Oceanic Accretion Research Articles

Evidence for asymmetric nonvolcanic rifting and slow incipient oceanic accretion from seismic reflection data on the Newfoundland margin

Prestack depth migrations of seismic reflection data collected around the Ocean Drilling Program (ODP) Leg 210 transect on the Newfoundland nonvolcanic margin delineate three domains: (1) extended continental crust, (2) transitional basement, and (3) apparent slow spreading oceanic basement beyond anomaly M3 and indicate first‐order differences between this margin and its well‐studied conjugate, the Iberia margin. Extended continental crust thins abruptly with few observed faults, in stark contrast with the system of seaward dipping normal faults and detachments imaged within continental crust off Iberia. Transition zone basement typically appears featureless in seismic reflection profiles, but where its character can be discerned, it does not resemble most images of exhumed peridotite off Iberia. Seismic observations allow three explanations for transitional basement: (1) slow spreading oceanic basement produced by unstable early seafloor spreading, (2) exhumed, serpentinized mantle with different properties from that off Iberia, and (3) thinned continental crust, likely emplaced by one or more detachment or rolling‐hinge faults. Although we cannot definitively discriminate between these possibilities, seismic reflection profiles together with coincident wide‐angle seismic refraction data tentatively suggest that the majority of transitional basement is thinned continental crust emplaced during the late stages of rifting. Finally, seismic profiles image abundant faults and significant basement topography in apparent oceanic basement. These observations, together with magnetic anomaly interpretations and the recovery of mantle peridotites at ODP Site 1277, appear to be best explained by the interplay of extension and magmatism during the transition from nonvolcanic rifting to a slow spreading oceanic accretion system.

Oppositely directed pairs of propagating rifts in back‐arc basins: Double saloon door seafloor spreading during subduction rollback

When a continent breaks up into two plates, which then separate from each other about a rotation pole, it can be shown that if initial movement is taken up by lithospheric extension, asthenospheric breakthrough and oceanic accretion propagate toward the pole of rotation. Such a propagating rift model is then applied to an embryonic centrally located rift which evolves into two rifts propagating in opposite directions. The resultant rhombic shape of the modeled basin, initially underlain entirely by thinned continental crust, is very similar to the Oligocene to Burdigalian back‐arc evolution of the Valencia Trough and the Liguro‐Provencal Basin in the western Mediterranean. Existing well and seismic stratigraphic data confirm that a rift did initiate in the Gulf of Lion and propagated southwest into the Valencia Trough. Similarly, seismic refraction, gravity, and heat flow data demonstrate that maximum extension within the Valencia Trough/Liguro‐Provencal Basin occurred in an axial position close to the North Balearic Fracture Zone. The same model of oppositely propagating rifts, when applied to the Burdigalian/Langhian episode of back‐arc oceanic accretion within the Liguro‐Provencal and Algerian basins, predicts a number of features which are borne out by existing geological and geophysical, particularly magnetic data. These include the orientation of subparallel magnetic anomalies, presumed to be seafloor spreading isochrons, in both basins; concave‐to‐the‐west fracture zones southwest of the North Balearic Fracture Zone, and concave‐to‐the‐east fracture zones to its northeast; a spherical triangular area of NW oriented seafloor spreading isochrons southwest of Sardinia; the greater NW extension of the central (youngest?) magnetic anomaly within this triangular area, in agreement with the model‐predicted northwestward propagation of a rift in this zone; successively more central (younger) magnetic anomalies abutting thinned continental crust nearer to the pole of rotation in the Liguro‐Provencal Basin. The latter feature demonstrates that a rift also propagated northeast in the Liguro‐Provencal Basin, at least in its oceanic accretion phase of development. An adaptation of an existing model for subduction slab detachment occurring along the North African margin in the late Burdigalian/Langhian, proposes propagation in opposite directions of the slab tear. The resultant rhombic slab detachment is closely associated in space and time with the rhombic form of the Algerian/Liguro‐Provencal basins, suggesting a cause and effect relationship.

Phanerozoic geological evolution of the Equatorial Atlantic domain

The Phanerozoic geological evolution of the Equatorial Atlantic domain has been controlled since the end of Early Cretaceous by the Romanche and Saint Paul transform faults. These faults did not follow the PanAfrican shear zones, but were surimposed on Palæozoic basins. From Neocomian to Barremian, the Central Atlantic rift propagated southward in Cassiporé and Marajó basins, and the South Atlantic rift propagated northward in Potiguar and Benue basins. During Aptian times, the Equatorial Atlantic transform domain appeared as a transfer zone between the northward propagating tip of South Atlantic and the Central Atlantic. Between the transform faults, oceanic accretion started during Late Aptian in small divergent segments, from south to north: Benin-Mundaú, deep Ivorian basin-Barreirinhas, Liberia-Cassiporé. From Late Aptian to Late Albian, the Togo-Ghana-Ceará basins appeared along the Romanche transform fault, and Côte d’Ivoire-Parà-Maranhão basins along Saint Paul transform fault. They were rapidly subsiding in intra-continental settings. During Late Cretaceous, these basins became active transform continental margins, and passive margins since Santonian times. In the same time, the continental edge uplifted leading either to important erosion on the shelf or to marginal ridges parallel to the transform faults in deeper settings.

A wide ocean-continent transition along the south-west Australian margin: first results of the MARGAU/MD110 cruise

Syn-rift exhumation of mantle rocks in a continental breakup zone was highlighted along the present-day west Iberian passive margin [e.g. Boillot et al., 1988, 1995; Whitmarsh et al., 1995, 2001; Beslier et al., 1996; Brun and Beslier, 1996; Boillot and Coulon, 1998; Krawczyk et al., 1996; Girardeau et al., 1998] and along the fossil Tethyan margins [e.g. Froitzheim and Manatschal, 1996; Manatschal and Bernoulli, 1996; Marroni et al., 1998; Muntener et al., 2000; Desmurs et al., 2001]. Along the west Iberian margin, serpentinized peridotite and scarce gabbro and basalt lay directly under the sediments, over a 30 to 130 km-wide transition between the thinned continental crust and the first oceanic crust [Girardeau et al., 1988, 1998; Kornprobst and Tabit, 1988; Boillot et al., 1989; Beslier et al., 1990, 1996; Cornen et al., 1999]. The formation of a wide ocean-continent transition (OCT), mostly controlled by tectonics and associated with an exhumation of deep lithospheric levels, would be an essential stage of continental breakup and a characteristic of magma-poor passive margins. The southwest Australian margin provides an opportunity to test and to generalize the models proposed for the west Iberian margin, as both margins present many analogies. The south Australian margin formed during the Gondwana breakup in the Mesozoic, along a NW-SE oblique extension direction [Willcox and Stagg, 1990]. From north to south, the continental slope is bounded by (1) a magnetic quiet zone (MQZ) where the nature of the basement is ambiguous [Talwani et al., 1979; Tikku and Cande, 1999; Sayers et al., 2001], (2) a zone where the basement shows a rough topography associated with poorly expressed magnetic anomalies [Cande and Mutter, 1982; Veevers et al., 1990; Tikku and Cande, 1999; Sayers et al., 2001], and which is the eastward prolongation of the Diamantina Zone, and (3) an Eocene oceanic domain. The continental breakup zone is believed to be located near or at the southern edge of the MQZ [Cande and Mutter, 1982; Veevers et al., 1990; Sayers et al., 2001]. Breakup is dated at 125 Ma [Stagg and Willcox, 1992], 95 ± 5 Ma [Veevers, 1986] or at 83 Ma [Sayers et al., 2001], and followed by ultra-slow seafloor spreading until the Eocene (43 Ma), and fast spreading afterwards [Weissel and Hayes, 1972; Cande and Mutter, 1982; Veevers et al., 1990; Tikku and Cande, 1999]. The western end of the margin (fig. 1) is starved and bounded in the OCT by basement ridges where peridotite, gabbro and basalt were previously dredged [Nicholls et al., 1981]. Altimetry data [Sandwell and Smith, 1997] show that some of these ridges are continuous over 1500 km along the OCT of the south Australian margin and of the conjugate Antarctic margin. The objectives of the MARGAU/MD110 cruise (May-June 1998; [Royer et al., 1998]; fig. 2) were to define the morpho-structure and the nature and evolution of the basement in the SW Australian OCT. An area of 180 000 km2 was explored with swath bathymetry. Gravimetric data (11382 km) were simultaneously recorded whereas few single channel seismic (1353 km) and magnetic (5387 km) data were obtained due to technical difficulties. Crystalline basement rocks, made of varied and locally well-preserved lithologies, were dredged at 11 sites located on structural highs.

From partial to full strain partitioning along the Indo-Burmese hyper-oblique subduction

The Andaman–Nicobar Trench and its onshore prolongation—the Indo-Burmese wedge—is the least studied segment of the India–Australia subduction. New offshore geological and geophysical data have recently been collected along the Burma scarp during a marine survey conducted with the R/V Marion Dufresne (Andaman Cruise). Swath bathymetric mapping combined with shallow and deep seismic show that the dominant active tectonics is dextral strike-slip faulting accompanied by various amounts of shortening. The southern portion of the Burma scarp is transpressive (north Andaman Islands), the central portion is pure strike-slip on NNE-oriented segments (southern Myanmar), the northern portion shows sedimentary wedge growth along NW segments, both onshore and offshore (Arakan Yoma prism). Further north, the wedge bends to the west, while dextral shear faults more or less parallel to the Sagaing Fault develop within the internal part of the wedge and elongated NS folds form in its external part, abutting onto the Shillong Plateau. We propose a simple kinematic model involving evolution from partial to full partitioning from south to north along the West Burma Scarp (WBS), and we test it quantitatively using the most recent geodetic results. In the south, half of the 3.5 cm/year of India motion is taken at the trench itself, the other half being accommodated onto a single shear fault, the Sagaing Fault in Myanmar. In the north, where the Bangladesh fold system developed, dextral strike-slip faults are activated within the Arakan Yoma belt and at the accretionary prism–backstop contact, resulting in full partitioning there. Faults accommodating the oblique component of motion of India are progressively migrating in space from far field faults (Andaman–Sagaing) to near trench faults (Arakan Yoma belt and trench itself), the Burma sliver being “buttressed” by the Eastern Himalayas. This kinematic extends back to 4 Ma, which is the time of initiation of the last pulse of oceanic accretion in the Andaman Basin.

Continental breakup and the onset of ultraslow seafloor spreading off Flemish Cap on the Newfoundland rifted margin

Prestack depth-migrated seismic reflection data collected off Flemish Cap on the Newfoundland margin show a structure of abruptly thinning continental crust that leads into an oceanic accretion system. Within continental crust, there is no clear evidence for detachment surfaces analogous to the S reflection off the conjugate Galicia Bank margin, demonstrating a first-order asymmetry in final rift development. Anomalously thin (3–4 km), magmatically produced oceanic crust abuts very thin continental crust and is highly tectonized. This indicates that initial accretion of the oceanic crust was in a magma-limited setting similar to present-day ultraslow spreading environments. Seaward, oceanic crust thins to <1.3 km and exhibits an unusual, highly reflective layering. We propose that a period of magma starvation led to exhumation of mantle in an oceanic core complex that was subsequently buried by deep-marine sheet flows to form this layering. Subsequent seafloor spreading formed normal, ∼6-km-thick oceanic crust. This interpretation implies large fluctuations in the available melt supply during the early stages of seafloor spreading before a more typical slow-spreading system was established.

Crustal structure of the ocean‐continent transition at Flemish Cap: Seismic refraction results

We conducted a seismic refraction experiment across Flemish Cap and into the deep basin east of Newfoundland, Canada, and developed a velocity model for the crust and mantle from forward and inverse modeling of data from 25 ocean bottom seismometers and dense air gun shots. The continental crust at Flemish Cap is 30 km thick and is divided into three layers with P wave velocities of 6.0–6.7 km/s. Across the southeast Flemish Cap margin, the continental crust thins over a 90‐km‐wide zone to only 1.2 km. The ocean‐continent boundary is near the base of Flemish Cap and is marked by a fault between thinned continental crust and 3‐km‐thick crust with velocities of 4.7–7.0 km/s interpreted as crust from magma‐starved oceanic accretion. This thin crust continues seaward for 55 km and thins locally to ∼1.5 km. Below a sediment cover (1.9–3.1 km/s), oceanic layer 2 (4.7–4.9 km/s) is ∼1.5 km thick, while layer 3 (6.9 km/s) seems to disappear in the thinnest segment of the oceanic crust. At the seawardmost end of the line the crust thickens to ∼6 km. Mantle with velocities of 7.6–8.0 km/s underlies both the thin continental and thin oceanic crust in an 80‐km‐wide zone. A gradual downward increase to normal mantle velocities is interpreted to reflect decreasing degree of serpentinization with depth. Normal mantle velocities of 8.0 km/s are observed ∼6 km below basement. There are major differences compared to the conjugate Galicia Bank margin, which has a wide zone of extended continental crust, more faulting, and prominent detachment faults. Crust formed by seafloor spreading appears symmetric, however, with 30‐km‐wide zones of oceanic crust accreted on both margins beginning about 4.5 m.y. before formation of magnetic anomaly M0 (∼118 Ma).

The syn-rift sedimentary cover of the North Biscay Margin (bay of Biscay): from new reflection seismic data

Abstract The Armorican Basin is a deep sedimentary basin lying at the footside of the North Bay of Biscay. From previous scattered inadequate data, the age and nature of this basin, oceanic domain or deep part of the Armorican margin itself were largely speculated. From this new seismo-stratigraphic study based on a dense seismic cover, the sedimentation within the Armorican Basin is beginning in the Aptian times, during the last tectonic rifting episode of the margin. The first sediments formation identified as the « 3B layer » is characterised on the profiles by a chaotic and transparent seismic facies and was emplaced by slumping process when the margin collapsed, at the final rifting phase, just before the oceanic accretion. The new seismic reflection data give also some informations on the polyphased evolution of the North Biscay Margin during the rifting period. Two main events occurred during the Lower Cretaceous times (the first one is pre-Berriasian, the second is Aptian), separated by a quiet tectonic period including the Upper Berriasian and Lower Aptian times. The first event is responsible of the margin tectonic structuration in some blocks, the second of collapsing and the emplacement of the allochthonous sediments (3B layer) in the Armorican Basin.

From rifting to Alpine inversion: Mesozoic and Cenozoic subsidence history of some Moroccan basins

Present-day Morocco constitutes the junction between the Atlantic and the Mediterranean domains. Its past evolution was driven by the development and the ultimate closure of the Tethys ocean. In order to constrain the evolution of the subsidence of this western part of the Tethyan margin, five regions of Morocco have been modelled, the Essaouira and Missour basins, the Middle and High Atlas and the High Plateaux. The subsidence history of these regions during the Triassic–Jurassic is linked to the evolution of the Atlantic and Tethyan oceans, from rifting to oceanic accretion. The subsidence history of the Essaouira Basin is the most directly linked to the evolution of the Atlantic Ocean. This basin records strong subsidence from Trias to Liassic time as well as a lesser pulse of subsidence during the Malm. After this time, the subsidence rate in the Essaouira Basin dramatically decreased and its present-day architecture was already acquired in the Upper Cretaceous, except for local uplifts. The Atlas intramarginal-rifted basins show strong subsidence at the beginning of the Lias, and a further episode of rapid subsidence during the Dogger. As soon as the alpine cycle started, topography resulting from tectonic inversion of the Atlas basins modified the pattern of sedimentary distribution, and the products of erosion were deposited in the border basins such as Missour, High Plateaux, Souss and Ouarzazate. The data available to us did not allow us to pinpoint the timing of the Atlas uplift between the Cretaceous and the Present, and we found no arguments to support the most likely Late Cretaceous/Tertiary age for the beginning of this uplift. Analysis of subsidence in the Missour Basin and in the High Plateaux confirms their permanent structural high position during Mesozoic time with respect to the High and Middle Atlas domains, and shows that these basins were individualised only after inversion of the Atlas region.

The South Caspian Basin: a review of its evolution from subsidence modelling

The basement surface of the South Caspian depression lies at a depth of 20–25 km, making it one of the deepest basins in the world. It occupies the southern, deep-water, part of the Caspian Sea and two adjacent lowlands: the West Turkmenia in the east and the Lower Kura in the west. The basin can be subdivided into several sub-basins with two main depocentres, one in the northern part of the basin, just on the southern flank of the Apsheron Sill, and one, called the Pre-Alborz trough, located in the south-eastern part of the marine basin. The sedimentary fill of the South Caspian Basin has been significantly deformed. Part of it is allochthonous and folded, overlying a ductile detachment zone within the Maikop shale (Oligocene–Early Miocene). The folded succession is unconformably overlain by Upper Pliocene–Quaternary neo-autochthonous sediments. An intense shortening event, related to the NNE–SSW convergence of the Arabian plate with Eurasia, affected the region during the Pliocene–Pleistocene. The thickness of Pliocene–Quaternary sediments alone reaches 10 km. They were deposited in a rapidly subsiding basin and were sourced from the surrounding Caucasus, Alborz, and Kopet-Dagh orogens as well as from the nearby Russian Platform. The thickness of the crust beneath the western central part of the basin is as little as 8 km in the western central part of the basin but exceeds 15 km in the eastern part. Geophysical data and gravimetric modelling provide evidence that the basement of the marine part of the basin comprises a high-velocity, thin complex crust. Subsidence of the basin is in part due to profound thinning of continental crustal or, more likely, to the formation of oceanic crust. This took place in Middle–Late Jurassic times, in the context of back-arc basin development, with possible reactivation during the Cretaceous. However, there remains controversy regarding the timing of oceanic accretion and deep-water deposition in the South Caspian Basin. The present results are based on subsidence analysis complemented by geological data from tectonic units surrounding the South Caspian Basin and on its margins. An additional mechanism, nevertheless, must be invoked to explain the younger, much more rapid Pliocene–Quaternary phase of subsidence that occurred simultaneously with the subsidence of Caucasus-related molasse basins and the uplift and erosion of the Caucasus Orogen. This rapid subsidence phase is probably of compressional origin and a simple elastic model in compression provides comparable amplitudes of subsidence. In addition, the South Caspian Basin is surrounded by orogenically loaded crust that adds to basin downwarping. To the north, the basin is bounded by a subduction zone.

Comparison of oceanic crust accretion features with low-velocity and ultralow-velocity spreading

Comparison of oceanic crust accretion features with low-velocity and ultralow-velocity spreading

Petrology and geochemistry of exhumed peridotites and gabbros at non-volcanic margins: ODP Leg 173 West Iberia ocena-continent transition zone

Abstract Ultramafic and mafic rocks recovered at Holes 1068A and 1070A were drilled during Leg 173 of the Ocean Drilling Program (ODP) in the ocean-continent transition zone of the Iberia Abyssal Plain. Peridotites show contrasting petrographic characteristics. Hole 1068A peridotites are fine grained and show a well-defined high-temperature foliation marked by elongated pyroxene as well as aligned spinels. Hole 1068A peridotites are strongly serpentinized. Hole 1070A peridotites are coarse grained and show little evidence of high-temperature foliation. The degree of serpentinization is lower and relicts of silicate minerals are preserved. In both sets of recovered material, spinels show a wide range of composition and suggest a complex magmatic evolution. Gabbros dykes, which are found only in Hole 1070A, are very coarse grained and are locally sheared and/or crushed. Magmatic amphiboles are kaersutites and Ti-rich tschermakites that are partially replaced by hornblende and actinolite, and are associated with plagioclase of intermediate composition. Peridotites and pyroxenite have low TiO 2 , Al 2 O 3 and CaO contents in carbonate-free samples. Ni and Cr contents fall into the upper-mantle array. On the other hand, gabbros have relatively high TiO 2 and V contents reflecting modal ilmenite, and suggesting that they are relatively differentiated. This paper presents the very first geochemical data on platinum group elements (PGE) of peridotites and gabbros from passive margins. Peridotites and gabbros show low PGE (25.83 ppb and 1.44 ppb), Pd (2.75 ppb and 0.15 ppb), Pd/Ir ratios (1.45 and 1.3) and mafic index. Pyroxenite has the highest PGE (27.97 ppb), Pd/Ir (19.87) and Pt/Ir (10.25). Interelemental correlation and observation of PGE-bearing sulphide phases suggest that the PGE are hosted by single sulphide phases. From a PGE point of view, extraction of magmas involved very PGE-depleted liquids similar to gabbroic veins cutting the peridotites at Hole 1070A. Partial melting is interpreted as occurring just before oceanic accretion. Geochemical attributes suggest that the peridotites belong to the Ronda and Beni Bousera peridotitic depleted end-member clan. Thus they are believed to be of subcontinental origin. Deformation and retrograde metamorphism of peridotites and gabbros are consistent with exhumation in a rift environment post-dating the 120 Ma magmatic stage.

Les extrémités orientale et occidentale du bassin de Manus, Papouasie-Nouvelle-Guinée, explorées par submersible : la campagne Manaute

The objective of the Manaute cruise of the R.V. L'Atalante with the submersible Nautile was the in situ study of accretion processes, magmatic activity and hydrothermalism in the Manus Basin. Twelve dives and 18 dredges, focusing on the eastern and western tips of the basin, allowed us to demonstrate that in the eastern part of the basin the oceanic accretion is reduced to two axes propagating between Djaul and Weitin FZ. However the magmatic and hydrothermal activity is widely spread along the axes but also on volcanoes and ridges aligned on a N110 oblique direction. In the western part of Manus basin the oceanic accretion is located along two axes propagating rapidly to the southwest. The magmatic and hydrothermal activities are variable and diffuse over the whole area. The effect of the subduction of the Australian Plate in the New Britain Trench is evident throughout the basin.

The relative timing of serpentinisation and mantle exhumation at the ocean–continent transition, Iberia: constraints from oxygen isotopes

Legs 173 and 149 of the Ocean Drilling Program profiled a zone of exhumed mantle peridotite at the ocean–continent transition (OCT) beneath the Iberia Abyssal Plain. The zone of exhumed peridotite appears to be tens of kilometers wide and is situated between blocks of continental crust and the first products of ocean accretion. Exhumed peridotite is 95–100% serpentinised to probable depths of 2–3 km. Down core oxygen isotope profiles of serpentinised peridotite at Sites 1068 and 1070 (Leg 173) show evidence for two fluid infiltration events. The earlier event involved pervasive infiltration of comparatively warm (>175°C) sea water and accompanied serpentinisation. The later event involved structurally focused infiltration of comparatively cool (<50–150°C) sea water and accompanied active mantle exhumation. We therefore conclude that the uppermost mantle was serpentinised before it was exhumed at the Iberian OCT. Implicit to this conclusion is that a sizeable region of serpentinised mantle existed directly beneath thinned but intact continental crust. Serpentinite has comparatively low density, low frictional strength and low permeability. The presence of such a ‘soft’ layer may have localised deformation and consequently promoted detachment-style exhumation of the uppermost mantle. The low permeability of a serpentinite ‘cap’ layer might help to explain the lack of observed melt at the Iberian OCT.

Drilling reveals transition from continental breakup to early magmatic crust

For the first time in an ocean‐continent transition zone (OCT), drilling has sampled rocks above and below what appears to be a synrift tectonic crust‐mantle contact. Research in the West Iberia OCT is assembling important data on the histories and ages of the basement rocks there. The principal result has been to show that evolution of the margin was dominated by extensional tectonics with only slight evidence of possible synrift melting.A significant preliminary finding of the recently completed Leg 173 of the Ocean Drilling Program (ODP) is that many, if not all, fault blocks underlain by a low‐angle reflector on multichannel seismic profiles under the 5000‐m deep Iberia Abyssal Plain consist of continental crust and are quite thin. The drilling has also shown that lower crust and peridotite became exposed in the OCT between the blocks of continental crust and the first products of oceanic accretion over a zone that appears to be tens of kilometers wide. This supports the hypotheses that predict a broad zone of upper mantle basement beneath the sediments in the development of the West Iberia zone. Preliminary observations of textural and compositional variations of the peridotite from east to west suggest that further studies will document the evolution of the mantle from continental breakup to oceanic accretion.

Structure of oceanic crust adjacent to a transform margin segment: the Côte d’Ivoire–Ghana transform margin

The structure of the oceanic crust adjacent to the Cote d’Ivoire–Ghana transform margin is deduced from multichannel seismic reflection and seismic wide-angle data, showing crustal heterogeneities within oceanic basement; the oceanic crust adjacent to the transform margin is half as thick as standard Atlantic oceanic crust. Refraction data indicate a gradual velocity transition towards typical mantle velocities. Such an abnormal oceanic crustal structure appears quite similar to crustal structures known along transform faults. This crustal thinning may be related to thermal effects of the nearby continental crust, on the oceanic accretion processes. We did not find geophysical evidence for oceanic crust contamination by continental lithosphere.

The dynamics of back-arc extension: an experimental approach to the opening of the Tyrrhenian Sea

The E-W-opening Tyrrhenian Sea developed after the Cretaceous-Palaeogene Alpine collision, nearly perpendicular to the motion of the African plate, as a back-arc of the Adria-Ionian westward subduction. Three driving mechanisms have been proposed to explain the dynamic evolution of the Tyrrhenian-Apennine system: (1) the northward indentation of the African plate; (2) the retreating subduction of the Adria-Ionian lithosphere; and (3) the gravitational collapse of the Alpine post-collisional wedge. In order to define the relative contribution of each of these mechanisms in the Neogene dynamic of the Tyrrhenian-Apennine system, we performed 3-D laboratory experiments, in which we simulated a retreating subduction process in a compressional regime oriented perpendicularly to the direction of subduction; in this framework we also tested the influence of the gravitational collapse of the overriding plate. Experiments were constructed using dry sand and silicone putties to simulate brittle upper crust and ductile lower crust/upper mantle, respectively; these layers floated on a high-density, low-viscosity glucose syrup which simulated the asthenosphere. The main conclusion of our experiments is that large-scale continental extension, similar to that observed in the Tyrrhenian area, could be reproduced perpendicular to the shortening direction induced by the indentation of the African plate; in this framework, extensional processes are indeed possible if the trench retreat velocity is higher than the rate of shortening induced by the advancing African plate. Our experimental results indicate that this high trench retreat velocity could be explained by the coexistence of the gravitational collapse of the post-Alpine wedge with a slab-pull process, linked to the retreating subduction of the Adria-Ionian plate. While the first mechanism is predominant in the Northern Tyrrhenian area, the second one seems to be important in the latest stage of extension and oceanic accretion of the Southern Tyrrhenian area.

Heat flux and timing of the drifting stage in the Ligurian-Provençal basin (northwestern Mediterranean)

The surface thermal flux in the Ligurian-Provençal basin is accounted for by a three-component model for the continental margins and a cooling model for the oceanic domain. In the continental margins the surface heat-flux data, corrected for the main perturbations, range from 65–70 to 85–90 mW m −; the background heat flowing out from the asthenosphere is 38 mW m −. Heat-flux data from the oceanic domain indicate that the drifting stage should have begun 4–5 Ma earlier and ended 2–3 Ma later than previously inferred, being consistent with recent paleomagnetic results. A decoupling of Corsica from Sardinia during drifting is also validated by the different periods of oceanic accretion which, in the northern sector, seems to be shorter. The rift stage started in the Oligocene and ended about 27 Ma ago between Sardinia and the Gulf of Lion. The rotation of Corsica and Sardinia, which was accompanied by westward crustal accretion, ended about 17 Ma ago. Such a wider time span for drifting involves a more reliable average spreading rate of 2.5 cm a −. An outline of the development of the basin during each evolutionary stage is drawn.

Special collection on the June 1993 volcanic eruption on the CoAxial segment, Juan de Fuca Ridge

The mid‐ocean ridge system is the primary site of oceanic crustal accretion and a major source of chemical/thermal flux from the Earth's interior to the hydrosphere. Although geological mapping, geophysical exploration, and hydrographie/chemical surveys have provided important insights into accretionary processes, until recently there has been no method of monitoring the dynamics of these systems over long periods and large areas. Insights into spreading center dynamics have depended on serendipitous discoveries of evidence of activity based on water column anomalies [Baker et al., 1987], bathymetric differences [Fox et al., 1992], or direct field observations [Haymon et al., 1993]. In situ instrumentation has been used to monitor dynamics at a few selected sites for long periods, but these instruments are generally unable to telemeter data in real time.

Evolution of nonvolcanic rifted margins: New results from the conjugate margins of the Labrador Sea

The crust across each margin of the Labrador Sea, a conjugate rift margin pair, consists of three well-defined zones: thinned continental crust, 70–80-km-wide transitional crust, and oceanic crust. The transition zone is characterized by a low-velocity upper crust (4–5 km/s) underlain by a 6.4–7.7 km/s layer. We propose that the lower layer is serpentinized upper mantle; it is less likely to be gabbroic igneous lower crust. The low-velocity upper crust may be either an oceanic basaltic layer or very thin (<2 km) continental crust. Results from this first combined seismic reflection and refraction study of a conjugate margin pair support very slow initial continental rifting (∼1 mm/yr on average) resulting in an asymmetric crustal-thinning profile, with breakup occurring against the West Greenland shelf. This asymmetry may be an important feature of rifting at nonvolcanic margins and may be related to strain hardening of the cooling and thinning lithosphere or to small-scale convection in the lower lithosphere and asthenosphere at the sides of the rift. The thin crust in the transition zone implies that the amount of melt generated in the upwelling mantle is negligible, consistent with slow spreading or rifting. The transition from this slow formational period to normal oceanic accretion processes is marked by a sharp decrease in basement depth, signaling an increase to normal oceanic crustal thickness.