Jean-Louis Olivet

The lost Inca Plateau : cause of flat subduction beneath Peru?

Abstract Since flat subduction of the Nazca Plate beneath Peru was first recognized in the 1970s and 1980s a satisfactory explanation has eluded researchers. We present evidence that a lost oceanic plateau (Inca Plateau) has subducted beneath northern Peru and propose that the combined buoyancy of Inca Plateau and Nazca Ridge in southern Peru supports a 1500 km long segment of the downgoing slab and shuts off arc volcanism. This conclusion is based on an analysis of the seismicity of the subducting Nazca Plate, the structure and geochemistry of the Marquesas Plateau as well as tectonic reconstructions of the Pacific-Farallon spreading center 34 to 43 Ma. These restore three sub-parallel Pacific oceanic plateaus; the Austral, Tuamotu and Marquesas, to two Farallon Plate counterparts; the Iquique and Nazca Ridges. Inca Plateau is apparently the sixth and missing piece in an ensemble of ‘V-shaped’ hotspot tracks formed at on-axis positions. We argue the mirror image of the Inca Plateau, the Marquesas Plateau, is an ancient edifice overprinted by recent volcanism, in disagreement with the widely accepted young (

South Atlantic fits and intraplate boundaries in Africa and South America

Abstract The misfit problems raised by the pre-drift reconstructions of the South and Equatorial Atlantic compel us to resort to intraplate deformations. It is shown that acceptable deformations of the African plate in line with the Benue Trough and the Transafrican rifts and shear zones provide a far from satisfactory solution to the geometrical misfit problems in the South Atlantic. These problems can instead be alleviated by considering a degree of deformation of the South American plate along an intraplate boundary in line with the Parana Province, the Rio Grande Rise and the Walvis Ridge. In spite of the scarcity of field observations in this area of South America, we show that such a deformation cannot be ruled out, but it will have to be associated with deformations in the Equatorial Atlantic and in other areas of the African and South American plates.

La Mediterranee occidentale depuis l'Oligocene Schema d'evolution

-The evolution of the Western Mediterranean basin during Tertiary time is discussed. It is proposed that it was created by a continental drift process during Middle Oligocene time, synchronously with the creation of the grabens over the continents. The significance of the Messinian evaporitic sedimentary episode is discussed in liaison with the problem of large scale subsidence which has been at least six kilometers since Oligocene time.

The crustal structure of the Moroccan continental margin from wide-angle and reflection seismic data

SUMMARY The Atlantic margin off Morocco with its neighbouring Jurassic oceanic crust is one of the oldest on earth. It is conjugate to the Nova Scotia margin of North America. The SISMAR marine seismic survey acquired deep reflection seismic data as well as wide-angle seismic profiles in order to image the deep structure of the margin, characterize the nature of the crust in the transitional domain and define the geometry of the synrift basins. We present results from the combined interpretation of the reflection seismic, wide-angle seismic and gravity data along a 440-km-long profile perpendicular to the margin at 33‐34 ◦ N, extending from nearly normal oceanic crust in the vicinity of Coral Patch seamount to the coast at El Jadida and approximately 130 km inland. The shallow structure is well imaged by the reflection seismic data and shows a thick sedimentary cover that is locally perturbed by salt tectonics and reverse faulting. The sedimentary basin thickens from 1.5 km on the normal oceanic crust to a maximum thickness of 6 km at the base of the continental slope. Multichannel seismic (MCS) data image basement structures including a few tilted fault blocks and a transition zone to a thin crust. A strong discontinuous reflection at 12 s two-way travel-time (TWT) is interpreted as the Moho discontinuity. As a result of the good data quality, the deep crustal structure (depth and velocity field) is well constrained through the wide-angle seismic modelling. The crust thins from 35 km underneath the continent to approximately 7 km at the western end of the profile. The transitional region has a width of 150 km. Crustal velocities are lowest at the continental slope, probably as a result of faulting and fracturing of the upper crust. Uppermantle velocities could be well defined from the ocean bottom seismometer (OBS) and land station data throughout the model.

40Ar/39Ar ages for the alkaline volcanism and the basement of Gorringe Bank, North Atlantic ocean

Abstract Gorringe Bank is situated on the Europe-Africa plate boundary at the eastern end of the Azores-Gibraltar fracture zone. It has two summits, Gettysburg Bank to the Southwest and Ormonde Bank to the northeast. We applied the 40 Ar/ 39 Ar stepwise heating method to date six samples of the alkaline volcanic rocks, two gabbros from the Ormonde Bank and a dolerite from the Gettysburg Bank. The results that the alkaline volcanism lasted probably for less than 6 Ma(66-60 Ma). Although the nature of this volcanism precludes any subduction feature during its setting, the alkaline volcanism of Ormonde is probably linked to Upper Cretaceous/Eocene compressive tectonic events. The basement rocks of Gorringe Bank reveal distrubed 40 Ar/ 39 Ar age spectra. One plagioclase and one biotite from a gabbro give evidence for a thermic event whose age is tentatively estimated at about 75 Ma, and related to a variation in the direction of the relative movement between Europe and Africa. The more probable age given by a plagioclase of another gabbro and by a dolerite (110 Ma) corresponds to tilting northeastward of the Gorringe massif.

Large-scale chemical and thermal division of the Pacific mantle

Isotope analyses of mid-ocean-ridge basalts have led to the identification of large-scale geochemical provinces, with a clear distinction between the Pacific and the Atlantic or Indian Ocean basins,. It is widely believed that Pacific ridges are formed from a single, fairly well mixed mantle reservoir, extending from the Australian–Antarctic discordance to the Juan de Fuca ridge and representing one of the largest chemically coherent mantle domains on the Earth,. However, the evidence for this conception is mostly based on samples from the northern Pacific ridges. Here we report Sr, Nd and Pb isotope data from the Pacific Antarctic ridge that reveal different isotopic signatures north and south of the Easter microplate (25° S). The evidence for two large-scale geochemical domains is further strengthened by the observation of different average depths of the ridge axes north and south of the 25° S boundary. This boundary is located at the southeastern end of the Darwin rise/Pacific Superswell area, which is interpreted as a zone of upwelling from the lower mantle that has persisted since Cretaceous times. We propose that this upwelling has led to the separation into two mantle domains with their own convective histories, producing slight differences in their average isotopic signatures and thermal regimes.

Location of Louisville hotspot and origin of Hollister Ridge: geophysical constraints

Abstract The application of a new geometric technique [P. Wessel, L. Kroenke, A geometric technique for relocating hotspots and refining absolute plate motions, Nature 387 (1997) 365–369] recently pointed to a recent change in the Pacific plate absolute motion and suggested that the Louisville hotspot could now be located underneath the Hollister Ridge, south of the Eltanin fault system. However, the pole that was proposed for the last 3 Ma does not fit the trend of most Pacific volcanic alignments, supporting geochemical evidence [I. Vlastelic, L. Dosso, H. Guillou, L. Geli, H. Bougault, J. Etoubleau, J.-L. Joron, Geochemistry of the Hollister Ridge: relation with the Louisville hotspot and the Pacific–Antarctic Ridge, Earth Planet. Sci. Let. 160 (1998) 777–793] that does not favor a genetic relationship between the Louisville hotspot and the Hollister Ridge. We propose a pole near 57°N, 100°W that reconciles kinematic models with a previously proposed location [P. Lonsdale, Geography and history of the Louisville hotspot chain in the Southwest Pacific, J. Geophys. Res 93 (1988) 3078–3104] for the Louisville hotspot (near a Pleistocene volcano dredged at 50.5°S, 139.2°W) and claim that the Hollister Ridge most probably results from intraplate deformation processes.

SINGLE-BUBBLE MARINE SOURCE OFFERS NEW PERSPECTIVES FOR LITHOSPHERIC EXPLORATION

Abstract We present deep penetration seismic time sections obtained in the last two years with the “single-bubble” pulse generating method using marine seismic airguns. Whole crustal penetration and resolution have been obtained on the Ionian quasi oceanic basin and its margin, on the continental crust of the Aegean Sea, on the continental margins, and oceanic basin of the Gulf of Biscay. We show that this method is more efficient than others in radiating the lower frequencies returned by deep structure, without loosing resolution. Hence it allows deep crustal imaging from relatively low-power academic ships, and if the method is applied to powerful industrial vessels, it may open the subcrustal lithosphere to imaging.

A 3D model of the India-Asia collision at plate scale

Abstract We present a 3D model of the evolution of the India-Asia collision-subduction system from the collision (55 Ma) until now. The pre-collisional history (120–55 Ma) is relaitvely simple and well constrained by the analysis of the oceanic magnetic anomalies which allow a Greater India to be defined and to determine precisely its position during its northward drift. The post-collisional history is complex because, during the subduction of India beneath Asia, the boundary between the two continents changed continuously in response to the penetration of India into Asia, producing intense shortening, rotations of the structures and erosion of the continental crust. We propose some 3D images of this history from the collision to present time, from geological and geophysical evidence and particularly seismic tomography.

Le detroit sardano-tunisien et la zone de fracture nord-tunisienne

Abstract The paper, based upon 1100 km of unpublished seismic profiles, presents: 1. (1) A structural description of the Sardinia-Tunisia strait. Evidence for the large extent of the Sardinian and Tunisian continental blocks is presented. The North Tunisia fracture zone shows a series of heights, striking N40° E, oblique with respect to the Sardinian and Tunisian margins. By analogy with the Emile Baudot and Alboran rises, these heights are assumed of volcanic origin. The aeromagnetic map of Le Borgne et al. (1972) to the west, and the offsets in the continental margin to the east, suggest a pattern of fracture zones paralleling the North Tunisia fracture zone. 2. (2) An interpretation of the sedimentary cover: Plio-Quaternary unconsolidated sediments blanket almost completely the Sardinian and Tunisian shelves and slopes, they rest, either on the Palaeozoic basement, or on the Numidian nappe. In the central Channel, Messinian evaporites are present, while on the northern Tunisian shelf the uppermost Miocene seems to be non marine. 3. (3) A model in which the Sardinian and Tunisian basements are in contact in the Sardinia-Tunisia strait. The North Tunisia fracture zone would appear to be the flow line along which the western edge of the Kabyle internal zones could have drifted toward the: southwest during Early to Middle Miocene. The motion would have resulted in the creation of the North African basin according to the model developed for the Pacific marginal basins.