continental oceanic crustal transition off southwest africa more than 1 426 mi 2 300 km of 48 trace 12 fold seismic reflection profiles were used to examine the nature of the continent ocean boundary off southwest africa south of the orange river faulted blocks which we interpret as rifted continental crust can be traced seaward to the 3 km isobath the transition to oceanic basement occurs in a zone 19 to 25 mi 30 to 40 km wide beneath the continental rise although structural details are obscured by a thick seaward dipping wedge of pre aii pre late early cretaceous overburden oceanic crust at the presumed contact is not older than magnetic anomaly m9 126 to 121 m y b p north of the orange river a pronounced hinge in continental crust correlates with magnetic anomaly g of rabinowitz 1976 seaward of the hinge a complex fault block terrane is evident oceanic basement can not be traced with any certainty landward of anomaly m4 intracontinental stretching and associated volcanism appear to have been important in the early history of the cape basin rifting and local dike intrusion may explain the presence of some lineated magnetic anomalies previously attributed to sea floor spreading related extrusives form at least part of the pre aii wedge just seaward of the hinge zone our interpretation of the seismic data suggests that the initiation of normal spreading in the cape basin postdates by 4 to 9 m y the valanginian age derived from prevailing plate tectonic reconstructions of the south atlantic

the tectonic evolution of the south atlantic from late jurassic to present an improved tectonic database for the south atlantic has been compiled by combining magnetic anomaly geosat altimetry and onshore geologic data we used this database to obtain a revised plate kinematic model starting with a new fit reconstruction for the continents around the south atlantic we present a high resolution isochron map from chron m4 to present fit reconstructions of south america and africa that require rigid continental plates result in substantial misfits either in the southern south atlantic or in the equatorial atlantic to achieve a fit without gaps we assume a combination of complex rift and strike slip movements 1 along the south american parana chacos basin deformation zone 2 within marginal basins in south america salado colorado basin and 3 along the benue trough niger rift system in africa these faults are presumed to have been active before or during the breakup of the continents our model describes a successive unzipping of rift zones starting in the southern south atlantic between 150 ma tithonian and approximately 130 ma hauterivian rifting propagated to 38 s causing tectonic movements within the colorado and salado basins subsequently between 130 ma and chron m4 126 5 ma the tip of the south atlantic rift moved to 28 s resulting in intracontinental deformation along the parana chacos basin deformation zone between chron m4 and chron mo 118 7 ma rifting propagated into the benue trough and niger rift inducing rift and strike slip motion after chron mo the equatorial atlantic began to open while rifting and strike slip motion still occurred in the benue trough and niger rift since chron 34 84 ma the opening of the south atlantic is characterized by simple divergence of two rigid continental plates

extension of the parana basin to offshore brazil implications for coalbed methane evaluation abstract coalbed methane cbm is a worldwide exploration target of the petroleum industry in brazil the most important coal bearing succession is associated with the permian rio bonito formation of the parana basin the gas prone areas are located at the southeastern margin of the parana basin and possibly in the offshore region of the northern part of the pelotas basin coalfields end abruptly at the present day shoreline a result of rifting of gondwana and the evolution of the south atlantic ocean all geologic indicators suggest that in pre rift times the coal seams extended further eastwards probably now lying deeply buried below the sedimentary succession of the pelotas basin the present paper discusses structural stratigraphic seismic and aeromagenetic data that support the preservation of continental crust beneath ocean sediment if the coal beds had similar lateral extent to known onshore coals and coal beds extended across the projected extension of the parana basin and there was a conservative 5 m of cumulative coal thickness then a potential methane volume can be estimated for this newly inferred resource average onshore coal gas content is 32 scf ton 1 00 m 3 ton if this is similar in the offshore coal deposits then the hypothetical methane volume in the offshore area could be in excess of 1 9 10 12 scf 56 10 9 m 3 metamorphism from dikes associated with rifting are potential complicating factors in these deposits and since no borehole reaching the deep lying strata in the offshore area are available this is a hypothetical gas resource with a certain level of uncertainty which should be tested in the future by drilling a deep borehole

geology and geochemistry of the yuanjiacun banded iron formation in shanxi province china constraints on the genesisthe genesis of the yuanjiacun bify men et al

tibetan tectonic evolution inferred from spatial and temporal variations in post collisional magmatism cenozoic magmatism on the tibetan plateau shows systematic variations in space and time that must be considered in models concerning tibetan tectonic evolution after the india asia collision which started in the early tertiary and terminated the gangdese arc magmatism in the lhasa terrane made of the southern tibetan plateau widespread potassium rich lavas and subordinate sodium rich basalts were generated from f50 to 30 ma in the qiangtang terrane of northern tibet subsequent post collisional magmatism migrated southwards producing ultrapotassic and adakitic lavas coevally between f26 and 10 ma in the lhasa terrane then potassic volcanism was renewed to the north and has become extensive and semicontinuous since f13 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations enable us to elaborate a geodynamic evolution model which depicts when and how the indian continental lithospheric mantle started thrusting under asia by involving rollback and breakoff of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root we propose that only after the lithospheric removal which occurred at f26 ma could the indian mantle lithosphere have commenced its northward underthrusting and henceforth served as a pivotal control to the himalayan tibetan orogenesis consequently the tibetan plateau is suggested to have risen diachronously from south to north whereas the southern part of the plateau may have been created and maintained since the late oligocene the northern plateau would have not attained its present day elevation and size until the mid miocene when the lower part of the western qiangtang and songpan ganze lithospheres began to founder owing to the push of the underthrust indian mantle lithosphere d 2004 elsevier b v all rights reserved

tectonic evolution of the colorado basin offshore argentina inferred from seismo stratigraphy and depositional rates analysis abstract based on a dense 2d seismic reflection dataset and information from 8 exploration wells we reinterpreted the stratigraphic evolution of the colorado basin the basin is located on the continental shelf and slope within 50 to 2250 m of bathymetry the total sediment fill can be up to 16 000 m seismic to well log correlations provide a chrono stratigraphic framework for the interpreted seismic sequences we show that the colorado basin records the development of a permian pre rift period a triassic jurassic to early cretaceous rift phase and a lower cretaceous to tertiary drift phase this passive margin represents the evolution of lithospheric extension from active rifting to the thermal subsidence drift stage several cretaceous to cenozoic slumping episodes were identified and related to progradation of the sequences and sediment build up in the slope as well as to the development of seaward dipping extensional faults assuming an accumulation time span of 25 ma for the syn rift succession which corresponds to the duration of extensional phases preceding the final break up of the south atlantic sedimentation rates are in the order of 118 m ma by late cretaceous the gondwana break up had stopped in almost all areas and post rift thermal subsidence dominated the basin development during the early phase of subsidence high sedimentation rates of up to 122 m ma are recorded an important decrease of sediment supply 30 m ma occurred until the end of the sag phase during the deposition of the colorado formation and sedimentation took place in elongated depocenters with oceanward progradation a second phase of increased sediment supply takes place during the oligocene and miocene resulting in the development of large basinward prograding sedimentary wedges this event seems to be associated to the interaction between continental dynamics subsidence and increased continental erosion during climate shift from greenhouse to icehouse conditions

slowdown of circumpolar deepwater flow during the late neogene evidence from a mudwave field at the argentine continental slope geochemical evidence from boreholes suggests enhanced transport of northern component water ncw to southern latitudes from about 6 ma onward however information on how this change in transport influenced the intensity and position of current systems is sparse here we use seismic reflection profiles interpreted together with bathymetric data to investigate current derived deposits at the central argentine margin upslope migrating mudwaves overlying a late miocene erosional unconformity provide evidence that circumpolar deepwater flow slowed down with the onset of ncw inflow during the last 3 ma changes in dimensions and migration rates of the waves are small indicating continuous bottom current flow conditions similar to today with only minor variations in flow speed suggesting that the deep western boundary current in the western south atlantic as observed today has been a pervasive feature of the global thermohaline circulation system during the plio pleistocene

the structure of the lower crust at the argentine continental margin south atlantic at 44 s abstract it is well established that the argentine passive margin is of the rifted volcanic margin type this classification is based primarily on the presence of a buried volcanic wedge beneath the continental slope manifested by seismic data as a seaward dipping reflector sequence sdrs here we investigate the deep structure of the argentine volcanic margin at 44 s over 200 km from the shelf to the deep oceanic argentine basin we use wide angle reflection refraction seismic data to perform a joint travel time inversion for refracted and reflected travel times the resulting p wave velocity depth model confirms the typical volcanic margin structure an underplated body is resolved as distinctive high seismic velocity vp up to 7 5 km s feature in the lower crust in the prolongation of a seaward dipping reflector sequence a remarkable result is that a second isolated body of high seismic velocity vp up to 7 3 km s exists landward of the first high velocity feature the centres of both bodies are 60 km apart the high velocity lower crustal bodies likely were emplaced during transient magmatic volcanic events accompanying the late rifting and initial drifting stages the lateral variability of the lower crust may be an expression of a multiple rifting process in the sense that the south atlantic rift evolved by instantaneous breakup of longer continental margin segments these segments are confined by transfer zones that acted as rift propagation barriers a lower crustal reflector was detected at 3 to 5 km above the modern moho and probably represents the lower boundary of stretched continental crust with this finding we suggest that the continent ocean boundary is situated 70 km more seaward than in previous interpretations

rocas verdes ophiolites southernmost south america remnants of progressive stages of development of oceanic type crust in a continental margin back arc basin

birth of a volcanic margin off argentina south atlantic 1 based on a dense grid of multichannel reflection seismic lines we study the evolution of a volcanic rifted margin segment off argentina the segment under study is located between 44 s and 41 s we describe in detail the facies of the extrusive basaltic complexes across the margin these include single and multiple inner seaward dipping reflectors sdrs with varying architecture along the margin outer highs and outer sdr wedges a detailed interpretation of a transitional zone between the inner and outer sdr wedges is presented multiple inner sdrs are concentrated at the southern part of the margin segment while in the north only one steeply dipping wedge is present the dip of the inner sdr wedges changes along the margin and we propose subsidence by loading as main cause the transitional zone between the inner and outer sdr wedges becomes narrower toward the north by correlating with magnetic data we conclude that the emplacement of the 30 100 km wide multiple inner sdrs occurred episodically we propose an injection center which migrated in a seaward direction with proceeding extension as the origin for the multiple inner sdrs a scissor like opening of the margin segment resulted in different amounts of stretching along strike of the margin segment and is likely the cause for the varying distribution of the extrusive basaltic complexes the varying extension rates within the margin segment contribute to decreasing volumes of melts generated in a northward direction within this rift segment

reassembling gondwana a new high quality constraint from vibroseis exploration of the sub ice shelf geology of the east antarctic continental margin the breakup of gondwana is manifested by coeval early jurassic karoo magmatism in south africa and east antarctica in south africa the large volumes of volcanic rocks of the adjoining lebombo and mwenetzi save monoclines represent a volcanic rift margin and in east antarctica a corresponding feature the explora wedge is buried below sediments and floating ice shelves on the continental margin of dronning maud land we use the seismic vibrator source to explore the sub ice geology in antarctica and the new seismic reflection and available regional aeromagnetic data enable us to outline a dogleg landward extent of the explora wedge in dronning maud land the congruent inboard wedge geometries on the two continents define a high quality constraint which facilitate for the first time a geologically consistent and tight reconstruction of africa relative to east antarctica within gondwana the uncertainties in correlations of major geological features mobile belts from one continent to the other may now be of the order of ten s of kilometers rather than hundreds of kilometers

depth and age in the north pacific the concept of the oceanic lithosphere as a cooling thermal boundary layer on top of a viscous upper mantle accounts for the increase in depth with age of the ocean floor analyses of the exact nature of this relation provide constraints on the rheology and structure of this layer in past studies problems with uncertainties of age and in sediment thickness have limited the value of this relation on older ocean floor in this study we evaluate different tectonic histories for the quiet zones and construct a revised isochron chart for the north pacific we check and modify the current sediment thickness charts using new deep sea drilling data and revised ideas concerning reflectors in the southwestern north pacific averaging the synbaps digital data base into half by half degree elements and correcting for sediment load we plot basement depth versus age for the entire north pacific because of the skewness in the data associated with seamounts we present the data as contours about the mode and find that two thirds of the observations lie within 300 m of this value on crust between 0 and 80 ma the ocean floor subsides with the square root of age on ocean floor older than 100 ma the depths in general flatten with age with areally correlateable swells and valleys the hawaiian and line island swells correlate positively and the mariana basin valley negatively with geoid height the flattening in the depths indicates that the thermal boundary layer has reached equilibrium conditions under older ocean floor further the correlation of the swells and valleys with geoid height presents evidence for organized convection in the upper mantle beneath older lithosphere in the north pacific

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

crust mantle mixing and crustal reworking of southern tibet during indian continental subduction evidence from miocene high silica potassic rocks in central lhasa block abstract collisional zones are commonly considered as important regions for crustal reworking but the reworking mechanism remains debated the well known himalayan southern tibetan orogen built by india asia collision and convergence has the thickest continental crust on earth and is therefore an ideal region for studying crustal reworking during collisional orogenesis here we revisit the miocene high silica potassic rocks trachytes in the konglong area of the central lhasa block southern tibet integrated studies of geochronology mineral compositions bulk rock major and trace element geochemistry and sr nd pb hf o isotopes unequivocally indicate that the konglong trachytes formed by mixing between enriched mantle derived ultrapotassic and thickened ancient crust derived magmas combined with post collisional magma mixing recently identified in the southern lhasa block we suggest that magma underplating and subsequent mantle crust interaction i e the matter and energy transfer from the mantle to the crust has been a common and important crustal reworking process in southern tibet during indian continental subduction this process may be related to indian plate flat subduction and subsequent foundering during the post collisional stage in combination with the nature of cenozoic magmatism in the himalaya block we suggest that in addition to partial melting of the subducted continental crust magma underplating and subsequent crust mantle mixing beneath the obducted continent has also played an important role in crustal reworking of the collisional zone

age and composition of dikes in southern tibet new constraints on the timing of east west extension and its relationship to postcollisional volcanism controversy exists over whether east west extension in southern tibet is related to plateau uplift or to the accommodation of plate boundary forces relationships between the onset of extension plateau uplift and the thermal state of the lithosphere are critical to this debate we present new data on postcollisional north south trending dikes in southern tibet their ages range from 18 3 2 7 ma to 13 3 0 8 ma and define the onset of regional east west extension in southern tibet dikes are compositionally indistinguishable from postcollisional lavas in southern tibet being either ultrapotassic having a source in the subcontinental lithospheric mantle or calc alkaline with a dominantly crustal origin the ultrapotassic dikes demonstrate that east west extension and subcontinental lithospheric mantle derived magmatism were temporally and spatially linked supporting models that relate the latest phase of plateau uplift to subcontinental lithospheric mantle thinning thus the onset of extension by 18 3 2 7 ma represents the time at which the potential energy of the plateau exceeded convergent boundary forces this places a new age limitation on the attainment of high elevation in southern tibet with implications for models that relate cenozoic monsoon intensification to plateau uplift

risks inherent in the cenozoic stratigraphic plays in basins of the uruguayan continental margin abstract the genesis and evolution of the uruguayan continental margin are linked to the geological processes that resulted in the breakup of the western gondwana and the opening of the atlantic ocean intense hydrocarbon exploration over the last decade has identified several plays in the mesozoic cenozoic sedimentary basin infill the main exploration milestone was the drilling of the raya x 1 well in the pelotas basin which targets an oligocene turbidite deposit the well encountered no hydrocarbon accumulations but has contributed to the knowledge of a frontier area and has raised questions regarding the geological risks associated with the cenozoic prospects in the uruguayan continental margin in this study the exploratory potential of a set of stratigraphic plays identified in the cenozoic sedimentary succession of the uruguayan continental margin basins was analysed through the interpretation of 2d seismic sections a significant risk appears to be related to migration and charge of the cenozoic reservoirs from cretaceous source rocks because of the lack of major faults and the development of thick transgressive sedimentary packages especially in the pelotas basin the best areas for vertical charge of the cenozoic reservoirs resides on the southwest of the uruguayan continental margin distal part of the punta del este basin and in a portion of the central region southernmost part of the uruguayan pelotas basin

early cenozoic crust at the norwegian continental margin and the conjugate jan mayen ridge multichannel seismic profiles at the voring plateau margin off norway and the northern jan mayen ridge have provided a framework for the early tertiary evolutionary history of these areas which were adjacent prior to the opening of the norwegian sea we propose that their paleogene evolution is quite similar and that the two acoustic basement reflectors ee and jo were formed in the early eocene by extrusion of flow basalts from the time of opening to about anomaly 23 time oceanic seafloor was generated by icelandic type spreading during that time the central jan mayen fracture was the main transform the seaward dipping reflector sequences are flow basalts generated by the icelandic spreading the existence of less well developed dipping sequences seaward of the primary ones probably reflects that these regions submerged later because of higher initial elevation during the early eocene subaerial spreading episode which is related to the north atlantic volcanic province basalt flows also covered the adjacent thinned and heavily intruded continental crust it is proposed that the continent ocean boundary is located just landward of anomaly 24b a short distance seaward of the voring plateau escarpment reflector k is only recognized landward of this boundary in a region of transitional crust the escarpment experienced syndepositional faulting and the most elevated parts of the voring marginal high did not subside below shallow water depths before oligocene miocene time we suggest that the voring margin and the jan mayen ridge represent volcanic passive margins the evolution of this margin type may relate to initial uplift due to rifting in previously thinned crust

insights from recent gravity satellite missions in the density structure of continental margins with focus on the passive margins of the south atlantic abstract we focus on new gravity and gravity gradient data sets from modern satellite missions goce grace and champ and their geophysical interpretation at passive continental margins of the south atlantic both sides south africa and south america have been targets of hydrocarbon exploration and academic research of the german priority program sample south atlantic margin processes and links with onshore evolution the achievable spatial resolution driven by goce is 70 80 km therefore most of the geological structures which cause a significant gravity effect by both size and density contrast can be resolved however one of the most important aspects is the evaluation of the omission error which is not always in the focus of interpreters it results from high frequency signals of very rough topographic and bathymetric structures which cannot be resolved by satellite gravimetry due to the exponential signal attenuation with altitude the omission error is estimated from the difference of the combined gravity model eigen 6c4 and the satellite only model goco05s it can be significantly reduced by topographic reductions simple 2d density models and their related mathematical formulas provide insights in the magnitude of the gravity effect of masses that form a passive continental margin they are contrasted with results from satellite only and combined gravity models example geophysical interpretations are given for the western and eastern margin of the south atlantic ocean where standard deviations vary from 25 to 16 mgal and 21 11 mgal respectively it could be demonstrated that modern satellite gravity data provide significant added value in the geophysical gravity data processing domain and in the validation of heterogeneous terrestrial data bases combined models derived from high resolution terrestrial gravity and homogeneous satellite data will lead to more detailed and better constrained lithospheric density models and hence will improve our knowledge about structure evolution and state of stress in the lithosphere

sedimentary growth pattern on the northern argentine slope the impact of north atlantic deep water on southern hemisphere slope architecture large sedimentary deposits consisting of several major contourite drifts were studied by means of high resolution multichannel seismic data at the middle slope along the northern argentina continental margin to determine their evolutionary stages as well as to identify and assess the possible impact of northern source deep water nsdw on the slope architecture the imaged contouritic sediments allow decoding on the regional paleo oceanographic setting of the last 32 ma earliest contouritic sedimentation can be observed close to the eocene oligocene boundary based on an aggradational stacking pattern with a complex and wavy seismic facies pointing toward a hydrodynamically turbulent flow pattern this facies is most likely related to the opening of the drake passage associated with global cooling and a strengthening of surface intermediate and deep ocean currents in the southern ocean during the middle miocene plastered drift sequences with an aggradational reflection pattern were deposited their depositional style indicates weak non turbulent current conditions which are interpreted to be related to a vertical shift of water mass interfaces caused by the first formation of nsdw during the mid miocene climatic optimum on top the formation of plastered drift sequences led to the modern extent of the ewing terrace which was probably controlled by the continuous strengthening and thickening of nsdw until the final closure of the central american seaway cas during the pliocene and quaternary after the complete closure of the cas and under the influence of the full force of the nsdw mounded plastered drift sequences are built upon the ewing terrace generating the modern slope morphology therefore we suggest that deep water production in the northern hemisphere plays a significant role by controlling the shape of the continental slopes in the southwestern south atlantic since the middle miocene highlights slope of northern argentine continental margin is current controlled since 32 ma variability of northern source deep water nsdw controls sedimentary processes sedimentary processes are susceptible to changes of the brazil malvinas confluence impact of nsdw on slope processes is underestimated in the southern hemisphere

pn uppermost mantle tomography of central tibet implication for mechanisms of n s rifts and conjugate faults abstract the north south convergence between the indian and eurasian continents since cenozoic has produced the tibetan plateau and east west extension structures have also developed since late cenozoic whether these extensional structures are limited to the crust or extend to the deep lithosphere remains controversial the uppermost mantle serves as a link between the crust and deep lithosphere the velocity structure of which provides an important constraint on exploring the mechanism of extensional structures based on pn traveltime data recorded by the sandwich network in central tibet we use the interstation traveltime difference method to obtain the uppermost mantle velocity structure our results show significant velocity differences for the uppermost mantle beneath the conjugated strike slip faults and rifts which indicates their different formation mechanisms the high pn velocity beneath the conjugated strike slip faults support an eastward horizontal shear at the base of the upper crust rather than the lower crust has contributed to the development of the conjugated strike slip faults the low pn velocity beneath the rifts in lhasa may provide evidence for the rifts cutting through the entire crust and reflect the connection between the formation of the rifts and deep tectonic activities the most prominent features in our results are the low pn velocity zones beneath the tangra yum co rift tyr the pumqu xianza rift pxr and the zone between the two rifts and we explain the low pn velocity as the top of the mantle wedge upon the subducting indian lithospheric slab

gravimetric determination of the continental oceanic boundary of the argentine continental margin from 36 s to 50 s fil arecco maria alejandra universidad de buenos aires facultad de ingenieria argentina

geology and seismic structure of the northern section of the oman ophiolite in the north oman mountains a continuous ophiolite succession is exposed from tectonized harzburgites and dunites at the base through layered gabbros and peridotites high level gabbros and plagiogranite to a dike swarm and pillowed volcanics overlain by pelagic shales the upper part of this sequence possesses a static metamorphic overprint which passes downward from greenschist facies in the lowermost volcanics and most of the dike swarm to amphibolite facies in the lowermost dike swarm and in the high level intrusives these pervasively altered rocks are underlain by layered gabbros and peridotites where hydration is restricted to fractures compressional and shear wave velocities have been measured to confining pressures of 6 kbar for 139 water saturated cores from this sequence the samples were field oriented and collected from known stratigraphic levels which allows the construction of velocity depth profiles and an examination of anisotropy within the ophiolite compressional wave velocities measured for the basalt section are quite variable ranging from 4 5 to 6 0 km s at in situ pressures within the sheeted dike section compressional and shear wave velocities vary from 5 4 to 6 3 km s and from 2 9 to 3 6 km s respectively from the basalt sediment contact through the dike section velocities increase rapidly with depth this gradient is related to increasing metamorphic grade and a decrease in grain boundary porosity the top of seismic layer 3 is located near the lower boundary of the sheeted dike swarm and marks the transition from greenschist facies to amphibolite facies metamorphics the dikes decrease in abundance downward into the high level intrusives coarse grained gabbroic and granitoid rocks which often contain abundant hornblende with isotropic textures and lacking compositional layering mean compressional and shear wave velocities increase uniformly from 6 7 and 3 6 km s respectively in the highlevel intrusives to 7 5 and 3 9 km s respectively at the base of the layered sequence measured velocities of the ultramafic tectonites are highly variable because of the anisotropy and serpentinization petrofabrics of relic olivine in the wadi ragmi region show strong preferred fabrics for the tectonites and calculated compressional wave velocities for the serpentine free rocks range from 7 8 to 8 5 km s with the fast direction parallel to the direction of spreading inferred from dike orientations in excellent agreement with observed upper oceanic mantle seismic anisotropy the crust mantle boundary which is defined by the abrupt increase in seismic velocities coincides with the petrological contact between layered gabbros and peridotites and tectonized harzburgite and dunites

north atlantic volcanic margins dimensions and production rates early tertiary lithospheric breakup between eurasia and greenland was accompanied by a transient 3 m y igneous event emplacing both the onshore flood basalts of the north atlantic volcanic province navp and huge extrusive complexes along the continent ocean transition on the rifted continental margins seismic data show that volcanic margins extend 2600 km along the early eocene plate boundary in places underlain by high velocity 7 2 7 7 km s lower crustal bodies quantitative calculations of navp dimensions considered minimum estimates reveal an areal extent of 1 3 106 km2 and a volume of flood basalts of 1 8 106 km3 yielding a mean eruption rate of 0 6 km3 yr or 2 4 km3 yr if two thirds of the basalts were emplaced within 0 5 m y the total crustal volume is 6 6 106 km3 resulting in a mean crustal accretion rate of 2 2 km3 yr thus navp ranks among the world s larger igneous provinces if the volcanic margins are considered the velocity structure of the expanded crust seaward of the continent ocean boundary differs from standard oceanic and continental crustal models based on seismic velocities this volcanic margin crust can be divided into three units of which the upper unit corresponds to basaltic extrusives the regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust including the high velocity lower crust which extends some distance landward of the continent ocean boundary was emplaced during and subsequent to breakup the volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling probably reflecting the interaction of a mantle plume and a lithosphere already extending

a late cretaceous mixed turbidite contourite system along the uruguayan margin sedimentary and palaeoceanographic implications abstract the late cretaceous is a critical period in the evolution of the south atlantic significant changes in the tectonic configuration of continental landmasses and marine basins during this time lead to major shifts in the sedimentary and oceanographic regimes using a combination of 2d and 3d seismic data with well data from the uruguayan margin this work provides a new highly detailed description of the late cretaceous sedimentary evolution of the margin and its regional and global palaeoceanographic implications the polonio drift system pds occupies the middle to lower slope of the uruguayan margin the pds is characterised by four channel drifts which propagate obliquely 120 170 from the palaeoslope and exhibit over 800 m of local relief relative to the surrounding seafloor seismic interpretation identified four seismic units su 1 su 4 separated by five major discontinuities r 1 r 5 these record the onset albian upper turonian 113 89 8 ma growth coniacian upper maastrichtian 89 8 66 ma and burial paleocene 66 56 ma phases for the pds during the onset stage down slope processes dominate but three major shifts in the sedimentary stacking pattern occur these coincide with palaeoceanographic changes in the south atlantic during the cenomanian turonian 100 5 89 9 ma campanian maastrichtian 83 6 66 ma and paleocene 66 56 ma following the opening of the falkland malvinas agulhas seaway fas the subsidence of the walvis ridge and the opening of the vema channel bottom current influence first occurs during the growth stage coniacian campanian when mounded deposits shift from aggradational and slightly progradational towards the n su 2 to clear lateral migration towards the s su 3 this sedimentary pattern indicates a regional palaeoceanographic shift in bottom current flow from northerly to a subsequent southerly direction bottom currents do not appear to have actively eroded sediment although they apparently had the potential to pirate and entrain fine grained material from turbidity plumes which build on underlying bathymetric features such as basement horsts and channel levees these produce large mounded morphologies adjacent to channels drift growth ceases following the termination of shelf progradation r 4 enabling sluggish bottom currents contributed to a homogeneous sedimentary drape bottom currents intensify in the beginning of the paleocene forming contourite depositional systems with well developed contourite terrace along the middle slope further research can elucidate the nature and mechanics of turbidite contourites interactions along continental margins this work offers new insights for the implications and consequences of the interaction of turbidites and contourites which can improve the characterisation of deep water deposits and understanding palaeoceanographic interpretations

two dimensional seismic reflection modeling of the inferred fossil oceanic crust mantle transition in the bay of islands ophiolite we investigate the origin and character of oceanic moho reflections by computing two dimensional synthetic seismogram profiles of the inferred fossil oceanic crust mantle transition observed in the bay of islands ophiolite to simulate a seismic reflection experiment we calculated near vertical incidence seismograms at a horizontal spacing of 500 m for three separate sections of the ophiolite totaling 64 km in length in the synthetic profiles the moho reflection event varies from a single phase to two or more phases of up to 1 s two way travel time total duration individual phases show lateral variation in amplitude and their two way travel times vary by as much as 0 25 s over horizontal distances as short as 10 km lateral discontinuity of phases results in abrupt variations in the travel times of first arriving high amplitude moho phases the geological structures generating the highest amplitude moho reflections vary from high and low velocity lenses of mafic and ultramafic material in the lower crust and upper mantle to interlayered mafic and ultramafic lithologies in the moho transition zone reflection amplitudes from the residual upper mantle are insignificant and our modeling suggests that using the first arriving high amplitude moho phase to estimate thickness of magmatic material might result in errors of up to 1 s two way travel time 3 4 km multichannel seismic data from both the western pacific and western north atlantic show moho travel time variations similar to those observed in the synthetic profiles the western north atlantic data also show multiphase moho reflection events that are laterally discontinuous on a scale similar to that observed in the synthetic data suggesting that the structures observed in the inferred fossil crust mantle transition of the ophiolite are characteristic of oceanic lithosphere

origin composition and relative timing of seaward dipping reflectors on the pelotas rifted margin abstract the mechanism by which seaward dipping reflectors sdrs are formed is a topic of debate two contrasting models exist for their formation the volcanic faulting model and the volcanic loading model each of these models has important implications for the processes which control the structure and formation of magma rich rifted continental margins we have examined high quality deep seismic reflection data across the pelotas basin offshore brazil these data reveal a remarkable set of sdrs for which we have investigated the likely nature of their formation the total package of sdrs has an across strike width of 200 km and a variable vertical thickness of 10 17 km previously interpreted as volcanic flows detailed observations however show changes in seismic character and geometry within the sdr package which suggest a complex and varied evolution we have used gravity anomaly inversion and seismic observations together to investigate the likely composition of the sdrs by determining the proportion of basaltic material to sedimentary volcaniclastic material basalt fraction within the sdrs this has been achieved by minimising the difference between the depth of the gravity moho and seismic moho in order to quantify the lateral variation in basalt fraction taken to be proportional to the bulk density of the package the density of the sdr package together with seismic interpretation is then used to infer the composition depositional environment source and time of formation relative to breakup our analysis suggests that the overall sdr basalt fraction and bulk density decrease oceanwards possibly due a change in the type of volcanic deposits from predominantly subaerial to volcaniclastics possibly deposited subaqueously the sdrs can be split into three sub packages the two inner sdr packages are interpreted to consist of lava flows sourced from syn tectonic subaerial eruptions associated with the onshore parana large igneous province flowing eastwards into an extensional basin the outer sdr package shows reflector geometries that progressively offlap oceanwards interpreted as extrusives sourced from an eastwards migrating newly formed ocean ridge our analysis suggests that both the volcanic faulting and volcanic loading models for sdr formation are applicable to the pelotas rifted margin recording distinct syn rift and syn breakup magmatic events we show that both sdr formation models can be recognised in a naturally occurring example and can coexist on the same margin

a contourite depositional system along the uruguayan continental margin sedimentary oceanographic and paleoceanographic implications abstract for the first time a multidisciplinary approach to evaluate the influence of bottom currents in the uruguayan continental margin is presented bathymetric data and multichannel 2d and 3d seismic reflection profiles were used to construct a morphosedimentary map to interpret and decode sedimentary and oceanographic processes along the uruguayan continental margin based on these results an extensive contourite depositional system on the margin is described which contains an impressive array of large erosive depositional drifts and mixed terrace features which have been generated primarily by the near bottom flows associated with water masses of antarctic and subantarctic origin from the eocene oligocene boundary up to present time the long term influence of water masses from higher southern latitudes in combination with down slope sedimentary processes have strongly controlled the overall margin morphology most of the features described here were formed during the middle late miocene epoch due to paleoceanographic shifts that include the arrival of antarctic intermediate water along the margin which in combination with deeper antarctic bottom water are fundamental in the margin evolution in combination with quaternary climatic and eustatic changes in sea level fluctuations of the brazil malvinas confluence influenced subsequently glacial and interglacial stages as recognized in sedimentary features defined here these paleoceanographic changes controlled the sedimentary stacking pattern and the locations of high amplitude reflections along the contourite terraces which could be associated with sandy deposits a more detailed understanding of the margin will improve interpretations of variations in the south atlantic subtropical gyre and further constrain general climatic and ocean circulation models

crustal structure across the colorado basin offshore argentina summary the geology of the wide shelves surrounding the south atlantic is closely linked to the kinematics and history of the opening of the ocean however several wide sedimentary basins which developed along the margins show peculiarities that are not yet understood in the context of the evolution of the south atlantic the colorado basin a wide sedimentary basin on the broad shelf of argentina extends in ew direction the basin s evolution oblique or orthogonal to the continent ocean boundary indicates that it is not a product of simple progressive extension and crustal thinning in addition a basement high paralleling the continental margin and separating the colorado basin from the deep sea basin is a common interpretation these findings are hardly in accordance with the idea that the colorado basin is an extensional basin that developed in conjunction with the early e w opening phase of the south atlantic in the late jurassic early cretaceous the composition type and structure of the basement key points for the evaluation of the basins evolution are widely speculative in this context multichannel seismic reflection data from the argentine shelf and a 665 kmlong onshore offshore refraction profile running across the colorado basin onto the coast are discussed in combination with gravity data the stratigraphy for the sedimentary successions was adopted from the literature and the reflection seismic marker horizons formed besides the interval velocities the input for the starting model for refraction seismic traveltime modelling the modelling strategy was an iterative procedure between refraction seismic traveltime and gravity modelling the preparation of the density models was coarsely orientated on published velocity density relations the modelling results are in favour of a continuation of the main onshore geological features beneath the sedimentary infill of the colorado basin we interpret the basement along the line from west to east as offshore continuation of the ventana hills the

mantle inputs to himalayan anatexis insights from petrogenesis of the miocene langkazi leucogranite and its dioritic enclaves abstract oligocene and miocene himalayan anatexis is generally thought to have been induced by intracrustal heating or processes without the involvement of mantle derived heat and materials suggesting that the himalayan leucogranites are typical examples of purely crustal melts this study focuses on a miocene leucogranite at langkazi within the himalayan orogen an intrusion that contains a large number of dioritic enclaves these enclaves have typical igneous textures contain acicular apatites and have back veining structures quenched margins and crystallization ages identical to the hosting two mica granites indicating that these enclaves are magmatic although the enclaves are evolved the most primitive samples contain high concentrations of mgo up to 4 3 wt cr up to 159 ppm and ni up to 102 ppm are strongly enriched in large ion lithophile elements are depleted in high field strength elements have negative e nd t values 8 6 to 6 1 and have relatively high 87 sr 86 sr i values 0 7085 0 7137 suggesting that they were derived from a relatively enriched region of the lithospheric mantle source whole rock geochemical data indicate that the hosting langkazi leucogranite formed from magmas generated by the partial melting of metapelite material within the high himalaya crystalline sequence and these magmas subsequently mixed with mantle derived melts that are now represented by the langkazi dioritic enclaves this indicates that mantle derived material played an important role in the generation of the langkazi intrusions the whole rock geochemical compositions of samples from the study area also indicate that the primary melts that formed the langkazi enclaves were significantly contaminated by the relatively juvenile himalayan lower crustal material suggesting in turn that these mantle derived magmas underwent mash crustal melting melt assimilation magma storage and homogenization processes at the base of crust introducing heat to the lower crust this mantle derived heat may have induced partial melting of the himalayan lower crust forming adakite like magmas although the small volumes of these dioritic enclaves suggest that this heat may have assisted the genesis of the north himalayan granites and the high himalayan leucogranites whereas in situ crustal radiogenic heating shear heating and decompression melting dominated the melting processes in this region

palaeogeographic consequences of conservational models in the south atlantic ocean conservational models like simple shear pure shear or polyphase models that exclude exchanges between the lower continental crust and upper mantle are usually proposed to explain the lithospheric stretching and consequent crustal thinning of passive continental margins these models need large amounts of horizontal movement and have therefore important implications for plate kinematic reconstructions and intraplate deformation in this paper we propose to show these implications in the central segment of the south atlantic ocean in the angola brazilian system these models imply about 240 km of horizontal movement this movement can be com pensated by two end member mechanisms 1 an intraplate deformation located in africa and 2 an intraplate deformation located in south america we detail for each solution the strong geological and geodynamical implications and discuss the consequences for the genesis of passive continental margins

reef building and carbonate production modes in the west central tethys during the cenozoic abstract changing components rock textures lithofacies platform types and architecture throughout time are unique characteristics of carbonate rocks characterizing these attributes has been approached by 1 building reference models for specific phanerozoic intervals 2 recognizing the climatic impact in modulating carbonate production and 3 analyzing the influence of changing bio geochemical conditions the reference model approach is mostly based on biological evolution the climatic impact approach emphasizes temperature and the bio geochemical approach considers the changes in mg ca ratios and ca concentrations in the oceans to date however an analysis integrating all of these factors is still missing the analysis presented here includes all these factors but also co 2 which is fundamental for both photosynthesis and caco 3 precipitation here we analyze the waxing and waning of cenozoic reef limestones from the central tethys region through several steps 1 on the basis of rock volume rock textures associated sediments and light dependent skeletal components as records of light penetration and wave energy depth 2 on global environmental conditions 13 c 18 o p co 2 temperature and 3 on the basis of functionality nutritional requirements and available resources through the cenozoic water motion whether induced by surface or internal waves or by currents increased as the thermal gradients strengthened both with depth and with latitude active water motion is essential for plankton catchers such as corals but less so for many larger benthic foraminifers lbf pycnoclines in the meso oligophotic zone would then favor the benthic plankton catchers such as corals but would be detrimental for many lbf warm temperatures favored lbf the eocene lbf families predominated during lowering of atmospheric p co 2 by using respiratory co 2 to enhance the symbiont production of photosynthates under oligotrophic conditions and limited turbulence whereas the miocene families had to adapt to a progressive increase in turbulence the eurythermal coralline red algae however became preponderant producers in the mesophotic zone during times when the 13 c was relatively high this explains two apparent paradoxes 1 corals thrive best when the earth s high latitudes cool and 2 the dominance of corals and lbf is inversely correlated despite they both require tropical conditions and have similar trophic strategies mixotrophy

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

origin of seaward dipping reflectors in oceanic crust off the norwegian margin by subaerial sea floor spreading a remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the norwegian continental margin it comprises a seaward dipping complex of reflectors in the form of a wedge we suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean basin genesis

the crustal structure of the southern argentine margin summary multichannel reflection seismic profiles combined with gravimetric and magnetic data provide insight into the crustal structure of the southernmost argentine margin at the transition from a rifted to a transform margin and outline the extent of the north falkland graben based on these data we establish a regional stratigraphic model for the post rift sediments comprising six marker horizons with a new formation in the barremian lower cretaceous our observations support that a n s trending subsidiary branch of the north falkland graben continues along the continental shelf and slope to the argentine basin during the rift phase a wide shelf area was affected by the e w extension subsequently forming the north falkland graben and the subsidiary branch along which finally breakup occurred we propose the division of the margin in two segments a n s trending rifted margin and an e w trending transform margin this is further underpinned by crustal scale gravity modelling three different tectono dynamic processes shaped the study area 1 the triassic early jurassic extensional phase resulting in the formation of the north falkland graben and additional narrower rift grabens ended synchronously with the breakup of the south atlantic in the early valanginian 2 extensional phase related to the opening of the south atlantic 3 the transform margin was active in the study area from about hauterivian times and activity lasted until late cretaceous early cenozoic both the rifted margin and the transform margin are magma poor very limited structures may have a volcanic origin but are suggested to be post rift the oceanic crust was found to be unusually thin indicating a deficit in magma supply during formation these findings in combination with the proposed breakup age in the early valanginian that considerably predates the formation of the parana etendeka continental flood basalt provinces in brazil and namibia question the influence of the tristan da cunha hotspot during the initial formation of the south atlantic

implications of shortening in the himalayan fold thrust belt for uplift of the tibetan plateau 1 recent research in the himalayan fold thrust belt provides two new sets of observations that are crucial to understanding the evolution of the himalayan tibetan orogenic system first u pb zircon ages and sm nd isotopic studies demonstrate that 1 greater himalayan medium to high grade metasedimentary rocks are much younger than true indian cratonic basement and 2 these rocks were tectonically mobilized and consolidated with the northern margin of gondwana during early paleozoic orogenic activity these observations require that greater himalayan rocks be treated as supracrustal material in restorations of the himalayan fold thrust belt rather than as indian cratonic basement in turn this implies the existence of greater himalayan lower crust that is not exposed anywhere in the fold thrust belt second a regional compilation of shortening estimates along the himalayan arc from pakistan to sikkim reveals that 1 total minimum shortening in the fold thrust belt is up to 670 km 2 total shortening is greatest in western nepal and northern india near the apex of the himalayan salient and 3 the amount of himalayan shortening is equal to the present width of the tibetan plateau measured in an arc normal direction north of the indus yalu suture zone this information suggests that a slab of greater indian lower crust composed of both indian cratonic lower crust and greater himalayan lower crust with a north south length of 700 km may have been inserted beneath the tibetan crust during the cenozoic orogeny we present a modified version of the crustal underthrusting model for himalayan tibetan orogenesis that integrates surface geological data recent results of mantle tomographic studies and broadband seismic studies of the crust and upper mantle beneath the tibetan plateau previous studies have shown that incremental mesozoic and early cenozoic shortening had probably thickened tibetan crust to 45 55 km before the onset of the main cenozoic orogenic event thus the insertion of a slab of greater indian lower crust with maximum thickness of 20 km tapering northward could explain the cenozoic uplift of the tibetan plateau the need for tibetan crust to stretch laterally as the greater indian lower crust was inserted may explain the widespread east west extension in the southern half of the plateau our reconstruction of the himalayan fold thrust belt suggests that indian cratonic lower crust of presumed mafic composition and high strength should extend approximately halfway across the tibetan plateau to the banggong suture from there northward we predict that the tibetan plateau is underlain by more felsic and therefore weaker lower crust of greater himalayan affinity two slab break off events are predicted by the model the first involved neotethyan oceanic lithosphere that foundered 45 35 ma and the second consisted of greater indian lithosphere most likely composed of greater himalayan material that delaminated and foundered 20 10 ma asthenospheric upwelling associated with the break off events may explain patterns of cenozoic volcanism on the tibetan plateau although the model predicts a northward migrating topographic front due solely to insertion of greater indian lower crust the actual uplift history of the plateau was complicated by early middle tertiary shortening of tibetan crust

new constraints on the age and style of continental breakup in the south atlantic from magnetic anomaly data abstract we present new constraints on the opening of the south atlantic ocean from a joint interpretation of marine magnetic anomaly grids and forward modelling of conjugate profiles we use 45 000 km of recently collected commercial ship track data combined with 561 000 km of publically available data the new data cover the critical ocean continental transition zones and allow us to identify and downgrade some poorly navigated older ship tracks relied upon in earlier compilations within the final grids the mean cross over error is 14 nt computed from 8 227 ship track intersections the forward modelling used uniformly magnetised bodies whose shapes were constrained from coincident deep seismic reflection data we find the oldest magnetic anomalies to date from m10r 134 2 ma late valanginian north of the falkland agulhas fracture zone and m3 129 3 ma barremian south of the rio grande fracture zone hence assuming the gpts used is correct continental breakup was contemporaneous with the parana and etendeka continental flood basalts many of the landward linear anomalies overlap seismically mapped seaward dipping reflectors sdrs we interpret this to mean that a significant portion of the sdrs overlay crust formed by subaerial seafloor spreading here crustal accretion is envisaged to be similar to that at mid ocean ridges but sheet lava flows that later form the sdrs rather than pillow basalts form the extrusive component segmentation of the linear anomalies generated implies that this stage of continental breakup is organised and parallels the seafloor spreading centre that follows our results call into question the common assumption that at volcanic continental margins the first linear magnetic anomalies represent the start of conventional submarine oceanic crustal generation

kinematic keys of the santos namibe basins understanding the genesis of the very peculiar 600 km wide santos basin sa o paulo plateau system and its narrow conjugate namibe margin is a kinematic and structural problem several hypotheses have been proposed in order to explain the genesis of this system that imply the same amount of horizontal movement we investigate the consequences of the horizontal movement in the santos basin based in plate kinematic reconstructions the kinematic history of this system that we present here based on the interpretation of seismic profiles and kinematic constraints has the following consequences 1 there is no evidence of a ridge jump sensu stricto but rather a southwards propagation in the central segment of the south atlantic that starts in the northern part between the ne brazilian and gabonese margins 2 the namibe margin evolved as a transform passive margin 3 the opening direction of the santos basin sa o paulo plateau system is oblique to the general opening motions of the south american and african plates and 4 this opening is younger 6 ma than those of the other basins of the central segment of the south atlantic

ophiolites synthetic seismograms and ocean crustal structure 1 comparison of ocean bottom seismometer data and synthetic seismograms for the bay of islands ophiolite a series of synthetic seismograms based on the bay of islands ophiolite data are compared with ocean bottom seismometer seismic refraction observations which sample 60 m y crust in the northeastern pacific the general agreement between the synthetics for the ophiolite and the observations support the idea that some ophiolites are representative of normal ocean crust this support extends to similarities in complex second arrivals in the synthetics and observations which often are not used in the interpretation of marine seismic data the use of the ophiolite model also allows us to show how the interpretation of seismic data is affected by experimental design and interpretation procedures as the sampling density increases and as more phases in the observations are accounted for the velocity depth model more closely approaches that found for the ophiolite the results of this modeling support sharper p velocity transitions within the crust than are found in young 12 m y age oceanic crust the need for a thin approximately 1 km thick p low velocity zone at the base of the crust is interpreted as support for serpentinization at the crust mantle boundary serpentinization of the ultramafic component in the bay of islands ophiolite therefore may include a small preemplacement component

magmatism at rifted continental margins the intense volcanism and uplift observed on many rifted continental margins forming basaltic seaward dipping reflector sequences is accompanied by the emplacement of a thick igneous section at depth partial melting by decompression of passively upwelling asthenosphere that is hotter than normal because it is near a hotspot explains both the thickness of the new igneous crust and the initial elevation of the rifted margins

age progressive volcanism in the new england seamounts and the opening of the central atlantic ocean radiometric ages k ar and oar 39ar methods have been determined on dredged volcanic rocks from seven of the new england seamounts a prominent northwest southeast trend ing volcanic lineament in the northwestern atlantic ocean the oar 39ar total fusion and increnental heating ages show an increase in seamount construction age from southeast to northwest that is consistent with northwestward motion of the north american plate over a new england hot spot between 103 and 82 ma a linear volcano migration rate of 4 7 cm yr fits the seamount age distribution these ages fall within a longer age progression from the corner seamounts 70 to 75 ma at the eastern end of the new england seamounts to the youngest phase of volcanism in the white mountain igneous province new england 100 to 124 ma the new england hot spot estimated from the new radio metric ages and motion of north america in the hot spot reference frame to be near 28on 33ow probably generated a short line of mid to late tertiary age seamounts on the african plate but appears to be presently inactive the hot spot reference frame is used to calculate the motion of the north american plate away from the african plate from early cretaceous time to the present rominent magnetic anomalies recorded in the central atlantic seafloor give the positions of the spreading ridge for a bange of known ages hot spots that now underlie the african plate in the eastern central atlantic new england canary madeira cape verde azores could have produced cretaceous seamount and island chains on the north american plate during the early opening of the central atlantic each of these hot spots has been overridden by spreading ridges at pre dictable times some of these hot spot crossings are expressed as geochemical anomalies in the oceanic crust now far removed from spreading ridges

the argentine continental margin north of 48 s sedimentary successions volcanic activity during breakup abstract new seismic data and geophysical results from the outer argentine continental margin and from seismic flow line transects across the oceanic argentine brazil and cape angola basins are presented the results document that the early cretaceous south atlantic continental breakup and initial sea floor spreading were accompanied by large scale transient volcanism emplacing voluminous extrusive constructions on the conjugate outer continental margins of the south atlantic on the argentine margin we interpret three major tectono volcanic crustal units beneath a thick and tectonically undisturbed sedimentary succession of cretaceous and tertiary age 1 syn rift basins and even pre rift features are present on the outer shelf 2 a deeply buried 60 120 km wide and several thousand metres thick volcanic wedge occupies most of the slope the wedge is characterized by seaward dipping reflectors and is associated with the distinct magnetic anomaly g 3 the seaward adjacent oceanic crust of cretaceous age shows isochronous changes of the seismic images along our flow line transects suggesting some form of episodicity of rich magma and poor to moderate magma supply at the pre existing mid ocean ridge of the south atlantic during cretaceous time

origin of cenozoic alkaline potassic volcanic rocks at konglongxiang lhasa terrane tibetan plateau products of partial melting of a mafic lower crustal source abstract it is generally believed that cenozoic potassic and ultrapotassic volcanic rocks of the tibetan plateau were generated by partial melting of enriched mantle this paper reports new data regarding the elemental and isotopic compositions of the konglongxiang volcanic rocks kvr k2o na2o 2 and low mgo contents in the middle western lhasa terrane tibetan plateau ar ar isotope dating of a sample from the kvr yields an age of 21 3 ma indicating that the kvr lavas erupted during the early miocene the kvr consists mainly of trachyte and rhyolite a comparison of the kvr with cenozoic potassic volcanic rocks of the tibetan plateau and the results of melting experiments involving k rich volcanic rocks as well as other k rich volcanic rocks from throughout the world reveals that the kvr trachytes originated from a mafic lower crust containing k rich minerals rather than from a mantle source the kvr rhyolites were probably derived from the middle upper crust based on age data compositional data and the regional tectonic setting we propose that the formation of the kvr and associated n s grabens was triggered by early miocene e w regional extension in the area of the southern tibetan plateau

tearing of the indian lithospheric slab beneath southern tibet revealed by sks wave splitting measurements a b s t r a c t shear wave birefringence is a direct diagnostic of seismic anisotropy it is often used to infer the northern limit of the underthrusting indian lithosphere based on the seismic anisotropy contrast between the indian and eurasian plates most studies have been made through several near north south trending passive source seismic experiments in southern tibet to investigate the geometry and the nature of the underthrusting indian lithosphere an east west trending seismic array consisting of 48 seismographs was operated in the central lhasa block from september 2009 to november 2010 splitting of sks waves was measured and verified with different methods along the profile the direction of fast wave polarization is about 60 in average with small fluctuations the delay time generally increases from east to west between 0 2 s and 1 0 s and its variation correlates spatially with north south oriented rifts in southern tibet the sks wave arrives 1 0 2 0 s later at stations in the eastern part of the profile than in the west the source of the anisotropy estimated by non overlapped parts of the fresnel zones at stations with different splitting parameters is concentrated above ca 195 km depth all the first order features suggest that the geometry of the underthrusting indian lithospheric slab in the himalayan tibetan collision zone beneath southern tibet is characterized by systematic lateral variations a slab tearing and or breakoff model of indian lithosphere with different subduction angles is likely a good candidate to explain the observations

south atlantic volcanic margins the early cretaceous south atlantic continental break up and initial sea floor spreading were accompanied by large scale transient volcanism emplacing the parana etendeka continental flood basalts and voluminous extrusive constructions on the conjugate margins south of the torres arch abutment plateau on the north namibia margin we interpret four main tectono magmatic crustal units 1 oceanic crust 2 thickened oceanic crust covered by huge seaward dipping wedges 3 a c 150 km wide break up related rift zone partly covered by the dipping wedges and 4 thicker continental crust partly deformed by palaeozoic extension east of the early cretaceous rift similar settings also characterize other south atlantic margin segments we infer an up to 300 km wide and 2400 km long rift zone representing lithospheric extension leading to breakup and formation of the south atlantic volcanic margins comparison with other volcanic margins demonstrates in spite of local and regional differences gross similarities in tectono magmatic style crustal units and dimensions

deep structure of the argentine margin inferred from 3d gravity and temperature modelling colorado basin abstract following previous work on the colorado basin using a 3d crustal structural model we now investigate the presence of lower crustal bodies at the base of the crust using 3d lithospheric gravity modelling and calculations of the conductive thermal field our first study highlighted two fault directions and depocentres associated with thinned crust nw se in the west and ne sw at the distal margin fault relative chronology argues for two periods of extension 1 nw se faulting and thinning in the western colorado basin and 2 ne sw faulting and thinning related to the continental breakup and formation of the ne sw striking volcanic margins of the atlantic ocean in this study the geometry of modelled high density lower crustal bodies lcbs enables the reproduction of the gravimetric field as well as of the temperature measured in wells down to 4500 m the modelled lcbs correlate with geological observations 1 nw se lcbs below the deepest depocentres in the west 2 ne sw lcbs below the distal margin faults and the seaward dipping reflectors thus the proposed poly phased evolution of the margin could as well correspond to two emplacement phases of the lcbs the calculated conductive thermal field fits the measured temperatures best if the thermal properties thermal conductivity and radiogenic heat production assigned to the lcbs correspond to either high grade metamorphic rocks or to mafic magmatic intrusions to explain the possible lithology of the lcbs we propose that the two successive phases of extension are accompanied by magma supply emplaced 1 in the thinnest crust below the older nw se depocentres then 2 along the ne sw continentward boundary of the distal margin and below the volcanic seaward dipping reflectors the south african conjugate margin records only the second ne sw event and we discuss hypotheses which could explain these differences between the conjugate margins

crustal structure of the southeast georgia embayment carolina trough preliminary results of a composite seismic image of a continental suture and a volcanic passive margin new deep penetration multichannel seismic reflection data combined with refraction results and magnetics modeling support a hypothesis that the carolina trough is a mesozoic volcanic passive margin exhibiting a seaward dipping wedge and associated underplating the structure of carolina platform continental crust is consistent with the late paleozoic continental collision that produced the appalachians but imbrication has had no obvious effect on shallower structures produced by mesozoic extension and volcanism the origin of prominent magnetic anomalies crossing the southeast georgia embayment can be explained by processes attending mesozoic separation of africa and north america and is not related to a paleozoic continental suture as previously postulated

backward modelling of the subsidence evolution of the colorado basin offshore argentina and its relation to the evolution of the conjugate orange basin offshore sw africa abstract in this study we focus on reconstructing the post rift subsidence evolution of the colorado basin offshore argentina we make use of detailed structural information about its present day configuration of the sedimentary infill and the crystalline crust this information is used as input in a backward modelling approach which relies on the assumption of local isostasy to reconstruct the amount of subsidence as induced by the sedimentary load through different time stages we also attempt a quantification of the thermal effects on the subsidence as induced by the rifting here included by following the uniform stretching model of lithosphere thinning and exponentially cooling through time based on the available information about the present day geological state of the system our modelling results indicate a rather continuous post rift subsidence for the colorado basin and give no significant evidence of any noticeable uplift phase in a second stage we compare the post rift evolution of the colorado basin with the subsidence evolution as constrained for its conjugate sw african passive margin the orange basin despite these two basins formed almost coevally and therefore in a similar large scale geodynamic context their post rift subsidence histories differ based on this result we discuss causative tectonic processes likely to provide an explanation to the observed differences we therefore conclude that it is most probable that additional tectonic components other than the ridge push from the spreading of the south atlantic ocean are required to explain the observed differences in the subsidence of the two basins along the conjugate passive margins such additional tectonic components might be related to a dynamic mantle component in the form of either plume activity africa or a subducting slab and the presence of an ongoing compressional stress system as revealed for different areas in south america