the north side of the range we speculate that this uplift is the result of convergence and slip partitioning associated with local geometric complexities along this strike slip system transpression thus appears to have been accommodated by both vertical displacement within the san andreas fault zone and thrusting on adjacent structures

regional analysis of bedrock stream long profiles evaluation of hack s sl form and formulation and assessment of an alternative the ds form the equilibrium form of the fluvial long profile has been used to elucidate a wide range of aspects of landscape history including tectonic activity in tectonic collision zones and in continental margin and other intraplate settings as well as other base level changes such as due to sealevel fluctuations the hack sl form of the long profile which describes a straight line on a log normal plot of elevation normal versus distance logarithmic is the equilibrium long profile form that has been most widely used in such studies slope area analysis has also been used in recent years we show that the sl form is a special case of a more general form of the equilibrium long profile here called the ds form that can be derived from the power relationship between stream discharge and downstream distance and the dependence of stream incision on stream power the ds form provides a better fit than the sl form to river long profiles in an intraplate setting in southeastern australia experiencing low rates of denudation and mild surface uplift we conclude that if an a priori form of the long profile is to be used for investigations of regional landscape history the ds form is preferable in particular the ds form in principle enables equilibrium steepening due to an increase in channel substrate lithological resistance parallel shift in the ds plot to be distinguished from disequilibrium steepening due to long profile rejuvenation disordered outliers on the ds plot slope area analysis and the slope distance ds approach outlined here are complementary approaches reflecting the close relationship between downstream distance and downstream catchment area copyright 2006 john wiley sons ltd

forward modeling and interpretation of u th he ages soon after the initial studies on he diffusion in apatite pointed out the use of the u th he system for low temperature geochronology zeitler et al 1987 wolf et al 1996 warnock et al 1997 the potential value of forward modeling for refined interpretation of u th he dating was indicated wolf et al 1998 while the first studies provided evidence for the low closure temperature t c dodson 1973 of the u th he system in apatite for monotonously cooling systems the latter study investigated the sensitivity of the u th he system for a wide range of geologically feasible heating cooling scenarios forward modeling of he accumulation in minerals expanded the use of the u th he geochromometer to geological systems that do not fit one of the main conditions for the meaningful application of the closure temperature concept i e monotonous cooling at a constant rate through t c forward modeling can help to give a meaning to apparent u th he ages from rocks with cooling histories with protracted residence time in the helium partial retention zone heprz e g 40 85 c in apatite wolf et al 1998 where the closure temperature is undefined fig 1 figure 1 apatite he age evolution curves dotted lines for several representative time temperature paths solid lines forward modelling was conducted using the crank nicolson method for spheres of 60 m radius note that only for history 2 the closure temperature concept dodson 1973 is defined and the age obtained relates to passing through a point t c on the time temperature path for histories that are similar to 3 5 which know a protracted residence time in the heprz there is no direct geological meaning of he ages forward modelling is required to give a geological meaning to the apparent ages obtained from he and u th concentrations in samples used with permission of elsevier from wolf

denudation rates for the southern drakensberg escarpment se africa derived from in situ produced cosmogenic 36c1 initial results cosmogenic 36 c1 concentrations in basalt samples from the drakensberg escarpment on the se african passive margin give quantitative estimates of denudation and scarp retreat rates over the 10 4 10 6 year timespan addressed by these data the calculated escarpment retreat rate has been 50 95 m ma 1 and the average summit denudation rate 6 m ma 1 the scarp retreat rate is an order of magnitude less than previously suggested the rate of summit lowering is sufficient to prevent the long term intact survival of erosion cycle surfaces formed in the mesozoic that were previously inferred for this region

thermochronological data from sudan in the frame of the denudational history of the nubian red sea margin low temperature thermochronology provides information on the timing of rifting and denudation of passive margins and the red sea with its well exposed young rift margins is a suitable setting for its application here we present new apatite fission track aft data from sudan northern hinterland and red sea coastal areas from the former region we obtained ages between 270 2 ma ad 253 53 ma and from the coastal belt between 83 8 ma and 39 7 ma these data prompted a review and comparison with low temperature thermochronological data from the whole nubian red sea margin and a discussion on their implication in assessing the margin evolutionary style aft data are available for egypt and eritrea as well as apatite u th he ahe ages for two transects transversal to the margin in eritrea both in egypt and eritrea aft data record a cooling event at about 20 25 ma early miocene and an earlier more local cooling event in egypt at about 34 ma early oligocene the thermal modeling of the sudan samples provides an indication of a rapid cooling in miocene times but does not support nor rules out an early oligocene cooling phase the re assessment of new and existing thermochronological data within the known geological framework of the nubian and conjugate arabian margins favours the hypothesis that early rifting stages were affecting the whole gulf of suez red sea gulf of aden system since the oligocene these precocious more attenuated phases were followed by major extension in miocene times as to the mode of margin evolution aft age patterns both in egypt and eritrea are incompatible with a downwarp model the distribution of ahe ages across the eritrean coastal plain suggests that there the escarpment was evolving predominantly by plateau degradation copyright 2009 john wiley sons ltd

postrift history of the eastern central atlantic passive margin insights from the saharan region of south morocco the passive margin of south morocco is a low elevated passive margin it constitutes one of the oldest margins of the atlantic ocean with an early jurassic breakup and little geological data are available concerning its postrift reactivation so far we investigated the postrift thermal history of the onshore part of the margin with low temperature thermochronology on apatite crystals fission track and u th sm he ages we obtained are significantly younger than the breakup 190 ma fission track ages range from 107 8 to 175 16 ma with mean track lengths from 10 7 0 3 to 12 5 0 2 m u th sm he ages range from 14 1 to 185 15 ma using inverse modeling of low temperature thermochronological data we demonstrate that the south moroccan continental margin underwent a complex postrift history with at least two burial and exhumation phases the first exhumation event occurred during late jurassic early cretaceous and we attribute this to mantle dynamics rather than to intrinsic rifting related processes such as flexural rebound the second event from late cretaceous to early paleogene might record the onset of africa europe convergence we show a remarkably common behavior of the whole moroccan passive margin during its early postrift evolution the present day differences result from a segmentation of the margin domains due to the africa europe convergence finally we propose that varying retained strengths during rifting and also the specific crustal lithospheric geometry of stretching explain the difference between the topographical expressions on the continental african margin compared to its american counterpart

quantitative constraints on the rate of landform evolution derived from low temperature thermochronology in recent years the nature of the complex interactions between the solid earth and the overlying atmosphere has been the subject of much debate england and molnar 1990 whipple et al 1999 braun et al 1999 does erosion play an important role in the dynamics of mountain building is there a strong feedback into the balance of forces at the crustal scale from mass transport by erosional and depositional processes at the earth s surface or does erosion passively react to changes in the earth s surface topography to shape mountains with tectonic forces originating in the underlying convecting mantle dominating the dynamics of plate interactions and crustal deformation at convergent plate margins the answers to these fundamental questions lie in our ability to determine the rate at which surface processes can react to tectonic forcing whipple et al 1999 where erosion is efficient the coupling will be strong as the rate of change of surface loading by mass transport will be similar to the rate of creation of topography by crustal deformation this may lead to a concentration of deformation and thus exhumation in regions of intense erosion koons 1990 willett et al 1993 beaumont et al 2001 koons et al 2002 2003 braun and pauselli 2004 where erosion is relatively inefficient mountain building is the result of a balance between internal driving forces and those originating from the deflection of an upper free surface there is no feedback from erosional processes low temperature thermochronology such as u th he he or fission track ft dating in apatite and zircon provides information on the rate at which rocks cool as they are exhumed towards the cold earth surface duddy et al 1988 warnock et al 1997 farley 2000 reiners and farley 1999 because the

thermal evolution of rifted continental margins new evidence from fission tracks in basement apatites from southeastern australia the palaeozoic granitic basement of southeastern australia shows a regular pattern of apatite fission track ages which is dominated by a rapid decrease in apparent age towards the rifted continental margin apparent ages away from the margin vary between about 230 and 360 myr but drop sharply to minimum values of around 80 myr near the coast just south of the sydney basin and 150 175 myr elsewhere along the coast fission track length information allows the age pattern to be interpreted as the result of partial to complete resetting of the older ages by a widespread thermal event at about 80 100 myr this event accompanied the rift phase of passive margin development and peaked just before the onset of sea floor spreading in the adjacent tasman sea the zone of reduced apatite ages is deeply embayed around the gippsland basin supporting the suggestion that it originated as a cretaceous failed arm of the tasman rift system the thermal event has affected apatites up to about 130 km from the margin and has produced a characteristic set of bimodal track length distributions in apatites with partially reset ages the bimodality gradually disappears towards the oldest apatite ages which have negatively skewed unimodal distributions the youngest apatites also tend towards unimodal length distributions indicating that thermal annealing of pre existing tracks in these samples went almost to completion maximum temperatures reached in samples presently exposed in the coastal area were thus probably about 110 120 c thermal activity was accompanied by considerable uplift across the region followed by gradual erosion to expose the present land surface post breakup erosion was generally greater towards the new coastline with near coastal samples being previously buried a probable 1 5 2 5 km

an empirical test of helium diffusion in apatite borehole data from the otway basin australia we have analyzed helium ages of apatites from several boreholes in the otway basin australia to evaluate whether laboratory helium diffusivity can be accurately extrapolated to conditions relevant in nature downhole apatite helium ages define a broad swath of values from 78 71 ma at the surface 15 c to nearly zero at depths corresponding to ambient temperatures of 80 c the width of the swath results from uncertainties in corrected borehole temperatures differences in the thermal history experienced by the various boreholes and possibly from slightly different helium diffusivities among the detrital apatite grains studied in the eastern otway basin the shape and position of the helium age profile is in good agreement with predictions based on the extrapolation of laboratory diffusivity data for durango apatite coupled with published thermal histories for this part of the basin in contrast helium ages are much younger than predicted in the western otway basin based on measured ages from otway sediments which have been essentially isothermal over the last few million years an empirical diffusivity d 2 of 2 10 15 s 1 can be assigned to apatites residing at downhole temperatures of 67 97 c this empirical diffusivity is consistent with laboratory diffusion measurements demonstrating that such measurements are reasonably accurate and can be applied with confidence to natural geologic settings given this confirmation of the laboratory diffusivity data the discrepancy between the observed and modeled helium age profiles in the western otway basin suggests that these sediments recently experienced higher temperatures than presently supposed

constraining the post emplacement evolution of the hebridean igneous province hip using low temperature thermochronology how long has the hip been cool abstract the thermal history of the hebridean igneous province has been determined through the application of low temperature thermochronology to the four central complexes in the province the zircon u th he age 59 4 3 3 ma 1 n 18 and the ages from each complex 60 7 2 3 ma for skye 58 0 0 4 ma for mull 55 9 3 2 ma for ardnamurchan 55 6 2 7 ma for rum are indistinguishable from the crystallization ages apatite fission track ages 61 2 57 2 ma mean of 59 3 3 4 ma from the major plutonic units also overlap crystallization ages implying that on a regional scale the hebridean igneous province cooled rapidly to near surface temperatures immediately after emplacement however apatite fission track ages and track lengths and apatite u th he ages from some small volume intrusions in the skye and rum central complexes identify localized mid eocene 45 47 ma cooling forward and inverse modelling suggests a discrete heating cooling event at c 47 ma which may have been caused by structurally controlled localized advection of heat above shallow emplacement this is the first suggestion of eocene magmatism in the hebridean igneous province

quantifying passive margin denudation and landscape development using a combined fission track thermochronology and cosmogenic isotope analysis approach abstract a new approach to empirically constraining numerical models of denudation and landscape development across passive continental margins is presented involving a combination of apatite fission track thermochronology and cosmogenic isotope analysis this integrated strategy is applied to the namibian sector of the south west african margin where the conventional landscape evolution model of inland retreat of an escarpment initiated at the coast at break up implies a mean rate of retreat of 1 km ma 1 the thermochronological and cosmogenic isotope data are incompatible with a simple escarpment retreat model since they show that denudation rates oceanward of the present escarpment position have been low 1 since the end of the eocene and the estimated rate of escarpment retreat has been only 10 m ma 1 this low rate of retreat is however consistent with numerical landscape evolution models where the escarpment is pinned at an inland drainage divide

stable isotope geochronology of the australian regolith australian regolith profiles can be assigned to one of three post palaeozoic age groups on the basis of the oxygen isotope composition of authigenic clay minerals developed in the profile systematic variations in the isotopic composition of meteoric waters and therefore of the authigenic regolith minerals that formed in equilibrium with them are a result of the continent s drift from high to low latitudes and changes in global climate residual clays collected in situ from regolith profiles of post mid tertiary age have 18o values between 17 5 and 21 3 with the exception of samples from northern australia north of approximately 20 s which have anomalously low values because meteoric waters in northern australia are largely derived from monsoonal rain the high 18o values of post mid tertiary residual clays are consistent with australia s northward drift to low warm latitudes during this time compared with the much higher latitudes it occupied for most of its post palaeozoic history late tertiary residual clays have 18o values that are indistinguishable from all other post mid tertiary clays indicating that further subdivision of the sample groups is not feasible on a continent wide scale comparison with hypothetical modern clays in equilibrium with modern meteoric waters confirms that there has been very little change in the isotopic composition of meteoric waters in australia since the mid tertiary pre mid tertiary clays have lower 18o values between 10 0 and 17 5 which are consistently lower than post mid tertiary clays 18o values of less than 14 to 15 are thought to reflect weathering events during much earlier geologic periods perhaps early or mid mesozoic when australia was at high latitude and the australian climate was humid and cool to cold several workers have documented the existence of very old regolith profiles and land surfaces in australia and support for the above hypothesis comes from three analyses of clays from profiles of stratigraphically demonstrable pre late mesozoic age which have 18o values between 10 2 and 12 3 regolith profiles composed of low 18o clays 15 are widespread in australia and it is possible that a much greater part of the modern landscape than previously recognized developed in the early or mid mesozoic the 18o values of transported i e sedimentary kaolinites reveals that the major source for many tertiary kaolinitic sediments was this postulated pre late mesozoic low 18o regolith the stableisotope evidence that much of the australian regolith formed in comparatively cold conditions suggests that contrary to some traditional interpretations lateritization and deep weathering phenomena are not solely the result of weathering in tropical or sub tropical climates the hydrogen isotope composition of the clays range from 115 to 50 however it is inferred that the majority of clays have undergone some post formational hydrogen isotope exchange which renders their d composition unsuitable for dating purposes

denudation along the atlantic passive margin new insights from apatite fission track analysis on the western coast of south africa apatite fission track aft data from two traverses across the great escarpment of the western coast of south africa are used to reconstruct the tectonic evolution and denudation history of this sector of the atlantic passive margin fission track ages range between 180 and 86 ma modelling of this data identifies two distinct cooling events the first event between 160 and 138 ma is recorded only by the rocks above the escarpment in the karoo area and is tentatively linked to post karoo magmatism 180 ma thermal relaxation the second between 115 and 90 ma results instead from a tectonically induced denudation episode responsible for the removal of up to 2 5 km of crust across the coastal zone in front of the escarpment and less than 1 km on the elevated interior plateau based on these results it is suggested that the cretaceous is the time when most of the elevated topography of southern africa was generated with only a minor cenozoic contribution

wearing down wearing back and gorge extension in the long term denudation of a highland mass quantitative evidence from the shoalhaven catchment southeast australia the long standing issue of the dominance of scarp retreat versus summit lowering in the denudation of a highland mass is investigated with supporting evidence provided by tertiary basalts throughout the shoalhaven catchment in southeast australia both of these forms of denudation are found to be insignificant compared to the role of fluvial gorge extension over the last 30 m y headward advancement of the shoalhaven gorge has been occurring at approximately 15 times the rate of major escarpment retreat 250 times the average rate of summit lowering and over 500 times the rate of interfluve consumption these estimates show that the headward erosion of gorges is the most important process denuding the highlands in the shoalhaven region and possibly elsewhere in the highlands of eastern australia over the long term the highlands in this region will become considerably more dissected well before they decrease substantially in height or are narrowed this conclusion has important implications for models p

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

escarpment evolution on high elevation rifted margins insights derived from a surface processes model that combines diffusion advection and reaction experiments with a surface processes model of large scale 1 1000 km long term 1 100 m y erosional denudation are used to establish the controls on the evolution of a model escarpment that is related to the rifting of a continent the model describes changes in topographic form as a result of simultaneous short and long range mass transport representing hillslope diffusive processes and fluvial transport advection respectively fluvial entrainment is modeled as a first order kinetic reaction which reflects the credibility of the substrate and therefore the fluvial system is not necessarily carrying at capacity one dimensional and planform models demonstrate that the principal controls on the evolution of an initially steep model escarpment are 1 antecedent topography drainage 2 the timescale or equivalently a length scale in the fluvial entrainment reaction 3 the flexural response of the lithosphere to denudation and 4 the relative efficiencies of the short and long range transport processes when rainfall and substrate lithology are uniform a significant amount of discharge draining over the escarpment top causes it to degrade only when the top of the model escarpment coincides with a drainage divide can escarpment retreat occur for these conditions an additional requirement for retreat of a model escarpment without decline is a long reaction time scale for fluvial entrainment this corresponds to a substrate that is hard to detach by fluvial erosion and therefore to fluvial erosion that is not transport limited continuous backtilting of an escarpment due to flexural isostatic uplift in response to denudational unloading helps maintain the scarp top as a divide it is essential if the escarpment gradient is to be preserved during retreat in a uniform lithology low flexural rigidities promote steep and slowly retreating escarpments for given rainfall and substrate conditions the morphology of a retreating model escarpment is determined by the ratio of the short range diffusive and long range advective transport efficiencies a low ratio which is interpreted to correspond to a relatively arid climate and weathering limited conditions promotes steep sharp topped escarpments with straight main slopes and escarpment retreat occurs over a wide range of height scales a high ratio interpreted to correspond to a more humid temperate climate produces a convex upper slope and concave lower slope morphology and only major escarpments are predicted to preserve a high scarp gradient lithological contrasts in the model produce more complex morphologies and predict the formation of scarps crowned by an erosionally resistant caprock however resistant caprocks are not an essential requirement for model scarps to retreat we conclude that the inferred controls and model behavior are both consistent with the present day morphology of rifted continental margins and with modern conceptual models of landscape evolution

granite landform evolution factors and implications most granite landforms are due to the exploitation of weaknesses especially fractures by meteoric waters and particularly groundwaters moisture attack produces grus which commonly forms the bulk of the regolith in granitic terrains the transition from weathered to fresh rock is typically abrupt and takes the form of a narrow transition zone called the weathering front

heatflow density distribution in the red sea abstract a new heatflow density hfd map of the red sea was compiled from 467 heatflow density values collected by various organisations during the last 20 years the map shows that the complicated heatflow distribution can only be explained if we assume considerable convective transport of heat through water circulating in the sediments and crust as well as convection in small volumes of mobilized magma associated with the deeps along the morphologic depression of the red sea the hfd values are twice as high as the world mean at the flanks of the rift and reach values of 5 to 8 times the world mean in the graben and the deeps one dimensional stationary computations of the distribution of the isotherms showed that at 15 to 18 km depth the temperatures are sufficiently high to partially mobilize the upper mantle distribution of the hfd anomalies can only be explained if we assume that part of the rift development can be accounted for by the development of pull apart basins that dominated the early stages of rifting and pure shear combined with wrench faulting active now

controls on the erosion of the continental margin of southeast brazil from cosmogenic 10be in river sediments abstract the atlantic ocean coast region of southeast brazil contains two coast parallel mountain ranges the serra do mar and serra da mantiqueira generated by tectonic activity pulses tens of millions years after the main continental rift event occurred around 120 ma although the short term erosion rates for the region are established the relative importance of the factors controlling erosion is poorly constrained we combine new and published catchment averaged erosion rates n 48 using in situ produced 10be concentrations in quartz from river sediments to establish the regional erosion pattern the river catchments are i escarpment topography ii high altitude low relief and iii mixed topography which record how escarpment fronts are migrating inland ocean facing coastal escarpment catchments of the serra do mar e 18 53 m ma can be eroded approximately twice as fast as continent facing escarpment catchments in the serra do mar and serra da mantiqueira e 7 24 m ma the correlation between the normalized channel steepness index ksn and slope angle indicates that river incision and hillslope erosion processes combine to maintain the high relief the serra do mar catchments define a mean slope angle threshold indicating that landslides are the dominant erosional process when slope angles in excess of 30 tectonic activity is low and plays no significant role in driving erosion a first order relationship between erosion rate and precipitation temperature across the region implies that climate plays a key role in soil production river incision and in triggering erosional processes although the high topographic relief is a pre condition for the occurrence of significant erosion the climatic condition is the outlining factor of the regional variation in erosion rates

the influence of rifting on escarpment migration on high elevation passive continental margins 1 using numerical models that couple surface processes flexural isostasy faulting and the thermal effects of rifting we show that fault bounded escarpments created at rift flanks by mechanical unloading and flexural rebound have little potential to survive as retreating escarpments if the lower crust under the rift flank is substantially stretched in this configuration a drainage divide that persists through time appears landward of the initial escarpment in a position close to a secondary bulge that is created during the rifting event at a distance that depends on the flexural rigidity of the upper crust moreover the migration of the escarpment to the secondary bulge occurs when the pre rift topography dips landward otherwise the evolution of the escarpment is guided by the pre existing inland drainage divide to illustrate this new mechanism for the evolution of passive margins we study the examples of southeastern australia and southeastern brazil we propose that a pre existing inland drainage divide with rift related flank uplift can produce the double drainage divide observed in southeastern australia on the other hand we conclude that it is possible that the serra do mar escarpments on the southeastern brazilian margin originated as a secondary flexural bulge during rifting that persisted through time in both cases the retreating escarpment scenario is unlikely and the present day margin morphology can be explained as resulting from rift related vertical motions alone without requiring significant post rift rejuvenation

natural age dispersion arising from the analysis of broken crystals part ii practical application to apatite u th he thermochronometry abstract we describe a new numerical inversion approach to deriving thermal history information from a range of naturally dispersed single grain apatite u th he ages the approach explicitly exploits the information about the shape of the 4he diffusion profile within individual grains that is inherent in the pattern of dispersion that arises from the common and routine practice of analysing broken crystals additional dispersion arising from differences in grain size and in u and th concentration of grains and the resultant changes to helium diffusivity caused by differential accumulation and annealing of radiation damage is explicitly included in this approach we calculate the ingrowth and loss due to both thermal diffusion and the effects of ejection of helium over time using a finite cylinder geometry broken grains are treated explicitly as fragments of an initially larger crystal the initial grain lengths l0 can be treated as unknown parameters to be estimated although this is computationally demanding a practical solution to the problem of solving for the unknown initial grain lengths is to simply apply a constant and sufficiently long l0 value to each fragment we found that a good value for l0 was given by the maximum fragment length plus two times the maximum radius of a given set of fragments currently whole crystals and fragments with one termination are taken into account a set of numerical experiments using synthetic fragment ages generated for increasingly complex thermal histories and including realistic amounts of random noise 5 15 are presented and show that useful thermal history information can be extracted from datasets showing very large dispersion these include experiments where dispersion arises only from fragmentation of a single grain length 400 m and radius 75 m c 6 50 dispersion including the effects of grain size variation for spherical equivalent grain radii between 74 and 122 m c 10 70 dispersion and the combined effects of fragmentation grain size and radiation damage for eu between 5 and 150 ppm c 10 107 dispersion additionally we show that if the spherical equivalent radius of a broken grain is used as a measure of the effective diffusion domain for thermal history inversions then this will likely lead to erroneous thermal histories being obtained in many cases the viability of the new technique is demonstrated for a real data set of 25 single grain u th he apatite ages obtained for a gabbro sample from the bk 1 bierkraal borehole drilled through the bushveld complex in south africa the inversion produces a well constrained thermal history consistent with both the u th he data and available fission track analysis data the advantage of the new approach is that it can explicitly accommodate all the details of conventional schemes such as the effects of temporally variable diffusivity zonation of u and th and arbitrary grain size variations and it works equally effectively for whole or broken crystals and for the most common situation where a mixture of both are analysed for the routine application of the apatite u th he thermochronometry technique with samples where whole apatite grains are rare our experiments indicate that 15 20 single grain analyses are typically required to characterise the age dispersion pattern of a sample the experiments also suggest that picking very short crystal fragments as well as long fragments or even deliberately breaking long crystals to maximise the age dispersion in some cases would ensure the best constraints on the thermal history models the inversion strategy described in this paper is likely also directly applicable to other thermochronometers such as the apatite rutile and titanite u pb systems where the diffusion domain is approximated by the physical grain size