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 A fracture zone, with limited magma supply, has been replaced by a segment centre with a high magmatic budget.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

Processes of magmatic and tectonic accretion of oceanic lithosphere at mid-ocean ridges - Constraints from a seismic refraction study at the Mid-Atlantic Ridge near 21.5° N

Mid-oceanic ridges are plate boundaries where new oceanic crust is created. Especially slow-spreading ridges, like the Mid-Atlantic Ridge (MAR), reveal a complex structure denoted by magmatic and tectonic processes. In the working area of this seismic refractions study both types of crustal accretion are present. The northern segment (22.2° N) compensates tectonically the tensional stresses caused by the plate tectonic movements of the African and the Northern American plates. So called detachment faults or oceanic core complexes (OCCs) develop during that tectonic phase. In the meantime the southern segment (21.5° N) is a magmatically robust segment. The peculiarity of this segment is that it growths south- and northwards along the ridge axis, starting at about 5 m.y. ago. Ridge propagation was strong enough to break through a stable small offset transform fault. During propagation the transform migrated southwards, leaving behind a V-shaped structure the so called inner and outer pseudofaults. From five seismic refraction and wide-angle profiles, ridge-parallel and ridge-crossing, the seismic velocity structure was observed. The results show a strong crustal variation. The ridge-crossing profiles illustrate the temporal evolution of the crustal accretion within the magmatic robust segment. Past magmatic activities can be reconstructed. The different morphological and geological features of the area required different inversion and modelling procedures. A broad variety of methods for interpretation of the collected geophysical data were applied to gain a subsurface image and to allow a geological reconstruction. First arrival seismic tomography, joint refraction and reflection tomography, and joint seismic and gravimetric tomography were used. Along the northern profile tomography for the near offset travel time arrivals was used, yielding the shallow part of the subsurface. Joint forward modelling of seismic travel times and gravimetric data made it possible to resolve the structure at greater depth. The southern and hence magmatically dominated ridge segment shows crustal thickening along the ridge axis from 4 km at the segment ends to about 8 km in the segment centre whereas the crust in the northern basin thins more than beneath the southern ridge tip. Layer 2 is rather constant and the main thickening is taken by layer 3. The seismic velocities in the ridge tip tend to be lower, which could be caused by strong fracturing and partial alteration. In the seismic velocity models crustal thinning has been observed also with increasing distance to the spreading axis. The latter suggests intensified magmatic activity with focussed melt supply in the segment centre leading to an upwelling of the seafloor and an hourglass shaped bathymetry with a small axial valley at the segment centre that widens towards its ends. Melts are transported laterally at crustal levels towards the segment ends, preferable towards the southern ridge tip, while the larger part remains at the segment centre. The northern segment has a much larger variation of the crustal thickness across the ridge axis. Tectonically dominated crust thins extremely to approximately 40% of average oceanic crust at the western ridge flank near 22°19’. Partly the upper crust is completely missing and high seismic velocities of 7 km/s are reached already a few hundred metres below the seafloor. The asymmetric crustal accretion is also reflected in the seismic velocities that reach a level of normal oceanic young crust on the eastern ridge crest. This long lived detachment fault shifted the plate boundary towards the west. However, it does not expose mantle material in its central surface. This can be caused at least by two factors: 1) during the tectonic phase the area is magmatically starved but still magmatic accretion occurs. 2) The detachment fault is a steep normal fault, marked by higher seismicity, near the ridge axis and is rotated based on the “rolling-hinge” model to a shallow low-angle fault caused by the slip and the tensional stresses. If the fault is rotated from an optimum angle a new fault will be generated and this fault block (rider or rafted block) stays on the surface of the detachment fault. A petrologic survey detected serpentinised mantle at the steep southern wall of the core complex facing towards the southern segment end. This suggests a three-dimensional structure of the core complex with a detachment fault rooted in an intrusive zone in the mid-segment setting, exposing gabbroic rocks, and a detachment fault rooted near crust-mantle boundary zone towards the segment end unroofing mantle rocks. The uplift of the massif can not be only explained by flexural rotation caused by the tension of the plate tectonic processes. There has to be an additional force. This could be a result of lower dense serpentinised mantle. The density difference will be compensated by an uplift to reach the isostatic equilibrium

Crustal structure from teleseismic P-wave receiver function analysis in the Maule Region, Central Chile

EGU2011-12780 A temporary passive seismic network of 31 broad-band stations was deployed in the region around Talca and Constitución between 35°S to 36°S latitude and 71°W to 72.5°W longitude. The network was operated between March and October 2008. Thus, we recorded data prior the magnitude Mw=8.8 earthquake of 27 February 2010 at a latitude of the major slip and surface uplift. The experiment was conducted to address fundamental questions on deformation processes, crustal and mantle structures, and fluid flow. We present results of a teleseismic P receiver function study that covers the coastal region and reaches to the Andes. The aim is to determine the structure and thickness of the continental crust and constrain the state of hydration of the mantle wedge. The P-wave receiver function technique requires large teleseismic earthquakes from different distances and backazimuths. A few percent of the incident P-wave energy from a teleseismic event will be converted into S-wave (Ps) at significant and relatively sharp discontinuities beneath the station. A small converted S phase is produced that arrives at the station within the P wave coda directly after the direct P-wave. The converted Ps phase and their crustal multiples contain information about crustal properties, such as Moho depth and the crustal vp/vs ratio. We use teleseismic events with magnitudes mb > 5.5 at epicentral distances between 30° and 95° to examine P-to-S converted seismic phases. Our preliminary results provide new information about the thickness of the continental crust beneath the coastal region in Central Chile. At most of the stations we observed significant energy from P to S converted waves between 4 and 5 s after the direct P-wave within a positive phase interpreted as the Moho, occurring at 35 to 40 km. The great Maule earthquake of 27 February 2010 nucleated up-dip of the continental Moho. The rupture of this earthquake seems to have propagated down-dip of the Moho. The Moho reflection show a positive polarity, indicating that the mantle is either dry or only moderately hydrated. We observed converted energy from an intracrustal boundary at around 2 s that disappears near the coast. Further, positive polarity peaks occur that are possibly caused by the down going plate

Basin inversion: reactivated rift structures in the central Ligurian Sea revealed using ocean bottom seismometers

The Alpine orogen and the Apennine system are part of the complex tectonic setting in the Mediterranean Sea caused by the convergence between Africa and Eurasia. Between 30 Ma and 15 Ma, the Apennines-Calabrian-Maghrebides subduction retreated in a southeast direction pulling Corsica and Sardinia away from the Eurasian landmass, opening the Ligurian Sea. In this extensional setting, the Ligurian Sea was formed as a back-arc basin. The northern margin of the Ligurian Basin shows notable seismicity at the Alpine front, including frequent magnitude 4 events. Seismicity decreases offshore towards the basin center and Corsica, revealing a diffuse distribution of low-magnitude earthquakes. Within the framework of the AlpArray research initiative, a long-term amphibious seismological experiment was conducted in the Ligurian Sea to investigate the lithospheric structure and the seismicity in the Ligurian Basin. The passive seismic network consisted of 29 broad-band ocean bottom stations from Germany and France next to permanent and temporary broad-band land stations. The ocean bottom stations were in operation between June 2017 and February 2018. Two clusters consisting of 18 earthquakes occurred between ∼ 10 km to ∼ 16 km depth below the sea surface, within the lower crust and uppermost mantle, in the centre of the basin. Thrust faulting focal mechanisms indicate compression and tectonic inversion of the Ligurian Basin, which is an abandoned Oligocene–Miocene rift basin. The basin inversion is suggested to be related to the Africa–Europe plate convergence. The locations and focal mechanisms of seismicity suggest reactivation of pre-existing rifting-related structures. Slightly different striking directions of presumed rifting-related faults in the basin center compared to faults further east and hence away from the rift basin may reflect the counter-clockwise rotation of the Corsica–Sardinia block. Mantle refractions Pn and Sn have apparent velocities of 8.2 km/s and 4.7 km/s. The low Vp-Vs-ratio of 1.72 indicates a more brittle behavior of the mantle material. This supports the hypothesis of strengthening of crust and uppermost mantle during the Oligocene–Miocene rifting-related extension and thinning of continental crust. This project is part of the DFG Priority Program “Mountain Building Processes in Four Dimensions (4DMB)”. This research has been supported by the Deutsche Forschungsgemeinschaft (grant nos. TH_2440/1-1, KO_2961/6-1, and LA_2970/4-1) and the Agence Nationale de la Recherche (grant no. ANR-15-CE31- 0015)

Ligurian Ocean Bottom Seismology and Tectonics Research (LOBSTER)

The LOBSTER project constitutes the offshore component of the DFG Priority Program “Mountain Building Processes in Four Dimensions” (SPP 2017, 4DMB) and aimed to expand the densely spaced AlpArray broadband seismic network to the offshore domain in the Ligurian Sea. The LOBSTER program encompassed research cruises on the French RV Pourquoi Pas? in 2017 to deploy a long-term ocean bottom seismology network that was recovered using the German RV Maria S. Merian in 2018 (Fig. 1). The LOBSTER long-term seismic network consisted of 7 French (from IPGP) and 22 German (from the DEPAS pool and from GEOMAR) stations. During the second cruise an active seismic experiment was conducted to complement the passive seismology study. The refraction seismic data acquisition was conducted along two wide-angle profiles: P01 runs from the Gulf of Lion to Corsica and P02 trends parallel to the center of the Ligurian Basin in a NE-SW direction. Both profiles were analyzed using a travel time tomography (Dannowski et al., 2020 and in prep). The combined data set in addition to high-resolution bathymetry data shed light on today’s active deformation of the Ligurian Sea (Thorwart et al., 2021). In addition, the 3-D crustal and upper mantle structure of the Ligurian Basin was inferred from surface wave tomography (Wolf et al., 2021). The main technical aim of the LOBSTER project is to provide consistent data that can be smoothly integrated with the onshore seismology data. Key features in the data pre-processing are the correct timing, determining of the orientation of the horizontal seismometer components, and the searchability and availability of the data based on FAIR data standards. LOBSTER studied the Ligurian Sea at the transition from the western Alpine orogen to the Apennine system. This complex geodynamic setting is manifested in pronounced variations in crustal thickness. Topographic gradients in the area are the largest for the entire Alpine-Mediterranean domain, rising from -2500 m in the Ligurian basin to > +3000 m in the Alpine-Apennine orogen over a distance of less than 100 km. The Ligurian Basin is a back-arc basin opened by the south-eastward trench retreat of the Apennines-Calabria-Maghrebides subduction zone, which also triggered the opening of the adjacent western Mediterranean basins. The recent deformation in the Ligurian Sea results from compression along its northern margin (0.3 - 1.5 mm/year shortening), but no significant convergence is evident from GPS data, and rates of deformation are very low. The LOBSTER data set offers a better understanding of the complex geodynamic setting of the Ligurian Sea, which is characterized by pronounced variations in crustal thickness. Based on the LOBSTER data the following conclusions were documented: - Extension in the Ligurian Basin led to stretched and very thin continental crust or exhumed, partially serpentinised mantle. - Continental crustal thinning from north to south is related to the increase of extension with increasing distance from the rotation pole of the anticlockwise rotation of the Corsica–Sardinia block. - Seafloor spreading and formation of mantle-derived oceanic crust was not initiated during the extension of the Ligurian Basin. - The Ligurian Sea is currently closing while Africa and Eurasia are converging. Part of the stresses are taken up in the basin center through re-activation of extension-related faults. Data analysis is still ongoing and further results are expected from local earthquake tomography in the area of the Alps-Ligurian Junction conducted with the data from the long-term ocean bottom seismometer deployment

Land seismic data of the ALPHA amphibious controlled source experiment - Report

Raw-, SEG-Y and other supplementary data of the landside deployment from the amphibious wide-angle seismic experiment ALPHA are presented. The aim of this project was to reveal the crustal and lithospheric structure of the subducting Adriatic plate and the external accretionary wedge in the southern Dinarides. Airgun shots from the RV Meteor were recorded along two profiles across Montenegro and northern Albania

CAYSEIS - magma-starved oceanic crustal accretion and transform margin formation in the Cayman Trough revealed by seismic and seismological data - Cruise No. M115, April 1 - April 28, 2015 - Kingston (Jamaica) - Pointe-à-Pitre (Guadeloupe)

About 57% of the Earth’s outer surface is oceanic crust and new ocean floor is continuously created along the 55,000-60,000 km long mid-ocean ridge (MOR) system. About 25% of MORs spread at an ultra-slow spreading rate of < 20 mm/yr. Most ultra-slow spreading ridges occur in areas of the world that are difficult to reach, like the Gakkel Ridge in the Arctic Ocean and the Southwest Indian Ridge in the Indian Ocean. It has long been recognized that crustal accretion at ultra-slow spreading rates is fundamentally different from crust generated at faster spreading rates. However, due to the remoteness of ultra-slow ridges, the formation of crust at these magma-starved centres is yet not well understood. During the CAYSEIS cruise we surveyed lithospheric formation at ultra-slow spreading rates at the Mid-Cayman spreading centre (MCSC) in the Caribbean Sea, where oceanic crust is formed at a full rate of ~17 mm/yr. To the northeast and southwest, the MCSC is bound by two major transform faults. Using active-source wide-angle seismic imaging and passive local earthquake monitoring we, studied the balance between magmatic accretion and tectonic stretching (and hence oceanic core complex formation) and the relationship between faulting and hydrothermal activity at ultra-slow spreading rates. In addition, we explored transform margin formation at a unique setting, occurring at the southern terminus of the MCSC. In total, six seismic lines surveyed crust formed at the MCSC, two of these profiles also crossed the Swan Island transform fault. The project was a collaboration between German, British and American scientists

Potential impacts of gas hydrate exploitation on slope stability in the Danube deep-sea fan, Black Sea

Highlights • The Danube deep-sea fan offers best conditions for hydrate production. • Gas production out of a hypothetical methane hydrate reservoir was simulated. • Hazard assessment to investigate the hazard of production-induced slope failures. • Factor of Safety against slope failure is not affected by the production process. • Mobilized mass could hit the production site if landslide were to happen. Methane production from gas hydrate reservoirs is only economically viable for hydrate reservoirs in permeable sediments. The most suitable known prospect in European waters is the paleo Danube deep-sea fan in the Bulgarian exclusive economic zone in the Black Sea where a gas hydrate reservoir is found 60 m below the seafloor in water depths of about 1500 m. To investigate the hazards associated with gas production-induced slope failures we carried out a slope stability analysis for this area. Screening of the area based on multibeam bathymetry data shows that the area is overall stable with some critical slopes at the inner levees of the paleo channels. Hydrate production using the depressurization method will increase the effective stresses in the reservoir beyond pre-consolidation stress, which results in sediment compaction and seafloor subsidence. The modeling results show that subsidence would locally be in the order of up to 0.4 m, but it remains confined to the immediate vicinity above the production site. Our simulations show that the Factor of Safety against slope failure (1.27) is not affected by the production process, and it is more likely that a landslide is triggered by an earthquake than by production itself. If a landslide were to happen, the mobilized sediments on the most likely failure plane could generate a landslide that would hit the production site with velocities of up to 10 m s-1. This case study shows that even in the case of production from very shallow gas hydrate reservoirs the threat of naturally occurring slope failures may be greater than that of hydrate production itself and has to be considered carefully in hazard assessments

Seismicity cluster below the Moho indicates thrust faulting in the central Ligurian Basin

International audienceThe Alpine orogen and the Apennines system are part of the complex tectonic settings in the Mediterranean Sea caused by the convergence between Africa and Eurasia. Between 30 Ma and 15 Ma, the Calabrian Subduction retreated in southeast direction pulling Corsica and Sardinia away from the Eurasian continent. In this extensional setting, the Ligurian Sea was formed as a back-arc basin. The rifting jumped 15 MA ago to the Tyrrhenian Sea leaving Corsica and Sardinia in a stable position relative to Eurasia as observed by GPS measurements.Within the framework of the AlpArray research initiative and its German component "4D Mountain building" (SPP2017 4D-MB) a long-term experiment was conducted in the Ligurian sea to investigate the lithosphere structure and the seismicity in the Ligurian basin. The passive seismic network was operated by France and Germany and consisted of 29 broad-band ocean bottom stations. It was in operation between June 2017 and February 2018. At the end of the experiment two active seismic profiles were conducted additionally.A cluster of 15 events with magnitudes lower than 2.5 occurred in the centre of the Ligurian Basin. The earthquakes are located at a depth of 20 km to 35 km, i.e. 10 - 25 km below the Moho. The cluster was not continuously active but had several active periods which lasted between 2 and 5 days.A fault plane solution could be determined of the larger events in the cluster. The mechanism is a thrust faulting. Smaller events should have a similar mechanism due to the highly coherent waveforms. A similar mechanism was observed for the Mw=4.9 earthquake on 07.07.2011 which occurred 50 km east of the cluster. Both solutions show a SW-NE striking, northwest dipping fault plane. This indicates a convergence in NW-SE direction between Corsica and Eurasia