crustal thinning in the northern tyrrhenian rift insights from multichannel and wide angle seismic data across the basin extension of the continental lithosphere leads to the formation of rift basins or rifted continental margins if breakup occurs seismic investigations have repeatedly shown that conjugate margins have asymmetric tectonic structures and different amount of extension and crustal thinning here we compare two coincident wide angle and multichannel seismic profiles across the northern tyrrhenian rift system sampling crust that underwent different stages of extension from north to south and from the flanks to the basin center tomographic inversion reveals that the crust has thinned homogeneously from 24 km to 17 km between the corsica margin and the latium margin implying a factor of 1 3 1 5 on the transect 80 km to the south the crust thinned from 24 km beneath sardinia to a maximum of 11 km in the eastern region near the campania margin factor of 2 2 the increased crustal thinning is accompanied by a zone of reduced velocities in the upper crust that expands progressively toward the southeast we interpret that the velocity reduction is related to rock fracturing caused by a higher degree of brittle faulting as observed on multichannel seismic images locally basalt flows are imaged intruding sediment in this zone and heat flow values locally exceed 100 mw m2 velocities within the entire crust range 4 0 6 7 km s which are typical for continental rocks and indicate that significant rift related magmatic underplating may not be present the characteristics of the pre tectonic syn tectonic and post tectonic sedimentary units allow us to infer the spatial and temporal evolution of active rifting in the western part of the southern transect thick postrift sediments were deposited in half grabens that are bounded by large fault blocks fault spacing and block size diminish to the east as crustal thinning increases recent tectonic activity is expressed by faults cutting the seafloor in the east near the mainland of italy the two transects show the evolution from the less extended rift in the north with a fairly symmetric conjugate structure to the asymmetric margins farther south this structural evolution is consistent with w e rift propagation and southward increasing extension rates

crustal taper and topography at passive continental margins terra nova 23 349 361 2011 abstract the long term evolution of kilometre s high seaward facing escarpments at passive margins is linked directly to the crustal thinning gradient the development of post rift faults in the onshore parts of the margin and the associated distribution of drainage patterns landscape types and sediment dispersal patterns reflect this linkage for the seismically well imaged norwegian margin as well as for a number of passive margins worldwide we identify a scaling relationship that correlates escarpment height with the distance to the highly extended distal margin or more specifically the taper break the highest escarpments and the most asymmetric margin topography are found inboard of sharply tapering crystalline crust independent of margin age conversely escarpments are lower where the taper is gentle thus the topography of passive margins does not primarily reflect age magmatism climate or mantle convection but more probably the response to loading of the laterally variable fault controlled architecture of the crystalline crust

structural style formation of low angle normal fault and its controls on the evolution of baiyun rift northern margin of the south china sea abstract recent studies of high resolution seismic reflection profiles in the northern rifted margin of the south china sea revealed that low angle normal faults dominated the structural evolution of deepwater basins rather than high angle normal faults solely as was previously assumed however none of the studies proposed kinematic restorations of the fault system and the relationship to continental margin formation the baiyun rift is located in the slope of the northern margin of the south china sea underlain by extremely thinned continental crust it is filled with a relatively complete sequence of cenozoic sediments up to 10 km and has been considered the best documented deepwater basin basin wide 3d seismic data together with the available deepwater well data have allowed the structural framework to be built and tectono stratigraphic history of the baiyun rift to be traced in detailed from the rift stage a quantitative study of the primary geometry and kinematics of the major faults can be performed consequently results show the inception of the rift is characterized by faults slipped at high angles the later stage mid eocene of intensive extension is dominated by a fault system with large horizontal displacements 10 km and initial low dips

interaction between prerift salt and detachment faulting in hyperextended rift systems the example of the parentis and mauleon basins bay of biscay and western pyrenees prerift salt layers associated with extensional detachment faults exhuming mantle and deeper crustal rocks at the sea floor are observed in the parentis and arzacq mauleon basins located at the eastern termination of the bay of biscay how detachment faults interact with salt in hyperextended rift systems is yet little understood based on field observations and drill hole and seismic data we propose a new model to explain the interaction between salt tectonics and extensional detachment systems we demonstrate that the presence of a thick prerift salt layer in an area undergoing extreme crustal thinning can control the geometry and evolution of rift systems and obscure the rift related structures in the underlying basement during an initial stage of rifting prerift salt layers act as a decoupling horizon between sub and suprasalt units and hinder crustal detachment faults to cut through the salt layers and form breakaways at the sea floor as a consequence they sole out along the ductile salt layer and no subsalt material can be exhumed to the sea floor thus sub and suprasalt layers deform by different deformation modes which makes that detachment fault difficult to identify on seismic images in a later stage when salt has migrated and thinned out sub and suprasalt layers can locally couple and detachment faults can be exposed at the surface resulting in windows of exhumed basement surrounded by extensional allochthons formed by suprasalt sedimentary units

contourite drifts on early passive margins as an indicator of established lithospheric breakup the albian cenomanian breakup sequence bs offshore northwest iberia is mapped described and characterised for the first time in terms of its seismic and depositional facies the interpreted dataset comprises a large grid of regional 2d seismic reflection profiles complemented by industry and odp dsdp borehole data within the bs are observed distinct seismic facies that reflect the presence of a black shales and fine grained turbidites b mass transport deposits mtds and coarse grained turbidites and c contourite drifts borehole data show that these depositional systems developed as mixed carbonate siliciclastic sediments proximally and as organic carbon rich mudstones black shales distally on the northwest iberia margin mtds and turbidites tend to occur on the continental slope frequently in association with large scale olistostromes distally these change into interbedded fine grained turbidites and black shales showing widespread evidence of deep water current activity towards the top of the bs current activity is expressed by intra bs erosional surfaces and sediment drifts the results in this paper are important as they demonstrate that contourite drifts are ubiquitous features in the study area after aptian albian lithospheric breakup therefore we interpret the recognition of contourite drifts in northwest iberia as having significant palaeogeographic implications contourite drifts materialise the onset of important deep water circulation marking the establishment of oceanic gateways between two fully separated continental margins as a corollary we postulate the generation of deep water geostrophic currents to have had significant impact on north atlantic climate and ocean circulation during the albian cenomanian with the record of such impacts being preserved in the contourite drifts analysed in this work

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 geodynamics of lithospheric extension lithospheric extension is a fundamental plate tectonic process controlling the collapse of mountain belts the break up of continents and the formation of new oceanic basins this review summarises major advances on understanding a the nature of the ocean continent transition b the origin and evolution of detachment faults c the dynamic strength of continental lithosphere during extension and d the role of magmatism during extension major steps have been made in mapping out the complexities of the ocean continent transition zone particularly in magma poor rifted margins where extensional structures are not masked by voluminous magmatism the role of detachment faulting during rifting seems to be crucial and explains observations on depth dependent stretching of the lithosphere despite new insights into the dynamic behaviour of the lithosphere its rheological response remains a major uncertainty in modelling the evolution of extensional systems likewise the multiple roles that basaltic magmatism may have in modifying extension remain a key question future insights into the global behaviour of extensional systems will be gained by constraining lithospheric rheology through integrating structural evolution and magmatism in the field geodetic measurements of active extension thermochronology seismic surveys and drilling programs 2008 elsevier b v all rights reserved

deep structure and metallogeny of early proterozoic mobile belts in the light of superdeep drilling in russia abstract superdeep drilling in russia and ukraine provides new direct information about the internal structure of the continental crust its evolution and associated ore forming processes deep levels of early proterozoic mobile belts have been investigated with two superdeep boreholes in russia and ukraine 1 in the kola peninsula in the baltic shield and 2 in the krivoy rog basin in the ukrainian shield the investigations of the kola superdeep borehole invalidate previous hypotheses on the conrad discontinuity they prove intensive proterozoic tectonic and metamorphic reworking of archaean rocks and reveal mineralizations of sulphide copper nickel and gold at depth in the light of the new evidence two geotectonic models of the pechenga imandra varzuga belt are reviewed 1 an intracontinental rift subject to the later compression and 2 an ocean opening followed by development of a collisional suture zone the krivoy rog belt has traditionally been described as a deep synclinorium filled with banded iron formations and cut in the west by a steep regional fault the krivoy rog superdeep hole was predicted to intersect all members of the early proterozoic krivoy rog group and to meet tonalites of the archaean basement at 9 10 km depth contrary to expectations however the borehole reached the basement complex at a depth of 2350 m the drilling stimulated geological and geophysical studies of adjacent territories and the results are reviewed in the light of two geotectonic models for the central part of the ukranian shield 1 a plate tectonic model involving westward subduction of oceanic crust and 2 a model implying a rise of asthenolite during early proterozoic times west of the krivoy rog basin the superdeep borehole data lead to better understanding of the structures of the continental crust and its metallogeny but at the same time pose new questions to which answers have yet to be found

hyperextension serpentinization and weakening a new paradigm for rifted margin compressional deformation the early eocene magma rich northeast atlantic rifted margins contain a large number of pre breakup and post breakup compressional structures located in abandoned early cretaceous hyperextended basins with crustal stretching factors of 3 4 or more the deformation both predates and postdates the magma rich breakup the hyperextended basins are often underlain by high velocity lower crustal bodies which we argue represent partially serpentinized upper mantle long lived lithospheric weakening and proneness to deformation is proposed to relate to crustal hyperextension probably enhanced by mantle hydration

strain migration during multiphase extension observations from the northern north sea many rifts develop through multiphase extension it can be difficult however to determine how strain is distributed during reactivation because structural and stratigraphic evidence associated with earlier rifting is often deeply buried using 2 d and 3 d seismic reflection and borehole data from the northern north sea we examine the style magnitude and timing of reactivation of a preexisting permian triassic rift phase 1 fault array during a subsequent period of middle jurassic to early cretaceous rift phase 2 extension we show that rift phase 2 led to the formation of new n s striking faults close to the north viking graben but did not initially reactivate preexisting rift phase 1 structures on the horda platform we suggest that at the beginning of rift phase 2 strain was focused in a zone of thermally weakened lithosphere associated with the middle jurassic north sea thermal dome rather than reactivating extant faults diachronous reactivation of the permian triassic fault network eventually occurred with those faults located closer to the middle jurassic to early cretaceous rift axis reactivating earlier than those toward the eastern margin this diachroneity may have been related to flexural down bending as strain became focused within the north viking graben and or the shifting of the locus of rifting from the north sea to the proto north atlantic our study shows that the geometry and evolution of multiphase rifts is not only controlled by the orientation of the underlying fault network but also by the thermal and rheological evolution of the lithosphere and variations in the regional stress field

active normal faulting and crustal extension a world wide review of fault plane solutions and focal depths for large normal faulting earthquakes on the continents shows that the overwhelming majority of such earthquakes nucleate in the depth range 6 15 km on faults dipping between 30 and 60 in the few cases where levelling or seismic data are good enough these normal faults are shown to be approximately planar from the surface to their nucleation depth at the base of the brittle crust there is evidence that in some cases as a result of the enormous transitory increase in strain rate during large earthquakes rupture continues into the normally ductile lower crust on surfaces with substantially gentler dips these low angle surfaces may be analogous to some of the detachments seen in metamorphic core complexes of the western usa but the nature of the motion on them depends on strain rate as well as on rheological contrasts between the detachments and the blocks on either side such contrasts however are unlikely to introduce substantial curvature to an originally planar shear zone if the observed spread in active normal fault dips is caused by rotation of the faults and the blocks they bound during extension then a maximum value of 1 7 can be accommodated by seismic activity on a single generation of normal faults with continued extension either a new generation of steeper faults cutting the rotated first faults is likely to form or the deformation will continue aseismically on faults dipping at less than 30 large earthquakes have not been observed to nucleate on very low angle 20 normal faults within the continental crust anywhere in the world such faults can of course move aseismically but are unlikely to do so on a large scale within the upper crust in areas where steep normal faults are seismically active thus extensional models that require concentrated simple shear on large sub horizontal faults within the brittle upper crust will also require a spatial separation between aseismic very low angle faulting and seismic highangle faulting since seismogenic high angle faults dominate the topography of extending regions upper crustal very low angle faulting if it occurs on a large scale presumably does so in areas that are relatively fiat

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

pre existing normal faults have limited control on the rift geometry of the northern north sea abstract many rifts develop in response to multiphase extension with numerical and physical models suggesting that reactivation of first phase normal faults and rift related variations in bulk crustal rheology control the evolution and final geometry of subsequent rifts however many natural multiphase rifts are deeply buried and thus poorly exposed in the field and poorly imaged in seismic reflection data making it difficult to test these models here we integrate recent 3d seismic reflection and borehole data across the entire east shetland basin northern north sea to constrain the long term regional development of this multiphase rift we document the following key stages of basin development i pre triassic to earliest triassic development of multiple sub basins controlled by widely distributed nnw to ne trending east and west dipping faults ii triassic activity on a single major ne trending west dipping fault located near the basins western margin and formation of a large half graben and iii jurassic development of a large e dipping n to ne trending half graben near the eastern margin of the basin which was associated with rift narrowing and strain focusing in the viking graben in contrast to previous studies which argue for two discrete periods of rifting during the permian triassic and late jurassic early cretaceous we find that rifting in the east shetland basin was protracted from pre triassic to cretaceous we find that during the jurassic most pre jurassic normal faults were buried and in some cases cross cut by newly formed faults with only a few being reactivated previously developed faults thus had only a limited control on the evolution and geometry of the later rift we instead argue that strain migration and rift narrowing was linked to the evolving thermal state of the lithosphere an interpretation supporting the predictions of lithosphere scale numerical models our study indicates that additional regional studies of natural rifts are required to test and refine the predictions of physical and numerical models more specifically our study suggests models not explicitly recognising or including thermal or rheological effects might over emphasise the role of discrete pre existing rift structures such as normal faults

amount of extension on small faults an example from the viking graben extension estimates based on palinspastic restoration of only the largest faults in a deformed terrane may be significantly deficient due to neglect of small faults i e unsampled faults most of which have smaller displacements than do sampled faults small faults individually account for little extension but there may be large numbers of them the cumulative extension on small faults can be calculated from the fractal size distributions of fault displacements independent extension estimates support the inference from fault population statistics that small faults account for significant amounts of extension for example restorations of mesozoic faults interpreted from seismic reflection profiles across the viking graben in the north sea yield extension estimates that are only 40 75 of independent estimates the fractal size distribution of fault displacements in the viking graben is consistent with the hypothesis that the remaining 25 60 of extension resulted from displacements on small faults

hydrate occurrence in europe a review of available evidence abstract large national programs in the united states and several asian countries have defined and characterised their marine methane hydrate occurrences in some detail but european hydrate occurrence has received less attention the european union funded project marine gas hydrate an indigenous resource of natural gas for europe migrate aimed to determine the european potential inventory of exploitable gas hydrate to assess current technologies for their production and to evaluate the associated risks we present a synthesis of results from a migrate working group that focused on the definition and assessment of hydrate in europe our review includes the western and eastern margins of greenland the barents sea and onshore and offshore svalbard the atlantic margin of europe extending south to the northwestern margin of morocco the mediterranean sea the sea of marmara and the western and southern margins of the black sea we have not attempted to cover the high arctic the russian ukrainian and georgian sectors of the black sea or overseas territories of european nations following a formalised process we defined a range of indicators of hydrate presence based on geophysical geochemical and geological data our study was framed by the constraint of the hydrate stability field in european seas direct hydrate indicators included sampling of hydrate the presence of bottom simulating reflectors in seismic reflection profiles gas seepage into the ocean and chlorinity anomalies in sediment cores indirect indicators included geophysical survey evidence for seismic velocity and or resistivity anomalies seismic reflectivity anomalies or subsurface gas escape structures various seabed features associated with gas escape and the presence of an underlying conventional petroleum system we used these indicators to develop a database of hydrate occurrence across europe we identified a series of regions where there is substantial evidence for hydrate occurrence some areas offshore greenland offshore west svalbard the barents sea the mid norwegian margin the gulf of cadiz parts of the eastern mediterranean the sea of marmara and the black sea and regions where the evidence is more tenuous other areas offshore greenland and of the eastern mediterranean onshore svalbard offshore ireland and offshore northwest iberia we provide an overview of the evidence for hydrate occurrence in each of these regions we conclude that around europe areas with strong evidence for the presence of hydrate commonly coincide with conventional thermogenic hydrocarbon provinces

mesozoic exhumation history and palaeolandscape of the iberian massif in eastern galicia from apatite fission track and u th he data apatite fission track aft and u th he ahe data combined with time temperature inverse modelling reveal the cooling and exhumation history of the iberian massif in eastern galicia since the mesozoic the continuous cooling at various rates correlates with variation of tectonic boundary conditions in the adjacent continental margins the data provide constraints on the 107 timescale longevity of a relict paleolandscape aft ages range from 68 to 174 ma with mean track lengths of 10 7 2 6 to 12 6 1 8 m and ahe ages range from 73 to 147 ma fastest exhumation 0 25 km ma occurred during the late jurassic to early cretaceous main episode of rifting in the adjacent western and northern margins exhumation rates have decreased since then and have been approximately one order of magnitude lower across inland galicia the aft data are consistent with early cretaceous movement on post variscan ne trending faults this is coeval with an extensional episode offshore the ahe data in this region indicate less than 1 7 km of denudation in the last 100 ma this low exhumation suggests the attainment of a mature landscape during late cretaceous post rift tectonic stability whose remains are still preserved the low and steady rate of denudation prevailed across inland galicia despite minor n s shortening in the northern margin since 45 ma ago in north galicia rock uplift in response to nw strike slip faulting since early oligocene to early miocene has caused insufficient exhumation 3 km to remove the mesozoic cooling signal recorded by the aft data

segmentation and along strike asymmetry of the passive margin in socotra eastern gulf of aden are they controlled by detachment faults on the island of socotra the southern passive margin of the gulf of aden displays along its strike two different types of asymmetric structures western socotra is made up of a series of southward tilted blocks bounded by consistently northward dipping normal faults eastern socotra consists of a broad asymmetric anticline with a steep northern limb and a gently dipping southern limb a zone of ne sw striking strike slip and normal faults separates the two areas the overall structure is interpreted as representing two rift segments separated by a transfer zone the along strike juxtaposition of crustal scale asymmetric structures on the southern margin of the gulf of aden is complemented by the asymmetry of the conjugate margins on either side of the gulf whereas the western socotra margin is narrow and characterized by oceanward dipping normal faults the conjugate oman margin is broader and dominated by horsts and graben considering that asymmetric structures in the upper crust are often associated with synthetic shear zones at deeper ductile levels we propose that the western and eastern socotra margin segments were controlled at depth by two detachment faults with opposite dips and senses of shear the normal faults of western socotra would sole out into a top to the north ductile shear zone whereas the asymmetric anticline of eastern socotra would be associated with a top to the south detachment fault

the long term topographic response of a continent adjacent to a hyperextended margin a case study from scandinavia we present data that link scandinavia s passive margin domains under a unified system invoking isostatically driven postextension phase vertical adjustments to severe crustal thinning topographic and geological data indicate that the relative location of the first landward occurrence of total crustal embrittlement or deformation coupling the taper break controlled and continues to control scandinavia s post thinning geomorphic evolution formed during late jurassic or early cretaceous thinning yet marked today by seismicity the taper break closely approximates the boundary between 1 less stretched lithosphere that increases in rigidity both toward land and through postrift time and 2 the highly attenuated pervasively faulted permanently weakened lithosphere of the distal margin following the stretching thinning and exhumation phases proposed by other workers an accommodation phase is warranted commencing during sag basin time and continuing today it is probably driven by thermal cooling and mass transfer from the escarpment to the basins offshore the accommodation phase does not entirely coincide with the traditional postrift phase as the former may contain the latter during accommodation the original synrift escarpments can be eroded to very low base levels sharply tapered margin segments can undergo subsequent rejuvenation by out of sequence normal faulting and footwall uplift probably in response to tensile bending stresses engendered by lithospheric scale flexure accommodation phase uplift at passive margins is the inexorable and penultimate phase of hyperextension and may perhaps be followed by the onset of subduction localized by the weakened lithosphere of the distal margin and the ocean continent transition

spatial and temporal evolution of rifting and continental breakup in the eastern black sea basin revealed by long offset seismic reflection data the age and distribution of the syn rift and early post rift infill records the spatial and temporal distribution of extension and breakup processes in a rift basin the eastern black sea basin ebsb is thought to have formed by back arc extension during cretaceous to early cenozoic time however a lack of direct constraints on its deep stratigraphy leaves uncertainties over the time duration and location for rifting and breakup processes in the basin here we use the enhanced imaging provided by 2d long offset seismic reflection profiles to analyze the deep structural and stratigraphic elements of the ebsb based on these elements we infer the presence of two distinct late cretaceous syn rift units recording initial extension rift stage 1 over the continental highs shatsky ridge and the mid black sea high followed by strain localization along the major basin bounding faults and rift migration towards the basin axis rift stage 2 overlying these units palaeocene eocene and oligocene units show a syn kinematic character in the nw with evidence for ongoing extension until oligocene time towards the se these sequences are instead post kinematic directly overlaying a basement emplaced during breakup we interpret the palaeocene oligocene units to record the time spanning from the initiation of breakup late cretaceous palaeocene in the se to the end of extension oligocene in the nw the first ubiquitously post rift infill is the lower miocene maykop formation our results highlight the along strike temporal variability of extension and breakup processes in the ebsb

rifted margins ductile deformation boudinage continentward dipping normal faults and the role of the weak lower crust the stunningly increased resolution of the deep crustal levels in recent industrial seismic profiles acquired along most of the world s rifted margins leads to the unraveling of an unexpected variety of structures it provides unprecedented access to the processes occurring in the middle and lower continental crust we present a series of so far unreleased profiles that allows the identification of various rift related geological processes such as crustal boudinage ductile shear and low angle detachment faulting and a rifting history that differs from the classical models of oceanward dipping normal faults the lower crust in rifted margins appears much more intensely deformed than usually represented at the foot of both magma rich and magma poor margins we observe clear indications of ductile deformation of the deep continental crust along large scale shallow dipping shear zones these shear zones generally show a top to the continent sense of shear consistent with the activity of continentward dipping normal faults cdnf observed in the upper crust this pattern is responsible for a migration of the deformation and associated sedimentation and or volcanic activity toward the ocean we discuss the origin of these cdnf and investigate their implications and the effect of sediment thermal blanketing on crustal rheology in some cases low angle shear zones define an anastomosed pattern that delineates boudin like structures the maximum deformation is localized in the inter boudin areas the upper crust is intensely boudinaged and the highly deformed lower crust fills the inter boudins underneath the boudinage pattern controls the position and dip of upper crustal normal faults we present some of the most striking examples from the margins of uruguay west africa south china sea and barents sea and discuss their implications for the time temperature history of the margins

deep crustal structure of the conjugate margins of the sw south china sea from wide angle refraction seismic data abstract the south china sea is the largest marginal basin of se asia yet its mechanism of formation is still debated a 1000 km long wide angle refraction seismic profile was recently acquired along the conjugate margins of the sw sub basin of the south china sea over the longest extended continental crust a joint reflection and refraction seismic travel time inversion is performed to derive a 2 d velocity model of the crustal structure and upper mantle based on this new tomographic model northern and southern margins are genetically linked since they share common structural characteristics most of the continental crust deforms in a brittle manner two scales of deformation are imaged and correlate well with seismic reflection observations small scale normal faults grabens horsts and rotated faults blocks are often associated with a tilt of the velocity isocontours affecting the upper crust the mid crust shows high lateral velocity variation defining low velocity bodies bounded by large scale normal faults recognized in seismic reflection profiles major sedimentary basins are located above low velocity bodies interpreted as hanging wall blocks along the northern margin spacing between these velocity bodies decreases from 90 to 45 km as the total crust thins toward the continent ocean transition the continent ocean transitions are narrow and slightly asymmetric 60 km on the northern side and no more than 30 km on the southern side indicating little space for significant hyper stretched crust although we have no direct indication for mantle exhumation shallow high velocities are observed at the continent ocean transition the moho interface remains rather flat over the extended domain and remains undisturbed by the large scale normal faults the main decollement is thus within the ductile lower crust

lower crustal extension across the northern carnarvon basin australia evidence for an eastward dipping detachment asymmetry and strain partitioning along conjugate margins are often explained in terms of detachment faults nevertheless cogent evidence for their existence remains limited furthermore even when inferred detachment faults are imaged seismically it is difficult to demonstrate that differential displacement has occurred across these surfaces we present tectonic and stratigraphic evidence from the northern carnarvon basin northwest australia that documents the existence of an intracrustal eastward dipping detachment by integrating sequence stratigraphy with kinematic and isostatic models of basin development we conclude that the northern carnarvon basin was formed as a consequence of four extension events 1 a broadly distributed late permian event 2 a predominantly localized rhaetian event responsible for the inception of the barrow and dampier subbasins 3 a localized callovian fault reactivation within the barrow dampier subbasins and 4 a tithonian valanginian event that generated large post valanginian regional subsidence across the northern carnarvon basin with only minor accompanying brittle deformation and erosional truncation the regional distribution and amplitude of the post valanginian subsidence are not consistent with the minor amounts of tithonian valanginian brittle upper crustal extension observed on the margin large portions of the platform were emergent or at very shallow water depths prior to the tithonian valanginian extension to match the distribution and magnitude of the post valanginian thermal type subsidence requires significant lower crustal and mantle extension across the northern carnarvon basin such a distribution of extension implies the existence of an eastward dipping intracrustal detachment having a ramp flat ramp geometry that effectively thinned the lower crust and lithospheric mantle the detachment breached the surface close to the position of the continent ocean boundary west of the exmouth plateau the flat component of the detachment occurred at midcrustal depths 15 km across the plateau and ramped beneath the australian continent lower crustal ductile extension provides a viable mechanism to generate large regional subsidence with little attendant brittle deformation which may explain the paradox that both sides of many conjugate margins appear to be the upper plate

styles of extensional decoupling simplified models of continental lithospheric extension demonstrate that the strength of the lower crust is an important factor in the evolution of rifting when the lower crust is strong both the crust and the mantle lithosphere should extend in the same place when the lower crust is weak however the upper crust can mechanically decouple from the lithospheric mantle during extension two distinct styles of extensional decoupling can be recognized diffuse decoupling occurs when the lower crust flows laterally in response to topographically induced pressure gradients offset decoupling occurs when stretching of the upper crust is horizontally displaced from mantle lithospheric stretching we show that diffuse decoupling is expected for a range of possible crustal mineralogies but that offset decoupling only occurs for extremely weak mineralogies

evidence for shear zones in the lower crust offshore britain lower crustal reflections seen on british institutions reflection profiling syndicate birps deep seismic reflection data recorded offshore the united kingdom are commonly short convex up and crosscutting consequently the lower crust should not be described as layered as much of the reflecting structure is not fully resolved and may be quite complex moreover the lower crustal reflective zone lcrz is not a uniformly reflective layer but contains reflective bands that wrap around nonreflective transparent zones because transparent zones probably correspond to a lack of a reflecting fabric understanding why these features are unreflective may help constrain the cause of the reflectivity of the lower crust although transparent zones may have a variety of explanations e g large shear pods or frozen magma bodies the geometry of reflective bands and transparent zones is perhaps best explained by the geometry of shear zones and low strain lozenges predicted both by combining theoretical lithospheric strength profiles with slip line field theory and by drawing analogies with structures developed in highly tectonised high grade metamorphic terrains this interpretation implies that lower crustal reflections may be from shear zones and also has implications for the way the lower crust deforms in particular extension may be accommodated by the relative movement of low strain lozenges along intervening zones of simple shear the lower crust may be stretched during extension but it must also transfer deformation between the upper crust and the mantle the observation on birps data of mantle reflections interpreted as mantle shear zones implies that deformation in the mantle is localised however the manue reflections are never colinear with faults in the upper crust requiring at least locally a component of sub horizontal bulk strain within the lower crust which may greatly increase the amount of shearing therein and hence perhaps may increase the reflectivity of the lcrz

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

structure and evolution of the galicia interior basin atlantic western iberian continental margin abstract an industrial multichannel seismic reflection data set is presented on the galicia interior basin g i b a sedimentary basin that has been poorly studied until now seismic stratigraphic units defined in other areas of the western iberian continental margin as well as some deeper reflections can also be identified within this basin although they show some particular characteristics the g i b is bounded to the east by a major fault identified beneath the continental shelf and to the west by an alignment of highs western banks the structural framework of the area consists of nnw sse trending normal faults cross cut by ne sw transfer faults the inception of the g i b probably ocurred during triassic times within the late jurassic early cretaceous rifting phase it probably experienced several extensional episodes the most intense one being during valanginian times evidence for compressional tectonic activity in this basin during cenozoic times has also been found when comparing the g i b with the adjacent deep galicia margin a gap in the timing of the main extensional events as well as some differences in the structural styles have been found in a wider regional context an integration of data about the timing of the processes of continental rifting and oceanic spreading along the whole western iberian margin is consistent with a northward propagating rift for the late jurassic early cretaceous phase

the importance of rift history for volcanic margin formation rifting and magmatism are fundamental geological processes that shape the surface of our planet a relationship between the two is widely acknowledged but its precise nature has eluded geoscientists and remained controversial largely on the basis of detailed observations from the north atlantic ocean mantle temperature was identified as the primary factor controlling magmatic production with most authors seeking to explain observed variations in volcanic activity at rifted margins in terms of the mantle temperature at the time of break up however as more detailed observations have been made at other rifted margins worldwide the validity of this interpretation and the importance of other factors in controlling break up style have been much debated one such observation is from the northwest indian ocean where despite an unequivocal link between an onshore flood basalt province continental break up and a hot spot track leading to an active ocean island volcano the associated continental margins show little magmatism here we reconcile these observations by applying a numerical model that accounts explicitly for the effects of earlier episodes of extension our approach allows us to directly compare break up magmatism generated at different locations and so isolate the key controlling factors we show that the volume of rift related magmatism generated both in the northwest indian ocean and at the better known north atlantic margins depends not only on the mantle temperature but to a similar degree on the rift history the inherited extensional history can either suppress or enhance melt generation which can explain previously enigmatic observations

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

oxygen consuming processes at the profundal and littoral sediment water interface of a small meso eutrophic lake lake vechten the netherlands oxygen consumption in the profundal gyttja and littoral sands of lake vechten was determined with pore water analyses and core incubations methane 58 64 ammonium lo 13 iron 2 6 and sulfide 12 oxidation accounted for 75 of the oxygen consumed in the profundal sediments almost the complete diffusive flux of ch out of the anoxic layer into the oxic surface layer 7 9 9 4 mmol mm2 d l was oxidized while only 27 36 of the nh flux 5 1 5 2 mmol m i d l was oxidized in the oxic surface layer and 64 73 diffused into the overlying water in contrast to the profundal sediments oxidation of reduced end products of anoxic respiration was of minor importance 15 in littoral sandy sediments also sediment oxygen consumption rate was higher in the profundal gyttja 29 mmol m d l at 7 c during overturn than in the littoral sandy sediments in both winter 8 7 mmol m 2 d at 7 c and summer 18 6 mmol me2 d at 22 c poisoning the sediment cores with formaldehyde stopped bacterial activity but oxygen profile measurements showed that this treatment does not reveal the bacterial or chemical nature of the oxidative processes

polyphase faulting during the development of the west galicia rifted margin rifted margins are commonly characterised by an extension discrepancy the amount of extension measurable from the observed faulting is far less than that required to explain the crustal thinning it is shown here that polyphase faulting may provide a simple explanation for this paradox but can be very hard to recognise on seismic sections however at the west galicia rifted margin the galicia interior basin between the galicia bank and the mainland and the deep galicia margin to the west of the galicia bank a combination of high quality depth images seismic velocity information and stratigraphic control through odp drilling and submersible sampling provide complementary evidence for polyphase faulting berriasian hauterivian rifting in the galicia interior basin occurred along two sets of faults the first unroofed deep crustal rocks evidenced by high seismic velocities close to top basement the second cut and dismembered these early faults further rifting up until the aptian then focussed west of the galicia bank where two further phases of faulting can be inferred from the diachronous nature of seismostratigraphic units tilted within fault blocks removal of the latest phase of faulting aligns discontinuous reflections within the fault blocks into an anastomosing network of earlier faults restoration along these brings the crust back to its early hauterivian state similar to the present structure of the interior basin

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

extension of continental crust at the margin of the eastern grand banks newfoundland seismic and gravity observations from the rifted margin of the eastern grand banks newfoundland support a new model for extension of the continental crust from the shelf edge to odp site 1277 where mantle rocks are exhumed we find that the largest decrease in crustal thickness from about 28 km to 6 km occurs beneath the continental slope of the grand banks over a distance of just 20 km this rapid decrease in crustal thickness coincides with anomalously high seismic velocities 7 0 7 2 km s 1 in the lower crust of the shelf edge the thin crust of the continent ocean transition cot in this area has a smooth basement surface void of upper crustal blocks and prerift sediments we compare our geophysical results with a geodynamical model that represents rifting of a relatively hot continental lithosphere and with another numerical model that represents rifting of a cold lithosphere both geodynamic models suggest that crustal thinning beneath the continental slope was achieved by extensional faulting in the upper crust and ductile shear zones in the middle crust the geodynamic models provide an explanation for the formation of distinct continental slopes at rifted margins beneath the continental shelf of the grand banks the moho and the strong lower crust rotated upwards toward to a 50 dip without visible internal deformation the presence of these strong lower crustal rocks at shallow depth in the rift flank subsequently helped to localize the extension farther seaward with ongoing extension some high angle normal faults may have rotated to a sub horizontal orientation which would explain the lack of brittle deformation visible in the seismic reflection data the two geodynamic models produce different amounts of extension of continental crust in the distal margins the hot rifting model localizes strain much more rapidly leaving narrow zones of extended continental crust and it produces a relatively large amount of melt 30 in the final stages of rifting continental breakup may occur rapidly in hot lithosphere 5 myr on the other hand a cold extension model extends the continental crust to a thickness smaller than 10 km over a width of 50 km in the distal margin similar to what we inferred at the eastern grand banks the cold lithospheric model requires about 23 myr of extension before continental breakup and it predicts much less melting in the mantle 13 the long rift duration wide zones of thinned continental crust and small amount of magmatism make the cold rifting model the most applicable to newfoundland iberia rift

extreme crustal thinning in the south porcupine basin and the nature of the porcupine median high implications for the formation of non volcanic rifted margins the southern porcupine basin is characterized by axial stretching factors that are greater than six and typical of rifted margins as such the basin can be regarded as a natural laboratory to investigate the evolution and symmetry of rifting leading towards continental separation and breakup a bright reflection here named p cuts down to the west from the base of the sedimentary section is overlain by small fault blocks and appears to represent a detachment fault p may in part follow the top of partially serpentinized mantle this interpretation is consistent with gravity modelling and with numerical models of crustal embrittlement and mantle serpentinization during extension furthermore p closely resembles the s reflection west of iberia where such serpentinites are well documented although overall the basin remains symmetrical the consistent westward structural dip of the detachment implies that at high stretching factors extension became asymmetric farther south the porcupine median high appearing lens shaped in cross section overlies the tilted fault blocks and is onlapped by postrift sediment despite no evidence for synrift magmatism this high has previously been interpreted as a basaltic structure however it develops above the line of intersection of the crust mantle boundary with the p detachment and hence may be related to the spatial limit of serpentinization the median high may represent a serpentinite mud volcano or diapir we suggest that such structures produce the serpentinite breccias found within the rifted continent ocean transition of nonvolcanic margins

the s reflector west of galicia spain evidence from prestack depth migration for detachment faulting during continental breakup the mechanism of continental extension and breakup and the role of detachment faults in these processes are currently the subject of intense debate one possible detachment fault is the s reflector imaged as an undulating in time locally discontinuous reflection on existing seismic profiles west of iberia here we present new images in depth of the four margin normal profiles across the west galicia rifted margin where the s reflector was originally defined and is best imaged it is shown that faults bounding wedge shaped units of late synrift sediments which hence were active during rifting immediately prior to breakup appear to detach at shallow levels onto the s reflector s itself appears as a continuous locally domal feature and does not generally appear offset the waveform of s is compatible with a reflection from a single interface such as a sharp tectonic boundary the depth sections show that s was active at 1 3 km below the seafloor during final rifting s is interpreted as a brittle detachment fault which controlled the final breakup of the continent west of galicia furthermore the data provide constraints on the sense of shear of s analogy with detachment terranes the present synrift and structural dips of s and the identification of a breakaway to s imply that s accommodated top to the west shear toward the east of the profiles s becomes more complex possibly because of different phases of detachment faulting and the development of both incisement and excisement structures from the geometry of wedge shaped sedimentary units deposited during faulting above s it also appears that s was active at an angle of 20 or less and hence may be considered a genuine low angle normal fault

plate motions and continental extension at the rifting to spreading transition in woodlark basin papua new guinea can oceanic plate kinematics be extended into continental rifts abstract woodlark basin provides a unique opportunity to compare brittle extension subsidence and extension predicted from long term plate motions at the rifting to spreading transition of a non volcanic margin detailed analysis of reflection seismic data near the current rifting to spreading transition yields 111 km 23 km of brittle extension subsidence along the same transect predicts 115 km 47 km of extension long term plate motions derived from seafloor spreading and transform azimuths in the eastern portion of woodlark basin predict 220 km of extension in this area these results imply no large discrepancy between brittle extension and extension predicted from subsidence contrary to observations at ancient rifted margins however there remains a pronounced two fold discrepancy between post breakup plate motions and extension during rifting this implies that continental extension rates do not match seafloor spreading rates and requires that the extension within the rift can only be modeled using euler pole kinematics after the initiation of seafloor spreading this is hypothesized to result from a decoupling of the lithosphere in which the upper crust moves more slowly than the mantle lithosphere pre breakup plate motions are accommodated primarily by extension of the mantle lithosphere as a result of rollback of the solomon sea plate after the initiation of seafloor spreading spreading rates are controlled by the motion of the mantle lithosphere and the upper crust moves at the same rate as the rest of the lithosphere

active normal faulting along the seerteng shan north china geometry and kinematics abstract although rotation of fault blocks during seismic and long term periods has long been recognized in extensional tectonic regimes the geometry and kinematic behaviors of normal faults at outcrop scale have scarcely been investigated in sufficient detail in this study combining with the analysis of high resolution topography acquired by small unmanned aerial vehicle suav using the structure from motion sfm technique we present the results of our field observations on the structural characteristics of the seertengshan piedmont fault spf a major active normal fault bounding the northern margin of the hetao graben north china although previous studies have reported the recent tectonic activity of this fault including timing of most recent earthquake average slip rate and repeat time of paleo earthquakes few of them have focused on geometric patterns of the outcropping fault zones which provide excellent outcrop scale examples to investigating geometry and kinematics of active normal faults besides the observed prominent fault scarps indicative of recent tectonic activity along the spf outcrop observations indicate that in some cases both planar and curved normal faults are developed forming complex fault zones sequential development of normal faults may have rotated the early formed fault blocks resulting in shallowly dipping faults and highly tilted strata collectively structural similarities probably exit among normal faults at multiple spatial scales we suggest that the results of this study are also helpful for a better understanding of structural development and evolution in other actively extending areas

the baiyun and liwan sags two supradetachment basins on the passive continental margin of the northern south china sea abstract detachment faults have been reported on the northern and southern margins of the south china sea scs but have not been recognized in the paleocene eocene baiyun and liwan sags of the pearl river mouth basin prmb this is despite the well known very thin crust beneath the two sags in this study we use 2d and 3d seismic data to interpret the sequence stratigraphy as well as identify faults and other important structural features in the baiyun and liwan sags we show that high amplitude seismic reflectors corresponding to detachment faults in the two sags occur in multistep ramp flat geometries associated with important structures including 1 alternating sub sags and sub uplifts developing on the hanging walls 2 secondary faults 3 dome structures and 4 extensional allochthons the detachment faults dip landward and seaward in the baiyun and liwan sags respectively indicating paired detachment systems hence integrated with thin crust and subsidence centers offset at great distances from the fault bounded margins the baiyun and liwan sags are demonstrated to be supradetachment basins seated on the distal domain of the northern scs s passive continental margin the well correlated sequence stratigraphic framework in all sub sags shows that the detachment faults fit the standard detachment model in which faults are continuously active furthermore two detachment basins are inferred to have developed over hot ductile crust resulting from plate subduction in the mesozoic since the upper crust stretches far less than the middle lower crust the fault controlled extension is proposed to have had a smaller contribution to the thinned crust than a ductile extension did within the middle lower crust in the study area the recognition of detachment faults and supradetachment basins is a significant contribution towards understanding the formation of the northern passive continental margin of the scs

structural comparison of archetypal atlantic rifted margins a review of observations and concepts abstract in this study we compare three pairs of conjugate rifted margins that are often referred to as archetypes of rift systems despite numerous differences a remarkable first order structural similarity appears between the magma poor iberia newfoundland the magma rich mid norway east greenland and the sediment rich angola brazil rifted margins typical is the seaward arrangement of distinct domains proximal necking distal and outer that each present specific comparable structural characteristics our study also suggests that magma rich systems may go through a stage of hyper extension indicating that extreme crustal thinning does not preclude a magmatic breakup in this contribution we clarify the definition of a number of terms introduced recently in rifted margin studies we review the major features constitutive of these key referenced systems discuss their similarities and differences and examine how the related deformation modes develop in the life cycle of a rift that goes to seafloor spreading we conclude that the distinct domains observed in the margin architecture represent distinct stages in the evolution of rifted margin independently of their later evolution into magma poor or magma rich environments

unravelling the architecture and evolution of the inverted multi stage north iberian bay of biscay rift abstract current rift models propose polyphase rift evolutions that develop with the same kinematic framework and accommodate continuous and sequential extension through the stacking of deformation modes several rifted margins show however evidence for multiple and out of sequence rift events which developed within different kinematic frameworks with a complex spatial and temporal evolution in this work we address the problem of multi stage rifting based on the study of the central north iberian margin located at the southern bay of biscay triangular oceanic domain this magma poor rifted margin recorded three major mesozoic rift events and a subsequent alpine compressional reactivation representing a unique setting to study the architecture of multi stage rift systems and the exerted control on subsequent reactivation relying on a dense set of 2d seismic reflection profiles boreholes and published velocity models we map and describe structural domains and major extensional and compressional structures and we construct depth and thickness maps of syn rift units these new maps display the geometry and spatial distribution of major rift basins and bounding structures distinctive scenarios resulting from this tectono stratigraphic approach led us to define three rift systems a first diffuse and widespread triassic system with classical fault bounded half graben basins a second narrow deep and localised late jurassic to barremian transtensional system including laterally confined pull apart basins and a third widely distributed aptian to cenomanian hyperextended system our results show that each rift system controlled the successive extensional events and the subsequent contractional deformation resulting in a complex 3d template deciphering the tectono stratigraphic architecture of such multi stage systems provide key insights on the spatial temporal and thermal evolution of divergent plate boundaries it is likewise indispensable to propose and test kinematic plate deformable models on conjugate rifted margins and to comprehend the implications for their reactivation

new constraints on the formation of the non volcanic continental galicia flemish cap conjugate margins numerous multichannel seismic data acquired on the starved continental margin off galicia show that the prominent intra crustal s reflector is observed only below the oceanward portion of the continental margin close to the ocean continent boundary superficial extension has been computed from the geometry of the well imaged tilted fault blocks of the entire margin by using a series of parallel seismic lines specifically designed for this purpose the pure shear model clearly accounts for the lithospheric extension of the continentward portion of the continental margin but not for the oceanward portion of the continental margin where the s reflector is present the s reflector is a detachment zone which presents a decreasing offset in the direction of the continent and this detachment zone has been active since the beginning of rifting recent kinematic reconstructions show that the conjugate margin to galicia is the southeastern flemish cap margin from the few seismic profiles available on this canadian margin a major master fault is observed beneath the continental slope and is interpreted as the outcropping portion of a main detachment zone identified on the galicia margin as the s reflector the proposed mechanism of extension for these two conjugate margins is pure shear which affects the whole lithosphere but in the upper part of the crust the rifting mechanism is either pure shear for the continentward portion of the galicia margin or simple shear for the oceanward portion of the galicia and the whole southeastern flemish cap margins where the detachment zone is active i consider that pure shear is the basic lithospheric mechanism and that simple shear is a mechanism which affects only the upper crust