Description: The formation of Cenozoic mountain belts in the Mediterranean realm was preceded by tens of millions of years of subduction, forming volcanic arcs, and frontal contractional systems. In addition, subduction usually involves slab rollback and formation of oceanic backarcs. Although such structure must have influenced the orogeny of Mediterranean mountain belts, no active analog has been mapped with modern crustal-scale seismic methods. Here, we study the entire Calabrian subduction system to map the structure resulting from Tethys lithosphere subduction and slab rollback, in a process that must be akin to that operating during a phase of the formation of the Mediterranean orogenic belts. We present a crustal-scale cross section of the entire Calabrian subduction system obtained from on- and off-shore wide-angle seismic data. The 2D P-wave velocity section shows spatially abrupt (〈5 km of profile distance) structural and petrological transitions from the Ionian sedimentary wedge and Calabrian arc, to the rifted NW Calabrian margin, where the Quaternary Aeolian arc is emplaced. The margin, then, transitions northwards into the Marsili backarc region, where exhumed mantle and localized volcanism occurred during its formation. This complex structure implies rapid temporal and spatial changes between magmatic and amagmatic processes, and between compressional and extensional regimes during the evolution of this subduction system. We find that some terranes involved in the Alpine orogeny share petrological and tectonic similarities with some domains of the Calabrian subduction system. Based on the results of this study we propose the Calabrian Arc system as an analog for the subduction structuration that preceded the formation of Alpine orogenic systems.

Description: Published

Description: 116480

Description: 1T. Struttura della Terra

Description: JCR Journal

Description: Highlights • We identify Volcano F as the source of the August 2019 pumice raft in Tonga. • Satellite and seismic data give constraints on the timing of the submarine eruption. • 2.5–12.3*106 m3 estimated eruption volume, corresponding to VEI 2–3. • First report of the morphology and geology of Volcano F. Abstract In August 2019 a large raft of pumice appeared in the territorial waters of Tonga. As in many other cases, this pumice raft was the only surface expression of a major submarine volcanic eruption. Discolored water and reconstruction of the drift path of the pumice raft using satellite imagery points towards ‘Volcano F’ in the Tofua Arc NW of the island of Vava’u as the most likely volcanic source. Here we present imagery from ESA’s Sentinel-2 satellite that captured the start of the submarine eruption on 6 August 2019 and the waning of the eruption on 8 August, followed by observations of the drifting pumice raft until 14 August. This start time is consistent with T-phase records at the seismic stations on Niue Island and Rarotonga and the signal delay time of 733 s between the two stations is consistent with an origin at or at least near Volcano F. On 8 August, a 〉136.7 km2 large raft of pumice appears at the sea surface. The modelled minimum raft volume is 8.2–41.0*106 m3, which is equivalent to 2.5–12.3*106 m3 dense rock. The eruption thus corresponds to a volcanic explosivity index (VEI) 2–3 eruption in the submarine environment. Prior to the volcanic eruption, a series of earthquakes close to Volcano F was recorded. The series started on 5 August with a Mb 4.7 event, followed by at least six shallow earthquakes (Mb 〉3.9) on 6 August. In December 2018 and January 2019, we surveyed the seafloor around Volcano F with multibeam sonar. Combining our data with pre-existing information, we present the first comprehensive bathymetric map of the volcanic edifice and its geologic setting. We show that Volcano F represents a major arc volcanic complex that is situated in an extensional setting. The basal diameter of the volcanic apron is 〉50 km with a large central, 8.7 x 6 km caldera with a floor at ∼700 m water depth. The top of the post-caldera constructional cone complex had a summit depth of 35 m below sea level in 2004. The volcano shows geochemical differences to the adjacent arc volcanoes on Fonualei and Late islands. The volcano’s pristine volcanic morphology and two documented eruptions (2001 and 2019) indicate a highly active volcanic system that warrants further scientific attention.

Description: Highlights • We present the first modern amphibious seismic experiment conducted across Calabria. • The section shows the forearc-to-backarc Vp structure of the subduction system. • We infer mantle exhumation in the Marsili backarc basin, in the Tyrrhenian. • The system is marked by spatially rapid petrological and tectonic changes. • An analog of Tethys subduction systems formed by slab rollback is proposed. Abstract The formation of Cenozoic mountain belts in the Mediterranean realm was preceded by tens of millions of years of subduction, forming volcanic arcs, and frontal contractional systems. In addition, subduction usually involves slab rollback and formation of oceanic backarcs. Although such structure must have influenced the orogeny of Mediterranean mountain belts, no active analog has been mapped with modern crustal-scale seismic methods. Here, we study the entire Calabrian subduction system to map the structure resulting from Tethys lithosphere subduction and slab rollback, in a process that must be akin to that operating during a phase of the formation of the Mediterranean orogenic belts. We present a crustal-scale cross section of the entire Calabrian subduction system obtained from on- and off-shore wide-angle seismic data. The 2D P-wave velocity section shows spatially abrupt (〈5 km of profile distance) structural and petrological transitions from the Ionian sedimentary wedge and Calabrian arc, to the rifted NW Calabrian margin, where the Quaternary Aeolian arc is emplaced. The margin, then, transitions northwards into the Marsili backarc region, where exhumed mantle and localized volcanism occurred during its formation. This complex structure implies rapid temporal and spatial changes between magmatic and amagmatic processes, and between compressional and extensional regimes during the evolution of this subduction system. We find that some terranes involved in the Alpine orogeny share petrological and tectonic similarities with some domains of the Calabrian subduction system. Based on the results of this study we propose the Calabrian Arc system as an analog for the subduction structuration that preceded the formation of Alpine orogenic systems.

Description: Highlights • The crustal thickness of the Zhongsha Block ranging from ~6 to ~25 km. • Rapid transition from the Zhongsha Block to the adjacent oceanic basins was revealed. • Different oceanic structures were observed in the adjacent oceanic basins. • The pre-rift lithospheric configuration may affect the process of rifting and seafloor spreading. Abstract Continental rifting, break-up, and onset of seafloor spreading are inherently controlled by the segmentation and structure of the continental domain suffering from extension. Today, the Zhongsha Atoll (ZS) is wedged between the Northwest Sub-basin (NWSB) and the Southwest Sub-basin (SWSB), two oceanic abyssal basins of the South China Sea (SCS). The nature of the crust and the structure of the transition from continental to oceanic domain are key to revealing the processes and dynamics during the rifting and break-up of the Zhongsha block. In this paper, we present a P-wave velocity model obtained from both forward modeling and tomographic inversion of wide-angle seismic line OBS2017-2. The results support the continental nature of the Zhongsha Block with a thickness of up to ~25 km. However, the transition from the thick continental domain of the ZS into both adjacent abyssal basins shows clear differences. To the north, a ~120 km wide domain of extended continental crust was observed. Farther north, the NWSB is characterized as a narrow basin with typical oceanic crust. The transitional domain between the continental and oceanic crust shows a ~30–40 km wide region with a high-velocity lower crust reflecting excessive magmatism. In contrast, the SWSB is characterized by a sharp transition from the thick continental crust of the ZS to thin oceanic crust which is probably underlain by serpentinized mantle. The strong rheological properties of the pre-rift crust in the western part of the SCS margin may be the reason that rifting concentrated on narrow rifts and thinning focused on necking domains, while the ZS avoided any intense extension. The configuration of rigid blocks thereafter affected the break-up position and the style of oceanic crust.

Description: Marine gravity data can provide information on the distribution of mass anomalies in the oceanic crust and upper mantle. Computing corresponding gravity anomalies, especially so-called ‘residual’ gravity anomalies that directly reflect variations in the crustal structure, relies on gravity corrections of both seafloor relief and lithospheric thermal structure. The lithospheric thermal gravity correction involves either a plate cooling approximation or a mantle flow model with the latter typically done using simplified assumptions on mantle rheology. However, a detailed study of how differing rheological models affect the computed gravity anomalies is still missing. Here, we systematically examine the differences in residual mantle Bouguer anomalies (RMBA) caused by differing assumptions on mantle rheology for 16 mid-ocean ridge – transform fault systems. Our calculations show that isoviscous models tend to underpredict RMBA values within the transform deformation zone and overpredict them in the far field at older plate ages, when compared to plate cooling and nonlinear viscoplastic models. This discrepancy stems from isoviscous models failing to capture plate-like deformation, as well as their inability to resolve brittle failure and the associated strain localization that leads to warm upwelling beneath the transform fault. By exploring a wide parameter range, we find that the importance of mantle rheology scales with plate tectonic parameters at the mid-ocean ridge – transform fault system such as transform age offset, spreading rate, and transform fault length. These findings suggest that gravity thermal corrections at the intrinsically three-dimensional ridge – transform systems should employ mantle flow models that resolve plate-like deformation and brittle failure.