sub aerial denudation of passive margins regional elevation versus local relief models abstract variations in denudation rates which are an important factor in the modification of the sub aerial topography of passive margins have frequently been modelled as being proportionate to regional elevation to be valid this assumption requires that mean slope gradient and the related property of local relief show a strong positive relationship to regional elevation since slope angle is a primary determinant of denudation rate analysis of the relationships between these variables based on the 1 250 000 scale topographic map coverage of southern africa demonstrates that regional modal elevation explains only 0 5 of the variance in mean sine slope gradient and only 0 14 of the variance in local relief measured over 15 15 areas these results demonstrate that regional elevation cannot be used to predict denudation rates for passive margins such as those of southern africa where a dissected coastal zone is separated from a high interior plateau by a major escarpment on the basis of a previous regression analysis relating local relief to sediment and solute transport by rivers the data for southern africa indicate denudation rates of 80 m ma 1 over much of the coastal zone and 1 across most of the elevated interior extrapolation of these results over geological time however necessitates either assumptions about the local relief of palaeotopography or independent evidence of long term denudation rates

plate tectonic development of the south west indian ocean a revised reconstruction of east antarctica and africa a revised sequence of reconstructions for east antarctica relative to africa is generated from published seafloor spreading and fracture zone anomalies these date from the cretaceous tertiary boundary to the late jurassic and lead to a revised predrift reconstruction for east and west gondwanaland euler pole 1 67 s 35 99 e rotation angle 53 43 this reconstruction derived wholly from marine data places east antarctica adjacent to the lebombo mountains and south of and subparallel to the sabi acid volcanic suite supporting the postulate that these rocks represent mesozoic plate boundaries this refit implies that part of the coastal plain of mozambique is underlain by oceanic or very highly extended continental crust the reconstruction is supported by the alignment of 1 the cape fold belt with its antarctic equivalents and 2 the western limit of pan african reactivation of the mozambique mobile belt with the western limit of the sverdrupfjella metamorphic suite antarctica initially moved along a transform fault lying immediately east of and subparallel to the lebombo mountains the eastern face of the tugela cone the southeastern face of the falkland plateau in its reconstructed position and the eastern flank of the agulhas plateau in its reconstructed position at m10 time a triple junction formed near the tip of the falkland plateau implying that the oldest lineated magnetic anomalies in the eastern weddell sea associated with this triple junction cannot predate m10 at m2 time the mozambique ridge rifted from the astrid ridge in a ridge jump episode such an event may also explain the presence of magnetic anomalies in the southern mozambique basin gunnerus ridge conrad rise and del cano rise are all tectonically associated with the madagascar ridge

denudational isostatic rebound of intraplate highlands the lachlan river valley australia a range of evidence from the lachlan valley in the southeast australian highlands is consistent with neogene isostatic rebound in response to denudational unloading this evidence is found along the inland edge of the highlands in the transition zone between the highlands proper and the lachlan s inland alluviated valley and the intracratonic murray basin the amounts and rates of uplift indicated by offsets of suballuvium bedrock profiles and the long profiles of tertiary valley filling basalts are consistent with modelling of denudational rebound using known rates of highland denudation and basinal sedimentation and reasonable crustal properties the modelling shows that weak to moderately strong strong lithosphere effective elastic thickness te 1 25 km and strong lithosphere te 100 km are all consistent with the observed amounts of rebound strong lithosphere must be broken however to be consistent with the field data even in the australian setting which is characterized by very low rates of denudation isostatic rebound in response to denudational unloading must be a significant factor in maintaining highland elevation and must be incorporated in models of long term landscape evolution it would be expected that denudational isostatic rebound would be an even more significant component of long term landscape evolution in areas of higher denudation rates

the timing of uplift volcanism and rifting peripheral to the red sea a case for passive rifting prior to the formation of the red sea the northeastern afro arabian continent had low relief and was largely below sea level from the late cretaceous to the early oligocene the events leading to the formation of the red sea followed the sequence 1 alkaline volcanism and rifting beginning about 30 32 ma affecting a narrow linear zone in the continent 2 rotational block faulting and detachment faulting well underway by 25 ma 3 gabbro and diorite magmatism andesite to rhyolite volcanism and fine grained nonmarine sedimentation in the rift between 20 and 25 ma 4 fine grained marine sedimentation in the rift as the early shelves started to subside in the middle miocene and 5 uplift of the adjacent continents about 3 km and subsidence of the shelves about 4 km between 13 8 and 5 ma the youth of the uplift is suggested by 44 fission track dates on apatites from rocks of the proterozoic arabian shield that range in age from 13 8 to 568 ma the youngest of these ages coupled with the present high relief along the arabian escarpment and published heat flow measurements indicate that 2 5 4 km uplift has occurred in the last 13 8 m y the sequence volcanism rifting followed by uplift leads to our adoption of a passive mantle model for rift origin models that require uplift to create the rift are rejected because of the late uplift we advocate a model of lithospheric extension caused by two dimensional plate stress over those requiring tractional drag at the base of the lithosphere caused by vigorous flow in the asthenosphere it is acknowledged that traction models could explain the observed data but they imply a rigid static lithosphere and seem to require a link between the direction of flow in the asthenosphere and plate motions neither requirement is necessary in the extension model the rift starts with mechanical extension in a narrow zone of lithosphere between 25 32 ma in our model the thinned lithosphere is replaced by upwelling asthenosphere and by rocks from the adjacent deep continental lithosphere which flow into the rift ductile flow of the deep continental lithosphere is accelerated by partial melting as rocks flow upward toward the rift axis once partially melted rocks formerly part of the continental lithosphere join the upwelling asthenosphere resulting in a rapid erosion of the lithospheric mantle beneath the continent near the rift edge the resulting density decrease explains the uplift we think that the red sea began as a consequence of changing plate geometries resulting from the collision of india and eurasia after the collision the segment of the owens fracture zone north of the carlsberg ridge became locked forcing the northeast corner of afro arabia to rotate with the indian plate away from the rest of africa

testing the applicability of morphometric characterisation in discordant catchments to ancient landscapes a case study from southern africa the ancient landscapes south of the great escarpment in southern africa preserve large scale geomorphological features despite their antiquity this study applies and evaluates morphometric indices such as hypsometry long profile analysis stream gradient index and linear areal catchment characteristics to the gouritz catchment a large discordant catchment in the western cape spatial variation of morphometric indices were assessed across catchment trunk rivers and subcatchment scales the hypsometric curve of the catchment is sinusoidal and a range of curve profiles are evident at subcatchment scale hypsometric integrals do not correlate to catchment properties such as area circularity relief and dissection and stream length gradients do not follow expected patterns with the highest values seen in the mid catchment areas rock type variation is interpreted to be the key control on morphometric indices within the gouritz catchment especially hypsometry and stream length gradient external controls such as tectonics and climate were likely diminished because of the long duration of catchment development in this location while morphometric indices can be a useful procedure in the evaluation of landscape evolution this study shows that care must be taken in the application of morphometric indices to constrain tectonic or climatic variation in ancient landscapes because of inherited tectonic structures and signal shredding more widely we consider that ancient landscapes offer a valuable insight into long term environmental change but refinements to geomorphometric approaches are needed

constraints on the late thermotectonic evolution of the western alps evidence for episodic rapid uplift forty two new apatite and zircon fission track ages are presented for samples from the western alps in southern switzerland northern italy and southeastern france measured ages plotted against assumed closure temperatures yield cooling patterns for the final cooling uplift and exhumation of the western alps similar fission track zircon ages in the penninic gran paradiso massif dent blanche nappe sesia lanzo zone and ivrea zone indicate cooling of all four units to 225 c by 33 ma differences in apatite ages reveal differential cooling of the four blocks between 33 ma and the present in the sesia lanzo zone similarity of apatite ages regardless of elevation together with near volcanic confined fission track length patterns suggest rapid cooling and uplift at 25 ma compared with slow cooling of other western alps units around 12 ma uplift is thus not continuous but episodic often over a short time interval beyond the resolution of other methods such episodes of uplift as revealed here in the sesia lanzo zone may be the rule rather than the exception

heat flow anomaly in lesotho implications for the southern boundary of the kaapvaal craton heat flow studies in southern africa reveal a pattern of low heat flow in archean cratons compared with proterozoic mobile belts that provides grounds for modeling in which cratons have lower mantle heat flux and greater lithospheric thickness there is little detailed information regarding thermal transitions across craton boundaries but new data from lesotho provide insights into the nature of the boundary between the kaapvaal craton and the natal belt measurements at nine localities in a 90 km long north south traverse show an increase of heat flow from typically cratonic values of about 45 mw m 2 in the north to about 80 mw m 2 where the craton abuts on the natal belt in the south the change occurs within 30 km suggesting that the anomaly largely reflects an increase in radioactive heat from the upper crust in the orogenic front the change in mantle heat flux across the boundary is probably considerably smaller these and other results suggest that the surficial craton boundary is further north than usually depicted

role of in situ cosmogenic nuclides 10be and 26al in the study of diverse geomorphic processes the central premises of applications of the in situ cosmogenic dating method for studying specific problems in geomorphology are outlined for simple and complex exposure settings in the light of these general models we discuss the information that can be derived about geomorphic processes utilizing concentrations of in situ produced cosmogenic radionuclides 10be half life 1 5 ma and 26a1 half life 0 7 ma in a variety of geomorphic contexts glacial polish and tills meteorite impact craters alluvial fans paleo beach ridges marine terraces sand dunes and bedrock slopes we also compare 10be 26al data with results obtained by other dating methods we conclude that the technique of measuring in situ cosmic ray produced nuclides holds promise for quantitative studies of processes and time scales in a wide range of geomorphological problems

backstacking apatite fission track stratigraphy a method for resolving the erosional and isostatic rebound components of tectonic uplift histories measuring tectonic uplift of the mean surface of earth using thermochronological data to document uplift histories is often plagued by confusion arising from the independent nature of the thermal and sea level frames of reference the technique of backstacking apatite fission track stratigraphy differentiates between vertical movement of rocks toward earth9s surface due to erosional or tectonic denudation and tectonic uplift of earth9s surface apatite fission track analysis can also be used to identify episodes of erosion where no other evidence for the erosional event exists in addition to providing information about the timing magnitude and geographic distribution of erosion

predicting and testing continental vertical motion histories since the paleozoic abstract dynamic topography at the earth s surface caused by mantle convection can affect a range of geophysical and geological observations including bathymetry sea level change continental flooding sedimentation and erosion these observations provide important constraints on and test of mantle dynamic models based on global mantle convection models coupled with the surface plate motion history we compute dynamic topography and its history for the last 400 ma associated with pangea assembly and breakup with particular focus on cratonic regions we propose that burial unroofing histories of cratons inferred from thermochronology data can be used as a new diagnostic to test dynamic topography and mantle dynamic models our models show that there are currently two broad dynamic topography highs in the pacific and africa for the present day earth that are associated with the broad warm structures i e superplumes in the deep mantle consistent with previous proposals of dynamical support for the pacific and african superswells our models reveal that pangea assembly and breakup by affecting subduction and mantle upwelling processes have significant effects on continental vertical motions our models predict that the slave craton in north america subsides before pangea assembly at 330 ma but uplifts significantly from 330 ma to 240 ma in response to pre pangea subduction and post assembly mantle warming the kaapvaal craton of africa is predicted to undergo uplift from 180 ma to 90 ma after pangea breakup but its dynamic topography remains stable for the last 90 ma the predicted histories of elevation change for the slave and kaapvaal cratons compare well with the burial unroofing histories inferred from thermochronology studies thus supporting our dynamic models including the development of the african superplume mantle structure the vertical motion histories for other cratons can provide further tests of and constraints on our mantle dynamic models

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

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

the thermal history of the western irish onshore we present here a low temperature thermochronological study that combines the apatite fission track and u th he dating methods with a pseudo vertical sampling approach to generate continuous and well constrained temperature time histories from the onshore irish atlantic margin the apatite fission track and u th he ages range from the late jurassic to early cretaceous and the mean track lengths are relatively short thermal histories derived from inverse modelling show that following post orogenic exhumation the sample profiles cooled to c 75 c a rapid cooling event to surface temperatures occurred during the late jurassic to early cretaceous and was diachronous from north to south it was most probably caused by c 2 5 km of rift shoulder related exhumation and can be temporally linked to the main stage of mesozoic rifting in the offshore basins a slow phase of reheating during the late cretaceous and early cenozoic is attributed to the deposition of a thick sedimentary sequence that resulted in c 1 5 km of burial our data imply a final pulse of exhumation in neogene times probably related to compression of the margin however it is possible that an early cenozoic cooling event compatible with our data but not seen in our inverse models accounts for part of the cenozoic exhumation supplementary material details on the apatite fission track and u th he methods are available at www geolsoc org uk sup18765

late cretaceous unroofing of the white mountains new hampshire usa an episode of passive margin rejuvenation the growing recognition that many passive margins have experienced periods of post rift uplift has raised new questions about the nature of the tectonic processes that drive such uplift the eastern north america margin is of particular interest because it is a classic atlantic style margin that has long been suspected of tectonic rejuvenation yet the timing of post rift uplift events remains elusive to address this question we present apatite u th he and fission track ages from a 1300 m vertical transect in the white mountains of new hampshire usa including a 910 m deep bore hole thermal modeling suggests a period of accelerated denudation from ca 85 to 65 ma this timing is broadly similar to the timing of accelerated exhumation and compression on passive margins elsewhere in the atlantic raising the question of whether a common mechanism such as a change in lateral stresses changes in mantle convection or climate change could be responsible

passive margins getting squeezed in the mantle convection vice 1 passive margins often exhibit uplift exhumation and tectonic inversion we speculate that the compression in the lithosphere gradually increased during the cenozoic as seen in the number of mountain belts found at active margins during that period less clear is how that compression increase affects passive margins in order to address this issue we design a 2 d viscous numerical model wherein a lithospheric plate rests above a weaker mantle it is driven by a mantle conveyor belt alternatively excited by a lateral downwelling on one side an upwelling on the other side or both simultaneously the lateral edges of the plate are either free or fixed representing the cases of free convergence and collision or slab anchoring respectively this distinction changes the upper mechanical boundary condition for mantle circulation and thus the stress field between these two regimes the flow pattern transiently evolves from a free slip convection mode toward a no slip boundary condition above the upper mantle in the second case the lithosphere is highly stressed horizontally and deforms for a constant total driving force compression increases drastically at passive margins if upwellings are active conversely if downwellings alone are activated compression occurs at short distances from the trench and extension prevails elsewhere these results are supported by earth like models that reveal the same pattern where active upwellings are required to excite passive margins compression our results substantiate the idea that compression at passive margins is in response to the underlying mantle flow that is increasingly resisted by the cenozoic collisions

influence of radiation damage on titanite he diffusion kinetics titanite is a common u th bearing accessory mineral that previous study suggested has a u th he closure temperature tc of 210 175 c although it has been recognized that radiation damage influences the he retentivity of apatite and zircon this effect has not been documented in titanite we acquired 51 the analyses from 11 archean basement samples across the kaapvaal craton south africa the the dates range from 20 to 1200 ma and are negatively correlated with their 10 970 ppm span of effective uranium concentration eu raman spectra acquired for 4 of these samples display peak broadening with increasing eu 4he diffusion experiments on the same 4 samples suggest tc values that vary by 175 c estimated alpha dose versus tc for our results and published the diffusion data document tc values of 150 210 c at alpha doses 50 1016 g with a sharp decrease in he retentivity above this threshold this damage diffusivity pattern is similar to zircon but the damage level at which the reduction in retentivity occurs appears to be lower for titanite because titanite typically has substantially lower eu than zircon for the same protracted thermal history the dates are likely to record higher temperature portions of the history than zircon u th he dates the results demonstrate that the dating can access a much larger temperature range and potentially be used to decipher more detailed time temperature paths than previously thought

estimating uplift rate histories from river profiles using african examples 1 we describe and apply a method for estimating uplift rate histories from longitudinal river profiles our strategy is divided into three parts first we develop a forward model which calculates river profiles from uplift rate histories height variation along a river profile is controlled by uplift rate and moderated by the erosional process we assume that the erosional process can be represented by a combination of advection and diffusion which are parameterized using four erosional constants second we have posed and solved the geologically more interesting inverse problem which uplift rate history minimizes the misfit between calculated and observed river profiles the inverse algorithm has been tested on synthetic river profiles which demonstrates that uplift rate histories can be reliably retrieved our tests show that the erosional process is dominated by advection i e knickpoint retreat and that changes in lithology and discharge play a secondary role in determining the transient form of a river profile finally we have inverted river profiles from a series of african topographic swells namely the bie south african namibian hoggar and tibesti domes fits between calculated and observed river profiles are excellent calculated uplift rate histories suggest that these domes grew rapidly during the last 30 40 million years uplift rate histories vary significantly from dome to dome but cumulative uplift histories agree closely with independent geologic estimates

isopach and borehole data as indicators of rifted margin evolution in southwestern africa abstract isopach and borehole data are used to document post rifting sediment accumulation along the western margin of southern africa the isopach data reveal an increasing rate of sediment accumulation up to the end of the eocene but the borehole data generally show the opposite trend an integrated analysis of both types of data however is consistent with a general model of post rifting drainage development in which the area supplying sediment expanded significantly after breaching of a linear upwarp along the margin allowed a dominant drainage network to develop the borehole data suggest that such a major sediment outlet from the interior of the sub continent the palaeo orange became established in the late cretaceous coniacian in the vicinity of the present orange river mouth after conversion to equivalent rock volumes the isopach data can be combined with the general drainage model to infer the likely spatial and temporal patterns of erosion over southern africa during the accumulation of four offshore sedimentary sequences beginning with the onset of rift related sedimentation and extending up to the present this analysis using three variant forms of the general model to encompass a wide range of drainage development possibilities indicates that the average rate of erosion for catchment areas draining to the western margin has declined from 41 82 m ma 1 during the first sedimentary sequence 152 to 113 ma to about 7 9 m ma 1 during the last sequence 37 to 0 ma maximum depths of post rifting erosion of several kilometres appear to have occurred in a narrow zone bordering the margin primarily during its early development while depths of erosion in some interior areas have probably been an order of magnitude less the average depth of post rifting erosion for the present day catchment as a whole is estimated to be about 1 8 km and the erosion rate over this period has averaged about 12 m ma 1

strong rocks sustain ancient postorogenic topography in southern africa the cape mountains of southern africa exhibit an alpine like topography in conjunction with some of the lowest denudation rates in the world this presents an exception to the often cited coupling of topography and denudation rates and suggests that steep slopes alone are not sufficient to incite the high denudation rates with which they are commonly associated within the cape mountains slope angles are often in excess of 30 degrees and relief frequently exceeds 1 km yet be 10 based catchment averaged denudation rates vary between 2 32 0 29 m m y and 7 95 0 90 m m y we attribute the maintenance of rugged topography and suppression of denudation rates primarily to the presence of physically robust and chemically inert quartzites that constitute the backbone of the mountains be 10 based bedrock denudation rates on the interfluves of the mountains vary between 1 98 0 23 m m y and 4 61 0 53 m m y the close agreement between the rates of catchment averaged and interfluve denudation indicates topography in steady state these low denudation rates in conjunction with the suggestion of geomorphic stability are in agreement with the low denudation rates 20 m m y estimated for southern africa during the late cenozoic by means of cosmogenic nuclide thermochronology and offshore sedimentation analyses accumulatively these data suggest that the coastal hinterland of the subcontinent may have experienced relative tectonic stability throughout the cenozoic

erosion at extended continental margins insights from new aerogeophysical data in eastern dronning maud land abstract modelling rock cooling sedimentation and exposure based interpretations of the mechanisms by which topography evolves at extended continental margins vary widely observations from the margin of dronning maud land antarctica have until now not strongly contributed to these interpretations here we present new airborne gravity and radar data describing the eastern part of this margin inland of a tall 2 5 km great escarpment a plateau topped by a branching network of valleys suggests preservation of a fluvial landscape with sw directed drainage beneath a cold based ice sheet the valley floor slopes show that this landscape was modified during a period of alpine style glaciation prior to the onset of the current cold based phase around 34 ma the volume of sediments in basins offshore in the riiser larsen sea balances with the volume of rock estimated to have been eroded and transported by north directed drainage from between the escarpment and the continental shelf break the stratigraphy of these basins shows that most of the erosion occurred during the 40 myr following late jurassic continental breakup this erosion is unlikely to have been dominated by backwearing because the required rate of escarpment retreat to its present location is faster than numerical models of landscape evolution suggest to be possible we suggest an additional component of erosion by downwearing seawards of a pre existing inland drainage divide the eastern termination of the great escarpment and inland plateau is at the west ragnhild trough a 300 km long 15 20 km wide and up to 1 6 km deep subglacial valley hosting the west ragnhild glacier numerous overdeepened by 300 m segments of the valley floor testify to its experience of significant glacial erosion thick late jurassic and early cretaceous sediments fanning out from the trough s mouth into the eastern riiser larsen sea betray an earlier history as a river valley the lack of late jurassic relief forming processes in this river s catchment in the interior of east antarctica suggests this erosion was related to regional climatic change

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

a record of appalachian denudation in postrift mesozoic and cenozoic sedimentary deposits of the u s middle atlantic continental margin abstract the complex interplay between source terrain uplift basin subsidence paleoclimatic shifts and sea level change left an extensive sedimentary record in the contiguous offshore basins of the u s middle atlantic margin salisbury embayment baltimore canyon trough and hatteras basin isopach maps of 23 postrift lower jurassic to quaternary a allostratigraphic units coupled with a revised stratigraphic framework reveal that tectonism by regulating sediment supply accumulation rate dominated the interplay of forcing mechanisms tectonic pulses are evidenced by abruptly accelerated sediment accumulation marked latitudinal shifts in the location of depocenters and regional changes in lithofacies relatively rapid tectonic subsidence during the early and middle jurassic history of the basins may have enhanced sediment accumulation rates beginning in the late jurassic however subsidence rates decreased significantly though occasional short pulses of subsidence may have effected relative sea level rises sea level change heavily influenced the distribution and redistribution of sediments one they reached the basins and paleoclimate regulated the relative abundance of carbonates and evaporites in the basins we conclude that source terrains of the central appalachian highlands were tectonically uplifted intensely weathered and rapidly eroded three times since the late triassic 1 early to middle jurassic aalenian to callovian 2 mid early cretaceous barremian and 3 late cenozoic middle miocene intervals of tectonic quiescence following these three tectonic pulses provided conditions suitable for the formation of regional erosion surfaces geomorphic features commonly reported to characterize the central appalachian highlands this series of three irregularly spaced tectonic quiescent cycles does not however match the traditional four cycle concept of post triassic appalachian peneplanation

morphotectonic evolution of rifted continental margins inferences from a coupled tectonic surface processes model and fission track thermochronology we use a numerical model to study the topographic evolution and erosional history of rifted continental margins the model combines a kinematic description of lithospheric extension tectonic model with a surface processes model that includes short range hillslope and long range fluvial transport the tectonic model predicts the evolution of the lithospheric temperature and strength elastic thickness distribution as well as the tectonic uplift through time this is input into the surface processes model which calculates the degradation of topography and associated isostatic rebound modeled denudation histories across the margin are used to predict apatite fission track age and length patterns modeling results indicate that depending on the adopted parameters an uplifted rift flank should degrade by erosion within 50 100 m y without significant retreat of the topographic elevation maximum the development of an escarpment system at rifted continental margins is in itself not an indication of tectonic rift flank uplift but results from the existence of a high elevation interior plateau erosional base level lowering as a consequence of rifting and regional isostatic response to erosion of the margin however apatite fission track thermochronology reveals that the areas seaward of the escarpment at a number of rifted margins have been exhumed from several kilometers depth such amounts of denudation cannot be accommodated with isostatic rebound alone and require additional tectonic uplift of the rift flanks modeling of apatite fission track patterns suggests that fission track thermochronology dates the onset of rapid erosion which coincides with the initiation of strong relief i e initiation of rifting fission track ages which are younger than the age of rifting thus cannot be unambiguously interpreted as excluding prerift uplift the timing of margin uplift can be established only by careful track length analysis and integration with regional stratigraphic data the model is applied to the saudi arabian red sea margin and the southeastern australian highlands where it is constrained by present day topography and apatite fission track data as well as seismic and gravity data for the saudi arabian red sea margin synrift regional plateau uplift with a magnitude of approximately 1 km is inferred possibly as a result of asthenospheric upwelling flexurally induced tectonic uplift of the rift flank with a magnitude of 2 km is superimposed on this regional uplift the relatively high elevation of the southeastern australian highlands their steep front and the relatively high amounts of erosion suggest that apart from mesozoic synrift flexural uplift tertiary rejuvenation of topography has occurred possibly as a result of renewed lithospheric thinning and underplating the low elevation of the australian interior would inhibit the evolution of a major escarpment in the absence of such renewed uplift

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

a users guide to fission track dating calibration abstract misconceptions regarding the fission track dating method prompt the description of five handling procedures and the derivation of commonly used system calibrations problems of registration geometry inhomogeneous uranium distribution accumulated radiation damage and anisotropic etching largely invalidate all but the population and external detector methods and impose strict limitations upon even their routine application complexities of unique absolute thermal neutron dosimetry are illustrated by the calibration and use of the nbs pre irradiated glass srm 962 the 20 disparity in measured f decay constant values cannot be isolated from neutron fluence measurement and calibration frequently a calibration ratio f is evaluated against an age standard and then split into its component parts to illustrate this interdependence of f and zircon fission track ages in agreement with independent k ar ages are obtained by calculating the same track count data with each of the preferred values of f f 7 03 10 17 yr 1 and8 46 10 17 yr 1 together with appropriate selected neutron dosimetry schemes an alternative approach is presented formally relating unknown ages of samples to known ages of standards either by direct comparison of standard and sample track densities or by the repeated calibration of a glass against age standards practical recommendations are given for the reporting and critical assessment of fission track data

assessment of u th he thermochronometry the low temperature history of the san jacinto mountains california u th he ages have been measured on igneous apatites from the san jacinto mountains a high region at the junction of the peninsular and transverse ranges to investigate the potential of this technique for thermochronometry of slowly cooled rocks helium ages from 79 to 17 ma are younger than ages obtained by other dating techniques including apatite fission track counting and are consistent with laboratory experiments that indicate this system has a uniquely low closure temperature helium ages are strongly correlated with elevation and record the latest low temperature thermal evolution of the range they suggest relative tectonic quiescence in the latest cretaceous through mid tertiary and provide no evidence for rapid unroofing of the block during this period nor do they obviously require a large degree of uplift associated with convergence between the transverse and peninsular ranges in the last few million years helium ages document modest westward tilting of the block 7 and a significant vertical component of motion on the block s bounding faults after helium retention began this work suggests that apatite helium ages record low temperature tectonic and thermal histories that are not apparent from other dating techniques

erosional dynamics flexural isostasy and long lived escarpments a numerical modeling study erosional escarpments are common features of high elevation rifted continents fission track data suggest that these escarpments form by base level lowering and or marginal uplift during rifting followed by lateral retreat of an erosion front across tens to hundreds of kilometers previous modeling studies have shown that this characteristic pattern of denudation can have a profound impact upon marginal isostatic uplift and the evolution of offshore sedimentary basins yet at present there is only a rudimentary understanding of the geomorphic mechanisms capable of driving such prolonged escarpment retreat in this study we present a nonlinear two dimensional landscape evolution model that is used to assess the necessary and sufficient conditions for long term retreat of a rift generated escarpment the model represents topography as a grid of cells with drainage networks evolving as water flows across the grid in the direction of steepest descent the model accounts for sediment production by weathering fluvial sediment transport bedrock channel erosion and hillslope sediment transport by diffusive mechanisms and by mass failure numerical experiments presented explore the effects of different combinations of erosion processes and of dynamic coupling between denudation and flexural isostatic uplift model results suggest that the necessary and sufficient conditions for long term escarpment retreat are 1 incising bedrock channels in which the erosion rate increases with increasing drainage area so that the channels steepen and propagate headward 2 a low rate of sediment production relative to sediment transport efficiency which promotes relief generating processes over diffusive ones 3 high continental elevation which allows greater freedom for fluvial dissection and 4 any process including flexural isostatic uplift that helps to maintain a drainage divide near an escarpment crest flexural isostatic uplift also facilitates escarpment retreat by elevating topography in the vicinity of an eroding escarpment thereby increasing channel gradients and accelerating erosion which in turn generates additional isostatic uplift of all the above conditions high continental elevation is common to most rift margin escarpments and may ultimately be the most important factor

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

alteration in the irdp drill hole compared with other drill holes in iceland the overall alteration pattern in the drill hole at reydarfjordur is very similar to alteration patterns observed in icelandic geothermal areas and in low grade metamorphosed basalts in deep crustal sections elsewhere in iceland however more detail is obtained by the study of the irdp drill core than by study of drill cuttings sampled in previous drill holes in iceland a comparatively high fossil thermal gradient is obtained at reydarfjordur by a combination of mineral stability data and the observed occurence of secondary minerals this high gradient is consistent with the measured dike dilation at the drill site and the location of the drill site adjacent to a central volcano

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

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

the importance of apatite composition and single grain ages when interpreting fission track data from plutonic rocks a case study from the coast ranges british columbia abstract fission track results determined from granitic rocks exposed within the coast ranges of british columbia indicate that significant variations in apatite chemistry may occur in small plutonic rock samples to test ages determined earlier by parrish using the population method of fission track dating new apparent apatite ages were completed using the external detector method so that single grain ages could be generated in most cases the new apatite ages were similar to the earlier ones within 2 however in many cases it was found that individual samples contained significant variations in single grain apatite ages microprobe analyses showed the spread in ages were directly related to variations in apatite chemistry such that chlorine rich apatites retained older ages than fluorine rich grains examination of the single grain age data suggested that the spread in single grain ages provided information which constrained the thermal history of the sample and that distributions of single grain ages reflected the temperatures at which they resided during cooling these results suggest that it is important to generate more than a few single grain fission track ages when dating apatite from granitic rocks since variations in the chlorine concentration in apatite grains may exist in kg sized granitic rock samples therefore it is no longer a valid assumption that each apatite grain separated from a small granite sample will have a similar chlorine concentration and in future apatite fission track studies of granitic terrains it is important to recognize that the apatite composition might be heterogeneous when interpreting results

three and half million year history of moisture availability of south west africa evidence from odp site 1085 biomarker records ocean drilling program site 1085 provides a continuous marine sediment record off southern south west africa for at least the last three and half million years the n alkane 13 c record from this site records changes in past vegetation and provides an indication of the moisture availability of sw africa during this time period very little variation and no apparent trend is observed in the n alkane 13c record suggesting stable long term conditions despite significant changes in east african tectonics and global climate slightly higher n alkane 13c values occur between 3 5 and 2 7 ma suggesting slightly drier conditions than today between 2 5 and 2 7 ma there is a shift to more negative n alkane 13c values suggesting slightly wetter conditions during a 0 2 ma episode that coincides with the intensification of northern hemisphere glaciation inhg from 2 5 to 0 4 ma the n alkane 13c values are very consistent varying by less than 0 5 and suggesting little or no long term change in the moisture availability of south west africa over the last 2 5 million years this is in contrast to the long term drying trend observed further north offshore from the namib desert and in east africa a comparison of the climate history of these regions suggests that southern africa may have been an area of long term stability over the last 3 5 myrs

post rift stratigraphic evolution of the atlantic margin of namibia and south africa implications for the vertical movements of the margin and the uplift history of the south african plateau abstract the aim of this study is to constrain the post rift deformations of the atlantic passive margin of namibia and south africa using an extensive industrial 2d reflection seismic dataset calibrated by wells and onshore outcrops that have been revaluated in age biostratigraphy in order to discuss the evolution of the south african plateau uplift the first order evolution of the margin is tectonically driven and can be divided into three principal phases the first 131 93 5 ma comprises an overall retrogradational trend that results from a rate of accommodation created by the thermal flexure of the margin that is higher than the sediment supply the second 93 5 66 ma comprises an overall aggradational progradational trend that results from a relative increase in sediment supply due to an uplift and subsequent erosion of the margin and the inland domain the third 66 0 ma is retrogradational again in response to a decrease in sediment supply induced by a relative stability of the margin and climate conditions this study demonstrates that the present day configuration of the margin is acquired during the late cretaceous from 81 to 66 ma with a strike long wavelength and seaward tilt of the margin which could be related to the growth of the onshore main escarpment we also characterize a regional oligocene unconformity marked by a significant downward shift of the shoreline which suggests a moderate uplift of the inland domain and could be associated with a reactivation of the relic late cretaceous relief

scale dependence of hypsometric integrals an analysis of southeast african basins abstract previous studies have shown that the hypsometry of individual drainage basins is sensitive to both tectonic uplift rates and variations in erosional resistance of different lithological units however basin hypsometry has also been found to be related to the size the shape and relief of the sampled area as well as factors such as the type of dominant erosional process this study extends the evaluation of the controls on a basin s hypsometry by assessing factors that might influence the spatial distribution of hypsometric integrals at different scales the method is applied to catchments draining the southeast margin of southern africa kwazulu natal region because of the region s rather uniform runoff a rather well constrained denudational history and a relatively simple coastward sequence of bedrock geology several sets of basins were extracted from an srtm derived 90 m resolution dem one set of main basins those that have outlets at sea level and six sets of sub basins of different size based on strahler order the mean area of the sub basins sampled ranged from 9 km2 for strahler order 2 through to 3215 km2 for strahler order 6 in contrast to previous studies data from this study fail to show a correspondence between hypsometric integral and indices of basin dimension such as basin area and basin relief rather basin hypsometry in se africa varies spatially and with scale in particular a mountain range scale positive hypsometric integral anomaly situated between 10 and 100 km of the coast line was observed but only in basins of strahler order 5 and less this anomaly is compatible with moderate crustal displacement focussed midway between the coast and the drakensburg escarpment some intermediate scale positive hypsometric integral anomalies in basins of strahler order 4 and less was also observed in some cases these anomalies correspond to differences in erosional resistance among major lithological groups finally numerous local basin scale positive hypsometric integral anomalies in basins of strahler order 3 and less were also observed in many cases these latter anomalies are related to local differences in lithological resistance e g the presence of dolerite sills and dikes the analysis of the spatial variation of hypsometric integrals in basins of different size ranges is therefore a useful first step in assessing factors that control landscape development

the african landscape through space and time it is generally accepted that cenozoic epeirogeny of the african continent is moderated by convective circulation of the mantle nevertheless the spatial and temporal evolution of africa s basin and swell physiography is not well known here we show how continental drainage networks can be used to place broad constraints on the pattern of uplift through space and time first we assemble an inventory of 710 longitudinal river profiles that includes major tributaries of the 10 largest catchments river profiles have been jointly inverted to determine the pattern of uplift rate as a function of space and time our inverse model assumes that shapes of river profiles are controlled by uplift rate history and modulated by erosional processes which can be calibrated using independent geologic evidence e g marine terraces volcanism and thermochronologic data our results suggest that modern african topography started to develop 30 myr ago when volcanic swells appeared in north and east africa during the last 15 20 myr subequatorial africa was rapidly elevated culminating in the appearance of three large swells that straddle southern and western coasts our results enable patterns of sedimentary flux at major deltas to be predicted and tested we suggest that the evolution of drainage networks is dominated by rapid upstream advection of signals produced by a changing pattern of regional uplift an important corollary is that with careful independent calibration these networks might act as useful tape recorders of otherwise inaccessible mantle processes finally we note that there are substantial discrepancies between our results and published dynamic topographic predictions

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

denudational history along a transect across the drakensberg escarpment of southern africa derived from apatite fission track thermochronology 1 the denudational history of a 500 km long transect across the drakensberg escarpment on the high elevation passive margin of se africa is quantified on the basis of thermal history modeling of apatite fission track data for 15 deep borehole samples supplemented by an additional 10 outcrop samples a minimum of 4 5 km of denudation since formation of the margin 130 myr ago is estimated for the coastal zone with a marked early cretaceous episode of accelerated denudation broadly coincident with continental breakup samples from the swartberg borehole sw 1 67 located 30 km seaward of the present position of the drakensberg escarpment indicate a total depth of denudation of 3 1 1 2 km since 91 myr with a phase of accelerated denudation of 2 1 0 9 km at a mean rate of 95 43 m myr between 91 and 69 myr samples from the ladybrand borehole la 1 68 west of the lesotho highlands indicate 1 7 0 5 km of denudation since 78 myr with a phase of accelerated denudation at 82 43 m myr from 78 to 64 myr average denudation rates declined to about 10 m myr during much of the tertiary although the apatite fission track data do not provide any direct constraints on the denudational history of the lesotho highlands interpolation between the two boreholes constrained by geological evidence and extrapolated in situ produced cosmogenic 36cl derived denudation rate estimates suggests a pattern of denudation compatible with numerical modeling studies of escarpment evolution involving rapid river incision seaward of a preexisting inland drainage divide these patterns of denudation are incompatible with constant retreat of the drakensberg escarpment from an initial position near the present coast we suggest that the drakensberg escarpment formed by rapid post breakup river incision seaward of a preexisting drainage divide located just east of the present escarpment location and became pinned at this divide with subsequent retreat rates of only 100 200 m myr

timing and structural relations of regional zeolite zones in basalts of the east greenland continental margin zeolite isograds delineating regionally extensive metamorphic mineral zones occur throughout most of the early tertiary flood basalts in central east greenland mineralogic zones defined by isograds are essentially uniform in thickness throughout the province parallel paleotopographic surfaces and transgress lava stratigraphy the distribution of index zeolite minerals is consistent with a thermal gradient of 40 5 c km and heat flow of 1 7 0 5 heat flow units zeolite facies metamorphism occurred rapidly during and just after the bulk of volcanism 57 55 ma and prior to hydrothermal alteration associated with emplacement of the ca 55 ma skaergaard intrusion and mafic dikes of the coastal dike swarm consequently zeolite isograds are temporal markers for distinguishing synvolcanic and postvolcanic deformation events discordance between dips of zeolite isograds and lava flows suggests a pre metamorphic and probably synvolcanic component to tilting of the lava pile later deformation likely associated with dikes and faults near the continental margin tilts the zeolite isograds 2 5 se

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

exploiting radiation damage control on apatite u th he dates in cratonic regions abstract apatites from four pairs of samples of precambrian basement from the western canadian shield were analyzed by u th he thermochronometry to test for the influence of radiation damage on apatite u th he dates in this cratonic region recent studies have demonstrated that the accumulation of radiation damage increases the apatite he retentivity so that apatites with a span of effective u concentrations eu that experienced the same thermal history may be characterized by a range of closure temperatures in this investigation each sample pair consisted of a mafic dike cross cutting felsic gneisses from a single outcrop or nearby outcrops that contained apatites with a span of eu the apatites yielded u th he dates from 846 to 123 ma and were positively correlated with eu within each sample pair these results can be explained using a model that tracks the evolution of he mobility in response to the accumulation of radiation damage when coupled with regional geological constraints the data appear to require partial to complete he loss due to burial and reheating in phanerozoic time new apatite fission track dates and length data were obtained for five of these samples the apatite fission track dates are proterozoic regardless of apatite eu thermal history simulations indicate that the apatite fission track data are compatible with the u th he results although the thermal histories are not identical in detail and the fission track results alone do not require phanerozoic heating together the data are consistent with burial of this region by 1 km of phanerozoic strata that were subsequently denuded thus pointing toward significant phanerozoic deposition in the north american cratonic interior hundreds of kilometers east of where previously documented the results suggest that exploiting radiation damage control on apatite u th he dates through investigation of surface sample apatites with a span of closure temperatures can impose tighter restrictions on thermal histories in low relief cratonic landscapes than was previously possible

resolution of the age structure of the detrital zircon populations of two lower cretaceous sandstones from the weald of england by fission track dating modes in the frequency of distribution of fission track ages obtained from detrital zircon grains may prove characteristic of individual sandstone bodies supporting the identification of the sources from which a particular flow of sedimentary detritus was derived and thus allowing new inferences to be made concerning palaeogeography a computer program has been written and used to identify modes in the zircon fission track age distribution within two lower cretaceous sandstone samples from the weald of southern england pronounced modes appear in one rock around 119 ma 160 ma 243 ma and 309 ma and in the other around 141 ma 175 ma 257 to 277 ma and 394 to 453 ma the geological implications of these quite dissimilar zircon age spectra are discussed it is concluded that they support the palaeogeographical models of allen 1981 and indicate that the provenance of the first sample from the top ashdown sandstone member at dallington in east sussex was almost entirely southerly while that of the second from the netherside sand member at northchapel in west sussex was more varied but predominantly westerly and northerly

early tectonic extension between the agulhas bank and the falkland plateau due to the rotation of the lafonia microplate abstract along the southeast african continental margin a deep sedimentary basin and marginal ridge bordering the agulhas fracture zone formed coevally with south american counterparts underlying the falkland plateau extension in both the southern outeniqua and the falkland basin is related to large angle 100 rotation of the lafonia falkland islands block in middle late jurassic times during attempted propagation of the proto indian ocean across the active circum pacific margin of gondwanaland in the early cretaceous the jurassic microplate boundaries were partly reactivated and partly transected by a right lateral shear along the 1200 km long falkland agulhas transform fault preceded by substantial vorticity of the surrounding crustal blocks this boundary segment separating the south american and african plates was probably generated by complex geodynamic interactions between oceanic plate subduction continental to back arc rift propagation and the impingement of deep mantle plumes on the lithosphere

uplift history of a transform continental margin revealed by the stratigraphic record the case of the agulhas transform margin along the southern african plateau the south and southeast coast of southern africa from 28 s to 33 s forms a high elevated transform passive margin bounded to the east by the agulhas falkland fracture zone affz we analysed the stratigraphic record of the outeniqua and durban thekwini basins located on the african side of the affz to determine the evolution of these margins from the rifting stage to present day the goal was to reconstruct the strike slip evolution of the agulhas margin and the uplift of the inland high elevation south african plateau the agulhas transform passive margin results from four successive stages 1 rifting stage from late triassic to early cretaceous 200 134 ma punctuated by three successive rifting episodes related to the gondwana breakup 2 wrench stage 134 131 ma evidenced by strike and dip slip deformations increasing toward the affz 3 active transform margin stage 131 92 ma during which the falkland malvinas plateau drifts away along the affz with an uplift of the northeastern part of the outeniqua basin progressively migrating toward the west 4 thermal subsidence stage 92 0 ma marked by a major change in the configuration of the margin onset of the shelf break passive margin morphology two main periods of uplift were documented during the thermal subsidence stage of the agulhas margin 1 a 92 ma short lived margin scale uplift followed by a second one at 76 ma located along the outeniqua basin and 2 a long lasting uplift from 40 to 15 ma limited to the durban thekwini basin this suggests that the south african plateau is an old upper cretaceous relief 90 70 ma reactivated during late eocene to early miocene times 40 15 ma

modeling postbreakup landscape development and denudational history across the southeast african drakensberg escarpment margin 1 we employ a numerical surface processes model to study the controls on postbreakup landscape development and denudational history of the southeast african margin apatite fission track data presented in the companion paper suggest that the drakensberg escarpment formed by rapid postbreakup river incision seaward of a preexisting drainage divide located close to its present position and subsequently retreated at rates of only 100 m m y 1 numerical modeling results support such a scenario and show that the prebreakup topography of the margin has exerted a fundamental control on subsequent margin evolution the rheology of the lithosphere lithological variations in the eroding upper crust and inland base level falls provided secondary controls a relatively low flexural rigidity of the lithosphere te 10 km is required to explain the observed pattern of denudation as well as the observed geological structure of the southeast african margin lithological variations have contributed to the formation of flat topped ridges buttressing the main escarpment as well as major fluvial knickpoints both these features have previously been interpreted as supporting significant cenozoic uplift of the margin an inland base level fall possibly related to back cutting of the orange river drainage system and occurring 40 50 m y after breakup explains the observed denudation inland of the escarpment as well as the development of inland drainage parallel to the escarpment our model results suggest that in contrast to widely accepted inferences from classical geomorphic studies the southeast african margin has remained tectonically stable since breakup and escarpment retreat has been minimal 25 km

u th he thermochronometry constraints on unroofing of the eastern kaapvaal craton and significance for uplift of the southern african plateau the timing and causes of the 1 0 km elevation gain of the southern african plateau since paleozoic time are widely debated we report the first apatite and titanite u th he thermochronometry data for southern africa to resolve the unroofing history across a classic portion of the major escarpment that encircles the plateau the study area encompasses 1500 m of relief within archean basement of the barberton greenstone belt region of the eastern kaapvaal craton titanite dates are neoproterozoic apatite dates are cretaceous with most results clustering at ca 100 ma thermal history simulations confirm mesozoic heating followed by accelerated cooling in mid to late cretaceous time the lower temperature sensitivity of the apatite u th he method relative to previous thermochronometry in southern africa allows tighter constraints on the cenozoic thermal history than past work the data limit cenozoic temperatures east of the escarpment to 35 c and appear best explained by temperatures within a few degrees of the modern surface temperature these results restrict cenozoic unroofing to less than 850 m and permit negligible erosion since the cretaceous if substantial uplift of the southern african plateau occurred in the cenozoic as advocated by some workers then it was not responsible for the majority of post paleozoic unroofing across the eastern escarpment significant mesozoic unroofing is coincident with large igneous province activity kimberlite magmatism and continental rifting within and along the margins of southern africa compatible with a phase of plateau elevation gain due to mantle buoyancy sources associated with these events

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

early cretaceous uplift and erosion of the northern appalachian basin new york based on apatite fission track analysis abstract the thermal history of outcropping devonian sediments of the northern appalachian basin new york has been investigated using fission track analysis of detrital apatites from 57 sandstone samples based on lengths and apparent age measurements using fission tracks in apatite it is concluded that lower devonian sediments presently at the surface in the catskill region were cooled rapidly from temperatures higher than about 110 c during early cretaceous times 120 140 ma ago in the western part of new york wellsville buffalo data from late devonian sediments are consistent with cooling at the same time as that identified for the catskill region but from lower temperatures in the range of approximately 80 110 c the maximum temperature these sediments experienced since deposition for a pre uplift paleogeothermal gradient of 25 35 c km the confined track length data indicates uplift and erosion of 2 3 km for western new york and greater than 3 4 km for the catskill region a differential uplift pattern which is consistent with the historical stratigraphic data from the region this conclusion is at variance with earlier interpretations put forth by others rapid broad scale uplift and erosion of the scale identified imply that large volumes of sediment could have been supplied from the northern appalachian basin during the early cretaceous this timing for the dominant post devonian cooling phase in the basin is not accounted for by recent models of the tectonic evolution of the appalachian orogen but is compatible with the change from carbonate to siliciclastic deposition in the atlantic coastal plain it is suggested that this style of broad regional uplift without significant deformation is characteristic of a tectonic regime associated with and subsequent to continental rifting apatite fission track analysis is shown to be a basic tool in providing fundamental limits for thermal history assessment in regional tectonic problems

a tenfold increase in the orange river mean holocene mud flux implications for soil erosion in south africa soil erosion poses a major threat to sustainable agriculture in southern africa but is difficult to quantify one measure of soil erosion is the sediment flux of rivers the orange river is the principal source of sediment to the western margin of south africa with an estimated mean mud flux over the last 11 500 years the holocene epoch of 5 1 3 2 7 4 million metric tons year mt yr a total of 43 gigatons gt 10 15 g representing 72 of the holocene mud flux has accumulated on the shelf in the orange river prodelta and mudbelt a clayey fine silt deposit focused on the inner to middle shelf only 8 5 gt of the mud flux occurs in holocene calcareous ooze on the slope comparison of the clay to mud ratio of offshore deposits with orange river suspended sediment and catchment soils indicates that 20 11 gt of the holocene mud flux has been lost as clay beyond the margin the orange river mud flux prior to the building of large dams 1930 1969 is ten times greater than the mean holocene mud flux and is reconciled with estimates of soil erosion within the catchment a tenfold increase in the orange river mud flux implies up to a hundredfold increase in total soil erosion depending on the extent of mud storage over periods of decades to centuries within the catchment erosion has shifted from areas of high relief and rainfall of the drakensberg escarpment during the holocene to intensely cultivated lands of low relief having moderate to high rainfall in the eastern catchment and to a lesser extent grazing areas of the southern orange river catchment

post breakup evolution of the namibian margin constraints from numerical modeling abstract the namibian margin evolution started 130 myr ago after the breakup between south america and southern africa today it shows specific features i an elevation and a bathymetry about 1000 m higher than the surrounding areas ii a significant sediment supply during the late cretaceous following the first sedimentary peak that occurred during rifting and just afterwards and iii a long term progradation since the mid cretaceous this present configuration results in couplings between several processes and the inherited structure we constrained this evolution using a numerical simulation applied both onshore and offshore integrating different couplings i a thermal adjustment of a lithosphere driven by conductivity ii a flexural isostasy and iii the loading unloading effects of surface matter transfer three processes were investigated rock uplift a change in the erosion efficiency and a sea level fall the results were compared to some pertinent observations made on the namibian margin such as the vertical displacements the sediment transfers the denudation and the stratigraphic architecture in the marginal basin in each simulation rift related reliefs were rapidly relaxed resulting in an exponentially decreasing denudation and sediment accumulation sixty myr after the breakup we simulated an additional uplift an increase in the transfer efficiency of the sediment and a sea level fall the subsidence increased by loading inducing a new progradational trend nonetheless the three types of events differed in the duration of this progradation trend the larger the additional uplift the longer the progradation the transfer efficiency increase and sea level fall only resulted in a transient progradation trend from these results we suggest that the post rift evolution of the namibian margin was associated during the upper cretaceous to the rejuvenation of its continental relief triggered by a climate change during the turonian and by a slow long term rock uplift during the cenozoic

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

cenozoic denudation of corsica in response to ligurian and tyrrhenian extension results from apatite fission track thermochronology 1 the island of corsica france occupies a unique position in the western mediterranean since it has recorded both the cenozoic alpine orogenic history of the area as well as subsequent extensional collapse and oceanic basin formation we present 41 new apatite fission track aft ages and 23 measurements of track length distributions from corsica in order to elucidate its cenozoic thermal and morphological evolution aft ages vary from 10 5 0 8 ma to 53 8 4 1 ma and form a clear spatial pattern oldest ages are encountered in the southwest of the island with a broad band of ages between 20 and 30 ma running across the mountainous central area and ages 20 ma confined to the eastern half of the island samples along the western and northwestern coasts record kilometer scale erosional denudation linked to rifting in the ligurian provencal basin whereas samples from close to the extensionally inverted alpine deformation front record a later cooling phase related to tyrrhenian extension the eastward younging pattern of aft ages suggests the migration of a wave of erosional denudation from west to east across the island apparently controlled by the migrating locus of extension our aft data therefore support models of mediterranean extension controlled by slab roll back

determination of lateral inhomogeneities in reflection seismics by inversion of traveltime residuals lateral inhomogeneities generate fluctuations in the traveltime of seismic waves by evaluation of these traveltime fluctuations from different source and receiver positions lateral inhomogeneities can be located using a pseudo inverse matrix method aki christoffersson and husebye 1977 the formulation of the problem is possible for transmitted waves as well as for reflected and refracted waves in reflection seismics this method is of importance if no reflections from the inhomogeneities themselves but only reflections from lower boundaries can be observed the basic assumptions for the mathematical formulation are 1 the average velocities and depths of the reflecting horizons are known already from standard processing methods and 2 the traveltime residuals are due to lateral velocity changes between different reflectors or between reflectors and the surface the area of the earth to be considered is divided into layers and the layers into rectangular blocks the parallel displacement of a ray after passing a disturbed block is neglected only the traveltime residual is taken into account in this paper the method and its application to data obtained with two dimensional models are described

relative sea level fall since the last interglacial stage are coasts uplifting worldwide the growing interest in quantification of vertical ground motion stems from the need to understand in detail how the earth s crust behaves for both scientific and social reasons however only recently has the refinement of dating techniques made possible the use of paleoshorelines as reliable tools for tectonic studies although there are many local studies of quaternary vertical motions of coastlines we know of no comprehensive worldwide synthesis here we provide a compilation of 890 records of paleoshoreline sequences with particular emphasis on the last interglacial stage marine isotopic stage mis 5e 122 ka the quality of dating mis 5e makes it a reliable marker to evaluate vertical ground motion rates during the late quaternary on a global scale the results show that most coastal segments have risen relative to sea level with a mean uplift rate higher than 0 2 mm yr i e more than four times faster than the estimated eustatic drop in sea level the results also reveal that the uplift rate is faster on average for active margins than for passive margins neither dynamic topography nor glacio hydro isostasy may explain sustained uplift of all continental margins as revealed by the wide distribution of uplifted sequences of paleoshorelines instead we suggest that only plate tectonic processes reconcile all observations of quaternary coastal uplift we propose that long term continental accretion has led to compression of continental plates and uplift of their margins therefore this study concludes that plate tectonics processes impact all margins and emphasizes the fact that the notion of a stable platform is unrealistic these results therefore seriously challenge the evaluation of past sea levels from the fossil shoreline record