rift propagation along the southern dead sea rift gulf of elat abstract the gulf of elat is located at the southern part of the dead sea rift and at the northern part of the red sea geophysical data suggest that it can be divided into three distinct segments northern central and southern on the basis of shallow and deep structures the shallow structure of the gulf of elat as well as of other parts of the dead sea rift is dominated by the presence of grabens which are usually interpreted as pull apart basins formed by the horizontal motion on faults along the dead sea rift this rift is considered to be a plate boundary of the transform type various geophysical data including heat flow gravity magnetics and seismic refraction indicate systematic changes along the length of the gulf of elat the rifting activity is probably more advanced in the southern portion of the gulf this advanced rifting activity propagate from south northward within the southern portion of the gulf of elat the transition from the structure of the red sea to that of the dead sea rift actually takes place of the entire length of the dead sea rift only in the southern part and possibly also in the central part of the gulf of elat is actual mantle upwelling now taking place this upwelling is also associated with a widening of the rift zone at depth the two processes the transform motion and the propagation of rifting activity probably proceed independently

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

a punctuated late ordovician and early silurian deglaciation and transgression evidence from the subsurface of northern saudi arabia abstract the lower section of the lower silurian qusaiba member qalibah formation is characterized by regionally developed organic rich shales that have sourced many of the large paleozoic petroleum systems of saudi arabia in northern saudi arabia these high total organic carbon toc horizons are being assessed for their unconventional shale gas potential the initial phase of exploration drilling which resulted in a quadrupling of the number of penetrations in northern saudi arabia had the dual purpose of 1 assessing the high toc horizons as an unconventional resource play and 2 acquiring the fundamental data required to understand the geologic development of the zones of interest within the lower qusaiba the availability of numerous new cores from across northern saudi arabia enabled an extensive refinement of the existing biostratigraphy and enhanced integration between graptolite and palynomorph biozonation systems in cores from the study area four distinct sedimentary facies are recognized 1 pyritic siltstone 2 black mudstone 3 black chert and 4 gray shale representing distinct paleoenvironmental conditions related to the stepped latest ordovician and early silurian gondwanan deglaciation the failure of the gondwanan ice sheet was not a simple short lived consistent melting and associated flooding of a flat continental shelf this study highlights the complex interplay of sea floor topography ocean currents sediment supply and variations in the rate of melting of the ice sheet with the associated rising ambient temperatures there are 1 increasing clay concentrations associated with intensifying chemical weathering of the exposed land mass and 2 progressive lowering of the carbonate compensation depth as water temperatures rise enabling the preservation of carbonate shell material

on the prediction of channel heads in a complex alpine terrain using gridded elevation data 1 threshold conditions for channel initiation are evaluated by using gridded elevation data derived from a lidar survey a reliable algorithm for the determination of surface flow paths and field observations of channel heads for a study area located in the eastern italian alps these threshold conditions are determined by considering the channel heads observed across a portion of the study area and computing the related values of 1 drainage area a 2 area slope function as2 with s being the local slope and 3 strahler order of surface flow paths extracted from gridded elevation data attention is focused on the dependence of the obtained threshold values on the size of grid cells involved and on the ability of the identified threshold conditions to provide reliable predictions of channel heads across the entire study area the results indicate that the threshold values of a as2 and are all significantly dependent on grid cell size and the uncertainty in the determination of threshold values of is significantly smaller than that affecting the determination of threshold values of a and as2 the comparison between predicted and observed channel heads indicates that the considered methods display variable reliability and sensitivity over different drainage basins and grid cell sizes with a general tendency to predict more channel heads than can be observed in the field acceptable predictions are normally obtained where channel heads are formed essentially by surface runoff more comprehensive methods seem however to be needed to predict channel heads affected by groundwater seeping upward

evaluation of a landscape evolution model to simulate stream piracies insights from multivariable numerical tests using the example of the meuse basin france in landscapes dominated by fluvial erosion the landscape morphology is closely related to the hydrographic network system in this paper we investigate the hydrographic network reorganization caused by a headward piracy mechanism between two drainage basins in france the meuse and the moselle several piracies occurred in the meuse basin during the past one million years and the basin s current characteristics are favorable to new piracies by the moselle river network this study evaluates the consequences over the next several million years of a relative lowering of the moselle river and thus of its basin with respect to the meuse river the problem is addressed with a numerical modeling approach landscape evolution model hereafter lem that requires empirical determinations of parameters and threshold values classically fitting of the parameters is based on analysis of the relationship between the slope and the drainage area and is conducted under the hypothesis of equilibrium application of this conventional approach to the capture issue yields incomplete results that have been consolidated by a parametric sensitivity analysis the lem equations give a six dimensional parameter space that was explored with over 15 000 simulations using the landscape evolution model golem the results demonstrate that stream piracies occur in only four locations in the studied reach near the city of toul the locations are mainly controlled by the local topography and are model independent nevertheless the chronology of the captures depends on two parameters the river concavity given by the fluvial advection equation and the hillslope erosion factor thus the simulations lead to three different scenarios that are explained by a phenomenon of exclusion or a string of events

late cretaceous sediment sample from the amirante passage western indian ocean erosion associated with deep thermohaline flow through the amirante passage western indian ocean has caused upper cretaceous sediments to become exposed at the sea floor the age of a core of these sediments is the strongest direct evidence to date with which to support interpretations of the sea floor spreading history of the area based on geophysical data

cuyania an exotic block to gondwana review of a historical success and the present problems abstract a review of the early history of the cuyania terrane and the numerous pioneering works of the past century provides the present robust framework of evidence supporting a derivation from laurentia travel towards gondwana as an isolated microcontinent and final amalgamation to the protomargin of western gondwana in middle to late ordovician times the major remaining uncertainties and inconsistencies such as the time of deformation and collision with gondwana the lack of evidence of famatinian derived zircons the effects of strike slip displacements proposed along the suture as well as the potential sutures defined by ophiolite assemblages are discussed the precise boundary along the northern and southern limits is not yet well defined the most suitable hypothesis based on present data is that cuyania originated as a conjugate margin of the ouachita embayment south of the appalachian platform during early cambrian times the subsequent travel toward the gondwana protomargin is clearly depicted by the changing faunal assemblages in the carbonate platform new geochemical and age data on k bentonites presented by several authors reinforce the strong connection between cuyania ash fall tuffs and famatina volcanics by 468 470 ma indicating cuyania and gondwana were in close proximity at that time extension related to flexural subsidence preceded by the drowning of the carbonate platform in early llanvirnian times is recorded by abrupt facies changes in the sedimentary cover during late llanvirnian and early caradocian times this episode marked the beginning of contact between cuyania and gondwana the subsequent evolution of the foreland basin indicates that deformation lasted until latest silurian early devonian times the time of collision is tracked by the cessation of arc related magmatic activity in the upper plate gondwana protomargin at about 465 ma in western sierras pampeanas and ages around 454 ma corresponding to syncollisional and postcollisional magmatism the age of the collision is also preserved in the lower plate cuyania where both angular unconformities in the sedimentary cover and the ages of peak of regional metamorphism in the basement rocks point to 460 ma as the most probable age for the beginning of the collision evidence from the upper plate is essentially identical with an age of 463 ma thermal gradients along this suture vary from 13 c km in the lower plate to 18 c km in the fore arc upper plate reaching more than 30 c km along the famatinian arc based on these data a llandelian caradocian age for the collision can be postulated on firm grounds deformation continued through most of the early paleozoic until amalgamation of the chilenia terrane by the late devonian

earthquakes in the davie ridge madagascar region and the southern nubian somalian plate boundary the largest earthquake sequence mb up to 6 4 and m0 up to 5 1018 nm to have struck eastern africa within the past 60 years occurred in may and june of 1985 off the coast of the tanzania mozambique border the epicenters are located on the northern extension of the davie ridge a prominent north south trending bathymetrie feature accompanied by distinct gravity anomalies a joint hypocenter location of the 14 largest events in this sequence appears as a cluster of epicenters with no apparent lineation al teleseismic distances p and sh waves generated by the two largest events of this sequence have nearly identical waveforms on the long period records of the worldwide standard seismograph network a formal inversion of the data shows a pure normal faulting mechanism on nodal planes striking slightly west of north at least two point sources are necessary to model each event one at a depth of 40 10 km and the other at about 20 5 km on a regional scale normal faulting seems to characterize the scattered seismicity throughout the region between madagascar and the eastern arm of the east african rift zone even though oceanic lithosphere dominates part of the region and no topographic expression of a rift system can be identified since normal faulting events of up to 25 5 km deep also characterize the scattered seismicity west of the topographic expression of the western arm of the rift zone at about the same latitude we propose that the southern termination of the nubian somalian plate boundary consists of a diffuse zone of east west extension up to 2000 km wide

motion between the indian antarctic and african plates in the early cenozoic we used a three plate best fit algorithm to calculate four sets of euler rotations for motion between the india capricorn africa somali and antarctic plates for 14 time intervals in the early cenozoic each set of rotations had a different combination of data constraints the first set of rotations used a basic set of magnetic anomaly picks on the central indian ridge cir southeast indian ridge seir and southwest indian ridge swir and fracture zone constraints on the cir and seir but did not incorporate data from the carlsberg ridge and did not use fracture zones on the swir the second set added fracture zone constraints from the region of the bain fracture zone on the swir which were dated with synthetic flowlines based on the first data set the third set of rotations used the basic constraints from the first rotation set and added data from the carlsberg ridge the fourth set of rotations combined both the swir fracture zone constraints and the carlsberg ridge constraints data on the indian plate side of the carlsberg ridge arabian basin were rotated to the capricorn plate before being included in the constraints plate trajectories and spreading rate histories for the cir and swir based on the new rotations document the major early cenozoic changes in plate motion on the cir and seir there was a large but gradual slowdown starting around chron 23o 51 9 ma and continuing until chron 21y 45 3 ma followed 2 or 3 myr later by an abrupt change in spreading azimuth which started around chron 20o 42 8 ma and which was completed by chron 20y 41 5 ma no change in spreading rate accompanied the abrupt change in spreading direction on the swir there was a continuous increase in spreading rates between chrons 23o and 20o and large changes in azimuth around chrons 24 and 23 and again at chron 21 unexpectedly we found that the two sets of rotations constrained by the carlsberg ridge data diverged from the other two sets of rotations prior to anomaly 22o when compared to rotations for the cir that are simultaneously constrained by data from all three branches of the indian ocean triple junction there is a progressively larger separation of anomalies on the carlsberg ridge with a roughly 25 km misfit for anomaly 23o and increasing to over 100 km for anomaly 26y these data require that there was previously unrecognized convergence somewhere in the plate circuit linking the indian capricorn and somali plates prior to chron 22o we quantify this motion by summing our new capricorn somalia rotations with previously published rotations for neogene india capricorn motion and for early cenozoic somali india motion based solely on carlsberg ridge data the most likely possibility is that there was motion within the somalia plate due to a distinct seychelles microplate as young as chron 22o the sense of the misfit on the carlsberg ridge is consistent with roughly 100 150 km of convergence across a boundary passing through the amirante trench and extending north to the carlsberg ridge axis between anomalies 26y and 22o alternatively there may have been convergence within the indian plate either along the western margin of indian or east of the cir in the region of the current capricorn indian diffuse plate boundary our work sharpens the dating of the two major eocene changes in plate motion recognized in the indian ocean

drainage of a deep magma reservoir near mayotte inferred from seismicity and deformation the dynamics of magma deep in the earth s crust are difficult to capture by geophysical monitoring since may 2018 a seismically quiet area offshore of mayotte in the western indian ocean has been affected by complex seismic activity including long duration very long period signals detected globally global navigation satellite system stations on mayotte have also recorded a large surface deflation offshore here we analyse regional and global seismic and deformation data to provide a one year long detailed picture of a deep rare magmatic process we identify about 7 000 volcano tectonic earthquakes and 407 very long period seismic signals early earthquakes migrated upward in response to a magmatic dyke propagating from moho depth to the surface whereas later events marked the progressive failure of the roof of a magma reservoir triggering its resonance an analysis of the very long period seismicity and deformation suggests that at least 1 3 km3 of magma drained from a reservoir of 10 to 15 km diameter at 25 to 35 km depth we demonstrate that such deep offshore magmatic activity can be captured without any on site monitoring recent seismicity near mayotte in the indian ocean is due to dyke propagation from and drainage of a 25 35 km deep magma reservoir according to an analysis of earthquake and deformation data

modelling landscape evolution geomorphology is currently in a period of resurgence as we seek to explain the diversity origins and dynamics of terrain on the earth and other planets in an era of increased environmental awareness yet there is a great deal we still do not know about the physics and chemistry of the processes that weaken rock and transport mass across a planet s surface discovering and refining the relevant geomorphic transport functions requires a combination of careful field measurements lab experiments and use of longer term natural experiments to test current theory and develop new understandings landscape evolution models have an important role to play in sharpening our thinking guiding us toward the right observables and mapping out the logical consequences of transport laws both alone and in combination with other salient processes improved quantitative characterization of terrain and process and an ever improving theory that describes the continual modification of topography by the many and varied processes that shape it together with improved observation and qualitative and quantitative modelling of geology vegetation and erosion processes will provide insights into the mechanisms that control catchment form and function this paper reviews landscape theory in the form of numerical models of drainage basin evolution and the current knowledge gaps and future computing challenges that exist copyright 2010 john wiley sons ltd

madagascar s escape from africa a high resolution plate reconstruction for the western somali basin and implications for supercontinent dispersal accurate reconstructions of the dispersal of supercontinent blocks are essential for testing continental breakup models here we provide a new plate tectonic reconstruction of the opening of the western somali basin during the breakup of east and west gondwana the model is constrained by a new comprehensive set of spreading lineaments detected in this heavily sedimented basin using a novel technique based on directional derivatives of free air gravity anomalies vertical gravity gradient and free air gravity anomaly maps also enable the detection of extinct mid ocean ridge segments which can be directly compared to several previous ocean magnetic anomaly interpretations of the western somali basin the best matching interpretations have basin symmetry around the m0 anomaly these are then used to temporally constrain our plate tectonic reconstruction the reconstruction supports a tight fit for gondwana fragments prior to breakup and predicts that the continent ocean transform margin lies along the rovuma basin not along the davie fracture zone dfz as commonly thought according to our reconstruction the dfz represents a major ocean ocean fracture zone formed by the coalescence of several smaller fracture zones during evolving plate motions as madagascar drifted southwards and offshore tanzania is an obliquely rifted rather than transform margin new seismic reflection evidence for oceanic crust inboard of the dfz strongly supports these conclusions our results provide important new constraints on the still enigmatic driving mechanism of continental rifting the nature of the lithosphere in the western somali basin and its resource potential

channel changes in badlands stripping of the vegetation and soil from a 13 hectare site in virginia underlain by coastal plain sediments created a rapidly evolving badland topography two types of channels developed 1 sand bed alluvial channels were graded to transport the bed material load supplied from slope erosion with available runoff but they also generally eroded their beds slowly and 2 steeper bedrock floored channels incised rapidly in bedrock channels the erosion rate was proportional to the 4 9ths power of drainage area and the 2 3rds power of gradient these exponents are consistent with a model in which the erosion rate is proportional to the bed shear during high flows due to rapid mass wasting and reduced runoff the alluvial channels became as much as 50 steeper during the winter than the summer with an attendant yearly cycle of winter aggradation and summer entrenchment the gradients their seasonal variability and their downstream hydraulic geometry were consistent with the predictions of total load transport formulas for sand beds and high loads the hydraulic geometry of alluvial channels in the virginia badlands were similar to that on the morrison formation in the western united states

magnetostratigraphy paleomagnetic correlation and deformation of pleistocene deposits in the south central puget lowland washington 1 paleomagnetic results from pleistocene sedimentary deposits in the central puget lowland indicate that the region has experienced widespread deformation within the last 780 kyr three oriented samples were collected from unaltered fine grained sediments mostly at sea level to determine the magnetostratigraphy at 83 sites of these 47 have normal 18 have reversed and 18 have transitional 8 localities polarities records of reversed to normal polarity transitions of the geomagnetic field were found in thick sections of silt near the eastern end of the tacoma narrows bridge and again at wingehaven park near the northern tip of vashon island the transitional horizons probably related to the bruhnes matuyama reversal apparently fall between previously dated pleistocene sediments at the puyallup valley type section all reversed polarity to the south and the whidbey island type section all normal polarity to the north the samples in general are of sufficient quality to record paleosecular variation psv of the geomagnetic field and a statistical technique is used to correlate horizons with significant agreement in their paleomagnetic directions our data are consistent with the broad structures of the seattle uplift inferred at depth from seismic reflection gravity and aeromagnetic profiles but the magnitude of vertical adjustments is greatly subdued in the pleistocene deposits index terms 1520 geomagnetism and paleomagnetism magnetostratigraphy 1522 geomagnetism and paleomagnetism paleomagnetic secular variation 1525 geomagnetism and paleomagnetism paleomagnetism applied to tectonics regional global 1535 geomagnetism and paleomagnetism reversals process timescale magnetostratigraphy keywords puget lowland pleistocene magnetostratigraphy paleosecular variation tectonic deformation

fluvial system response to middle and upper pleistocene climate change in the meurthe and moselle valleys eastern paris basin and rhenish massif abstract research in the meurthe and moselle valleys downstream from the vosges massif has resulted in the definition of the terrace system and in the recognition of the climatic influence on its formation eight alluvial terraces were recognized at less than 100 m relative height an absolute chronology was established for the youngest alluvial terraces based on infrared optically stimulated luminescence and radiocarbon dating this provides evidence that each terrace corresponds to a single glacial interglacial cycle this result is confirmed by the study of numerous sections which present a constant succession of lithofacies in each valley part four alluvial sequences were also defined along the meurthe and moselle valleys reflecting middle and late pleistocene climate change their upper basin developed in the vosges massif was actually covered by glaciers during the pleistocene cold periods the differences between these four alluvial sequences are attributed to the unequal expansion of ice in the upper basins of the meurthe and moselle rivers and their tributaries in spite of these differences the sequences show important similarities main aggradation events correlate with cold periods pleniglacial and lateglacial whereas erosional phases occurred at the warm to cold transitions and secondarily at the pleniglacial lateglacial transitions this evolution is comparable with those already described in other northwestern european valleys especially since the upper saalian

shape and amount of the quaternary uplift of the western rhenish shield and the ardennes western europe article i nfo a good evaluation of the quaternary uplift of the rhenish shield is a key element for the understanding of the cenozoic geodynamics of the western european platform in front of the alpine arc previous maps of the massif uplift relied on fluvial incision data since the time of the rivers younger main terrace to infer a maximum post 0 73 ma uplift of 290 m in the se eifel here we propose a new interpretation of the incision data of the intra massif streams where anomalies in the terrace profiles would result from knickpoint retreat in the tributaries of the main rivers rather than from tectonic deformation we also use additional geomorphological data referring to 1 deformed tertiary planation surfaces 2 the history of stream piracy that severely affected the meuse basin in the last 1 ma and 3 incision data outside the rhenish shield a new map of the post 0 73 ma uplift of the rhenish shield is drawn on the basis of this enlarged dataset it reduces the maximum amount of tectonic uplift in the se eifel to 140 m and modifies the general shape of the uplift namely straightening its e w profile it is also suggested that an uplift wave migrated across the massif starting from its southern margin in the early pleistocene and currently showing the highest intensity of uplift in the northern ardennes and eifel these features seem to favour an uplift mechanism chiefly related to lithospheric folding and minimize the impact on the topography of a more local eifel plume

the tectonic setting of the seychelles mascarene and amirante plateaus in the western equatorial indian ocean abstract a granitic crustal fragment occurs within a system of the marine plateaus in the western equatorial indian ocean the plateaus form an arcuate series of wide shallow banks with small islands in places in spite of the geographical proximity and the morphological similarity the geological structure and origin of the seychelles bank amirante arc and mascarene plateau contrast sharply the seychelles bank is founded on precambrian granite basement the mascarene plateau from saya de malha bank to mauritius island is founded on paleocene basalts and upper cretaceous basalts prevail in the amirante arc geological and geophysical data suggest that the precambrian granite of the seychelles bank is a fragment separated from the indian continental plate during the paleocene this break up was associated with the volcanic activity that built the mascarene plateau the occurrence of middle eocene syenite diorite and microgranite in the western part of the seychelles bank could indicate that crustal accretion across the northwestern indian ocean ridge apparently drove the seychelles block southwestwards causing it to collide with the northeastern edge of the mascarene basin this collision could have led to an early tertiary subduction and subsequently to the development of the amirante trough the absence of post eocene igneous suites in the seychelles could indicate that a change in the spreading pattern of the northwestern indian ocean during the late eocene probably terminated the amirante subduction

mesoproterozoic chronostratigraphy of the southeastern llano uplift central texas the llano uplift of central texas exposes mesoproterozoic crystalline rocks affected by a grenvillian orogeny between 1 2 and 1 1 ga we report four new u pb zircon protolith ages of 1247 4 1257 3 1272 8 5 and 1366 3 ma for four quartzofeldspathic rocks from the regionally defined packsaddle schist and valley spring gneiss in the southeastern part of the uplift the 1366 3 ma gneiss also yields a metamorphic age of 1325 5 ma these new u pb ages in conjunction with previous u pb age data demonstrate that several regionally defined map units contain constituents with widely disparate ages we have identified four age suites for protoliths of metamorphic rocks in the southeastern part of the llano uplift that appear to represent rock packages of distinct tectonic origins 1 the northern 1366 1272 ma felsic gneisses of volcanic plutonic and continentally derived sedimentary origin valley spring gneiss and an older gneiss component 2 the geographically intermediate ca 1257 1247 ma and possibly older basinal sequence that formed along a continental shelf and slope near an arc packsaddle schist 3 the southern 1326 1275 ma remnant of an exotic ensimatic arc complex big branch gneiss and coal creek serpentinite and plutonic complex and 4 the 1239 1232 ma tectonized felsic rocks that intruded the first two suites the first three suites of rocks were structurally imbricated during grenville orogenesis and the fourth suite may either represent early synorogenic crustal melts or be related to the volcanic rocks in the packsaddle schist in addition these new data show that contrary to previous reports younger packsaddle schist lies in structural contact above older components of the valley spring gneiss moreover one of the dated meta igneous units is the oldest unit yet found in the uplift and records an early period of metamorphism prior to formation of most llano uplift rocks its 1366 3 ma protolith age is coeval with rocks of the western granite rhyolite terrane age and geochemical similarities link this unit of unknown regional extent with the western granite rhyolite terrane of known laurentian affinity thus this component of the llano uplift may be the basement on which the younger components of the valley spring gneiss and packsaddle schist formed in a tectonic setting proximal to laurentia and south of the western granite rhyolite terrane

the extent of continental crust beneath the seychelles abstract the granitic islands of the seychelles plateau have long been recognised to overlie continental crust isolated from madagascar and india during the formation of the indian ocean however to date the extent of continental crust beneath the seychelles region remains unknown this is particularly true beneath the mascarene basin between the seychelles plateau and madagascar and beneath the amirante arc constraining the size and shape of the seychelles continental fragment is needed for accurate plate reconstructions of the breakup of gondwana and has implications for the processes of continental breakup in general here we present new estimates of crustal thickness and v p v s from h stacking of receiver functions from a year long deployment of seismic stations across the seychelles covering the topographic plateau the amirante ridge and the northern mascarene basin these results combined with gravity modelling of historical ship track data confirm that continental crust is present beneath the seychelles plateau this is 30 33 km thick but with a relatively high velocity lower crustal layer this layer thins southwards from 10 km to 1 km over a distance of 50 km which is consistent with the seychelles being at the edge of the deccan plume prior to its separation from india in contrast the majority of the seychelles islands away from the topographic plateau show no direct evidence for continental crust the exception to this is the island of desroche on the northern amirante ridge where thicker low density crust consistent with a block of continental material is present we suggest that the northern amirantes are likely continental in nature and that small fragments of continental material are a common feature of plume affected continental breakup

long range sonar observations of sedimentary facies patterns in and around the amirante trench western indian ocean long range side scan sonographs are used to map two distinct acoustic facies in the amirante trench western indian ocean a structural barrier formed by a seamount within the trench separates the two facies types the acoustic facies developed s of the seamount is characterized by weak reflectivity returned from sediments deposited by the flow of the deep western boundary undercurrent through the channels of the amirante passage into the southern part of the amirante trench n of the seamount the acoustic facies is characterized by high reflectivity and is confined to the trench axis it arises from sediments episodically deposited by turbidity currents or debris flows possibly originated by slumping of reef talus from the atolls surmounting the adjacent amirante ridge

cretaceous ocean floor in the amirante passage tectonic and oceanographic implications abstract core v34 76 obtained near the summit of a basement ridge in the axis of the amirante passage recovered in situ pelagic chalk of latest cretaceous age abathomphalus mayaroensis zone ca 66 ma this minimal age for the nne trending basement ridges places important constraints on the crustal age of the amirante passage region in the western indian ocean and indicates that the passage has provided a deep water connection between the mascarene and somali basins throughout the entire cenozoic

multi process late quaternary landscape evolution modelling reveals lags in climate response over small spatial scales landscapes evolve in complex non linear ways over quaternary timespans integrated geomorphological field studies usually yield plausible hypotheses about timing and impact of process activity landscape evolution models lems have the potential to test and falsify these landscape evolution hypotheses despite this potential lems have mainly been used with hypothetical data and rarely to simulate the evolution of an actual landscape in this paper we use a lem lapsus landscape process modelling at multi dimensions and scales to explore if it is possible to test and falsify conclusions of an earlier field study on 50 ka landscape evolution in okhombe valley kwazulu natal south africa in this lem five landscape processes interact without supervision water driven erosion and deposition creep solifluction biological weathering and frost weathering calibration matched model results to three types of qualitative fieldwork observations individual process activity over time relative process activity over time and net landscape changes over time results demonstrate that landscape evolution of the okhombe valley can be plausibly simulated a particularly interesting and persistent feature of model results are erosional and depositional phases that lag climatic drivers both by decades and by several ka within a few hundred meters the longer lag has not been reported for this spatial scale before and may be an effect of slow landscape soil vegetation feedbacks the combined modelling and fieldwork results allow a more complete understanding of these responses to climate change and can fill in hiatuses in the stratigraphical record suggestions are made for methodological adaptations for future lem studies

the crustal structure of the southern davie ridge offshore northern mozambique a wide angle seismic and potential field study abstract some of the oldest surviving oceanic basins in the world the mozambique and west somali basins were created during the breakup of gondwana starting around 180 ma between the two basins relative movements of west gondwana and east gondwana including madagascar created a shear zone the davie fracture zone dfz with a topographic elevation davie ridge dr marking its centre the crustal composition of the dfz and dr is a subject of speculation and debate in this study we present seismic refraction data across the prominent topography of the southern dr ray tracing of the wide angle data as well as additional seismic amplitude modelling and 2 5d density modelling constrain its crustal structure and architecture the data indicate that in the mozambique channel the dr consists of fragments of continental crust with a thickness of 10 to 12 km an oceanic crust indenter extends northward from the mozambique basin into the area between the dr and the east african margin at 16 5 s northeast of the dr at 41 8 w 14 5 s the somali basin is probably floored by 6 km thick oceanic crust hence the continental dr separates oceanic crust of the somali and mozambique basins the transitional crustal area at the central mozambican margin is underlain by high velocity lower crust hvlc the hvlc has velocities up to 7 3 km s and extents along the margin vanishing northward between 16 5 and 14 5 s at the madagascan side of the dr at 16 5 s the highly intruded stretched continental crust is 9 km thick and possibly underlain with a smaller hvlc of 2 9 km thickness and an e w extent of 120 km the oceanic crust at 14 5 s represents the oldest part the somali basin which formed after the initial nw se rifting between east and west gondwana

spatial and temporal patterns of the upper pleistocene alluvial fill deposits of the upstream seine river alluvial plain la bassee france abstract this study focuses on the contrasted architectural elements of the two late pleistocene alluvial fill sequences t1 and t2 attributed to the late saalian and weichselian of the upstream seine river alluvial plain la bassee and constitutes the first estimation of the sediment volumes associated to these glacial cycles along this river estimations of the incised valley and valley fill spatial distribution are obtained by cokriging of data from 546 wells collected from the french national repository and a digital elevation model dem of the present topography the well stratigraphic attribution is given by the homogenised geomorphological map of la bassee we calculated the sediment volume of eroded material for the t1 sequence to be 1 70 109 m3 and of valley fill deposits to be 1 18 109 m3 this corresponds to a mean thickness of 6 1 m by contrast volumes of the t2 sequence are one third smaller 1 23 109 m3 the t2 mean thickness is very similar with 5 8 m the alluvial plain is divided into four reaches representing a typical assemblage of the t1 and t2 incision style and valley fill deposits along the downstream reaches 1 montereau bazoches 2 bazoches grisy the t2 proportion per kilometer are close to the alluvial plain mean value reach 1 76 reach 2 69 la bassee 71 the t2 mean incised valley width corresponds to three quarters of the t1 4 km in the middle upstream reach 3 grisy courceroy the t2 proportion is the largest 92 the t2 mean incised valley width is the largest and comparable to the t1 5 5 km along the upstream reach 4 courceroy romilly the t2 proportion is the lowest 53 and the t2 incised valley 2 km wide is enclosed within the t1 incised valley 4 km wide the rate of exported material though is the same for each sequence in any reach 31 the spatial pattern of the t1 t2 deposits coincides largely with the presence of a knickpoint along the t2 paleothalweg downstream part of reach 4 slope face 1 5 suggesting that the knickpoint had a large influence on the fluvial processes the toe of the knickpoint reaches 3 to 1 slope 0 24 was the site of the most efficient erosion of the t1 deposits we propose that the change of slope favored diversion of the channel path upstream of the knickpoint face upstream part of reach 4 thalweg slope 0 34 the incision processes were dominated by incision avulsions or multithreading with limited lateral migration as indicated by the numerous t1 relicts preserved across valley the sediment export corresponds to a mean incision rate of 40 m ma during the last 300 kyr a fairly low value for the paris basin suggesting a low uplift area north of the massif central such contrast between the fluvial processes during the t1 and t2 sequences is most likely to be looked for in the climatic conditions that existed at the onset of the incision as no local controls for the knickpoint location could be identified such as active faults substrate heterogeneity or flow modifications the lower sediment flux and the knickpoint associated to the t2 sequence suggest a transient river profile and valley incision at the end of the climatic transition associated to the weichselian t2

evolution of the continental margin of western india new evidence from apatite fission track dating apatite fission track dating in southwestern india yields middle triassic to middle jurassic ages that correlate with the breakup of eastern gondwanaland and africa the later india madagascar split is not recorded in the apatite ages the apatite ages are not a reflection of thermal events in the present location of this region but are associated with uplift and erosion in the upper paleozoic early mesozoic eastern gondwana africa separation track length measurements indicate that cooling through the 100 c isotherm began in the upper paleozoic time temperature modeling suggests that a period of slow cooling during the mesozoic was followed by relatively rapid cooling to the present

late thermal evolution of proterozoic rocks in the northeastern llano uplift central texas abstract the llano uplift of central texas usa contains large exposures of polydeformed grenvillian metaigneous and metasedimentary rocks that were intruded by late to post orogenic granites new u pb and 40 ar 39 ar single mineral ages and rb sr mineral isochrons document the retrograde cooling history that followed grenville age metamorphism in the north central and northeastern llano uplift and explain discrepancies between previous u pb crystallization ages and younger determinations based on rb sr and k ar isotopic systems dating has been carried out in the valley spring gneiss vsg and the post tectonic lone grove pluton lgp in three areas of the llano uplift from west to east vsg rocks in the babyhead area north of llano texas yielding hornblende muscovite and biotite 40 ar 39 ar ages and rb sr mineral isochron ages from 1098 to 1014 ma a sample from the lgp at petrick quarry yielding u pb titanite and hornblende and biotite 40 ar 39 ar ages from 1096 to 1076 ma and vsg samples from the inks lake area in the eastern llano uplift yielding u pb titanite hornblende and biotite 40 ar 39 ar and rb sr mineral isochron ages from 1087 to 1029 ma titanite from the oatman granite to the southwest of these areas yields a u pb age of 1119 6 ma these ages combined with closure temperature estimates document cooling in the llano uplift from 1100 1000 ma at rates between 5 and 14 c ma 1 although rapid uplift following early high pressure metamorphism has important regional thermal effects the cooling rates documented in this study are interpreted to represent the relaxation of local geotherms following local magmatic activity the low temperature history documented in this study is probably unrelated to uplift rates this supports the suggestion that magmatic heating was an important factor in the late high temperature low pressure metamorphism of the llano region it further suggests that the late pressure and temperature conditions recorded in the rocks were likely not synchronous across the region

how well can hillslope evolution models explain topography simulating soil transport and production with high resolution topographic data the morphology of hillslopes is a direct refl ection of tectonic forcing and climatic and biologic processes that drive soil production mobilization and transport soil transport on hillslopes affects river incision by providing tools for channel abrasion and controls the distribution of sediment that infl uences aquatic habitat although numerous hillslope transport relationships have been proposed over the past 60 years a comprehensive analysis of model predictions for a real landscape has not been performed here we use high resolution topographic data obtained via airborne laser swath mapping alsm to simulate the long term evolution of oregon coast range hillslopes and test three published transport models and a new model that accounts for nonlinear depth and slopedependent transport analysis of one dimensional steady state solutions for these four models suggests that plots of gradient curvature may be diagnostic for distinguishing model predictions to evaluate two dimensional model predictions for our fi eld site we assumed local steady state erosion for a 72 000 m 2 sequence of hillslopes and valleys after calibrating each of the four models we imposed constant base level lowering for cells within the valley network simulated 500 000 yr of soil production and transport and determined which transport model best preserved morphologic patterns that describe the current landscape form models for which fl ux varies proportionally with hillslope gradient generated broadly convex hilltops inconsistent with the sharp crested steep sided slopes of our study site whereas the two nonlinear slope dependent models produced convex planar slopes consistent with current hillslope form our proposed nonlinear slope and depth dependent model accounts for how soil thickness controls the magnitude of biogenic disturbances that drive transport this model best preserved the current landscape form particularly the narrow sharply convex hilltops characteristic of the oregon coast range according to our formulation which provides an explicit linkage for relating the distribution of biota to hillslope processes the degree of hilltop convexity varies nonlinearly with the ratio of erosion rate to maximum soil production rate highlighting the profound infl uence of soil depth on hillslope evolution

drainage basin responses to climate change recent investigations have shown that the extent of the channel network in some drainage basins is controlled by a threshold for overland flow erosion the sensitivity of such basins to climate change is analyzed using a physically based model of drainage basin evolution the golem model simulates basin evolution under the action of weathering processes hillslope transport and fluvial bedrock erosion and sediment transport results from perturbation analyses reveal that the nature and timescale of basin response depends on the direction of change an increase in runoff intensity or a decrease in vegetation cover will lead to a rapid expansion of the channel network with the resulting increase in sediment supply initially generating aggradation along the main network followed by downcutting as the sediment supply tapers off by contrast a decrease in runoff intensity or an increase in the erosion threshold will lead to a retraction of the active channel network and a much more gradual geomorphic response cyclic changes in runoff intensity are shown to produce aggradational degradational cycles that resemble those observed in the field cyclic variations in runoff also lead to highly punctuated denudation rates with denudation concentrated during periods of increasing runoff intensity and or decreasing vegetation cover the sediment yield from threshold dominated basins may therefore exhibit significant variability in response to relatively subtle environmental changes a finding which underscores the need for caution in interpreting modern sediment yield data

characterization and tectonic evolution of a mesoproterozoic island arc in the southern grenville orogen llano uplift central texas abstract integrated field and u pb geochronologic data document the presence and tectonic evolution of a distinct lithotectonic terrane in the southeastern llano uplift here termed the coal creek domain ccd the ccd consists of tonalitic and amphibolitic gneisses foliated gabbroic tonalitic and granodioritic intrusions and the structurally emplaced coal creek serpentinite the oldest rocks in the ccd consist of tonalitic gneisses with protolith crystallization ages of 1326 ma and 1301 ma and pre 1292 ma amphibolitic gneisses all ages are u pb zircon they were intruded by dominantly mafic and intermediate with subordinate amounts of felsic low k plutonic rocks between 1286 ma and 1275 ma high temperature metamorphism and local anatexis that resulted in the formation of gneisses and migmatites occurred between 1301 ma and 1286 ma and is suggested to have occurred at 1292 ma metamorphism at 1256 ma resulted in the growth of zircons in mafic rocks the regional significance of this metamorphic event is unclear but it may be related to dynamothermal metamorphism of the ccd the contemporaneous emplacement of low k gabbro tonalite and granodiorite and their spatial association with serpentinite combined with published geochemical data strongly suggest that the ccd represents an ensimatic arc the coal creek serpentinite is interpreted as a fragment of oceanic crust that formed either basement to the arc or in a related back arc or intra arc setting combined field u pb geochronologic isotopic and geochemical data indicate that the magmatic metamorphic and structural history of the ccd differs from that of coeval rocks elsewhere in the llano uplift the boundary between the ccd and deformed and metamorphosed granitic and supracrustal rocks to the north is a zone of intense mylonitization and polyphase deformation this zone is interpreted as the structural boundary between the arc terrane and units to the north

the separation of madagascar and africa identification of a sequence of east west trending magnetic anomalies of mesozoic age in the western somali basin helps define the position of madagascar in the gondwana reconstruction the anomalies are symmetric about ancient ridge segments and are flanked to the north and south by the jurassic magnetic quiet zone the motion of madagascar relative to africa was from the north and began in the middle jurassic about the same time as the initial breakup of gondwanaland sea floor spreading ceased when madagascar assumed its present position in the early cretaceous

re examination of geophysical data off northwest india implications to the late cretaceous plate tectonics between india and africa abstract the gop and laxmi basins lying off northwest india have been assigned ambiguous crustal types and evolution mechanisms the chagos laccadive ridge clr complex lying along the southwest coast of india has been attributed to different evolutionary processes late cretaceous seafloor spreading between india and africa formed the mascarene basin and the plate reconstruction models depict unequal crustal accretion in this basin re interpretation of magnetic data in the gop and laxmi basins suggests that the underlying oceanic crust was accreted contemporaneously from 79 ma at slow half spreading rates 0 6 to 1 5 cm yr separating the seychelles laxmi ridge complex from india the spreading ridge became extinct at 71 ma in the central region between 19 20 n and 65 5 67 e and at 68 7 ma in the gop basin extinction progressed southwards with time until 64 1 ma at 14 5 n in the laxmi basin this spreading probably limited the seafloor spreading in the northern mascarene basin while normal to fast spreading continued in the south the fracture zone fz2 acted as a major boundary between the northern and southern mascarene basin and may have also restricted the southward propagation of the laxmi basin spreading ridge plate reconstruction models generated in this study accommodate several microplates between madagascar and india and trace their movements over time within the india africa spreading corridor furthermore these models suggest the formation of the seychelles laxmi ridge microplate by 79 ma in a complex three plate system this microplate may have existed up to 64 1 ma and experienced an anticlockwise rotation of about 50 under the influence of the deccan hotspot activity the laxmi basin spreading ridge jumped southwestward splitting the seychelles laxmi ridge complex and initiating the carlsberg ridge the presence of about 160 km of oceanic crust in the gop and laxmi basins partially reduces the space problem faced in the earlier plate reconstruction models

climatic influences on profile concavity and river incision 1 longitudinal profiles of alluvial and bedrock rivers are typically concave up and the precise shape of their concavity is known to integrate tectonic climatic and channel grain size distribution factors we isolate climatic hydrologic effects on profile concavity through a spatial analysis of large watersheds with mixed bedrock alluvial trunk channels spanning a steep climatic hydrologic gradient in a tectonically stable landscape forty watersheds on the eastern american high plains were selected to minimize variability in base level relief vegetation rock type and drainage area we calculate stream concavity by two complementary methods an area normalized stream concavity index sci and the slope of a line regressed through log log plot of channel slope and basin area the sci and covary more importantly a positive correlation exists between profile concavity and climatic hydrologic factors such as precipitation intensity and peak annual discharge however modeled profile steepness has no correlation to climate or watershed hydrology we conclude that in tectonically stable settings higher intensity rainfall and greater mean annual precipitation lead to more concave profiles we do not have data to know if the concavity changes reflect primarily bedrock or alluvial grain size processes but generally a doubling of rainfall intensity on the high plains leads to a tripling in concavity manifest as tens of meters of incision such climatically influenced profile concavity could explain some of the often cited late cenozoic incision and increase in local relief for the high plains and rocky mountains traditionally ascribed to tectonics

morphology sediments and structure of the amirante trench western indian ocean implications for trench origin abstract the amirante trench is a 600 km long and up to 5200 m deep topographic depression whose origin has remained enigmatic the trench contains 1 km of undisturbed ponded turbidites and mass transport deposits that have been periodically modified by northward flowing contour currents of the deep western boundary current dwbc morphologically the trench and adjacent amirante ridge resemble an arcuate oceanic arc trench system and some investigators have suggested that this complex represents some type of subduction zone however our analysis of bathymetric and structural data indicate that the trench is not a single continuous arcuate feature but is actually a compound feature composed of three discrete essentially linear segments of differing structural orientations and probable sea floor spreading origins the northern segment 4 20 6 20 s trends ne 030 and apparently represents a fracture zone or transform fault trend related to sea floor spreading in the mascarene basin during the late cretaceous the central segment 6 20 8 40 s trends nnw 350 and appears to represent a fracture zone lineament that is parallel to fracture zones and lineaments created by the opening of the somali basin during the late jurassic and early cretaceous however somali basin crust apparently does not extend eastward as far as this portion of the trench so the origin of this segment remains uncertain the southern segment 8 40 10 s trends nw 310 and may represent a tensional rift related to sea floor spreading within the mascarene basin during the early tertiary further our data show no convincing evidence that any portion of this feature was a subduction zone at any time during its development the data also essentially rule out an extraterrestrial impact origin for this region at the end of the cretaceous

the crustal structure of the central mozambique continental margin wide angle seismic gravity and magnetic study in the mozambique channel eastern africa abstract the continental margin of mozambique formed during the initial dispersal of gondwana about 180 ma due to the lack of deep seismic and dense potential field data many details of the timing and geometry of the early breakup in this region remained unknown to date to close this gap a research project mobamasis mozambique basin marine seismic survey with the french research vessel r v marion dufresne ii was conducted in 2007 this paper presents the results of p wave magnetic and 2d gravity modelling along two parallel seismic refraction profiles between 37 and 41 e crossing the mozambique rifted margin the crust shows the characteristics of normal to slightly thickened oceanic crust a lower crustal high velocity body with p wave velocities of 7 0 7 5 km s is observed along both profiles its origin is discussed in the context of upper mantle convection and thermal properties the existing magnetic anomaly identifications have been extended to older ages we postulate that the oldest oceanic crust near the central mozambique continental margin has been formed around m41n 166 ma closer to the coast a pronounced negative magnetic anomaly exists that we interpret to coincide with the continent ocean transition this implies that the position of the continent ocean transition is located significantly closer to the shoreline than proposed before

seaward extension of the east african rift the continental margin of northern mozambique is edged by a north south trending alignment of submarine peaks forming the davie ridge fig la from earlier studies1 5 the ridge has been interpreted as a remnant transform fault that controlled the late jurassic early cretaceous drift of madagascar and the opening of the somali basin6 8 during md40 macamo cruise june 1984 7 500 km of water gun seismic reflexion lines have been implemented throughout the region where two flat bottomed troughs the north south trending kerimbas and lacerda deep grabens interrupt the continental slope alongside the ridge these grabens cut into tertiary sediments correspond westward to a miocene tilting or a recent normal fault and eastward to an old discontinuity several times reactivated and located above an abrupt uplift of the moho the two grabens are separated from each other by a rigid region of thickened crust affected by intense seismicity and supporting volcanoes we report here that the eastern branch of the east african rift joins the submerged grabens across the tanzanian continental shelf fig 1b

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

reconstruction of madagascar and africa evidence from the davie fracture zone and western somali basin new seismic reflection gravity and magnetic data from offshore east africa allow the davie fracture zone to be traced from 11 s to its intersection with the kenyan coast at 2 s constraining the relative motion of madagascar and africa seasat derived free air gravity anomalies and slope rise positive magnetic anomalies observed in shipboard data help to locate the continent ocean boundaries cob off the shore of east africa and madagascar seismic reflection data further document a diapirprovince off madagascar presumably conjugate to that observed off kenya and somalia the dhow and very large crude carrier vlcc basement ridges are complex features and do not appear to be simple fracture zones owing their existence entirely to the separation of madagascar and africa from these data we determine a predrift fit of madagascar and africa involving a 14 2 rotation of madagascar to africa about a pole at 10 n 150 e the geometry of the reconstruction adheres to seismic and potential field data indicating the oceanic nature and extent of the comoros basin and of the somali basin between kenya and the seychelles and it does not conflict with onshore or offshore stratigraphy timing of the opening of the western somali basin is constrained by mesozoic marine magnetic anomalies and extrapolation to the interpreted cob and occurred between approximately 165 and 130 ma

somali basin chain ridge and origin of the northern somali basin gravity and geoid low the northern somali basin located between chain ridge and the horn of africa north of 4 n is characterized by a distinct 5 m geoid low and by large negative gravity anomalies the boundaries of the basin are marked by steep gradients in both gravity and geoid basement in northern somali basin is 1 2 km deeper than on the carlsberg ridge flank to the southeast or on the sheba ridge flank to the north with a sharp discontinuity across the boundary of the basin the western somali basin to the south was created in the late jurassic and early cretaceous by the movement of madagascar away from africa reinterpretation of the magnetic anomalies in the western somali basin shows that they record both limbs of a mid ocean ridge that was active by m22 time kimmeridgian and died soon after mo aptian magnetic and gravity data allow the relict ridge crest to be traced from davie ridge near the african coast to the dhow vlcc ars fracture zone complex at 50 e davie ridge is a transform fault connecting the western somali basin spreading center with a similar age spreading center in the mozambique basin the dhow vlcc ars complex can be shown to continue north of the 4 n bathymetric high separating the northern and western somali basins and to intersect the african margin near 7 n the northern somali basin thus appears to be the third of a series of oceanic basins separated by long transform faults created during movement between east and west gondwanaland the original northern somali basin was split apart by the northward motion of india in the late cretaceous and chain ridge formed along the new boundary thermal and gravity modeling shows that the flexure resulting from differential subsidence across chain ridge combined with the difference in lithospheric thermal structure late jurassic vs early tertiary on either side of it explains well the amplitude and shape of the observed geoid step and gravity anomalies across chain ridge the geoid step up from the basin up to the african coast can be modeled as an edge effect between the juxtaposed jurassic oceanic and african continental crust thus the geoid and gravity low over the northern somali basin results from the superposition of a continental edge effect anomaly and the fracture zone edge effect anomaly

dynamic reorganization of river basins river networks evolve as migrating drainage divides reshape river basins and change network topology by capture of river channels we demonstrate that a characteristic metric of river network geometry gauges the horizontal motion of drainage divides assessing this metric throughout a landscape maps the dynamic states of entire river networks revealing diverse conditions drainage divides in the loess plateau of china appear stationary the young topography of taiwan has migrating divides driving adjustment of major basins and rivers draining the ancient landscape of the southeastern united states are reorganizing in response to escarpment retreat and coastal advance the ability to measure the dynamic reorganization of river basins presents opportunities to examine landscape scale interactions among tectonics erosion and ecology

large scale geomorphology classical concepts reconciled and integrated with contemporary ideas via a surface processes model linear systems analysis is used to investigate the response of a surface processes model spm to tectonic forcing the spm calculates subcontinental scale denudational landscape evolution on geological timescales 1 to hundreds of million years as the result of simultaneous hillslope transport modeled by diffusion and fluvial transport modeled by advection and reaction the tectonically forced spm accommodates the large scale behavior envisaged in classical and contemporary conceptual geomorphic models and provides a framework for their integration and unification the following three model scales are considered micro meso and macroscale the concepts of dynamic equilibrium and grade are quantified at the microscale for segments of uniform gradient subject to tectonic uplift at the larger meso and macroscales which represent individual interfluves and landscapes including a number of drainage basins respectively the system response to tectonic forcing is linear for uplift geometries that are symmetric with respect to baselevel and which impose a fully integrated drainage to baselevel for these linear models the response time and the transfer function as a function of scale characterize the model behavior numerical experiments show that the styles of landscape evolution depend critically on the timescales of the tectonic processes in relation to the response time of the landscape when tectonic timescales are much longer than the landscape response time the resulting dynamic equilibrium landscapes correspond to those envisaged by hack 1960 when tectonic timescales are of the same order as the landscape response time and when tectonic variations take the form of pulses much shorter than the response time evolving landscapes conform to the penck type 1972 and to the davis 1889 1899 and king 1953 1962 type frameworks respectively the behavior of the spm highlights the importance of phase shifts or delays of the landform response and sediment yield in relation to the tectonic forcing finally nonlinear behavior resulting from more general uplift geometries is discussed a number of model experiments illustrate the importance of fundamental form which is an expression of the conformity of antecedent topography with the current tectonic regime lack of conformity leads to models that exhibit internal thresholds and a complex response

headward retreat of streams in the late oligocene to early miocene swiss alps this study uses the widths the spacing and the grain size pattern of oligo miocene alluvial fan conglomerates in the central segment of the swiss alpine foreland to reconstruct the topographic development of the alps these data are analysed with models of longitudinal stream profile development to propose that the alpine topography evolved from an early transient state where streams adjusted to rock uplift by headward retreat to a mature phase where any changes in rock uplift were accommodated by vertical incision the first stage comprises the time interval between ca 31 ma and 22 ma when the alpine streams deposited many small fans with a lateral spacing of 30 km in the north alpine foreland as the range evolved the streams joined and the fans coalesced into a few large depositional systems with a lateral spacing of ca 80 to 100 km at 22 ma the models used here suggest that the overall elevation of the alps increased rapidly within 5 myr the variability in pebble size increased either due to variations in sediment supply enhanced orographic effects or preferentially due to a change towards a stormier palaeoclimate by 22 ma only two large rivers carried material into the foreland fans suggesting that the major alpine streams had established themselves this second phase of stable drainage network was maintained until ca 5 ma when the uplift and erosion of the molasse started and streams were redirected both in the alps and in the foreland this study illustrates that sedimentological archives of foreland basins can be used to reconstruct the chronology of the topographic development of mountain belts it is suggested that the finite elevation of mountainous landscapes is reached early during orogeny and can be maintained for millions of years provided that erosion is efficient

grenville orogenic affinities in the red mountain area llano uplift texas the red mountain area in the southeast corner of the llano uplift contains evidence of a grenville age polyphase non coaxial deformational history four synmetamorphic deformational phases affected the precambrian click formation of the packsaddle group and a single shear related deformation is observed in the big branch and red mountain gneisses the first phase of deformation produced tight folds with an axial planar metamorphic foliation the second and most intense pervasive deformational phase produced the regional metamorphic layering isoclinal folds transposition of bedding and mylonitic textures in the gneisses the third deformation is characterized by tight to isoclinal north verging folds with an axial planar differentiation crenulation cleavage the fourth phase of deformation resulted in a north northwest trending differentiation crenulation cleavage that is axial planar to small east northeast verging folds the growth of cordierite over all foliations indicates that low pressure me

analysis and modeling of gully headcut dynamics north american high plains gullies are dynamic fluvial features that can be the primary driver for landscape dissection and sediment production in many settings this research exploits a well constrained field area near west bijou creek colorado u s in order to develop a natural experiment in which we explore gully headcut erosion rates the controls on gully headcut height and the morphology of gully longitudinal profiles analysis of headcut retreat using aerial photography and airborne lidar imagery indicates that headcut retreat rates correlate with the square root of drainage area approximately we investigate how a drainage area control on headcut retreat translates into the longitudinal profile morphology over time using a simple numerical model the model combines fluvial erosion deposition and headcut retreat to identify the necessary and sufficient conditions needed to reproduce longitudinal profiles observed in the field field profiles are typically concave upward predominantly aggradational channel profiles with retreating headcuts whose height varies with catchment position systematic variation of environmental parameters in the model showed that the most successful model was achieved when highly resistant vegetation is applied throughout the channel excluding a bare soil zone downstream of the headcut this model scenario maintained an abrupt headcut over hundreds of model years and produced a realistic longitudinal profile that aggrades downstream of the headcut over time the vegetation pattern used in the best model fit is observed at the field site where easily erodible sparsely vegetated soil downstream of the headcut grades into a more resistant grassy channel downstream

a cellular model of holocene upland river basin and alluvial fan evolution coulthard t j macklin m g kirkby m j 2002 a cellular model of holocene upland river basin and alluvial fan evolution earth surface processes and landforms 27 3 269 288

cretaceous to present kinematics of the indian african and seychelles plates an iterative inverse model of seafloor spreading data from the mascarene and madagascar basins and the flanks of the carlsberg ridge describes a continuous history of indian african plate divergence since 84 ma visual fit modelling of conjugate magnetic anomaly data from near the seychelles platform and laxmi ridge documents rapid rotation of a seychelles plate about a nearby euler pole in palaeocene times as the euler pole migrated during this rotation the amirante trench on the western side of the plate accommodated first convergence and later divergence with the african plate the unusual present day morphology of the amirante trench and neighbouring amirante banks can be related to crustal thickening by thrusting and folding during the convergent phase and the subsequent development of a spreading centre with a median valley during the divergent phase the model fits fracture zone trends in the north arabian and east somali basins suggesting that they formed in india africa plate divergence seafloor fabric in and between the basins shows that they initially hosted a segmented spreading ridge that accommodated slow plate divergence until 71 69 ma and that upon arrival of the deccan re union plume and an increase to faster plate divergence rates in the period 69 65 ma segments of the ridge lengthened and propagated ridge propagation into the indian continental margin led first to the formation of the laxmi basin which accompanied extensive volcanism onshore at the deccan traps and offshore at the saurashtra high and somnath ridge a second propagation episode initiated the ancestral carlsberg ridge at which seychelles india and india africa plate motions were accommodated with the completion of this propagation the plate boundaries in the mascarene basin were abandoned seafloor spreading between this time and the present has been accommodated solely at the carlsberg ridge

gondwana breakup no evidence for a davie fracture zone offshore northern mozambique tanzania and kenya plate tectonic reconstructions assume a major inactive transform fault the davie fracture zone in the west somali basin along which madagascar is thought to have migrated southwards following gondwana breakup in the mesozoic based on the interpretation of reflection seismic data we show that the walu ridge offshore kenya and the kerimbas basin offshore northern mozambique are tectonically unrelated to the southward motion of madagascar and correlate with late cretaceous volcanism and inversion in kenya and the evolution of the east african rift system respectively offshore tanzania geophysical data do not show basement structures indicating the presence of a major transform fault these results challenge the commonly supported transform margin concept and imply a more southerly pre breakup position of madagascar within gondwana opening of the west somali basin by sw propagating oblique rifting and seafloor spreading is proposed

petrological significance of prograde homogenization of growth zoning in garnet an example from the llano uplift in the llano uplift of central texas usa prograde homogenization of garnet growth zoning took place during moderate to high pressure dynamothermal metamorphism over a narrow temperature range near the transition from the amphibolite to the granulite facies this subtle record of early dynamothermal metamorphism survived subsequent static metamorphism at low pressures in the middle amphibolite facies despite the destruction of most high pressure mineral assemblages that originated in the early metamorphic episode geographically systematic variations in the degree of homogenization indicate that the uplift as a whole underwent high pressure metamorphism in accord with emerging tectonic models for the mid proterozoic evolution of the southern margin of the north american continent

mozambique upper fan origin of depositional units the upper mozambique fan includes a stable down stream region with a north south channel flanked by thick 1 5 sec two way traveltime asymmetric levees and a migrating upstream region where at least two main feeding paths have been successively dominant from the oligocene to early miocene the north south serpa pinto valley acted as the main conduit for the north mozambique terrigenous sediments from the middle miocene the west east zambezi valley became the dominant path and supplied the fan with sediments transported by the zambezi river from the central part of mozambique the transfer from one sediment feeding system to the other is related to the abandonment of the serpa pinto valley because of graben formation along the davie ridge which trapped the sediments and the increase of the zambezi river sediment supply because of the creation and erosion of the east african rift

stratigraphic analysis of upper cretaceous rocks in the mahajanga basin northwestern madagascar implications for ancient and modern faunas abstract upper cretaceous strata of the mahajanga basin northwestern madagascar yield some of the most significant and exquisitely preserved vertebrate fossils known from gondwana the sedimentology of these strata and their stratigraphic relations have been the focus of renewed geological investigations during the course of five expeditions since 1993 we here designate stratotypes and formalize the terrestrial maevarano formation with three new members masorobe anembalemba miadana and the overlying marine berivotra formation the maevarano formation accumulated on a broad semiarid alluvial plain bounded to the southeast by crystalline highlands and to the northwest by the mozambique channel the berivotra formation was deposited in an open marine setting that evolved from a clastic to a carbonate dominated shelf resulting in deposition of the overlying betsiboka limestone of danian age new stratigraphic data clearly indicate that the maevarano formation correlates at least in part with the

kinematic constraints on rodinia reconstructions from the core of the texas grenville orogen abstract mesoproterozoic metamorphic rocks exposed in the llano uplift central texas were involved in a 1 15 1 12 ga grenville age collisional orogenic event along the southern margin of laurentia this event is characterized by polyphase ductile deformation synchronous with transitional amphibolite granulite to eclogite facies metamorphism results of detailed structural mapping in the southeastern part of the llano uplift show that these rocks record a deformational history involving northeast directed ductile thrusting and regional scale polyphase folding all related to northward collision of a southern continent and an arc with the southern margin of laurentia we use the structural metamorphic and kinematic history of the southeastern llano uplift to critically evaluate collisional models for the grenville orogen in laurentia and discuss the implications for reconstructions of rodinia

three dimensional landscape process modelling the effect of dem resolution many landscape models have been developed over the past decades however relatively little is known about handling the effects of changing spatial and temporal resolutions therefore resolution effects remain a factor of uncertainty in many hydrological and geomorphological modelling approaches in this paper we present an experimental multi scale study of landscape process modelling emphasis was laid on quantifying the effect of changing the spatial resolution upon modelling the processes of erosion and sedimentation a simple single process model was constructed and equal boundary conditions were created using artificial digital elevation models dems eliminated effects of landscape representation the only variable factors were dem resolution and the method of flow routing both steepest descent and multiple flow directions our experiments revealed an important dependency of modelled erosion and sedimentation rates on these main variables the general trend is an increase of erosion predictions with coarser resolutions an artificial mathematical overestimation of erosion and a realistic natural modelling effect of underestimating resedimentation cause this increasing the spatial extent eliminates the artificial effect while at the same time the realistic effect is enhanced both effects can be quantified and are expected to increase within natural landscapes the modelling of landscape processes will benefit from integrating these types of results at different resolutions

application of ostracoda and charophyta from the late jurassic to early cretaceous tendaguru formation at tendaguru tanzania east africa biostratigraphy palaeobiogeography and palaeoecology abstract revisitation of the famous late jurassic early cretaceous dinosaur locality of tendaguru by a german tanzanian expedition in summer 2000 resulted in a new standard section micropalaeontologic samples from this section yielded a marine and nonmarine partially mixed ostracod fauna consisting of 40 taxa and some charophyte gyrogonites and utriculi a few marine taxa provided biostratigraphic information although comparable faunas from the west indian madagascan east african faunal province are stratigraphically older middle late jurassic fully marine and strongly endemic an middle oxfordian age is suggested for the lower part of the tendaguru formation nodosoclavatoroid utriculi charophyta suggest that the jurassic cretaceous boundary may be located lower in the section than supposed previously i e in the trigonia smeei member rather than in the upper saurian member in biogeographical terms the marine ostracod fauna is relatively endemic and has most similarities with that known from eastern tanzania only a few faunal links to madagascar northwest india and somalia exist and there are no links to south africa ostracods and charophytes also provided palaeoecological data for specific horizons in the lower part of the middle saurian member a regression is documented while the nonmarine late jurassic early cretaceous ostracods of tendaguru are not applicable to biostratigraphy so far they are important because the nonmarine setting is unique in this area of the tethys

grenville age orogeny in the llano uplift of central texas deformation and metamorphism of the rough ridge formation field and petrographic study of the precambrian rough ridge formation of the packsaddle group documents a more complex tectonic history for the llano uplift than has previously been reported the rough ridge formation was affected by at least two metamorphic events and five phases of deformation that may be part of a single progressive event the predominant mineral assemblages in pelitic and mafic rocks are characteristic of the amphibolite facies at relatively low pressure ubiquitous granoblastic polygonal textures overprinting multiple foliations indicate that post tectonic recrystallization followed dynamothermal metamorphism a probable consequence of regional reheating during later granite intrusion the most intense deformation produced isoclinal folds and pervasive axial planar foliation s2 resulting in complete transposition of original bedding this foliation cuts an earlier local foliation s1 and is folded by small f3 folds and a crenulation cleavage s3 both s2 and s3 are cut by a differentiated crenulation cleavage s4 the most prominent folds in the area and the associated pervasive axial planar cleavage s5 overprint all earlier structures in one pelitic unit oriented inclusions show that garnet formed after s3 and cordierite after s5 growth and recrystallization of micas quartz and feldspars span all phases of deformation the timing of tectonic and metamorphic events is bracketed by the pretectonic intrusion of the 1 167 15 m y old red mountain gneiss and the post tectonic intrusion of a 1 080 30 m y old melarhyolite dike structural and petrologic characteristics of the rough ridge formation indicate that the llano uplift underwent a complex tectonic and metamorphic development similar to that described for the grenville orogeny elsewhere in north america

observation of sections of oceanic crust and mantle cropping out on the southern wall of kane fz n atlantic the objective of the 20 nautile dives of the recent kanaut cruise was to study the southern wall of the kane fracture zone from its eastern intersection with the mid atlantic ridge mar to 5 myr in age the geological mapping shows four successive massifs wrench faulted and slightly tilted the transform facing walls of these massifs exhibit outcrops of fresh and serpentinized peridotites gabbros and basalts the entire crustal exposure is cataclased and metamorphosed to greenschist facies

the amirante ridge trough complex response to rotational transform rift drift between seychelles and madagascar the amirante ridge trough complex developed along the late cretaceous transform boundary that separated the seychelles india and madagascar somali basin plates motion between these plates was complex comprising sinistral n s strike slip movement coupled and coeval with counter clockwise rotation induced when seafloor spreading developed in only the southern portion of the transform the overall morphology of the complex comprises a series of arcuate ridge and trough segments these segments were successively produced by tectonic and volcanic activity within the zone of migrating plate contact adjacent to the rotational pivot where compression was replaced by extension along the transform boundary in the extensional regime to the south of this contact zone the mascarene oceanic basin developed whilst in the compressional zone further north island arcs developed and the ophiolites of baluchistan were obducted from the somali basin onto the leading edge of the seychelles india plate

constraints on the motion of madagascar with respect to africa abstract three constraints on the motion of madagascar relative to africa are investigated in order to arrive at a model for madagascar s drift these are 1 fracture zone morphology and direction in the somali basin and mozambique channel 2 the relative positions of the carboniferous to jurassic palaeomagnetic poles for madagascar and africa and 3 the crustal structure along a geophysical traverse between lamu kenya and mahe seychelles the evidence for the bunce and molnar 1977 pole for the rotation of madagascar relative to africa defined by the davie dhow vlcc and ars fracture zones is critically examined and accepted it is used to rotate madagascar northwards back to a position that brings the carboniferous to jurassic palaeomagnetic poles for madagascar and africa into good agreement the rotation of approximately 16 places madagascar in the north of the somali basin with its northern tip at the southwestern end of the chain ridge it moves madagascar almost exactly the length of the extended davie fracture zone accepting that this position may have been the pre drift position of madagascar some interesting correlations with the lamu mahe crustal section emerge continental crust appears to extend 300 km offshore from lamu and thus abuts precisely the continental crust of rotated madagascar between 300 km and 1200 km from lamu the crust is probably oceanic of jurassic cretaceous age this would have been left in the wake of madagascar as it drifted southwards four fracture zones are seen in this crust a major one appears in the crustal section about 700 km from lamu the dhow fracture zone at 850 km may be associated with a large step up to the east in basement which has acted as an efficient dam to sediments from africa and the vlcc and ars are smaller fracture zones the easternmost part of oceanic crust between 1100 and 1200 km may have been formed in the wake of the farquhar islands drifting southwards with madagascar more likely however it could have formed as the seychelles bank area drifted away from madagascar having previously moved southwards with madagascar eastwards from 1200 km thicker possibly continental crust adjoins the seychelles bank this model assumes that the comores basin is oceanic in origin which may not be the case if it is continental a rotation of madagascar of only 9 or 10 can be accommodated moving the island almost exactly the length of the topographic expression of the davie fracture zone but providing a poor fit of the palaeomagnetic poles

modelling sediment dynamics due to hillslope river interactions incorporating fluvial behaviour in landscape evolution model lapsus landscape evolution models lems simulate the three dimensional development of landscapes over time different lems have different foci e g erosional behaviour river dynamics the fluvial domain hillslopes or a combination lem lapsus is a relatively simple cellular model operating on timescales of centuries to millennia and using annual timesteps that has had a hillslope focus our objective was to incorporate fluvial behaviour in lapsus without changing the existing model equations the model should be able to reproduce alternating aggradation and incision in the floodplains of catchments depending on simulated conditions testing was done using an artificial digital elevation model dem and a demonstration of the ability for fluvial simulation was performed for a real landscape torrealvilla catchment southeast spain model equations to calculate sediment dynamics and water routing were similar for both hillslope and fluvial conditions but different parameter values were used for these domains defined based on annual discharge parameters changing between the domains are convergence factor p which is used in the multiple flow algorithm to route water and discharge and gradient exponents m and n used in transport capacity calculations erodibility and sedimentability factors k and p were changed between cold little vegetation high erodibility and warm conditions more vegetation lower erodibility results show that the adapted parameters reproduced alternating aggradation due to divergent flow in the floodplain and sediment supply under cold conditions and incision due to reduced sediment supply and resulting clean water erosion during simulated warm conditions the simulated results are due to interactions between hillslopes and floodplains as the former provide the sediments that are deposited in the latter similar behaviour was demonstrated when using the real dem sensitivity and resolution analysis showed that the model is sensitive to changes in m n and p and that model behaviour is influenced by dem resolution copyright c 2012 john wiley sons ltd

post mining landform evolution modelling 1 derivation of sediment transport model and rainfall runoff model parameters data were collected from three sites on the waste rock dump at era ranger mine 1 the cap site which was unvegetated and unripped with a surface slope of 2 8 per cent 2 the batter site surface slope 20 7 per cent also unvegetated and unripped but with a covering of coarse rock material and 3 the soil site surface slope 1 2 per cent which had c 90 per cent vegetation cover of low shrubs and grasses and had been topsoiled and surface ripped natural rainfall events were monitored on the sites to collect rainfall runoff and soil loss data to parameterize a sediment transport model of the form t s q dt and the distfw rainfall runoff model low frequency high intensity events resulted in the greatest soil loss to accurately predict sediment loss during high intensity events storms with a range of intensities were selected to derive the sediment transport model distfw hydrology model parameters were derived by fitting four monitored events simultaneously the selected parameters for the vegetated and ripped case may overpredict discharge for some rainfall events resulting in conservative design of erosion control features on rehabilitated landforms copyright 2000 environmental research institute of the supervising scientist commonwealth of australia

earthquakes associated with diffuse zones of deformation in the oceanic lithosphere some examples abstract we review the results of source parameters of earthquakes associated with the eastern end of the azores gibraltar plate boundary the davie ridge near madagascar and the horizon bank in the north fiji basin to characterize the nature of present day tectonics in diffuse zones of deformation in the oceanic lithosphere in comparison with typical plate boundaries diffuse zones of deformation are in general characterized by complex morphological expressions and scattered seismicity of up to several hundred kilometers width although the average rate of displacement across these regions is not resolvable by current models of global plate reconstruction the deformation is not truly intraplate in nature because strain is concentrated and often distinct tectonic boundaries can be identified earthquakes with seismic moments as large as 8 10 20 nm have occurred in bands of scattered seismicity that are associated with diffuse zones of deformation in the oceanic lithosphere in two cases the zones of diffuse deformation continue into the continental lithosphere east of gibraltar and north of the davie ridge however the largest earthquakes occur in the oceanic part of these two zones in contrast to previous reports the depth of earthquakes is quite shallow in regions where a mixture of focal mechanisms occur the consistent parameter is usually the orientation of the p or the t axes instead of that of the nodal planes or slip vectors thus as in the continental lithosphere zones of diffuse deformation in the oceanic lithosphere seem to be better described by a regional compression or extension field rather than by displacement on discrete faults

gravity models of the amirante arc western indian ocean abstract a free air gravity anomaly chart has been compiled for the amirante arc amirante basin and the western margin of the seychelles plateau the new gravity data obtained during r r s discovery cruise 101 has tracks orthogonal to the amirante arc at approximately 40 km separation and detailed coverage of the south and west margins of the seychelles plateau that has been merged with earlier data the free air anomaly across the southern amirante arc has a wavelength close to 75 km and an average amplitude of 215 mgal three profiles are interpreted using two dimensional models with the moho at 14 km depth in the amirante and south somali basins and a significant variation in the depth to the mantle of between 11 and 25 km beneath the arc trough structure the density distributions show evidence of partial subduction analogous to that of the north spanish margin and high lithostatic stress reflecting the absence of local isostatic compensation

a catchment scale evaluation of the siberia and caesar landscape evolution models landscape evolution models provide a way to determine erosion rates and landscape stability over times scales from tens to thousands of years the siberia and caesar landscape evolution models both have the capability to simulate catchment wide erosion and deposition over these time scales they are both cellular operate over a digital elevation model of the landscape and represent fluvial and slope processes however they were initially developed to solve research questions at different time and space scales and subsequently the perspective detail and process representation vary considerably between the models notably caesar simulates individual events with a greater emphasis on fluvial processes whereas siberia averages erosion rates across annual time scales this paper describes how both models are applied to tin camp creek northern territory australia where soil erosion rates have been closely monitored over the last 10 years results simulating 10 000 years of erosion are similar yet also pick up subtle differences that indicate the relative strengths and weaknesses of the two models the results from both the siberia and caesar models compare well with independent field data determined for the site over different time scales representative hillslope cross sections are very similar between the models geomorphologically there was little difference between the modelled catchments after 1000 years but significant differences were revealed at longer simulation times importantly both models show that they are sensitive to input parameters and that hydrology and erosion parameter derivation has long term implications for sediment transport prediction therefore selection of input parameters is critical this study also provides a good example of how different models may be better suited to different applications or research questions copyright 2010 john wiley sons ltd and commonwealth of australia

evidence for nonlinear diffusive sediment transport on hillslopes and implications for landscape morphology steep soil mantled hillslopes evolve through the downslope movement of soil driven largely by slope dependent transport processes most landscape evolution models represent hillslope transport by linear diffusion in which rates of sediment transport are proportional to slope such that equilibrium hillslopes should have constant curvature between divides and channels on many soil mantled hillslopes however curvature appears to vary systematically such that slopes are typically convex near the divide and become increasingly planar downslope this suggests that linear diffusion is not an adequate model to describe the entire morphology of soil mantled hillslopes here we show that the interaction between local disturbances such as rainsplash and biogenic activity and frictional and gravitational forces results in a diffusive transport law that depends nonlinearly on hillslope gradient our proposed transport law 1 approximates linear diffusion at low gradients and 2 indicates that sediment flux increases rapidly as gradient approaches a critical value we calibrated and tested this transport law using high resolution topographic data from the oregon coast range these data obtained by airborne laser altimetry allow us to characterize hillslope morphology at 2 m scale at five small basins in our study area hillslope curvature approaches zero with increasing gradient consistent with our proposed nonlinear diffusive transport law hillslope gradients tend to cluster near values for which sediment flux increases rapidly with slope such that large changes in erosion rate will correspond to small changes in gradient therefore average hillslope gradient is unlikely to be a reliable indicator of rates of tectonic forcing or baselevel lowering where hillslope erosion is dominated by nonlinear diffusion rates of tectonic forcing will be more reliably reflected in hillslope curvature near the divide rather than average hillslope gradient

effects of holocene climate and sea level changes on coastal gully evolution insights from numerical modelling gullies are known to be sensitive to a wide range of environmental disturbances so their geomorphology can provide insight into the environmental history of the surrounding landscape coastal gullies are of particular interest in that they are strongly influenced by both terrestrial and marine processes for example the coastal gullies found on the isle of wight uk known locally as chines are highly dynamic with episodes of sea cliff erosion frequently causing the rejuvenation of the channel network consequently a key factor in the long term evolution of the chines is the relative balance between rates of cliff retreat driven primarily by holocene sea level rise and headwards incision caused by knickpoint migration driven primarily by holocene climate via its impact on runoff in this paper we explore the holocene erosional history of the chines using a numerical landscape evolution model that has been modified to include a cliff recession function knickpoint recession rates are simulated using a detachment limited channel erosion law wherein erosion rate is a power function of drainage area and stream gradient with model parameters defined using empirically derived data hindcast simulations from 12000 cal years bp to present are undertaken for a range of scenarios of holocene climate change and sea level rise plausible erosional histories are extracted from scenarios in which simulated and observed chine and landscape forms match the results suggest that the rate of sea level rise is the key control on chine formation and that it is only in this late holocene period and specifically in the last 2000 years that sea level rise has slowed sufficiently for knickpoint recession rates to exceed cliff recession rates and create sustainable gully networks our interpretation that the isle of wight gullies are of relatively modern is in agreement with previous studies finally the simulations also indicate that contemporary chine gully systems are close to a critical threshold suggesting that future gully evolution is likely to be sensitive to small changes in future rates of effective precipitation and or sea level rise