gravity anomalies and segmentation of the continental margin offshore west africa the free air edge effect anomaly at passive continental margins provides constraints on the thermal and mechanical properties of the lithosphere in extensional regions like many margins the edge effect high offshore gabon west africa is located at the shelf break in the region of maximum sediment thickness gabon differs however from other margins in that the edge effect low is displaced from over the continental rise to the slope the origin of the low has been investigated by backstripping the sediments at the margin we have used the backstrip to calculate the amount of crustal thinning assuming airy isostasy such a model implies zero strength during rifting alternatively this amount of crustal thinning can be explained by a strong margin and a depth of necking that exactly balances the upward and downward forces that act on the crust during rifting we have developed a method which takes into account variations in the strength during rifting and the depth of necking to compute the thinning directly from the sediment backstrip the backstrip and the crustal thinning have been used to calculate the rifting gravity anomaly differences between the backstrip and the present day water depth give the geometry of the sediment load and hence the contribution to the gaavity anomaly of sedimentation by comparing the combined rifting and sedimentation anomaly to the observed free air gravity anomaly we have been able to constrain the flexural rigidity and equivalent elastic thickness te of the lithosphere during and after rifting the best fitting model is one in which the low is caused by a 125 km wide zone of relatively thin i e 15 km continental crust which was weak i e 0 te 10k during rifting and has remained so since then we speculate that the weak zone extends for 350 400 km along strike of the gabon margin and that passive margins may be highly segmented as regards their long term strength

gravity anomalies and segmentation at the east coast usa continental margin summary the free air gravity edge effect anomaly at rifted continental margins has generally been attributed to the transition between thick continental and thin oceanic crust while crustal thinning is a major contributor recent studies suggest that sediment loading and magmatism may significantly modify the edge effect anomaly and cause it at some margins to be highly segmented along their strike in this paper we use a combined 3 d flexural backstripping and gravity anomaly modelling technique to determine the role that sediment loading has played in controlling the segmentation of atlantic type continental margins we focus on the east coast usa since a substantial amount of high quality seismic reflection and refraction gravity and magnetic data already exists for this margin by calculating the gravity anomaly associated with rifting and sediment loading and iteratively comparing it to the observed free air anomaly we have determined the best fit elastic thickness te structure of the margin we show that 0 10 5 a integrated strength of the lithosphere these results imply that weak regions abut strong ones at the east coast usa margin te generally decreases with increase in the amounts of crustal thinning and the flexed basement curvature k suggesting it is controlled at least in part by the mechanical structure of the pre rift lithosphere and yielding due to flexural loading however there is considerable scatter suggesting other factors such as along strike changes in crustal composition irrespective we show that an isostatic anomaly that takes into account the best fit te distribution is significantly reduced in spectral power compared with one which is computed assuming only airy compensation i e te 0 km this is not to imply that rifting and sediment loading completely accounts for the anomalies along strike and across strike the east coast usa margin to the contrary significant isostatic anomaly highs and lows persist especially in inner and middle shelf regions one of the most prominent is an arcuate 670 km long high with flanking lows offshore carolina that we attribute to magmatism during the initial stages of continental break up

a tidal model for eastern juan de fuca strait and the southern strait of georgia a three dimensional barotropic finite element model is used to calculate the tidal flows in eastern juan de fuca strait and the southern strait of georgia the harmonics of eight constituents are computed and compared with those from previous finite difference models and those from historical tide gauge and current meter observations root mean square differences between observed and calculated sea level amplitudes are within 2 0 cm for all constituents and the phases are within 4 0 for all constituents except k2 horizontal currents from the model are found to reproduce the observed vertical variations in shear except in regions where stratification effects and internal tides exist in particular the model representation of currents at six stations in the canadian tide and current tables volume 5 is accurate the pattern of tidal residual currents has much more detail than previous coarser resolution models and numerous eddies are predicted computed energy flux fields reveal that over a 29 day period only 38 of the tidal power entering juan de fuca strait is transmitted into the southern strait of georgia and of the 39 entering haro strait 36 is dissipated within the strait itself

iron oxide copper gold iocg deposits through earth history implications for origin lithospheric setting and distinction from other epigenetic iron oxide deposits the iron oxide copper gold iocg group of deposits initially defined following discovery of the giant olympic dam cu u au deposit has progressively become too embracing when associated deposits and potential end members or analogs are included the broader group includes several low ti iron oxide associated deposits that include iron oxide p rich iron oxide f and ree rich fe or cu au skarn high grade iron oxide hosted au cu carbonatite hosted cu ree and f rich and iocg sensu stricto deposits consideration of this broad group as a whole obscures the critical features of the iocg sensu stricto deposits such as their temporal distribution and tectonic environment thus leading to difficulties in developing a robust exploration model the iocg sensu stricto deposits are magmatic hydrothermal deposits that contain economic cu and au grades are structurally controlled commonly contain significant volumes of breccia are commonly associated with presulfide sodic or sodic calcic alteration have alteration and or brecciation zones on a large commonly regional scale relative to economic mineralization have abundant low ti iron oxides and or iron silicates intimately associated with but generally paragenetically older than fe cu sulfides have lree enrichment and low s sulfides lack of abundant pyrite lack widespread quartz veins or silicification and show a clear temporal but not close spatial relationship to major magmatic intrusions these intrusions where identified are commonly alkaline to subalkaline mixed mafic even ultramafic to felsic in composition with evidence for mantle derivation of at least the mafic end members of the suite the giant size of many of the deposits and surrounding alteration zones the highly saline ore fluids and the available stable and radiogenic isotope data indicate release of deep volatile rich magmatic fluids through devolatization of causative mantle derived magmas and variable degrees of mixing of these magmatic fluids with other crustal fluids along regional scale fluid flow paths precambrian deposits are the dominant members of the iocg group in terms of both copper and gold resources the 12 iocg deposits with 100 tonnes t resources are located in intracratonic settings within about 100 km of the margins of archean or paleoproterozoic cratons or other lithospheric boundaries and formed 100 to 200 m y after supercontinent assembly their tectonic setting at formation was most likely anorogenic with magmatism and associated hydrothermal activity driven by mantle underplating and or plumes limited amounts of partial melting of volatile rich and possibly metal enriched metasomatized early precambrian subcontinental lithospheric mantle sclm fertilized during earlier subduction probably produced basic to ultrabasic magmas that melted overlying continental crust and mixed with resultant felsic melts with devolatilization and some penecontemporaneous incorporation of other lower to middle crustal fluids to produce the iocg deposits preservation of near surface deposits such as olympic dam is probably due to their formation above buoyant and refractory sclm which resisted delamination and associated uplift most precambrian iron oxide p rich or magnetite apatite kiruna type deposits have a different temporal distribution apparently forming in convergent margin settings prior to or following supercontinent assembly it is only in the phanerozoic that iocg and magnetite apatite deposits are roughly penecontemporaneous in convergent margin settings the phanerozoic iocg deposits such as candelaria chile occur in anomalous extensional to transtensional zones in the coastal cordillera which are also the sites of mantle derived mafic to felsic intrusions that are anomalous in an andean context this implies that special conditions possibly detached slabs of metasomatized sclm may be required in convergent margin settings to generate world class iocg deposits it is likely that formation of giant iocg deposits was mainly a precambrian phenomenon related to the extensive mantle underplating that impacted on buoyant metasomatized sclm generally smaller and rarer phanerozoic iocg deposits formed in tectonic settings where conditions similar to those in the precambrian were replicated

the use of multifrequency and polarimetric sir c x sar data in geologic studies of bir safsaf egypt abstract bir safsaf within the hyperarid core of the sahara in the western desert of egypt was recognized following the sir a and sir b missions in the 1980s as one of the key localities in northeast africa where penetration of dry sand by radar signals delineates previously unknown sand buried paleodrainage valleys radar rivers of middle tertiary to quaternary age the bir safsaf area was targeted as a focal point for further research in sand penetration and geologic mapping using the multifrequency and polarimetric sir c x sar sensors analysis of the sir c x sar data from bir safsaf provides important new information on the roles of multiple sar frequency and polarimetry in portraying specific types of geologic units materials and structures mostly hidden from view on the ground and on landsat tm images by a relatively thin but extensive blanket of blow sand basement rock units granitoids and gneisses and the fractures associated with them at bir safsaf are shown here for the first time to be clearly delineated using c and l band sar images the detectability of most geologic features is dependent primarily on radar frequency as shown for wind erosion patterns in bedrock at x band 3 cm wavelength and for geologic units and sand and clay filled fractures in weathered crystalline basement rocks at c band 6 cm and l band 24 cm by contrast quaternary paleodrainage channels are detectable at all three radar frequencies owing among other things to an usually thin cover of blow sand the sir c x sar data investigated to date enable us to make specific recommendations about the utility of certain radar sensor configurations for geologic and paleoenvironmental reconnaissance in desert regions

crustal anatomy of a transform continental margin abstract this paper describes new deep seismic reflection results across the major transform margin southwest of the grand banks off eastern canada an interpretation of these results is presented supplemented with previous seismic refraction gravity and magnetic measurements the results show a fairly sharp transition between oceanic and continental regions at the transform margin a narrow zone about 40 km wide separates the two regions and is interpreted to be a zone in which shearing has destroyed the original fabric of the continental crust faults in this region extend throughout the crust and there is some evidence that a major strike slip fault may extend some tens of kilometres into the upper mantle magmatism may have further modified the crust in this shear zone which is bounded on the oceanic side by a small seamount the continental crust is thinned by a factor of 3 or more in this zone which may be due to erosion or to flow in the lower crust during shearing the oceanic crust does not appear to thin significantly as the transform margin is approached crustal scale dipping reflectors over a confined region below the shelf are interpreted to represent the contact between two appalachian terranes the meguma and avalon terranes this supports earlier interpretations of a large magnetic lineament in the region as marking this boundary these crustal seismic data crossing a large transform margin are the first of their kind and will help us to constrain evolutionary models of transform margins the picture they provide is different in many respects from that across rifted margins and similar in others in some cases it might be difficult to distinguish between these two classes of atlantic type margins on the basis of crustal structure

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

evidence for transform margin evolution from the ivory coast ghana continental margin the relative motions between lithospheric plates can be reduced to three major types convergence divergence and transform motion convergence leads to an active margin and eventual collision divergence results in the construction of a rifted passive margin transform motion generates a transform or sheared margin although passive margins have been extensively studied1 2 and many models have been proposed for their origin and subsequent evolution3 5 little is known about transform margins with the exception of a few studies of their morphologies6 7 shallow structures or crustal sections8 9 here we present the main conclusions derived from a recent study of the northern gulf of guinea margins particularly off the eastern ivory coast and ghana where the continental margin is one of the best preserved examples of an extinct transform margin10 12 our observations support a four stage model for transform margin evolution technically active transform contacts first between normal continental crusts and then between thinned margins induce characteristic structures such as pull apart grabens and shear folds the next stage in which thermal exchange between oceanic and continental lithos pheres controls a complex subsidence is followed by the transition to a true intra oceanic fracture zone

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

crustal structure at the blake spur fracture zone from expanding spread profiles we present results from wkbj and reflectivity synthetic seismogram modeling of 10 reversed expanding spread profiles esps along flow lines parallel to the blake spur fracture zone across 140 ma atlantic crust these profiles provide detailed constraints on variations in crustal structure at the fracture zone seven profiles on either side of the fracture zone show a normal structure for old oceanic crust a 2 3 km thick upper layer with a steep velocity gradient is underlain by 4 5 km of crust with velocities of 6 5 7 2 km s and low gradient and a sharp transition to upper mantle velocities of 8 km s several velocity discontinuities were detected within the upper 2 3 km but these do not generally coincide with intracrustal reflectors detected by simultaneous normal incidence reflection profiles this structure shows little regional variation toward the fracture zone despite the small magnetic anomaly offset 12 km and the indistinct topographic signature of the fracture zone three esps within a 10 20 km wide ribbon centered on the fracture zone trough show clearly anomalous crustal structure relative to normal oceanic crust a 2 4 km thick high gradient upper layer is underlain by a thick prism of material with a velocity of 7 2 7 6 km s and high poisson s ratio probably at least 0 29 which is consistent with 15 30 serpentinization of upper mantle peridotites this interpretation requires the action of off axis hydrothermal circulation penetrating the cracked and relatively permeable fracture zone lithosphere to a depth of at least 7 km the original igneous crust in a narrow region close to the fracture zone is thus inferred to have been much thinner than adjacent normal crust which may imply a sharp reduction in the magma budget at the ends of the adjacent spreading segments

crustal structure of the french guiana margin west equatorial atlantic summary geophysical data from the amazon cone experiment are used to determine the structure and evolution of the french guiana and northeast brazil continental margin and to better understand the origin and development of along margin segmentation a 427 km long combined multichannel reflection and wide angle refraction seismic profile acquired across the southern french guiana margin is interpreted where plate reconstructions suggest a rift type setting the resulting model shows a crustal structure in which 35 37 km thick pre rift continental crust is thinned by a factor of 6 4 over a distance of 70 km associated with continental breakup and the initiation and establishment of seafloor spreading the ocean continent boundary is a transition zone up to 45 km in width in which the two layered oceanic type crustal structure develops although relatively thin at 3 5 5 0 km such thin oceanic crust appears characteristic of the margin as a whole there is no evidence of rift related magmatism either as seaward dipping sequences in the reflection data or as a high velocity region in the lower crust in the p wave velocity model and as a such the margin is identified as non volcanic in type however there is also no evidence of the rotated fault block and graben structures characteristic of rifted margins consequently the thin oceanic crust the rapidity of continental crustal thinning and the absence of characteristic riftrelated structures leads to the conclusion that the southern french guiana margin has instead developed in an oblique rift setting in which transform motion also played a significant role in the evolution of the resulting crustal structure and along margin segmentation in structural style

from continental extension to seafloor spreading crustal structure of the goban spur rifted margin southwest of the uk we present a 169 km wide angle velocity model across the goban spur rifted margin southwest of the uk a 120 km wide intermediate region is identified between the first clear seafloor spreading magnetic anomaly anomaly 34r and thinned continental crust where velocities increase from 4 5 km s1 to 6 8 km s1 in the top 4 km beneath acoustic basement at depth it can be divided into a region where a 1 5 km thick high velocity layer 7 2 7 6 km s1 exists and a region where this layer is absent but velocities of 7 km s1 are present wide angle pmp arrivals are observed across the whole of the intermediate region but a normal incidence reflection moho is not present based on these velocities and combined with seismic reflection gravity and magnetic modelling along this transect we interpret the intermediate region to consist of a 70 km wide zone of exhumed mantle with high poisson s ratio at top basement 0 34 an extremely low topographic expression and a high velocity deep layer empirical relationships between velocity and degree of serpentinization suggest that serpentinite content decreases with depth from 100 per cent at top basement to 25 per cent from 5 7 km into basement the observed magnetic anomaly is best fit by a thin magnetized layer 1 km of magnetization 2 3 a m1 the intense magnetization may be due to magnetite formation during a prolonged interaction of serpentinite with sea water between this exhumed mantle and anomaly 34r is a 50 km wide region raised 400 m above the adjacent exhumed mantle consisting of a series of basement ridges poisson s ratio is not well defined in this region which may be composed of anomalous oceanic crust or exhumed mantle

structure and evolution of the eastern gulf of aden insights from magnetic and gravity data encens sheba md117 cruise summary magnetic and gravity data gathered during the encens sheba cruise 2000 june in the eastern gulf of aden provide insights on the structural evolution of segmentation from rifted margins to incipient seafloor spreading in this study we document the conjugate margins asymmetry confirm the location of the ocean continent transition oct previously proposed by seismic data and describe its deep structure and segmentation in the oct gravity models indicate highly thinned crust while magnetic data indicate presence of non oceanic high amplitude magnetic anomalies where syn rift sediments are not observed thus the oct could be made of ultra stretched continental crust intruded by magmatic bodies however locally in the north the nature of the oct could be either an area of ultra slow spreading oceanic crust or exhumed serpentinized mantle between the alula fartak and socotra fracture zones the non volcanic margins and the oct are segmented by two n027 e trending transfer fault zones these transfer zones define three n110 e trending segments that evolve through time the first evidence of oceanic spreading corresponds to the magnetic anomaly a5d and is thus dated back to 17 6 ma at least reconstruction of the spreading process suggests a complex non uniform opening by an arc like initiation of seafloor spreading in the oct the early segmentation appears to be directly related to the continental margin segmentation the spreading axis segmentation evolved from three segments 17 6 to 10 95 ma to two segments 10 95 ma to present at the onset of the spreading process the western segment propagated eastwards thus reducing the size of the central segment the presence of a propagator could explain the observed spreading asymmetry with the northern flank of the sheba ridge being wider than the southern one

the ocean continent boundary off the western continental margin of iberia crustal structure west of galicia bank a seismic refraction transect across the galicia bank continental margin shows that the original continental crust thins westward from 17 to 2 km immediately east of a margin parallel peridotite ridge pr immediately west of the pr oceanic crust is only 2 5 3 5 km thick but farther west oceanward it thickens to 7 km the pr caps a similar to 60 km wide lens shaped serpentinized peridotite body underlying both thinned continental and thin oceanic crust when superimposed on a reflection time version of the velocity model the s reflector is clearly intracrustal at its east end westward s cuts down to lower crustal levels eventually coinciding with the top of the serpentinized peridotite lens original crust mantle boundary these observations render almost impossible the seafloor exposure of the pr by s acting as a top to the west detachment fault numerical models of melting and borehole subsidence information constrain our rifting model the easternmost continental crust experienced a total stretching factor of 4 3 most likely in two stages it probably occurred over similar to 25 m y with the highest fate of stretching at the beginning of the main earlier rift phase valanginian 141 135 ma the 3 4 7 km thick continental crust depending on whether serpentinized peridotite is assigned to crust or mantle which may include melt products requires stretching factors 6 more than 11 7 and a rift duration of more than 25 13 m y the thin oceanic crust immediately west of the pr is explained by conductive cooling of the mantle during the long prebreakup stretching phase which temporarily caused reduced melting immediately after breakup

the structure of the rockall trough imaged by deep seismic reflection profiling westline is a 450 km long near normal incidence seismic reflection profile crossing the rockall trough from the rockall bank to the irish coast beneath rockall bank and the irish shelf unreflective upper crust overlies reflective lower crust and upper mantle crustal reflectors beneath the irish shelf are interpreted as caledonian the nw slope of the trough is dissected by a series of east dipping faults forming small sediment filled half grabens the steep se slope is formed by the erris ridge the floor of the trough is formed from rotated blocks separated by mainly east dipping faults wedges of sediment fill half grabens formed by this faulting there is no evidence for oceanic crust or a 9reflection moho9 within the trough the floor and slopes of the trough are blanketed by c 4 km of parallel bedded sediments the lower part of the parallel bedded sequence is intruded by sills and dykes that do not extend above a minor unconformity it is suggested that the bulk of the sediment preserved in the rockall trough accumulated during and following a major rifting event in the early or mid cretaceous

crustal geometry of the northeastern gulf of aden passive margin localization of the deformation inferred from receiver function analysis summary here we use receiver function analysis to retrieve crustal thickness and crustal composition along the 35 my old passive margin of the eastern gulf of aden our aims are to use results from the 3 d seismic array to map crustal stretching across and along the aden margin in southern oman the array recorded local and teleseismic events between 2003 march and 2004 march seventy eight events were used in our joint inversions for vp vs ratio and depth the major results are 1 crustal thickness decreases from the uplifted rift flank of the margin towards the sheba mid ocean ridge we found a crustal thickness of about 35 km beneath the northern rift flank this value decreases sharply to 26 km beneath the post rift subsidence zone on the salalah coastal plain this 10 km of crustal thinning occurs across a horizontal distance of less than 30 km showing a localization of the crustal thinning below the first known rifted block of the margin 2 a second rift margin transect located about 50 km to the east shows no thinning from the coast to 50 km onshore the lack of crustal thickness variation indicates that the maximum crustal stretching could be restricted to offshore regions 3 the along strike variations in crustal structure demonstrate the scale and longevity of the regular along axis rift segmentation 4 extension is still observed north of the rifted domain 70 km onshore from the coast making the width of margin larger than first expected from geology 5 the crust has a felsic to normal composition with a probably strong effect of the sedimentary layer on the vp vs ratio comprised between 1 67 and 1 91

geophysical evidence for a transform margin offshore western algeria a witness of a subduction transform edge propagator for the first time a deep seismic data set acquired in the frame of the algerian french spiral program provides new insights regarding the origin of the westernmost algerian margin and basin we performed a tomographic inversion of traveltimes along a 100 km long wide angle seismic profile shot over 40 ocean bottom seismometers offshore mostaganem northwestern algeria the resulting velocity model and multichannel seismic reflection profiles show a thin 3 4 km thick oceanic crust the narrow ocean continent transition less than 10 km wide is bounded by vertical faults and surmounted by a narrow almost continuous basin filled with miocene to quaternary sediments this fault system as well as the faults organized in a negative flower structure on the continent side marks a major strike slip fault system the extremely sharp variation of the moho depth up to 45 3 degrees beneath the continental border underscores the absence of continental extension in this area all these features support the hypothesis that this part of the margin from oran to tenes trending n65 n70 degrees e is a fossil subduction transform edge propagator fault vestige of the propagation of the edge of the gibraltar subduction zone during the westward migration of the alboran domain

crustal structure of the central nova scotia margin off eastern canada summary the central nova scotia margin off eastern canada is located at a transition from a volcanic margin in the south to a non volcanic margin in the north in order to study this transition a wide angle refraction seismic line with dense airgun shots was acquired across the central nova scotia margin the 500 km long transect is coincident with previous deep reflection profiles across the lahave platform and extending into the sohm abyssal plain a p wave velocity model was developed from forward and inverse modelling of the wide angle data from 21 ocean bottom seismometers and coincident normal incidence reflection profiles the velocity model shows that the continental crust is divided into three layers with velocities of 5 5 6 9 km s 1 the maximum thickness is 36 km a minor amount 5 km of thinning occurs beneath the outer shelf while the major thinning to a thickness of 8 km occurs over the slope region the seaward limit of the continental crust consists of 5 km thick highly faulted basement there is no evidence for magmatic underplating beneath the continental crust on the contrary a 4 km thick layer of partially serpentinized mantle 7 6 7 95 km s 1 begins beneath the highly faulted continental crust and extends 200 km seawards forming the lower part of the oceancontinent transition zone the upper part of the transition zone consists of the highly faulted continental crust and 4 to 5 km thick initial oceanic crust the continent ocean boundary is moved 50 km farther seawards compared to an earlier interpretation based only on reflection seismic data the oceanic crust in the transition zone consists of layer 2 and a high velocity lower crustal layer layer 2 is 1 3 km thick with velocities of 5 6 6 0 km s 1 the high velocity lower crustal layer is 1 2 km thick with velocities of 7 25 7 4 km s 1 suggesting a composite layer of serpentinized peridotite and gabbroic layer 3 oceanic crust with normal thickness of 5 7 km and more typical layer 3 with velocities of 6 95 7 3 km s 1 is observed at the seaward end of the profile

characterizing the galicia bank southern iberia abyssal plain rifted margin segment boundary using multichannel seismic and ocean bottom seismometer data 1 we present multichannel seismic reflection and ocean bottom seismometer reflection refraction data from ise 9 a margin parallel north south oriented profile 200 km west of the portuguese coast ise 9 images the boundary between two distinct segments of the iberia nonvolcanic rifted margin galicia bank gb and the southern iberia abyssal plain siap the bathymetric contrast between gb 2 km depth and siap 4 5 km depth spans only 25 km the crustal thickness transition however spans 137 km from 13 18 km thick beneath gb to 2 km thick beneath siap we define this crustal thickness transition as the segment boundary crustal structure along the segment boundary tilted blocks bounded by normal faults is surprisingly similar to crustal structure observed along orthogonal east west profiles of the iberia margin the apparent north south extension is similar in magnitude to previously calculated east west extension implying an overall northeast southwest extension however paleoreconstructions and rift basin orientations constrain lithospheric extension to a nearly east west direction we speculate that north south extension is limited to the crust and is caused by large scale mass wasting sometime between the tithonian and valanginian this rotational slump spans the 137 km wide modern segment boundary emplacing gb continental crust directly onto exhumed serpentinized mantle of siap palinspastic reconstruction restores the southern edge of the blocks to coincide within 10 km of a steep moho transition and a near vertical fault which extends through the slump and offsets moho we interpret this location as the preslump segment boundary that accommodated transform motion between the two rifted margin segments

crustal structure across the southwest newfoundland transform margin a seismic refraction survey providing deep crustal structure information of the continent ocean boundary across the south west newfoundland transform margin was carried out using large air gun sources and ocean bottom seismometer receivers continental crust 30 km thick beneath the southern grand banks p wave velocity 6 2 6 5 km s thins oceanward to a 25 km wide transition zone in the transition zone paleozoic basement of the grand banks 5 5 5 7 km s is replaced by a basement of oceanic volcanics and synrift sediments 4 5 5 5 km s seaward of the transition zone the crust is oceanic in character with a velocity gradient from 4 7 to 6 5 km s and a thickness of 7 8 km oceanic layer 3 is absent no significant thickness of intermediate velocity 7 km s material is present at the continent ocean transition indicating that no under plating of continental crust has taken place the continent ocean transition across the transform margin is much narrower than across rifted margins supporting the

recent rapid uplift in the bolivian andes evidence from fission track dating apatite and zircon fission track cooling ages constrain the tertiary cooling and uplift history of the eastern cordillera and altiplano of bolivia fission track data are from two triassic plutons and surrounding paleozoic metasedimentary rocks in the eastern andes north of la paz zircon cooling ages indicate that the roof of the huayna potosi pluton was emplaced in the zircon partial annealing zone and that the zongo pluton was emplaced entirely in the zircon total annealing zone apatite cooling ages for both plutons record uplift in the past 5 15 m y and zircon cooling ages from the zongo pluton reflect uplift in the past 25 45 m y uplift rates calculated by these apatite and zircon cooling ages suggest that uplift rates were 0 1 0 2 mm yr between 20 and 40 ma and increased significantly between 10 and 15 ma by 3 ma uplift rates may have been as high as 0 7 mm yr

transform marginal plateaus abstract numerous submarine plateaus form highstanding bathymetric highs at continent to ocean transitions due to their proximity to continents they have been frequently labelled marginal plateaus although this term has not been clearly defined or associated with a specific geology or geodynamic process until now these elevations have been interpreted as submerged thinned continental fragments detached from continents basaltic buildups formed by hotspots volcanic margins or oceanic plateaus many of these plateaus formed at transform margins connecting oceanic basins of contrasted ages we propose for the first time to define and review a class of marginal plateaus related to a specific tectonic setting transform marginal plateaus tmps based on a compilation of 20 tmps around the world we show that most of them have a polyphased history and have undergone at least one major volcanic phase our review highlights in particular a hitherto unrecognized close link between hotspots volcanic activity and transform margins we also propose that due to their polyphased history tmps may contain several successive basins and overlooked long lived sedimentary archives we finally highlight that because these tmps were transform plate boundaries perpendicular or oblique to surrounding rifts many of them were close to last contact points during final continental breakup and may have formed land bridges or bathymetric highs between continents therefore we discuss broader scientific issues such as the interest of tmps in recording and studying the onset and variations of oceanic currents or past biodiversity growth bio connectivity and lineage evolution

lateral dispersion over the continental shelf analysis of dye release experiments lateral dispersion over the continental shelf was examined using dye studies performed as a part of the coastal mixing and optics experiment four experiments performed at intermediate depths each lasting 2 5 5 days were examined in some cases the dye patches remained fairly homogeneous both vertically and horizontally throughout an experiment in other cases significant patchiness was observed on scales ranging from 2 to 10 m vertically and a few hundred meters to a few kilometers horizontally the observations showed that the dye distributions were significantly influenced by shearing and straining on scales of 5 10 m in the vertical and 1 10 km in the horizontal superimposed on these larger scale distortions were simultaneous increases in the horizontal second moments of the dye patches with corresponding horizontal diffusivities based on a fickian diffusion model of 0 3 4 9 m 2 s 1 analysis of the dye data in concert with shear estimates from shipboard acoustic doppler current profiler observations showed that the existing paradigms of shear dispersion and dispersion by interleaving water masses cannot account for the observed diffusive spreading of the dye patches this result suggests that some other mechanisms provided an additional diffusivity of order 0 2 4 6 m 2 s 1 an alternative mechanism dispersion by vortical motions caused by the relaxation of diapycnal mixing events may explain the observed dispersion in some cases

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

crustal structure of the goban spur rifted continental margin ne atlantic summary we present results that constrain the crustal structure across a typical non volcanic rifted continental margin the goban spur in the ne atlantic traveltime and amplitude modelling of wide angle ocean bottom seismograph profiles shows that the continental crust thins in a seaward direction to about 7 km before it breaks to allow a new oceanic spereading centre to develop the crustal velocity structure the stretching factors and the density distribution inferred from gravity anomalies are consistent with bulk pure shear stretching for the development of the rifted continental margin the seismic velocity structure of the thinned continental crust suggests that thinning was accompanied by only limited igneous intrusion and extrusion in the upper crust the adjacent oceanic crust is abnormally thin 5 4km this is explained most readily by accretion at a very slow spreading centre approx 11 mm yr full rate although additional cooling by conductive heat loss from the parent mantle as it rose slowly beneath the stretched and thinned continental lithosphere together with minor igneous intrusion into and surface flows onto the adjacent continental crust may also have reduced the oceanic igneous thickness immediately adjacent to the continent ocean transition

petrology of the alkaline core of the messum igneous complex namibia evidence for the progressively decreasing effect of crustal contamination the messum complex of nw namibia a part of the parana introduction etendeka volcanic province consists of a dominantly felsic central the etendeka igneous province in the damaraland core surrounded by older gabbros the igneous rocks of the core region of northwest namibia fig 1 contains numerous can be divided in order of decreasing age into 1 a sub alkaline anorogenic igneous complexes of mesozoic age martin suite 2 an outer quartz syenite suite and 3 an inner silicaet al 1960 piranjo 1994 one of which is the messum undersaturated suite dominated by nepheline syenite compositional complex the range in chemical composition both within differences within the quartz syenite suite can be explained by and between the various complexes is remarkable and fractional crystallization but srand o isotope data indicate that includes both silica oversaturated and silica underthese rocks contain a significant crustal component the younger saturated mafic and felsic rocks including carbonatite central nepheline syenites trend towards the 0 1 gpa phonolite messum was classified as a mixed complex by harris minimum and appear to have evolved by closed system fractional 1995 as it contains substantial amounts of both silicacrystallization of a mantle derived magma whereas earlier nepheline undersaturated and silica oversaturated rock types syenites appear to be contaminated with quartz syenite the dating of several damaraland complexes yields ages decreasing effects of crustal contamination with time may reflect that lie between 137 and 124 ma siedner miller progressive armouring of the magmatic plumbing system by less 1968 milner et al 1995b renne et al 1996 the contaminated material basanite dykes that intrude the plutonic alignment of the complexes along two parallel ne sw rock of the core show trace element and ndand sr isotope trends suggests that they represent a hotspot trace caused characteristics that are similar to those of basanites from tristan by the motion of the african plate over a fixed mantle da cunha this suggests that the mantle derived component of the plume e g rhodes 1971 because there is no systematic age progression along the lineament this view has genmessum core rocks came from the tristan plume erally been discounted marsh 1973 recent ideas that large continental flood basalt cfb provinces such as the parana etendeka province are manifestations of mantle plume activity white mackenzie 1989 has refocused attention on the relationship between plumes and linear arrays of complexes in cfb provinces milner

the circular uneged uul structure east gobi basin mongolia geomorphic and structural evidence for meteorite impact into an unconsolidated coarse clastic target abstract the uneged uul structure is a 10 km circular complex multi ridged domal feature in the unegt subbasin of the east gobi basin southeastern mongolia as revealed by remote sensing and recent field reconnaissance the central part of the uneged uul structure comprises a complex central peak of outward radiating curved ridges composed of stratigraphically uplifted greenschist facies basement schists surrounded by an annular moat the most prominent feature of the structure is a central annular ridge 3 km in diameter composed of pebble boulder conglomerates and gravels of the upper jurassic sharilyn formation surrounded by three outer domal ridges composed of lower cretaceous conglomeratic sandstones and gypsum clays jurassic conglomerates forming the main part of the central annular ridge show effects of severe internal deformation the original population of pebbles cobbles and boulders appears moderately displaced and mostly broken but nowhere aligned along shear planes or foliated primary sedimentary features such as cross lamination or imbrication have been obliterated we explain this penetrative brecciation as a result of dissipative shearing caused by a strong and rapid singular event that in magnitude was beyond the range of the common crustal tectonics recorded elsewhere in this region disrupted and chaotically distributed conglomeratic sandstone beds in the central annular ridge dip in highly variable directions on a local scale but show an apparent se nw trend of bedding plane alignment further outside the tilted and uplifted upper jurassic to lower cretaceous strata of the domal area are overlain by the flat lying upper cretaceous which stratigraphically constrains the timing of deformation at the uneged uul structure to most likely the early cretaceous endogenic formation models such as magmatism and salt gypsum or mud diapirism fail to explain the nature of the uneged uul structure the uneged uul structure bears a set of geomorphic and structural features resembling those at some eroded complex impact structures on earth morphologically similar central peaks are observed at the spider and matt wilson impact structures in australia the central annular ridge reminds of that at gosses bluff in australia the outer domal ridges might correspond to ring like features as known from tin bider in algeria we therefore cautiously propose that an impact may have produced the uneged uul feature causing structural uplift 1000 m of basement rocks at its center so far no convincing evidence for shock metamorphism could be proven by field work and petrographic analyses however it is likely that at the time of the deformation event the unconsolidated conglomerates were highly porous and possibly immersed in groundwater buffering the propagation of sudden stress reducing deformation further studies will be in order to unravel the nature of the uneged uul structure which should be considered a promising possible impact structure

the crustal structure across the transform continental margin off ghana eastern equatorial atlantic forward modeling of a suite of wide angle seismic lines across the transform continental margin at the eastern end of the romanche fracture zone off ghana has shown the transition from continental to oceanic crust to be confined to a narrow 6 to 11 km wide zone located at the foot of the steep continental slope the structure of the adjacent oceanic and continental crusts has also been resolved these results are confirmed and enhanced by gravity and magnetic models the crust of the ocean continent transition zone is characterized by high velocities 5 8 7 3 km s 1 a high density 3 10 mg m 3 and high magnetizations 1 1 1 25 a m 1 this rules out a purely continental origin for the zone and suggests that it may be formed of basic igneous rocks intruded when the hot oceanic spreading center migrated along the margin the oceanic crust within 70 km of the margin is abnormally thin compared to normal atlantic oceanic crust and shows an average thickness of just 4 4 km we suggest that the region of abnormally thin oceanic crust is the result of a reduced magma supply due to a combination of closely spaced fracture zones and the conductive loss of heat from the upwelling oceanic mantle in small basins surrounded on at least three sides by cold continental lithosphere the preferred seismic model also shows a new fracture zone 30 km southeast of the ocean continent transition which is characterized by low velocities in the upper crust a lens shaped layer with velocities of 7 2 7 4 km s 1 at the base of the crust and a crustal thickness of just 3 4 km the continental crust appears largely unaffected by the proximity of the adjacent oceanic crust there is no evidence for underplating of the continental crust adjacent to the ocean continent transition zone

evidence for unusually thin oceanic crust and strong mantle beneath the amazon fan we used seismic and gravity data to determine the structure of the crust and mantle beneath the amazon fan seismic data suggest that the crust is of oceanic type and is unusually thin 4 km compared to elsewhere in the atlantic we attribute the thin crust to ultraslow seafloor spreading following the breakup of south america and africa during the early cretaceous gravity data suggest that the fan was emplaced on lithosphere that in creased its elastic thickness te and hence strength following rift ing the increase from 10 km to 40 km is greater however than would be expected if te were determined by a single controlling isotherm based on a cooling plate model hence we conclude that the amazon fan has been emplaced on and is supported by un usually thin oceanic crust and strong mantle

endogenous and nonimpact origin of the arkenu circular structures al kufrah basin se libya the twin arkenu circular structures acs located in the al kufrah basin in southeastern libya were previously considered as double impact craters the arkenu craters the acs consist of a ne arkenu 1 and a sw structure arkenu 2 with approximate diameters of about 10 km they are characterized by two shallow depressions surrounded by concentric circular ridges and silica impregnated sedimentary dikes cut by local faults our field petrographic and textural observations exclude that the acs have an impact origin in fact we did not observe any evidence of shock metamorphism such as planar deformation features in the quartz grains of the collected samples and the previously reported shatter cones are wind erosion features in sandstones ventifacts conversely the acs should be regarded as a paired intrusion of porphyritic stocks of syenitic composition that inject the nubia formation and form a rather simple and eroded ring dike complex stock emplacement was followed by hydrothermal activity that involved the deposition of massive magnetite hematite horizons typical of iron oxide copper gold deposits their origin was nearly coeval with the development of silicified dikes in the surroundings plugs of tephritic phonolitic rocks and lamprophyres monchiquites inject the nubian sandstone along conjugate fracture zones trending nnw sse and ne sw that crosscut the structural axis of the basin

distribution timing and causes of andean deformation across south america the andean orogeny in south america has lasted over 100 ma it comprises the peruvian incaic and quechuan phases the nazca and south american plates have been converging at varying rates since the palaeocene the active tectonics of south america are relatively clear from seismological and global positioning system gps data horizontal shortening is responsible for a thick crust and high topography in the andes as well as in se brazil and patagonia we have integrated available data and have compiled four fault maps at the scale of south america for the mid cretaceous late cretaceous palaeogene and neogene periods andean compression has been widespread since the aptian the continental margins have registered more deformation than the interior for the peruvian phase not enough information is available to establish a tectonic context during the incaic phase strike slip faulting was common during the quechuan phase crustal thickening has been the dominant mode of deformation to investigate the mechanics of deformation we have carried out 10 properly scaled experiments on physical models of the lithosphere containing various plates the dominant response to plate motion was subduction of oceanic lithosphere beneath continental south america however the model continent also deformed internally especially at the margins and initial weaknesses

from rifting to spreading in the eastern gulf of aden a geophysical survey of a young oceanic basin from margin to margin a geophysical survey in the eastern gulf of aden between the alula fartak 52 e and the socotra 55 e transform faults was carried out during the encens sheba cruise the conjugate margins of the gulf are steep narrow and asymmetric asymmetry of the rifting process is highlighted by the conjugate margins horst and graben in the north and deep basin in the south two transfer fault zones separate the margins into three segments whereas the present day sheba ridge is divided into two segments by a transform discontinuity therefore segmentation of the sheba ridge and that of the conjugate margins did coincide during the early stages of oceanic spreading extensive magma production is evidenced in the central part of the western segment anomaly 5d was identified in the northern and southern parts of the oceanic basin thus confirming that seafloor spreading in this part of gulf of aden started at least 17 6 ma ago

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

spaceborne radar subsurface imaging in hyperarid regions imaging data acquired with the shuttle imaging radar sir a over the hyperarid region of egypt sudan clearly show surface penetration through the sand cover even though absorption does occur in the sand layer surface refraction leads to a steeper incidence angle at the sand bedrock interface resulting in a stronger backscatter a simple backscatter model shows that for a low loss thin sand layer the presence of the covering layer enhances the capability to image the subsurface interface particularly at large incidence angles and hh polarization

new field evidence bearing on the origin of the el laco magnetite deposit northern chile a reply the el laco magnetite deposit has been interpreted as lava flows and feeder dikes formed from iron oxide magma but more recently as a product of metasomatic replacement open pit exposure created at laco sur during the 1990s reveals that the massive magnetite contains magnetite veined blocks and smaller fragments of altered andesitic volcanic rock clearly supporting the replacement origin open space growth of magnetite and pyroxene along the walls of chimney structures and veins within magnetite indicates precipitation from aqueous fluid rather than from gas alone the native sulfur bearing cristobalite alunite alteration widespread at el laco and broadly contemporaneous with magnetite formation is recognized as part of a steam heated horizon generated in the vadose zone above a paleowater table consequently the magnetite replacement must have taken place beneath the paleowater table but probably 300 m beneath the paleosurface thus magnetite deposition at or immediately beneath the paleosurface is precluded the main outstanding question is how hot hypersaline brine the probable ore forming fluid at el laco attained such shallow crustal levels within a high volcanic edifice without undergoing the extensive groundwater dilution documented from most shallowly preserved hydrothermal systems

mode of extension and rifting history of upper tiburon and upper delfin basins northern gulf of california 1 the crustal structure of the northern gulf of california transtensional margin has been investigated by a 280 km long nw se profile including deep multichannel seismic reflection and densely sampled refraction wide angle reflection seismic information combined with gravity modeling the seismic and gravity modeling constrains two thinned crustal areas corresponding to the upper delfin and the upper tiburon basins on both sides of the profile toward the baja california peninsula and the mexico mainland a progressive thickening of the continental crust is observed our results indicate that the crustal thickness is 19 km below the coastline and it decreases to 14 and 17 km below the upper delfin and upper tiburon basins respectively in the area between both basins the crust thickens to 19 5 km there are significant lateral thickness variations for the different levels of the crust the interpreted structure is consistent with the existence of an aborted rift below the upper tiburon basin prominent dipping reflections in the multichannel data under upper tiburon basin and the ridge between upper tiburon and upper delfin basins can be explained as a mylonite like zone related to a detachment fault this interpretation suggests that the structural evolution of upper tiburon basin could be controlled by a major fault that cuts through the upper crust and merges into a zone of subhorizontal reflections in the lower crust the mode and locus of extension have evolved from a core complex in upper tiburon to a narrow rift mode in upper delfin basin

structure and variability of oceanic crust on the flanks of the east pacific rise between 11 and 13 n we use over 500 seismograms collected by five ocean bottom seismometers to examine the structure and variability of 0 5 m y old crust in the rivera ocean seismic experiment rose area of the east pacific rise between 11 and 13 n there are no significant differences among the first arrival travel times for 10 independent refraction line segments along the isochron amplitude patterns recorded on the individual lines however suggest that structural boundaries within the upper crust vary by hundreds of meters in depth over lateral distances of tens of kilometers within this 200 km segment of lithosphere on the basis of the modeling of amplitude patterns at ranges of 5 10 km and of comparisons with the upper crust sampled at deep sea drilling project hole 504b and in ophiolite complexes layer 2 can be divided into three units 2a corresponds to extrusive volcanics with a steep velocity gradient 2b to extrusives with a near zero velocity gradient and 2c to a complex region of transition from extrusives to sheeted dikes the total thickness of layer 2 varies by about 0 7 km over the length of the line another amplitude peak which varies in range between 14 and 21 km may be related to the downward transition from sheeted dikes to isotropic gabbros within layer 3 the variability of these two amplitude patterns provides direct evidence that the along strike structure of oceanic crust in the rose area is definably heterogeneous and that the process of crustal accretion is likewise variable on the scale resolved by amplitude and travel time information

iron oxide copper gold deposits an andean view iron oxide copper gold iocg deposits defined primarily by their elevated magnetite and or hematite contents constitute a broad ill defined clan related to a variety of tectono magmatic settings the youngest and therefore most readily understandable iocg belt is located in the coastal cordillera of northern chile and southern peru where it is part of a volcano plutonic arc of jurassic through early cretaceous age the arc is characterised by voluminous tholeiitic to calc alkaline plutonic complexes of gabbro through granodiorite composition and primitive mantle derived parentage major arc parallel fault systems developed in response to extension and transtension induced by subduction roll back at the retreating convergent margin the arc crust was attenuated and subjected to high heat flow iocg deposits share the arc with massive magnetite deposits the copper deficient end members of the iocg clan as well as with manto type copper and small porphyry copper deposits to create a distinctive metallogenic signature

geomorphic indicators of holocene winds in egypt s western desert abstract geomorphic mapping of egypt s western desert from landsat mss images reveals oriented aeolian landforms that record in part holocene winds wind directions reconstructed from these landforms indicate the dominance of n s airflow from 30 n to 20 n turning clockwise southward to ne sw conformable with modern circulation a second direction appears over western egypt w between 30 n and 26 n nw between 26 n and 20 n cross cutting aeolian landforms show that w nw winds are older than the n ne winds geomorphic evidence abundant south to 26 n and less abundant to 20 n also indicates that w and nw winds were early holocene palaeowesterlies some evidence also indicates that they extended eastward to at least 30 e perhaps to the red sea these winds steered moist atlantic mediterranean air masses to egypt sustaining early holocene lakes and playas north of the limit of tropical monsoonal rainfall at 20 n upon aridification beginning after 5 kyr bp yardangs oriented west to east were eroded in early holocene basinal sediments in western egypt indicating that these winds continued there for 1 2 kyr until 3 4 kyr bp optically stimulated luminescence osl ages of surface sand sheet in southern egypt indicate that the present north south winds were established ca 3 4 kyr bp at the same time as the northern savanna boundary was stabilized at its present position

shock metamorphism of quartz in nature and experiment ii significance in geoscience the occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies its utility is due to its almost ubiquitous occurrence in terrestrial rocks its overall stability and the fact that a variety of shock metamorphic effects occurring over a range of shock pressures have been well documented these shock effects have been generally duplicated in shock recovery experiments and thus serve as shock pressure barometers after reviewing the general character of shock effects in quartz the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored the short pulse lengths in experiments may account for the difficulty in synthesizing the high pressure polymorphs coesite and stishovite compared to natural occurrences in addition post shock thermal effects are possible in natural events which can affect shock altered physical properties such as refractive index and cause annealing of shock damage and recrystallization the orientations of planar microstructures however are unaffected by post impact thermal events except if quartz is recrystallized and provide the best natural shock barometer in terms of utility and occurrence the nature of planar microstructures particularly planar deformation features pdfs is discussed in some detail and a scheme of variations in orientations with shock pressure is provided the effect of post impact events on pdfs is generally limited to annealing of the original glass lamellae to produce decorated pdfs resulting from the exsolution of dissolved water during recrystallization basal 0001 pdfs differ from other pdf orientations in that they are multiple mechanical brazil twins which are difficult to detect if not partially annealed and decorated the occurrence and significance of shock metamorphosed quartz and its other phases namely coesite stishovite diaplectic glass and lechatelierite are discussed for terrestrial impact structures in both crystalline non porous and sedimentary porous targets the bulk of past studies have dealt with crystalline targets where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures in sedimentary targets the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets this is discussed along with examples of variations in the relative abundances of planar microstructures and their orientations which are attributed to textural variations in sedimentary target rocks examples of the shock metamorphism of quartz in distal ejecta such as at the k t boundary and from nuclear explosions are illustrated and are equivalent to that of known impact structures except with respect to characteristics that are due to long term post shock thermal effects finally the differences between the deformation and phase transformation of quartz by shock and by endogenic tectonic and volcanic processes are discussed we confirm previous conclusions that they are completely dissimilar in character due to the vastly different physical conditions and time scales typical for shock events compared to tectonic and volcanic events well characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment

south atlantic fits and intraplate boundaries in africa and south america abstract the misfit problems raised by the pre drift reconstructions of the south and equatorial atlantic compel us to resort to intraplate deformations it is shown that acceptable deformations of the african plate in line with the benue trough and the transafrican rifts and shear zones provide a far from satisfactory solution to the geometrical misfit problems in the south atlantic these problems can instead be alleviated by considering a degree of deformation of the south american plate along an intraplate boundary in line with the parana province the rio grande rise and the walvis ridge in spite of the scarcity of field observations in this area of south america we show that such a deformation cannot be ruled out but it will have to be associated with deformations in the equatorial atlantic and in other areas of the african and south american plates

lateral reynolds stress and eddy viscosity in a coastal strait abstract the lateral reynolds stress u representing the transfer of along strait momentum toward the sides of juan de fuca strait is measured with an array of adcps the contributions to the stress from a number of frequency bands are analyzed to highlight the roles of different processes motion in a frequency band that includes internal waves and doppler shifted subinertial eddies gives a reynolds stress that when scaled by the observed shear is consistent with an eddy viscosity of o 10 m2 s 1 this viscosity acts on the tides and the estuarine flow the tides can also impart a viscous stress upon the low frequency estuarine flow these tidal currents although strong are largely nondivergent within the area of the array and thus appear to be less important for the cross strait transfer of momentum the long term mean estuarine circulation can be acted upon by meanders of the estuarine flow defined as features with periods of 3 5 days these meanders are found to also have a horizontal eddy visc

segmentation in gravity and magnetic anomalies along the u s east coast passive margin implications for incipient structure of the oceanic lithosphere segmentation is a characteristic feature of seafloor spreading along the global mid ocean ridge system while segmentation of active spreading centers has been the focus of much recent research the process by which a rifted continental margin develops into a segmented mid ocean ridge is still poorly understood in this study we investigate the segmentation character of the u s east coast margin through a modeling study of the margin basement structure magnetics and gravity anomalies the east coast margin is of particular interest because it is one of several rifted continental margins that display thick sequences of high seismic velocity igneous crust presumably formed during rifting the east coast magnetic anomaly ecma a distinct total field magnetic high running offshore along the margin is commonly located seaward of the thickest sections of the high velocity crust and displays segmentation on length scales 100 120 km similar to the segmentation observed at the mid atlantic ridge mar isostatic gravity anomalies were calculated by removing from free air gravity the predicted effects of seafloor sediments and a crust mantle model assuming local isostatic compensation the resultant residuals show a corridor of high anomaly running along the margin situated close to the maximum thickness of the high seismic velocity crust as determined from the two available seismic refraction lines reduction to the pole r t p of the total field magnetic anomaly shows that after the removal of skewness from the ecma the location of the isostatic gravity high is closely correlated to the ecma the isostatic gravity high is also segmented but in two distinct wave bands 100 150 km and 300 500 km the short wavelength 100 150 km segmentation in the r t p magnetic and isostatic gravity anomalies is similar in wavelength to segmentation in magnetization and mantle bouguer anomaly observed along the present day mar the 300 500 km segmentation in the along margin isostatic gravity anomaly is similar in length scale to both intermediate wavelength tectonic segmentation observed in the south atlantic and variations in lithospheric strength observed along the african margin furthermore two of the intermediate wavelength 300 500 km isostatic gravity lows correspond to the early traces of the kane and atlantis fracture zones suggesting that these two fracture zones may define boundaries of a single tectonic corridor in the north atlantic we hypothesize that the direct cause of the intermediate wavelength segmentation may be along margin variations in both the amount of underplated igneous crust and the strength of the lithosphere although the relative importance of these two effects remains unresolved our results imply that segmentation is an important feature of margin development and that segmentation at mature oceanic spreading centers may be directly linked to segmentation during continental rifting

structure and evolution of the eastern gulf of aden conjugate margins from seismic reflection data the gulf of aden is a young and narrow oceanic basin formed in oligo miocene time between the rifted margins of the arabian and somalian plates its mean orientation n75 e strikes obliquely 50 to the n25 e opening direction the western conjugate margins are masked by oligo miocene lavas from the afar plume this paper concerns the eastern margins where the 19 35 ma breakup structures are well exposed onshore and within the sediment starved marine shelf those passive margins about 200 km distant are non volcanic offshore during the encens sheba cruise we gathered swath bathymetry single channel seismic reflection gravity and magnetism data in order to compare the structure of the two conjugate margins and to reconstruct the evolution of the thinned continental crust from rifting to the onset of oceanic spreading between the alula fartak and socotra major fracture zones two accommodation zones trending n25 e separate the margins into three n110 e trending segments the margins are asymmetric offshore the northern margin is narrower and steeper than the southern one including the onshore domain the southern rifted margin is about twice the breadth of the northern one we relate this asymmetry to inherited jurassic cretaceous rifts the rifting obliquity also influenced the syn rift structural pattern responsible for the normal faults trending from n70 e to n110 e the n110 e fault pattern could be explained by the decrease of the influence of rift obliquity towards the central rift and or by structural inheritance the transition between the thinned continental crust and the oceanic crust is characterized by a 40 km wide zone our data suggest that its basement is made up of thinned continental crust along the southern margin and of thinned continental crust or exhumed mantle more or less intruded by magmatic rocks along the northern margin abstract from author

cote d ivoire ghana margin seismic imaging of passive rifted crust adjacent to a transform continental margin i i summary during may 1990 and january february 1991 an extensive geophysical data set was collected over the cote d ivoire ghana continental margin located along the equatorial coast of west africa the ghana margin is a transform continental margin running subparallel to the romanche fracture zone and its associated marginal ridge the cote d ivoire ghana ridge from this data set an explosive refraction line running 150 km ene wsw between 3 55 n 3 21 w and 4 23 n 2 4 w has been modelled together with wide angle airgun profiles and seismic reflection and gravity data this study is centred on the cote d ivoire basin located just to the north of the cote d ivoire ghana ridge where bathymetric data suggest that a component of normal rifting occurred rafher than the transform motion observed along the majority of the equatorial west african margin traveltime and amplitude modelling of the ocean bottom seismometer data shows that the continental moho beneath the margin rises in an oceanward direction from 24 km below sea level to 17 km in the centre of the line where the crust thins most rapidly there exists a region of anomalously high velocity at the base of the crust reaching some 8km in thickness this higher velocity region is thought to represent an area of localized underplating related to rifting modelling of marine gravity data collected coincident meth the seismic line has been used to test the bestfitting seismic model this modelling has shown that the observed free air anomaly is dominated by the effects of crustal thickness and that a region of higher density is required at the base of the crust to fit the observed data this higher density region is consistent in size and location with the high velocities required to fit the seismic data

deep structure of the west african continental margin congo zaire angola between 5 s and 8 s from reflection refraction seismics and gravity data summary the deep structure of the west african continental margin between 5 s and 8 s was investigated using vertical reflection and wide angle reflection refraction techniques during the zaiango project a joint programme conducted in 2000 april by ifremer and totalfinaelf to penetrate below the salt layer a non conventional low frequency seismic source was used in the single bubble mode together with ocean bottom instruments hydrophones and seismometers and a 4 5 km long streamer that recorded multichannel seismic reflection mcs the data show that the continental crust thins abruptly over a lateral distance of less than 50 km from 30 km thick below the continental platform based on gravity data to less than 4 km thick below the lower congo basin that formed prior to the aptian salt deposition this subsalt sedimentary basin 180 km wide 4 km thick with velocities varying from 4 7 km s 1 to 5 8 km s 1 at the bottom is located between the foot of the continental slope and the oceanic domain it is underlain by crust of an intermediary or transitional type between continental crust and what can be recognized as oceanic crust in the transitional zone a crustal upper layer is present below the pre salt sedimentary basin 3 to 7 km thick with velocities increasing from 5 8 km s 1 at the top to 6 8 km s 1 at the bottom of the layer this layer appears to thin regularly from 6 7 km thick below the depocentre of the pre salt sedimentary basin to 3 4 km thick below the western termination of the basin below this upper crustal layer an anomalous velocity layer 7 2 to 7 8 km s 1 is documented below the eastern side of the basin where the crustal thinning is at a maximum the origin of this layer is unknown several arguments like rifting duration between 15 ma and 30 ma or the absence of seaward dipping reflectors precludes the hypothesis of underplated mantle material but other hypotheses such as serpentinized material or high grade metamorphic crustal rocks or a mixture of mafic and ultramafic crustal rocks are plausible near the ocean termination of the basin the transitional zone is bounded to the west by a basement ridge that is clearly documented on two profiles 7 11 and 14 having a dense ocean bottom seismometer hydrophone obs obh spacing on these profiles an anomalous velocity layer is present in the westernmost part of the transitional zone below the basement ridge and in the oceanic domain this layer absent on profile 3 may be related either to oceanization and slow seafloor spreading processes or to a consequence of the rifting process

oblique rifting and segmentation of the ne gulf of aden passive margin 1 the gulf of aden is a young obliquely opening oceanic basin where tectonic structures can easily be followed and correlated from the passive margins to the active mid oceanic ridge it is an ideal laboratory for studies of continental lithosphere breakup from rifting to spreading the northeastern margin of the gulf of aden offers the opportunity to study on land the deformation associated with oblique rifting over a wide area encompassing two segments of the passive margin on either side of the socotra fracture zone exhibiting distinct morphologic stratigraphic and structural features the western segment is characterized by an elevated rift shoulder and large grabens filled with thick synrift series whereas the eastern segment exhibits low elevation and is devoid of major extensional structures and typical synrift deposits though the morphostructural features of the margin segments are different the stress field analysis provides coherent results all along the margin four directions of extension have been recognized and are considered to be representative of two tensional stress fields with permutations of the horizontal principal stresses 2 and 3 the two dominant directions of extension n150 e and n20 e are perpendicular to the mean trend of the gulf of aden n75 e and parallel to its opening direction n20 e n30 e respectively unlike another study in the western part of the gulf our data suggest that the n150 e extension stage is older than the n20 e extension stage these conflicting chronologies which are nowhere unambiguously established suggest that the two extensions coexisted during the rifting on land data are compared with offshore data and are interpreted with reference to oblique rifting the passive margin segmentation represents a local accommodation of the extensional deformation in a homogeneous regional stress field which reveals the asymmetry of the rifting process the first order segmentation of the sheba ridge is inherited from the prior segmentation of the passive margin

are buoyancy forces important during the formation of rifted margins summary profiles of crustal thickness across rifted continental margins are examined in an attempt to understand the key observations and controlling parameters crustal stretching factor profiles from rifted continental margins supplemented by isochron data for early seafloor spreading have been used to determine a correlation between strain rate e and stretching factor despite the different methods assumptions and data sources our e relationship for rifted margins is consistent with that observed by newman and white for intracontinental rift basins the e relationship we derive is also consistent with the dynamic models of newman and white which include thermorheological strain hardening and strain softening but omit crustal buoyancy forces generated by lateral crustal thickness variations whilst crustal buoyancy forces are not included in the above dynamic models the e data do not necessarily preclude their importance simple numerical models of buoyancy force evolution show that for the first 30 myr after rifting the thermally derived buoyancy forces within the lithosphere that assist extension are larger than the crustal buoyancy forces that oppose extension this rift push force acts as a positive feedback mechanism is of the order of 3 10 12 nm 1 and dominates over the opposing crustal buoyancy forces immediately after rifting it is therefore clear that the delocalising effects of the crustal buoyancy force are dominant over a restricted range of conditions namely at low strain rate and at long times after rifting histograms of the lateral pressure gradients derived from crustal thinning factors along rifted margins show a dominant peak at 0 05 0 3 kpa m 1 and a significant secondary peak at 0 8 0 3 kpa m 1 the lower lateral pressure gradient peak corresponds to thinned parts of the continental crust which is adjacent to unstretched continental crust and may define the edge of a zone of thermal strain softening independent observations show that narrow margins are associated with rapid strain rates and are consistent with thermal strain softening predicted by thermorheological models however the dominant near zero pressure gradient peak is consistent with the operation of crustal buoyancy force processes during rifting which attempt to remove variations in crustal thickness

fylla bank structure and evolution of a normal to shear rifted margin in the northern labrador sea summary cenozoic seafloor spreading between greenland and north america is generally considered a major right stepping ridge transform ridge system between nw se trending spreading ridge segments in the labrador sea and baffin bay the ridges were linked by n s nne ssw trending transform motions in the davis strait in particular expressed by the 1000 km long ungava fault zone fylla bank part of the southern west greenland continental margin is located in the northernmost labrador sea at the transition between the normal and shear rifting regimes of the labrador sea and davis strait as such the bank may be compared with the demerara plateau part of the french guinea northeast brazil continental margin seismic reflection interpretations presented in this study show that fylla bank is situated above an extensive basin complex herein referred to as fylla structural complex which contains an up to 5 km thick cretaceous cenozoic sedimentary succession above an inferred pre cretaceous basement seismic mapping of basement structures show that the complex is dominated by nnw nw striking rift basins in its southern part and nne striking rift basins in its northern part the rift basins are interpreted to be the result of an initial late early cretaceous rift phase which mainly resulted in the formation of the nnw nw striking structures and a subsequent early campanian rift phase mainly resulting in the formation of large nne striking rotated fault blocks resumed rifting in the early cenozoic deepened the nne striking rift basins the nne oriented structures have previously been interpreted to initiate during the latest cretaceous however this study suggests that they initiated transfer faults already during the late early cretaceous rift phase and possibly correlate with along strike discontinuities in oceanic crust in the labrador sea to define margin segmentation in southern west greenland including the borders of fylla bank a structural kinematic model presented here thus suggests that the cretaceous cenozoic poly phase rifting to some extent was controlled by pre existing crustal fabric combined with an interpreted interplay between normal stresses in the labrador sea and oblique shear stresses in the davis strait this resulted in a very complex structural tectonic evolution and the formation of several distinct structural styles the seismic interpretations are supported by maps of the moho topography and crustal thickness which were compiled from results of pseudo 3 d gravity modelling the maps show minimum crustal thicknesses 11 km and maximum moho uplifts in areas where the nnw nw and nne striking structures interact moreover a strong correlation is found between moho topography crustal attenuation rift enforced thermal uplift and erosion and post rift subsidence in the area this is interpreted to be a result of thermally controlled basin dynamics

the jebel hadid structure al kufrah basin se libya a possible impact structure and potential hydrocarbon trap abstract the jebel hadid structure that formed in the nubian sandstone of the southern al kufrah basin is a 4 7 km circular feature with a set of multiple concentric annular ridges the age of the structure is probably post early cretaceous to pre pliocene as the prevalent geological processes that usually account for the formation of circular structures magmatism diapirism sand volcanism karst dissolution glacial erosion can be widely excluded in this region we suggest that the jebel hadid structure might represent an eroded complex impact structure in particular the jebel hadid structure bears a strong morphological resemblance to the impact structure of tin bider in algeria general host rock properties may be compared to those at the upheaval dome impact structure usa numerous terrestrial impact structures are hosts to hydrocarbon reservoirs this investigation provides a first prospective approach based on remote sensing discussing the economic potential of a circular structure of suspected impact origin the still underexplored al kufrah basin is considered as a high potential area for future hydrocarbon exploration if we consider a possible impact origin of the jebel hadid structure this feature might provide local secondary pathways and traps for hydrocarbon migration accumulation in the nubian series and thus ought to be considered as a preferential site for oil and gas exploration

structure and evolution of the demerara plateau offshore french guiana rifting tectonic inversion and post rift tilting at transform divergent margins intersection we present the structure and evolution of the eastern part of the demerara plateau offshore french guiana from the analysis of geophysical data collected during guyaplac cruise this area is located at the intersection of a transform segment and a divergent segment of a continental margin related to the early cretaceous opening of the equatorial atlantic the main structures are nne ssw to nnw sse trending normal faults on the eastern edge of the plateau and wnw ese to nw se trending acoustic basement ridges on its northern edge when replaced in their albian position these structures appear to be parallel to the coeval oceanic accretion axis and transform faults respectively the most striking structures are related to a post rift but syn transform tectonic inversion producing e w to wnw ese trending folds sealed by a regional unconformity this shortening can not be related to ridge push but is probably related to a plate kinematic change 105 my ago that modified the deformation in the vicinity of the transform fault late post rift evolution also includes a significant tertiary oceanward tilt of the edge of the demerara plateau the driving mechanism of this late tilt is unclear but may be related to a lithospheric flexure resulting from the loading of the abyssal plain by the orinoco and amazon deep sea fans

aorounga and gweni fada impact structures chad remote sensing petrography and geochemistry of target rocks shuttle radar topography mission data was used to investigate the aorounga and gweni fada impact structures in chad as part of a new remote sensing study we believe the results of various data treatments provide extensive new perspective on the macro structural and topographic divisions for these two impact structures our remote sensing studies indicate revised diameters of aorounga and gweni fada of 16 and 22 km respectively we selected samples from these two structures for their petrographic geochemical and rb sr and sm nd isotopic characteristics in samples from both structures evidence for shock metamorphism was found in the form of single or multiple sets of planar deformation features in quartz which confirms the impact origin for both the aorounga and gweni fada structures the crystallographic orientations of pdfs indicate maximum shock levels of 20 30 gpa for samples from the central parts of both structures the pdf orientations are characteristic for the orientations observed elsewhere in shocked sandstones with the higher angles in the orientation histograms being fairly abundant geochemically the rocks are typical upper crustal sandstones

the s reflector west of galicia the seismic signature of a detachment fault the so called s reflector is a group of bright continuous reflections underlying the landward tilted fault blocks of the west galicia rifted margin s has been interpreted as the brittle ductile transition the lop of an intrusion a detachment fault and the crust mantle boundary to constrain the internal structure of the reflector we have carried out detailed analyses of these reflections we compare the waveforms of the seafloor reflection and its first multiple both to determine the amplitude of the seafloor reflection and to show hat the seafloor is in effect a spike in the reflectivity series so that the seafloor reflection can be used as the far field wavelet including both source and receiver ghosts we compare he waveform of the seafloor and 5 and show that within the resolution of our data s is a reflection from a step increase in acoustic impedance this result is confirmed through complex trace analysis and in particular the determination of the apparent polarity of s and the examination of the instantaneous frequency function s is consistently positive polarity and shows no significant frequency anomaly simple modelling shows that s is very unlikely to be a reflection from a thin layer we thus conclude that s is probably a single steplike interface from the varying frequency content of the data we determine a value for the effective q between s and the seafloor and use this to assess the amplitude loss due to attenuation and scattering we use a comparison between the seafloor and the s reflection to constrain the amplitude of s and estimate a reflection coefficient for s of at least 0 2 in places decreasing landwards by analogy with structures developed in the highly extended regions of the western united states we consider that the most likely interpretation of s is as a sharp west dipping detachment fault separating a granitic upper plate from a higher velocity lower plate locally probably serpentinized mantle

continent ocean transition on the voring plateau ne atlantic derived from densely sampled ocean bottom seismometer data 1 a 180 km long seismic wide angle profile was acquired across the voring plateau ne atlantic using a tuned air gun array and three component ocean bottom seismometers deployed with 5 km spacing the seismic p wave data have been modeled by ray tracing inversion and the model has been constrained by s wave and gravity modeling the data and modeling have allowed us to depict the crustal structure and nature of the continent ocean transition cot in a classical volcanic margin case the p wave velocity near the top of the main crustal layer is estimated to 6 0 km s landward of the 25 km wide cot the seaward increase to 6 5 km s in the cot is conformable with heavily intruded continental crust within this zone farther seaward the velocity increase to 6 9 km s in the same layer suggests the presence of oceanic crust the abundant magmatism landward of and within the cot is primarily observed as extrusives forming wedges of seaward dipping reflectors and mafic lower crustal intrusions underplating the maximum thickness of the oceanic crust is measured to 23 5 km at the seaward termination of the cot

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

seismic structure gravity anomalies and flexure of the amazon continental margin ne brazil 1 seismic and gravity data have been used to determine the structure of the sediments crust and upper mantle that underlie the amazon continental margin offshore ne brazil seismic reflection profile data reveal a major unconformity at 7 s two way travel time twtt which we interpret as marking the onset of the transcontinental amazon river and the formation of the amazon deep sea fan system in the late miocene seismic refraction data show mean sediment velocities that decrease by 1 5 km s 1 in a seaward direction we attribute this decrease to facies changes associated with sediment progradation and the development of topset foreset and bottomset beds seismic refraction data show that the sediments are underlain by oceanic crust that has a similar velocity structure compared to elsewhere in the atlantic ocean but is unusually thin 4 2 km we attribute the thin crust to either slow seafloor spreading or a limited magma supply during the initial rifting of south america and africa in the early cretaceous the seismic data have been used to construct a new sediment thickness grid that together with gravity anomaly data suggests the amazon fan loaded lithosphere with an unusually high flexural strength while a high strength lithosphere explains the overall depth of the seismic moho there are discrepancies of up to 2 km beneath the upper fan where the modeled flexed moho is shallower than the seismic moho and beneath the middle fan where it is deeper gravity and seismic modeling suggest these discrepancies are caused by lateral changes in subcrustal density such that the mantle underlying the upper fan is denser than it is beneath the middle fan we attribute these lateral density differences to proximity to the ceara rise which is believed to have formed during the late cretaceous in a mid ocean ridge setting fan loading of a relatively strong dense and hence cold lithosphere predicts stress orientations that are consistent with borehole breakout data and the location and height of the gurupe arch onshore despite its proximity to leaky transform faults the margin that underlies the amazon fan appears to be of nonvolcanic origin the main differences with other nonvolcanic margins such as west iberia and newfoundland are a greater sediment accumulation a narrower zone of transitional crust and a lack of any evidence for extreme extension and mantle serpentinization

demerara plateau the structure and evolution of a transform passive margin summary the amazon cone experiment acquired two transects profile sd a across the demerara plateau part of the french guiana northeast brazil continental margin to better understand rift and transform style margin evolution profile a images an intermediate type margin formed as a result of trans tensional extension in this paper we describe the modelling of wide angle and multichannel seismic and gravity data from profile d to reveal whole crustal structure and features exhibiting transform characteristics combining these results with other studies in the region and comparing our results with young rift analogues we develop a model of along margin segmentation that explains the evolution of the west equatorial atlantic interpretation of the velocity depth model for profile d shows a 35 37 km thick continental crust which thins to 10 11 km over a distance of 320 km this thinning is accommodated in two regions the narrowest region associated with the ocean continent transition is interpreted to have formed by dextral shearing of the margin along major transform zones that accommodated the initial break up geometry of the central atlantic given the orientation of the margin relative to local fracture zone traces it is likely that the second region of thinning located 162 km landward of the ocean continent transition is a result of rifting suborthogonal to the profile orientation there is no evidence of rotated faulted blocks half graben structures or riftrelated magmatism manifest as either seaward dipping reflectors or as a high velocity region within the lower crust the demerara plateau is therefore interpreted as a margin segment comprising thinned continental crust bound to the north and south by transform type zones in which trans tensional extension is accommodated in contrast to profile a modelling suggests that the eastern margin exhibits a relatively broad region of crustal thinning associated with extension consistent with a rift type setting offshore unusually thin oceanic crust of 3 3 5 7 km thickness is identified which is consistent with similar observations further south in the absence of identifiable magnetic anomalies best estimates of the initial half spreading rate of 20 mm yr 1 suggest that the thin crust throughout the region is unlikely to be a result of ultra slow spreading but instead it is most likely due to a reduced magma supply at numerous long lived transform faults reflected by those presently offsetting the mid atlantic ridge in this equatorial setting

oxygen isotope geochemistry of the mesozoic anorogenic complexes of damaraland northwest namibia evidence for crustal contamination and its effect on silica saturation fifty six new oxygen isotope analyses of minerals separated from nine mesozoic anorogenic complexes of damaraland in northwest namibia have been used to estimate the 18o values of the original magmas magma these complexes range in composition from nepheline syenite to alkaline and peraluminous granites often with a variety of rock types present at a single centre the silica undersaturated rock types show a relatively small spread of values of magma from 6 0 to 7 0 mean 6 6 which is consistent with their derivation from the mantle with little or no subsequent crustal contamination the silica oversaturated rocks show a wide range of magma values from 4 9 to 12 0 with a mean value of 8 3 the high values of magma in the silica oversaturated rocks can only be explained with considerable involvement of the continental crust in their petrogenesis and those rocks with magma 10 are interpreted as essentially crustal melts it is generally accepted that the damaraland complexes were generated as a result of rifting across the tristan plume with the plume providing both mantle derived magma and with it the heat required for crustal melting in addition to their mantle like oxygen isotope ratios the undersaturated rocks of the damaraland complexes possess the trace element characteristics e g low zr nb ratios of ocean island basalt suggesting that their parental magmas were produced from the plume itself in contrast the oversaturated complexes generally have higher zr nb ratios that are consistent with a larger crustal input the highest values of magma in the damaraland complexes are found in granitic rocks that intrude the central zone of the pan african damara orogen where presumably there is a substantial component of sedimentary origin in the lower to middle crust

non impact origin of the crater like structures in the gilf kebir area egypt implications for the geology of eastern sahara several small crater like structures occur in gilf kebir region sw egypt it has been previously suggested that they could be the result of meteoritic impacts here we outline the results of our geological and geophysical survey in the area the proposed impact origin for these structures is not supported by our observations and analyses and we suggest an alternative interpretation the crater like structures in gilf kebir area are likely related to endogenic processes typical of hydrothermal vent complexes in volcanic areas which may reflect the emplacement of subvolcanic intrusives

mechanism of thin skinned detachment in the amazon fan assessing the importance of fluid overpressure and hydrocarbon generation abstract since about 10 ma a wedge of clastic sediment up to 10 km in thickness has accumulated near the mouth of the amazon river forming the amazon fan thin skinned extensional and contractional structures root into a common basal detachment hundreds of kilometers long extensional faults have formed landward of the prograding delta slope and contractional folds and faults have formed seaward of it by plan view restoration we show that extension and contraction balance we therefore infer that slope instability has provided the driving force for thin skinned deformation on seismic data the detachment appears as a sharp surface within late cretaceous strata however there is no evidence at that level for soft layers such as evaporites or muds abnormal fluid pressure provides a more reasonable mechanism for the detachment at depths of several thousand meters deep wells have encountered fluid overpressures amongst the possible causes of abnormal fluid pressures are compaction and hydrocarbon generation although compaction may have contributed to fluid overpressures encountered in wells we argue that hydrocarbon generation provides the best explanation for detachment in cretaceous rocks which were old and lithified when it occurred in particular the broadly triangular area of detachment coincides with an inferred pod of cenomanian turonian source rock which is interpreted to have been in the gas window during detached deformation we have developed new techniques of sandbox modelling to investigate the geometries of fault patterns produced by fluid flow through porous media for the amazon fan we have been able to reproduce the structural pattern on the assumption that fluid overpressures existed over a broadly triangular area beneath the detachment

re evaluation of the petroleum potential of the kufra basin se libya ne chad does the source rock barrier fall abstract the kufra basin is a large underexplored palaeozoic intracratonic sag basin in se libya and ne chad with extensions into nw sudan and sw egypt the basin fill consists of shallow marine to fluvial deposits ranging in age from infracambrian to cretaceous geologically the basin is very similar to the murzuq basin in sw libya which recently presented libya with its largest oil discovery for over a decade most of the hydrocarbon play elements known from the murzuq basin also occur in the kufra basin thick porous cambro ordovician sandstones are present and would form good reservoirs lower silurian shales may act as effective seals and there are potential structural traps in seismically defined fault blocks however the source rock availability in the kufra basin is currently unclear one of the two main source rock candidates in the basin is a lower silurian shale unit tanezzuft formation the tanezzuft shales have been described as being up to 130 m thick in outcrops at the basin margins but the shales were found to be replaced by siltstones and sandstones in two dry exploration wells drilled in the northern part of the basin by agip between 1978 and 1981 hot shales developed at the base of this widespread silurian shale unit form important source rocks in many areas of north africa and arabia these hot shales are interpreted to have been deposited in palaeodepressions such as incised valleys of the preceding lowstand or intrashelf basins during the initial transgression after the melting of the late ordovician ice cap the areal distribution of the organic rich unit is therefore discontinuous fieldwork in the kufra basin has shown that the basal tanezzuft horizon is not exposed on the northern and eastern margins of the basin deep infracambrian rift grabens have been interpreted on seismic lines from the kufra basin and in analogy to oman and algeria could contain organic rich infracambrian deposits the infracambrian succession in the kufra basin may contain a second major potential source rock and warrants further investigation

mass transport deposits and fluid venting in a transform margin setting the eastern demerara plateau french guiana abstract the eastern demerara plateau offshore french guiana was surveyed in 2003 during the guyaplac cruise multibeam bathymetry and acoustic imagery 6 channel seismic reflection and 3 5 khz echo sounding the data show the post transform cenozoic that the series located on the outer part of the plateau below c 2000 m contain at least twelve stacked mass transport deposits mtds that have recorded a history of large scale slope failure as well as two main normal fault sets that provide possible pathways for upward fluid migration through the series reaching at high as the uppermost mtds seabed data show that the area above the failures is characterized by circular to elongate slope parallel depressions interpreted as fluid seeps pockmarks some of them have been modified by along slope currents we suggest that the development of the mtds to results from the combinaiton of the presence of fluid overpressure at depth the geometry of the margin s deep structure in particular the existence of a free borderlateral border on the outermost plateau our results also emphasise the role of stratigraphic decollements within the cenozoic series

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

numerical simulations of a buoyancy driven coastal countercurrent off vancouver island abstract a three dimensional prognostic numerical model has been developed to study the ocean circulation around vancouver island british columbia in a series of simulations the model is applied to examine the role of buoyancy forcing in the dynamics of the summer coastal countercurrent found off the west coast of vancouver island the forcing is provided by the fraser river discharge into the strait of georgia an estuarine circulation establishes itself in juan de fuca strait from which a distinctive right bounded current is formed and advances along the coast sensitivity studies are conducted to determine the robustness of this current to initial conditions opposing wind enhanced vertical mixing and grid resolution finally various characteristics of the numerically modeled coastal flow are compared with observations