Description: The Romanche transform offsets the Mid‐Atlantic Ridge (MAR) axis by about 950 km in the equatorial Atlantic. Multibeam and high‐resolution multichannel seismic reflection surveys as well as rock sampling were carried out on the eastern part of the transform with the R/V Akademik Strakhov as part of the Russian‐Italian Mid‐Atlantic Ridge Project (PRIMAR). Morphobathymetric data show the existence on the northern side of the transform of a major 800‐km‐long aseismic valley oriented 10° to 15° from the active valley; it disappears about 150 km from the western MAR segment. The aseismic valley marks probably the former location of the Romanche transform (“PaleoRomanche”) that was active up to roughly 8–10 Ma, when the transform boundary migrated to its present position. A temporary microplate developed during the migration and reorientation of the transform. This microplate changed its sense of motion as it was transferred from the South American to the African plate. A prominent transverse ridge extends for several hundred kilometers parallel to the transform on its northern side, reaching its shallowest part (shallower by over 4 km than the predicted thermal contraction depth) in a zone opposite the eastern MAR axis/transform intersection (RTI). Flat‐top peaks on the summit of the transverse ridge are capped by acoustically transparent, weakly stratified, shallow water platfonn/lagunal/reef limestones. This limestone unit is a few hundred meters thick and overlies igneous basement. Evaluation of the seismic reflection data as well as study of samples of carbonates, ventifact basaltic pebbles and gabbroic, peridotitic and basaltic rocks recovered at different sites on the transverse ridge, suggest that (1) the summit of the transverse ridge was above sea level at and before about 5 Ma; (2) the transverse ridge subsided since then at an average rate 1 order of magnitude faster than the predicted thermal contraction rate; its summit was flattened by erosion at sea level during subsidence; (3) the transverse ridge is an uplifted sliver of lithosphere and not a volcanic constructional feature; and (4) transtensional and transpressional tectonics have affected the transverse ridge. Hypotheses on the origin of the Romanche transverse ridge include (1) lateral heat conduction across the RTI; (2) shear heating; (3) lithospheric flexure due to thermal stresses in the cooling lithosphere; (4) viscoelastic deformation of the lithosphere; (5) hydration/dehydration of mantle peridotites; and (6) longitudinal flow of melt and igneous activity across the RTI. These processes cannot by themselves explain the transverse ridge, although some of them could contribute to its formation to a small extent. Vertical tectonics due to transpressional and transtensional events related to a nonstraight transform boundary and to regional changes in ridge/transform geometry is probably the primary process that gave rise to the uplift of the transverse ridge and to its recent subsidence. Uplift may have been caused primarily by thrust faulting induced by transpression related to the oblique impact of the lithospheric plate against the former (PaleoRomanche) and the younger transform boundaries, before and during the transition to the present boundary. After migration of the transform boundary to its present position, transpression was replaced by transtension and by subsidence of the transverse ridge. An aseismic axial rift valley impacting against the transform valley about 80 km west of the present RTI suggests eastward ridge jumping that probably followed transform migration. Localized transtension or transpression due to bends in the orientation of the transform may have caused intense although localized vertical movements, such as those that formed an ultradeep (〉7800 m) pull‐apart basin along the transform valley.

Description: Hydrothermal alteration processes involve mineralogical, chemical, and textural changes as a result of hot aqueous fluid-rock interaction under evolving boundary conditions. These changes affect the physico-chemical properties of the rocks, enabling high-resolution geophysical prospecting to be an important tool in the detection of seafloor hydrothermal alteration. Here we present the results of recent geophysical investigations of the Marsili and Palinuro volcanic complexes, southern Tyrrhenian Sea, during the 2010 TIR10 and 2011 MAVA2011 cruises by the R/V Urania . The new dataset includes a dense grid of multibeam bathymetry; seafloor reflectivity, magnetic and gravity lines; and high-resolution single (CHIRP) and multichannel seismic profiles. The surveys were focused on areas known to host intense hydrothermal alteration in order to provide a more detailed description of the Marsili and Palinuro hydrothermal systems. Ground-truthing was based on earlier discoveries of hydrothermal vents and their associated deposits, and on direct observations made by ROV dives. High-resolution morpho-bathymetry, sonar reflectivity, rock magnetization, and density distribution together enabled us to assess the extent of seafloor hydrothermal alteration and its relationship to local volcanic and tectonic structures. Hydrothermal alteration associated with the Marsili seamount is largely distributed along primary volcano-tectonic structures at the ridge crest. By contrast, at Palinuro hydrothermal alteration is mostly associated with secondary volcanic structures such as collapsed calderas and volcanism reactivation along ring faults. In particular, evidence for intense hydrothermal activity occurs at Palinuro where volcanotectonic features interact with regional tectonic structures.

Description: Near-bottom magnetic field data were collected using a towed magnetometer over selected parts of Palinuro and Marsili submarine volcanoes in the southern Tyrrhenian Sea, Italy. We obtained equivalent magnetizations maps at these sites by inverting the corresponding magnetic anomalies, highlighting the seafloor expression of hydrothermal alteration. Zones of reduced magnetization are interpreted as evidence for alteration related to hydrothermal processes; they are associated with water-column and seafloor observations of hydrothermal activity and altered host rocks. At Marsili volcano, a large elliptical area of reduced magnetization is located south of the summit cone and perpendicular to the trend of Marsili volcano itself. This confirms the presence of a large hydrothermal system centered over the more recent eastern volcanic ridge and its associated magma chamber. Similarly, areas of reduced magnetization over Palinuro caldera are consistent with hydrothermal venting occurring along the caldera walls, consistent with permeability structures related to caldera ring faults providing preferred pathways for the upflow of hydrothermal fluids.

Description: New multichannel seismic reflection profiles were acquired to unravel the structure of a portion of the eastern margin of the Tyrrhenian basin. This extensional feature is part of an Oligocene to Present back-arc basin in the hangingwall of the west directed Apennines subduction system. The basin provides excellent conditions to investigate the early stage processes leading to the development of rifted passive margins and to the emplacement of oceanic crust in an oblique setting. The interpreted post-stack-migrated seismic profiles highlight the geometry and kinematics of the Pontine escarpment that connects the Latium-Campania continental margin to the Vavilov basin. The latter is the main feature of the area, related to the early Pliocene extension of the Tyrrhenian Sea. Several morphological variations are pointed out along strike, mirroring different structural settings of the margin itself: a steeper margin to the north corresponds to high-angle possibly transtensional faults, whereas a smooth slope in the southern portion corresponds to several more distributed listric faults. This work contributes to the understanding of the interplay between extensional and transtensional tectonics along the margins of an oblique back-arc basin.

Description: Published

Description: 78–107

Description: 3A. Geofisica marina

Description: 2TR. Ricostruzione e modellazione della struttura crostale

Description: JCR Journal

Description: Two major deformation belts occur in the portion of the Adriatic Sea offshore the Gargano Promontory. The NE-SW - trending Tremiti Deformation Belt, located north of the Gargano Promontory, originated during the Plio- Quaternary, while the E-W-trending South Gargano Deformation Belt, located south of the Gargano Promontory, formed in a time span from Eocene to Early Pliocene. These deformation belts may have originated by tectonic inversion of Mesozoic extensional faults. This inversion tectonics, of Tertiary age, can be related to the evolution of the fold-and thrust belts surrounding the Adriatic Sea. The whole of the study area is, at present, seismically active and represents a preferential site of deformation.

Description: Published

Description: 573-578

Description: open

Description: Multichannel reflection seismic data were acquired south of the Salento peninsula, in an area where crustal seismicity has been recorded. Seismic profiles show the presence of small grabens bounded by extensional faults with NW-SE direction. These grabens are filled with Plio-Quaternary sediments and represent the prolongation of the grabens located onshore in the Salento peninsula. Outer arc extension due to flexuring of the Adriatic-Apulian lithosphere under the double load of the Hellenides and Apennines-Calabrian arc is thought to have originated these grabens. The Adriatic-Apulian continental lithosphere presents a very small radius of curvature and a decoupling between upper crust and mantle lithosphere is expected. Inner arc compression within the upper crust may be responsible for the seismicity recorded in the area.

Description: JCR Journal

Description: open