Oceanic Core Complex die off and generation of enhanced mantle upwelling on the Mid-Atlantic Ridge - 22° N

EGU2011-13199 Images of crustal construction provide a key to understand the interplay of magmatism and tectonism while oceanic crust is build up. Bathymetric data show that the crustal construction is highly variable. Areas that are dominated by magmatic processes are adjacent to areas that are highly tectonised and where mantle rocks were found. The Mid-Atlantic Ridge at 22°N shows this high variability along the ridge axis, within the TAMMAR segment, and from segment to segment. However, this strong variability occurs also off-axis, spreading parallel, representing different times in the same area of the ridge. A fracture zone, with limited magma supply, has been replaced by a segment centre with a high magmatic budget. Roughly 4.5 million years ago, the growing magmatic active TAMMAR segment, propagated into the fracture zone, started the migration of the ridge offset to the south, and stopped the formation of core complexes. We present data from seismic refraction and wide-angle reflection profiles that surveyed the crustal structure across the ridge crest of the TAMMAR segment. These yield the crustal structure at the segment centre as a function of melt supply. The results suggest that crust is ~8 km thick near the ridge and decreases in thickness with offset to the ridge axis. Seismic layer 3 shows profound changes in thickness and becomes rapidly one kilometre thicker approx. 5 million years ago. This correlates with gravimetric data and the observed “Bull’s eye” anomaly in that region. Our observations support a temporal change from thick lithosphere with oceanic core complex formation to thin lithosphere with focussed mantle upwelling and segment growing. The formation of ‘thick-crust’ volcanic centre seems to have coincided with the onset of propagation 4.5 million years ago

Role of the Alboran Sea volcanic arc choking the Mediterranean to the Messinian salinity crisis and foundering biota diversification in North Africa and Southeast Iberia

The Mediterranean Sea desiccated 5.96 million years ago when it became isolated from the world oceans during the Messinian salinity crisis. This event permitted the exchange of terrestrial biota between Africa and Iberia contributing to the present rich biodiversity of the Mediterranean region. The cause chocking the Mediterranean has been proposed to be tectonic uplift and dynamic topography but the driving mechanism still remains debated. We present a new wide-angle seismic profile that provides a detailed image of the thickness and seismic velocity distribution of the crust in the eastern Alboran basin. The velocity model shows a characteristic structure of a subduction-related volcanic arc with a high-velocity lower crust and a 16-18 km total-thickness igneous crust that magmatic accreted mostly between 10-6 Ma across the eastern Alboran basin. Estimation of the isostatically corrected depth of the arc crust taking into account the original thermal structure and sediment-loading subsidence since 6 Ma places a large area of the eastern Alboran basin above sea level at the time. This estimation is supported by geophysical data showing subaereal erosional unconformities for that time. This model may explain several up-to-now-disputed features of the Messinian salinity crisis, including: the progressive isolation of the Mediterranean since 7.1 Ma with the disappearance of open marine taxa, the existence of evaporites mostly to the east of the volcanic arc, the evidence that the Gibraltar straits were not a land bridge offered by continuous Messinian open marine sediments at ODP site 976 in the western Alboran basin, the importance of southeastern Iberia and North Africa as centres of biota diversification since before the salinity crisis, and patterns of speciation irradiating from SE Iberia and the eastern Rif in some taxons. Guillermo Booth-Rea (1), Cesar R. Ranero (2), and Ingo Grevemer (3

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

The complex 3-D transition from continental crust to backarc magmatism and exhumed mantle in the Central Tyrrhenian basin

Geophysical data from the MEDOC experiment across the Northern Tyrrhenian backarc basin has mapped a failed rift during backarc extension of cratonic Variscan lithosphere. In contrast, data across the Central Tyrrhenian have revealed the presence of magmatic accretion followed by mantle exhumation after continental breakup. Here we analyse the MEDOC transect E–F, which extends from Sardinia to the Campania margin at 40.5°N, to define the distribution of geological domains in the transition from the complex Central Tyrrhenian to the extended continental crust of the Northern Tyrrhenian. The crust and uppermost mantle structure along this ∼400-km-long transect have been investigated based on wide-angle seismic data, gravity modelling and multichannel seismic reflection imaging. The P-wave tomographic model together with a P-wave-velocity-derived density model and the multichannel seismic images reveal seven different domains along this transect, in contrast to the simpler structure to the south and north. The stretched continental crust under Sardinia margin abuts the magmatic crust of Cornaglia Terrace, where accretion likely occurred during backarc extension. Eastwards, around Secchi seamount, a second segment of thinned continental crust (7–8 km) is observed. Two short segments of magmatically modified continental crust are separated by the ∼5-km-wide segment of the Vavilov basin possibly made of exhumed mantle rocks. The eastern segment of the 40.5°N transect E–F is characterized by continental crust extending from mainland Italy towards the Campania margin. Ground truthing and prior geophysical information obtained north and south of transect E–F was integrated in this study to map the spatial distribution of basement domains in the Central Tyrrhenian basin. The northward transition of crustal domains depicts a complex 3-D structure represented by abrupt spatial changes of magmatic and non-magmatic crustal domains. These observations imply rapid variations of magmatic activity difficult to reconcile with current models of extension of continental lithosphere essentially 2-D over long distances

Focussing on Proto-Seismogenic Zone of erosional Convergent Margin

Great earthquakes in subduction zones occur after stable slip in the proto-seismogenic zone transitions to the unstable slip that characterizes seismogenic zones. Subducted material input to seismogenic zones affects this transition. Material structure, lithology and physical properties change progressively during subduction, and according to current hypotheses, specific material transformations trigger the stable to unstable slip transition.Where accretion dominates a convergent margin, material input is trench sediment that is easily drill-sampled. However, where erosion dominates a margin, material input is unknown because it originates along the base of the upper plate and alters differently. The depth at which material is eroded lies beyond the sampling capabilities of past scientific ocean drilling, so the protoseismogenic zone transformed material has never been drill-sampled; nor does geophysics resolve its structure, lithology, and physical properties. The Japanese riser drill ship Chikyu in the Integrated Ocean Drilling Program (IODP) overcomes this difficulty. Preparing a site for deep drilling is a much greater task than preparing the shallower sites of past programs, so this is accomplished during workshops

The velocity structure of the Alboran Basin (westernmost Mediterranean) along a north-south transect

European Geosciences Union (EGU) General Assembly 2018, 8-13 April 2018, Vienna, Austria.-- 1 page.The deep structure of the Alboran Basin (westernmost Mediterranean) has been under debate since the first surveys in the late 1980’s. Although the regional crustal and upper mantle structure has been extensively studied recently (i.e. global tomography, receiver function analysis or full-waveform inversion) the lithosphere underneath the Alboran Basin remains inadequately imaged, as the resolution of the crustal model is limited due to sparse data, inhibiting a precise characterization of the crust underneath the basin. The deep structure of the Alboran Basin is directly linked to the seismogenic potential of this area, as the main faults seem to be located at the edges of the crustal domains. Thus, an accurate seismic characterization of the crustal domains of the Alboran Basin and the transition between them is key to improve our understanding of the seismogenic potential of the faults in this complex area. Based on multichannel seismic reflection studies (i.e. TOPOMED project), with seafloor and basement groundtruthing through available drill and dredge samples, four different crustal domains have been defined for the area: (i) a thin continental domain (West Alboran and Malaga basins), (ii) the magmatic arc domain (East Alboran Basin), (iii) the North-African continental domain (South Alboran, Pytheas and Habibas basins) and (iv) oceanic crust (Algero-Balearic Basin). Unfortunately, basement dredges and well data are scarce, and, to a better understand of the deep structure of the Alboran Basin, wide-angle seismic deep-penetrating data are needed. Here we present the velocity and density model obtained from the inversion of profile P01 (WESTMED project), running from the south of Iberia (Almeria) to the north African margin (east of Nador). This profile includes 24 ocean bottom hydrophones and seismometers (OBH/OBS) with a spacing <2.5km, and 13 onshore geophones located on both margins. Velocity modelling has been carried out using the Korenaga inversion method. The result resolves the crustal structure and the Moho depths across the Alboran Basin. This profile runs coincident with a crustal-scale multichannel seismic reflection profile, allowing the association of the velocity variations and anomalies with their tectonic origin. Further, the comparison between 1D vertical Vp structure and density models with empirical Vp and density relationships sheds light on the petrological nature of the basement, fulfilling the crustal domain characterization. The analysis of the refraction seismic profile will advance the characterization of the deep basin structure and its fault geometry and hence contribute to an improved hazard assessment and refined prevention plans for the area.Peer Reviewe