Yves Lagabrielle

Ultramafic and gabbroic exposures at the Mid-Atlantic Ridge: geological mapping in the 15°N region

Abstract The outcrops of mantle-derived ultramafic rocks in the 15°N region are the most extensive yet reported for the Mid-Atlantic Ridge. North of the Fifteen Twenty fracture zone, these outcrops form a belt at least 20 km long along the west wall of the axial valley and also crop out on the east axial valley wall. Ultramafic rocks also crop out extensively south of the Fifteen Twenty fracture zone. Based on dredging and on a morphological analysis of the bathymetric map, we propose that ultramafic outcrops may be common in the crust formed between 14°30′N and 15°50′N during at least the past 2.4 m.y. Moderately dipping fault planes and large expanses of tectonic breccia have been observed during dives on the ultramafic outcrops. Diving observations also show that the ultramafic rocks are capped, in stratigraphic contact, by a thin layer of basalt. This suggests that these rocks were tectonically emplaced at the axial seafloor, or very close to it, then uplifted in the footwall of the faults that bound the axial valley. The occurrence of ultramafic rocks on both walls of the axial valley may be due to frequent changes of faulting polarity in the axial region: instead of one master shear zone, there would be a complex array of cross-cutting conjugate faults and shear zones that could jump inward in the axial domain as spreading proceeds.

Recent tectonic, magmatic, and hydrothermal activity on the East Pacific Rise between 17°S and 19°S: Submersible observations

The objective of the Naudur cruise (December 1993) of the submersible Nautile was to study the interaction among magmatic, tectonic, and hydrothermal processes at a very fast spreading mid-ocean ridge axis. Twenty-three dives were completed, both along and across the axis, in four areas located between 17°10′ and 18°45′S on the East Pacific Rise. Rock, sulfides, water, and biological samples have been collected along each of the segments. Two main types of segments have been distinguished, characterized either by the predominance of present-day volcanic activity or by predominant tectonic activity. Linked to both types of activity, 69 hydrothermal sites have been discovered and sampled. They comprise four types, interpreted as successive evolutionary stages. The first are shimmering water sites which occur immediately after the formation of lava lakes and are characterized by large surface area and poorly developed associated fauna. The second, in areas dominated by recent volcanic activity, have waters venting directly from lava fissures and more focused discharge areas through black smoker chimneys. The third stage is represented by more mature hydrothermal vents and deposits, along the faults bounding the eastern side of the axial graben in tectonic-dominated areas. The associated fauna is well developed. The fourth stage corresponds to the reactivation of volcanic activity with lava flows, young black smokers, and diffuse venting associated with the faults bounding the axial graben. Fluids collected range from 200° to 340°C and show a wide variability in chemical and gas composition. Within each of the explored areas, evidence of recent volcanic activity has been observed.

SUBDUCTION EROSION RELATED TO SPREADING-RIDGE SUBDUCTION: TAITAO PENINSULA(CHILE MARGIN TRIPLE JUNCTION AREA)

During the Pliocene, subduction of the Chile ridge beneath the South American margin was coeval with the emplacement of the Cabo Raper pluton located at the seaward edge of the Taitao peninsula. The chemical characteristics of the Cabo Raper pluton combined with the available tectonic data allow us to reconstruct the paleogeometry of the Chile margin 3–4.2 m.y. ago. When compared to the modern configuration, the volume of material removed by subduction erosion can be estimated quantitatively. From 3–4.2 to 1.5–1.6 Ma, 625 km 3 of rock were removed along each kilometre of margin along the Taitao peninsula transect. This leads to a conservative subduction-erosion rate of 231–443 km 3 ċkm −1 ċm.y. −1 , significantly higher than those calculated along the Japan and Peru convergent margins. It is proposed that subduction of the Chile ridge contributes fluids in addition to the subducted sediment, resulting in a higher rate of subduction erosion.

New geochemical constraints for the origin of ridge-subduction-related plutonic and volcanic suites from the Chile triple junction (Taitao peninsula and Site 862, LEG ODP141 on the Taitao ridge)

Abstract Several features of Neogene and Quaternary magmatism in the region south of the present-day Chile Triple Junction (CTJ) at 46°12′S are directly related to the migration of the triple junction. Due to the obliquity of the ridge orientation with respect to the subduction front, the triple junction migrated from South to North during the last 14 Ma. The Taitao Peninsula — the westernmost promontory of the Chile coast — and the Taitao Ridge — a submarine promontory north of the Taitao Peninsula — provide the most complete collection of ridge subduction-related magmatic products in the region. The emplacement of near-trench volcanics, the intrusion of a variety of plutonic rocks and the related hydrothermal activity at these two sites have been interpreted as resulting from magma interactions between subducted ridge segments of the Chile spreading centre and the continental crust. We present new field observations and geochemical data that help to better constrain the problem of the sources and evolution of the Taitao magmas. The new geochemical data were obtained on samples collected from the Taitao Peninsula during a field expedition in 1995, and from samples of the Taitao Ridge during Leg ODP141, Site 862, which have been re-sampled in 1996 by one of us. Selected major- and trace-element compositions of 20 volcanic rocks from the Taitao Ridge are discussed together with 53 analyses from different rock types from the Taitao Peninsula including 24 unpublished analyses. Nd and Sr isotopic compositions were obtained from 5 whole rocks and separated minerals of the Taitao Peninsula together with the oxygen isotope composition of four separated clinopyroxenes. Six main magmatic types are identified: (1) N-type MORB; (2) E-type MORB; (3) LREE-depleted MORB showing some trace-element features typical of arc basalts; (4) moderately Nb-depleted E-MORB; (5) calc-alkaline andesites, dacites and rhyolites; and (6) andesites and dacites with adakitic signature. Chemical similarities exist between some forearc magmas of the Taitao Ridge and the Taitao Peninsula and magmas emplaced at the Chile active spreading ridge. One important result, based on isotope data, is that the lavas emplaced over the continental crust (Taitao Peninsula) did not originate from melting of continental crust nor from extensive assimilation of such a crust by mantle-derived magmas. The likely source of these basalts could be the hot convective oceanic mantle of the southern Chile spreading ridge buried at moderate depth (10–30 km).

Alpine, Corsican and Apennine ophiolites: the slow-spreading ridge model

Abstract Ophiolites of the Alps, Corsica and the Apennines (ACA) derive from the Jurassic-Cretaceous Ligurian Tethys ocean, which was located between Europe and Africa. They are characterized by a very small volume of gabbros and basalts, by the absence of any dyke complex, and by the frequent stratigraphic contact of sediments resting directly upon mantle-derived peridotites or gabbros. Two types of ACA ophiolites can be distinguished: magma rich (MR) and magma poor (MP). They show strong affinities with the oceanic lithosphere found respectively at the centres and tips of 20–100 km long segments which characterize the suds of the slow-spreading Mid Atlantic Ridge.

Volcanic record of the subduction of an active spreading ridge, Taitao Peninsula (southern Chile)

During the Pliocene, the Taitao Peninsula was the locus of the subduction of the Chile active spreading ridge beneath the South American margin. New field data allow us to distinguish within the Taitao ophiolite a genuine ophiolitic body of undetermined age and two volcanic and volcaniclastic units. One unit shows evidence for deposition in a shallow-water environment, and it unconformably overlies the Chilean continental basement. Its established Pliocene age demonstrates that it was deposited during the subduction of the Chile ridge. Lavas from both units include enriched mid-oceanic ridge basalts (MORBs) together with Nb-depleted basalts, andesites, and dacites. La/Nb ratios vary randomly from 1 to 4 in both units and are positively correlated with SiO 2 content. These chemical characteristics are consistent with upper-crustal contamination of MORB-type basalts by the Chilean crust as already suggested from isotopic data. We consider these volcanic suites to be the result of eruptions of MORB magmas that originated from the downgoing, shallowly buried, active spreading center and were contaminated during their rise through the Chilean continental basement.

Multiple active spreading centres in the hot North Fiji Basin (Southwest Pacific): a possible model for Archaean seafloor dynamics?

Abstract The North Fiji Basin (NFB), in the Southwest Pacific, is one of the most evolved back-arc basins. Within the basin, active spreading is taking place along several ridges, some of which display the characteristics of spreading ridges observed in mature large oceans characterized by a range of spreading rates (segmented accretion centres, organized magnetic lineations and propagators, for example). However, the great cumulate length of the spreading centres of the NFB relative to the Basins surface makes it an unique example among the worlds oceanic basins. We have reviewed a comprehensive body of geological, geophysical and geochemical evidence (multiple spreading centres of various trends, high heat flow, high geoid and slow upper mantle seismic velocities) which collectively demonstrate that a hot upper mantle underlies the North Fiji Basin. Active upper mantle convection is the key process controlling the large crustal and lithospheric production in the NFB, which would be thus largely independent from the evolution of even the largest structural features, such as the Vitiaz and New Hebrides subduction zones. Petrological studies of the Archaean greenstone belts have led to a similar picture for the entire Earth during the Archaean: numerous short active ridges connected by unstable triple junctions would have, at this time, provided an efficient convective mechanism allowing for a large heat dissipation. We propose that the presently active processes in the North Fiji basin are modern analogues to Archaean seafloor dynamics.

High-resolution imaging of the Pyrenees and Massif Central from the data of the PYROPE and IBERARRAY portable array deployments

The lithospheric structures beneath the Pyrenees, which holds the key to settle long-standing controversies regarding the opening of the Bay of Biscay and the formation of the Pyrenees, are still poorly known. The temporary PYROPE and IBERARRAY experiments have recently filled a strong deficit of seismological stations in this part of western Europe, offering a new and unique opportunity to image crustal and mantle structures with unprecedented resolution. Here we report the results of the first tomographic study of the Pyrenees relying on this rich data set. The important aspects of our tomographic study are the precision of both absolute and relative traveltime measurements obtained by a nonlinear simulated annealing waveform fit and the detailed crustal model that has been constructed to compute accurate crustal corrections. Beneath the Massif Central, the most prominent feature is a widespread slow anomaly that reflects a strong thermal anomaly resulting from the thinning of the lithosphere and upwelling of the asthenosphere. Our tomographic images clearly exclude scenarios involving subduction of oceanic lithosphere beneath the Pyrenees. In contrast, they reveal the segmentation of lithospheric structures, mainly by two major lithospheric faults, the Toulouse fault in the central Pyrenees and the Pamplona fault in the western Pyrenees. These inherited Hercynian faults were reactivated during the Cretaceous rifting of the Aquitaine and Iberian margins and during the Cenozoic Alpine convergence. Therefore, the Pyrenees can be seen as resulting from the tectonic inversion of a segmented continental rift that was buried by subduction beneath the European plate.

Glacial‐interglacial trench supply variation, spreading‐ridge subduction, and feedback controls on the Andean margin development at the Chile triple junction area (45–48°S)

During the Chile triple junction (CTJ) cruise (March–April 1997), EM12 bathymetry and seismic reflection data were collected in the vicinity of the Chile triple junction (45-480S), where an active spreading ridge is being subducted beneath the Andean continental margin. Results show a continental margin development shaped by tectonic processes spanning a spectrum from subduction-erosion to subduction-accretion. The Andean continental margin and the Chile trench exhibit a strong segmentation which reflects the slab segmentation and the Chile triple junction migration. Three segments were identified along the Andean continental margin: the presubduction, the synsubduction, and the postsubduction segments, from north to south. Both climate-induced variations of the sediment supply to the trench and the tectonic reorganization at the Nazca-Antarctica plate boundary involving postsubduction ridge jump are the two main factors that control the tectonic regime of this continental margin. Along the survey area we infer the succession of two different periods during the last glacial-interglacial cycle: a glacial period with ice-rafted detrital discharges restricted to the shoreline area and low river output and a warmer period during which the Andean ice cap retreat allowed the Andes to be drained off. During these warm periods, rapid increase in trench deposition caused the margin to switch from subductionerosion or nonaccretion to subduction-accretion: (1) along the presubduction segment after the last deglaciation and (2) along the postsubduction segment after the interglacial episode at 130–117 ka. Conversely, a nonaccretion or subduction-erosion mode characterized the presubduction and postsubduction segments during glacial maximums. The major effects of subduction of the buoyant Chile ridge include a shallow trench which diverts trench sediment supply and tectonic instabilities at the Nazca-Antarctica plate boundary. We suggest that a postsubduction westward jump of the Chile ridge occurred during the past 780 kyr. It produced slab fragmentation and individualization of an ephemeral microplate north of the Taitao fracture zone: the Chonos microplate. In 780 kyr, two episodes of subduction-accretion separated by an episode of subduction-erosion occurred in relation with the Chonos microplate individualization and subduction. The current northward migration of the triple junction along the Chonos microplate-South America plate boundary introduces a sharp change in the tectonic mode from subduction-erosion to the north to subduction-accretion to the south. The data collected along the Taitao ridge have revealed the complex three-dimensional structure of an accretionary wedge which includes a midslope thrust sheet exhibiting the characteristics of an ophiolite: the Taitao Ridge ophiolite. No connection exists between the Taitao Ridge ophiolite and the Bahia Barrientos ophiolite cropping out onland in the Taitao peninsula.

Thermal control on the modes of crustal thinning leading to mantle exhumation. Insights from the Cretaceous Pyrenean hot paleomargins.

The pre-rift Mesozoic sequences of the Cretaceous passive margins fossilized in the North Pyrenean Zone (NPZ) are characterized by high temperature deformation in relation with thinning of the continental basement. Our compilation of chronological and geological data confirms a clear correlation between the distribution of the highest paleotemperatures in the pre-rift sedimentary cover and the loci of extreme crustal stretching. Geological evidences such as the occurrence of peridotite bodies directly underlying metamorphic pre-rift sediments indicate an early attenuation of the rifted continental crust. This leads us to propose a mechanism of rifting involving boudinage of the continental crust. The lateral extraction of the Paleozoic basement occurred under the pre-rift cover that is decoupled on the Triassic clays and evaporates. The thermal conditions allowing coeval ductile deformation of the crust and of the pre-rift sediments leaded to the widening of basins devoid of large faulted blocks. We discuss the implications on the origin and significance of the granulites and the relations between flysch deposition and high temperature metamorphism of the pre-rift sediments. In the NPZ, Albian-Cenomanian flysch sequences were deposited synchronously with the syn-metamorphic ductile deformation of the pre-rift sequences. Since the base of the flysch deposits also recorded locally the high-temperature tectonic event, we propose an original mechanism for the evolution of the basins involving continuous deformation of the pre-rift metamorphic sediments. At the scale of the Pyrenean domain, our results suggest a strong lateral variability in the tectonic style of passive margins, in direct link with their thermic pattern