A wide-angle seismic survey across the Southern Tyrrhenian basin and the Northwestern Ionian (CHIANTI experiment): data and preliminary results

In this work we present first results of two wide-angle seismic transects acquired in the Southern Tyrrhenian basin and Northwestern Ionian during the CHIANTI experiment (July 2015). The first transect runs NW to SE starting in the Vavilov basin, crossing the Marsili basin, the currently active volcanic arc of the Aeolian Islands and the Calabrian arc, ending in the accretionary prism of the NW Ionian. This transect is >500 km long and includes 46 OBS and 5 landstations. The second transect crosses the Vavilov basin from N to S at a longitude of 12.5ºE. This one is 180 km long and includes 15 OBS. The preliminary interpretation of the OBS data clearly shows that the crustal structure is very similar in the Marsili and Vavilov basins. They show no crust-mantle boundary reflections and high apparent velocities of up to 8 km/s a few kms below the top of the basement. These results are in good agreement with previous ones obtained in the central Tyrrhenian during the MEDOC-2010 experiment, in which a transition from extended continental crust to magmatically-affected back-arc crust to exhumed mantle that challenges current conceptual models of back-arc extension, has been interpreted. The combination of the results of these two experiments is providing a new view of the nature and configuration of the geological domains in the whole Tyrrhenian basin, giving first order constraints on the processes that have controlled its geodynamic evolution

Calabrian Arc Hazards in Ionian and Tyrrhenian Seas: First results from the CHIANTI cruise

The main objective of the CHIANTI cruise was to collect geophysical marine data to determine the deep crustal structure and plate geometry across the subduction system of the Ionian and Tyrrhenian Seas, from the frontal wedge to the arc and back-arc. The goal is to study the processes that operated during the subduction of the Ionian slab of oceanic crust under Calabria, which lead to the development of the Aeolian volcanic arc, and the subsequent opening of the Tyrrhenian basin, and are responsible of the geological hazards that threaten the region. The CHIANTI cruise onboard the Spanish R/V BO Sarmiento de Gamboa started in Barcelona (Spain) on July 12, and finished in Catania (Italy), on August 28, 2015. It consisted of four legs devoted to acquisition of data with different seismic/acoustic techniques in the Tyrrhenian and Ionian Seas. Leg 1 and 2 were focused on the acquisition of deep penetrating Wide-Angle Reflection and Refraction Seismic (WAS) data, Leg 3 on Multichannel Seismic (MCS) Reflection data and finally Leg 4 was devoted to sidescan imaging, coring and single channel seismic acquisition. During the entire cruise, complementary acoustic data (i.e. multibeam bathymetry and sub-bottom profiler) were acquired simultaneously. In this presentation we focus on the seafloor mapping and processed multichannel seismic reflection grid collected on the IONIAN prism. The data show abundant evidence of ongoing widespread deformation across the entire region from the deformation front to the uppermost slope and extending into the Calabrian emerged region. The seafloor mapping shows numerous mud volcanoes associated to fault activity. The seismic images display deformational features active across the entire prims at different locations extending the definition of structures described in previous works of the region with fewer areal coverage. The data show a prism tectonic structure that is distinct from the structure of prism in other subduction systems worldwide

Magmatic and non-magmatic history of the Tyrrhenain backarc Basin: new constraints from geophysical and geological data

The Western Mediterranean region is represented by a system of backarc basins associated to slab rollback and retreat of subduction fronts. The onset of formation of these basins took place in the Oligocene with the opening of the Valencia Through, the Liguro-Provençal and the Algero-Balearic basins, and subsequently, by the formation of the Alboran and Tyrrhenian basins during the early Tortonian. The opening of these basins involved rifting that in some regions evolved until continental break up, that is the case of the Liguro-Provençal, Algero-Balearic, and Tyrrhenian basins. Previous geophysical works in the first two basins revealed a rifted continental crust that transitions to oceanic crust along a region where the basement nature is not clearly defined. In contrast, in the Tyrrhenian Basin, recent analysis of new geophysical and geological data shows a rifted continental crust that transitions along a magmatic-type crust to a region where the mantle is exhumed and locally intruded by basalts. This basement configuration is at odds with current knowledge of rift systems and implies rapid variations of strain and magma production

Improvements of Travel-time Tomography Models from Joint Inversion of Multi-channel and Wide-angle Seismic Data

Commonly multichannel seismic reflection (MCS) and wide-angle seismic (WAS) data are modeled and interpreted with different approaches. Conventional travel-time tomography models using solely WAS data lack the resolution to define the model properties and, particularly, the geometry of geologic boundaries (reflectors) with the required accuracy, specially in the shallow complex upper geological layers. We plan to mitigate this issue by combining these two different data sets, specifically taking advantage of the high redundancy of multichannel seismic (MCS) data, integrated with wide-angle seismic (WAS) data into a common inversion scheme to obtain higher-resolution velocity models (Vp), decrease Vp uncertainty and improve the geometry of reflectors. To do so, we have adapted the tomo2d and tomo3d joint refraction and reflection travel time tomography codes (Korenaga et al, 2000; Meléndez et al, 2015) to deal with streamer data and MCS acquisition geometries. The scheme results in a joint travel-time tomographic inversion based on integrated travel-time information from refracted and reflected phases from WAS data and reflected identified in the MCS common depth point (CDP) or shot gathers

Seismic Oceanography in the Tyrrhenian Sea – Thermohaline Staircases, Eddies and Internal Waves

We use seismic oceanography to document and analyze oceanic thermohaline finestructure across the Tyrrhenian Sea. Multichannel seismic (MCS) reflection data were acquired during the MEDiterranean OCcidental survey in April-May 2010. We deployed along-track expendable bathythermograph probes simultaneous with MCS acquisition. At nearby locations we gathered conductivity-temperature-depth data. An autonomous glider survey added in-situ measurements of oceanic properties. The seismic reflectivity clearly delineates thermohaline finestructure in the upper 2,000 m of the water column, indicating the interfaces between Atlantic Water/Winter Intermediate Water, Levantine Intermediate Water, and Tyrrhenian Deep Water. We observe the Northern Tyrrhenian Anticyclone, a near-surface meso-scale eddy, plus laterally and vertically extensive thermohaline staircases. Using MCS we are able to fully image the anticyclone to a depth of 800 m and to confirm the horizontal continuity of the thermohaline staircases of more than 200 km. The staircases show the clearest step-like gradients in the center of the basin while they become more diffuse towards the periphery and bottom, where impedance gradients become too small to be detected by MCS. We quantify the internal wave field and find it to be weak in the region of the eddy and in the center of the staircases, while it is stronger near the coastlines. Our results indicate this is because of the influence of the boundary currents, which disrupt the formation of staircases by preventing diffusive convection. In the interior of the basin the staircases are clearer and the internal wave field weaker, suggesting that other mixing processes such as double-diffusion prevail. Synopsis We studied the internal temperature and salinity structure of the Tyrrhenian Sea (Mediterranean) using the multichannel seismic reflection method (the same used in the hydrocarbon industry). Low frequency sound (seismic) waves are produced at the surface with an explosive air source and recorded by a towed cable containing hydrophones (underwater microphones). The data are processed to reveal 'stratigraphy' that result from contrasts in density that are themselves caused by changes in temperature and salinity. In this way we can map ocean circulation in two-dimensions. We also deployed in situ oceanographic probes to measure temperature and salinity in order to corroborate and optimize the processing of the seismic data. We then quantified the internal gravity wave field by tracking the peaks of seismic trace wavelets. Our results show that the interior of the Tyrrhenian Sea is largely isolated from internal waves that are generated by a large cyclonic boundary current that contains waters from the Atlantic ocean and other parts of the Mediterranean. This isolation allows the thermohaline finestructure to form, where small scale vertical mixing processes are at play. Understanding these mixing processes will aid researchers study global ocean circulation and to add constraints that can help improve climate models

The structure of the Calabrian subduction system from the fore-arc to the back-arc: new insights from wide-angle seismic data

American Geophysical Union Fall Meeting, 11-15 December 2017, New OrleansThe Calabrian arc is a Neogene-Quaternary arcuate orogen result from the subduction of the Ionian Lithosphere under Calabria. The SE migration of this subduction system, triggered by slab rollback, caused the opening of the Tyrrhenian back-arc basin. The large-scale lithospheric structure of the subduction system is mostly imaged by regional earthquake tomography studies. The limited resolution of these studies, however, hinders the definition of smaller-scale details on the location, nature and transition of different lithospheric domains, which are crucial to study the geodynamic evolution of the system. Here we perform travel-time tomography of offshore and onshore active-source wide-angle seismic data to define the 2D Vp structure of the entire Calabrian subduction system. The data were acquired along a 550 km-long transect that extends from the Tyrrhenian back-arc domain to the fore-arc in the Ionian Sea, across Calabria. From NW to SE, the tomographic model shows abrupt variations of the velocity structure. In the back-arc system, particularly in the Vavilov and Marsili basins, OBS sections lack PmP-like arrivals and the velocity structure shows a continuous and strong vertical velocity gradient of ~1 s-1. These results strongly support the presence of a basement made of exhumed mantle rocks. Between the Vavilov and Marsili basins, a relatively thick, low-velocity block is interpreted to be of continental affinity. The transition between Marsili Basin and Calabria is marked by a steep Moho geometry that shallows from SE to NW, revealing a dramatic crustal thinning along the N Calabrian margin. The lower crust of the margin has localized Vp of ~7 km/s under the submarine volcanic arc. SE Calabria, the model shows a strong horizontal velocity gradient that is interpreted as the backstop of the subduction. In the Ionian, a 3-5 km thick sedimentary wedge thickens towards the NW. The frontal part of the wedge shows sub-vertical low-velocity anomalies indicating the presence of fluid-saturated large thrusts faults.Peer Reviewe

Recent inversion of the Tyrrhenian Basin

The Tyrrhenian Basin is a region created by Neogene extensional tectonics related to slab rollback of the east-southeast–migrating Apennine subduction system, commonly believed to be actively underthrusting the Calabrian arc. A compilation of >12,000 km of multichannel seismic profiles, much of them recently collected or reprocessed, provided closer scrutiny and the mapping of previously undetected large compressive structures along the Tyrrhenian margin. This new finding suggests that Tyrrhenian Basin extension recently ceased. The ongoing compressional reorganization of the basin indicates a change of the regional stress field in the area, confirming that slab rollback is no longer a driving mechanism for regional kinematics, now dominated by the Africa-Eurasia lithospheric collision

The continent-to-ocean transition in the Iberia Abyssal Plain

Conceptual models of magma-poor rifting are strongly based on studies of the nature of the basement in the continent-to-ocean transition of the Iberia Abyssal Plain, and suggest that exhumed mantle abuts extended continental crust. Yet, basement has only been sampled at a few sites, and its regional nature and the transition to seafloor spreading inferred from relatively low-resolution geophysical data are inadequately constrained. This uncertainty has led to a debate about the subcontinental or seafloor-spreading origin of exhumed mantle and the rift-related or oceanic nature of magmatic crust causing the magnetic J anomaly. Different interpretations change the locus of break-up by >100 km and lead to debate of the causative processes. We present the tomographic velocity structure along a 360-km-long seismic profile centered at the J anomaly in the Iberia Abyssal Plain. Rather than delineating an excessive outpouring of magma, the J anomaly occurs over subdued basement. Furthermore, its thin crust shows the characteristic layering of oceanic crust and is juxtaposed to exhumed mantle, marking the onset of magma-starved seafloor spreading, which yields the westward limit of an ~160-km-wide continent–ocean transition zone where continental mantle has been unroofed. This zone is profoundly asymmetric with respect to its conjugate margin, suggesting that the majority of mantle exhumation occurs off Iberia. Because the J anomaly is related to the final break-up and emplacement of oceanic crust, it neither represents synrift magmatism nor defines an isochron, and hence it poorly constrains plate tectonic reconstructions