Alessio Sanfilippo

Detrital zircon provenance from the Neuquén Basin (south-central Andes): Cretaceous geodynamic evolution and sedimentary response in a retroarc-foreland basin

The surface response, in terms of drainage pattern changes, to the Cretaceous geodynamic reorganization of the Andean subduction zone between 36°S and 41°S is reconstructed through the geochronology-based provenance study of alluvial detrital zircons. The age spectra obtained by 500 spot U-Pb ages record an eastward provenance of detritus coming from the foreland during the Early Cretaceous backarc extensional stage, followed by westward-sourced clastics coming from the Cordillera during the Cenomanian. This drainage pattern reversal fits the regional unconformity in the sedimentary record that is linked to the geodynamic reorganization of the continental margin from an extensional to a compressional regime, forcing the Neuquen Basin to evolve from a retroarc to a foreland stage. After this inversion, the clastic systems progressively returned to be mainly fed by the foreland, due to the uplift of the peripheral bulge as a consequence of the Late Cretaceous thrust front migration. This tectonic evolution of the Neuquen Basin and the related response of the drainage pattern are thought to be the surface expression of the dip decrease of the Benioff subduction zone.

Development and evolution of detachment faulting along 50 km of the Mid‐Atlantic Ridge near 16.5°N

A multifaceted study of the slow spreading Mid-Atlantic Ridge (MAR) at 16.5°N provides new insights into detachment faulting and its evolution through time. The survey included regional multibeam bathymetry mapping, high-resolution mapping using AUV Sentry, seafloor imaging using the TowCam system, and an extensive rock-dredging program. At different times, detachment faulting was active along ∼50 km of the western flank of the study area, and may have dominated spreading on that flank for the last 5 Ma. Detachment morphologies vary and include a classic corrugated massif, noncorrugated massifs, and back-tilted ridges marking detachment breakaways. High-resolution Sentry data reveal a new detachment morphology; a low-angle, irregular surface in the regional bathymetry is shown to be a finely corrugated detachment surface (corrugation wavelength of only tens of meters and relief of just a few meters). Multiscale corrugations are observed 2–3 km from the detachment breakaway suggesting that they formed in the brittle layer, perhaps by anastomosing faults. The thin wedge of hanging wall lavas that covers a low-angle (6°) detachment footwall near its termination are intensely faulted and fissured; this deformation may be enhanced by the low angle of the emerging footwall. Active detachment faulting currently is limited to the western side of the rift valley. Nonetheless, detachment fault morphologies also are present over a large portion of the eastern flank on crust >2 Ma, indicating that within the last 5 Ma parts of the ridge axis have experienced periods of two-sided detachment faulting.

Rhenium-osmium isotope fractionation at the oceanic crust-mantle boundary

It has been proposed that the oceanic crust-mantle boundary may partly form through interactions between melts and the shallow mantle. Although this process may have a key role in defining the chemical signature of the oceanic crust, the effect of such melt-mantle interactions on the isotopic composition of the reacting melts has rarely been investigated. Here we report Re-Os isotopes and platinum group element (PGE) compositions of residual mantle harzburgites (olivine forsterite, Fo 91–90 ) and troctolites (olivine Fo 89–87 ) likely produced by melt-rock reactions at the Central Indian Ridge crust-mantle boundary. Although the harzburgites have high Os (4000–2500 pg/g) and unradiogenic isotopic signatures ( 187 Os/ 188 Os = 0.128–0.124), the troctolites show variable Os (383–16 pg/g) and highly fractionated 187 Os/ 188 Os ratios (0.129–0.140), which correlate with mineral (e.g., olivine Fo) and bulk-rock (e.g., CaO, Ni, and Cu) compositions. We relate this isotopic variability to the melt-mantle interaction process by which the troctolites generated. In particular, we suggest that the radiogenic Os signature was produced by selective assimilation of radiogenic interstitial sulfides from the reacted mantle. This mechanism is able to mimic the isotope variability of erupted melts, sustaining the idea that the crust-mantle boundary can act as a reactive filter for the melts extracted from the source region.

Compositional variations in spinel-hosted pargasite inclusions in the olivine-rich rock from the oceanic crust–mantle boundary zone

The crust–mantle boundary zone of the oceanic lithosphere is composed mainly of olivine-rich rocks represented by dunite and troctolite. However, we still do not fully understand the global variations in the boundary zone, and an effective classification of the boundary rocks, in terms of their petrographical features and origin, is an essential step in achieving such an understanding. In this paper, to highlight variations in olivine-rich rocks from the crust–mantle boundary, we describe the compositional variations in spinel-hosted hydrous silicate mineral inclusions in rock samples from the ocean floor near a mid-ocean ridge and trench. Pargasite is the dominant mineral among the inclusions, and all of them are exceptionally rich in incompatible elements. The host spinel grains are considered to be products of melt–peridotite reactions, because their origin cannot be ascribed to simple fractional crystallization of a melt. Trace-element compositions of pargasite inclusions are characteristically different between olivine-rich rock samples, in terms of the degree of Eu and Zr anomalies in the trace-element pattern. When considering the nature of the reaction that produced the inclusion-hosting spinel, the compositional differences between samples were found to reflect a diversity in the origin of the olivine-rich rocks, as for example in whether or not a reaction was accompanied by the fractional crystallization of plagioclase. The differences also reflect the fact that the melt flow system (porous or focused flow) controlled the melt/rock ratios during reaction. The pargasite inclusions provide useful data for constraining the history and origin of the olivine-rich rocks and therefore assist in our understanding of the crust–mantle boundary of the oceanic lithosphere.

The Ligurian Ophiolites: a journey through the building and evolution of slow spreading oceanic lithospher

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