Petrological and geochemical study of spilites and associated dike rocks from the Virgin Island core (Caribbean Island arc)

A continuous section of extrusive and intrusive rocks have been recovered through a drill core of about 2h30 feet that was taken at St. John, U. S. Virgin Islands. The present study is confined to the upper 1180 feet and comprises a cummulative total of 950 feet of albitized mafic extrusive rocks (spilite). This portion of the core is also intruded by four dikes: two amphibole porphyryes (129-13h and 732-7&2 feet): an albite-diabase (177-226 feet) and an andesine-diabase (8h7-1003 feet). The spilite is a fine—grained rock with pilotaxitic and intersertal textural features. Throughout the flow, the major mineralogical assemblages consist of partly albitized Ca-plagioclase, chlorite, epidote and quartz. The upper part of this flow shows a concentration of dark colored and fine-grained patches set in a green, fine-grained, and abundantly recrystallized matrix. This green matrix is characterized by glassy textural features and lower temperature assemblages (epidote, quartz, chlorite) than the dark patchy material. The patchiness decreases with depth and below 650 feet disappears, being replaced by a homogeneous, fine-grained spilite. ’Below the non-patchy spilite, there is a sheared and recrystallized spilitic zone which contains epidote, quartz, chlorite and breccia fragments. After extrusion of the spilite flows onto the surface, the upper part of the core solidified faster than the underlying non-patchy zone. The dark patches represent centers of crystallization which solidified before the surrounding green matrix. Once the lava flow solidified, it was intruded by several dikes and a later faulting gave rise to the bottom brecciated zone. Alkali migrations occur throughout the rocks studio. The K20 content in the upper part of the spilite flow is higher and the Na2O content lower than the bottom of the flow. Local variations of alkalis show also that the dark patches are higher in Na20 than the surrounding green matrices. The alkali migration throughout the different lithological units of the core could have been caused by both magmatic and metamorphic processes. During the last stage of crystallization, hydrothermal solutions might have concentrated some of the K20 in the green matrix and the Na2O in the adjacent dark patches. Burial metamorphism also affected the core and caused. the disappearance of prehnite and pumpellyite with depth. The parental magma of the spilite is believed to be derived from the partial melting of amphibolite tapped in the lower Crust or upper Mantle (extrapolated from experimental data) beneath the Virgin Islands

Updated Interpretation of Magnetic Anomalies and Seafloor Spreading Stages in the South China Sea : Implications for the Tertiary Tectonics of Southeast Asia

International audienceWe present the interpretation of a new set of closely spaced marine magnetic profiles that complements previous data in the northeastern and southwestern parts of the South China Sea (Nan Hai). This interpretation shows that seafloor spreading was asymmetric and confirms that it included at least one ridge jump. Discontinuities in the seafloor fabric, characterized by large differences in basement depth and roughness, appear to be related to variations in spreading rate. Between anomalies 11 and 7 (32 to 27 Ma), spreading at an intermediate, average full rate of ~50 mm/yr created relatively smooth basement, now thickly blanketed by sediments. The ridge then jumped to the south and created rough basement, now much shallower and covered with thinner sediments than in the north. This episode lasted from anomaly 6b to anomaly 5c (27 to ~16 Ma) and the average spreading rate was slower, ~35 mm/yr. After 27 Ma, spreading appears to have developed first in the eastern part of the basin and to have propagated towards the southwest in two major steps, at the time of anomalies 6b-7, and at the time of anomaly 6. Each step correlates with a variation of the ridge orientation, from nearly E-W to NE-SW, and with a variation in the spreading rate. Spreading appears to have stopped synchronously along the ridge, at about 15.5 Ma. From computed fits of magnetic isochrons we calculate 10 poles of finite rotation between the times of magnetic anomalies 11 and 5c. The poles permit reconstruction of the Oligo-Miocene movements of Southeast Asian blocks north and south of the South China Sea. Using such reconstructions, we test quantitatively a simple scenario for the opening of the sea in which seafloor spreading results from the extrusion of Indochina relative to South China, in response to the penetration of India into Asia. This alone yields between 500 and 600 km of left-lateral motion on the Red River-Ailao Shan shear zone, with crustal shortening in the San Jiang region and crustal extension in Tonkin. The offset derived from the fit of magnetic isochrons on the South China Sea floor is compatible with the offset of geological markers north and south of the Red River Zone. The first phases of extension of the continental margins of the basin are probably related to motion on the Wang Chao and Three Pagodas Faults, in addition to the Red River Fault. That Indochina rotated at least 12° relative to South China implies that large-scale "domino" models are inadequate to describe the Cenozoic tectonics of Southeast Asia. The cessation of spreading after 16 Ma appears to be roughly synchronous with the final increments of left-lateral shear and normal uplift in the Ailao Shan (18 Ma), as well as with incipient collisions between the Australian and the Eurasian plates. Hence no other causes than the activation of new fault zones within the India-Asia collision zone, north and east of the Red River Fault, and perhaps increased resistance to extrusion a long the SE edge of Sundaland, appear to be required to terminate seafloor spreading in the largest marginal basin of the western Pacific and to change the sense of motion on the largest strike-slip fault of SE Asia

Primitive layered gabbros from fast-spreading lower oceanic crust

Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks-in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas-provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt

MAHOU-HÉKINIAN Valentin : Quelle gestion des risques naturels en Arménie ? Prévention des risques, gestion de crise, résilience : Focus sur l'aléa sismique

Valentin MAHOU-HÉKINIAN (2015). Master 2 de Géographie DYNARISK (Université Paris Panthéon-Sorbonne) Télécharger Quelle gestion des risques naturels en Arménie ? Prévention des risques, gestion de crise, résilience : Focus sur l'aléa sismiqu

Spreading-rate dependence of the extent of mantle melting beneath ocean ridges

Abyssal peridotites and mid-ocean-ridge basalts (MORBs) are complementary products of the mantle melting and melt-extraction processes that create the ocean crust. Studies of abyssal peridotites and MORBs have showed that the extent of mantle melting is high beneath hotspot-influenced shallow ridges, and is low beneath deep ridges away from hotspots. These results have led to the recognition of a global correlation of MORB composition with ridge depth, and to the notion that mantle temperature variation exerts the primary control on the extent of melting beneath ocean ridges. This conclusion is, however, based largely on data from slow- spreading ridges in the Atlantic and Indian oceans. At the fast-spreading East Pacific Rise (EPR), there is little correlation between MORB chemistry and ridge depth, an observation that has proved puzzling. Here we show that abyssal peridotites from the EPR are extremely depleted in basaltic major- element components-significantly more so than peridotites from ridges away from hotspots in the i Atlantic and Indian oceans-indicating that the EPR peridotites are residues of the highest extents of melting. These abyssal peridotite data and existing MORB major-element data both suggest that the extent of mantle melting beneath normal ocean ridges increases with increasing spreading rate

Ridge suction drives plume-ridge interactions

Deep-sourced mantle plumes, if existing, are genetically independent of plate tectonics. When the ascending plumes approach lithospheric plates, interactions between the two occur. Such interactions are most prominent near ocean ridges where the lithosphere is thin and the effect of plumes is best revealed. While ocean ridges are mostly passive features in terms of plate tectonics, they play an active role in the context of plume-ridge interactions. This active role is a ridge suction force that drives asthenospheric mantle flow towards ridges because of material needs to form the ocean crust at ridges and lithospheric mantle in the vicinity of ridges. This ridge suction force increases with increasing plate separation rate because of increased material demand per unit time. As the seismic low-velocity zone atop the asthenosphere has the lowest viscosity that increases rapidly with depth, the ridge-ward asthenospheric flow is largely horizontal beneath the lithosphere. Recognizing that plume materials have two components with easily-melted dikes/veins enriched in volatiles and incompatible elements dispersed in the more refractory and depleted peridotitic matrix, geochemistry of some seafloor volcanics well illustrates that plume-ridge interactions are consequences of ridge-suction-driven flow of plume materials, which melt by decompression because of lithospheric thinning towards ridges. There are excellent examples: (1) The decreasing La/Sm and increasing MgO and CaO/Al2O3 in Easter Seamount lavas from Salas-y-Gomez Islands to the Easter Microplate East rift zone result from progressive decompression melting of ridge-ward flowing plume materials. (2) The similar geochemical observations in lavas along the Foundation hotline towards the Pacific-Antarctic Ridge result from the same process. (3) The increasing ridge suction force with increasing spreading rate explains why the Iceland plume has asymmetric effects on its neighboring ridges: both topographic and geochemical anomalies extend 1500 km along the faster (20 to 25 mm/yr southward) spreading Reykjanes Ridge. (4) The spreading-rate dependent ridge suction force also explains the first-order differences between the fast-spreading East Pacific Rise (EPR) and the slow-spreading Mid-Atlantic Ridge (MAR). Identified mantle plumes/hotspots are abundant near the MAR (e.g., Iceland, Azores, Ascension, Tristan, Gough, Shona and Bouvet), but rare along the entire EPR (notably, the Easter hotspot at ~ 27°S on the Nazca plate). Such apparent unequal hotspot distribution would allow a prediction of more enriched MORB at the MAR than at the EPR. However, the mean compositions between MAR-MORB and EPR-MORB are the same in terms of incompatible element abundances, and are identical in terms of Sr-Nd-Pb isotopic ratios. This suggests similar extents of mantle plume contributions to EPR and MAR MORB. We consider that the apparent rarity of near-EPR plumes/hotspots results from fast spreading. The fast spreading creates large ridge suction forces that do not allow the development of surface expressions of mantle plumes as such, but draw plume materials to a broad zone of sub-ridge upwelling, giving rise to random distribution of abundant enriched MORB and elevated and smooth axial topography along the EPR (vs. MAR). One of the important implications is that the asthenospheric flow is necessarily decoupled from its overlaying oceanic lithospheric plate. This decoupling increases with increasing spreading rate

Basaltic liquids and harzburgitic residues in the Garrett Transform: A case study at fast-spreading ridges

The peridotite-basalt association in the Garrett Transform, ∼13°28′S, East Pacific Rise (EPR), provides a prime opportunity for examining mantle melting and melt extraction processes from both melts and residues produced in a common environment beneath fast-spreading ridges. The peridotites are highly depleted, clinopyroxene-poor, harzburgites. Residual spinel, orthopyroxene and clinopyroxene in these harzburgites are extremely depleted in AlO, and plot at the most depleted end of the abyssal peridotite array defined by samples from slow-spreading ridges (including samples from hotspot-influenced ridges), suggesting that these harzburgites are residues of very high extents of melting. The residual peridotites from elsewhere at the EPR (i.e., Hess Deep and the Terevaka Transform) also are similarly depleted. This suggests that the extent of melting beneath the EPR is similar to, or even higher than, beneath ridges influenced by hotspots (e.g., Azores hotspot in the Atlantic Ocean and Bouvet hotspot in the Indian Ocean), and is significantly higher than ≤10%, a value that has been advocated to be the average extent of melting beneath global ocean ridges. Many of these harzburgite samples, however, show whole-rock incompatible element abundances higher than expected. These same samples also have various amounts of excess olivine with forsterite contents as low as Fo. The total olivine modes correlate inversely with olivine forsterite contents, and positively with whole-rock incompatible element abundances. These correlations suggest that the excess olivine and incompatible element enrichment are both the result of melt-solid re-equilibration. The buoyant melts that ascend through previously depleted residues crystallize olivine at shallow levels as a result of cooling. Entrapment of these melts leads to whole-rock incompatible element enrichment. These observations contrast with the notion that melts formed at depth experience little low pressure equilibration during ascent