An alternative model of plate tectonics

International audienc

Pétrographie des ignimbrites et des dépôts volcanoclastiques associés dans le Cambrien de l'Est du massif armoricain

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Westward migration of oceanic ridges and related asymmetric upper mantle differentiation

International audienceCombining geophysical, petrological and structural data on oceanic mantle lithosphere, underlying astheno-sphere and oceanic basalts, an alternative oceanic plate spreading model is proposed in the framework of the westward migration of oceanic spreading ridges relative to the underlying asthenosphere. This model suggests that evolution of both the composition and internal structure of oceanic plates and underlying upper mantle strongly depends at all scales on plate kinematics. We show that the asymmetric features of lithospheric plates and underlying upper asthenosphere on both sides of oceanic spreading ridges, as shown by geophysical data (seismic velocities, density, thickness, and plate geometry), reflect somewhat different mantle compositions , themselves related to various mantle differentiation processes (incipient to high partial melting degree, percolation/reaction and refertilization) at different depths (down to 300 km) below and laterally to the ridge axis. The fundamental difference between western and eastern plates is linked to the westward ridge migration inducing continuing mantle refertilization of the western plate by percolation-reaction with ascending melts, whereas the eastern plate preserves a barely refertilized harzburgitic residue. Plate thickness on both sides of the ridge is controlled both by cooling of the asthenospheric residue and by the instability of pargasitic amphibole producing a sharp depression in the mantle solidus as it changes from vapour-undersaturated to vapour-saturated conditions, its intersection with the geotherm at~90 km, and incipient melt production right underneath the lithosphere-asthenosphere boundary (LAB). Thus the intersection of the geotherm with the vapour-saturated lherzolite solidus explains the existence of a low-velocity zone (LVZ). As oceanic lithosphere is moving westward relative to asthenospheric mantle, this partially molten upper asthenosphere facilitates the decoupling between lower asthenosphere and lithosphere. Thereby the westward drift of the lithosphere is necessarily slowed down, top to down, inducing a progressive decoupling within the mantle lithosphere itself. This intra-mantle decoupling could be at the origin of asymmetric detachment faults allowing mantle exhumation along slow-spreading ridges. Taking into account the asymmetric features of the LVZ, migration of incipient melt fractions and upwelling paths from the lower asthenosphere through the upper asthenosphere are oblique, upward and eastward. MORB are sourced from an eastward and oblique, near-adiabatic mantle upwelling from the lower as-thenosphere. This unidirectional mantle transfer is induced by isostatic suction of the migrating spreading ridge

Structural and paleostress analysis within a fossil slow-spreading ridge: Tectonic processes involved during ocean expansion

International audienceThis study aims to decipher the spatio-temporal chronology of tectonic processes leading to ocean widening at a slow-spreading oceanic ridge axis, involving mantle and gabbro exhumation and volcanism. Structural and petrographic studies of deformed lithologies were performed on peridotites, gabbros and basalts from the Chenaillet ophiolite (Alps, France). Inversion of fault-slip data reveals N030° and N060° σ3 paleostress trends. The dominant N030° extension, is consistent with the NNW-SSE direction of volcano feeder-dykes and ENE dipping low-angle normal faults crosscutting the mantle. The N060° extension accords with the mantle dome structure formation to the east. Low-angle faults intersect roots of volcanoes that thus belong to their hanging walls. Hydrothermalism is contemporaneous with or postdates low-angle faulting. A feeder-dyke major virgation northwards, synchronous with eruptions, suggests a dextral transform fault consistent with a N120° σ2. Oceanic expansion mechanisms are clarified. At the surface, magma eruption occurs along on-axis active high-angle normal faults, their footwalls enabling mantle exhumation. At depth, off-axis high-angle faults become low-angle faults as they spread at shallow level. With westward drift of the lithosphere, the uppermost levels of the ridge shift westward faster, such that volcanoes move to an off-axis position while their roots are cut by low-angle faults

Melting of plagioclase+spinel lherzolite at low pressures (0.5GPa): An experimental approach to the evolution of basaltic melt during mantle refertilisation at shallow depths

International audienceThe presence of plagioclase + spinel lherzolites among ocean floor samples and in some ophiolite complexes invites speculation on their origin and relationships to processes of magmatism and lithosphere refertilisation beneath mid-ocean ridges. In an experimental approach to their petrogenesis, we have determined the compositions of liquids and co-existing minerals in the six phase assemblage [liquid + olivine + orthopyroxene + clinopyroxene + plagioclase + spinel] at 0.5 GPa and 1100 °C to 1200 °C. In our experimental approach we maintained the olivine Mg# [Mg / (Mg + Fe)] close to 90 (i.e., 88.8-95.5) but varied plagioclase from anorthite to albite. The major variations in liquid compositions are related to plagioclase composition. Liquids have much lower MgO and FeO and higher SiO2 and Al2O3 than liquids in the 6-phase plagioclase + spinel lherzolite at 0.75 GPa and 1 GPa. Liquids are quartz-normative (silica-oversaturated) for plagioclase that are more calcic than An40 but nepheline-normative (critically silica-undersaturated) for plagioclase that are more sodic than An25. Liquid compositions are quite unlike natural MORB glasses with similar Mg# (i.e., compatible with parental magmas from lherzolitic mantle with Mg# ≈ 90). Our study provides no support for models of MORB petrogenesis which suggest extraction of near-solidus melts from plagioclase lherzolite at low pressure. Similarly, referring to numerical models of melting volumes beneath mid-ocean ridges (Langmuir et al., 1992; McKenzie and Bickle, 1988) in which melt increments are calculated for different sites and these increments pooled to form MORB, our data argue that melts equilibrated with plagioclase ± spinel lherzolite at < 1 GPa cannot be significant components of such ‘pooled melt’ focussed from within the melting volume. The compositions of minerals from plagioclase ± spinel lherzolite at Lanzo (northern Italy; Piccardo et al., 2007) are compared with our experimental assemblages at 0.5, 0.75 and 1 GPa, leading to the conclusion that the Lanzo plagioclase ± spinel lherzolites equilibrated at pressures between 0.75 and 1 GPa, at temperatures ~ 100-200 °C below the solidus. Field, petrological and geochemical studies argue that the Lanzo plagioclase ± spinel lherzolites are ‘refertilised’ by the reaction of residual harzburgite or lherzolite with percolating intergranular basaltic magma (Piccardo et al., 2007). The experimental study suggests that the process of refertilisation took place at depths of 25-30 km. Our experimental data also define the co-variance of Na2O in coexisting plagioclase (An25 to An94) and clinopyroxene at 0.5 and 0.75 GPa. From these data, the Na2O content of clinopyroxene can be used as a predictor for the co-existing plagioclase composition in the very common occurrences of partially serpentinised peridotite in which plagioclase is completely saussuritised