underlying fracture zone nature of astrid ridge off antarctica s queen maud land new geophysical data are used to refine the extent and setting of astrid ridge a volcanic edifice lying off the princess astrid coast section 5 e to 20 e of queen maud land in antarctica new findings presented here include a well defined fracture zone occurring as a narrow basement ridge which extends northeast from astrid ridge and an adjacent set of mesozoic magnetic anomalies m0 to m9 which are offset by 120 km from an identical previously described set these two sets of anomalies are combined with the conjugate mozambique basin anomalies to define an africa antarctica rotation pole at 10 s 29 w for anomaly m2 time the northern edge of astrid ridge and the sharp southern margin of the mozambique ridge on the african plate are probably related to the same cessation of excess volcanic activity near the end of the cretaceous normal polarity interval both features had their eastern and western limits confined by a contiguous pair of fracture zones a possible major swing in fracture zone trend prior to anomaly m9 is also suggested

new somali basin magnetic anomalies and a plate model for the early indian ocean abstract the oldest portions of the indian ocean formed via the breakup of gondwana and the subsequent fragmentation of east gondwana we present a constrained plate model for this early indian ocean development for the time period from gondwana breakup until the start of the cretaceous normal superchron the motions of the east gondwana terranes are determined using new geophysical observations in the somali basin and existing geophysical interpretations from other coeval indian ocean basins within the somali basin recent satellite gravity data clearly resolve traces of an east west trending extinct spreading ridge and north south oriented fracture zones a thorough compilation of somali basin ship track magnetic data allows us to interpret magnetic anomalies m24bn through m0r about this extinct ridge our magnetic interpretations from the somali basin are similar in age spreading rate and spreading directions to magnetic anomalies previously interpreted in the neighboring mozambique basin and riiser larsen sea the similarity between the two magnetic anomaly datasets allows us to match both basin s older magnetic anomaly picks by defining a pole of rotation for a single and cohesive east gondwana plate however following magnetic anomaly m15n we find it is no longer possible to match magnetic picks from both basins and maintain plausible plate motions in order to match the post m15n geophysical data we are forced to model the motions of madagascar india and east antarctica australia as independent plates the requirement to utilize two independent plates after anomaly m15n provides strong circumstantial evidence that suggests east gondwana breakup began around 135 ma

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

contributions of cretaceous quiet zone natural remanent magnetization to magsat anomalies in the southwest indian ocean the magsat magnetic anomalies over the southwest indian ocean are modeled using a combination of induced plus viscous remanent magnetization im vrm and natural remanent magnetization nrm two broad roughly parallel sw to ne trending triple peaked positive anomalies dominate the region one lying south of africa and the other north of antarctica although these anomaly peaks generally correspond with the agulhas plateau maud rise mozambique plateau astrid ridge and madagascar ridge conrad rise conjugate pairs the im vrm contribution from structural characteristics i e crustal thickness accounts for only about 20 of the anomaly amplitudes a spatially variable but observationally constrained nrm contribution in cretaceous quiet zone kqz crust is required to account for the location shape and amplitude contrast of these anomalies many crustal features in the southwest indian ocean near antarctica have little geophysical data to constrain their structure but do have tectonic conjugates near africa for which much more geophysical data are generally available using geophysical and geological constraints from one member to model the magnetization structure of its conjugate reproduces the observed magsat reduced to pole anomalies over both structures very well this suggests that no significant alteration in their magnetization structure has occurred since the features split models of these conjugate structures show that im vrm reproduces the magsat anomalies associated with non kqz crust but that both im vrm and a dominant nrm component are required to explain the anomalies associated with kqz crust

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

hot spot activity and the break up of pangea abstract the mesozoic and cenozoic positions of the continents that formed pangea in the triassic jurassic were derived from paleomagnetic and intraplate volcanic data paleoclimatic observations such as reef and fossil flora distribution and geological observations major hot spots helped to determine the longitudinal position of pangea and to construct a model of plate motion during the pangean break up the position of the northern part of pangea was constrained using iceland and jan mayen hot spots the iceland hot spot was traced from its present day position to greenland in the paleocene to baffin bay in the late cretaceous to the alpha ridge in the early cretaceous to the chukchi borderland in the middle late jurassic to franz joseph land in the early jurassic the yenisei khatanga trough in the middle triassic and finally to west siberia in the late permian early triassic the hot spot activity is expressed by eastern and western greenland volcanics the siberian trap basalts and perhaps by alpha ridge and chukchi borderland volcanics the chukchi borderland volcanics are related to the early stage of the opening of the canadian basin the position of the southern part of pangea was constrained using the bouvet hot spot this hot spot was tracked from its present day position to western antarctica in the early cretaceous late jurassic and to south africa in the early jurassic late triassic this hot spot activity produced the ferrar and karoo volcanics the model of plate motion obtained agrees with other data on intraplate volcanics which are also related to hot spots at the time of the opening of the central atlantic the cape verde and canary island hot spots were located along the ocean s spreading axis the long lasting location of hot spot and associated mantle upwelling plumes could help to explain forces driving the tethys transit plates opening of the ligurian ocean and eastern mediterranean the european hot spots are related to the several mesozoic phases of rifting for example the rhine graben hot spot may have been located at the rifting axis in the north sea several authors have already discussed the contribution of hot spots to the opening of the south atlantic and indian oceans mantle plumes associated with hot spots play an active role in rifting and initial phases of spreading the lithospheric displacement caused by upwelling combined with sometimes remote collisional forces on the other side of continental plate may result in compression and basin inversion

early permian to middle jurassic rifting and sedimentation in east africa and madagascar pre drift sediments of madagascar early permian middle jurassic have been studied palynologically these studies resulted in a more precise dating of individual stratigraphic units and the recognition of minor and major breaks in the succession palynostratigraphic and physical evidence of unconformities have been used to subdivide the pre drift sediments into depositional cycles and to infer rifting events a comparison with equivalent strata of mozambique tanzania kenya ethiopia and somalia shows a general correspondence and provides additional information for the construction of a generalized framework for the east african madagascan region which demonstrates the relationships between rifting and sedimentation and elucidates the prehistory of the break up of gondwana into a western and eastern fragment during the middle jurassic the predrift succession of east africa madagascar can be subdivided into two major sequences a karoo sequence cycles 1 5 asselian early smithian and a transitional sequence cycles 6 9 ladinian early bajocian the two sequences are separeted by a late scythian to anisian hiatus which indicates extensive uplift and erosion before a major rifting event initiated the resumption of sedimentation in the ladinian this middle triassic event marks the transition from the intracratonic rift stage to the pericratonic basin stage and correlates with a significant event in the pangaean history the transition from final coalescence to initial dispersal the onset of the southward drift of madagascar is believed to have occurred about 60 ma later near the aalenian bajocian boundary contemporaneous with or immediately after the deposition of syntectonic sediments of cycle 9

early cretaceous uplift in the ellsworth mountains of west antarctica apatite fission track analysis of samples covering a 4 2 kilometer vertical section from the western flank of vinson massif antarctica s highest mountain indicates that the ellsworth mountains were uplifted by 4 kilometers or more during the early cretaceous following the initial separation of east and west gondwana and accompanying the opening of the weddell sea relief of at least 1 8 kilometers has persisted in the ellsworth mountains since the early cretaceous and a maximum of 3 kilometers of uplift has occurred since that time

marine magnetic anomalies palaeomagnetism and the drift history of gondwanaland abstract the results of palaeomagnetic surveys of mesozoic and tertiary rocks from gondwanaland can be reconciled with the results of modelling the evolution of oceanic floors from analyses of marine magnetic anomalies previous inconsistencies were mainly due to errors apparent in the australian cenozoic palaeomagnetic data an alternative tertiary apparent polar wander path apwp has been constructed from an analysis of all published laterite and weathered profile data palaeomagnetic results for africa antarctica australia india madagascar and south america are compared for rotations corresponding to marine magnetic anomalies 16 22 28 34 and m1 and for fit india has been selected as the reference continent since it provides the most detailed apwp having drifted about 50 of latitude since breakup

differential deformation of gravity driven deep water fold and thrust belts along the passive continental margin of east africa and their impact on petroleum migration and accumulation abstract passive margin deep water fold and thrus belts dwftbs have been the focus of a number of studies in recent decades because many oil and gas fields have been discovered in traps associated with them nevertheless the impact of dwftbs on petroleum plays remains unclear in this study we developed models for petroleum migration and accumulation in the offshore rovuma basin and lamu basin dwftbs located along the passive continental margin of east africa based on an integrated analysis of seismic geochemical and geological data using available high quality seismic data in the depth domain we were able to quantify the evolution of thrust anticline growth by applying the area depth strain ads method our results indicate that the offshore lamu basin dwftb was active from the late cretaceous period to the early miocene period while the offshore rovuma basin dwftb has been active from the oligocene period geochemical parameters and basin modeling show that widespread lower jurassic mudstone is the dominant source of petroleum resources with peaks of oil and gas generation in the cretaceous and cenozoic periods respectively based on this we suggest that the traps within the offshore lamu basin dwftb contain oil generated from lower jurassic sources that migrated along faults and accumulated during the late cretaceous period and gas generated from the same sources during the cenozoic period conversely there is an insufficient amount of oil in the offshore rovuma basin dwftb owing to the absence of oil migration pathways or traps as a result of the dwftb s more recent formation from the oligocene period

subsidence history of the tethys himalaya abstract this article reconstructs the evolution of the passive northern margin of peninsular india facing the neotethys from the earliest rifting stages in the carboniferous to final collision with the active southern margin of asia in the early eocene classical techniques of basin analysis were applied to an extensive and coherent stratigraphic data base facies analysis biostratigraphic dating and palaeoenvironmental interpretation of an over 10 km thick sedimentary succession from eight main composite sections allowed us to draw accurate sedimentation and backstripped subsidence curves and to place quantitative constraints on the palaeotectonic scenario inferred from the sedimentary record in each of the investigated composite sections three major stratigraphic gaps are consistently recognised in the rift sequence the rift unconformity mississippian the carboniferous permian hiatus largely kasimovian to asselian and the break up unconformity mid sakmarian two or even all three gaps may merge into a single hiatus spanning up to 70 ma overall gaps are associated with rift shoulder uplift which after calculation of backstripped subsidence sums up to 600 m at least in three of the investigated sections thermal subsidence and low sedimentation rates are documented by the mid permian to middle triassic drift sequence deposited while the neotethys was expanding between northern gondwana and the detached peri gondwanan blocks a marked increase in accumulation rates is recorded in the carnian norian but calculations of uniform lithospheric stretching suggest that extension was minor and associated with tectonic processes affecting distant parts of gondwana the tethys himalayan margin was uplifted around the jurassic cretaceous boundary at the onset of a major volcanic episode finally uplift exceeding in magnitude all previous events is documented by the uppermost units of the stratigraphic column deposited during the earliest stages of collision between india and asia in the latest paleocene to early eocene

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

crustal structure of the murray ridge northwest indian ocean from wide angle seismic data the murray ridge dalrymple trough system forms the boundary between the indian and arabian plates in the northern arabian sea geodetic constraints from the surrounding continents suggest that this plate boundary is undergoing oblique extension at a rate of a few millimetres per year we present wide angle seismic data that constrains the composition of the ridge and of adjacent lithosphere beneath the indus fan we infer that murray ridge like the adjacent dalrymple trough is underlain by continental crust while a thin crustal section beneath the indus fan represents thinned continental crust or exhumed serpentinized mantle that forms part of a magma poor rifted margin changes in crustal structure across the murray ridge and dalrymple trough can explain short wavelength gravity anomalies but a long wavelength anomaly must be attributed to deeper density contrasts that may result from a large age contrast across the plate boundary the origin of this fragment of continental crust remains enigmatic but the presence of basement fabrics to the south that are roughly parallel to murray ridge suggests that it separated from the india seychelles madagascar block by extension during early breakup of gondwana

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

fluid seepage associated with slope destabilization along the zambezi margin mozambique abstract evidences for active fluid seepages have been discovered along the zambezi continental slope offshore southern mozambique these seepages are mostly associated with pockmarks which are aligned along a trend parallel to the slope and running closely upstream of the headwall scarp of a wide zone of slope destabilization fluid seepages are interpreted as a potential trigger for the slope destabilization acoustic anomalies within the water column have been interpreted as related to moderate bubble seepages mostly located outside and only punctually inside the destabilization zone exploration with the scampi towed camera system in the widest pockmark diameter 200 m wide has shown fluid seepages associated to authigenic carbonate crusts and possibly bacterial mats these fluid seepages are also associated to the presence of chemiosynthetic organisms vesicomyidae and thyasiridae bivalves siboglinidae tubeworms the sampled gas in the sediment corresponds mainly to ch4 of microbial origin generated by hydrogenotrophic methanogenesis from a substrate of organic origin i e a conventional process of genesis of microbial gas in the marine domain no evidence for thermogenic gas was detected another type of pockmarks has been observed within the core of the slope destabilization zone most of these pockmarks are inactive in terms of fluid seepage at present time and are associated to carbonate buildups forming chimney geometries they probably correspond to diagenetic chimneys of former fluid migration pathways that have been exhumed during the mass sliding and the surrounding depression are related to recurrent activity of strong lateral slope currents which have scoured the sediments around the spatial organization of the slope destabilization features is considered as representative of the temporal evolution of the landslide giving information about the dynamics of slope instability processes this proposed evolution started by scattered seepages of formation water with dissolved gas then free gas seepages appeared notably in the upper part of the slope this was followed by progressive shallow deformation in the sediments downslope of the main gas seepages finally the whole slope was destabilized forming imbricated landslides exhuming locally former diagenetic chimneys

plate tectonics and orogenic research after 25 years a tethyan perspective abstract orogeny the process by which the earth s prominent mountain ranges are constructed has been a central topic of interest in the earth sciences since at least the end of the 18th century the recognition that strains and displacements of very considerable magnitude occur along all of the three dimensions within an orogenic belt during its evolution has grown gradually during the last two centuries emphasis on primary vertical movements dominated the ideas on the nature of orogeny during the first half of the 19th century whereas compression and consequent uplift across mountain belts were believed to be the main cause for their origin during the subsequent one hundred years or so that were spent under the dominance of the fixist contraction theory mobilist tectonicians realised that continental drift in places also required motion along the trend of orogenic belts but this view did not gain general acceptance recognition of significant strike slip motion parallel or subparallel with mountain ranges evolved independently and mostly within the fixist camp by the 1960 s presence of important motions both along and across mountain belts had become common knowledge but no theoretical basis existed to account for them all

the rovuma delta deep water fold and thrust belt offshore mozambique abstract we interpret two dimensional seismic reflection data from the rovuma delta basin deep water fold and thrust belts two major arcuate complexes with different architecture and extent are identified while in the northern palma arcuate complex a multitude of steep east dipping thrust related fold anticlines formed above a single main detachment in the southern mocimboa arcuate complex multiple detachments resulted in the formation of thrust duplexes in between the two arcuate domains only few thrust related fold anticlines developed our interpretation of the rovuma basin is a linked system of up dip extension and down dip compression that is mainly driven by gravity tectonics sediment loading and a hinterland uplift due to the development of the east african rift system since the oligocene is proposed as origin of the delta it is shown that the main seaward dipping detachment in early cenozoic strata is likely under compacted and overpressured shale conversely shale diapirism is questionable since the shape and location of such structures in the fold and thrust belts appear simply indicating steeply dipping imbricated folds rooted by a near vertical thrust we suggest that mainly a different rheology and thickness and thus efficiency of the shale detachment across the delta resulted in different morphologies and geometries of the deep water fold and thrust belts

crustal structure in the western somali basin as part of integrated marine geophysical studies in the western somali basin we performed 118 sonobuoy experiments to define better the crustal structure of the margins and basin created by the separation of madagascar and africa after using t2 x2 conventional slope intercept methods and slant stacked t p techniques to analyse the data we combined our solutions with all previous velocity information for the area velocity functions were derived for the sediment coiumn and we detected a high velocity 4 58 0 29 km s 1 sediment layer overlying acoustic basement we confirmed that the crust is indeed seismically oceanic and that it may be considered either in terms of a layered model layers 2b 5 42 0 19 km s 1 2c 6 23 0 22 km s 1 3 7 03 0 25 km s 1 and mantle 7 85 0 32 km s 1 were identified or a more complex gradient model in which layer 2 is marked by a steeper velocity gradient than underlying layer 3 integrated igneous crustal thicknesses 1 62 0 22 s 5 22 0 64 km are significantly less than what is considered normal we present a revised seismic transect across the east african margin as well as total sediment thickness depth to basement and crustal thickness maps

salt diapirs bordering the continental margin of northern kenya and southern somalia the presence of newly discovered diapirs of presumed salt origin is documented for the continental margin of northeastern kenya and southeastern somalia these structures are probably a manifestation of a significant thickness of lower jurassic evaporites deposited during the rift and early drift stages of the separation of madagascar from the african continent

reconstruction of the paleoenvironmental changes around the miocene pliocene boundary along a west east transect across the mediterranean abstract in order to reconstruct the environmental changes at the end of the messinian salinity crisis a multidisciplinary study has been carried out with a high sampling resolution of the late messinian early zanclean zone mpl 1 sediments along a west east mediterranean transect the studied examples comprise sections from southern spain vera almanzora balearic basin odp site 975 tyrrhenian basin odp site 974 sicily eraclea minoa zakynthos kalamaki corfu aghios stefanos crete aghios vlasis previously analyzed sections from the levantine basin cyprus and odp sites 968 and 969 are used for comparison the sections have been correlated using planktonic foraminiferal assemblages sedimentological and stable isotope variations and compared to the astronomical cyclicity defined in the miocene pliocene boundary stratotype of eraclea minoa sicily variations of caco3 content stable isotopes of carbonates 18o 13c and foraminiferal assemblages indicate similar environmental transition at the miocene pliocene boundary in all of the investigated sections the latest messinian deposits are barren of fossils or characterized by only reworked planktonic foraminifers except for the sporadic presence of ammonia tepida brackish or lacustrine ostracods and brackish mollusks typical of the lago mare facies the oxygen and carbon isotopic compositions of carbonates usually exhibit large variations with dominantly low values indicating freshwater dilution the earliest pliocene mpl 1 cycle 1 shows a rapid and progressive increase of the 18o values which indicates the restoration of marine conditions after the lago mare event normal marine environments were definitely established and stabilized at the top of cycle 1 these data confirm that the inflow of marine waters occurred contemporaneously within the whole mediterranean at the base of pliocene although stable marine conditions occurred only about 20 kyrs later

influence of carbonate content on the stress strength behaviour of neogene marls from the betic cordillera spain in cu triaxial tests using a quasilinear elastic hyperbolic model the stress strain behaviour of 135 samples of calcareous silt and clay soil known as marl from three quarries is presented and discussed in connection with its adjustment to a hyperbolic non elastic constitutive model the marl samples were classified and identified following conventional soil mechanics procedures and tested under triaxial cu consolidated undrained stresses also the carbonate sulphate and soluble salts contents were analysed by chemical and x ray methods considering the obtained strength parameters and chemical compositions a high correlation between the carbonate content and the parameters of the hyperbolic model was observed indicating a parallel increase in the brittle behaviour and carbonate content with a slight increase in bearing capacity nevertheless when all the samples were considered the parameters of the hyperbolic model seemed not to be clearly related to the carbonate content this being interpreted as expressing a hyperbolic model influenced not only by the carbonate content but also by several other variables such as the percentage of fine particles or the dilatationing behaviour of the soil particles expressed by the dry density given the extensive use of marly soils in the construction of earth dam cores and slope banks the main interest of the results is to present new insights into its mechanical behaviour not usually considered in common engineering practice

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

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

a coarsening upward megasequence generated by a gilbert type fan delta in a tectonically controlled contex upper miocene guadix baza basin betic cordillera southern spain abstract the 100 m thick succession at bodurria of a gilbert type fan delta constitutes an upper tortonian coarsening and thickening upward megasequence it lies uncofor mably converted by the continental pliocene neogene materials of the basin infill and is uncomformably covered by the continental pliocene the succession was deposited on one margin of an extension basin with ac tive sysedimental techonics and rosing sea level in these circumtances the accommodation space and the sudiment supply increased throughhout time this determined the development of the coarsening and thickening upwward megasequence resulting from the vertial stacking of four smaller sequences which in turn were built by staking of gilbert tpe fan delta units and were bounded by calcarenite facies deposited on a shallow shelf this sequential organisation and internal structure were controlled by the tectonic behaviour of the faults on the basin margin in particular large scale movements with intervals of stability which respectively created accommodation space for the deltaic sequences and conditions for development of calcarenite shelf facies

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

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

mesozoic and early tertiary rift tectonics in east africa abstract a complex history of crustal extension occurred in east and central africa during the mesozoic and early tertiary beginning in the late jurassic this resulted in a large system of rifts the central african rift system that spanned from central sudan to southern kenya late jurassic rifting is best documented in the white and blue nile rifts of the sudan and records east west extension in half graben that were connected by large scale shear zones and pull apart basins early cretaceous rifting re activated jurassic basins and spread to the large south sudan rifts and anza rift in kenya by the late cretaceous the extension direction shifted to the ne sw and the presently observed large scale rift geometry was established in the early tertiary some mesozoic basins were again reactivated while other regions underwent wrench faulting and basin inversion the large number of basins preserved in the central african rift system can be used to construct an evolutionary model of continental rift tectonics early phases of extension at low strains produced alternating half graben accommodation zone geometries similar to those observed in most young and active continental rifts at higher strains some border faults were abandoned so that through going simpler active fault systems could evolve this is interpreted as representing a switch from complex oppositely dipping detachment structures with strike dimensions of 50 150 km to regional detachment structures that continue for hundreds of kilometers parallel to the rift this change in the type of detachment was accompanied by a shift in the position of the subsidence away from the breakaway to a position focused further within the regional upper plate non rotational high angle normal faulting dominates in the development of these late basin geometries deciphering similar rift basin histories from passive continental margins may in many cases exceed the limits of available reflection seismic data east west striking shear zones including the umm hani and south sudan shears served as basin terminating structures in the late jurassic rift system these shear zones were later reactivated during the cretaceous and early tertiary controlling the positions of some accommodation zones and jogs apparent offsets in rift trends the south sudan shear may have continued to play a role in neogene extensional tectonics as it connects jogs in both the abu gabra anza rift jurassic to early tertiary and turkana chew bahir rift neogene to recent

the proto indian ocean and a probable paleozoic mesozoic triradial rift system in east africa abstract a revised paleozoic mesozoic stratigraphy of coastal kenya including in particular the karroo based on current geological mapping near mombasa is briefly described this stratigraphy provides the geological framework for proposals concerning the proto indian ocean and the tectonic setting of the karroo depositional basins recent geophysical evidence suggests that within gondwanaland madagascar was situated off east africa near kenya tanzania the southern limits of the marine lower jurassic and southern limits of the marine middle and upper jurassic are in similar positions in mainland africa and madagascar using the latter reconstruction these paleogeographic limits also define the position during the jurassic of an embayment from an ocean to the north regional geological similarities also support this reconstruction and are reinforced by paleocurrent data from the karroo of kenya indicating drainage north northeast during the permian and triassic and possibly the lower jurassic marine connections during karroo times appear to be of different ages in kenya tanzania somalia and madagascar probably reflecting physical limitations to marine access in fault separated basins the above embayment encroached across the karroo depositional basins from northeast kenya to southern tanzania during the lower and middle jurassic i e from the direction towards which the karroo drainage had been previously directed marine conditions remain to the present day so this embayment can be considered the proto indian ocean for east africa the marine incursion took place before the breakup of gondwanaland suggesting that during the jurassic the proto indian ocean in east africa was an epicontinental sea and not a true ocean i e floored by simatic crust the epicontinental nature of this sea is confirmed by the lithologies of the associated sediments paleontological data indicate that this sea was an arm of tethys true oceanic conditions could not have been established until the displacement of madagascar away from africa probably in the cretaceous accepting the above northern position of madagascar the writers also postulate that in east africa the fault bounded karroo depositional basins troughs were located within a major triradial rift system extending from lake malawi at least as far as eastern kenya some 1600 km this rift system if valid was established within gondwanaland over a period 100 m y in the paleozoic mesozoic pre breakup in marked contrast to the east african rift system classical rift valleys which is mainly a cainozoic phenomenon post breakup it is therefore considered that there is a fundamental difference in origin between the two rift systems

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

active fault system across the oceanic lithosphere of the mozambique channel implications for the nubia somalia southern plate boundary abstract seismic reflection and multibeam echosounder data were acquired in the mozambique channel in 2014 and 2015 during the ptolemee pamela moz02 and moz04 marine surveys aboard the rv atalante and pourquoi pas these data revealed that an active fault system is deforming the oceanic lithosphere of the mozambique basin which has developed during jurassic cretaceous times the correlation between the fault system and the arrangement of earthquake epicenters suggests that this tectonically active zone directly connects northward with the southern part of the eastern branch of the east african rift system which corresponds to the seismically active graben system bounding the northern part of the davie ridge the fault zone extends southwestward of the mozambique ridge along the same trend as the agulhas falkland transform fault zone the general organization of the fault zone shows the characteristics of an extensional system north of the mozambique channel north of the europa island and a right lateral transtensional system with coeval normal faults and strike slip faults south west of europa this tectonic activity is associated with volcanic activity since at least late miocene times our findings emphasize that the eastern branch of east african rift system is extending largely toward the south not only in continental domains but also through the oceanic lithosphere of the mozambique basin this fault zone is participating to the complex plate boundary between the main african continent nubia plate and madagascar somalia plate

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

evolution of the glorieuses seamount in the sw indian ocean and surrounding deep somali basin since the cretaceous abstract little is known about the geological history of the glorieuses seamount including basic information about its age and origin related to the regional evolution of the southern tip of the somali basin this study focused on describing and reconstructing the long term stratigraphic evolution of the glorieuses seamount sw indian ocean to identify the mechanisms that have occurred through time to finally shape the emerged modern islands distinct terrace levels currently submerged along the flanks of the seamount and surrounding seamounts have already been interpreted as resulting from successive carbonate development and back stepping episodes over the last 62 myr new isotopic and biostratigraphic dating on the flanks of the seamount coupled with sequence stratigraphic interpretation of seismic profiles acquired in the adjacent basin provide new constraints for the late cretaceous and cenozoic vertical evolution of the seamount topped by carbonate platforms and sedimentation in the surrounding deep basin even if starved steep slopes prevent a straightforward source to sink continuity between the platform and the basin domains our findings propose a consistent chronostratigraphic framework for the identified seismic markers and sequences in the deep basin and discuss a long term geological model that includes the main driving factors behind deposition volcanic events subsidence vs uplift phases climate and hydro dynamism changes and their quantitative impact on the evolution of the isolated carbonate sedimentary system our results show that i the glorieuses volcanic seamount emerged from two successive late cretaceous magmatic pulses firstly during the turonian then during the maastrichtian ii at least two potential uplift phases are recognized during the tertiary paleogene and or the eocene and tortonian iii basinal sedimentation recorded an abrupt change probably related to major regional hydro dynamical changes in late eocene times in the western indian ocean iv the export of sediments from the platform towards the basin numerous gravity flow processes is strongly enhanced after the mid miocene and is probably linked to the onset of the asian monsoon winds and bipolar circulation finally the glorieuses seamount although located in the vicinity of the comoros islands appears to have a much longer history and is geologically more comparable to the nearby seychelles this long term study has enabled us to associate the glorieuses seamount with the sse nnw madagascar seychelles alignment rather than with the comoro hot spot evolution

sedimentation in an intracratonic extensional basin the karoo of the central morondava basin madagascar the late carboniferous to triassic karoo supergroup of madagascar is a sequence of predominantly continental clastic sediments deposited during a long period of regional crustal extension in the morondava basin of western madagascar the lower two divisions of the karoo sediments the sakoa and the sakamena are deposits of fluvial and lacustrine sedimentation systems supplied from the precambrian metamorphic basement terrain to the east east west crustal extension produced a series of graben and half graben structures after the sakoa period which were reactivated after the sakamena the position and orientation of these half graben which were marginal to a larger rift system to the west were partly controlled by the steep nne ssw mylonitic fabric in the metamorphic basement palaeocurrents in the braided river deposits of the sakoa and lower sakamena indicate flow to the southwest and west in both sequences the rivers followed a regional palaeoslope to the southwest west and were apparently not significantly influenced by the local structural trends which were oriented perpendicular to this slope the absence of local structural control is attributed to extensive erosional events which followed each tectonic episode and preceded the onset of further sedimentation which took place on an essentially peneplained surface the tectonic episodes brought about changes in base level which caused this part of the basin to fluctuate between erosion and deposition

development of the mozambique and ruvuma sedimentary basins offshore mozambique two major sedimentary basins have been identified on the continental margin of mozambique mozambique basin and ruvuma basin the formation of the basins is related to the break up of gondwana and opening of the western part of the indian ocean the basins are relatively young having developed discordantly to the structural plan of gondwana sedimentary basins the history of the formation of the east african continental margin sedimentary basins within mozambique has been studied on the basis of the present day concept of gondwana break up and madagascar s drift with respect to africa two stages in the history of the east african basins can be recognized late gondwana and post gondwana the late gondwana stage 303 157 ma is typified by sedimentary and volcanogenic rocks of the karoo group the post gondwana stage 157 0 ma corresponds to the period of active gondwana break up and the formation of indian ocean marginal basins the mozambique basin occupies both central and southern parts of the coastal plain of mozambique extending onto the continental shelf and slope the sedimentary fill is composed of upper jurassic cretaceous and cenozoic rocks which discordantly overly the karoo basalts the upper jurassic occurs as continental red beds mostly distributed within buried grabens cretaceous rocks occur as terrigenous sediments of continental and marine genesis cenozoic deposits are of predominantly marine and deltaic origin the ruvuma basin situated in the north of mozambique is part of an extended east african marginal basin which includes parts of the coastal plains and continental margins of tanzania and kenya the basement of the basin is composed of crystalline and metamorphic rocks of pre cambrian age the sedimentary fill is represented by continental terrigenous karoo sediments marine and lagoonal jurassic deposits and marine and deltaic rocks of cretaceous and cenozoic age the main difference between the sedimentary fill of the mozambique and ruvuma basins is related to the changing sedimentary environment in the period of gondwana break up jurassic and early cretaceous when the drift of madagascar and sea floor spreading took place and the marine basin gradually extended southwards between the east african margin and madagascar from the late cretaceous on the sedimentary environment within both basins was similar the mozambique and ruvuma basins forming a single system on the east african continental margin the main stages of the basins development are illustrated by a series of palaeogeographic reconstruction maps

late cretaceous early tertiary convergence between the indian and arabian plates recorded in ophiolites and related sediments abstract remnants of ocean floor forming the eastern ophiolite belt in oman and the western ophiolite belt in pakistan have a common plate tectonic history culminating in emplacement at the cretaceous tertiary boundary fragments of ocean floor in these two belts have ages between 150 and 65 ma and recorded tectonic events in the early indian ocean at 150 ma 130 120 ma 110 100 ma and 70 65 ma new radiometric and chronostratigraphic ages and paleomagnetic and sedimentary information are used to relocate these ophiolites in the frame of the evolving indian ocean which was characterised by the stepwise breakup of gondwana at 158 ma east and west gondwana 130 ma southern atlantic breakup of east gondwana 95 84 ma madagascar and india seychelles 65 ma india and seychelles and finally at 40 ma rifting between africa and arabia the 150 ma old oceanic rocks of masirah island in oman originally formed the extension of the basins now preserved along the eastern edge of the afro arabian plate masirah drifted together with india seychelles when a new ridge formed at approximately 130 ma separating microplates e g kabul block from the northern edge of greater india parts of this new ocean were later emplaced to form the western ophiolite belt in pakistan consumption of oceanic crust and emplacement of ophiolites during the closure of the neotethys occurred not only north of india seychelles but also along two or three subduction zones between the afro arabian plate and india seychelles documented by the formation of metamorphic soles beneath ophiolites relics of ocean floor formed approximately 65 70 ma ago now form the upper units in the western ophiolite belt of pakistan where they overlie an accretionary prism of pillow lavas of dominantly aptian albian age and sedimentary rocks the breakup of india and the seychelles at 65 ma was associated with the eruption of flood basalts deccan and seychelles and enhanced counter clockwise movement of india this breakup possibly caused the late jurassic early cretaceous fragments of ocean floor e g masirah to be thrust onto the eastern edge of oman the ophiolites which were emplaced onto the western edge of india drifted as part of this plate farther north where collision with eurasia and the accreted microplates occurred in the eocene 55 ma

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

stratigraphy depositional history and environments of deposition of cretaceous through tertiary strata in the lamu basin southeast kenya and implications for reservoirs for hydrocarbon exploration abstract the lamu basin which is characterized by an extensional tectonic style is the failed arm of a tri radial rift system and possesses tectonic and stratigraphic elements in the post rift series that are analogous to those of the west coast of africa development of the southern part of the basin as a passive margin is closely related to considerations of the pre drift position of madagascar and formation of the indian ocean basin during mesozoic time cretaceous and tertiary strata in the basin comprise an eastward thickening gross succession of sediments on which eustatic sea level fluctuations and a sequence of unconformities related to pulses of transgressive and regressive depositional trends are superimposed recognition of these trends has provided the basis for classification of the strata into megasequences representing distinct provinces with regard to time stratigraphy sedimentation tectonics depositional environments and hydrocarbon potential megasequence i karoo group and jurassic includes strata of the permian through jurassic the discussion of which is beyond the scope of this paper megasequence ii sabaki group includes strata of the cretaceous and early paleocene deposited in tide influenced shelf and marine settings these are in ascending order the ewaso sands the walu shale the hagarso limestone the freretown limestone and the kofia sands megasequence iii tana group includes strata of the eocene through oligocene which were deposited in fluvial deltaic and restricted shelf settings these are in ascending order the kipini formation the pate limestone the linderina limestone the dodori limestone and the barren beds formation megasequence iv coastal group includes strata of the miocene through pliocene which were deposited in restricted shelf middle to outer shelf deep marine and fluvial settings these are in ascending order the baratumu formation the lamu reefs the simba shales and the marafa formation determination and description of these lithostratigraphic units their distribution and palaeohistory have been imperative for predicting reservoirs for hydrocarbon exploration

a model of plate kinematics in gondwana breakup summary an accurate model of relative plate motions in gondwana breakup is based on visual fitting of seafloor isochrons and fracture zones fz from the riiser larsen sea and mozambique basin used predictively the model precisely locates kinematic markers in the west somali basin which allows the conclusion that the spreading centres in the west somali and mozambique basins and the riiser larsen sea formed parts of the boundary between the same two plates the locations of fz and less well defined isochrons from neighbouring regions are also consistent with their formation on other lengths of this same boundary and with its relocation from the west somali basin and northern natal valley to the west enderby basin and lazarev sea during chron m10n small independently moving plates thus played no role in the breakup of this core part of gondwana in an inversion procedure the data from these areas yield more precise finite rotations that describe the history of the two plates separation breakup is most simply interpreted to have occurred in coincidence with karoo volcanism and a reconstruction based on the rotations shows the lebombo and mateke sabi monoclines and the mozambique and astrid ridges as two sets of conjugate volcanic margins madagascar s pre drift position can be used as a constraint to reassess the positions of india and sri lanka in the supercontinent

the himalayan passive margin from precambrian to cretaceous times abstract the himalaya passive margin should be separated into the indian himalaya and the pakistan himalaya divided by the nanga parbat haramosh basement uplift in the northwestern syntaxes the indian passive margin shows a complex history involving splitting of microcontinents off the northern gondwana margin from early paleozoic times until the jurassic in contrast the pakistan himalaya formed part of the stable indian shelf until separation from africa started in the jurassic each margin has a distinct stratigraphic history until the mid mesozoic after which they show similar histories until the tertiary the himalaya mountains in india consist of three distinct tectonic units juxtaposed during the neogene by southward thrusting of the northern indian precambrian to mesozoic passive margin the lesser himalaya on the south forms a late precambrian passive margin usually directly overlain by late cretaceous to tertiary clastics its stratigraphy is closest to that of the indian shield in contrast the high himalaya has a thin ordovician to carboniferous shelf sequence deposited after early ordovician deformation and granite intrusion above this thick permian and mesozoic shelf sequences mark the separation of continental blocks off the northern indian margin and the opening of the neotethys ocean the north himalaya formed the slope and basin of this ocean and consists of reactivated paleozoic gneiss domes overlain by thick mesozoic sediments which pass in the northeastern himalaya into an enormously thick sedimentary sequence resting on oceanic crust paleomagnetic and structural evidence indicates at least 500 km of southward thrusting along the main central thrust between the lesser and high himalaya this thrusting together with a further 250 km of erosion at the front of the nappes has removed the entire inner shelf of the himalaya passive margin and can explain the startling contrast between the high and lesser himalaya stratigraphic sequences but there are still discrepancies particularly in the eastern himalaya where precambrian basement is juxtaposed with the north himalaya mesozoic slope these discrepancies can be partly resolved by strike slip movements roughly parallel to the himalaya which have removed parts of the northern indian passive margin such faults of the requisite orientation displacement and age occur in southeast asia where their cumulative displacements can add up to several thousand kilometres similar faults of undoubted pre tertiary age occur in the pakistan himalaya here the passive margin shows a much simpler history of jurassic to miocene subsidence

jurassic and cretaceous marine connections between the southeast pacific and tethys abstract the affinities of the ammonoid faunas from central west and southern south america are used to analyze the evolution of seaways and oceanic connections with tethys during the jurassic and cretaceous during the early and middle jurassic marine connection with eastern tethys was around the australian margin of gondwana from the pliensbachian onwards connection with western tethys occurred through the sporadically opened hispanic corridor in central america in late jurassic time the break up of gondwanaland was initiated and a seaway began to open between south america africa and antarctica connection of the eastern pacific with tethyan waters through southern africa was sporadic and was still mainly around australia and along the now open central american seaway throughout the early cretaceous the opening of the south atlantic proceeded from south to north and the first marine connection between the southern and northern atlantic took place in albian time a definite marine connection between the opened south atlantic and eastern tethys with faunal migration routes was established in late hauterivian barremian time and became fully open in the aptian albian successive stages in the opening of a south african seaway between tethys and the southeast pacific show an asymmetric development to the west and east of south africa during late cretaceous time oceanic circulation continued between the south atlantic and eastern tethys and true oceanic conditions developed between the southern and northern atlantic the antarctic peninsula was still located in a position adjacent to south america preventing open circulation between the south atlantic and pacific oceans

the magma poor somalian continental margin lower crustal boudinage and mantle exhumation abstract combined interpretation of 2d reflection seismic data and 2d gravity modelling reveals an iberian type magma poor margin across the obbia basin in offshore central somalia the configuration of the syn rift and post rift sediments as well as the upper crust the lower crust and the moho delineates four distinct marginal domains the proximal domain is characterized by thickest crust and narrow underfilled basins bounded by normal faults piercing the low reflective upper crust on top of the upper crust there is a thick package of karoo aged pre rift sediments 2 s two way time twt a major normal fault with a throw of 4 s twt dissects the sediments the crystalline crust and likely also the moho and represents the necking point the oceanward limit of the proximal domain and the transition to the necking domain the necking domain hosts deep half grabens filled with wedge shaped syn rift sediments of likely early jurassic age crustal thinning was predominantly accommodated by ductile extension of the high reflective lower crust resulting locally in lower crustal boudins with ongoing extension the upper crust may have become completely detached from the lower crust and rests on the upper mantle at present day within the subsequent hyperextended domain the moho shallows significantly and reaches the top basement level in the seaward portion of the line normal faults commonly penetrate through the entire crust and bound minor isolated syn rift deposits the termination of the hyperextended domain is marked by a major shallow dipping concave downward fault that apparently connects the moho with the top basement further seaward several small continental blocks so called extensional allochthones may be present in the transitional area between the coupling point and the exhumation point the absence of moho reflections in the east of the profile indicates that crust may be absent and the upper mantle is exhumed

geology basin analysis and hydrocarbon potential of mozambique and the mozambique channel abstract the phanerozoic structural and stratigraphic history of mozambique can be separated into a karoo and a post karoo phase outcrops of karoo rocks limited to the zambezi valley and small outliers in the mozambique belt to the north show an eastward facies change marked by an increased argillaceous content and the appearance of carbonates the wide coastal plain and immediate offshore of southern mozambique is marked by extensional horsts and grabens with a mesozoic fill covered by cenozoic sediments little is known of lower stratigraphic horizons in this coastal province there is a facies transition from continental clastics in the northwest and west to more lacustrine and marine deposits to the southeast and east upwards the succession passes into the cenozoic without a noticeable break in the offshore in the north seismic lines help define two basins the zambezi delta basin and the mozambique channel basin separated by the beira high in the south several offshore grabens have been defined one of which the south mozambique graben has been examined here by an integrated basin analysis technique the technique which uses available depositional structural and thermal data provides information on maturation and the timing of hydrocarbon maturation the analysis showed variation in the extension rate with time and two phases of rifting the greatest rate of extension accumulation and subsidence between 96 and 76 ma depends upon interpretational assumptions during this time interval fault blocks were systematically tilted domino fashion this event broadly coincides with the rifting between madagascar and antarctica a period of plate reorganization the earlier rifting phase was probably related to the strike slip separation of madagascar from africa modelling heat flow suggests that the later event is the more important from the maturation hydrocarbon standpoint the offshore and potentially the deeper onshore post karoo basins have traps reservoirs and seals with the presumed source rock within lower cretaceous or jurassic strata there is at the present time no means to evaluate the potential of the karoo rocks

geotechnical characterization of carbonate marls for the construction of impermeable dam cores abstract fine grained more or less cohesive carbonate materials are extremely widespread in terms of surface area and are therefore commonly used as materials to construct impermeable cores for dams however it has not been adequately documented whether the carbonate content in fine grained soils significantly affects their engineering behaviour the present study shows that the carbonate content substantially influences the engineering behaviour of clayey material for this we subjected 32 samples to different laboratory tests such as the normal proctor the atterberg limits granulometric analysis oedometric and undrained triaxial tests the resulting parameters were correlated with the carbonate content of the samples the materials studied in this work had been used in the construction of the impermeable core of the san clemente dam belonging to the hydrographic basin of the guadalquivir river southern spain these marls present as their prime characteristic a carbonate content of the fine fraction consistently exceeding 50 giving them special importance in the study of this phenomenon in this study a direct relationship was found between the geotechnical properties of the soils studied and their degree of compaction with the carbonate content and the type of minerals in the clay being the main factors determining the behaviour of these soils finally we conclude that the percentage of carbonates should be used as a classification criterion for the soils used to construct the cores of earth filled dams

engineering geological characterization of neogene marls in the southeastern granada basin spain analysis of 171 samples taken from the neogene cohesive soils of the southeastern edge of the granada basin shows inverse correlation between carbonate content and dispersion index and swelling behaviour and direct correlations between carbonate content and shear strength this paper shows that carbonate content and clay fraction activity have a great influence on the compaction characteristics of soils marls of the middle and upper tortonian age lower marls are inadequate for use as subgrade because of their high plasticity in addition marls of lower and upper messinian age upper marls are inadequate for use as subgrade because of their high carbonate content the relationship between carbonate content and geotechnical properties is particularly important because the changes caused by pedogenic dissolution and precipitation processes lead to changes in mechanical behaviour

the break up of east gondwana along the northeast coast of oman evidence from the batain basin recent detailed studies on the batain nappes northeast coast of oman which represent a special part of the so called oman exotics have led to a better understanding of the neotethyan geodynamic evolution the batain exotics bear witness to volcanic activity sea level changes tectonic instability rifting and oceanization along the eastern oman margin during late palaeozoic and mesozoic times they allow definition of the batain basin as an aborted permian branch of neotethys this marine basin was created in early permian times extending southward to the east african madagascar region and was linked to the karoo rift system the presented revised classification of the batain nappes considers the batain basin to be no longer a part of the hawasina basin and the neotethyan margin proper we attribute the batain basin to a mozambique somali masirah rift system somoma this system started in early permian times creating a marine basin between arabia and india madagascar rifting in the late triassic and oceanization during late jurassic times led to the separation of east gondwana

a time slice approach to the history of cenozoic sedimentation in the indian ocean abstract results from drilling at over 100 sites have been combined with a new plate tectonic model to produce a series of palinspastic maps which represent snap shots of the changing patterns of sediment accumulation in the indian ocean through the cenozoic the maps clearly delineate the main sedimentary depocenters and show how the sedimentary record has responded on an oceanwide scale to major changes in paleogeography tectonics paleocirculation and or climate paleogene sediment accumulation rates were generally low with significant accumulation in only limited areas suggesting weak ocean circulation and stable well stratified conditions by contrast the vigorous thermohaline circulation of the neogene resulted in substantial widespread sedimentation accumulation rates were generally high and displayed a wide range of values following rapid growth of the east antarctic ice sheet in the mid miocene carbonate sedimentation has been progressively restricted to the shallow ridges and plateaus of the western indian ocean but accumulates there with locally high rates the link between tectonics and terrigenous sedimentation is exemplified by the major deep sea fans off east africa zambezi fan and india indus and bengal fans which developed in response to uplift and erosion beginning in late oligocene time in the east african rift system and the himalayas respectively although maps from only five widely spaced times are shown the methodology is such that a continuous series of maps at intervals as close as 1 m y can readily be generated ultimately the combination of such time slice maps with the results of detailed time series studies will provide a powerful tool for understanding past oceanic environments

isostatic response of the lithosphere beneath the mozambique ridge sw indian ocean and geodynamic implications summary the sw indian ocean is characterized by the presence of several aseismic features the mozambique ridge an elongated feature lying roughly parallel to the se coast of africa is by far the least known of those structures mainly due to the scarcity of marine data to date the crustal nature and the origin of the ridge are still controversial points since knowledge of the origin of the mozambique ridge is important for a better understanding of the evolution of the sw indian ocean the isostatic response of the lithosphere beneath the ridge is analysed in order to characterize its effective elastic thickness and the emplacement process of the feature two different approaches are applied the direct computation of the geoid anomaly over the ridge by means of a 2 5 dimensional method and the computation of the admittances between the bathymetry and both the geoid and gravity anomalies both approaches point to a local isostatic response of the lithosphere the crustal thickness beneath the mozambique ridge ranges from 17 to 30 km and the average density from 2 78 to 2 7 103 kg m 3 varying with locality in good agreement with the few existing refraction profiles based on our results on the geochemical similarity between the basalts cored at the dsdp site 249 and the mor basalts erlank reid 1974 thompson et al 1982 and on the present knowledge of the sw indian ocean kinematics martin hartnady 1986 we propose an on ridge origin for the mozambique ridge the ridge would have been formed by the anomalous activity of a spreading axis linking the northern mozambique and the transkei basins accretion centres between m10 and m2 times at the m2 epoch a ridge jump event would have caused the spreading to cease the astrid ridge a poorly known aseismic structure lying close to the coast of antarctica near 15 e may be the antarctic counterpart of the mozambique ridge formed by the same accretionary phenomenon the two ridges breaking apart at m2 time due to the ridge jump event however any admittance computation at such high latitudes is impossible at present due to the very poor quality of both bathymetry and altimetry and so this hypothesis cannot be verified

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

an oceanwide view of palaeogene plate tectonic events abstract the tectonic events taking place during the palaeogene are discussed ocean by ocean three predominant periods of tectonic changes are determined separating periods of continuous stable seafloor spreading these periods of change occur at the palaeocene eocene boundary 56 53 m y at the eocene oligocene boundary 42 35 m y and during the early middle eocene 35 30 m y during the course of writing this review it became very evident how much marine surveying and data analysis took place during the early 1970s this decade showed great leaps forward in our understanding of seafloor anatomy and tectonics

the davie fracture zone and adjacent basins in the offshore mozambique margin a new insights for the hydrocarbon potential abstract the interpretation of 2 d seismic reflection data provides a modern structural framework including hydrocarbon potential in the present day stratigraphic and structural traps of both the davie fracture zone and the adjacent nacala and angoche basins possible stratigraphic traps were identified in submarine fan and channel depositional environments during cretaceous to tertiary times structural traps are mostly defined within compressional structures formed by a variety of fault related folds and rift grabens within the jurassic and cretaceous successions the nacala and angoche basins form two depressions separated by the davie compressional zone this compressional structure is a prominent interior high running approximately north south an event of transpression and contraction characterizes the main tectonic setting commonly hosting several detached compressional structures along the western edge of the transform zone both basins are associated with the late jurassic early cretaceous rifting during the opening of the mozambique channel the angoche basin is proposed here to have formed by the earliest stage of break up in mid jurassic time the basin is bounded landward by the angoche volcanic zone a dyke swarm branch oriented n64dege forming part of the karoo and dronning maud land magmatism at c 180 ma subsequent rifting and break up led to the drift of east gondwana southwards along the dextral strike slip davie fracture zone at about 150 ma tithonian east gondwana appears to have rotated slightly clockwise about a pivot in the proximity of the angoche basin leading to extension and rifting in the rovuma basin to the north of the pivot point and compression west of the davie fracture zone to the south consequently the eastern boundary of the angoche basin was compressed developing a typical growth wedge of massive thrust imbrication structures while extensional tectonics created several depressions and rift grabens forming the nacala and quirimbas basins basin stratigraphy is interpreted along seismic reflection lines and correlated to the regional stratigraphic information and wells from the zambezi delta and rovuma basins

pre mesozoic basement rocks offshore nova scotia canada new constraints on the accretion history of the meguma terrane basement samples from offshore wells and geophysical data show that the meguma terrane forms the basement of mesozoic cenozoic sedimentary basins as far as 200 km offshore nova scotia the data provide constraints on avalon meguma terrane interaction during middle to late paleozoic events in the appalachian orogen the basement consists of metasedimentary rocks intruded by late paleozoic granitoid plutons principally magnetite bearing granodiorites which contrast with more peraluminous ilmenite bearing devonian granodiorite and granite onshore offshore tonalites have a subduction related trace element signature that is similar to the signature in minor devonian gabbro and tonalite intrusions on land nd sr and pb isotopes suggest three sources for meguma terrane plutons 1 2 ga basement possibly saharan shield and sediments derived therefrom this basement may be present beneath the southwestern scotian shelf 2 late proterozoic underthrust avalonian basement and basement autochthonous to the meguma terrane and 3 melts from mantle last remobilized in the late proterozoic pan african orogeny seismic reflection profiles show meguma terrane thrust northward over a wedge of avalon basement extending 30 km south of the surface suture there is no evidence that the devonian tonalites are directly related to subduction the lack of either ophiolites or paleozoic pelagic sediment suggests that convergence of the avalon and meguma terranes was achieved principally by strike slip motion resulting from low angle continent continent convergence with only a minor thrust component the widespread devonian carboniferous igneous activity of the amalgamated meguma and avalon terranes was a consequence of regional extension which resulted in decompression melting of upper mantle and production of mafic magmas variably mixed with crustal material

cretaceous plate tectonic reconstructions abstract plate tectonic reconstructions for each cretaceous stage are produced as base maps for cretaceous paleogeography and for oceanic and continental data both geologic and geophysical data are utilized to determine the relative rotations of cretaceous plates with respect to north america a sea floor spreading history is generated from the relative rotations this history is consistent with data from the deep sea drilling project paleolatitude lines are derived using average paleomagnetic pole positions these reconstructions are significantly more comprehensive than previous global reconstructions although there are still many areas which remain controversial