the crustal structure of beira high central mozambique combined investigation of wide angle seismic and potential field data abstract the timing and geometry of the initial gondwana break up between africa and east antarctica is still poorly known due to missing information about the continent ocean boundaries along the rifted margins in this context the beira high off central mozambique forms a critical geological feature of uncertain crustal fabric based on new wide angle seismic and potential field data across beira high a p wave velocity model supported by amplitude and gravity modelling provides constraints on the crustal composition of this area in the mozambique basin mainly normal oceanic crust of 5 5 7 km thickness with velocities of 6 5 7 0 km s in the lower crust is present a sharp transition towards beira high marks the continent ocean boundary here the crust thickens to 23 km at maximum a small velocity depth gradient and a constant increase in velocity with basal velocities of maximum 7 0 km s are in good agreement with typical velocities of continental crust and continental fragments the density model indicates the existence of felsic material in greater depths and supports a fabric of stretched but highly intruded continental crust below beira high a gradual decrease in crustal thickness characterizes the transition towards the mozambican shelf area here in the zambezi delta depression 12 km of sediments cover the underlying 7 km thick crust the presence of a high velocity lower crustal body with velocities of 7 1 7 4 km s indicates underplated magmatic material in this part of the profile however the velocity structure in the shelf area allows no definite interpretation because of the experimental setup thus the crustal nature below the zambezi delta and consequently the landward position of the continent ocean boundary remains unknown the difference in stretching below the margins of beira high suggests the presence of different thinning directions and a rift jump during the early rifting stage

new global marine gravity model from cryosat 2 and jason 1 reveals buried tectonic structure gravity models are powerful tools for mapping tectonic structures especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment we combined new radar altimeter measurements from satellites cryosat 2 and jason 1 with existing data to construct a global marine gravity model that is two times more accurate than previous models we found an extinct spreading ridge in the gulf of mexico a major propagating rift in the south atlantic ocean abyssal hill fabric on slow spreading ridges and thousands of previously uncharted seamounts these discoveries allow us to understand regional tectonic processes and highlight the importance of satellite derived gravity models as one of the primary tools for the investigation of remote ocean basins

crustal structure and rift architecture across the krishna godavari basin in the central eastern continental margin of india based on analysis of gravity and seismic data abstract the krishna godavari basin is a rifted passive margin basin that developed orthogonally to the nw se trending pranhita godavari graben along the central eastern continental margin of india in response to the continental rifting process and the subsequent seafloor spreading between india and eastern antarctica during the early cretaceous period the 3 d gravity interpretation of both the onshore and offshore sections of the basin integrated with detailed seismic reflection and refraction data provided significant new information about the crustal architecture and the early breakup history of the basin the gravity derived crustal models indicate that the crust at the eastern indian shield margin is 39 41 km thick and thins to as much as 20 23 km at the ocean continent transition oct in the offshore there is a significant variation in the nature of crust and the configuration along the margin between the three crustal domains encompassing the basin the cuddapah basin pranhita godavari graben and the eastern ghat mobile belt that indicates the lateral segmentation of the margin a zone of high density 3 0 g cm3 crustal material at the oct separates the pure continental and oceanic crusts on either side the moho is shallowest along this zone and brought the upper mantle notably close to the surface which indicates that it could be comprised of proto oceanic crustal rocks furthermore the present study highlights a major basement structural high in the deep offshore area of the margin the geophysical signatures over this structure suggest that it is a crustal scale feature that is characterized by intrusive volcanic rocks comprised of low density crust and lies close to the oct towards offshore based on the inferred tectonic reconstruction we believe that this structural high could be a continental fragment that was left behind during the process of breakup between india and the elan bank

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

ophiolitic basement to the great valley forearc basin california from seismic and gravity data implications for crustal growth at the north american continental margin the nature of the great valley basement whether oceanic or continental has long been a source of controversy a velocity model derived from a 200 km long east west reflection refraction profile collected south of the mendocino triple junction northern california in 1993 further constrained by density and magnetic models reveals an ophiolite underlying the great valley great valley ophiolite which in turn is underlain by a westward extension of lower density continental crust sierran affinity material we used an integrated modeling philosophy first modeling the seismic refraction data to obtain a final velocity model and then modeling the long wavelength features of the gravity data to obtain a final density model that is constrained in the upper crust by our velocity model the crustal section of great valley ophiolite is 7 8 km thick and the great valley ophiolite relict oceanic moho is at 11 16 km depth the great valley ophiolite does not extend west beneath the coast ranges but only as far as the western margin of the great valley where the 5 7 km thick great valley ophiolite mantle section dips west into the present day mantle there are 16 18 km of lower density sierran affinity material beneath the great valley ophiolite mantle section such that a second deeper present day continental moho is at about 34 km depth at mid crustal depths the boundary between the eastern extent of the great valley ophiolite and the western extent of sierran affinity material is a near vertical velocity and density discontinuity about 80 km east of the western margin of the great valley our model has important implications for crustal growth at the north american continental margin we suggest that a thick ophiolite sequence was obducted onto continental material probably during the jurassic nevadan orogeny so that the great valley basement is oceanic crust above oceanic mantle vertically stacked above continental crust and continental mantle

seismic stratigraphic framework and depositional history for cretaceous and cenozoic contourite depositional systems of the mozambique channel sw indian ocean this study describes previously unrecognized contourite depositional systems cdss in the mozambique channel which constrain palaeoceanographic models for this area the stratigraphic stacking patterns record nine seismic units su1 to su9 separated by eight major discontinuities a to h oldest to youngest key seismic markers in cds evolutionary history occur during aptian albian 122 ma late cenomanian 94 ma early 38 2 36 2 ma and late 25 23 ma oligocene and early middle miocene 17 15 ma epochs these record onset 122 to 94 ma growth 94 to 25 23 ma maintenance 25 23 to 17 15 ma and burial 17 15 ma to the actual time stages for cdss cdss first develop during the onset stage which coincides with the opening and deepening of the african southern ocean gateway at 122 and 100 ma respectively the growth stage beginning in the late cenomanian 94 ma correlates with the opening and deepening of the equatorial atlantic gateway during the growth stage two major shifts in sedimentary stacking pattern occur which coincide with palaeoceanographic changes during the early 38 2 36 2 ma and late 25 23 ma oligocene these in turn coincide with the onset and local enhancement of antarctic water masses cds growth continued until the early middle miocene during the maintenance stage 17 15 ma most cds growth ceased at the end of the maintenance stage circulation of the north atlantic water mass into the southern hemisphere led to a deepening of antarctic water masses in the area

the agulhas plateau structure and evolution of a large igneous province summary large igneous provinces lip are of great interest due to their role in crustal generation magmatic processes and environmental impact the agulhas plateau in the southwest indian ocean off south africa has played a controversial role in this discussion due to unclear evidence for its continental or oceanic crustal affinity with new geophysical data from seismic refraction and reflection profiling we are able to present improved evidence for its crustal structure and composition the velocity depth model reveals a mean crustal thickness of 20 km with a maximum of 24 km where three major units can be identified in the crust in our seismic reflection records evidence for volcanic flows on the agulhas plateau can be observed the middle crust is thickened by magmatic intrusions the up to 10 km thick lower crustal body is characterized by high seismic velocities of 7 0 7 6 km s 1 the velocity depth distribution suggests that the plateau consists of overthickened oceanic crust similar to other oceanic lips such as the ontong java plateau or the northern kerguelen plateau the total volume of the agulhas plateau was estimated to be 4 10 6 km 3 of which about 10 per cent consists of extruded igneous material we use this information to obtain a first estimate on carbon dioxide and sulphur dioxide emission caused by degassing from this material the agulhas plateau was formed as part of a larger lip consisting of the agulhas plateau itself northeast georgia rise and maud rise the formation time of this lip can be estimated between 100 and 94 5 ma

the relationship between seismic velocity and density in the continental crust a useful constraint summary plots of seismic velocity and density of rock samples show that a range of densities is possible for rocks of each seismic velocity and vice versa although a single linear relationship is often assumed in crustal gravity calculations because of the scatter whenever rocks of known seismic velocity are converted to density using this relationship a reduction is made to the resolving power of the resulting gravity calculation if these rocks reach thicknesses of more than a few kilometres then the uncertainties become significant when compared with the size of commonly observed gravity anomalies examples are considered from the north sea mississippi and carolina trough it is concluded that the use of a seismic velocity measurement as the only indication of rock density does not provide a useful constraint when attempting to reproduce observed gravity variations an appropriate model for isostatic compensation is probably the most important factor for successful predictions of crustal structure on the basis of gravity data

gravity anomalies flexure and crustal structure at the mozambique rifted margin abstract the free air gravity edge effect anomaly associated with passive continental margins is one of the most distinctive features of the marine gravity field early studies attributed the edge effect to the juxtaposition of thick continental with thin oceanic crust while thinning of the crust due to rifting at the time of continental break up is a major contributor it is now recognised that geological processes such as sedimentation and magmatic underplating also contribute the different amounts of sediment and underplate at rifted margins help in fact to explain the diversity of the edge effect although they obscure the mechanics styles and geometry of rifting in this paper combined 2 dimensional flexural backstripping and gravity modelling techniques are used to quantify the contribution of rifting sediment loading and magmatic underplating to the edge effect anomaly at the mozambique margin this margin formed as the result of a two stage break up the first of which resulted in the separation of africa and madagascar in the early jurassic by comparing the observed edge effect to calculations based on simple elastic plate models we have been able to constrain the elastic thickness te and hence flexural rigidity of the lithosphere that underlies the sediments which accumulated at the mozambique margin following break up the model that best explains the shape i e amplitude and wavelength of the edge effect high is one in which the earliest jurassic early cretaceous sediments were deposited on relatively weak lithosphere te 17 5 km while the later miocene recent sediments accumulated on relatively rigid lithosphere te 40 km these results suggest that sedimentation at the mozambique margin has involved lithosphere which responds to loads in a similar manner as would oceanic lithosphere therefore the thinned transitional crust that underlies much of the coastal plain of the mozambique margin is probably of oceanic rather than continental origin

crustal thinning mantle exhumation and serpentinization in the porcupine basin offshore ireland evidence from wide angle seismic data new wide angle seismic data were gathered along a 230 km long profile that runs east west across a deep structural feature in the porcupine basin offshore ireland known as the porcupine arch ocean bottom seismometers were deployed at 3 4 km intervals and seismic sources fired every 120 m along it prominent primary and secondary arrivals indicate that the continental crust is extremely thin locally less than 2 km across the basin centre the sedimentary succession is up to 12 km thick and comprises three distinctive seismic layers the two uppermost layers are interpreted as mostly a post rift succession of cretaceous and cenozoic strata the lowest layer thins rapidly towards the basin centre and is interpreted as a succession of predominantly jurassic synrift sediments a strong asymmetry in both the geometry of the crust and the sedimentary layers is probably related to a simple shear mode of extension and the subsidence that it induced crustal thinning is far greater than in the adjacent rockall basin and local exhumation of continental mantle lithosphere may have occurred in parts of the porcupine basin low p n velocities beneath the porcupine arch are compatible with larger amounts of mantle serpentinization than in the rockall basin

advanced insights into magmatism and volcanism of the mozambique ridge and mozambique basin in the view of new potential field data summary a new plate tectonic model is presented which describes the emplacement of the mozambique ridge off southeast africa as the result of long lasting volcanic activity 140 122 ma during the initial opening between africa and antarctica thus an oceanic origin for the mozambique ridge is proposed this model is based on a new and systematic high resolution magnetic anomaly data set acquired across the mozambique ridge and throughout the mozambique basin data from the mozambique basin allow the identification of mesozoic magnetic anomalies from m0r to m26 124 61 155 3 ma with previously unmatched accuracy small scale fracture zones are recognized by offsets of magnetic anomalies in the westernmost part of the basin additionally a bend in the major fracture zones f and e between m17r and m18n 142 84 144 04 ma and a recognized sinusoidal change in spreading direction with an amplitude of about 15 indicate that the basin experienced several small scale changes in spreading direction through time a maximum change in spreading direction to almost 0 at around m11n 135 69 ma is followed by a short lived increase in spreading half rate from 23 5 km ma 1 to about 27 5 km ma 1 in the time frame from m10r to m9n 134 30 132 83 ma we propose that this is related to the initial opening of the south atlantic ocean represented by the onset of seafloor spreading between the falkland plateau and africa in the conjugate georgia and natal basins across the mozambique ridge high amplitude magnetic anomalies at major structural boundaries suggest that the different plateaus of the ridge were formed at different times a simple 2 d gravity and magnetic model for the ridge supports the hypothesis of multiple volcanic episodes which formed the ridge though long lasting volcanic activity between about 140 and 122 ma together with new and verified rotation parameters from the mozambique basin and its conjugate the riiser larsen sea antarctica a series of plate tectonic reconstructions are presented which demonstrate when and how the different parts of the ridge evolved through time

mesozoic and early cenozoic sediment influx and morphology of the mozambique basin abstract the mozambique basin is one of the oldest extensional sedimentary basins developed along the eastern african margin the basin hosts a continuous record of sediments since the jurassic separation of antarctica from africa the objectives of this study were to extend the regional stratigraphic framework north of the zambezi delta into the deep abyssal plains and review the early evolution of the mozambique basin using nine multi channel seismic reflection profiles we identify six major stratigraphic units that were deposited in jurassic early cretaceous late cretaceous paleogene neogene and quaternary times mesozoic sedimentation rates of 5 10 cm kyr and 1 3 cm kyr during the paleogene are calculated in the deeper basin the presence of shales in neighbouring wells on the shelf implies an euxinic environment in the rapidly subsiding basin until early cretaceous times the mesozoic sediments have a high seismic velocity that exceeds 4 5 km s except in a distinct early cretaceous low velocity 3 7 km s zone that may indicate the presence of undercompacted overpressured shales in spite of the fact that the zambezi catchment was much smaller in pre miocene times the high late cretaceous sedimentation rates can be attributed to rapid denudation of the african continent after a major tectonic uplift episode at approximately 90 ma increased sediment influx into the basin from the zambezi in late cretaceous times resulted in the formation of an elongated submarine fan lobe into the mozambique channel north of beira high strong north south bottom currents commenced within the channel in late cretaceous times forcing the aggradation of sediments on the southern flank of the lobe in addition we observe several current controlled sediment deposits in the deeper basin that are influenced by north south bottom currents low paleogene sedimentation rates are attributed to a sediment starved basin during a relative quiet tectonic phase onshore

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

the southern mozambique basin the most promising hydrocarbon province offshore east africa recent offshore acquisition of approximately 12 800 km 7 950 mi of seismic reflection data with some gravity and magnetic profiles encompassing the southern half of the mozambique basin reveals new facets of the subsurface geology integrated interpretation of these new geophysical data with old well information allows the development of depositional and tectonic models that enhance the basin s hydrocarbon potential previous drilling was sparse however predating modern seismic technology and exploration philosophy and left the area classified as a frontier province despite some encouraging hydrocarbon shows previous operators considered the entire basin gas prone and immature recent interpretation suggests that the basin s geothermal history must have been more favorable than generally was inferred from present day gradients and oil prone source rocks may be buried below the sequences drilled during wildcatting also marine oil prone source rocks may be present furthermore the liquid hydrocarbon sourcing potential of the karroo coal measures should not be underestimated wildcatters have not in retrospect optimally positioned exploratory wells in the vicinity of ample structural closures nor have they drilled to sufficient depth owing to poorly resolved outdated seismic data and to erroneous age assignments to lava flows therefore the viability of prospects mapped along a major offshore extension of the east african rift system delineated by this new survey has not been adequately assessed finally a well remains to be drilled within the rift to assess the presence of reservoirs and the maturity richness and volume of source rock sequences not necessarily represented over the eroded structures drilled to date end page 399

crustal structure beneath orphan basin and implications for nonvolcanic continental rifting a wide angle seismic profile was acquired across the northeast newfoundland margin seaward to orphan knoll the profile is complemented by existing multichannel reflection data two online deep wells and other deep seismic reflection and refraction lines modeling of data from 15 ocean bottom seismometers provides the seismic velocity structure for a significant portion of the 400 km wide stretched continental crust on this margin prerift metasedimentary rocks have a topographic relief of up to 5 km with a velocity of 5 2 km s increasing to 5 9 km s unaltered crystalline crust below the metasedimentary section has a velocity range of 6 1 6 5 km s most of the lower crust landward of orphan knoll has a typical continental lower crustal velocity 6 8 7 0 km s and the lower crust shallows significantly beneath orphan knoll our model shows no evidence for the 5 km thick 7 35 km s layer modeled from data of an earlier experiment and interpreted as magmatic underplating but instead indicates that thinned continental crust extends seaward for 360 km without underplating implying nonvolcanic rifting for the ne newfoundland margin we suggest that continental stretching persisted from 180 ma to chron 34n time 101 ma when the final breakup between canada and western europe took place leaving a 400 km wide zone of thinned continental crust underneath the shelf and deep water area of the orphan basin on the landward side of the basin gravity modeling indicates a zone of very thin 6 8 km crust possibly a failed rift center formed as a result of the northward progression of nonvolcanic rifting between canada and europe a possible sub moho reflector suggests generally uniform stretching of the crust and upper mantle and the absence of volcanics and underplating is in contrast to the observations on related rift basins of the grand banks

mesozoic magnetic anomalies in the southern natal valley the recognition of mesozoic magnetic anomalies m0 m12 within the submarine natal valley south west indian ocean provides a further constraint on the relative motion between south america falkland plateau and africa during the early opening phase of west gondwanaland fig 1 colinear with m12 magnetic anomaly g ref 1 is delineated north of the south tugela ridge2 an acoustic basement high which is suspected to have been emplaced within continental crust immediately north of the continental oceanic crust boundary cob during the initial rifting phase starting from a revised west gondwana reconstruction3 and using palaeo poles of rotation1 3 the falkland plateau can be rotated into the natal valley superposing the corresponding georgia basin and natal valley magnetic anomalies this achieves an optimum continuity of inter continental structural lineaments3 by overlapping of an anomaly intermediate to m10 and m11 m10n4 this suggests that the natal valley cob is located between anomalies m10 and m11 implying that in this zone of south west gondwana seafloor spreading commenced at 125 myr

the initial gondwana break up a synthesis based on new potential field data of the africa antarctica corridor abstract the initial opening of the africa antarctica corridor in the heart of gondwana is still enigmatic due to missing information on the origin of major crustal features and the exact timing of the onset of the first oceanic crust in the jurassic therefore in 2014 new ship borne magnetic data were systematically acquired in the northern mozambique basin and across beira high which we merged with all accessible magnetic data in the mozambique basin herein distinct magnetic lineations are observed which allow a refined identification of a whole set of jurassic magnetic spreading anomalies constraining the timing of the onset of oceanization beginning at m38n 2n 164 1 ma in combination with high resolution potential field data from the conjugate antarctic margin well expressed fracture zones can be traced throughout the africa antarctica corridor and allow the precise rotation of antarctica back to africa the initial fit depicts striking continuations of onshore tectonic features across the plate boundaries taking onshore aeromagnetic data of both margins into account within a tight gondwana fit the beira high can be restored along the major sinistral namama orvin shear zone of the east african antarctic orogen the beira high represents a continental block which was detached from antarctica by 157 ma at the latest simultaneously the antarctic plate cleared the area of the mcp however the crustal nature of the southern mcp remains ambiguous the northern natal valley and the mozambique ridge consist of thick oceanic crust being emplaced between m26r m18n 157 1 144 ma and m18n m6n 144 131 7 ma respectively about the half of this crust was won from the antarctic plate by a series of southwards directed ridge jumps to the northern boundary of the explora wedge a refined kinematic break up model constrained by the most extensive magnetic dataset is presented describing consistently the initial opening of the africa antarctica corridor and the somali basin

crustal structure of the basin and ridge system west of new caledonia southwest pacific from wide angle and reflection seismic data 1 during the zoneco 11 marine geophysical survey september 2004 two deep reflection seismic profiles recorded by ocean bottom seismometers were acquired in the offshore domain west of new caledonia the northern profile crosses the new caledonia basin the fairway ridge the fairway basin and the lord howe rise the southern profile crosses the norfolk rise south of new caledonia the new caledonia basin the fairway ridge and basin and ends at the foot of lord howe rise on the northern profile the lord howe rise has a crustal thickness of 23 km and exhibits seismic velocities and velocity gradients characteristic of continental crust the crust thins to 12 15 km in the neighboring fairway basin which is interpreted to be of thinned continental origin based on the seismic velocities the crustal thickness of the fairway rise is 22 km and it is also interpreted to be of continental origin the new caledonian basin is underlain by crust of 10 km thickness which shows unusally high velocities between 7 0 and 7 4 uncharacteristic for either thinned continental or oceanic crust on the southern profile the norfolk rise is also found to be of continental nature here the new caledonia basin shows velocities crustal thickness and basement roughness characteristic of typical oceanic crust the crust in the fairway basin shows higher velocities than on the northern profile which could be caused by volcanic intrusions into the crust during extension a deep reflector in the upper mantle was imaged underneath the new caledonian basin on the northern profile

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

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

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

mechanisms of extension at nonvolcanic margins evidence from the galicia interior basin west of iberia 1 we have studied a nonvolcanic margin the west iberia margin to understand how the mechanisms of thinning evolve with increasing extension we present a coincident prestack depth migrated seismic section and a wide angle profile across a mesozoic abandoned rift the galicia interior basin gib the data show that the basin is asymmetric with major faults dipping to the east the velocity structure at both basin flanks is different suggesting that the basin formed along a paleozoic terrain boundary the ratios of upper to lower crustal thickness and tectonic structure are used to infer the mechanisms of extension at the rift flanks stretching factor 2 the ratio is fairly constant indicating that stretching of upper and lower crust was uniform toward the center of the basin 3 5 5 5 fault block size decreases as the crust thins and faults reach progressively deeper crustal levels indicating a switch from ductile to brittle behavior of the lower crust at 3 5 faults exhume lower crustal rocks to shallow levels creating an excess of lower crust within their footwalls we infer that initially extension occurred by large scale uniform pure shear but as extension increased it switched to simple shear along deep penetrating faults as most of the crust was brittle the predominant brittle deformation might have driven small scale flow 40 km of the deepest crust to accommodate fault offsets resulting in a smooth moho topography the gib might provide a type example of nonvolcanic rifting of cold and thin crust

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

the crustal structure of the madagascar ridge summary the madagascar ridge is an elongated aseismic plateau in the southwest indian ocean which separates two ocean basins of mid to late cretaceous age from interpretations of a reversed refraction line on the ridge and a continuous gravity profile across it we conclude that it formed during the cretaceous as thickened oceanic crust at a mid ocean ridge which possibly overlay a mantle hot spot the seismic data consist of record sections from five recording sonobuoys forty one shots were fired along a 210 km long nne ssw line across the central part of the ridge simple horizontal layer analysis of first arrivals reveals a lower crustal refractor with vp 7 kms 1 at a depth of 8 8 5 km and the moho at an apparent depth of 18 km but fails to characterize the crustal structure as either oceanic or continental we obtain a definitively oceanic structure by using synthetic seismogram techniques together with tau p analysis and second arrival correlations the model which best fits the data consists of 2 2 km of water and 1 km of sediments overlying a velocity gradient from 4 to 7 kms 1 beneath this at a depth of 7 5 km is a 9 5 km thick layer with vp 6 9 7 1 kms 1 a 7 5 kms 1 layer not observed in the first arrival analysis is found at the base of the crust the moho is a transition zone 1 km thick and occurs at 25 km depth the gravity data consist of a continuous ne sw profile across the ridge and on to the flanking basins two dimensional gravity modelling allows a structure consistent with the synthetic seismogram results and supports the existence of a high density basal layer in the crust we use in addition a time series analysis approach to estimate the depth of compensation assuming an airy model and lithospheric thickness assuming an elastic plate model convolution of bathymetry along the profile with a range of model filters gives minimum rms gravity residuals for a crustal compensation depth of 23 5 km and a plate thickness of less than 2 5 km

crustal transect from the north atlantic knipovich ridge to the svalbard margin west of hornsund the crustal structure along a 312 km transect stretching from the axial mountains of the north atlantic knipovich ridge to the continental shelf of svalbard has been obtained using seismic reflection data and wide angle obs data the resulting seismic vp and vs models are further constrained by a 2 d gravity model the principal objective of this study is to describe and resolve the physical and compositional properties of the crust in order to understand the processes and creation of oceanic crust in this extremely slow spreading counterpart of the north atlantic ridge systems vp is estimated to be 3 50 6 05 km s for the upper oceanic crust oceanic layer 2 with a marked increase away from the ridge the measured vp of 6 55 6 95 km s for oceanic layer 3a and 7 10 7 25 km s for layer 3b both with a vp vs ratio of 1 81 except for slightly higher values at the ridge axis does not allow a clear distinction between gabbro and mantle derived peridotite 10 40 serpentized the thickness of the oceanic crust varies a lot along the transect from the minimum of 5 6 km to a maximum of 8 1 km the mean thickness of 6 7 km for the oceanic crust is well above the average thickness for slow spreading ridges 10 mm year half spreading rate the areas of increased thickness could be explained by large magma production rates found in the zones of axial highs at the ridge axis which also have generated the off axial highs adjacent the ridge we suggest that these axial and off axial highs along the ridge control the lithological composition of the oceanic crust this approach suggests normal gabbroic oceanic crust to be found in the areas bound by the active magma segments the axial and off axial highs and mantle derived peridotite outside these zone

relationship between indosinian yanshanian tectonic framework and large superlarge mineral deposits in qinling qilian orogenic belt it is obvious that the indosinian yanshanian tectono magmatic activities controlled and affected the ore forming process of the qinling qilian orogenic belt this finds expression in the fact that the indosinian yanshanian ne trending structures not only controlled the mineralization of gold and other metallic minerals but also caused the considerable reformation and enrichment of the earlier formed copper zinc lead and other minerals where the ne trending structures intersected or are superimposed on the earlier formed nw and nearly ew trending structures or ore bearing formations the large and superlarge mineral deposits are likely to be formed in this paper the characteristics of the indosinian yanshanian tectono magmatic belt which controlled and affected the formation of large superlarge mineral deposits are analyzed and discussed and new measures and thinking are put forward for carrying out a new round of mineral exploration in this region

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

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

a geotectonic history of the northern mozambique basin including the beira high a contribution for the understanding of its development abstract the interpretation of a comprehensive two dimensional 2d seismic reflection data set discloses several rifting events for a typical passive rifted margin two major phases have been identified rift i phase is in agreement with the initial rifting of gondwana postulated for the early jurassic this rift resulted in break up stage 1 governed by a north eastern motion of antarctica along the reactivated pebane shear zone indications for lava flows were identified in the seismic profiles along the offshore zambezi depression they form a link between early rifting the initial break up and early sea floor spreading the nature of these lava flows may be associated with the emplacement of thick volcanic dykes during post rift magmatism that occurred when the antarctica plate with the beira high drifted dextrally from the west to east until the mid jurassic times it is more likely that the break up in stage 1 resulted from high tension due to strain relief of rift i phase above all the extensional deformation occurred in a narrow rift mode therefore the v shape of the offshore zambezi depression suggests a possible rift failure structure this is best explained by the reeves and de wit model 2000 this model postulates that the motion of antarctica changed to southward direction at around 170 ma consequently a rift jump from the offshore zambezi depression in the northwest to the south eastern edge of the beira high occurred and rift ii phase may have started leading to the break up in stage 2 during this stage rift ii phase the extension migrated towards the east thus thinning the crust and exhuming the sub continental mantle in the continental ocean transition zone the rift ii phase shows a sequence of half graben morphologies confining the syn rift infill that is subdivided in three units syn rift i syn rift ii and rift sag all three units appear to have developed under minor extensional regimes in the crust evidenced by gently dipping low angle detachment faults with respect to the presence of the two rift phases it is deduced that break up and sea floor spreading are diachronous within rift i and rift ii segments

crustal structure of the flemish cap continental margin eastern canada an analysis of a seismic refraction profile summary the crustal structure of the ne flemish cap margin was determined along a 460 km long refraction wide angle reflection seismic transect flame line to define the thickness structure and composition of the crust and uppermost mantle along the line a p wave velocity model was developed from forward and inverse modelling of dense airgun shots recorded by 19 ocean bottom seismometers a coincident multichannel seismic profile was used to guide the modelling as reflections could be identified down to moho the model displays a sediment cover of up to 3 6 km thick subdivided into three layers with velocities of 1 8 1 9 km s 1 2 8 3 1 km s 1 and 4 7 4 8 km s 1 for the western part of the flame line over flemish cap the p wave velocity model displays an up to 32 km thick three layer continental crust the continental crust has velocities of 5 8 6 1 km s 1 6 3 6 45 km s 1 and 6 65 6 85 km s 1 and thicknesses of about 5 km 7 km and 20 km in the upper middle and lower layers respectively the thick continental crust thins to a two layer 6 km thick crust upper layer is 5 55 6 0 km s 1 and the layer below is 6 65 6 8 km s 1 over a distance of 45 km s wave velocities are determined in the upper layer of the thick continental crust over flemish cap and the transition zone by assigning poisson s ratios in the p wave velocity model comparison of calculated to observed arrival times gives a poisson s ratio of 0 27 in the upper layer and 0 28 in the layer below which suggests that the composition of the crust is primarily continental in both the thick crust and the thin crust of the transition zone the thin continental crust is stretched over a width of 80 km and is underlain by a layer with velocities of 7 5 7 9 km s 1 we interpret this layer as partially serpentinized mantle which is consistent with observationsfromthenewfoundlandmargintothesouth theserpentinizedmantleterminates 30 km seaward of the thick continental crust at the seaward most end of the thin continental crust a prominent ridge feature is observed the seismic refraction and multichannel seismic data results indicate a mixed character between serpentinized mantle with volcanic extrusions or continental crust the reflection seismic data show a high relief basement from the ridge feature and seaward the flame line crosses magnetic anomaly 34 and extends another 50 km seaward well into oceanic crust the ridge is flanked seaward by a two layer oceanic crust the upper layer layer 2 has velocities of 4 8 5 0 km s 1 for the landward most 35 km of oceanic crust and 4 8 6 2 km s 1 for the seaward 60 km the average thickness of layer 2 is 2 km the lower layer layer 3 has velocities of 6 7 7 2 km s 1 and a thickness of 3 5 km the velocity model is consistent with a sharp onset of seafloor spreading seaward of the ridge feature

a refraction seismic transect from the faroe islands to the hatton rockall basin 1 the crustal structure of the faroe rockall plateau was studied by a 790 km long refraction seismic transect consisting of two intersecting lines the air gun shot spacing was 200 m and the signals were recorded by 77 ocean bottom seismometers a p wave velocity model was developed from forward and inverse modeling of the wide angle seismic data and incorporation of coincident multichannel reflection seismic data continental crust with velocities ranging from 5 6 to 6 8 km s can be traced from the faroe islands across the banks to the sw of the faroes and into the hatton rockall basin the thickness of the subvolcanic crust is up to 25 km on the banks but is as little as 8 km in the channels between the banks the thinning in the channels may be related to nw trending shear zones extending from major lineaments in ne rockall trough basalt layers are found along the entire transect with a total thickness of up to 4 km two layers with velocities of 4 9 5 2 and 5 3 5 6 km s are thought to represent paleogene flood basalts that can be correlated from the faroe islands to george bligh bank close to george bligh bank an 80 km wide and up to 9 km thick body with velocities of 6 5 km s is interpreted as intrusion a 5 km thick high velocity lower crustal layer 7 3 km s extends from the area of the intrusion into the northern hatton rockall basin at the northern flank of lousy bank the transition zone to oceanic crust was encountered

corrigendum to a geotectonic history of the northern mozambique basin including the beira high a contribution for the understanding of its development mar pet geol 36 2012 1 12

the jurassic history of the africa antarctica corridor new constraints from magnetic data on the conjugate continental margins abstract finding the best fit for east and west gondwana requires a detailed knowledge of the initial jurassic movements between africa and antarctica this study presents results of systematic and densely spaced aeromagnetic measurements which have been conducted in 2009 2010 across the astrid ridge antarctica and in the western riiser larsen sea to provide constraints for the early seafloor spreading history between both continents the data reveal different magnetic signatures of the northern and southern parts of the astrid ridge which are separated by the astrid fracture zone the southern part is weakly magnetised corresponding to the low amplitude anomaly field of the southwestern riiser larsen sea in contrast the northern astrid ridge bears strong anomalies of positive value furthermore several sets of trends are visible in the data in the mozambique channel we extended the existing magnetic spreading anomaly identifications combined with the spreading anomalies in the conjugate riiser larsen sea they were used to establish a new model of the relative movements of africa and antarctica after the breakup of gondwana in jurassic times a detailed model for the emplacement of the mozambique ridge is now incorporated the model postulates a tight fit between africa and antarctica and two stages of breakup the first of which lasting until 159 ma m33n during this stage antarctica rotated anticlockwise with respect to africa the grunehogna craton cleared the coastal plains of mozambique to a position east of the mozambique fracture zone the southern astrid ridge is interpreted to consist of oceanic crust also formed during this first stage prior to the riiser larsen sea during the second stage antarctica moved southward with respect to africa forming the mozambique basin and the conjugate riiser larsen sea the mozambique ridge and the northern natal valley were formed at different spreading centres being active subsequently

compressional and shear wave velocity versus depth relations for common rock types in northern california abstract this article presents new empirical compressional and shear wave velocity vp and vs versus depth relationships for the most common rock types in northern california vp versus depth relations were developed from borehole laboratory seismic refraction and tomography and density measurements and were converted to vs versus depth relations using new empirical relations between vp and vs the relations proposed here account for increasing overburden pressure but not for variations in other factors that can influence velocity over short distance scales such as lithology consolidation induration porosity and stratigraphic age standard deviations of the misfits predicted by these relations thus provide a measure of the importance of the variability in vp and vs caused by these other factors because gabbros greenstones basalts and other mafic rocks have a different vp and vs relationship than sedimentary and granitic rocks the differences in vs between these rock types at depths below 6 or 7 km are generally small the new relations were used to derive the 2005 u s geological survey seismic velocity model for northern california employed in the broadband strong motion simulations of the 1989 loma prieta and 1906 san francisco earthquakes initial tests of the model indicate that the vp model generally compares favorably to regional seismic tomography models but that the vp and vs values proposed for the franciscan complex may be about 5 too high

the crustal structure of the continental margin east of the falkland islands abstract the 1500 km long falkland plateau is the most prominent morphological structure in the southern south atlantic ocean which crustal composition and development is mainly unknown at the westernmost boundary of the plateau the falkland islands precambrian geology provides the only insight into basement structure and age the question of whether continental basement of a similar age and origin underlies the falkland plateau further east is strongly disputed we present new high quality constraints on the crustal fabric of the plateau east of the falkland islands based on wide angle seismic and potential field data acquired in 2013 the p wave velocity model supported by amplitude and density modelling shows that the falkland plateau basin is filled with 8 km of sediments continental crust of 34 km thickness underlies the falkland islands the eastern continental margin of the falkland islands can be classified as a volcanic rifted margin the falkland plateau basin is floored by up to 20 km thick oceanic crust the exceptionally thick igneous crust and its high lower crustal velocities up to 7 4 km s indicate the influence of a regional thermal mantle anomaly during its formation which provided extra melt material the wide angle model revises published crustal models which predicted thin oceanic or thick extended continental crust below the falkland plateau basin our results provide a sound basis for future tectonic interpretations of the area

saucer shaped intrusions occurrences emplacement and implications abstract magmatic intrusions and fluidized injected sands represent the two main types of sheet intrusions in the earth s crust in this paper we show that both intrusion types often display a saucer like geometry as revealed by 3d seismic imaging and field observations saucer shaped intrusions are fairly common in sedimentary basins as for example offshore in the norwegian and north sea basins and onshore in the karoo basin of south africa the formation of the saucer geometry is controlled by the low viscosity of the injected fluid and by the interaction between a growing shallow hydraulic fracture and overburden deformations statistics gathered from observations and modelling show a linear relationship between the depth of emplacement and the size of the saucer shaped intrusions we anticipate future cross disciplinary studies aiming to discover other occurrences of saucer shaped intrusions and to identify the physical processes controlling the development of this fundamental geometry