Conjugate Margins Research Articles

A review of the NE Atlantic conjugate margins based on seismic refraction data

Abstract The NE Atlantic region evolved through several rift episodes, leading to break-up in the Eocene that was associated with voluminous magmatism along the conjugate margins of East Greenland and NW Europe. Existing seismic refraction data provide good constraints on the overall tectonic development of the margins, despite data gaps at the NE Greenland shear margin and the southern Jan Mayen microcontinent. The maximum thickness of the initial oceanic crust is 40 km at the Greenland–Iceland–Faroe Ridge, but decreases with increasing distance to the Iceland plume. High-velocity lower crust interpreted as magmatic underplating or sill intrusions is observed along most margins but disappears north of the East Greenland Ridge and the Lofoten margin, with the exception of the Vestbakken Volcanic Province at the SW Barents Sea margin. South of the narrow Lofoten margin, the European side is characterized by wide margins. The opposite trend is seen in Greenland, with a wide margin in the NE and narrow margins elsewhere. The thin crust beneath the basins is generally underlain by rocks with velocities of >7 km s −1 interpreted as serpentinized mantle in the Porcupine and southern Rockall basins; while off Norway, alternative interpretations such as eclogite bodies and underplating are also discussed.

An evaluation of Mesozoic rift-related magmatism on the margins of the Labrador Sea: Implications for rifting and passive margin asymmetry

The Labrador Sea is a small (∼900 km wide) ocean basin separating southwest Greenland from Labrador, Canada. It opened following a series of rifting events that began as early as the Late Triassic or Jurassic, culminating in a brief period of seafloor spreading commencing by polarity chron 27 (C27; Danian) and ending by C13 (Eocene-Oligocene boundary). Rift-related magmatism has been documented on both conjugate margins of the Labrador Sea. In southwest Greenland this magmatism formed a major coast-parallel dike swarm as well as other smaller dikes and intrusions. Evidence for rift-related magmatism on the conjugate Labrador margin is limited to igneous lithologies found in deep offshore exploration wells, mostly belonging to the Alexis Formation, along with a postulated Early Cretaceous nephelinite dike swarm (ca. 142 Ma) that crops out onshore, near Makkovik, Labrador. Our field observations of this Early Cretaceous nephelinite suite lead us to conclude that the early rift-related magmatism exposed around Makkovik is volumetrically and spatially limited compared to the contemporaneous magmatism on the conjugate southwest Greenland margin. This asymmetry in the spatial extent of the exposed onshore magmatism is consistent with other observations of asymmetry between the conjugate margins of the Labrador Sea, including the total sediment thickness in offshore basins, the crustal structure, and the bathymetric profile of the shelf width. We propose that the magmatic and structural asymmetry observed between these two conjugate margins is consistent with an early rifting phase dominated by simple shear rather than pure shear deformation. In such a setting Labrador would be the lower plate margin to the southwest Greenland upper plate.

A reconstruction of the North Atlantic since the earliest Jurassic

AbstractThe stretched continental margins of the North Atlantic region record a plate kinematic history dominated by major episodes of extension since the Late Palaeozoic. Accounting for the restoration of this stretched continental crust across the region, and the subsequent derivation of plausible full‐fit configurations between these continents, prior to extension, still remains unresolved. Previous plate reconstructions have highlighted difficulties such as determining the amount of extension to be distributed across the multiple episodes of rifting, or defining the distribution of extension across intraplate deformation occurring adjacent to the rifting of two major continents. Here, we implement a new approach to derive a set of total reconstruction poles based on a full‐fit, palinspastic restoration of the conjugate margins that considers the rifting evolution of the North Atlantic in a regional plate kinematic context since the Earliest Jurassic. Gravity inversion forms the basis of our regional crustal thickness estimates, and aids in the identification of thinned continental crust. Our crustal restoration estimates are computed in multiple phases along margin segments in accordance with the timing of their major rifting episodes. Our model predicts a full‐fit, prerift, palaeogeographic position of all the major continents across the North Atlantic; and predicts a time‐dependent evolution of multiple phases of extension including regional divergence directions, consistent with previous observations. Our plate model represents a new approach to plate kinematic reconstructions incorporating the application of a multiphase restoration methodology applied in a major regional context, constrained by the synthesis of several different geological and geophysical data sets.

An overview of the Upper Palaeozoic–Mesozoic stratigraphy of the NE Atlantic region

Abstract This study describes the distribution and stratigraphic range of the Upper Palaeozoic–Mesozoic succession in the NE Atlantic region, and is correlated between conjugate margins and along the axis of the NE Atlantic rift system. The stratigraphic framework has yielded important new constraints on the timing and nature of sedimentary basin development in the NE Atlantic, with implications for rifting and the break-up of the Pangaean supercontinent. From a regional perspective, the Permian–Triassic succession records a northwards transition from an arid interior to a passively subsiding, mixed carbonate–siliciclastic shelf margin. A Late Permian–earliest Triassic rift pulse has regional expression in the stratigraphic record. A fragmentary paralic to shallow-marine Lower Jurassic succession reflects Early Jurassic thermal subsidence and mild extensional tectonism; this was interrupted by widespread Mid-Jurassic uplift and erosion, and followed by an intense phase of Late Jurassic rifting in some (but not all) parts of the NE Atlantic region. The Cretaceous succession is dominated by thick basinal-marine deposits, which accumulated within and along a broad zone of extension and subsidence between Rockall and NE Greenland. There is no evidence for a substantive and continuous rift system along the proto-NE Atlantic until the Late Cretaceous.

Rift–drift transition in a magma-rich system: the Gop Rift–Laxmi Basin case study, West India

Abstract This paper studies the magma-rich Gop Rift–Laxmi Basin, West India, which underwent the mantle first–crust second break-up mode. It draws from reflection seismic and gravity data from this abandoned system. Seismic images document that the crustal necking was associated with the development of seawards-dipping reflector wedges deformed by landwards-dipping detachment faults. A wide crustal necking zone indicates that the ductile lower crust was still present during necking. Observed uneven detachment fault spacing indicates the effect of upper-crustal anisotropy. Comparison of the seismic images through progressively more mature stages of the rift–drift transition documents that the final stages of thinning represented the time period when the upper-crustal wide and symmetrical rift architecture changed to the asymmetrical one, and the decoupled system to the coupled one. It further indicates that the last crustal layer was broken with a convex-up fault that was associated with an excess magmatic event. The fault propagation represented the first spontaneous deformation unaffected by the pre-existing anisotropy. Subsequent drift of the two plates was associated with melt-assisted spreading and spontaneous faulting. The faulting geometry and sequence controlled which of the conjugate margins ended up with a volcanic outer high, representing the record of the break-up-locating excess magmatism.

Moho and basement depth in the NE Atlantic Ocean based on seismic refraction data and receiver functions

Abstract Seismic refraction data and results from receiver functions were used to compile the depth to the basement and Moho in the NE Atlantic Ocean. For interpolation between the unevenly spaced data points, the kriging technique was used. Free-air gravity data were used as constraints in the kriging process for the basement. That way, structures with little or no seismic coverage are still presented on the basement map, in particular the basins off East Greenland. The rift basins off NW Europe are mapped as a continuous zone with basement depths of between 5 and 15 km. Maximum basement depths off NE Greenland are 8 km, but these are probably underestimated. Plate reconstructions for Chron C24 ( c. 54 Ma) suggest that the poorly known Ammassalik Basin off SE Greenland may correlate with the northern termination of the Hatton Basin at the conjugate margin. The most prominent feature on the Moho map is the Greenland–Iceland–Faroe Ridge, with Moho depths >28 km. Crustal thickness is compiled from the Moho and basement depths. The oceanic crust displays an increased thickness close to the volcanic margins affected by the Iceland plume.

Asthenosphere and lithosphere structure controls on early onset oceanic crust production in the southern South Atlantic

The southern South Atlantic has often been considered a classic example of continental break-up in the presence of a starting mantle plume. Evidence for a mantle plume includes the Paranà-Etendeka continental flood basalts, which are associated with the Rio Grande Rise and Walvis Ridge, and the wide-spread presence of seaward dipping reflectors and high-velocity lower-crustal bodies along the conjugate margins. Observations from seaward dipping reflector distributions suggested that lithospheric segmentation played a major role in the pattern of volcanism during break-up in this region, and consequent numerical modelling was used to test this. We tested this hypothesis ourselves by measuring the thickness of the earliest oceanic crust generated. This was done through the use of 37 measurements of initial oceanic crustal thickness from wide-angle and multichannel seismic profiles collected along the conjugate margins. These measurements show that at 450kmsouth of the Paranà-Etendeka flood basalts the oceanic crust is thicker than the global average at 11.7km. Farther south the oceanic crust thins, reaching 6.1km at a distance of 2300km along-strike. Overall, the along-strike trend of oceanic crustal thickness is linear with a regression coefficient of 0.7 and little indication of segmentation. From numerical models representing extension of the lithosphere, we find that observed melt volumes are matched with the presence of a hot layer. If we assume this region of hot mantle has a thickness of 100km, its excess temperature relative to the asthenosphere has to decrease from 200 to 50°C, north to south. This decrease in temperature, also seen in published thermobarometry results, suggests that temperature was the primary control of volcanism during the opening of the southern South Atlantic.

Constraining lithosphere deformation modes during continental breakup for the Iberia–Newfoundland conjugate rifted margins

A kinematic model of lithosphere and asthenosphere deformation has been used to investigate lithosphere stretching and thinning modes during continental rifting leading to breakup and seafloor spreading. The model has been applied to two conjugate profiles across the Iberia–Newfoundland rifted margins and quantitatively calibrated using observed present-day water loaded subsidence and crustal thickness, together with observed mantle exhumation, subsidence and melting generation histories. The kinematic model uses an evolving prescribed flow-field to deform the lithosphere and asthenosphere leading to lithospheric breakup from which continental crustal thinning, lithosphere thermal evolution, decompression melt initiation and subsidence are predicted. We explore the sensitivity of model predictions to extension rate history, deformation migration and buoyancy induced upwelling. The best fit calibrated models of lithosphere deformation evolution for the Iberia–Newfoundland conjugate margins require; (1) an initial broad region of lithosphere deformation with passive upwelling, (2) lateral migration of deformation, (3) an increase in extension rate with time, (4) focussing of the deformation and (5) buoyancy induced upwelling. The model prediction of exhumed mantle at the Iberia–Newfoundland margins, as observed, requires a critical threshold of melting to be exceeded before melt extraction. The preferred calibrated models predict faster extension rates and earlier continental crustal separation and mantle exhumation for the Iberia Abyssal Plain–Flemish Pass conjugate margin profile than for the Galicia Bank–Flemish Cap profile to the north. The predicted N–S differences in the deformation evolution give insights into the 3D evolution of Iberia–Newfoundland margin crustal separation.

Demerara Rise, offshore Suriname: Magma-rich segment of the Central Atlantic Ocean, and conjugate to the Bahamas hot spot

The Demerara Rise is a prominent bathymetric feature that has been considered as a broad expression of shallow continental basement and used in conjunction with the Guinea Plateau as a pinning point for circum-Atlantic plate reconstructions. Previously, shallow-penetration, poorly imaged seismic data over the Demerara Rise were modeled with the lower sequences interpreted as continental crust at relatively shallow depths. However, new long-offset, deeply penetrating seismic data provide evidence that basement nearly or entirely comprises excessively thick volcanic strata (approximately 21 km). Seismic character and geometry, 2D gravity modeling, and volcanic margin analogs were used to identify unfaulted, convex-upward seaward dipping reflector (SDR) packages. These steeply dipping (approximately 20°) igneous successions are westwardly divergent, and occur as offlapping reflector sets in trains as long as 250 km. This rift-related volcanism now recognized at the Demerara Rise was probably conjugate to syn-rift volcanism in South Florida/Great Bahama Bank, and from this we have predicted a volcanic element for the Guinea Plateau. This volcanism could be linked to a Bahamas hot spot at the initial opening of the Central Atlantic. Six SDR packages have been interpreted below the Late Jurassic-Early Cretaceous carbonate section of the rise, indicating that the early volcanism produced a marine substrate upon which the subsequent carbonate bank section developed. We have inferred that this Early Cretaceous volcanic/carbonate margin continued into the Guinea Plateau of West Africa. The pre-Aptian section was inverted and peneplained with a strong angular unconformity prior to the Early Cretaceous opening of the Equatorial Atlantic seaway. The newly identified Central Atlantic volcanic margin of the Demerara Rise holds implications of a volcanic origin for its conjugate margins. We have confirmed a voluminous magma-rich opening of the southeastern Central Atlantic.

The missing piece of the South Atlantic jigsaw: when continental break-up ignores crustal heterogeneity

Abstract Crustal heterogeneity is considered to play a critical role in the position of continental break-up, yet this can only be demonstrated when a fully constrained pre-break-up configuration of both conjugate margins is achievable. Limitations in our understanding of the pre-break-up crustal structure in the offshore region of many margins preclude this. In the southern South Atlantic, which is an archetypal conjugate margin, this can be achieved because of the high confidence in plate reconstruction. Prior to addressing the role of crustal heterogeneity, two questions have to be addressed: first, what is the location of the regionally extensive Gondwanan Orogeny that remains enigmatic in the Orange Basin, offshore South Africa; and, second, although it has been established that the Argentinian Colorado rift basin has an east–west trend perpendicular to the Orange Basin and Atlantic spreading, where is the western continuation of this east–west trend? We present here a revised structural model for the southern South Atlantic by identifying the South African fold belt offshore. The fold belt trend changes from north–south to east–west offshore and correlates directly with the restored Colorado Basin. The Colorado–Orange rifts form a tripartite system with the Namibian Gariep Belt, which we call the Garies Triple Junction. All three rift branches were active during the break-up of Gondwana, but during the Atlantic rift phase the Colorado Basin failed while the other two branches continued to rift, defining the present day location of the South Atlantic. In addressing these two outstanding questions, this study challenges the premise that crustal heterogeneity controls the position of continental break-up because seafloor spreading demonstrably cross-cuts the pre-existing crustal heterogeneity. Furthermore, we highlight the importance of differentiating between early rift evolution and subsequent rifting that occurs immediately prior to seafloor spreading.

Petroleum systems asymmetry across the South Atlantic Equatorial Margins

Abstract We describe an examination of two lines of evidence, tectono-structural evolution and hydrocarbon geochemistry, of asymmetric opening of the Atlantic Equatorial Margin. Our structural mapping used compilations of geophysical data and a review of both published literature and oil company public presentations. Geochemically, we accessed regional non-exclusive oil studies of the conjugate margins of Africa and South America, plus considerable published material. A group of non-exclusive oils was refined to 286, which clustered into five families, all represented along the NE Brazil margin but only one along the West African Transform (WAT) margin. Multiple lacustrine-sourced oils were seen around the South Atlantic, including NE Brazil, but a rich, oil-prone lacustrine source was not indicated offshore Ivory Coast and Ghana. Despite minor evidence of mixed source, possibly lacustrine stringers within an alluvial to marine setting, the predominant source is marine Cretaceous (Cenomanian–Turonian and possibly Albian). We find that opening asymmetry (a) biased the location of lacustrine (Early to mid-Cretaceous prerift to early synrift) source rocks to the NE Brazil margin and (b) locally narrowed the width of the optimal marine (Mid-Late Cretaceous postrift) WAT Margin source kitchens. Burial of the latter has aggravated the risk of late charge from light (condensate and gas) hydrocarbons.

Deep structure of the Argentine margin inferred from 3D gravity and temperature modelling, Colorado Basin

Following previous work on the Colorado Basin using a 3D crustal structural model, we now investigate the presence of lower crustal bodies at the base of the crust using 3D lithospheric gravity modelling and calculations of the conductive thermal field.Our first study highlighted two fault directions and depocentres associated with thinned crust (NW–SE in the West and NE–SW at the distal margin). Fault relative chronology argues for two periods of extension: (1) NW–SE faulting and thinning in the western Colorado Basin and (2) NE–SW faulting and thinning related to the continental breakup and formation of the NE–SW-striking volcanic margins of the Atlantic Ocean.In this study, the geometry of modelled high-density Lower Crustal Bodies (LCBs) enables the reproduction of the gravimetric field as well as of the temperature measured in wells down to 4500m. The modelled LCBs correlate with geological observations: (1) NW–SE LCBs below the deepest depocentres in the West, (2) NE–SW LCBs below the distal margin faults and the seaward dipping reflectors. Thus the proposed poly-phased evolution of the margin could as well correspond to two emplacement phases of the LCBs. The calculated conductive thermal field fits the measured temperatures best if the thermal properties (thermal conductivity and radiogenic heat production) assigned to the LCBs correspond to either high-grade metamorphic rocks or to mafic magmatic intrusions. To explain the possible lithology of the LCBs, we propose that the two successive phases of extension are accompanied by magma supply, emplaced (1) in the thinnest crust below the older NW–SE depocentres, then (2) along the NE–SW continentward boundary of the distal margin and below the volcanic seaward dipping reflectors. The South African conjugate margin records only the second NE–SW event and we discuss hypotheses which could explain these differences between the conjugate margins.

Conjugate rifted margins width and asymmetry: The interplay between lithospheric strength and thermomechanical processes

AbstractNumerical experiments have been used to relate the range in the distribution and the style of deformation observed in rifted margins to localizing/delocalizing thermomechanical processes. The experiments give rise to four end‐members of margins for varying initial lithospheric strength and extension rates. The first two end‐members are narrow and asymmetric and narrow and near‐symmetric, conjugate margins. The third end‐member is asymmetric conjugate margins, wherein one side is <100 km wide and the other is >100–300 km wide. Lastly, we explore wide rift systems that may form very asymmetric conjugate margins with one narrow margin and a very wide conjugate, 200 km to > 350 km across. With initial and boundary conditions close to that inferred from the North and South Atlantic margins, we find that not all margins experience a polyphase rifting history of stretching‐thinning‐exhumation. Instead, the stretching mode can be very short or protracted, and the thinning or the exhumation modes can be incomplete or absent. The deformation localization of the thinning mode is in places associated with the formation of a keystone block or “block H.” A new mechanism for the formation of the unstable crustal root under block H is described, wherein the bounding border faults lead to differential thinning of the crust and mantle lithosphere. Nonuniform extension also occurs in both types of wide rift systems and is related to the sequential deformation migration outward of an initial graben, associated with effective lithospheric strengthening that occurs during crustal thinning and bending.

How can asymmetric detachment faults generate symmetric Ocean Continent Transitions?

AbstractAsymmetry or symmetry of magma‐poor rifted margins refers commonly to the crustal architecture and the occurrence or absence of large‐scale extensional detachment faults. While distal parts of magma‐poor rifted margins are often considered to be asymmetric, the observation of downlapping sedimentary sequences over exhumed mantle domains at conjugate margins suggests a symmetric evolution during mantle exhumation. On the basis of seismic observations along the Iberia–Newfoundland and Australia–Antarctica margins, we propose that their most distal parts show evidence for the development of multiple, out‐of‐sequence asymmetric detachment faults. We present evidence for cyclic delocalization and re‐localization of deformation, resulting in an apparent symmetry of the exhumed mantle domain. The interaction between out‐of‐sequence detachment systems and the successive rise of the asthenosphere may explain the observed transition from fault‐controlled to magma‐controlled strain accommodation and the transition to more symmetric and localized accretion associated with the formation of a stable spreading center.

Conjugate margin pattern of the Southwest Sub‐basin, South China Sea: insights from deformation structures in the continent‐ocean transition zone

AbstractEight cross‐sections from the continent‐ocean transition zone of the Southwest Sub‐basin, South China Sea, have been used to compare conjugate rifted margins that are typical of magma‐poor rift systems. The geological timing of the break‐up unconformity in the continental margins shows that the Reed Bank and the Dangerous Grounds in the south continental margin connect with the Zhongsha Islands and the western Phu Khanh Basin, which experienced northwest‐southeast oriented movement after the opening of the Southwest Sub‐basin. Detailed interpretations identify a range of deformational and sedimentary features in the continent‐ocean transition zones with and without rigid blocks. Remarkable structural similarities appear in the transition zone lying within the convex‐shaped area shaped by rigid blocks of the conjugate margin (e.g. the Zhongsha Islands and the Reed Bank) or within concave‐shaped areas sandwiched by rigid blocks. Rigid tectonic blocks such as the Zhongsha Islands and the Reed Bank played important roles in both the shaping of the continent‐ocean boundary and the initial continental break‐up. Copyright © 2015 John Wiley & Sons, Ltd.

Foz do Amazonas Basin – A case for oil generation from geothermal gradient modelling

With the recent Zaedyus-1 well oil discovery in French Guiana and the recent oil discoveries in the offset conjugate margin area in Africa (Sierra Leone-Liberia), it is no surprise that explorers are looking to continue this success into the Foz do Amazonas Basin of Brazil. Foz do Amazonas is the most northerly of the Brazilian equatorial margin basins with an area of 282,909 km², and water depths ranging from 50 m to greater than 3000 m. In this under-explored frontier basin, exploration drilling has been confined to the shelf, with 95 exploration wells drilled and ten of these with hydrocarbon shows. The 11th Exploration Licence Round focused on the potentially large reservoirs in distal Late Cretaceous/Palaeogene deep-water turbidite plays, following successful wells drilled in French Guiana. There are strong indicators for the presence of hydrocarbon-bearing reservoirs shown in recent publications, based on newly available data (Rodriguez et al., 2014). This study complements some of the evidence previously presented by analysing the prospectivity within the Foz do Amazonas Basin. We use temperature gradients obtained from exploration wells and Ocean Drilling Programme (ODP) wells, as well as values calculated from the Bottom Simulating Reflector (BSR) to model the geothermal gradient and present-day source rock maturity.

Fast Episodes of West-Mediterranean-Tyrrhenian Oceanic Opening and Revisited Relations with Tectonic Setting.

Extension and calc-alkaline volcanism of the submerged orogen of alpine age (OAA) initiated in Early Oligocene (~33/32 Ma) and reached the stage of oceanic opening in Early-Miocene (Burdigalian), Late-Miocene and Late-Pliocene. In the Burdigalian (~20–16 Ma) period of widespread volcanism of calcalkaline type on the margins of oceanic domain, seafloor spreading originated the deep basins of north Algeria (western part of OAA) and Sardinia/Provence (European margin). Conversely, when conjugate margins’ volcanism has been absent or scarce seafloor spreading formed the plains Vavilov (7.5–6.3 Ma) and Marsili (1.87–1.67 Ma) within OAA eastern part (Tyrrhenian Sea). The contrast between occurrence and lack of margin’s igneous activity probably implies the diversity of the geotectonic setting at the times of oceanization. It appears that the Burdigalian calcalkaline volcanism on the continental margins developed in the absence of subduction. The WNW-directed subduction of African plate probably commenced at ~16/15 Ma (waning Burdigalian seafloor spreading) after ~18/16 Ma of rifting. Space-time features indicate that calcalkaline volcanism is not linked only to subduction. From this view, temporal gap would exist between the steep subduction beneath the Apennines and the previous, flat-type plunge of European plate with opposite direction producing the OAA accretion and double vergence.

Full-fit reconstruction of the South China Sea conjugate margins

Although the South China Sea margins have been well-studied, there are no full-fit reconstructions that document the complex rifting history in a regional geodynamic context. We restored conjugate margins to their original Late Cretaceous unstretched geometries, and generated the first full-fit reconstructions of the South China Sea conjugate margins. The impact of magmatic underplating on restoration and full-fit reconstruction was estimated quantitatively. The full-fit reconstruction model includes restoration uncertainties and results in minimum gaps and overlaps between the restored conjugate margin geometries. Based on our restoration, the full-fit reconstruction and the continental basement thickness distribution, we estimate that there was more extension of continental basement on the western part of the conjugate margins compared with the eastern margin prior to the initiation of seafloor spreading. Therefore, the mid ocean ridge initially formed in the east and propagated westward to open the South China Sea. Seafloor spreading propagated along the Xisha Trough during NW Sub-basin formation, but continental breakup occurred along the southern side of the Zhongsha–Xisha Islands where the lithosphere was probably weaker than along the Xisha Trough, consistent with the geometry and evolution of the NW Sub-basin and the southward jump of the mid oceanic ridge.

The tectono-stratigraphic evolution of distal, hyper-extended magma-poor conjugate rifted margins: Examples from the Alpine Tethys and Newfoundland–Iberia

In order to capture the general tectono-stratigraphic framework of a magma-poor conjugate rifted margin, two examples have been analysed and compared, the seismically imaged and drilled Iberia–Newfoundland rifted margins (e.g. seismic sections SCREECH1-ISE1) and the fossil distal Tethys rifted margins exposed in the Alps. The Lower Austroalpine and the Ligurian Briançonnais units exposed in SE Switzerland and NW Italy respectively, are here considered and described for the first time as conjugate margins. They form, like the Iberia–Newfoundland rifted margins, conjugate upper and lower plate margin. Both pairs of margins show a similar first-order morpho-tectonic arrangement. Using the Top of Pre-Rift Succession (TPRS) and the Base of Post-Rift Successions (BPRS) as stratigraphic marker levels and two Syn-Tectonic Sequences (STS1&2) three major rift domains can be identified, which experienced different evolutions and preserved different stratigraphic architectures:i) The proximal margin, soled by normal, 30 ± 5 km thick continental crust. It is characterized by widespread fault-bounded graben/half graben basins filled by the STS1 showing growth structures followed by a succession (time-equivalent unit to STS2) that does not show any evidence for syn depositional tectonic activity.ii) The upper plate distal margin, floored by thinned to ultra-thinned continental crust. It can be subdivided into a distal “high” and a distal “low” separated by a major fault system. The “high” is characterized by the occurrence of a marked syn-rift unconformity leading to the erosion and/or non deposition of the whole syn-tectonic sequence (STS1 + STS2); the “low” displays complete stratigraphic successions in which the STS2 becomes very characteristic and exclusive of this part of the margin.iii) The lower plate distal margin, floored by hyper-extended crust, exhumed crust and subcontinental mantle. The pre-tectonic sequence and the STS1 are only present inside extensional allochthons that are excised from the upper plate and passively transported on top of major exhumation faults. The STS2, deposited during the formation of the most distal domain, is mostly formed by clasts derived from the dismantling of the extensional allochthons and of the surrounding exhumed basement. It is overlain by deep marine turbidites and hemi-pelagic sediments. The nature of basement (serpentinized mantle and hydrated upper or lower continental crust) will selectively condition the composition of the STS2 in the distal lower plate margin.The recognition of a complex rift stratigraphy in the two conjugate pairs of margins shows that simple stratigraphic models are unable to describe the complete evolution of rifted margins from their initial stage, preserved in the proximal margins to the final evolution leading to upper and lower plate margins in the conjugate distal margins. This paper describes for the first time the stratigraphic relationships at distal conjugate rifted margins, which is a key to understand the rift evolution and architecture of these yet little investigated systems.

Detrital zircon geochronology of the Cretaceous succession from the Iberian Atlantic Margin: palaeogeographic implications

Detrital zircon U–Pb data performed on eight Cretaceous sandstone samples (819 age isotopic results) from the Lusitanian basin (west Portugal) constrain the history of uplift and palaeodrainage of western Iberia following break-up of Pangaea and opening of the North Atlantic Ocean. We examined the links between shifts in provenance and known basinwide unconformities dated to the late Berriasian, Barremian, late Aptian and Cenomanian–Turonian. The detrital zircon record of sedimentary rocks with wider supplying areas is relatively homogenous, being characterized by a clear predominance of late Palaeozoic ages (c. 375–275 Ma) together with variable proportions of ages in the range c. 800–460 Ma. These two groups of ages are diagnostic of sources within the Variscan Iberian Massif. A few samples also reveal significant amounts of middle Palaeozoic (c. 420–385 Ma) and late Mesoproterozoic to early Neoproterozoic (c. 1.2–0.9 Ga) zircon, which are almost absent in the basement to the east of the Lusitanian basin, but are common in terranes with a Laurussia affinity found in NW Iberia and the conjugate margin (Newfoundland). The Barremian unconformity marks a sudden rise in the proportion of c. 375–275 Ma zircon ages accompanied by a decrease in the abundance of the c. 420–385 Ma and c. 1.2–0.9 Ga ages. This shift in the zircon signature, which is contemporaneous with the separation of the Galicia Bank from Flemish Cap, reflects increased denudation of Variscan crystalline rocks and a reduction in source material from NW Iberia and adjoining areas. The late Aptian unconformity, which represents the largest hiatus in the sedimentary record, is reflected by a shift in late Palaeozoic peak ages from c. 330–310 Ma (widespread in Iberia) to c. 310–290 Ma (more frequent in N Iberia). It is considered that this shift in the age spectra resulted from a westward migration of catchment areas following major uplift in northern Iberia and some transport southward from the Bay of Biscay under the influence of a well-established Atlantic circulation.