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International audience; A new high-resolution bathymetric map combined with a regional Digital Elevation Model (DEM) analysis reveal the modalities of occurrence and emplacement of post-spreading magmatism along the NNE-SSW oriented, 1000 km long Tore-Madeira Rise (TMR) as well as its relationship with the activity of major fault systems including the Estremadura Fault System (ESF) and the Azores-Gibraltar Fracture Zone (AGFZ). Morphological and structural analysis of the bathymetric map were performed to map volcanic features such as eruptive cones, vents and fissures together with faults along the TMR. The new bathymetric map shows that the main NNW-SSE seamount alignment is formed by three structurally distinct volcanic massifs, the Tore, the Josephine and the Southern Volcanic Groups. The majority of the volcanoes of each group emplaced within or along specific portion of pre-existing faults (ESF and AGFZ) including splay fault, releasing bend, fault tips and interaction zones between different segments. Magmas were channelled into sub-vertical pre-existing lithospheric faults that acted as preferential pathways for the vertical magma ascent. Migration and final eruption of magma are controlled by the local stress variation induced by complex fault geometries, change in plate kinematics as well as strong shear zone anisotropy as suggested by the emplacement within localised areas of transtension. We conclude that post-spreading magma emplacement in the southern part of the Iberia margin was related to the development of a transtensional plate boundary between the Iberian and African Plate during the Late Cretaceous. More generally, our findings emphasize that the distribution of volcanism as the expression of the interaction between shallow plate tectonic and mantle processes should be included in plate kinematic reconstruction. This study also demonstrates that the accurate mapping of oceanic seafloor is pivotal to better understand tectono-magmatic evolution of volcanic seamount chains and geological processes in oceanic domains.

The Tore–Madeira Rise is a seamount chain located 300 km off the Portugal and Morocco coasts attributed to hotspot activity. U–Pb ages of lavas from the northern and central Tore–Madeira Rise range between 103 and 80.5 Ma whereas 40 Ar/ 39 Ar ages from the central and southern Tore–Madeira Rise yield ages ranging from 94.5 to 0.5 Ma. We performed new 40 Ar/ 39 Ar measurements to better understand the geodynamic history of the Tore–Madeira Rise. Plagioclase ages from the Bikini Bottom and Torillon seamounts suggest ages of >90 Ma and ≥60 Ma, respectively. Amphiboles from the Seine seamount yield an age of 24.0 ± 0.8 Ma. Biotites from lavas of the Ashton seamount give ages of 97.4 ± 1.1 Ma and 97.8 ± 1.1 Ma. The geochronological database available on the Tore–Madeira Rise has been filtered on statistical criteria to eliminate unreliable ages. The resulting database reveals three pulses of alkaline magmatism on the Tore–Madeira Rise at 103–80.5 Ma, at c . 68 Ma and between 30 Ma and the present. The magmatism was continuous from 103 Ma until c . 68 Ma and from c . 30 Ma until the present on the Tore–Madeira Rise, the surrounding seamounts and the Portugal coast. We suggest that the space–time distribution of this magmatism results from the interaction between a wide thermal anomaly emitting magmatic pulses and the complex motion of the Iberian plate. Supplementary material: A detailed Ar measurements dataset is available at http://www.geolsoc.org.uk/SUP18359.

The Seram-Ambon ophiolitic series comprise peridotites, websterites, gabbros and lavas. Petro-geochemical data show that the peridotites are weakly depleted rocks, except for the rare Cpx-free harzburgites. They underwent a sub-solidus metamorphic re-equilibration in the plagioclase field. The associated websterites and gabbros display various chemical features, allowing to define 3 types of websterites and 2 groups of gabbros. They have mostly BAB characteristics (presence of negative anomalies in Nb, Zr, Ti and Y), except the group 2 gabbros which have N-MORB features and the type 3 websterites which bear adakitic affinities. Lavas also display a variety of compositions, including high-Mg IAT and Mg-rich BABB with sub-alkaline affinities. Both IAT and BABB display high Th/Nb ratios which support an origin close to a continental crust environment. Our 20 to 15 Ma 40K/40Ar ages calculated for the BABB and 15-9 Ma for the IAT show that the basin and arc formed in a very short span of time, before their obduction 9–7 Ma ago [Linthout et al., 1997]. Considering the paleogeographic situation in the Miocene [Haile, 1979 ; Haile, 1981] and our data, we propose that the Seram-Ambon ophiolites formed during the early Miocene in a small, short-lived (10 Ma), transtensive basin bordered on its east by an active margin and on its western part by a passive continental margin over which it was later obducted towards the SW direction.

New petrological data on magmatic rocks obtained from the Iberia Abyssal Plain and from the Gorringe Bank, combined with those already known on the Galicia Bank, allow to better constrain the formation of the West Iberian Margin. These three zones were sampled along East-West transects of the ocean-continent transition, immediately West or at the foot of the last tilted continental block of the margin.

A Nautile submersible investigation of Pito Seamount documents vigorous hydrothermal venting at 23° 19.65'S, 111° 38.41'W and at a depth of 2270 m. The data indicate the volcano is young and recently active, as predicted from analyses of SeaMARC II side-scan and swath bathymetry, and geophysical data. Pito Seamount lies near Pito Deep (5980 m), which marks the tip of the northwestward propagating East rift of the Easter microplate. Bathymetry surrounding Pito Seamount consists of a series of ridges and valleys with relief up to ∼ 4 km. The 4-km submersible-transect to the summit of Pito Seamount crossed areas of very glassy basalt with little or no sediment cover, suggesting the lava flows are very young. Most of the lava samples from Pito Seamount are depleted normal MORB (mid-ocean ridge basalt). Lava samples associated with active and dead hydrothermal vents consist of phyric and aphyric transitional and enriched MORB. Sulfides consist primarily of sphalerite and pyrite, with traces of chalcopyrite. The active hydrothermal chimney on Pito Seamount has a small, undiversified biological community similar to northern East Pacific Rise vent sites (alvinellid worms, bythograeid crabs and bythitid fishes) and western Pacific back-arc basin sites (alvinocaridid shrimps). No vestimentiferan worms were observed. Previous geophysical data, and new geochemical data and visual observations, suggest that the vigorous black smoker is a result of deep, extensive crosscutting faults formed by extensive tectonic thinning of Pito Deep, and a very robust magmatic supply being supplied from upwelling asthenosphere. Although no biological or vent fluid samples were obtained, geological and biological observations, such as the large number of inactive chimneys, old hydrothermal deposits, and starfish, as well as the occurrence of dead mollusks (gastropod and mussels), suggest a recent waning of hydrothermal activity near the summit. The speculative interpretation that Pito Seamount is acting as a focal point for the formation of a new seafloor spreading axis trending northwest (310°) from the seamount summit towards Pito Deep is supported by the new data and observations reported here. These include the similar geochemistry of young lava samples obtained from Pito Seamount and from a small volcanic mound within Pito Deep and the strong SeaMARC II side-scan backscatter amplitudes along most of the ∼ 50 km rift zone connecting the summit of Pito Seamount to Pito Deep.

The 1981 French–Chinese expedition to Tibet focused on the Lhasa block, extending earlier coverage 400 km north of the Tsangpo suture. The Lhasa block stood between 10 and 15° N latitude over most of the Upper Cretaceous and Eocene and, if Gondwanian in origin, had detached from Gondwana by early Permian. Seismic profiles reveal a complex Moho topography resulting both from multiple continental thrusting and large-scale strike-slip faulting. Subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated. This study emphasizes the role of smaller blocks in the accretion of the continental mosaic.

During ODP Leg 103, serpentinized peridotite (clinopyroxene-spinel harzburgite) was cored within the basement approximatively at the boundary between the North Atlantic oceanic curst to the west, and the thinned continental crust of the Galicia passive margin (Spain) to the east. The exposure of mantle derived peridotite on the seafloor occurred at the end of the period of rifting, roughly 110 Ma ago. Ductile shear zones observed in the cored peridotite are consistent with movements along a deep low-angle, normal fault rooted within the upper mantle and dipping eastward, beneath the Galicia margin. To explain the tectonic denudation of the mantle at the ocean-continent boundary, we use a non-uniform stretching model for the lithosphere, set up from the Wernicke's model (1985).

Leg 101 of the Ocean Drilling Program (ODP) was devoted discovering the tectonic and sedimentary evolution of the Atlantic Margin of the lberian Peninsula. A transect of five sites, with a total of 14 drill-holes was undertaken to the South of the Galicia Bank on the seaward edge of the margin. The data obtained revealed a complex history of subsidence and rifting preceding the initiation of sea floor spreading between Newfoundland and Iberia. The main findings include: 1) The Upper Jurassic-Lowermost Cretaceous shallow-water carbonate platform are the first Messozoic deposits at the margin. The «basement seismic reflector» is made-up of these carbonates. 2) The platform drowning, tilting of fault blocks and rapid subsidence preceded the spreading by as much as 25 million years. 3) A ridge of serpentiniced peridotites is located near the boundary between the oceanic and continental crusts. 4) The seismic reflector «S» does not, as widely believed represent a ductile-brinle boundary within the continental crust but is instead a reflector at the base of the synrift sediments.La campaña oceanográfica 103 del Ocean Drilling Program (ODP) ha estado dedicada a dilucidar la evolución tectónica y sedimentación del Margen Atlántico-Ibérico. Se realizaron un total de 14 sondeos., en cinco puntos de posicionamiento, sobre el extremo más profundo del margen; al S del Banco de Galicia. Los resultados obtenidos revelan que previamente al inicio de la expansión oceánica entre Terranova e Iberia ocurrió una historia compleja de distensión cortical, fracturación y subsidencia asociadas. Los resultados fundamentales son los siguientes: 1) Carbonatos de plataforma marina somera, de edad Jurásico superior-Cretáceo basal constituyen los primeros depósitos mesozóicos en ese ámbito del margen y dan lugar a un reflector sísmico considerado interiormente como basamento, 2) El hundimiento de la plataforma, fallamiento y basculamiento de los bloques ocurre desde 25 m.a. antes de iniciarse la acreción oceánica. 3) En el límite entre corteza oceánica-corteza continental se ubica una cresta constituida por peridotitas serpetinizadas. 4) El reflector sísmico «S», generalmente considerado como el límite dúctil-frágil en la corteza continental, corresponde realmente a la base de los depósitos sinrift.

Results are reported from the 1980 joint French–Chinese field expedition in Tibet. The area covered was from the High Himalaya in the south, to the region of Nagqu ∼250 km north of Yangbajain. Ophiolites in the Zangbo valley represent remnants of the crust of an ocean basin which lay adjacent to the Gangdise granodiorite belt in the late Mesozoic. The ophiolites were thrust to the south onto the Cretaceous melange and Triassic flysch but the age of this event is unclear. In the Tertiary, sediments of the Indian margin and the Xigaze basin were folded with steeply dipping cleavage, which indicates some north–south shortening of the crust. However, the Tertiary volcanic sequence on the Lhasa block and further north shows comparatively little folding and thrusting. In the Quaternary, deformation of Tibet was characterized by east–west extension.

The Bangong-Nujiang suture zone is underlain by a discontinuous and roughly EW ophiolite belt. Around Donqiao–Gyanco, ophiolitic and metasedimentary Palaeozoic series form nappes thrusted over Jurassic flysch series. All these series have undergone a late symmetrical folding accompanied by generally northward reverse faulting. Although strongly dismembered, a complete ophiolitic sequence comprising harzburgites with large podiform-type chromite deposits, wehrlites, cumulate gabbros, sheeted dolerite dykes, foliated garnet-bearing amphibolites and pillow lavas, has been found.

Three tectonic units (the active palaeomargin, the palaeo-oceanic domain and the passive palaeomargin) are now juxtaposed and constitute the Yalu Tsangpo suture zone between India and Eurasia in southern Tibet. We examine the deformation, metamorphic and uplift histories of each of these units. Our observations are accounted for in terms of the processes whereby the crust was thickened to about 70 km. We finally discuss the implications for ancient continental collision orogens.

Introduction and geodynamic setting. – 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; Müntener 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. Main results. – The bathymetric map unveils three E-W domains (fig. 2). From north to south, they are the continental slope of Australia, prolonged westward by that of the Naturaliste Plateau, a 160 km-wide intermediate flat sedimented area corresponding to the MQZ, and a 100 km-wide zone of rough E-W oriented topography which continues the Diamantina Zone (fig. 3). The first two domains are cut through in three segments by two major fracture zones (FZ), the Leeuwin FZ along the eastern side of the Naturaliste Plateau, and the Naturaliste FZ along its western flank. These NW-SE trending FZ terminate north of the E-W trending fabric of the Diamantina Zone. Accordingly, extension occurred along the NW-SE direction during the formation of the slope and of the MQZ, and then turned to N-S during the formation of the Diamantina Zone. In the Diamantina Zone, the mantle rocks dredged at Site MG-DR02 are mainly lherzolites, rich in pyroxenitic micro-layers, and pyroxenites. They contain spinel rimmed by plagioclase and locally coronas of olivine + plagioclase between opx and spinel, which suggest that they underwent some subsolidus reequilibration in the plagioclase field (fig. 4C). Westward (Site DR09), the mantle rocks are harzburgitic, with lesser pyroxenitic bandings and no plagioclase. The rocks have coarse-grained porphyroclastic textures that are locally overprinted by narrow mylonitic shear bands, and then by a cataclastic deformation, which indicate decreasing temperatures and increasing stresses during their evolution. Basalts were sampled at Sites MG-DR01, −04, −05, and together with gabbros at Sites MG-DR02, -03, -09. They have a transitional composition as shown by their REE patterns, except one sample from site MG-DR-05 which is an alkaline basalt (fig. 5). The gabbros are clearly intrusive in the peridotite at Sites DR02 and -09. They contain olivine and clinopyroxene (cpx) at Site DR02, cpx, plagioclase and hornblende at Site DR03, and cpx and amphibole or orthopyroxene or olivine at Site DR09 (fig. 4D). At that site, a tonalite containing K-feldspar and biotite and alkaline in composition (fig. 5), has also been sampled. All these plutonic rocks display either their primary magmatic textures or secondary porphyroclastic ones that are locally overprinted by mylonitic shear zones (fig. 4E). Retrograde minerals of amphibolite to greenschist facies developed during the deformation. The basalts are clearly intrusive in the gabbros at Site DR03. They are altered and exhibit porphyric textures with abundant plagioclase and plagioclase + olivine phenocrysts at Sites DR03, -04, -08, -10, and have a transitional composition (fig. 5). The nature and evolution of the peridotites and associated gabbros are compatible with an exhumation under a rift zone, on both sides of the Leeuwin FZ. It includes a mylonitic deformation which attests that these rocks underwent a shearing deformation under lithospheric conditions, in probable relation with their exhumation during the early stages of the oceanic opening. The crustal rocks are represented only by intrusive gabbros and by transitional basalts. In the MQZ, the peridotites recovered at Site MG-DR06 are mainly spinel and plagioclase lherzolites (fig. 4B) and a few pyroxenites (fig. 4A) with high temperature porphyroclastic textures. Their discovery indicates that the basement in the MQZ is not exclusively formed of thinned continental crust. Lavas sampled westward of the Leeuwin FZ at Site DR10 have also transitional compositions (fig. 5). On the Australian slope, samples dredged at Site MG-DR07 are continental quartz-bearing rocks (mostly gneisses and rare granites), some showing a high grade paragenesis (upper amphibolite to granulite facies) marked by the presence of K-feldspar, biotite, sillimanite and/or kyanite and garnet, and without primary muscovite (fig. 4G). Some of these rocks underwent an intense mylonitic shear deformation followed by post-tectonic recrystallisation or migmatization. Depending on the age of the high grade evolution (metamorphism and shearing), these rocks document either the syn-rift exhumation of lower continental crust, or the formation of the older Australian craton. On the slope of the Naturaliste Plateau, at Site DR11, rocks of oceanic origin (gabbro-diorites/dolerite/basalt; fig. 4F) were dredged together with acid rocks (gneiss and granites) of probable continental origin, some having a quartz, K-feldspar, biotite and garnet metamorphic paragenesis (fig. 4H). At that site, the transitional basalts intrude the gabbros and associated dolerites. The presence of metamorphic acid rocks indicate that the Naturaliste Plateau is likely a continental fragment that was later intruded by mafic rocks, whose origin and ages of intrusion have to be determined. Discussion and conclusions. – The retrograde tectono-metamorphic evolution of the peridotites recovered in the MQZ, which includes a reequilibration in the plagioclase field (marked by the development of olivine and plagioclase after spinel and pyroxene), is compatible with an exhumation under a rift zone [Girardeau et al., 1988; Kornprobst and Tabit, 1988; Cornen et al., 1999]. By analogy with the Iberia Abyssal Plain, the MQZ could represent a wide OCT where the mantle was exhumed and stretched mostly by amagmatic extension before the initiation of oceanic accretion [Beslier et al., 1996; Boillot and Coulon, 1998] (fig. 6). This hypothesis is supported by the tectonic structures (horsts and grabens) imaged in the seismic data over the MQZ [Boeuf and Doust, 1975]. Accordingly, the limit of the continental crust would be located at the foot of the slope, i.e. 160 km (or 250 km in the NW-SE extension direction) northward of the assumed location of the OCT at the southern edge of the MQZ. The age of the Australia-Antarctic breakup would thus be (1) older than that inferred from the magnetic anomalies (circa 95 Ma [Cande and Mutter, 1982; Veevers, 1986]), which would rather date the onset of oceanic accretion, and (2) older than the age of the breakup unconformity estimated as Santonian (83 Ma), further east, in the Great Australian Bight [Sayers et al., 2001]. The origin of the Naturaliste Plateau, continental or oceanic, is still disputed. The discovery of metamorphic rocks of probable continental origin on the southern flank of the Plateau (Site DR11) shows that it consists at least partially of rocks of the Gondwana continent. All the samples from the Diamantina Zone confirm that its basement is made of a peridotite-gabbro-basalt assemblage. The nature and age of the peridotites and of the associated magmas will help understanding the origin of this domain, which can result either from Neocomian seafloor spreading with further remobilization during the Australia-Antarctic separation, or from post-Neocomian ultra-slow seafloor spreading. Because of the omnipresence of extensive tectonic structures (fig. 3) and of the relatively small proportion of crustal rocks relative to the mantle rocks, we argue that the formation of the Diamantina Zone was mainly controlled by tectonics rather than by magmatic processes. In conclusion, the data collected along the southwest Australian margin during the MARGAU/MD110 survey evidence two major tectonic phases with formation of a wide OCT where abundant mantle rocks, in association with few mafic rocks, outcrop or lay directly beneath the sediments. The evolution of the crystalline rocks is compatible with an exhumation under a rift zone during a phase of magma-poor extension primarily controlled by tectonic processes. The domains where basement highs were sampled seem to be continuous over more than 1500 km eastward along the south Australian margin. Additional evidence on such large-scale structural continuity and on the nature of the associated basement highs may help generalizing the models for continental breakup and formation of non-volcanic passive margins, where oceanic accretion does not immediately follow continental breakup.