Stochastic Heterogeneity Mapping around a Mediterranean salt lens

We present the first application of Stochastic Heterogeneity Mapping based on the band-limited von Kármán function to a seismic reflection stack of a Mediterranean water eddy (meddy), a large salt lens of Mediterranean water. This process extracts two stochastic parameters directly from the reflectivity field of the seismic data: the Hurst number, which ranges from 0 to 1, and the correlation length (scale length). Lower Hurst numbers represent a richer range of high wavenumbers and correspond to a broader range of heterogeneity in reflection events. The Hurst number estimate for the top of the meddy (0.39) compares well with recent theoretical work, which required values between 0.25 and 0.5 to model internal wave surfaces in open ocean conditions based on simulating a Garrett-Munk spectrum (GM76) slope of −2. The scale lengths obtained do not fit as well to seismic reflection events as those used in other studies to model internal waves. We suggest two explanations for this discrepancy: (1) due to the fact that the stochastic parameters are derived from the reflectivity field rather than the impedance field the estimated scale lengths may be underestimated, as has been reported; and (2) because the meddy seismic image is a two-dimensional slice of a complex and dynamic three-dimensional object, the derived scale lengths are biased to the direction of flow. Nonetheless, varying stochastic parameters, which correspond to different spectral slopes in the Garrett-Munk spectrum (horizontal wavenumber spectrum), can provide an estimate of different internal wave scales from seismic data alone. We hence introduce Stochastic Heterogeneity Mapping as a novel tool in physical oceanography

The Tyrrhenian Basin formation: from continental rifting to seafloor spreading, followed by mantle exhumation and late fissural volcanism

European Geosciences Union General Assembly 2014 (EGU2014), 27 april - 2 may 2014, Vienna, Austria.-- 1 pageWe present a new interpretation of the configuration of the geological domains and the processes of rifting forming the Tyrrhenian basin based on newly acquired geophysical data. The basin is not presently extending, but its crustal structure preserves information of the temporal evolution of rifting processes. We present P-wave velocity (Vp) models and seismic reflection images of data collected in a two-ship seismic experiment with Spanish R/V Sarmiento de Gamboa and the Italian R/V Urania carried out in spring 2010. We present five several-hundred-km-long wide-angle seismic (WAS) profiles crossing the entire basin at different transects and five Multichannel Seismic Reflection (MCS) profiles coincident with WAS profiles. The 5 transects provide the tectonic structure, the geometry of sedimentary deposits, and the Vp distribution of the crust and upper mantle. This information allows to interpret mechanisms of deformation, define the petrological nature and distribution of the geological domains, infer the importance and potential role of magmatism in the rifting process, and constrain the location of break up and the region of mantle exhumation. The basin has opened from north to south with different extension factors. The northern region stopped opening after a relatively low extension factors, but towards the south extension increased up to full crustal separation that produced first abundant magmatism and subsequently mantle exhumation in another region. Later fissural volcanism followed producing large volcanic ridges and tall seamounts. This sequence of events and the resulting configuration is in stark contrast with predictions based on conventional models of back-arc spreadingPeer Reviewe

A Cross-section of Crustal Domains and Tectonic Structure Across the Central Tyrrhenian Basin: From Back-arc Extension to Mantle Exhumation

European Geosciences Union General Assembly 2014 (EGU2014), 27 april - 2 may 2014, Vienna, Austria.-- 1 pageThe Tyrrhenian Sea constitutes a young, well-preserved example of Mediterranean back-arc oceanic basin. It opened mainly between Tortonian and mid-Pliocene as a response to the E-SE migration of the ApenninesCalabrian subduction system. We present a new interpretation of the crustal affinity and tectonic structure of the central Tyrrhenian basin, which considerably differs from previous ones, from two coincident wide-angle and multi-channel seismic reflection profiles and gravity data acquired in the MEDOC-2010 survey. The basin displays three distinct basement domains with different petrological affinity based on their velocity and velocity-derived density structure. The first domain includes the continental crust of Sardinia and the conjugate Campania margin. In the Sardinia margin extension has thinned the crust from ~20 km under the coastline to ~13 km in ~60 km. Similarly, the Campania margin is also affected by strong extensional deformation. The basement in the second domain, under the Cornaglia Terrace and its conjugate Campania Terrace, appears to be oceanic in nature. It shows differences with respect to the reference young Atlantic oceanic crust while it agrees with that described in back-arc oceanic settings. The high velocity and velocity gradient and the lack of crust-mantle reflections in seismic records of the third domain, which encompasses the Magnaghi and Vavilov basins, indicate that the basement is fundamentally made of exhumed mantle rocks, in accordance with previous observations from hole 651 at Ocean Drilling Program Leg 107. Several large seamounts of the third domain (e.g. Vavilov) are underlain by 10-20-km-wide, relatively low velocity anomalies interpreted as younger magmatic bodies locally intruding the exhumed mantle. We interpret that these domains correspond to different phases of back-arc spreading controlled by the variations on the relative location of the spreading axis and the active volcanic arc due to the migration of the subduction systemPeer Reviewe

Seismogenic zone structure of the southern Middle America Trench, Costa Rica

The shallow seismogenic portion of subduction zones generates damaging large and great earthquakes. This study provides structural constraints on the seismogenic zone of the Middle America Trench offshore central Costa Rica and insights into the physical and mechanical characteristics controlling seismogenesis. We have located ~300 events that occurred following the MW 6.9, 20 August 1999, Quepos, Costa Rica, underthrusting earthquake using a three-dimensional velocity model and arrival time data recorded by a temporary local network of land and ocean bottom seismometers. We use aftershock locations to define the geometry and characteristics of the seismogenic zone in this region. These events define a plane dipping at 19° that marks the interface between the Cocos Plate and the Panama Block. The majority of aftershocks occur below 10 km and above 30 km depth below sea level, corresponding to 30–35 km and 95 km from the trench axis, respectively. Relative event relocation produces a seismicity pattern similar to that obtained using absolute locations, increasing confidence in the geometry of the seismogenic zone. The aftershock locations spatially correlate with the downdip extension of the oceanic Quepos Plateau and reflect the structure of the main shock rupture asperity. This strengthens an earlier argument that the 1999 Quepos earthquake ruptured specific bathymetric highs on the downgoing plate. We believe that subduction of this highly disrupted seafloor has established a set of conditions which presently limit the seismogenic zone to be between 10 and 35 km below sea level

Seismogenic zone structure beneath the Nicoya Peninsula, Costa Rica, from three-dimensional local earthquake P- and S-wave tomography

The subduction plate interface along the Nicoya Peninsula, Costa Rica, generates damaging large (Mw > 7.5) earthquakes. We present hypocenters and 3-D seismic velocity models (VP and VP/VS) calculated using simultaneous inversion of P- and S-wave arrival time data recorded from small magnitude, local earthquakes to elucidate seismogenic zone structure. In this region, interseismic cycle microseismicity does not uniquely define the potential rupture extent of large earthquakes. Plate interface microseismicity extends from 12 to 26 and from 17 to 28 km below sea level beneath the southern and northern Nicoya Peninsula, respectively. Microseismicity offset across the plate suture of East Pacific Rise-derived and Cocos-Nazca Spreading Center-derived oceanic lithosphere is ∼5 km, revising earlier estimates suggesting ∼10 km of offset. Interplate seismicity begins downdip of increased locking along the plate interface imaged using GPS and a region of low VP along the plate interface. The downdip edge of plate interface microseismicity occurs updip of the oceanic slab and continental Moho intersection, possibly due to the onset of ductile behaviour. Slow forearc mantle wedge P-wave velocities suggest 20–30 per cent serpentinization across the Nicoya Peninsula region while calculated VP/VS values suggest 0–10 per cent serpentinization. Interpretation of VP/VS resolution at depth is complicated however due to ray path distribution. We posit that the forearc mantle wedge is regionally serpentinized but may still be able to sustain rupture during the largest seismogenic zone earthquakes

Integrated analysis of bacterial and microeukaryotic communities from differentially active mud volcanoes in the Gulf of Cadiz

The present study assesses the diversity and composition of sediment bacterial and microeukaryotic communities from deep-sea mud volcanoes (MVs) associated with strike-slip faults in the South-West Iberian Margin (SWIM). We used a 16S/18S rRNA gene based pyrosequencing approach to characterize and correlate the sediment bacterial and microeukaryotic communities from MVs with differing gas seep regimes and from an additional site with no apparent seeping activity. In general, our results showed significant compositional changes of bacterial and microeukaryotic communities in sampling sites with different seepage regimes. Sediment bacterial communities were enriched with Methylococcales (putative methanotrophs) but had lower abundances of Rhodospirillales, Nitrospirales and SAR202 in the more active MVs. Within microeukaryotic communities, members of the Lobosa (lobose amoebae) were enriched in more active MVs. We also showed a strong correlation between Methylococcales populations and lobose amoeba in active MVs. This study provides baseline information on the diversity and composition of bacterial and microeukaryotic communities in deep-sea MVs associated with strike-slip faults

Crustal thickness constraints on the geodynamic evolution of the Galapagos Volcanic Province

Earth and Planetary Science Letters, v. 214, n. 3-4, p. 545-559, 2003. http://dx.doi.org/10.1016/S0012-821X(03)00373-XInternational audienc

Geophysical evidence for hydration of the crust and mantle of the Nazca plate during bending at the north Chile trench

Geology, v. 32, n. 7, p. 549-552, 2004. http://dx.doi.org/10.1130/G20379.1International audienc

Direct temperature and salinity acoustic full waveform inversion

5 pages, 4 figuresRecent work has shown that Full Waveform Inversion could be suitable to extract physical properties such as sound speed (c), density (ρ), temperature (T), and salinity (S) from the weak impedance contrasts associated with the ocean's thermohaline fine structure.The seismic inversion approaches proposed so far are based on the iterative inversion of c from multichannel seismic data, while the rest of parameters (T,S, and ρ) are determined in a second step using two equations of state and a local T-S empirical relationship. In this work, we present an alternative to this approach. Using 1-D synthetic seismic data, we demonstrate that the direct full waveform inversion of T and S using adjoint methods is feasible without the use of any local T-S relationship and that the models of physical properties obtained with this approach are far more accurate than those inferred from c. Key Points T and S can be inverted simultaneously from ocean acoustic data using FWI Local T-S empirical relationships are not required for the inversion Our T and S results have a potential density error of 0.01 kg/m3. © 2013. American Geophysical Union. All Rights ReservedThis work has been fulfilled in the framework of the project POSEIDON (CTM2010-25169) and APOGEO (CTM2011-16001-E/MAR), both funded by the Spanish Ministry of Economy and Competitiveness (MINECO), and the Marie Curie project OCEANSEIS (FP7-PEOPLE-2010-IOF-271936-OCEANSEIS)Peer Reviewe