Ligi, Marco

The Pacific, Antarctic, and Macquarie lithospheric plates diverge from the Macquarie Triple Junction (MTJ) in the southwestern Pacific Ocean, south of Macquarie Island. Morphobathymetric, magnetic, and gravity data have been used to understand the evolution of the three accretionary/transform boundaries that meet at the MTJ. Plate velocities, estimated near the MTJ and averaged over the past 3 m.y., indicate an unstable ridge–fault–fault triple junction. The long life (>6 m.y.) of this configuration can be attributed to a rapid increase in spreading asymmetry along the Southeast Indian Ridge segment as it approaches the MTJ, and to transtension along the southernmost strand of the Macquarie–Pacific transform boundary. A major change in plate motion triggered the development of the Macquarie plate at ca. 6 Ma and makes clear the recent evolution of the MTJ, including (1) shortening of the Southeast Indian Ridge segment; (2) formation of the westernmost Pacific-Antarctic Ridge, which increased its length over time; and (3) lengthening of the two transform boundaries converging in the MTJ. The clockwise change of the Pacific-Antarctic motion (ca. 12–10 Ma) led to complex geodynamic evolution of the plate boundary to the east of the triple junction, with fragmentation of the long-offset Emerald transform fault and its replacement over a short time interval (1–2 m.y.) with closely spaced, highly variable transform offsets that were joined by short ridge segments with time-varying asymmetries in the spreading rates.

This study investigates crustal accretion processes along the northern stretch of the Mid-Atlantic Ridge (MAR) between the Charlie Gibbs (52°-53°N) and Bight (57°N) transforms. These long-lived transform systems, active for more than 40 Ma, bound a ~ 550 km-long MAR segment influenced to the South by the Azores and to the North by the Iceland mantle plumes. The Bight transform is located at the tip of the Reykjanes Ridge, where the spreading direction, influenced by the southward propagation of the Iceland plume, changes from oblique (30° to the axis) to perpendicular to the axis. Four hundred kilometres to the south, the MAR is offset by the Charlie Gibbs transform system consisting of two long-lived right-lateral transform faults linked by a short ~ 40 km-long spreading segment. Previous expeditions surveyed large areas of these two transform systems, defining their main morphological features. Based on these bathymetric data, Expedition V53 of the R/V A.S. Vavilov carried out an intense dredging program coupled with magnetic surveys in an area spanning from 57° to 52°N, covering both the Bight and the Charlie Gibbs transform systems. We collected 1850 kg of rock samples including limestones, basalts, gabbros and mantle peridotites from 27 dredging sites, along with two 6-m long sedimentary cores. The sampled lithologies are globally in agreement with the contrasting morphological features of the two transform faults. We discuss here and compare the geology of these two major transform systems and assess the influence of the Icelandic plume on seafloor morphology at the Bight Fracture Zone.

Based on morphobathymetric and seismic reflection data, we studied a large landslide body from the eastern Sea of Marmara (NW Turkey), along the main strand of the North Anatolian Fault, one of the most seismically active geological structures on Earth. Due to its location and dimensions, the sliding body may cause tsunamis in case of failure possibly induced by an earthquake. This could affect heavily the coasts of the Sea of Marmara and the densely populated Istanbul Metropolitan area, with its exposed cultural heritage assets. After a geological and geometrical description of the landslide, thanks to high-resolution marine geophysical data, we simulated numerically possible effects of its massive mobilization along a basal displacement surface. Results, within significant uncertainties linked to dimensions and kinematics of the sliding mass, suggest generation of tsunamis exceeding 15–20 m along a broad coastal sector of the eastern Sea of Marmara. Although creeping processes or partial collapse of the landslide body could lower the associated tsunami risk, its detection stresses the need for collecting more marine geological/geophysical data in the region to better constrain hazards and feasibility of specific emergency plans.

Half a century ago, our view of the Earth shifted from that of a Planet with fixed continents and ancient stable ocean basins to one with wandering continents and young, active ocean basins, reviving Wegener’s Continental Drift that had rested dormant for years. The lithosphere is the external, mostly solid and relatively rigid layer of the Earth, with thickness and composition different below the oceans and within the continents. We will review the processes leading to the generation and evolution of the Earth’s lithosphere that lies beneath the oceans. We will discuss how the oceanic lithosphere is generated along mid-ocean ridges due to upwelling of convecting hot mantle. We will consider in particular lithosphere generation occurring along the northern Mid Atlantic Ridge (MAR) from Iceland to the equator, including the formation of transform offsets. We will then focus on the Vema fracture zone at 10°–11° N, where a ~ 300 km long uplifted and exposed sliver of lithosphere allows to reconstruct the evolution of lithosphere generation at a segment of the MAR from 25 million years ago to the Present. This axial ridge segment formed 50 million years ago, and reaches today 80 km in length. The degree of melting of the subridge mantle increased from 16 million years ago to today, although with some oscillations. The mantle presently upwelling beneath the MAR becomes colder and/or less fertile going from Iceland to the Equator, with “waves” of hot/fertile mantle migrating southwards from the Azores plume. Scientific revolutions seem to occur periodically in the history of Science; we wonder when the next revolution will take place in the Earth Science, and to what extent our present views will have to be modified

The Doldrums Megatransform System (~7–8°N, Mid-Atlantic Ridge) shows a complex architecture including four intra-transform ridge segments bounded by five active transform faults. Lower crustal rocks are exposed along the Doldrums and Vernadsky transform walls that bound the northernmost intra-transform ridge segment. The recovered gabbros are characterized by variably evolved chemical compositions, ranging from olivine gabbros to gabbronorites and oxide gabbros, and lack the most primitive gabbroic endmembers (troctolites, dunites). Notably, the numerous recovered gabbronorites show up to 20 vol. % of coarse-grained orthopyroxene. Although covariations in mineral and bulk-rock chemical compositions of the olivine and oxide gabbros define trends of crystallization from a common parental melt, the gabbronorites show elevated light over heavy rare earth elements (LREE/HREE) ratios in both bulk-rock and mineral compositions. These features are not consistent with a petrological evolution driven solely by fractional crystallization, which cannot produce the preferential enrichments in highly incompatible elements documented in the orthopyroxene-bearing lithologies. We suggest that gabbronorites crystallized from evolved melts percolating and partly assimilating a pre-existing olivine gabbro matrix. Saturation in orthopyroxene and selective enrichments in LREE relative to M-HREE are both triggered by an increase in assimilated crystal mass, which ranges from negligible in the oxide-gabbros to abundant in the gabbronorites. This melt–rock reaction process has been related to lateral melt migration beneath ridge-transform intersections, where variably evolved melts injected from the peripheral parts of the melting region towards the transform zone may interact with a gabbroic crystal mush to form abundant oxide-bearing gabbronoritic associations.

The Charlie Gibbs offsetting by ~ 340 km the Mid Atlantic Ridge (MAR) axis at 52°-53° N is one of the main transform systems of the North Atlantic. Located between long mid-ocean ridge segments influenced from the south by the Azores and from the north by the Iceland mantle plumes, this transform system has been active since the early phases of North Atlantic rifting. Object of several surveys in the ’70 and ’80, Charlie Gibbs received great attention for its unique structure characterized by two long-lived right-lateral transform faults linked by a short ~ 40 km-long intra-transform spreading centre (ITR) with parallel fracture zone valleys extending continuously towards the continental margins. In October 2020 expedition S50 of the R/V A.N. Strakhov surveyed an area of 54,552 km2 covering the entire Charlie Gibbs transform system and the adjacent MAR spreading segments. We collected new bathymetric, magnetic and high-resolution single channel seismic data, along with basaltic, gabbroic and mantle rocks from 21 dredges. This work contains preliminary data from cruise S50 and discusses the large-scale architecture of this unique, long-lived transform 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.

The Palinuro volcanic chain (PVC) is located about 80 km offshore the Campania region (Italy) in the southern sector of the Tyrrhenian Sea. The chain consists of 15 volcanic edifices aligned in an E-W direction with two distinct major seamounts (Palinuro and Glabro). They cover a 90 km long and 20 kmwide area, with a present-day volume of 2700 km3. Palinuro volcanism emplaced between 0.8 and 0.3 Ma, although shallow seismicity and hydrothermalism indicate an ongoing volcanic activity. A geomorphological analysis of the volcanic chain and data from a multichannel seismic profile reveal large volumes of buried chaotic material suggesting gravity mass sliding from the volcano flanks and slide scars. A stability analysis of the Palinuro flanks has been carried out to determine the sectors potentially prone to sliding in case of shallow volcanic earthquakes. Landslides are simulated by adopting a scenario-based approach. Tsunamis induced by these mass movements and their propagation across the Tyrrhenian Sea are modeled. Results suggest that shallow earthquakes (M ~4.6–4.8) are able to destabilize the flanks of the volcanic chain generating slope failures. Sliding volumes in the order of 1.5 km3 and 2.4 km3 may induce waves as high as 1.5 and 6 m, respectively, along the peri-Tyrrhenian coast. Our results underline the need for further investigations on the stability of the submarine volcanoes of the Tyrrhenian basin. These volcanoes are still poorly known although their instability could trigger large tsunamis along the southern Italy coastal sectors. Our recommendation is that multiparamertic monitoring networks on PVC and periodic oceanic cruises should be put into action, and further that a systematic evaluation of the tsunami hazard related to possible sliding phenomena on the flanks of the Tyrrhenian seamounts should be performed

The Equatorial portion of the Mid Atlantic Ridge is displaced by a series of large offset oceanic transforms, also called “megatransforms”. These transform domains are characterized by a wide zone of deformation that may include different conjugated fault systems and intra-transform spreading centers (ITRs). Among these megatransforms, the Doldrums system (7-8ºN) is arguably the less studied, although it may be considered the most magmatically active. New geophysical data and rock samples were recently collected during the 45th expedition of the R/V Akademik Nikolaj Strakhov. Preliminary cruise results allow to reconstruct the large-scale structure and the tectonic evolution of this poorly-known feature of the Equatorial Atlantic. Swath bathymetry data, coupled with extensive dredging, were collected along the entire megatransform domain, covering an area of approximately 29,000 km2. The new data clearly indicate that the Doldrums is an extremely complex transform system that includes 4 active ITRs bounded by 5 fracture zones. Although the axial depth decreases toward the central part of the system, recent volcanism is significantly more abundant in the central ITRs when compared to that of the peripheral ITRs. Our preliminary interpretation is that a region of intense mantle melting is located in the central part of the Doldrums system as consequence of either a general transtensive regime or the occurrence of a more fertile mantle domain. Large regions of basement exposure characterize the transform valleys and the ridge-transform intersections. We speculate that different mechanisms may be responsible for the exposure of basement rocks. These include the uplift of slivers of oceanic lithosphere by tectonic tilting (median and transverse ridges formation), the denudation of deformed gabbro and peridotite by detachment faulting at inner corner highs, and the exposure of deep-seated rocks at the footwall of high-angle normal faults at the intersection of mid-ocean ridges with transform valleys.

The Doldrums transform system offsets the Equatorial Mid Atlantic Ridge by ~630 km at 7–8° N. This transform system consists of four intra-transform spreading centers (ITRs) bounded by five transform faults. The northernmost ITR is linked to the MAR axis by a ~ 180 km-long transform. Here, during two R/V A. N. Strakhov expeditions (S06 and S09), mantle peridotites were dredged along the transverse and median ridge of the transform, across the western flank of the ITR valley. Residual harzburgites were mainly sampled along the northern Doldrums transform valley, whereas plagioclase-bearing peridotites showing evidence for melt-rock interaction characterize the ITR domain. Petrological and geochemical observations reinforced by geochemical modelling are used to define the behaviour of trace elements during melt extraction and melt-rock reaction in our rocks. Results suggest that residual peridotites derive from mantle rocks that have undergone a degree of partial melting up to 12%, with melting likely starting at the transition of garnet-spinel stability fields, whereas peridotites which suffered melt-rock reactions have been divided into two types: (i) pl-impregnated peridotites, formed by migration of melts at high porosity and high melt-rock ratio; and (ii) refertilized peridotites, generated at reduced porosity, when small fractions of the same percolating melt crystallized clinopyroxene and minor plagioclase. We suggest that the refertilizing agent was a melt highly depleted in incompatible trace elements, in turn produced by an ultra-depleted mantle source. This mantle experienced previous degrees of melt extraction at the ridge axis, before being transposed laterally along the transform where it melted a second time during the opening of the intra-transform spreading segment.