temporal and spatial evolution of dynamic support from river profiles a framework for madagascar 1 we present a strategy for calculating uplift rates as a function of space and time from large sets of longitudinal river profiles this strategy assumes that the shape of a river profile is controlled by the history of uplift rate and moderated by the erosional process we assume that upstream drainage area is invariant the algorithm was tested on a set of 100 river profiles which were extracted from a digital elevation model of madagascar this set of profiles was simultaneously inverted to obtain uplift rate as a smooth function of space and time the fit between observed and calculated profiles is excellent and suggests that madagascar was uplifted by 1 2 km at rates of 0 2 0 4 mm yr during the last 15 myrs the location of madagascar suggests that its topographic elevation is maintained by convective circulation of the sub lithospheric mantle residual depth anomalies of oceanic fragments encompassing the island show that the island straddles a dynamic topographic gradient which generates asymmetric neogene uplift volcanism warped peneplains and uplifted marine terraces corroborate the existence of youthful uplift we suggest that sets of longitudinal river profiles contain useful information about the history of regional uplift which can be extracted by inverse modeling and calibrated by independent geologic observations

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

ability of a global three dimensional model to locate regional events 1 we assess the ability of a global three dimensional 3d seismic model of the crust and upper mantle cub2 0 to locate seismic events using regional travel time data alone epicentral distances 20 assessments are based on a ground truth gt database comprising nearly 1000 uncommonly well located events which occur in 23 event clusters across eurasia and north africa groomed p and pn arrival time data observed at regional distances from individual events and 1000 empirical phase path anomalies produced from event clusters evidence is presented that supports the conversion of the shear velocities vs in cub2 0 to compressional velocities vp in the upper mantle using the thermoelastic properties of an assumed mantle composition rather than traditional empirical scaling relationships two principal results are presented first this 3d vp model fits the empirical p and pn phase path anomalies very well in both pattern and absolute level second location assessments demonstrate that intrinsic regional location accuracy is 5 km using the 3d vp model and 10 km using the 1d model ak135 these findings highlight the importance of gt databases in assessing regional travel time models and establish that a global 3d vs model together with vp inferred from it provides a solid basis on which to build improvements in regional location capabilities

earth structure and instrumental seismicity of madagascar implications on the seismotectonics abstract the aim of this study is to improve the knowledge of the seismotectonics of madagascar we first investigate the structure of the earth beneath madagascar through the joint inversion of receiver functions and rayleigh wave group velocities then we use the obtained velocity models to relocate local earthquakes in order to analyse the distribution of seismicity finally we use structural models and earthquake coordinates to compute focal mechanisms our retrieved earth structure models confirm a thin lithosphere beneath madagascar when compared to the nearby east african rift the high plateau in the central region coincides with the thinnest lithosphere over the slowest asthenosphere our results are in good agreement with the gravity anomalies and likely confirm a localised asthenospheric upwelling beneath the central part of madagascar the surface expression of the asthenospheric upwelling consists in a horst graben structure the moderate seismicity is localised along pre existing structures reflecting an e w extension that is mostly accommodated in the lower crust

teleseismic traveltimes topography and the lithospheric structure across central mongolia 1 teleseismic travel times recorded along a 1000 km long n s transect across central mongolia are used together with topography and gravity data to constrain the deep lithospheric structure of this region time residuals appear positively correlated with the topography suggesting that p wave velocity changes correspond to density variations which in turn cause an isostatic uplift of the topography using a simple local isostasy model and a monte carlo inversion for the crust and asthenosphere velocities and density velocity conversion factors we determine best fitting moho and lithosphere asthenosphere boundary lab geometries which satisfyingly reproduce the observed topography and gravity data the model is validated using previously published 1d s wave velocity models obtained from receiver function analyses our results indicate that most of the long wavelength topography is supported by the asthenosphere buoyancy the hangai dome in the southern half of the transect appears mainly supported by a rather shallow 60 70 km asthenospheric uplift whereas the siberian platform at the extreme north of the profile is underlain by a very thick 200 km lithosphere misfits to the observed bouguer gravity anomaly occur near major faults probably because of lithospheric flexure

joint seismic geodynamic mineral physical modelling of african geodynamics a reconciliation of deep mantle convection with surface geophysical constraints recent progress in seismic tomography provides the first complete 3 d images of the combined thermal and chemical anomalies that characterise the unique deep mantle structure below the african continent we present a tomography based model of mantle convection that provides an excellent match to fundamental surface geodynamic constraints on internal density heterogeneity that includes both compositional and thermal contributions where the latter are constrained by mineral physics the application of this thermochemical convection model to the problem of african mantle dynamics yields a reconciliation of both surface gravity and topography anomalies to deep seated mantle flow under the african plate over a wider range of wavelengths than has been possible to date on the basis of these results we predict flow in the african asthenosphere characterised by a clear pattern of focussed upwellings below the major centres of late cenozoic volcanism including the kenya domes hoggar massif cameroon volcanicline cape verde and canary islands the flow predictions also reveal a deep seated large scale active hot upwelling below the western margin of africa under the cape verde islands that extends down to the core mantle boundary the scale and dynamical intensity of this west african superplume is comparable to the south african superplume that has long been assumed to dominate the large scale flow dynamics in the deep mantle under africa we evaluate the plausibility of the predicted asthenospheric flow patterns through a comparison with seismic azimuthal anisotropy derived from independent analyses of african shear wave splitting data

alkane and pah biomarkers as tracers of terrigenous organic carbon in arctic ocean sediments abstract we present the first sedimentary biomarker study encompassing the entire arctic ocean a large data set of organic markers for terrigenous petroleum and combustion inputs alkanes hopanes and steranes parent and alkyl polycyclic aromatic hydrocarbons pahs is examined for patterns in space and time using principal components analysis pca and partial least squares pls biomarker patterns reveal the central arctic ocean basin sediments to be compositionally distinct from those of the mackenzie river beaufort sea and barents sea but similar to those of the laptev sea pah distributions reflected in pah ratios and pca projections demonstrate that arctic ocean sediment is dominated by natural inputs to the extent that anthropogenic combustion pahs are not significant we find only modest changes between the glacial and post glacial sediments for atmospherically transported hydrocarbon biomarkers while particle associated biomarkers were captured strongly at basin edges during the glacial period and much more evenly transported across basins during the post glacial period the orders of magnitude decreases in particle associated petrogenic alkanes and pahs in central basins during glacial times coupled with the uniformity of most petrogenic biomarker parameters for most basin and shelf locations reflect a massive reduction in ice transport that makes the margins the most likely source of petrogenic material for the pleistocene holocene central arctic basins the proximity of large coal deposits of various maturity levels along the lena river the overlap in pah and biomarker composition of the laptev sea and surficial sediments from the central arctic ocean and the location of the laptev sea at the origin of the main transpolar drift all point to eroded coals from the lena river laptev sea as the likely source of petrogenic hydrocarbons to the central arctic ocean the ubiquitous presence of allochthonous coal in arctic ocean surface sediments provides a major constraint on the use of petrogenic biomarkers to infer the presence of subsurface petroleum reserves

the breakup of east gondwana assimilating constraints from cretaceous ocean basins around india into a best fit tectonic model published models for the cretaceous sea oor spreading history of east gondwana result in unlikely tectonic scenarios for at least one of the plate boundaries involved and or violate particular constraints from at least one of the associated ocean basins we link east gondwana spreading corridors by integrating magnetic and gravity anomaly data from the enderby basin off east antarctica within a regional plate kinematic framework to identify a conjugate series of east west trending magnetic anomalies m4 to m0 126 7 120 4 ma the mid ocean ridge that separated greater india from australia antarctica propagated from north to south starting at 136ma northwest of australia and reached the southern tip of india at 126 ma sea oor spreading in the enderby basin was abandoned at 115 ma when a ridge jump transferred the elan bank and south kerguelen plateau to the antarctic plate our revised plate kinematic model helps resolve the problem of successive two way strike slip motion between madagascar and india seen in many previously published reconstructions and also suggests that sea oor spreading between them progressed from south to north from 94 to 84 ma this timing is essential for tectonic ow lines to match the curved fracture zones of the wharton and enderby basins as greater india gradually began to unzip from madagascar from 100 ma in our model the 85 east ridge and kerguelen fracture zone formed as conjugate anks of a leaky transform fault following the 100ma spreading reorganization our model also identi es the afanasy nikitin seamounts as products of the conrad rise hotspot

shear velocity structure of the crust and upper mantle of madagascar derived from surface wave tomography the crust and upper mantle of the madagascar continental fragment remained largely unexplored until a series of recent broadband seismic experiments an island wide deployment of broadband seismic instruments has allowed the first study of phase velocity variations derived from surface waves across the entire island late cenozoic alkaline intraplate volcanism has occurred in three separate regions of madagascar north central and southwest with the north and central volcanism active until 1 ma but the sources of which remains uncertain combined analysis of three complementary surface wave methods ambient noise rayleigh wave cross correlations and two plane wave illuminate the upper mantle down to depths of 150 km the phase velocity measurements from the three methods for periods of 8 182 s are combined at each node and interpolated to generate the first 3 d shear velocity model for sub madagascar velocity structure shallow upper 10 km low shear velocity regions correlate well with sedimentary basins along the west coast upper mantle low shear velocity zones that extend to at least 150 km deep underlie the north and central regions of recent alkali magmatism these anomalies appear distinct at depths 100 km suggesting that any connection between the zones lies at depths greater than the resolution of surface wave tomography an additional low shear velocity anomaly is also identified at depths 50 150 km beneath the southwest region of intraplate volcanism we interpret these three low velocity regions as upwelling asthenosphere beneath the island producing high elevation topography and relatively low volume magmatism

peculiarities of surface wave propagation across central eurasia abstract the recent installation of six broadband digital iris ida seismic stations in the ussr has provided new opportunities for studying surface wave propagation across eurasia group velocities of fundamental rayleigh and love modes between epicenters and these stations were determined for 35 events that occurred since april 1989 to the middle of july 1990 near eurasia differential phase velocities were found for the same arrivals along paths between several pairs of stations group and phase velocities were obtained in the period range from 15 to 300 sec frequency time polarization analysis was used for studying polarization properties of surface waves in some cases significant anomalies in the particle motion for periods up to 100 sec were observed they are attributed to surface wave refraction and scattering due to lateral inhomogeneities at the boundaries and inside the eurasia continent

seismicity and seismotectonics of madagascar revealed by the 2011 2013 deployment of the island wide macomo broadband seismic array abstract the seismicity and seismotectonics of madagascar have been studied using an island wide distribution of broadband seismic stations the 28 station madagascar comoros mozambique macomo array was deployed for a 23 month period between 2011 and 2013 macomo data were supplemented by seven temporary stations from the seismological signatures in the lithosphere asthenosphere system of southern madagascar selasoma project ten temporary stations from the reunion hotspot and upper mantle reunions unterer mantel project rhum rum and 11 permanent stations a total of 695 earthquakes with magnitudes between ml1 and ml5 3 located within madagascar were recorded a new local magnitude scale was developed and focal mechanisms were determined for 23 well recorded events most of the seismicity is clustered within central madagascar forming nw se trends in the ankaratra region and ne sw trends in the alaotra and ankay regions that roughly align with extensional tectonic features the northern and southern parts of madagascar also show seismicity clusters that align parallel to existing tectonic features primarily precambrian shear zones focal mechanisms exhibit a wide orientation of nodal planes show predominantly normal faulting throughout madagascar and provide no evidence for a sharp east west striking plate boundary between the lwandle and somalian plates in the middle of the island however a diffuse plate boundary cannot be excluded we suggest that topographically generated extensional stresses is the cause of most seismicity given that significantly fewer earthquakes are located in the lower elevation areas of the island compared to regions of higher elevations the frequency magnitude distribution has a b value of 1 2 and indicates that events with magnitude ml5 or greater should occur with an average repeat time of 1 4 years however no earthquake exceeding ml6 has been recorded in the last century suggesting that the frequency magnitude distribution could be truncated

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

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

imaging of the moho depth in madagascar through the inversion of gravity data geodynamic implications several e w profiles of bouguer anomalies across the precambrian basement were inverted by applying a linearized inversion procedure in the spectral domain and using a simplified two layer model with a crust upper mantle interface of constant density contrast from these inverted profiles a contour map of moho topography has been constructed which covers the area of exposed precambrian basement in the eastern and central parts of the island such imaging of the moho depth in particular exhibits a n20 e trending zone of substantially thinned crust along the axis of the island this parallels the east coast margin which resulted from the northward motion of india relative to madagascar in the upper cretaceous from combined analysis of geological tectonic and metamorphic observations this prominent crustal feature in the precambrian basement of madagascar is interpreted as the relic of continental lithospheric extension and thinning possibly related to panafrican collapse tectonics on the eastern edge of the mozambique belt this extensional feature would have been reactivated only in part during the rifting and dislocation stages of the madagascar and india continental blocks as evidenced by an axis of cretaceous volcanism emphasized by the ankaratra and itasy massifs final separation nevertheless occurred further east at the site of the east coast margin

shakedown in madagascar occurrence of lavakas erosional gullies associated with seismic activity erosion via lavaka formation is widespread in madagascar but controls on why and where lavakas occur are not understood geographic information system analysis reveals a spatial correlation between lavaka abundance and the frequency of seismic events most lavakas occur in or near areas where recorded earthquakes magnitude 0 5 5 6 are most frequent this correlation explains the unevenness of lavaka distribution in the malagasy highlands and highlights the importance of natural factors in lavaka formation seismic activity appears to precondition the landscape to lavaka formation although the mechanism by which this happens is not yet known recognizing the connection however allows us to pinpoint areas prone to future lavaka development in zones of active deforestation areas with the greatest frequency of seismic events are most at risk for high density lavaka development

disentangling interglacial sea level and global dynamic topography analysis of madagascar abstract global inventories of stable sea level markers for the peak of the last interglacial period marine isotopic stage mis 5e play a pivotal role in determining sea level changes and in testing models of glacial isostatic adjustment here we present surveying and radiometric dating results for emergent terraces from northern madagascar which is generally regarded as a stable equatorial site fossil coral specimens were dated using conventional and open system corrected uranium series methods elevation of the upper undated terrace decreases from 33 8 m to 29 5 m over a distance of 35 km an intermediate terrace has an average radiometric age of 130 7 13 2 ka i e mis 5e its elevation decreases from 9 3 m to 2 8 m over a distance of 80 km the record of the lowest terrace is fragmentary and consists of beach rock containing rare corals with ages of 1 6 3 8 ka the spatial gradient of the mis 5e marker is inconsistent with glacio isostatic adjustment calculations instead we propose that variable elevations of this marker around madagascar and possibly throughout the indian ocean at least partly reflect spatial patterns of dynamic topography generated by sub plate mantle convection

age progressive volcanism in the comores archipelago western indian ocean and implications for somali plate tectonics abstract the comores islands together with the tertiary volcanic province of northern madagascar form a sublinear trend of alkali olivine basalt shield volcanoes across the northern entrance of the mozambique channel potassium argon dating of shield building lavas confirms an eastward increase in age of volcanism along the chain consistent with a hotspot origin for the lineament the velocity of the somali plate over the mantle magma source is 50 mm yr we use the distribution of ages along the comores madagascar chain in conjunction with existing age data for the reunion mascarene plateau hotspot track to model the absolute motion of the somali plate for the last 10 m y we calculate the relative motion across the east african rift by subtracting the somali plate absolute motion from african plate absolute motion during this period the model predicts 320 km of total separation across the east african rift during the past 10 m y which is greater than has been estimated from surface geological evidence the geometry of older portions of the comores and reunion trends indicates that there was no significant relative motion between the african and somali plates prior to about 10 m y ago

high resolution imaging of the pyrenees and massif central from the data of the pyrope and iberarray portable array deployments the lithospheric structures beneath the pyrenees which holds the key to settle long standing controversies regarding the opening of the bay of biscay and the formation of the pyrenees are still poorly known the temporary pyrope and iberarray experiments have recently filled a strong deficit of seismological stations in this part of western europe offering a new and unique opportunity to image crustal and mantle structures with unprecedented resolution here we report the results of the first tomographic study of the pyrenees relying on this rich data set the important aspects of our tomographic study are the precision of both absolute and relative traveltime measurements obtained by a nonlinear simulated annealing waveform fit and the detailed crustal model that has been constructed to compute accurate crustal corrections beneath the massif central the most prominent feature is a widespread slow anomaly that reflects a strong thermal anomaly resulting from the thinning of the lithosphere and upwelling of the asthenosphere our tomographic images clearly exclude scenarios involving subduction of oceanic lithosphere beneath the pyrenees in contrast they reveal the segmentation of lithospheric structures mainly by two major lithospheric faults the toulouse fault in the central pyrenees and the pamplona fault in the western pyrenees these inherited hercynian faults were reactivated during the cretaceous rifting of the aquitaine and iberian margins and during the cenozoic alpine convergence therefore the pyrenees can be seen as resulting from the tectonic inversion of a segmented continental rift that was buried by subduction beneath the european plate

role of mantle flow in nubia somalia plate divergence present day continental extension along the east african rift system ears has often been attributed to diverging sublithospheric mantle flow associated with the african superplume this implies a degree of viscous coupling between mantle and lithosphere that remains poorly constrained recent advances in estimating present day opening rates along the ears from geodesy offer an opportunity to address this issue with geodynamic modeling of the mantle lithosphere system here we use numerical models of the global mantle plates coupled system to test the role of present day mantle flow in nubia somalia plate divergence across the ears the scenario yielding the best fit to geodetic observations is one where torques associated with gradients of gravitational potential energy stored in the african highlands are resisted by weak continental faults and mantle basal drag these results suggest that shear tractions from diverging mantle flow play a minor role in present day nubia somalia divergence

id tims using the step wise dissolution technique versus ion microprobe u pb dating of metamict archean zircons from ne madagascar abstract a geochronological study using the step wise dissolution technique and in situ ion microprobe analysis is applied to late archean metamict zircons of granitoids from northeastern madagascar basement both techniques provide concordant u pb ages additionally the ion microprobe study provided in situ measurements on the pb isotopic composition of the metamict domains finally pb pb evaporation ages were found to be slightly but systematically younger than the dating obtained by the two other methods based on these data granites from the antongil belt were emplaced during at least three episodes separated by periods of magmatic inactivity of about 10 ma at 2513 2523 and 2532 ma respectively these results are consistent with previous whole rock rb sr and u pb zircon dating from this part of madagascar the occurrence of this late archean basement in northeastern madagascar can be related to southern india where large volumes of contemporaneous granitoids were emplaced

late cretaceous magmatism in madagascar palaeomagnetic evidence for a stationary marion hotspot late cretaceous basaltic volcanics in the morondava basin sw madagascar possess high quality and pre fold palaeomagnetic data declination 353 5 inclination 54 8 95 2 4 the palaeomagnetic data are all of normal magnetic polarity and remanence acquisition is linked to the terminal stages of the cretaceous normal superchron 83 ma this is sustained by an 40ar 39ar age of 83 6 1 6 ma from one of the tested basaltic flows a precise u pb zircon baddeleyite age from northeast madagascar demonstrates magmatism at least back to 91 6 0 3 ma thus reliable isotope ages for the madagascar cretaceous igneous province span a range of 8 million years late cretaceous palaeomagnetic data obtained from volcanics and dolerites all over madagascar are directionally concordant and the combined palaeomagnetic pole latitude 68 5 n longitude 230 3 e a95 5 5 n 8 studies sampling age range ca 84 90 ma represents one of the best late cretaceous poles for the former gondwanan elements the collective palaeomagnetic data yield a palaeolatitude of 45 3 s 5 3 4 7 for the proposed focal point of the marion plume volcan de l androy southeast madagascar during the late cretaceous this is in perfect agreement with hotspot controlled reconstructions that place the marion hotspot 46 0 s beneath southeast madagascar during the late cretaceous setting true polar wander aside hotspot movements in the indian ocean do not appear to exceed ca 0 75 cm yr and the marion hotspot appears stationary within the resolution power of palaeomagnetic data

fault related rocks suggestions for terminology many traditional terms for fault related rocks have undergone recent dynamic metamorphism under high pressure discussions by various groups of specialists a generally acceptable simplified framework encompassing these and associated structural terms is now needed for many geologic engineering and legal purposes such a framework is proposed here focusing on a rate of strain versus rate of recovery diagram and relating this framework to the products of brittle and ductile deformation along faults