crustal versus mantle core complexes deep crustal and mantle rocks are exhumed in core complex mode of extension in three types of structures metamorphic core complexes oceanic core complexes and magma poor passive margins using available analogue and numerical models and their comparison with natural examples the present paper reviews the mechanical processes involved in these different types of extensional setting three main aspects are considered i the primary role of lithosphere rheology ii the lithosphere scale patterns of progressive deformation that lead to the exhumation of deep metamorphic or mantle rocks and iii the initiation and development of detachment zones crustal core complexes develop in continental lithospheres whose moho temperature is higher than 750 c with upper crust dominated strength profiles contrary to what is commonly believed it is argued from analogue and numerical models that detachments that accommodate exhumation of core complexes do not initiate at the onset of extension but in the course of progressive extension when the exhuming ductile crust reaches the surface in models convex upward detachments result from a rolling hinge process mantle core complexes develop in either the oceanic lithosphere at slow and ultra slow spreading ridges or in continental lithospheres whose initial moho temperature is lower than 750 c with sub moho mantle dominated strength profiles it is argued that the mechanism of mantle exhumation at passive margins is a nearly symmetrical necking process at lithosphere scale without major and permanent detachment except if strong strain localization could occur in the lithosphere mantle distributed crustal extension by upper crust faulting above a decollement along the ductile crust increases toward the rift axis up to crustal breakup mantle rocks exhume in the zone of crustal breakup accommodated by conjugate mantle shear zones that migrate with the rift axis during increasing extension

transition from symmetry to asymmetry during continental rifting an example from the bight basin terre adelie australian and antarctic conjugate margins the rifting history of the magma poor conjugate margins of australia and antarctica is still a controversial issue in this article we present a model for lithosphere scale rifting and deformation history from initial jurassic rifting to late cretaceous breakup for the conjugate bight basin terre adelie section of the margin based on the interpretation of two regional conjugate seismic profiles of the margins and the construction of a lithosphere scale balanced cross section sequentially restored through time the model scenario highlights the symmetric pattern of initial stretching resulting from pure shear at lithospheric scale accompanied by the development of four conjugate detachments and crustal half graben systems this system progressively evolves to completely asymmetric shearing along a single south dipping detachment at the scale of the lithosphere antarctica plays the role of the upper plate and australia the lower plate the detachment accounts for the exhumation of the mantle part of the australian lithosphere and the isolation of a crustal klippe separated from the margin by a serpentinized peridotite ridge the total elongation amount of the australian antarctic conjugate system reaches 473km 178 elongation was partitioned through time 189 and 284km during symmetric and asymmetric stages respectively during the symmetric stage both margins underwent approximately the same degree of crustal stretching 105km 75 and 84km 67 for australia and antarctica respectively again both margins accommodated relatively the same elongation during the asymmetric stage the antarctic upper plate records an elongation amount of 284km 88 as crustal mantle stretching above the inferred low angle south dipping detachment zone whereas the australian lower plate underwent 270km 206 of elongation through mantle exhumation although the restoration process does not allow reconstruction of the precise geometry before deformation we propose that the jurassic early geometric evolution of the margins may have been controlled by the inherited structure or rheological heterogeneities of the continental crust its later evolution is thought to relate to the mechanical evolution of the crustal and mantle material during exhumation with a strong increase in localization of shear in the lower crust and mantle part of the australian margin the geometry of the rifted margins is comparable to other magma poor rifted margin such as the newfoundland iberia margins or the exhumed alpine tethys margin exposed in the central alps 2012 blackwell publishing ltd

contribution of deep seismic profiling to the knowledge of the lower crust in france and neighbouring areas abstract in france and neighbouring areas the crust is essentially formed of segments of the variscan orogen 380 300 ma and older formations cratonic basins began to form in the permian but extensive subsidence only occurred from the lias in basins such as the paris and aquitaine basins and also along the margin of the alpine tethys ocean the pyrenees and alps range eventually formed during the tertiary as a result of the collision between the european and african continental plates large grabens were also rifted in the rhine and rhone valleys deep seismic profiles show remarkable features common to the crust of the whole area although it underwent such a complicated geological history the most striking feature is the occurrence of conspicuous flat reflections densely distributed in the lower part of the crust below a comparatively transparent upper crust these reflections hereafter called laminations have been found to occur on most profiles within a large area covering western europe although they vary in depth thickness and energy dipping reflections have frequently been observed in the upper crust some of them extend across the lower crust and even penetrate into the upper mantle they may be reasonably interpreted as variscan compressional features by correlation with surface geology and comparison with similar features found in deep seismic profiles from other regions the origin of the flat laminations which generally lie in the lower part of the crust is more controversial this paper attempts to use the well known geological history of the region to place some constraints on the hypotheses which may be advanced to explain the formation of these laminations these features may be explained by the following tectonothermal events 1 1 compression which prevailed during the variscan orogeny may have formed the laminations as a result of delamination and shearing near the bottom of the crust associated with melting intrusion and metamorphism during the different stages of the crustal evolution 2 2 the thermal equilibration of the upper mantle which occurred in the permian at the end of the variscan orogeny may have caused metamorphism and magmatism in the overlying crust 3 3 a regional thermal event which might be associated with subordinate extension occurred during the rifting of the alpine tethys and the initiation of mesozoic cratonic basins in late triassic middle jurassic times it may also have caused metamorphism and magmatism in the lower crust while the first model implies conservation of the crust material the other two suggest that the crust mantle transition has been somewhat uplifted since the end of the variscan orogeny the latter hypothesis is one way to explain the undeniable occurrence of seismic events below the present moho reflection the above three models are not mutually exclusive and all these phenomena may well have affected the crust to some extent however the data at hand are still insufficient to determine what parts tectonic and nontectonic processes have played in the creation of the laminations in the lower crust and the moho further experiments are needed to check these hypotheses

central gulf of aden conjugate margins yemen somalia tectono sedimentary and magmatism evolution in hybrid type margins abstract magma rich and magma poor passive margins are generally studied separately yet the spatial evolution from one type to another is not well understood central gulf of aden margins are at the transition between these two types of margins based on new seismic data published wells and gravity data we determine seismic stratigraphy and structural patterns we map the distinct crustal domains continental ocean continent transition oct oceanic domains and propose a tectonic evolution of the yemeni and somalian conjugate margins in an oblique rifting context the most striking results are the significant segmentation narrowness and asymmetry of conjugate margins as well as the gradual eastward decrease in the intensity of magmatic activity of these hybrid type margins west of bosaso hami fracture zone bhfz the hami ahl medo segment presents magmatic type margin features in the distal yemeni margin related to the strong influence from the afar plume seaward dipping reflectors develop since the late syn rift stage 21 18 ma near the xiis al mukalla fracture zone conversely east of bhfz the bosaso sayhut margins segment is characterized by magma poor margins for which the exhumation stage is characterized by the unroofing of mantle along multiple detachment faults in the oct and shortly followed by diachronous volcanism 18 ma the central gulf of aden is affected by a major uplift during the final syn rift stage and up to oct formation this event is associated with the formation of erosional surfaces in the east and with gravitational deformation taking place on decoupling surfaces at the base of the syn rift sequence lower ghaydah fm that lead to the sliding of syn rift units on top of the exhumed mantle rocks in the oct

late quaternary evolution of a subantarctic paleofjord tierra del fuego lago roca lapataia valley 54 50 s 68 34 w is a paleofjord that was occupied by a valley glacier system during the glacial maximum of the late pleistocene estimated ca 18 20 ka bp deglaciation began before 10 080 270 bp the marine fauna in several marine terraces found in the area shows that early middle holocene climatic conditions were basically the same as at present species found are characteristic of cold and shallow waters although minor temperature fluctuations cannot be ruled out for this period a recent radiocarbon date of 7518 58 bp on chlamys patagonica nz 7730 confirms that lago roca was transformed into a fjord ca 7500 8000 bp the sea reached its maximum level of 8 10 m a s l around 6000 bp and at 4000 4500 bp was at least above 6 1 m a s l later when sea level fell lago roca was occupied by fresh water and was no longer tidal the relative land sea positions during this period are a consequence of combined eustatic and neotectonic processes

ocean continent boundary in the iberia abyssal plain from multichannel seismic data abstract the ocean continent boundary of the west iberia margin is marked by a basement ridge trending n s four segments of this ridge are recognized each of them being progressively offset westward from 40 n to 43 n because the setting and seismic character of the ridge in the iberia abyssal plain are similar to those of the galicia margin ridge which is made of serpentinized peridotite we think that the southern segments of the ridge are also made of the same mantle material the segmentation of the ridge suggests that the northward propagation of the continental break up during the north atlantic opening in early cretaceous times was discontinuous each segment possibly corresponding to a propagation step east of the ridge the basement of the whole iberia abyssal plain consists of highly thinned continental crust locally resting on a seismic reflector that resembles the s reflector previously recognized off galicia by analogy with the galicia margin we propose that the tilted crustal blocks lay on serpentinized peridotite derived from the upper mantle the s reflector corresponding to the contact between crustal rocks and serpentinite

the structure evolution and symmetry of the magma poor rifted margins of the north and central atlantic a synthesis abstract magma poor rifted margins consistently show extreme crustal thinning accompanied by normal faulting the serpentinization of the mantle beneath crust thinned to less than 8 2 km and the unroofing of a broad zone of mantle within the continent ocean transition accompanied by the development of detachment and other large offset faults these observations are the logical result of the progressive extension of cool lithosphere away from thermal anomalies such as plumes although the paucity of magmatism may be explained by depth dependent extension of the lithosphere and by pre depleted sub crustal lithosphere rifting above initially cool sub lithospheric mantle also may explain the subsidence deficit observed at some margins synrift subsidence is buffered by the simultaneous influx of warmer oceanic asthenosphere as it extends thins and cools ductile creeping layers in the mid and deep crust become progressively more brittle resulting in increased coupling between the upper and lower crust and eventually the embrittlement of the entire crust faults cutting from the surface across the moho bringing water into the mantle and causing its serpentinization increased coupling and the development of serpentine detachments predict the development of late stage asymmetry once the entire crust is brittle such detachments are imaged on some margins and inferred on others analysis of the crustal structure across conjugate margins shows that these are approximately symmetric until this late stage when they become markedly asymmetric similar analyses show that depth dependent stretching of the crust is insufficient to explain the discrepancy between the amount of the visible extension along faults and the amount of crustal thinning instead this extension discrepancy may be related to the complex evolution of brittle deformation through multiple phases and styles of faulting related to the changes in the rheological character and strength of the lithosphere as it is thinned complex polyphase faulting continues after complete crustal separation resulting in the exhumation of broad expanses of peridotitic basement the top of which is everywhere marked by an exhumed slip surface similar to the corrugated surface observed at mid ocean ridges the similarity in processes between mantle unroofing and seafloor spreading makes the distinction between the cot and true oceanic crust difficult and possibly moot

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

metamorphic evolution and oxygen isotope geochemistry of rift precursor amphibolites from hole 1067a odp leg 173 off west iberian galicia bank rifted margin abstract the paper documents permian igneous rocks in the vicinity of an ocean continent transition in the west iberian abyssal plain odp leg 173 hole 1067a was drilled through a sequence of foliated to massive amphibolites of gabbroic protolithic origin the 270 3 ma to 246 5 ma rock sequence experienced several metamorphic events during the interval of time spanning from 138 ma up to 55 ma metamorphic parageneses record two major thermobarometric events for which the temperature and pressure conditions are 670 40 c and 0 7 0 1 gpa and 550 50 c and 0 5 0 1 gpa according to radiometric dating these metamorphic events might have occurred around 138 136 ma and 113 110 ma respectively the low cl content and the preserved igneous 18 o of amphibole imply seawater absent 580 c isothermal plagioclase amphibole equilibrium related to the development of the strong foliation of the amphibolite during the second metamorphic event late isotopic equilibration involved lower temperature seawater derived fluid and is younger than 110 ma the gabbroic intrusion is believed to be a precursor for initiation of continental rifting melting of such metamorphosed rocks in a rift setting would generate magmatic rocks with complex oxygen isotopic signatures

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

role of the tectonic facies concept in orogenic analysis and its application to tethys in the eastern mediterranean region abstract the purpose of this paper is to advocate the use of a tectonic facies concept which aids systematic and objective analysis of orogenic belts the tectonic facies concept as discussed here focuses on the recognition of discrete tectonic settings mainly those exposed at high crustal levels in orogenic belts application of the tectonic facies concept can help field geologists to relate plate scale processes to outcrop geology tectonic facies are recognised based on knowledge of the modern oceans and margins and well constrained land settings tectonic facies analogous to sedimentary facies can be identified from integrated sets of field data and related laboratory studies a number of tectonic facies models can be recognised in this way their identification aids interpretation and comparisons of orogenic belts palinspastic reconstructions require additional information particularly age kinematic and palaeomagnetic data in this paper the tectonic facies concept is tested by application to a well documented mainly high level orogenic segment tethys in the eastern mediterranean area and is found to be an effective way both of the describing and interpreting this region other methods of tectonic analysis e g recognition of tectono stratigraphic terranes may also be found useful in certain settings and can be used in conjunction with the tectonic facies concept

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles ocean drilling program legs 149 and 173 transect we present a wide angle seismic refraction study of an 80 40 km region of the southern iberia abyssal plain south of galicia bank an intersecting grid of two e w and four n s wide angle reflection refraction profiles is used to define variations of the basement velocity structure within this unusually wide ocean continent transition oct these structures can be systematically linked to variations in acoustic basement morphology and to results from ocean drilling program odp boreholes lateral changes in the velocity structure of the basement occur abruptly over distances of 20 km where complex variations may be found thinned upper continental crust 2 5 km thick with velocities of 5 0 6 6 km s is limited to a series of n s fault blocks immediately south of galicia bank this crust is underlain by a high velocity layer 7 3 7 9 km s of weakly serpentinized i e 0 25 peridotite which exists throughout the eastern part of the survey area basement within the oct appears to consist dominantly of a broad region of exposed upper mantle that has been serpentinized heterogeneously both vertically and horizontally in the southeast sector of our survey where basement topography deepens and becomes subdued continental fault blocks are absent instead basement contains an upper layer of more pervasively serpentinized i e 25 45 peridotite that is 2 km thick this layer is characterized by low velocity at the top of basement 4 2 km s that increases rapidly with depth and it probably corresponds to a seismically unreflective layer previously identified in reflection profiles to the south of our survey in the western section of our survey beneath a series of elevated basement ridges velocities are reduced within both the upper basement layer 3 5 6 0 km s and lower layer 6 4 7 5 km s these changes suggest that both upper and lower layers have become more highly serpentinized with values of 60 100 in the upper layer and 25 45 in the lower layer probably during the last stages of rifting and immediately before formation of oceanic crust a normal or slow spreading oceanic crustal structure is not found within the survey region thus it appears that the onset of seafloor spreading occurs in the region west of the peridotite ridge sampled at odp site 897 and east of the j magnetic anomaly

lithospheric structure of the texas gulf of mexico passive margin from surface wave dispersion and migrated ps receiver functions the seismic velocity structure beneath texas gulf coastal plain gcp is imaged by migrating ps receiver functions with a seismic velocity model found by fitting surface wave dispersion we use seismic data from a linear array of 22 broadband stations spaced 16 20 km apart a common conversion point ccp stacking technique is applied to earthquake data to improve the s n ratios of receiver functions using an incorrect velocity model for depth migration of a stacked ccp image may produce an inaccurate image of the subsurface to find sufficiently accurate p and s velocity models we first apply a nonlinear modeling technique to fit rayleigh wave group velocity dispersion via very fast simulated annealing vs ranges from 1 5 km s in shallow layers of the gcp to 4 5 km s beneath the llano uplift and just outboard of the balcones fault zone bfz the bfz is characterized by slow velocities that persist to nearly 100 km depth in the stacked image the largest amplitude positive polarity event ranges from the surface at the llano uplift to a maximum depth of 16 km beneath matagorda island we attribute this event to the sediment basement contact which is expected to produce a large impedance contrast another large amplitude and positive polarity event at 35 km depth which likely marks the moho disappears outboard of the luling fault zone the disappearance of the moho beneath the gcp may be due to serpentinization of the upper mantle which would reduce the impedance contrast between the lower crust and upper mantle dramatically

deep seismic reflection profiling and the growth of the continental crust abstract deep seismic reflection profiling provides evidence for the style and magnitude of phanerozoic crustal growth by both lateral and vertical accretion profiles across convergent zones have been interpreted as showing tectonic underplating of the continental crust by fragmented oceanic lithosphere profiling across extensional zones provides evidence for magmatic underplating of the crust in addition to studies over active areas deep reflection profiling over large areas of western europe north america and australia shows that the lower crust is strongly reflective and seismically distinct from the upper crust and upper mantle the layering is detected beneath many different terranes of varying crustal type and basement age from archaean to cenozoic and is often apparently younger than the tectonic or metamorphic age of the exposed upper crust thus continental growth curves estimated from upper crustal measurements alone may underestimate phanerozoic crustal additions and mean crustal ages similarly estimated may be too high

crustal structure of a nonvolcanic rifted margin east of newfoundland a detailed seismic refraction study was undertaken of the rifted continental margin southeast of the grand banks of newfoundland along a preexisting deep multichannel seismic reflection profile the primary objectives were to determine the topography of moho across the margin to detect any evidence for crustal underplating and to determine the position of the continent ocean boundary data were obtained from nine ocean bottom seismometers deployed along the profile with an array of six 1000 inch3 air guns as the source two additional refraction lines were shot parallel to structure over the continental slope the resultant velocity model is well constrained and consistent with the observed gravity the crust thins abruptly beneath the continental slope to a major structural transition identified as the continent ocean boundary there is no significant magmatic underplating of the thinned continental crust seaward of the transition the seismic velocity structure is characterized by a 50 km wide zone of intermediate velocity 7 2 7 6 km s beyond this is a thin 3 5 km crust with low velocity 4 5 5 km s and high gradient over mantle with a velocity of 7 7 km s increasing to 8 0 km s the zone of intermediate velocity is interpreted to be highly serpentinized mantle material beneath a very thin crust suggesting little or no magmatic activity during and immediately after rifting this interpretation is consistent with results from adjacent and conjugate margins

rift tectonics on the passive continental margin off galicia spain abstract new migrated multichannel seismic profiles on the starved passive continental margin off galicia spain illustrate the tectonic style of the early cretaceous rifting event which preceded the separation of iberia and north america the main episode of rifting was preceded by other tensional episodes probably of earliest cretaceous and late jurassic age listric faulting associated with rifting was accompanied by large scale mass movements from the crests of the tilted blocks two subaerial erosional surfaces are visible one of late jurassic or earliest cretaceous age was affected by the final faulting episode the other was synchronous with the main phase of rifting and is present only on the upper part of the margin because the central trough was under deep water the significance of a ridge near the ocean continent boundary where altered lherzolite has been dredged and cored is discussed below the tilted blocks which include the basement rocks the seismic horizon s is visible as on the armorican margin its significance is also discussed

post spreading deformation and associated magmatism along the iberia morocco atlantic margins insight from submarine volcanoes of the tore madeira rise abstract a new high resolution bathymetric map combined with a regional digital elevation model dem analysis reveal the modalities of occurrence and emplacement of post spreading magmatism along the nne ssw oriented 1000 km long tore madeira rise tmr as well as its relationship with the activity of major fault systems including the estremadura fault system esf and the azores gibraltar fracture zone agfz morphological and structural analysis of the bathymetric map were performed to map volcanic features such as eruptive cones vents and fissures together with faults along the tmr the new bathymetric map shows that the main nnw sse seamount alignment is formed by three structurally distinct volcanic massifs the tore the josephine and the southern volcanic groups the majority of the volcanoes of each group emplaced within or along specific portion of pre existing faults esf and agfz including splay fault releasing bend fault tips and interaction zones between different segments magmas were channelled into sub vertical pre existing lithospheric faults that acted as preferential pathways for the vertical magma ascent migration and final eruption of magma are controlled by the local stress variation induced by complex fault geometries change in plate kinematics as well as strong shear zone anisotropy as suggested by the emplacement within localised areas of transtension we conclude that post spreading magma emplacement in the southern part of the iberia margin was related to the development of a transtensional plate boundary between the iberian and african plate during the late cretaceous more generally our findings emphasize that the distribution of volcanism as the expression of the interaction between shallow plate tectonic and mantle processes should be included in plate kinematic reconstruction this study also demonstrates that the accurate mapping of oceanic seafloor is pivotal to better understand tectono magmatic evolution of volcanic seamount chains and geological processes in oceanic domains

southeast baffin volcanic margin and the north american greenland plate separation plate breakup over plumes is characterized by the development of margins showing extensive magma production both underplated at moho level and extruded as thick piles of seaward dipping lava formations the disko svartenhuk area in west greenland is one of the few places in the world with exposed seaward dipping basalts forming a prism whose thickness increases seaward we present a quantitative tectonic study of this margin which we tentatively restored in its geodynamic position during the different stages of plate separation between greenland and north america our structural data are constrained with recently published 39ar 40ar and new and coherent 40k 40ar geochronology in dikes of different orientations the first order structure is that of a tectonically driven seaward crustal flexure linked to definitive plate breakup between greenland and baffin island during the eocene coeval with the formation of the upper part of the exposed seaward dipping volcanic prism this flexing is associated with a significant crustal stretching associated with arrays of continentward dipping normal faults this across strike structure is correlated to a fundamental along strike segmentation with the three segments adopting a zigzag strike there is a clear increase of extensional strain at the extremities of the segments eocene extension trended n060 on average in the northern and southern segments but was nw trending in the central ne trending nugssuaq segment we discuss the interpretation of such an extension perpendicular to the different margin segments from a regional point of view the n060 extension is distinct in orientation from the approximately n s trend of plate separation between north america and greenland in the baffin bay during the eocene however the extension recorded in the margin is nearly perpendicular to the obliquely spreading eocene accretion axis in the baffin bay this latter point suggests a mantle or ridge control of the development of the margin over regional far field lithospheric stress models

structure and growth of the izu bonin mariana arc crust 1 seismic constraint on crust and mantle structure of the mariana arc back arc system 1 a high resolution seismic velocity model is presented for the crust and upper mantle of the mariana arc back arc system mabs based on active source seismic profiling the major characteristics are 1 slow mantle velocity of 8 km s 1 in the uppermost mantle especially and deep reflectors under the mariana arc ma and the west mariana ridge wmr 2 a deep reflector in the upper mantle beneath the relative thick crust of the mariana trough mt axis 3 distribution of lower velocity lower crusts 6 7 6 9 km s 1 beneath the volcanic front and adjacent to the mt and 4 high velocity lower crust 7 2 7 4 km s 1 beneath the boundary regions between the ma and mt and between the wmr and the parece vela basin pvb adding to structural characteristics of crust and upper mantle beneath the mabs of the characteristics described above characteristic 1 suggests that the origins of the slow mantle velocity and the deep reflectors be explained by transfer of the lower crustal residues to the upper mantle across the moho considering that the wmr is extinct arc currently on the other hand characteristic 2 suggests that the origin of deep reflectors beneath the mt axis might be lower velocity materials due to the diffractive signals with strong amplitudes characteristic 3 suggests that the lower velocity lower crust advanced crustal growth and characteristic 4 suggests that the high velocity lower crust beneath arc back arc transition zone is composed of mafic ultramafic materials created by extensive partial melting of mantle peridotites or last stage of the arc magmatism rather than serpentinized peridotite

deep structure of the continental margin and basin off greater kabylia algeria new insights from wide angle seismic data modeling and multichannel seismic interpretation abstract during the algerian french spiral survey aimed at investigating the deep structure of the algerian margin and basin two coincident wide angle and reflection seismic profiles were acquired in central algeria offshore greater kabylia together with gravimetric bathymetric and magnetic data this 260 km long offshore onshore profile spans the balearic basin the central algerian margin and the greater kabylia block up to the southward limit of the internal zones onshore results are obtained from modeling and interpretation of the combined data sets the algerian basin offshore greater kabylia is floored by a thin oceanic crust 4 km with p wave velocities ranging between 5 2 and 6 8 km s in the northern hannibal high region the atypical 3 layer crustal structure is interpreted as volcanic products stacked over a thin crust similar to that bordering the margin and related to miocene post accretion volcanism these results support a two step back arc opening of the west algerian basin comprising oceanic crust accretion during the first southward stage and a magmatic and probably tectonic reworking of this young oceanic basement during the second westward opening phase the structure of the central algerian margin is that of a narrow 70 km magma poor rifted margin with a wider zone of distal thinned continental crust than on the other margin segments there is no evidence for mantle exhumation in the sharp ocean continent transition but transcurrent movements during the second opening phase may have changed its initial geometry the plio quaternary inversion of the margin related to ongoing convergence between africa and eurasia is expressed by a blind thrust system under the margin rising toward the surface at the slope toe and by an isostatic disequilibrium resulting from opposite flexures of two plates decoupled at the continental slope this disequilibrium is likely responsible for the peculiar asymmetrical shape of the crustal neck that may thus be a characteristic feature of inverted rifted margins

evidence of multi phase cretaceous to quaternary alkaline magmatism on tore madeira rise and neighbouring seamounts from 40ar 39ar ages abstract the tore madeira rise is a seamount chain located 300 km off the portugal and morocco coasts attributed to hotspot activity u pb ages of lavas from the northern and central tore madeira rise range between 103 and 80 5 ma whereas 40 ar 39 ar ages from the central and southern tore madeira rise yield ages ranging from 94 5 to 0 5 ma we performed new 40 ar 39 ar measurements to better understand the geodynamic history of the tore madeira rise plagioclase ages from the bikini bottom and torillon seamounts suggest ages of 90 ma and 60 ma respectively amphiboles from the seine seamount yield an age of 24 0 0 8 ma biotites from lavas of the ashton seamount give ages of 97 4 1 1 ma and 97 8 1 1 ma the geochronological database available on the tore madeira rise has been filtered on statistical criteria to eliminate unreliable ages the resulting database reveals three pulses of alkaline magmatism on the tore madeira rise at 103 80 5 ma at c 68 ma and between 30 ma and the present the magmatism was continuous from 103 ma until c 68 ma and from c 30 ma until the present on the tore madeira rise the surrounding seamounts and the portugal coast we suggest that the space time distribution of this magmatism results from the interaction between a wide thermal anomaly emitting magmatic pulses and the complex motion of the iberian plate supplementary material a detailed ar measurements dataset is available at http www geolsoc org uk sup18359

from rifting to passive margin the examples of the red sea central atlantic and alpine tethys abstract evolution of the red sea gulf of suez and the central atlantic rift systems shows that an initial transtensive rifting phase affecting a broad area around the future zone of crustal separation was followed by a pre oceanic rifting phase during which extensional strain was concentrated on the axial rift zone this caused lateral graben systems to become inactive and they evolved into rift rim basins the transtensive phase of diffuse crustal extension is recognized in many intra continental rifts if controlling stress systems relax these rifts abort and develop into palaeorifts if controlling stress systems persist transtensive rift systems can enter the pre oceanic rifting stage during which the rift zone narrows and becomes asymmetric as a consequence of simple shear deformation at shallow crustal levels and pure shear deformation at lower crustal and mantle lithospheric levels preceding crustal separation extensional denudation of the lithospheric mantle is possible progressive lithospheric attenuation entails updoming of the asthenosphere and thermal doming of the rift shoulders their uplift provides a major clastic source for the rift basins and the lateral rift rim basins their stratigraphic record provides a sensitive tool for dating the rift shoulder uplift asymmetric rifting leads to the formation of asymmetric continental margins corresponding in a simple shear model to an upper plate and a conjugate lower plate margin as seen in the central atlantic passive margins of the united states and morocco this rifting model can be successfully applied to the analysis of the alpine tethys palaeo margins such as rif and the western alps

response of a multi domain continental margin to compression study from seismic reflection refraction and numerical modelling in the tagus abyssal plain abstract the effects of the miocene through present compression in the tagus abyssal plain are mapped using the most up to date available to scientific community multi channel seismic reflection and refraction data correlation of the rift basin fault pattern with the deep crustal structure is presented along seismic line iam 5 four structural domains were recognized in the oceanic realm mild deformation concentrates in domain 1 adjacent to the tore madeira rise domain 2 is characterized by the absence of shortening structures except near the ocean continent transition oct implying that miocene deformation did not propagate into the abyssal plain in domain 3 we distinguish three sub domains sub domain 3a which coincides with the oct sub domain 3b which is a highly deformed adjacent continental segment and sub domain 3c the miocene tectonic inversion is mainly accommodated in domain 3 by oceanwards directed thrusting at the ocean continent transition and continentwards on the continental slope domain 4 corresponds to the non rifted continental margin where only minor extensional and shortening deformation structures are observed finite element numerical models address the response of the various domains to the miocene compression emphasizing the long wavelength differential vertical movements and the role of possible rheologic contrasts the concentration of the miocene deformation in the transitional zone tc which is the addition of sub domain 3a and part of 3b is a result of two main factors 1 focusing of compression in an already stressed region due to plate curvature and sediment loading and 2 rheological weakening we estimate that the frictional strength in the tc is reduced in 30 relative to the surrounding regions a model of compressive deformation propagation by means of horizontal impingement of the middle continental crust rift wedge and horizontal shearing on serpentinized mantle in the oceanic realm is presented this model is consistent with both the geological interpretation of seismic data and the results of numerical modelling

138 121 ma asthenospheric magmatism prior to continental break up in the north atlantic and geodynamic implications along the galicia and gorringe banks and in the iberia abyssal plain of the north atlantic unroofed sub continental mantle fills the gap between true oceanic crust and the continental crust margin these lithospheric peridotites are intruded by gabbros and dolerites and locally covered by basalts primary magmatic zircons extracted from gabbros and meta gabbros of the two banks were dated by the u pb chronometer and initial hafnium isotope signatures hfi were determined on the same grains for mt gettysburg at gorringe gabbro emplacement ages of 137 5 0 5 2 ma and 135 7 0 8 ma are obtained and corresponding hfi lie at 20 5 0 3 2 and 19 5 0 4 substantiating magma formation from severely lile depleted mantle domains gabbro zircons from mt ormonde at gorringe yield a much younger age of 77 1 0 4 ma and the hf isotopes document an intermediately lile depleted mantle source having a hfi of 7 6 0 4 given its age and hf signature emplacement of this rock can be ascribed to the alkaline magmatic event that also affected the iberian continent in upper cretaceous time concerning the galicia section zircons from a meta gabbro yield an emplacement age of 121 7 0 4 ma and a hfi of 14 0 0 2 and a hfi of 14 6 0 2 is obtained for zircons from a previously dated meta gabbro of identical age these results indicate magma extraction from mantle reservoirs that are slightly less lile depleted than those sampled by the about 20 myr older gorringe gabbros the data demonstrate that magmatism occurring prior to complete separation of europe from america was essentially of asthenospheric origin both the 138 135 ma ages for the gorringe gabbros and 122 ma ages for the galicia gabbros are at least 5 myr older than the oldest sediments on gorringe and the break up unconformity at the galicia bank respectively magma source signatures of the syn rift gabbros are in agreement with values expected for differently depleted cretaceous morb type mantle reservoirs and the age difference for magmatism between the gorringe and galicia banks suggests a rate of 4 4 0 3 cm yr for northward progression of continental rifting based on the new results a structural model for iberia america rifting is discussed putting forward the idea that magma emplacement produces a level of weakness and decollement between the rifting crust and its underlying lithospheric mantle

western extension of the iberian european plate boundary during the early cenozoic pyrenean convergence a new model abstract a dense seismic net work allows of drawing the western prolongation of the northern boundary of the iberian plate this early cenozoic deformed boundary is disrupted by two prominent nw se transform faults a kinematic reconstruction with a nw late cretaceous lower eocene motion of the iberian plate is proposed the azores biscay rise is interpreted as a short lived subduction zone whereas king s trough could be a transform fault

the importance of serpentinite mylonites for subduction and exhumation of oceanic crust abstract in the ultramafic erro tobbio unit voltri massif western alps a set of overprinting structures in serpentinite mylonites is related to alpine subduction to about 80 km depth and to subsequent exhumation antigorite mylonites are cut by en echelon olivine veins which in turn are dissected by multiple sets of shear bands containing olivine and titanian clinohumite the transition from olivine free to olivine bearing structures indicates recrystallization during prograde metamorphism all structures display the same top to the nw kinematics providing evidence for a continuous non coaxial deformation the serpentinite mylonites surround km scale bodies of pre alpine peridotite which show only minor alpine overprint this indicates that during subduction related deformation recrystallization and fluid flow were strongly localized within serpentinite mylonites olivine bearing discontinuous shear planes with top to the se sense of movement crosscut the prograde structures the inversion of shear sense suggests a change in position of the serpentinites relative to the downgoing slab i e from the subducted slab to the upper plate during accretion thus the shear sense inversion marks the change from burial to exhumation of the serpentinites the low density of antigorite serpentinites 2 75 g cm3 causes strong buoyancy thus providing a mechanism for the exhumation of deeply subducted rocks it is suggested that serpentinites may act as carriers for the uprise of eclogite bodies which have higher densities than the peridotitic upper mantle

crustal structure of the continent ocean transition off the cote d ivoire ghana transform margin implications for thermal exchanges across the palaeotransform boundary summary crustal structure of the continent ocean transition off the cote d ivoire ghana transform margin is determined by 2 d modelling of wide angle seismic data and gravity data the resulting models together with geological data recently collected by drilling on the margin are compared with available thermomechanical models of transform margins in particular we test the predicted effects on the transform margin crust of continental heating by the adjacent oceanic lithosphere these effects include i ductile flow of the lower continental crust either parallel to the transform direction or perpendicular to it from the continent towards the ocean and ii thermal uplift of the transform margin in the region of the cote d ivoire ghana transform margin the seismic and gravity data indicate that the continental crust remains uniform in thickness as the continent ocean boundary is approached the interpreted models lack a transitional domain between continental and oceanic crusts and abnormally thin oceanic crust is found along the margin these results exclude large scale ductile flow of lower continental crust during the margin formation the geological data are moreover not consistent with thermal uplift of the margin during the period of contact between the transform margin and the spreading centre our geophysical and geological data thus infer only moderate heating of the transform continental margin by the adjacent oceanic lithosphere moderate heating of the cote d ivoire ghana transform margin during margin formation implies that the thermal exchange between the transform margin and the adjacent spreading centre were lower than predicted by available thermal models two main factors may have contributed to reduce thermal exchanges across the continent ocean boundary the first factor is that the oceanic edge may have been colder than assumed by thermal models an abnormally cold spreading axis along the transform margin when the oceanic crust was emplaced is supported by the atypical structure of the oceanic crust along the cote d ivoire ghana transform margin the oceanic crustal structure is similar to that found along oceanic fracture zones where oceanic crust is also emplaced near a ridge transform intersection in both cases abnormally low thermal gradients at the spreading centre along the transform margin may be explained by the cumulative effect of i a cold edge effect of the adjacent continental lithosphere ii accretion at the end of a ridge segment away from the main magma supply point along the spreading axis and iii an efficient upward cooling of the oceanic lithosphere owing to deep hydrothermal circulation within the faulted lithosphere the second factor causing moderate thermal exchanges is the possibility of large amounts of water in oceanic lithospheric faults which reduces the lateral thermal conduction in the lithosphere since water is much less conductive than rock during the margin formation the oceanic edge thus seems to have acted as a thermal buffer between the cold continental margin and the hot spreading centre we propose that this thermal buffer hypothesis may be generalized to most transform margins implying a lower amount of continental heating than assumed by the available thermomechanical models

stress sensitivity of stylolite morphology abstract stylolites are rough surfaces that form by localized stress induced dissolution using a set of limestone rock samples collected at different depths from a vertical section in cirque de navacelles france we study the influence of the lithostatic stress on the stylolites morphology on the basis of a recent morphogenesis model we measured the roughness of a series of bedding parallel stylolites and show that their morphology exhibits a scaling invariance with two self affine scaling regimes separated by a crossover length l at the millimeter scale consistent with previous studies the importance of the present contribution is to estimate the stylolite formation stress from the sample position in the stratigraphic series and compare it to the crossover length l using the expected relationship l 2 we obtained a successful prediction of the crossover behavior and reasonable absolute stress magnitude estimates using relevant parameters depth of stylolite formation between 300 to 600 m with corresponding normal stress in the range of 10 18 mpa accordingly the stylolite morphology contains a signature of the stress field during formation and we thus suggest that stylolites could be used as paleo stress gauges of deformation processes in the upper crust

gabbro and related rock emplacement beneath rifting continental crust upb geochronological and geochemical constraints for the galicia passive margin spain the thinned continental crust of the west galicia margin is bound by a belt of serpentinized peridotites peridotite ridge lying about 300 km off the coast in the north atlantic ocean from this ridge a gabbro and a chlorite rock were studied in an attempt to substantiate rift related subcontinental magmatism occurring prior to sea floor spreading u pb dating of 13 different zircon fractions yields a precise age of 122 1 0 3 ma 2 for the emplacement of the chlorite rock protolith from which more than 50 of si and alkali calc alkali elements were lost during greenschist facies tectonometamorphism sr and nd isotope signatures suggest that the gabbro and chlorite rock protoliths were derived from mantle sources that were moderately depleted in lile relative to a chondritic reservoir no evidence for the presence of continental material in the magma source regions can be observed from the new zircon age of 122 1 0 3 ma and earlier determined39ar40ar age of 122 0 0 6 ma for amphibole from the same locality it can be documented that magma formation solidification and unroofing of the mantle rocks occurred during a short period of time of about 3 4 ma which means that the peridotite ridge detached from the continent and rose to the surface immediately after or even coevally with mantle melting

morphotectonics of the gorringe bank summit eastern atlantic ocean based on high resolution multibeam bathymetry abstract high resolution swath bathymetry of the gorringe bank located along the europe africa obliquely convergent plate boundary offshore sw portugal allowed investigation of its shallow depth morphotectonic structure as a contribution to understanding the interplay of sea level changes and active deformation of the region dominant ene wsw to nne wsw and subordinate nw se trending morphologic alignments were mapped on the digital terrain model dtm built from multibeam data for both the gettysburg and ormonde summits of the bank based on appraisal of regional structural and seismological data the dominant and subordinate sets are interpreted to reflect dip slip to left oblique reverse faults and normal faults or dyke like joints respectively several terrace remnants were mapped on the dtm and were grouped into five orders at depths ranging from 40 to 120 m morphological features suggest that the terraces were formed in a coastal environment likely during middle late pleistocene sea level changes the terraces are probably polycyclic implying minor vertical tectonic motion however the whole terrace set except the deeper and younger one which was mainly shaped during the lgm lowstand is consistently up to 10 m lower at ormonde in the east relative to gettysburg in the west suggesting displacement across a nne ssw striking fault mapped in the saddle between the two seamounts the structure assemblage is argued to represent the shallow splay system of a crustal scale fault ramp underlying the bank the ene wsw reverse faults have a southward polarity and together with the overall northwest tilt of the bank point to displacement toward the horseshoe abyssal plain to the south at least at upper crustal levels

tectonomagmatic evolution of the final stages of rifting along the deep conjugate australian antarctic magma poor rifted margins constraints from seismic observations the processes related to hyperextension exhumed mantle domains lithospheric breakup and formation of first unequivocal oceanic crust at magma poor rifted margins are yet poorly understood in this paper we try to bring new constraints and new ideas about these latest deformation stages by studying the most distal australian antarctic rifted margins we propose a new interpretation linking the sedimentary architectures to the nature and type of basement units including hyperextended crust exhumed mantle embryonic and steady state oceanic crusts one major implication of our study is that terms like prerift synrift and postrift cannot be used in such polyphase settings which also invalidates the concept of breakup unconformity integration and correlation of all available data particular seismic and potential field data allows us to propose a new model to explain the evolution of magma poor distal rifted margins involving multiple and complex detachment systems we propose that lithospheric breakup occurs after a phase of proto oceanic crust formation associated with a substantial magma supply first steady state oceanic crust may therefore not have been emplaced before 53 3 ma corresponding to magnetic anomaly c24 observations of magma amount and its distribution along the margins highlight a close magma fault relationship during the development of these margins

the formation of passive margins constraints from the crustal structure and segmentation of the deep galicia margin spain the crustal structure of the mesozoic deep galicia margin and adjacent ocean continent boundary ocb was investigated by seismic reflection including pre stack depth migration and attenuation of seismic waves with time the seismic data were calibrated using numerous geological samples recovered by drilling and or by diving with submersible the n s trending margin and ocb are divided in two distinct segments by ne sw synrift transverse faults locally reactivated and inverted by cenozoic tectonics the transverse faulting and ocb segmentation result from crustal stretching probably in a ne sw direction during the rifting stage of the margin in early cretaceous times the cenozoic tectonics are related to iberia eurasia convergence in palaeogene times pyrenean event in both segments of the deep margin the seismic crust is made of four horizontal layers 1 two sedimentary layers corresponding to post and syn rift sequences where velocity ranges from 1 9 to 3 5 km s 1 and where the q factor is low the two sedimentary layers being separated by a strong reflector marking the break up unconformity 2 a faulted layer where velocity ranges from 4 0 to 5 2 km s 1 and where the q factor is high this layer corresponds to the margin tilted blocks where continental basement and lithified pre rift sediments were sampled 3 the lower seismic crust where the velocity 7 km s 1 and more and the q factor are the highest this layer probably made of partly serpentinized peridotite is roofed by a strong s s seismic reflector and resting on a scattering poorly reflective moho a composite model based both on analogue modelling of lithosphere stretching and on available structural data accounts for the present structure of the margin and ocb stretching and thinning of the lithosphere are accommodated by boudinage of the brittle levels upper crust and uppermost mantle and by simple shear in the ductile levels lower crust and upper lithospheric mantle two main conjugate shear zones may account for the observations and seismic data one sz1 located in the lower ductile continental crust is synthetic to the tilting sense of the margin crustal blocks another sz2 located in the ductile mantle accounts for the deformation of mantle terranes and their final unroofing and exposure at the continental rift axis now the ocb the s s reflector is interpreted as the seismic signature of the tectonic contact between crustal terranes and mantle rocks partly transformed into serpentinite by syn rift hydrothermal activity it is probably related to both shear zones sz1 and sz2 the seismic moho is lower within the lithosphere at the fresh serpentinized peridotite boundary

crustal structure of the north iberian continental margin from seismic refraction wide angle reflection profiles abstract the structure and geodynamics of the southern margin of the bay of biscay have been investigated from a set of 11 multichannel seismic reflection profiles recorded also at wide angle offsets in an onshore offshore network of 24 obs obh and 46 land sites this contribution focuses on the analysis of the wide angle reflection refraction data along representative profiles the results document strong lateral variations of the crustal structure along the margin and provide an extensive test of the crustal models previously proposed for the northern part of the iberian peninsula offshore the crust has a typical continental structure in the eastern tip of the bay which disappears smoothly towards the nw to reach crustal thickness close to 10 km at the edge of the studied area 45 n 6 w the analysis of the velocity depth profiles altogether with additional information provided by the multichannel seismic data and magnetic surveys led to the conclusion that the crust in this part of the bay should be interpreted as transitional from continental to oceanic typical oceanic crust has not been imaged in the investigated area onshore the new results are in good agreement with previous results and document the indentation of the bay of biscay crust into the iberian crust forcing its subduction to the north the interpreted profiles show that the extent of the southward indentation is not uniform with an alpine root less developed in the central and western sector of the basque cantabrian basin n s to ne sw transfer structures seem to control those variations in the indentation degree

undercrusting by serpentinite beneath rifted margins the west galicia passive margin spain is bounded by a continuous belt of serpentinized peridotite here we present the results of petrological studies of the peridotite 39ar 40ar dating of high temperature ductile shearing of a dioritic dyke crossing the peridotite and regional seismic stratigraphy which demonstrate that the uplift ductile shearing and pervasive serpentinization of mantle rocks occurred at early cretaceous time during the rifting stage of the margin the serpentinite layer resting on fresh peridotite is probably several kilometres thick at present seismic data and gravity data cannot distinguish serpentinite from crustal material but our results from the galicia margin indicate that the seismic crust beneath rifted margin may include mantle material transformed into serpentinite by syn rift hydrothermal activity in some cases this undercrusting process might explain the young age and mobility of the moho beneath stretched continental basement

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

magmatism at the west iberia non volcanic rifted continental margin evidence from analyses of magnetic anomalies we discuss the magmatic development of the west iberia non volcanic rifted continental margin in the north atlantic ocean so called non volcanic rifted continental margins are characterized by a lack of syn rift magmatism and are considered to be largely amagmatic however this is clearly an oversimplification since seafloor spreading itself is a magmatic process and it is implausible that seafloor spreading begins instantaneously we concentrate our attention on the recently described zone of exhumed continental mantle zecm to investigate what magmatic processes accompanied the breakup of the continental lithosphere and the subsequent formation of the zecm leading to the onset of seafloor spreading we use magnetic anomalies supplemented by the interpretations of multichannel seismic reflection profiles and wide angle seismic experiments presented elsewhere forward and inverse modelling of a sea surface magnetic anomaly chart and of surface and deep towed magnetometer profiles shows that anomalies within the zecm differ in trend amplitude and source type from those in the adjacent oceanic crust and thinned continental crust the zecm anomalies appear to be caused by elongated source bodies within 8 km of the top of the acoustic basement aligned parallel to the margin we interpret such bodies as syn extensional intrusions that increase in volume oceanward they eventually merge in the vicinity of a margin parallel basement peridotite ridge to give rise to a continuous crust that records reversals in the earth s magnetic field from the time of anomaly m4 n m5 r i e to mark the onset of seafloor spreading we find no evidence for anomalies formed by seafloor spreading at either slow or ultraslow rates before m5 r 128 ma

ultramafic blocks from the ocean floor southwest of australia abstract samples dredged from the ocean floor near the junction of the naturaliste fracture zone and the diamantina zone 300 km southwest of cape leeuwin western australia contain 3 to 10 cm blocks of cr spinel lherzolites and al spinel olivine clinopyroxenites both assemblages show textural evidence for deformation and recrystallization with extensive development of kaersutite in one clinopyroxenite the blocks are enclosed by clay and fe mn oxide rich fragmental material which also contains a diverse detrital mineral suite and microfossils diopsides in the lherzolites are na and cr rich with marked similarities to those of lherzolite nodules in alkaline basaltic suites the al and ti rich diopsides of the clinopyroxenites are more magnesium than those of common pyroxene rich nodules the lherzolites are interpreted as upper mantle residues while the clinopyroxenites probably represent partly recrystallized cumulates from high temperature basaltic magmas traversing the lherzolite mantle both are thought to have been derived from an ultramafic body emplaced into the shallow crust near the cretaceous early tertiary continent ocean boundary off australia

heat flow and hydrothermal circulation at the ocean continent transition of the eastern gulf of aden abstract in order to investigate the importance of fluid circulation associated with the formation of ocean continent transitions oct we examine 162 new heat flow hf measurements in the eastern gulf of aden obtained at close locations along eight seismic profiles and with multi beam bathymetry the average hf values in the oct and in the oceanic domain 18 m y are very close to the predictions of cooling models showing that the overall importance of fluids remains small at the present time compared to oceanic ridge flanks of the same age however local hf anomalies are observed although not systematically in the vicinity of the unsedimented basement and are interpreted by the thermal effect of meteoric fluids flowing laterally we propose a possible interpretation of hydrothermal paths based on the shape of hf anomalies and on the surface morphology fluids can circulate either along dip or along strike but are apparently focussed in narrow pipes in several locations in the oct there is no detectable hf anomaly while the seismic velocity structure suggests serpentinization and therefore past circulation we relate the existence of the present day fluid circulation in the eastern gulf of aden to the presence of unsedimented basement and to the local extensional stress in the vicinity of the socotra hadbeen fault zone at the scale of rifted margins fluid circulation is probably not as important as in the oceanic domain because it can be inhibited rapidly with high sedimentation rates serpentinization and stress release after the break up

models of the development of the west iberia rifted continental margin at 40 30 n deduced from surface and deep tow magnetic anomalies the ocean continent transition oct on nonvolcanic rifted continental margins is important in that it contains evidence concerning the breakup of the continents and the onset of seafloor spreading the nature of the oct off western lberia has recently been attracting attention following seismic and other geophysical studies there and drilling of acoustic basement by leg 149 of the ocean drilling program here we concentrate on the interpretation of a new digital magnetic anomaly chart for the iberia abyssal plain between 39 5 o and 42 2 o n the most striking anomaly is a trough which exists immediately east of and locally parallel to anomaly j and is closely coincident with a basement peridotite ridge we conclude from modeling surface magnetic anomalies and a deep tow magnetometer profile that seafloor spreading began about 129 9 m y ago barremian at a rate of 10 0 mm yr consistent with drilling and seismostratigraphic results from the southern lberia abyssal plain a second feature is the existence of linear low amplitude isochron parallel magnetic anomalies east of the peridotite ridge trough and a clear change in the trend of these anomalies east of about 11 o 15 w it is possible to model these anomalies by a series of 7 to 30 km wide weakly magnetized crustal blocks of about the same thickness as the oceanic crust the west to east anticlockwise change in trend at 11 o 15 w suggests either a major change in stress regime and or in the type of magnetic anomaly source we speculate that the region between the peridotite ridge and 11 o 15 w represents a transitional crust whose formation was dominated by the same stress regime which accompanied eventual seafloor spreading we explain the change in trend at 11 o 15 w by a model featuring a rift which propagated through continental crust from south to north we consider three scenarios for the formation of the transitional zone crust first continental crust is surrounded and impregnated by intrusive and extrusive material formed from passive upwelling of magma created by the limited adiabatic decompression partial melting which occurs as the two continental lithospheric plates first begin to separate second such crust is surrounded by unroofed upper mantle material or third the zone is the product of ultraslow seafloor spreading present evidence tends to support the first scenario in this scenario the top of the asthenosphere eventually becomes shallow enough to enable magma to penetrate the thinnest continental crust fill the space caused by additional extension and form the transitional crust already described over a broad zone at least several tens of kilometers wide at breakup upper mantle rocks become exposed on the seafloor and undergo extensive serpentinization in a final stage we envisage focusing of the igneous activity leading to the onset of seafloor spreading initially producing thin oceanic crust because of a restricted melt supply the width of continuous tectonically extended continental crust in the southern iberia abyssal plain is less than was previously supposed

from unthinned continent to ocean the deep structure of the west iberia passive continental margin at 38 n abstract the crustal and lithospheric mantle structure at the south segment of the west iberian margin was investigated along a 370 km long seismic transect the transect goes from unthinned continental crust onshore to oceanic crust crossing the ocean continent transition oct zone the wide angle data set includes recordings from 6 obss and 2 inland seismic stations kinematic and dynamic modeling provided a 2d velocity model that proved to be consistent with the modeled free air anomaly data the interpretation of coincident multi channel near vertical and wide angle reflection data sets allowed the identification of four main crustal domains i continental east of 9 4 w ii continental thinning 9 4 w 9 7 w iii transitional 9 7 w 10 5 w and iv oceanic west of 10 5 w in the continental domain the complete crustal section of slightly thinned continental crust is present the upper ucc 5 1 6 0 km s and the lower continental crust lcc 6 9 7 2 km s are seismically reflective and have intermediate to low p wave velocity gradients the middle continental crust mcc 6 35 6 45 km s is generally unreflective with low velocity gradient the main thinning of the continental crust occurs in the thinning domain by attenuation of the ucc and the lcc major thinning of the mcc starts to the west of the lcc pinchout point where it rests directly upon the mantle in the thinning domain the moho slope is at least 13 and the continental crust thickness decreases seaward from 22 to 11 km over a 35 km distance stretched by a factor of 1 5 to 3 in the oceanic domain a two layer high gradient igneous crust 5 3 6 0 km s 6 5 7 4 km s was modeled the intra crustal interface correlates with prominent mid basement 10 15 km long reflections in the multi channel seismic profile strong secondary reflected pmp phases require a first order discontinuity at the moho the sedimentary cover can be as thick as 5 km and the igneous crustal thickness varies from 4 to 11 km in the west where the profile reaches the madeira tore rise in the transitional domain the crust has a complex structure that varies both horizontally and vertically beneath the continental slope it includes exhumed continental crust 6 15 6 45 km s strong diffractions were modeled to originate at the lower interface of this layer the western segment of this transitional domain is highly reflective at all levels probably due to dykes and sills according to the high apparent susceptibility and density modeled at this location sub moho mantle velocity is found to be 8 0 km s but velocities smaller than 8 0 km s confined to short segments are not excluded by the data strong p wave wide angle reflections are modeled to originate at depth of 20 km within the lithospheric mantle under the eastern segment of the oceanic domain or even deeper at the transitional domain suggesting a layered structure for the lithospheric mantle both interface depths and velocities of the continental section are in good agreement to the conjugate newfoundland margin a 40 km wide oct having a geophysical signature distinct from the oct to the north favors a two pulse continental breakup

the continent ocean crustal transition across the southwest greenland margin this paper examines the complete crustal transition across the nonvolcanic southwest greenland continental margin of the labrador sea using wide angle and coincident vertical incidence seismic profiles six ocean bottom seismometers and a sonobuoy record p and s wave first and multiple arrivals from the crust and upper mantle which are analyzed by two dimensional dynamic ray tracing and one dimensional reflectivity modeling the resulting seismic velocity model requires that the preexisting 30 km thick continental crust is thinned abruptly to 3 km across the continental slope primarily by removal of the lower crust farther seaward the crust thickens to 6 km primarily through the addition of a high velocity 7 0 7 6 km s layer in the lower crust this lower crustal layer is 4 5 km thick extends for a horizontal distance of 80 km and is interpreted as partially serpentinized upper mantle it is overlain by a low velocity 4 0 5 0 km s upper layer which is interpreted as highly fractured upper continental crust our model suggests that seafloor spreading did not start until chrons 27 28 13 ma younger than previously suggested this interpretation is supported by two dimensional modeling of gravity and magnetic data along the refraction line our results are consistent with a simple shear mechanism for the initial rifting with the sw greenland margin as the upper plate however a full characterization of the rifting mechanism must await comparison with a seismic model for the conjugate margin east of labrador

relationship between anisotropy of p and s wave velocities and anisotropy of attenuation in serpentinite and amphibolite velocities and q values of p and s waves as functions of pressure and temperature at 100 and 600 mpa are presented for a serpentinite and an amphibolite both rocks exhibit a strong lattice preferred orientation lpo of the major mineral phases antigorite and hornblende respectively velocities and q values increase with pressure the rate of increase is different in the three orthogonal directions normal and parallel to foliation and lineation and closely related to progressive closure of microcracks increasing temperature decreases velocities and q values only slightly as long as thermal cracking is prevented by the applied confining pressure substantial anisotropy of velocities and q in p and s waves is observed in both rocks but is found to be different in origin anisotropy of p and s wave velocities is highest at low pressure and basically caused by constructive interference of effects related to oriented microcracks and to the lpo of major minerals increasing confining pressure decreases velocity anisotropy at a smaller and smaller rate the residual anisotropy of p and s wave velocities shear wave splitting at high confining pressure is mainly a result of preferred mineral orientation by contrast anisotropy of q is very low at low confining pressure and markedly enhanced as pressure is increased at high confining pressure substantial anisotropy of q in p waves is apparent but reversed from that of p wave velocities qp is highest in the direction normal to the foliation plane whereas vp and vs is lowest in this direction the generation of a pronounced anisotropy of qp by increasing pressure is due to a directionally dependent increase of contact areas on the oriented grain boundaries of the platy minerals defining the foliation the increase of q with pressure in the direction normal to foliation is mainly caused by the decrease of energy loss due to compressive strain relative to shear strain the reverse is true for the x and y directions serpentinite and x direction amphibolite parallel to the foliation plane

the association of continental crust rocks with ophiolites in the northern apennines italy implications for the continent ocean transition in the western tethys abstract the late cretaceous sedimentary melanges from the external liguride units of the northern apennines include large slide blocks of ophiolites and lower and upper continental crust rocks representative of a continent ocean transition between the internal liguride oceanic domain and the thinned continental margin of the adria plate the slide blocks preserve a record of the long lived history of rifting which led to opening of the jurassic western tethys basin the external liguride ophiolites consist of 1 undepleted spinel peridoties partly re equilibrated under plagioclase facies conditions which were interpreted as unroofed subcontinental mantle 2 rare gabbroic rocks mainly troctolite to olivine bearing gabbro probably crystallised from n morb magmas and 3 basalts with n to t morb affinity covered by late callovian early oxfordian radiolarian cherts both gabbroic rocks and basalts locally intrude the mantle peridotites and postdate their re equilibration to plagioclase facies conditions the slide blocks of lower continental crust are composed of gabbro derived mafic granulites and felsic granulites the latter include quartzo feldspathic granulites and rare quartz poor to quartz free charnockitic rocks in both mafic and felsic granulites granulite facies re equilibration was followed by a retrograde metamorphic evolution to amphibolite greenschist and subgreenschist facies conditions retrogression is commonly accompanied by deformations progressively changing from plastic to brittle the upper crustal rocks occurring as slide blocks consist of hercynian granitoids with orogenic affinity mainly biotite bearing granodiorites and peraluminous two mica leucogranites locally the granitoids are intruded by basaltic dykes or capped by basaltic flows and radiolarian cherts the granitoids underwent polyphase brittle deformations under subgreenschist facies conditions which predated the basalt emplacement the tectono metamorphic evolution recorded by the slide blocks of the external liguride units started in the late carboniferous early permian about 290 ma with the emplacement at deep crustal levels of the gabbroic protoliths for the mafic granulites the associated felsic granulites likely represent the remnants of the lower continental crust intruded by the gabbro derived granulites mafic and felsic granulites subsequently underwent tectonic exhumation in permo triassic times as testified by the development of granulite to amphibolite facies ductile shear zones the granulites were finally exhumed to shallow levels probably in association with the subcontinental mantle in late triassic middle jurassic times the latter period was most likely characterized by extensive brittle faulting at shallow crustal levels thus giving rise to extensional allochthons formed by stretched slices of granitoids the western tethys opening is finally testified by the basalt intrusion and effusion in the late jurassic followed by deep sea pelagic sedimentation the external liguride crustal stratigraphy can be regarded as a fossil example of the transitional realm at the continent ocean boundary this reconstruction fits well with the available data on the present day continental margins derived from passive lithosphere stretching

reconstruction of the exhumed mantle across the north iberian margin by crustal scale 3 d gravity inversion and geological cross section recent models support the view that the pyrenees were formed after the inversion of a previously highly extended continental crust that included exhumed upper mantle rocks mantle rocks remain near to the surface after compression and mountain building covered by the latest cretaceous to paleogene sequences 3 d lithospheric scale gravity inversion demands the presence of a high density mantle body placed within the crust in order to justify the observed anomalies exhumed mantle having 50 km of maximum width continuously extends beneath the basque cantabrian basin and along the northern side of the pyrenees the association of this body with rift postrift and inversion structural geometries is tested in a balanced cross section across the basque cantabrian basin that incorporates a major south dipping ramp flat ramp extensional detachment active between valanginian and early cenomanian times results indicate that horizontal extension progressed 48 km at variable strain rates that increased from 1 to 4 mm yr in middle albian times low strength triassic keuper evaporites and mudstones above the basement favor the decoupling of the cover with formation of minibasins expulsion rollovers and diapirs the inversion of the extensional system is accommodated by doubly verging basement thrusts due to the reactivation of the former basin bounding faults in eocene oligocene times total shortening is estimated in 34 km and produced the partial subduction of the continental lithosphere beneath the two sides of the exhumed mantle obtained results help to pinpoint the original architecture of the north iberian margin and the evolution of the hyperextended aborted intracontinental basins

seismic images and magnetic signature of the late jurassic to early cretaceous africa eurasia plate boundary off sw iberia over the last two decades numerous studies have investigated the structure of the west iberia continental margin a non volcanic margin characterized by a broad continent ocean transition cot however the nature and structure of the crust of the segment of the margin off sw iberia is still poorly understood because of sparse geophysical and geological data coverage here we present a 275 km long multichannel seismic reflection mcs profile line ar01 acquired in e w direction across the horseshoe abyssal plain to partially fill the gap of information along the sw iberia margin line ar01 runs across the inferred plate boundary between the iberian and the african plates during the opening of the central atlantic ocean the boundary separates crust formed during or soon after continental rifting of the sw iberian margin from normal seafloor spreading oceanic crust of the central atlantic ocean line ar01 has been processed and pre stack depth migrated to show the tectonic structure of the crust across the palaeo plate boundary this boundary is characterized by a 30 40 km wide zone of large basements highs related to landward dipping reflections which penetrate to depths of 13 15 km and it marks a change in the character of the basement structure and relief from east to west in this study we have used pre stack depth migrated images the velocity model of line ar01 and magnetic data available in the area to show that the change in basement structure occurs across the fossil plate boundary separating african oceanic crust of the m series m21 m16 to the west from the transitional crust of the iberian margin to the east