Early-stage rifting of the northern Tyrrhenian Sea Basin: Results from a combined wide-angle and multichannel seismic study

Extension of the continental lithosphere leads to the formation of rift basins and ultimately may create passive continental margins. The mechanisms that operate during the early stage of crustal extension are still intensely debated. We present the results from coincident multichannel seismic and wide-angle seismic profiles that transect across the northern Tyrrhenian Sea Basin. The profiles cross the Corsica Basin (France) to the Latium Margin (Italy) where the early-rift stage of the basin is well preserved. We found two domains, each with a distinct tectonic style, heat flow and crustal thickness. One domain is the Corsica Basin in the west that formed before the main rift phase of the northern Tyrrhenian Sea opening (∼8–4 Ma). The second domain is rifted continental crust characterized by tilted blocks and half-graben structures in the central region and at the Latium Margin. These two domains are separated by a deep (∼10 km) sedimentary complex of the eastern portion of the Corsica Basin. Travel-time tomography of wide-angle seismic data reveals the crustal architecture and a subhorizontal 15–17 ± 1 km deep Moho discontinuity under the basin. To estimate the amount of horizontal extension we have identified the pre-, syn-, and post-tectonic sedimentary units and calculated the relative displacement of faults. We found that major faults initiated at angles of 45°–50° and that the rifted domain is horizontally stretched by a factor of β ∼ 1.3 (∼8–10 mm/a). The crust has been thinned from ∼24 to ∼17 km indicating a similar amount of extension (∼30%). The transect represents one of the best imaged early rifts and implies that the formation of crustal-scale detachments, or long-lived low-angle normal faults, is not a general feature that controls the rift initiation of continental crust. Other young rift basins, like the Gulf of Corinth, the Suez Rift or Lake Baikal, display features resembling the northern Tyrrhenian Basin, suggesting that half-graben formations and distributed homogeneous crustal thinning are a common feature during rift initiation

Formation and deformation of hyperextended rift systems: Insights from rift domain mapping in the Bay of Biscay-Pyrenees

International audienceThe Bay of Biscay and the Pyrenees correspond to a Lower Cretaceous rift system including both oceanic and hyperextended rift domains. The transition from preserved oceanic and rift domains in the West to their complete inversion in the East enables us to study the progressive reactivation of a hyperextended rift system. We use seismic interpretation, gravity inversion, and field mapping to identify and map former rift domains and their subsequent reactivation. We propose a new map and sections across the system illustrating the progressive integration of the rift domains into the orogen. This study aims to provide insights on the formation of hyperextended rift systems and discuss their role during reactivation. Two spatially and temporally distinct rift systems can be distinguished: the Bay of Biscay-Parentis and the Pyrenean-Basque-Cantabrian rifts. While the offshore Bay of Biscay represent a former mature oceanic domain, the fossil remnants of hyperextended domains preserved onshore in the Pyrenean-Cantabrian orogen record distributed extensional deformation partitioned between strongly segmented rift basins. Reactivation initiated in the exhumed mantle domain before it affected the hyperthinned domain. Both domains accommodated most of the shortening. The final architecture of the orogen is acquired once the conjugate necking domains became involved in collisional processes. The complex 3-D architecture of the initial rift system may partly explain the heterogeneous reactivation of the overall system. These results have important implications for the formation and reactivation of hyperextended rift systems and for the restoration of the Bay of Biscay and Pyrenean domain

Geophysical detection of underground cavities

International audienceIn this paper, we present a synthesis of the geophysical investigations conducted on experimental sites selected in the Lorraine salt basin and Haute-Normandie marlpits. These investigations were carried out in the frame of a partnership (scientific and industrial) research program on cavities detection and characterization using techniques of high resolution seismic, microgravity and electric resistivity. The objective of the research is three folds: (1) to develop and optimize P and S seismic vibratory sources with an enhanced procedure of generation and optimization of sweeps (2) for each technique, to define the privileged application field and limitations regarding the general context of the considered site (3) to progress in the joint interpretations of the geophysical data. Despite the good quality of the data, the results evidence the complexity in the interpretation of the geophysical response in cavities environment due mainly to the compromise between the resolution and the ratio depth/dimension of the cavities, and the nature of the filling (brine, water, air). In the case of the marlpit mines, the corresponding geophysical anomalies could be correlated to the exact position of the marlpit known according to the archive records. Drilling campaign has confirmed a local collapse of the marlpit identified on the only High Resolution Seismic dataCe papier présente une synthèse des investigations géophysiques menées sur des sites expérimentaux du bassin salifère lorrain et les marnières de Haute-Normandie. Ces travaux ont été réalisés dans le cadre d'un programme de recherche partenarial axé sur la détection et la caractérisation des cavités à partir des techniques de Sismique Haute résolution (SHR), de la microgravimétrie et de la résistivité électrique. L'objectif du programme est triple : (1) développer et optimiser des sources vibratoires en ondes P et S avec une Procédure de Génération et d'Optimisation de Sweep adapté ; (2) définir pour chaque technique le champ d'application privilégié et les limitations en fonction du contexte général des sites considérés ; (3) progresser dans les interprétations conjointes des données géophysiques. Malgré la bonne qualité des mesures, des difficultés ont été rencontrées lors des interprétations de la réponse géophysique de l'environnement des cavités avec, pour raison essentielle le compromis entre la résolution de l'imagerie et le rapport profondeur/dimension de la cavité ainsi que la nature du remplissage (saumure, air, eau). Dans le cas des marnières, les anomalies géophysiques ont été carrelées à la position exacte de marnière pilote bien documentée dans les archives. Une campagne de forage a permis de confirmer un éboulement local de la marnière observée sur les données de sismique haute résolutio

Deep structure of the North Tyrrhenian Sea from multi-channel seismic profiles and on land wide angle reflection/refraction seismic recording (LISA cruise): geodynamical implications

We present a lithospheric section of the northern Tyrrhenian basin based on principal observations along a transect from Corsica Island to Tuscany margin. Offshore, crustal structure observations come from the LISA deep seismic cruise, composed by Multi-Channel Seismic profiles and wide angle reflection/refraction seismic data. Other parts of the section are constrained by published data, from different geophysical sources. A great detachment between Alpine Corsica and Hercynian Corsica constitutes the base of the Corsica basin. The end of this detachment is probably located below the Elba Island in the ductile lower crust where we observed an east dipping reflector interpreted as ductile shear zone. Similar detachments have been observed beneath the Punta-Ala basin on the Tuscany margin. Our section shows a thin crust below Corsican Basin (15 km thick) and a thicker crust below the Pianosa ridge (25 km). Between Elba Island and the Tuscany coast the Moho reflections shallow to (23 km/depth) as identified on the MCS profiles and from the modelling of the wide-angle reflection/refraction seismic date. An asthenospheric mantle uplift produces a significant heat flow below the Tuscany margin. The base of the lithosphere is located at 75 km depth below the Corsica margin, at 50 km below the Corsica basin and as shallow as 30 km below the Tuscany margin. The shift observed between the Corsica Basin and the Tuscany coast suggests an asymmetric lithospheric thinning which involves a detachment fault located at the base of the Corsican basin

Location of microseismic swarms induced by salt solution mining

International audienceGround failures, caving processes and collapses of large natural or man-made underground cavities can produce significant socio-economic damages and represent a serious risk envisaged by the mine managements and municipalities. In order to improve our understanding of the mechanisms governing such a geohazard and to test the potential of geophysical methods to prevent them, the development and collapse of a salt solution mining cavity was monitored in the Lorraine basin in northeastern France. During the experiment, a huge microseismic data set (similar to 50 000 event files) was recorded by a local microseismic network. 80 per cent of the data comprised unusual swarming sequences with complex clusters of superimposed microseismic events which could not be processed through standard automatic detection and location routines. Here, we present two probabilistic methods which provide a powerful tool to assess the spatio-temporal characteristics of these swarming sequences in an automatic manner. Both methods take advantage of strong attenuation effects and significantly polarized P-wave energies at higher frequencies (>100 Hz). The first location approach uses simple signal amplitude estimates for different frequency bands, and an attenuation model to constrain the hypocentre locations. The second approach was designed to identify significantly polarized P-wave energies and the associated polarization angles which provide very valuable information on the hypocentre location. Both methods are applied to a microseismic data set recorded during an important step of the development of the cavity, that is, before its collapse. From our results, systematic spatio-temporal epicentre migration trends are observed in the order of seconds to minutes and several tens of meters which are partially associated with cyclic behaviours. In addition, from spatio-temporal distribution of epicentre clusters we observed similar epicentre migration in the order of hours and days. All together, we suggest that the recorded microseismicity mainly represents detachment and block breakage processes acting at the cavity's roof, indicating a zone of critical state of stress and where partial fractures cause chain reaction failures as a result of stress redistribution processes

Resolving source mechanisms of microseismic swarms induced by solution mining

International audienceIn order to improve our understanding of hazardous underground cavities, the development and collapse of a ∼200 m wide salt solution mining cavity was seismically monitored in the Lorraine basin in northeastern France. The microseismic events show a swarm-like behaviour, with clustering sequences lasting from seconds to days, and distinct spatiotemporal migration. Observed microseismic signals are interpreted as the result of detachment and block breakage processes occurring at the cavity roof. Body wave amplitude patterns indicated the presence of relatively stable source mechanisms, either associated with dip-slip and/or tensile faulting. Signal overlaps during swarm activity due to short interevent times, the high-frequency geophone recordings and the limited network station coverage often limit the application of classical source analysis techniques. To overcome these shortcomings, we investigated the source mechanisms through different procedures including modelling of observed and synthetic waveforms and amplitude spectra of some well-located events, as well as modelling of peak-to-peak amplitude ratios for the majority of the detected events. We extended the latter approach to infer the average source mechanism of many swarming events at once, using multiple events recorded at a single three component station. This methodology is applied here for the first time and represents a useful tool for source studies of seismic swarms and seismicity clusters. The results obtained with different methods are consistent and indicate that the source mechanisms for at least 50 per cent of the microseismic events are remarkably stable, with a predominant thrust faulting regime with faults similarly oriented, striking NW–SE and dipping around 35°–55°. This dominance of consistent source mechanisms might be related to the presence of a preferential direction of pre-existing crack or fault structures. As an interesting byproduct, we demonstrate, for the first time directly on seismic data, that the source radiation pattern significantly controls the detection capability of a seismic station and network