Unveiling the contourite depositional system in the Vema Fracture Zone (Central Atlantic).

A combination of a high sediment input and intense bottom currents often leads to the formation of contourites (sediments deposited or significantly reworked by bottom currents). Both of these components are present in the Vema Fracture Zone valley which is the most important passageway for the distribution of the Antarctic Bottom Water from the West to the North-East of the Atlantic. However, no contourite drifts, moats or contourite channels have been found in this region in more than half a century of research. The prevailing sedimentation paradigm postulates that turbidity currents have predominantly governed sedimentation in this region during the Pleistocene. This work describes the first example of contourite depositional system identified in the Vema Fracture Zone. The discovery was made through detailed high-resolution sub-bottom profiling, as well as numerical modeling and direct measurements of bottom current velocities. Such systems are exceptionally uncommon in fracture zones. This study highlights the importance of further research of contourites along the Vema Fracture Zone based on modern concepts of contourite and mixed depositional systems. The work also emphasizes the need to reevaluate the impact of bottom currents on sedimentation in this region, and particularly in the narrow segments of the fracture zone valley.

Neotectonics of the Sea of Galilee (northeast Israel): implication for geodynamics and seismicity along the Dead Sea Fault system.

The Sea of Galilee in northeast Israel is a freshwater lake filling a morphological depression along the Dead Sea Fault. It is located in a tectonically complex area, where a N-S main fault system intersects secondary fault patterns non-univocally interpreted by previous reconstructions. A set of multiscale geophysical, geochemical and seismological data, reprocessed or newly collected, was analysed to unravel the interplay between shallow tectonic deformations and geodynamic processes. The result is a neotectonic map highlighting major seismogenic faults in a key region at the boundary between the Africa/Sinai and Arabian plates. Most active seismogenic displacement occurs along NNW-SSE oriented transtensional faults. This results in a left-lateral bifurcation of the Dead Sea Fault forming a rhomb-shaped depression we named the Capharnaum Trough, located off-track relative to the alleged principal deformation zone. Low-magnitude (ML = 3-4) epicentres accurately located during a recent seismic sequence are aligned along this feature, whose activity, depth and regional importance is supported by geophysical and geochemical evidence. This case study, involving a multiscale/multidisciplinary approach, may serve as a reference for similar geodynamic settings in the world, where unravelling geometric and kinematic complexities is challenging but fundamental for reliable earthquake hazard assessments.

High H2O Content in Pyroxenes of Residual Mantle Peridotites at a Mid Atlantic Ridge Segment.

Global correlations of mid-ocean-ridges basalt chemistry, axial depth and crustal thickness have been ascribed to mantle temperature variations affecting degree of melting. However, mantle H2O content and elemental composition may also play a role. How H2O is distributed in the oceanic upper mantle remains poorly constrained. We tackled this problem by determining the H2O content of orthopyroxenes (opx) and clinopyroxenes (cpx) of peridotites from a continuous lithospheric section created during 26 Ma at a 11°N Mid-Atlantic Ridge segment, and exposed along the Vema Transform. The H2O content of opx ranges from 119 ppm to 383 ppm; that of cpx from 407 ppm to 1072 ppm. We found anomalous H2O-enriched peridotites with their H2O content not correlating inversely with their degree of melting, although H2O is assumed to be incompatible during melting. Inverse correlation of H2O with Ce, another highly incompatible component, suggests post-melting H2O enrichment. We attribute a major role to post-melting temperature-dependent diffusion of hydrogen occurring above the melting region, where water-rich melt flows faster than residual peridotites through dunitic conduits cross-cutting the uprising mantle. Accordingly, estimates of the H2O content of the MORB mantle source based on H2O in abyssal peridotites can be affected by strong uncertainties.

Fault-controlled deep hydrothermal flow in a back-arc tectonic setting, SE Tyrrhenian Sea.

Understanding magmatic systems and deep hydrothermal circulation beneath arc-volcanoes provides insights into deep processes associated with slab-subduction and mantle-wedge partial melting. Here we analyze hydrothermal flow below a structural high (Capo Vaticano Ridge, CVR) located offshore Capo Vaticano (western Calabria) and affected by magmatic intrusions generated from above the Ionian subducting-slab. In order to explain observations, we combine geophysical and numerical modelling results. Fluid-flow modelling shows that temperature distribution and geothermal gradient are controlled mainly by hydrothermal circulation, in turn affected by heat source, fault pattern, rock permeability, basement topography and sediment thickness. Two main faults, shaping the structural high and fracturing intensely the continental crust, enable deep hydrothermal circulation and shallow fluid discharge. Distribution of seismicity at depth supports the hypothesis of a slab below Capo Vaticano, deep enough to enable mantle-wedge partial melting above the subduction zone. Melt migration at shallow levels forms the magmatic intrusions inferred by magnetic anomalies and by δ3He enrichment in the discharged fluids at the CVR summit. Our results add new insights on the southern Tyrrhenian Sea arc-related magmatism and on the Calabrian inner-arc tectonic setting dissected by seismogenic faults able to trigger high-destructive earthquakes.

An updated reconstruction of basaltic crust emplacement in Tyrrhenian sea, Italy.

Basaltic crust is present in the oceans and marginal seas. Oceanic accretion from inception to ending may be usefully recognized in small basin setting like the Tyrrhenian. Alternating episodes of strong and moderate extensional tectonics characterized the small Tyrrhenian opening. Hyperextension (drifting) of late-Miocene and latemost Pliocene age was followed by Pliocene and Late Quaternary moderate extension (rifting). Early hyperextension (~7.5-6.3 Ma) acted in the submerged margin of Hercynian Sardinia. Sardinia offshore, E-directed low-angle faults were accompanied by MORB-like volcanism of non linear shape in the shallow Vavilov plain - inherited segment of alpine-age orogen. Late hyperextension (~1.9-1.7 Ma) acted along the central N-S lineament of Vavilov plain, former metamorphic core complex. At the lineament northern side, E-dipping detachment faulting exposed serpentinized peridotite. At the other side, Vavilov volcano was faulted and its east flank tilted westwards. At the same time, volcanism with affinity to transitional MORB induced opening of Marsili basin. The drift episodes were characterized by absence or scarcity of volcanic activity on the conjugated emerged margins. The rift episodes (respectively ~5-1.9 Ma, and ~1/0.8 Ma-Recent) saw growth of major north-south trending volcanoes in bathyal area as intense volcanic activity developed on the continental margins.

Post-mesozoic rapid increase of seawater Mg/Ca due to enhanced mantle-seawater interaction.

The seawater Mg/Ca ratio increased significantly from ~ 80 Ma to present, as suggested by studies of carbonate veins in oceanic basalts and of fluid inclusions in halite. We show here that reactions of mantle-derived peridotites with seawater along slow spreading mid-ocean ridges contributed to the post-Cretaceous Mg/Ca increase. These reactions can release to modern seawater up to 20% of the yearly Mg river input. However, no significant peridotite-seawater interaction and Mg-release to the ocean occur in fast spreading, East Pacific Rise-type ridges. The Mesozoic Pangean superocean implies a hot fast spreading ridge system. This prevented peridotite-seawater interaction and Mg release to the Mesozoic ocean, but favored hydrothermal Mg capture and Ca release by the basaltic crust, resulting in a low seawater Mg/Ca ratio. Continent dispersal and development of slow spreading ridges allowed Mg release to the ocean by peridotite-seawater reactions, contributing to the increase of the Mg/Ca ratio of post-Mesozoic seawater.

The tectonic puzzle of the Messina area (Southern Italy): insights from new seismic reflection data.

The Messina Strait, that separates peninsular Italy from Sicily, is one of the most seismically active areas of the Mediterranean. The structure and seismotectonic setting of the region are poorly understood, although the area is highly populated and important infrastructures are planned there. New seismic reflection data have identified a number of faults, as well as a crustal scale NE-trending anticline few km north of the strait. These features are interpreted as due to active right-lateral transpression along the north-eastern Sicilian offshore, coexisting with extensional and right-lateral transtensional tectonics in the southern Messina Strait. This complex tectonic network appears to be controlled by independent and overlapping tectonic settings, due to the presence of a diffuse transfer zone between the SE-ward retreating Calabria subduction zone relative to slab advance in the western Sicilian side.

Water-rich basalts at mid-ocean-ridge cold spots.

Although water is only present in trace amounts in the suboceanic upper mantle, it is thought to play a significant role in affecting mantle viscosity, melting and the generation of crust at mid-ocean ridges. The concentration of water in oceanic basalts has been observed to stay below 0.2 wt%, except for water-rich basalts sampled near hotspots and generated by 'wet' mantle plumes. Here, however, we report unusually high water content in basaltic glasses from a cold region of the mid-ocean-ridge system in the equatorial Atlantic Ocean. These basalts are sodium-rich, having been generated by low degrees of melting of the mantle, and contain unusually high ratios of light versus heavy rare-earth elements, implying the presence of garnet in the melting region. We infer that water-rich basalts from such regions of thermal minima derive from low degrees of 'wet' melting greater than 60 km deep in the mantle, with minor dilution by melts produced by shallower 'dry' melting--a view supported by numerical modelling. We therefore conclude that oceanic basalts are water-rich not only near hotspots, but also at 'cold spots'.

Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variations in the formation of oceanic lithosphere.

A 20-Myr record of creation of oceanic lithosphere is exposed along a segment of the central Mid-Atlantic Ridge on an uplifted sliver of lithosphere. The degree of melting of the mantle that is upwelling below the ridge, estimated from the chemistry of the exposed mantle rocks, as well as crustal thickness inferred from gravity measurements, show oscillations of approximately 3-4 Myr superimposed on a longer-term steady increase with time. The time lag between oscillations of mantle melting and crustal thickness indicates that the mantle is upwelling at an average rate of approximately 25 mm x yr(-1), but this appears to vary through time. Slow-spreading lithosphere seems to form through dynamic pulses of mantle upwelling and melting, leading not only to along-axis segmentation but also to across-axis structural variability. Also, the central Mid-Atlantic Ridge appears to have become steadily hotter over the past 20 Myr, possibly owing to north-south mantle flow.