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Gianreto Manatschal - One of the best experts on this subject based on the ideXlab platform.

  • Tectono-sedimentary evolution of a fossil Ocean-Continent Transition: Tasna nappe, central Alps (SE Switzerland)
    GSA Bulletin, 2019
    Co-Authors: Charlotte Ribes, Gianreto Manatschal, Benoît Petri, Jean-françois Ghienne, Federico Galster, Garry D. Karner, Patricio H. Figueredo, Christopher A. Johnson, Anne-marie Karpoff
    Abstract:

    Abstract Magma-poor Ocean-Continent Transitions at distal rifted margins record complex stratigraphic interactions engendered by extreme crustal thinning and mantle exhumation. The Tasna Ocean-Continent Transition, exposed in the Middle Penninic Tasna nappe in eastern Switzerland, is so far the only known example where the lateral Transition from continental crust to exhumed serpentinized mantle lithosphere is exposed and not overprinted by later Alpine deformation. This paper presents sedimentological, structural, and petrographical observations and detrital zircon provenance data to document: (1) the processes controlling continental hyperextension and mantle exhumation; and (2) the facies, depositional systems, sediment sources, delivery pathways, and depositional stacking patterns associated with magma-poor Ocean-Continent Transitions. Our results show that the basement units of the Tasna Ocean-Continent Transition are composed of prerift upper and lower crust and subcontinental mantle rocks juxtaposed as part of the continental crustal thinning process. The absence of pervasive, synrift deformation in the lower-crustal rocks indicates that the thinning was likely achieved by deformation along localized shear zones before being exhumed at the seafloor by brittle, late extensional detachment faulting and not by any form of lower-crustal flow. The age of the first sediments deposited on the continental crust and exhumed mantle, the so-called Tonschiefer Formation, is considered to be Late Jurassic. A key observation is that the restored morpho-tectonic and sedimentary evolution of the Tasna Ocean-Continent Transition shows the intercalation of downdip, transported platform-derived sediments and along-axis–derived siliciclastic sediments originating from the recycling of prerift sediments, local basement, and/or extra-Alpine sources.

  • Syntectonic carbonation during synmagmatic mantle exhumation at an Ocean-Continent Transition
    Geology, 2019
    Co-Authors: Rémi Coltat, Philippe Boulvais, Yannick Branquet, J. Collot, M.e. Epin, Gianreto Manatschal
    Abstract:

    A Jurassic extensional detachment associated with carbonated serpentinites and basalts is preserved at Falotta (Platta nappe, southeastern Switzerland). Structural data indicate that fluid circulation occurred during late increments of extension along the detachment plane at the contact between serpentinites and basalts. The homogeneity of the isotopic signatures (δ18O ~16‰ and δ13C centered on 0‰–1‰) can be best explained by a single, sudden seawater-derived carbonation event at temperatures of ~100 °C. Carbonation was focused in the high-permeability zone along the detachment. Our model yields new insights for carbonation processes related to mantle exhumation.

  • Preserved organic matter in a fossil Ocean Continent Transition in the Alps: the example of Totalp, SE Switzerland
    Swiss Journal of Geosciences, 2017
    Co-Authors: Tsvetomila Mateeva, Gianreto Manatschal, George A. Wolff, Suzanne Picazo, Nick J. Kusznir, John Wheeler
    Abstract:

    Evidence from ultraslow spreading mid-ocean ridges and both fossil and present-day Ocean–Continent Transitions (OCT) demonstrates that mantle serpentinization resulting from the interaction of mantle rock and water during tectonic exhumation is widespread. Observations at white smokers in modern ocean settings suggest that methane produced by serpentinization can support methanotrophic bio-systems, which use methane as the only source of carbon. An important question is whether such bio-systems are more generally pervasive in their association with serpentinized mantle in the subsurface. In this study, we examined whether there is evidence for such a methanotrophic system in exhumed serpentinized mantle at a magma-poor rifted continental margin, by probing for characteristic biological markers in these and associated sedimentary rocks in the Totalp unit of SE Switzerland. This unit represents a remnant of the former OCT of the southern Alpine Tethyan margin and was chosen because of its mild Alpine tectonic and low-grade metamorphic overprint during Alpine orogeny, hence giving potential for the preservation of indigenous organic matter (OM). Totalp samples are characterized by low organic carbon contents of 11–647 ppm. The majority of the samples contain hydrocarbons in the form of n -alkanes in the range C_17–C_36. Some sediments contain isoprenoids, for example pristane and phytane and a suite of steranes that are consistent with a marine origin for the OM preserved in the rocks. Traces of marine planktonic and bacterial OM are preserved in the serpentinized mantle and overlying sediments of this ancient Tethyan OCT, but there is no evidence that the OM has been generated from methanotrophic bio-systems.

  • Ocean-Continent-Transition at magma poor rifted margins, the magnetic signature of a magmatic breakup?
    2010
    Co-Authors: A. Bronner, Gianreto Manatschal, D. Sauter, G. Peron-pinvidic, Marc Munschy
    Abstract:

    Introduction Magnetic anomalies have been identified from M0 up to anomalies M20-M22 both in the Newfoundland basin and in the Iberian abyssal plain. A large amplitude magnetic anomaly (the J anomaly) marks the beginning of this M sequence of anomalies. These magnetic anomalies have been extensively used to constrain the position of Iberia relative to the European, African and American plates from the late Jurassique to the early Cretaceous times e.g. (Srivastava, 2000). However, the sources for these magnetic anomalies are strongly debated as they were not identified in typical oceanic crust but within the OCT (some have even shown afterwards to be located well within thinned continental crust (Funck, 2003)). They are thus different from standard seafloor spreading magnetic anomalies produced mainly by the cooling of basalts and interpreted as isochrones. The magnetic anomalies within the OCTs in the Iberian abyssal plain and Newfoundland basin are thought to be caused either by the serpentinization of the exposed mantle rocks (Sibuet, 2007) or by magmatic intrusions (Russell, 2003). We investigate the possibility that the magnetic lineation mapped as M0 is in fact not a seafloor spreading lineation. We propose an alternative model based on forward modeling of magnetic anomaly profiles using seismic refraction and drill hole data. We suggest that the main magnetic anomaly (J anomaly) in the OCTs of the Iberian-Newfoundland margins might be explained by underplated magmatic bodies and surface lava flows that post-date mantle exhumation. These magmatic bodies and lavas may result from a late Aptian tectono-magmatic event which marks the Transition to seafloor spreading.

  • From passive margins to orogens: The link between Ocean-Continent Transition zones and (ultra)high-pressure metamorphism
    Geology, 2010
    Co-Authors: Marco Beltrando, Daniela Rubatto, Gianreto Manatschal
    Abstract:

    A lithostratigraphic association consisting of serpentinized mantle rocks, continent-derived allochthons, mid-oceanic ridge gabbros of Jurassic age and post-rift sediments, typical of an Ocean-Continent Transition, is found in the eclogitic Piemonte units, in the Western Alps. In situ U-Pb geochronology was performed on zircons from an orthogneiss sampled at the bottom of a sliver of continental basement, in contact with serpentinites. Primary magmatic zircons of Permian age were overgrown by a second generation of zircon at ca. 166–150 Ma, likely related to melt infiltration associated with the intrusion of the underlying gabbroic body. This indicates that continental basement slices and oceanic basement rocks were already juxtaposed in the Jurassic and they were probably part of hyper-extended crust related to the opening of the Tethys. Therefore, the complex lithological association described here, which is also characteristic of several (ultra)high-pressure melange zones worldwide, was acquired prior to the orogenic event, during which it was only partly reworked. Ocean-Continent Transitions are in positions favorable to reach (ultra)high-pressure conditions, following negatively buoyant oceanic lithosphere into subduction, and then being accreted to the orogen, in response to the arrival of more buoyant continental lithosphere, resisting subduction. The Ocean-Continent Transition is now found in the immediate footwall of a 500-m-thick shear zone, which accommodated multiple episodes of deformation during Eocene–Oligocene time, suggesting an important link between Alpine deformation and rift-related structures.

R. B. Whitmarsh - One of the best experts on this subject based on the ideXlab platform.

  • deep structure of the ocean continent Transition in the southern iberia abyssal plain from seismic refraction profiles the iam 9 transect at 40 20 n
    Journal of Geophysical Research, 2000
    Co-Authors: S. M. Dean, R. B. Whitmarsh, T A Minshull, Keith E. Louden
    Abstract:

    We present a crust and mantle velocity structure for the West Iberia passive continental margin derived from a 320-km-long wide-angle seismic profile acquired in the southern Iberia Abyssal Plain. We observe a 170-km-wide Ocean-Continent Transition zone which includes a pair of overlapping peridotite ridges and is bounded by oceanic crust and landward by fault-bounded blocks of continental crust. The profile lies ∼40 km south of the transect sampled by Ocean Drilling Program (ODP) Legs 149 and 173. The Transition zone structure can be divided into an upper layer, 2–4 km thick with velocities of between 4.5 and 7.0 km s−1 and generally a high-velocity-gradient (1 s−1), and a lower layer up to 4 km thick with a velocity of ∼7.6 km s−1 and a low-velocity-gradient. A weak Moho reflection in this zone was seen only on wide-angle profiles at an offset of ∼30 km. The upper layer has a distinctly lower velocity than thinned continental crust adjacent to the continental slope. Conversely, the lower layer has too high a velocity to be magmatically intruded or underplated lower continental crust. On the coincident seismic reflection profile, fault-bounded crustal blocks, identified in unequivocal extended continental crust, are not observed in the Transition zone. The upper layer has velocity bounds and gradient similar to oceanic layer 2 observed west of the peridotite ridges, but no oceanic layer 3 velocity structure is present. While magnetic anomalies have been identified within the Transition zone, they have not been modeled successfully as seafloor spreading magnetic anomalies, nor do they generally form long linear margin-parallel features. Finally, ODP boreholes, ∼40 km north of our profile and within the interpreted Transition zone, have recovered up to 140-m-thick sections of serpentinite and serpentinized peridotites with little evidence of mafic igneous material. We conclude that the Transition zone cannot be dominantly composed of either extended continental crust or oceanic crust. Although current melting models predict a considerably thicker crust of decompression melt products, we interpret this region as exposed upper mantle peridotite with little or no synrift extrusive material and limited amounts of synrift material intruded within the serpentinized peridotite.

  • deep structure of the ocean continent Transition in the southern iberia abyssal plain from seismic refraction profiles ocean drilling program legs 149 and 173 transect
    Journal of Geophysical Research, 1999
    Co-Authors: Deping Chian, T A Minshull, Keith E. Louden, R. B. Whitmarsh
    Abstract:

    We present a wide-angle seismic refraction study of an 80×40 km region of the southern Iberia Abyssal Plain, south of Galicia Bank. An intersecting grid of two E-W and four N-S wide-angle reflection/refraction profiles is used to define variations of the basement velocity structure within this unusually wide Ocean-Continent Transition (OCT). These structures can be systematically linked to variations in acoustic basement morphology and to results from Ocean Drilling Program (ODP) boreholes. Lateral changes in the velocity structure of the basement occur abruptly over distances of ∼20 km where complex variations may be found. Thinned upper continental crust, 2–5 km thick with velocities of 5.0–6.6 km/s, is limited to a series of N-S fault blocks immediately south of Galicia Bank. This crust is underlain by a high-velocity layer (7.3–7.9 km/s) of weakly serpentinized (i.e., 0–25%) peridotite, which exists throughout the eastern part of the survey area. Basement within the OCT appears to consist dominantly of a broad region of exposed upper mantle that has been serpentinized heterogeneously both vertically and horizontally. In the southeast sector of our survey where basement topography deepens and becomes subdued, continental fault blocks are absent; instead, basement contains an upper layer of more pervasively serpentinized (i.e., 25–45%) peridotite that is ∼2 km thick. This layer is characterized by low velocity at the top of basement (4.2 km/s) that increases rapidly with depth, and it probably corresponds to a seismically unreflective layer, previously identified in reflection profiles to the south of our survey. In the western section of our survey, beneath a series of elevated basement ridges, velocities are reduced within both the upper basement layer (3.5–6.0 km/s) and lower layer (6.4–7.5 km/s). These changes suggest that both upper and lower layers have become more highly serpentinized (with values of 60–100% in the upper layer and 25–45% in the lower layer) probably during the last stages of rifting and immediately before formation of oceanic crust. A normal or slow spreading oceanic crustal structure is not found within the survey region. Thus it appears that the onset of seafloor spreading occurs in the region west of the peridotite ridge sampled at ODP Site 897 and east of the J magnetic anomaly.

  • Deep structure in the vicinity of the Ocean-Continent Transition zone under the southern Iberia Abyssal Plain
    Geology, 1998
    Co-Authors: Timothy A. Minshull, R. B. Whitmarsh, S. M. Dean, S. M. Russell, Keith E. Louden, D. Chian
    Abstract:

    We present results from a seismic and magnetic study of acoustic basement beneath the southern Iberia Abyssal Plain. An Ocean-Continent Transition zone (OCT), characterized by subdued basement relief and weak magnetization, lies seaward of thinned continental crust. Its western edge is a region with isochron-parallel ridges and generally higher basement magnetizations. The width of the OCT decreases to the north. The seaward change in basement morphology coincides with changes in seismic velocity structure, but in both regions, anomalously high velocities at shallow depths within acoustic basement suggest that the OCT consists largely of serpentinized peridotite. Our magnetic and seismic data support the hypothesis of exhumed upper mantle, more than that of ultraslow sea-floor spreading, for the origin of the OCT.

  • Synthesis of the crustal structure of the transform continental margin off Ghana, northern Gulf of Guinea
    Geo-Marine Letters, 1997
    Co-Authors: R. A. Edwards, R. B. Whitmarsh, Roger A. Scrutton
    Abstract:

    Results of a detailed geophysical transect across the transform continental margin off Ghana, at the eastern end of the Romanche Fracture Zone in the Equatorial Atlantic, are presented. Seismic refraction, single-channel seismic reflection, gravity, and magnetic data were collected, and seismic, gravity, and magnetic models along the transect are shown. The 6- to 11-km-wide ocean–continent Transition (OCT) is characterized by a high-velocity, high-density, high-magnetization crustal zone. The models show no evidence for any underplating of the continental crust adjacent to the margin but minor melting and intrusion of the continental crust may have occurred in the vicinity of the OCT.

  • The crustal structure across the transform continental margin off Ghana, eastern equatorial Atlantic
    Journal of Geophysical Research: Solid Earth, 1997
    Co-Authors: R. A. Edwards, R. B. Whitmarsh, Roger A. Scrutton
    Abstract:

    Forward modeling of a suite of wide-angle seismic lines across the transform continental margin at the eastern end of the Romanche fracture zone off Ghana has shown the Transition from continental to oceanic crust to be confined to a narrow, 6 to 11-km-wide zone located at the foot of the steep continental slope. The structure of the adjacent oceanic and continental crusts has also been resolved. These results are confirmed and enhanced by gravity and magnetic models. The crust of the Ocean-Continent Transition zone is characterized by high velocities (5.8–7.3 km s−1), a high density (3.10 Mg m−3), and high magnetizations (1.1–1.25 A m−1). This rules out a purely continental origin for the zone and suggests that it may be formed of basic igneous rocks intruded when the hot oceanic spreading center migrated along the margin. The oceanic crust within 70 km of the margin is abnormally thin compared to normal Atlantic oceanic crust and shows an average thickness of just 4.4 km. We suggest that the region of abnormally thin oceanic crust is the result of a reduced magma supply due to a combination of closely spaced fracture zones and the conductive loss of heat from the upwelling oceanic mantle in small basins surrounded, on at least three sides, by cold continental lithosphere. The preferred seismic model also shows a new fracture zone, 30 km southeast of the Ocean-Continent Transition, which is characterized by low velocities in the upper crust, a lens-shaped layer with velocities of 7.2–7.4 km s−1 at the base of the crust, and a crustal thickness of just 3.4 km. The continental crust appears largely unaffected by the proximity of the adjacent oceanic crust. There is no evidence for underplating of the continental crust adjacent to the Ocean-Continent Transition zone.

Othmar Müntener - One of the best experts on this subject based on the ideXlab platform.

  • Ancient depletion and mantle heterogeneity: Revisiting the Permian-Jurassic paradox of Alpine peridotites
    Geology, 2015
    Co-Authors: Anders Mccarthy, Othmar Müntener
    Abstract:

    The past decade has provided widespread evidence for isotopic disequilibrium between exhumed abyssal peridotites and associated gabbroic and basaltic bodies, calling into question the commonly held assumption of a direct genetic relationship between mantle rocks and spatially related melts. Alpine-Apennine ophiolites, dismembered remnants of the Jurassic Ligurian Tethys, are similar to present-day (ultra)slow spreading environments and Ocean-Continent Transition zones, and offer unique opportunities to study mantle processes. We present new results from a refractory peridotite within the Civrari Ophiolite (northern Italian Alps) containing clinopyroxene showing strongly radiogenic 143Nd/144Nd (0.5145–0.5147) and highly depleted 147Sm/144Nd (1.1–1.2) resulting from ∼13%–15% near-fractional melting. The isotopic compositions of Alpine-Apennine spinel peridotites that did not undergo Jurassic refertilization and impregnation show a pseudo-isochron age of 273 ± 24 Ma, which overlaps widespread Permian magmatic activity in Western Europe during post-Variscan extension. We propose that extremely refractory peridotites within Ocean-Continent Transition zones offer compelling evidence that mantle isotopic heterogeneity in (ultra)slow spreading environments is related to the exhumation of older and variably depleted rafts of subcontinental lithospheric mantle during continental breakup and rifting. We suggest that these residual peridotites offer snapshots of older partial melting events related to ancient crust-forming processes prior to their subsequent exhumation at the ocean floor.

  • A type sequence across an ancient magma-poor Ocean-Continent Transition: The example of the western alpine tethys ophiolites.
    Tectonophysics, 2008
    Co-Authors: Gianreto Manatschal, Othmar Müntener
    Abstract:

    Abstract The ophiolites from the Alpine Tethys are incompatible with the definition of the classical 3-layered Penrose ophiolite sequence, but they also show features that are inconsistent with ultraslow-spreading ridge sequences or transform settings. The existence of pre-rift contacts between subcontinental mantle and continental crust, the association of top-basement detachment faults with continent-derived blocks (extensional allochthons) and tectono-sedimentary breccias overlying subcontinental mantle, and a post-rift sedimentary evolution identical to that of the adjacent distal margin enable to characterize some of the Alpine Tethys ophiolites as remnants of a former Ocean Continent Transition (OCT). Therefore, we propose that at least some of the Alpine Tethys ophiolites are formed by remnants of an ancient Magma-Poor-Ocean Continent Transition, referred to as a MP-OCT sequence. The type sequence consists of the Platta, Tasna and Chenaillet ophiolite units, the former two representing the OCT of the ancient Adriatic and European/Brianconnais conjugate rifted margins, the latter representing a more developed “oceanic” domain. All three units escaped Alpine subduction and preserve pre-Alpine contacts between exhumed basement and a volcano-sedimentary cover sequence. These units preserve the structural, magmatic, hydrothermal and sedimentary record of continental breakup and early seafloor spreading. The observations compare well with those made along the magma-poor Iberia–Newfoundland rifted margins, which are the only example in an OCT where drill holes penetrated into basement. At present, magma-poor rifted margins form up to 50% of all rifted margins worldwide. We argue that MP-OCT sequences are more common in the geological record but were, in part mistaken as either Mid Ocean Ridge or tectonically dismembered Penrose-type ophiolite sections.

  • trace element chemistry and u pb dating of zircons from oceanic gabbros and their relationship with whole rock composition lanzo italian alps
    Contributions to Mineralogy and Petrology, 2008
    Co-Authors: Maryalix Kaczmarek, Othmar Müntener, Daniela Rubatto
    Abstract:

    The U–Pb ages and the trace element content of zircon U–Pb along with major and trace element whole rock data on gabbroic dikes from the Lanzo lherzolitic massif, N-Italy, have been determined to constrain crustal accretion in ocean–continent Transition zones. Three Fe–Ti gabbros were dated from the central and the southern part of the massif providing middle Jurassic ages of 161 ± 2, 158 ± 2 and 163 ± 1 Ma, which argue for magmatic activity over few millions of years. Zircon crystals are characterized by high but variable Th/U ratios, rare earth element patterns enriched in heavy rare earths, pronounced positive Ce and negative Eu-anomalies consistent with crystallization after substantial plagioclase fractionation. The zircon trace element composition coupled with whole rock chemistry was used to reconstruct the crystallization history of the gabbros. A number of gabbros crystallized in situ, and zircon precipitated from trapped, intercumulus liquid, while other gabbros represent residual liquids that were extracted from a cumulus pile and crystallized along syn-magmatic shear zones. We propose a model in which the emplacement mechanism of gabbroic rocks in ocean–continent Transition zones evolves from in situ crystallization to stratified crystallization with efficient extraction of residual liquid along syn-magmatic shear zones. Such an evolution of the crystallization history is probably related to the thermal evolution of the underlying mantle lithosphere.

  • What is the tectono-metamorphic evolution of continental break-up: The example of the Tasna Ocean–Continent Transition
    Journal of Structural Geology, 2006
    Co-Authors: Gianreto Manatschal, Anna Engström, Othmar Müntener, Laurent Desmurs, Urs Schaltegger, Mike Cosca, Daniel Bernoulli
    Abstract:

    Abstract The break-up of continental lithosphere in magma-poor margins is accompanied by the exhumation of mantle and crustal rocks in the footwall of large-scale detachment faults. Although these structures have been described from many modern and ancient margins, little is known about how they accommodate strain and evolve in time and space during continental break-up. The Tasna Ocean–Continent Transition (OCT) in southeastern Switzerland is one of the rare examples where such detachment faults are exposed and can be observed on a kilometre scale. In this paper we describe the deformation structures and their evolution observed along detachment faults in the Tasna OCT. Our results show that continental break-up was attained by a series of detachment faults. These detachment faults accommodated extensional strain in fault zones formed by a localized core zone that is surrounded by a several tens to hundred metres wide damage zones. The core zone corresponds to the zone of highest strain and is well defined structurally by the occurrence of gouges and/or foliated cataclasites and physically by separating a hanging wall from a footwall. Deformation in the fault zones occurred under greenschist facies to seafloor conditions and within the stability field of serpentine. U/Pb ages on zircon from a garnet-bearing pegmatite cross cutting high-temperature shear zones (upper amphibolite facies and higher) provide Carboniferous ages and demonstrate that these shear zones are neither kinematically nor genetically related to the detachment faults observed in the Tasna OCT. Ar/Ar ages on phlogopite from spinel websterite suggest that mantle exhumation occurred during Middle Jurassic time. Our data show that the detachment faults observed in the Tasna OCT formed during latest rifting, post-date major thinning of the crust and onset of mantle serpentinization. These results compare well with those from the deep Iberia margin. Our observations support the idea that rifting leading to continental break-up is a multi-phase process, and that serpentinization is the consequence rather than the reason for strain localization at non-volcanic margins. Apart from the more general implications for the tectonic evolution of continental break-up, our results have some important consequences for the palaeogeographic reconstruction of certain Alpine domains and question the existence of an independent Early Cretaceous Valais ocean in the Alpine realm.

  • MANTLE EXHUMATION PROCESSES IN THE OCEAN CONTINENT Transition OF MAGMA-POOR RIFTED MARGINS
    Ofioliti, 2005
    Co-Authors: Gianreto Manatschal, Othmar Müntener, Luc L. Lavier
    Abstract:

    Exhumation of mantle rocks to the seafloor is known to occur along oceanic transform faults, along slow and ultraslow spreading ridges, in back-arc basins and at Ocean-Continent Transitions (OCT) of magma-poor rifted margins. Good examples of OCT are described from the ancient Tethys margins exposed in the Alps as well as from the drilled and seismically imaged present-day Iberia margin. Indeed, in these margins, the Transition from continental to oceanic crust occurs across a zone consisting of serpentinized subcontinental mantle rocks that are intruded by minor volumes of gabbros. The rocks forming the basement are capped by a brittle fault zone and are overlain either by extensional allochthons, pillow basalts or deep-water sediments. The resulting ocean-floor sequences are very heterogeneous and characterized by the absence of sheeted dyke complexes and there is no genetic link between the magmatic rocks (basalts and gabbros) and the underlying mantle peridotites. In this presentation, we will focus to the tectonic processes related to mantle exhumation in OCT using examples from the Alps and the west Iberia margin. We discuss the rheological evolution of the extending lithosphere and examine the physical processes, which control mantle exhumation. The South-Pennine and Lower Austroalpine Platta and Err nappes in SE Switzerland preserve structural, sedimentological and petrological elements of an ancient OCT. The most prominent structure within the OCT is a detachment system that can be traced from the thinned continental crust towards oceanic crust. The occurrence of allochthons of continental origin directly emplaced over tectonized exhumed mantle rocks as well as the trace element composition of the mantle rocks and the observation that they preserve primary contacts to lower continental crust clearly indicate that the mantle in the OCT is of sub-continental origin forming the footwall of a major detachment system. In the OCT the mantle rocks change oceanwards form spinel lherzolites to plagioclase peridotites, the latter resulting from the reaction with infiltrating magmas. Increasing magmatic activity going oceanwards is also indicated by the observation that gabbroic bodies and pillow lavas generally become more voluminous and grade from T- to N-MORB oceanwards. Trace element compositions and Nd isotopic compositions of basalts and parental liquids of the gabbros are very similar and represent aggregated melts of low to moderate degrees of partial melting of an asthenospheric source. In the southwestern Iberia Abyssal Plain the architecture of the OCT is well imaged by reflection and refraction seismic sections and has been drilled during ODP Legs 149 and 173. The conspicuous analogies between drilled structures and lithologies off Iberia and those observed in the ancient Alpine margins suggest that both margins evolved in a similar way. Important insights into the tectonic processes related to mantle exhumation were obtained from the kinematic inversion of the Lusigal 12 seismic section in the Southern Iberia Abyssal Plain along which several ODP drill sites exist. The two key observations are: (1) the continental breakup is attained by a series of detachment faults that post-date thinning of the crust to less than 10 km, and (2) the geometry of the faults change form initial upwards to final downward concave faults that accommodated large (~10-20 km) offsets and exhumed the mantle rocks to the seafloor without causing a major submarine relief in the OCT. Drilling off Iberia and direct observations in the Alps confirm that the mantle in the OCT is capped by an exhumed top-to-the-ocean fault zone formed by serpentinite gouges and cataclasites. Gabbros and plagioclase peridotites, both of which document the presence of magma, formed during rifting and were exhumed in the footwall of a detachment to the seafloor. This suggests that detachment faulting is assisted by serpentinization and is concurrent with magmatic activity. It is important to note that hightemperature shear zones (upper amphibolite to granulite facies ; > 700°C) in the mantle are geometrically not related to the detachment faults and show an opposite sense of shear (top-to-the-continent). This may indicate that the detachment faults exhuming mantle rocks were only active at shallow levels within the stability field of serpentine, i.e. at less than ~550°C and that they were decoupled from the underlying hotter mantle along subhorizontal high-temperature shear zones that attenuated the ‘ductile’ deeper mantle. The relations between high and low-temperature deformation structures that may tell something about where the detachment faults are rooted, is, however, not yet fully understood. Our observations suggest that thinning of the crust to less than 10 km is a prerequisite to serpentinize the mantle and initiate magma emplacement and downward concave detachment faults that will eventually lead to continental break-up. Therefore serpentinization and magmatism are late events and are the consequence rather than the reason for initial strain localization at non-volcanic margins. This implies that mantle exhumation is a complex multi-phase process that is controlled by tectonic, magmatic and hydration processes that successively modify the rheology during rifting. While early rifting may depend mainly on the initial thermal conditions and the lithospheric structure, final break-up may be controlled by hydration reactions (e.g. serpentinization) and magmatic processes, as well as the rise of the asthenosphere (thermal erosion). It is clear that advective heating by ascending magmas and conductive cooling by exhumation and fluid flow are interacting during the final phase of rifting. However, little is known how these processes interact during final break-up and lots of observations need to be gathered and eventually modelled. Therefore, in order to understand these physical processes controlling the final phase of continental break-up, the community needs to concentrate its efforts on how melt/fluid/rock reactions and deformation processes are interacting and how these processes modify the rheology of the lithosphere.

Jeanclaude Sibuet - One of the best experts on this subject based on the ideXlab platform.

  • Tectonic Significance of the Taitung Canyon, Huatung Basin, East of Taiwan
    Marine Geophysical Researches, 2004
    Co-Authors: Jeanclaude Sibuet, Shu-kun Hsu, Alain Normand
    Abstract:

    Since the beginning of formation of Proto-Taiwan, the subducting Philippine (PH) Sea plate has moved continuously through time in the N307° direction with respect to Eurasia (EU), tearing the EU plate. The subducting EU plate includes a continental part in the north and an oceanic part in the south. The boundary B between these two domains corresponds to the eastern prolongation of the northeastern South China Sea Ocean-Continent Transition zone. In the Huatung Basin (east of Taiwan), the Taitung Canyon is N065° oriented and is close and parallel to B. Seismic profiles show that the southern flank of the canyon corresponds to a fault with a normal component of a few tens of meters in the sediments and possible dextral shearing. Several crustal earthquakes of magnitude >%6 are located beneath the trend of the Taitung Canyon and focal mechanisms suggest that the motion is right-lateral. Thus, faulting within the sedimentary sequence beneath the Taitung Canyon is a consequence of underlying dextral strike-slip crustal motions. As the continental part of the EU slab located north of B has been recently detached, some subsequent dextral strike-slip motion might be expected within the EU slab, along the Ocean-Continent Transition zone, which is a potential zone of weakness. We suggest that the dextral strike-slip motion along the Ocean-Continent boundary of the EU slab might trigger the observed dextral strike-slip motion within the overlying PH Sea crust and the associated faulting within the sediments of the Huatung Basin, beneath the Taitung Canyon.

  • Variations in heat flow across the ocean—continent Transition in the Iberia abyssal plain
    Earth and Planetary Science Letters, 1997
    Co-Authors: Keith E. Louden, Jeanclaude Sibuet, Francois Harmegnies
    Abstract:

    Abstract New heat flow observations have been made in the Iberia abyssal plain off the Galicia margin along the transect of Ocean Drilling Program Leg 149 drill sites, in order to investigate the nature of this unusually wide and deep continent-ocean Transition region. Our results indicate the presence of three separate zones. Average values of 47.5 ± 3 mW m−2 in the westernmost zone III agree with predictions of standard oceanic lithospheric models for its estimated age of 126 Ma. In contrast, the heat flow within zone II is 5–15 mW m−2 higher than predicted, assuming that the mantle heat flow remains constant across the basin. This region of high values is coincident with the location of a major intra-crustal “S”-type reflector east of ODP Site 900, and the anomaly is consistent with the presence of 2–3 km of primarily upper continental crust above the reflector, with concentrations of radiogenic components similar to those from granodiorite samples dredged off Galicia Bank. It is not, however, consistent with the low values of heat production measured on gabbroic samples from its western end at ODP Site 900. In zone I, detailed measurements across the tilted fault block south of ODP Site 901 show consistent variations which closely match predictions due to the effects of basement structure and sediment deposition. There is no evidence for variations due to vertical convective transport along the dipping basement fault block. Once corrected for these variations, measurements in zone 1 yield average values that agree quite well with previous measurements across Galicia Bank, indicating no systematic landward increase in heat flow with decreasing amounts of continental extension.

  • Thinning of the Goban Spur continental margin and formation of early oceanic crust: constraints from forward modelling and inversion of marine magnetic anomalies
    Geophysical Journal International, 1997
    Co-Authors: Veronique Louvel, J. Dyment, Jeanclaude Sibuet
    Abstract:

    SUMMARY The deep seismic reflection profile Western Approaches Margin ( WAM) cuts across the Goban Spur continental margin, located southwest of Ireland. This non-volcanic margin is characterized by a few tilted blocks parallel to the margin. A volcanic sill has been emplaced on the westernmost tilted block. The shape of the eastern part of this sill is known from seismic data, but neither seismic nor gravity data allow a precise determination of the extent and shape of the volcanic body at depth. Forward modelling and inversion of magnetic data constrain the shape of this volcanic sill and the location of the Ocean-Continent Transition. The volcanic body thickens towards the ocean, and seems to be in direct contact with the oceanic crust. In the contact zone, the volcanic body and the oceanic magnetic layer display approximately the same thickness. The oceanic magnetic layer is anomalously thick immediately west of the volcanic body, and gradually thins to reach more typical values 40 km further to the west. The volcanic sill would therefore represent the very first formation of oceanic crust, just before or at the continental break-up. The Ocean-Continent Transition is limited to a zone 15 km wide. The continental magnetic layer seems to thin gradually oceanwards, as does the continental crust, but no simple relation is observed between their respective thinnings.

  • thin crust at the western iberia ocean continent Transition and ophiolites
    Tectonics, 1993
    Co-Authors: R. B. Whitmarsh, P. R. Miles, L M Pinheiro, Maurice Recq, Jeanclaude Sibuet
    Abstract:

    Western Iberia is bounded by a nonvolcanic rifted continental margin made up of three apparently independent segments. The age of breakup decreases from south to north. Seismic refraction and reflection profiles, and magnetic and gravity data from each segment, show a consistent pattern of geophysical observations across the Ocean-Continent Transition (OCT) zone, which is a few tens of kilometers wide. We emphasize here the discovery of thin (2–4 km) oceanic crust underlain by 7.6 km s−1 material within the OCT. The available evidence favors the suggestion that the 7.6 km s−1 layer is serpentinized peridotite and that the thin oceanic crust is primarily the result of a poor magma supply for a few million years immediately after continental breakup. This thin crust may be the source of some ophiolites which exhibit thin crustal sections and continental margin affinities.

  • Thin crust at the western Iberia Ocean‐Continent Transition and ophiolites
    Tectonics, 1993
    Co-Authors: R. B. Whitmarsh, Luís M. Pinheiro, P. R. Miles, M. Recq, Jeanclaude Sibuet
    Abstract:

    Western Iberia is bounded by a nonvolcanic rifted continental margin made up of three apparently independent segments. The age of breakup decreases from south to north. Seismic refraction and reflection profiles, and magnetic and gravity data from each segment, show a consistent pattern of geophysical observations across the Ocean-Continent Transition (OCT) zone, which is a few tens of kilometers wide. We emphasize here the discovery of thin (2–4 km) oceanic crust underlain by 7.6 km s−1 material within the OCT. The available evidence favors the suggestion that the 7.6 km s−1 layer is serpentinized peridotite and that the thin oceanic crust is primarily the result of a poor magma supply for a few million years immediately after continental breakup. This thin crust may be the source of some ophiolites which exhibit thin crustal sections and continental margin affinities.

P. R. Miles - One of the best experts on this subject based on the ideXlab platform.

  • thin crust at the western iberia ocean continent Transition and ophiolites
    Tectonics, 1993
    Co-Authors: R. B. Whitmarsh, P. R. Miles, L M Pinheiro, Maurice Recq, Jeanclaude Sibuet
    Abstract:

    Western Iberia is bounded by a nonvolcanic rifted continental margin made up of three apparently independent segments. The age of breakup decreases from south to north. Seismic refraction and reflection profiles, and magnetic and gravity data from each segment, show a consistent pattern of geophysical observations across the Ocean-Continent Transition (OCT) zone, which is a few tens of kilometers wide. We emphasize here the discovery of thin (2–4 km) oceanic crust underlain by 7.6 km s−1 material within the OCT. The available evidence favors the suggestion that the 7.6 km s−1 layer is serpentinized peridotite and that the thin oceanic crust is primarily the result of a poor magma supply for a few million years immediately after continental breakup. This thin crust may be the source of some ophiolites which exhibit thin crustal sections and continental margin affinities.

  • Thin crust at the western Iberia Ocean‐Continent Transition and ophiolites
    Tectonics, 1993
    Co-Authors: R. B. Whitmarsh, Luís M. Pinheiro, P. R. Miles, M. Recq, Jeanclaude Sibuet
    Abstract:

    Western Iberia is bounded by a nonvolcanic rifted continental margin made up of three apparently independent segments. The age of breakup decreases from south to north. Seismic refraction and reflection profiles, and magnetic and gravity data from each segment, show a consistent pattern of geophysical observations across the Ocean-Continent Transition (OCT) zone, which is a few tens of kilometers wide. We emphasize here the discovery of thin (2–4 km) oceanic crust underlain by 7.6 km s−1 material within the OCT. The available evidence favors the suggestion that the 7.6 km s−1 layer is serpentinized peridotite and that the thin oceanic crust is primarily the result of a poor magma supply for a few million years immediately after continental breakup. This thin crust may be the source of some ophiolites which exhibit thin crustal sections and continental margin affinities.

  • The ocean–continent boundary off the western continental margin of Iberia—II. Crustal structure in the Tagus Abyssal Plain
    Geophysical Journal International, 1992
    Co-Authors: Luís M. Pinheiro, R. B. Whitmarsh, P. R. Miles
    Abstract:

    SUMMARY An 80 km long reversed seismic refraction line (Line 5) was shot over the Tagus Abyssal Plain off Portugal. The main P-wave reflected and refracted phases were modelled both for traveltime and amplitude. The resulting P-wave velocity/depth model has the following features: (a) an extremely thin crust of about 2 km; (b) the absence of oceanic layer 3; and (c) very low upper mantle velocities between 7.6 and 7.9 km s-'. This very unusual seismic velocity crustal structure is quite unlike thinned continental crust but is remarkably similar to the seismic crustal structures found at Atlantic fracture zones, and in particular to the structures found in profiles shot along the transform valley and near ridge-transform intersections. A magnetic anomaly chart seems to allow the possibility of several fracture zones one of which could intersect the centre of Line 5. As an alternative to the fracture zone hypothesis we show that if the oceancontinent Transition in the Tagus Abyssal Plain is located at about 11"30'W, in a symmetric position with respect to the Ocean-Continent Transition in the conjugate South Newfoundland Basin, then magnetic anomalies can be modelled simply by assuming sea-floor spreading west of 11'45'W at 10 mm yr-' beginning at M11 time (133 Myr BP), and blocks of rifted continental crust to the east. The location of the proposed Ocean-Continent Transition in the Tagus Abyssal Plain is marked by a well-defined N-S linear magnetic anomaly which is adjacent to the oldest sea-floor spreading block. East of the proposed Ocean-Continent Transition there is an increase in the depth to basement similar to that found east of the Ocean-Continent Transition in the Iberia Abyssal Plain and elsewhere. This model also allows us to explain why Purdy's (1975) seismic refraction line A-AR in the Tagus Abyssal Plain cannot be interpreted as a conventional reversed pair because most of Line A was shot over the Ocean-Continent Transition zone and most of Line AR over thinned continental crust. Remarkably similar velocity/depth structures to that under Line 5 are found close to the Ocean-Continent Transition zone off the whole of western Iberia, in areas which show no clear evidence of fracture zones. Therefore it appears more likely that the seismic structure of Line 5 is due to its proximity to the Ocean-Continent Transition than to a local association with a fracture zone and further, that its structure is typical of this Transition off the western margin of Iberia. We also suspect that the low upper mantle velocities associated with the Ocean-Continent Transition indicate the widespread occurrence of serpentinized peridotite.

  • The Ocean-Continent Transition of western Iberia
    1991
    Co-Authors: R. B. Whitmarsh, Luís M. Pinheiro, P. R. Miles, G. Boillot, M. Recq
    Abstract:

    The western continental margin of the Iberian peninsular has the characteristic of a rifted non-volcanic continental margin with half-graben and tilted fault blocks seen in several places on multichannel seismic reflection profiles. The Ocean-Continent Transition (OCT) is therefore expected to be where thinned continental crust and oceanic crust are juxtaposed, as elsewhere. The authors located the OCT off western Iberia in order to constrain the pre-lift fit of Iberia to North America. This fit is only marginally constrained by sea-floor spreading magnetic anomalies because the Cretaceous constant polarity interval is adjacent to the OCT. Thinned continental crust can be distinguished from oceanic crust by the nature of the lower crustal velocity structure. In 1986-1987, a series of seismic refraction profiles was shot across three parts of the Iberian Abyssal Plain, the OCT can be detected not only from seismic velocities but also by modeling magnetic anomalies. The chosen location of the OCT is consistent with the interpretation of subsequently acquired multichannel profiles. Off Galicia Bank, the OCT, recognized from seismic velocities and multichannel profiles, corresponds to a seabed peridotite ridge, which has been extensively sampled. In the Tagus Abyssal Plain, limited seismic data gives a less clear picture of themore » OCT.« less