ALBERT

Description: 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.

Description: Extension of the continental lithosphere leads to the formation of rift basins or rifted continental margins if breakup occurs. Seismic investigations have repeatedly shown that conjugate margins have asymmetric tectonic structures and different amount of extension and crustal thinning. Here we compare two coincident wide-angle and multichannel seismic profiles across the northern Tyrrhenian rift system sampling crust that underwent different stages of extension from north to south and from the flanks to the basin center. Tomographic inversion reveals that the crust has thinned homogeneously from ~24 km to ~17 km between the Corsica Margin and the Latium Margin implying a β factor of ~1.3–1.5. On the transect 80 km to the south, the crust thinned from ~24 km beneath Sardinia to a maximum of ~11 km in the eastern region near the Campania Margin (β factor of ~2.2). The increased crustal thinning is accompanied by a zone of reduced velocities in the upper crust that expands progressively toward the southeast. We interpret that the velocity reduction is related to rock fracturing caused by a higher degree of brittle faulting, as observed on multichannel seismic images. Locally, basalt flows are imaged intruding sediment in this zone, and heat flow values locally exceed 100 mW/m2. Velocities within the entire crust range 4.0–6.7 km/s, which are typical for continental rocks and indicate that significant rift-related magmatic underplating may not be present. The characteristics of the pre-tectonic, syn-tectonic and post-tectonic sedimentary units allow us to infer the spatial and temporal evolution of active rifting. In the western part of the southern transect, thick postrift sediments were deposited in half grabens that are bounded by large fault blocks. Fault spacing and block size diminish to the east as crustal thinning increases. Recent tectonic activity is expressed by faults cutting the seafloor in the east, near the mainland of Italy. The two transects show the evolution from the less extended rift in the north with a fairly symmetric conjugate structure to the asymmetric margins farther south. This structural evolution is consistent with W-E rift propagation and southward increasing extension rates.

Description: In this work we investigate the crustal and tectonic structures of the Central Tyrrhenian back-arc basin combining refraction and wide-angle reflection seismic (WAS), gravity, and multichannel seismic (MCS) reflection data, acquired during the MEDOC (MEDiterráneo OCcidental)-2010 survey along a transect crossing the entire basin from Sardinia to Campania at 40°N. The results presented include a ~450 km long 2-D P wave velocity model, obtained by the traveltime inversion of the WAS data, a coincident density model, and a MCS poststack time-migrated profile. We interpret three basement domains with different petrological affinity along the transect based on the comparison of velocity and velocity-derived density models with existing compilations for continental crust, oceanic crust, and exhumed mantle. 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 ~60 km seaward. Similarly, the Campania margin is also affected by strong extensional deformation. The second domain, under the Cornaglia Terrace and its conjugate Campania Terrace, appears to be oceanic in nature. However, it shows differences with respect to the reference Atlantic oceanic crust and agrees with that generated in back-arc oceanic settings. The velocities-depth relationships and lack of Moho reflections in seismic records of the third domain (i.e., the Magnaghi and Vavilov basins) support a basement fundamentally made of mantle rocks. The large seamounts of the third domain (e.g., Vavilov) are underlain by 10–20 km wide, relatively low-velocity anomalies interpreted as magmatic bodies locally intruding the mantle.

Description: Subduction erosion has dominated the evolution of the north Chile convergent continental margin since at least the Mesozoic. We investigate the structure of the Antofagasta (23°S) sector of this margin along a transect using coincident wide-angle and near-vertical seismic profiling and gravity data. A 2-D velocity field of the overriding and subducting plates was obtained using joint refraction and reflection travel time tomography. A velocity-derived density distribution was used to model marine gravity data and substantiate the velocity model. The gravity and velocity models imply that the overriding plate is mainly made of arc-type igneous basement. The upper plate is constructed of two main rock bodies separated by a subhorizontal layer defined by a velocity inversion, the top coincident with a reflection in near-vertical seismic images. The seismic boundary is interpreted as a detachment separating an upper extended domain with large-scale normal faulting from a lower domain apparently undergoing a different type of deformation. Velocity-derived porosity indicates that the front of the margin is probably fluid-saturated and disaggregated by fracturation as a consequence of frontal subduction erosion. Fluids carried into the subduction channel within slope debris filling underthrusting grabens reduce basal friction and probably induce hydrofracturing and basal erosion along the underside of the overriding plate. At depths greater than ∼20 km, porosity and density values imply that most fluids have been exhausted and the lower part of the upper plate is structurally coherent and little fractured. The change in physical properties leads to increased mechanical coupling along the plate boundary and occurs at the updip limit of the distribution of aftershocks of the 1995 Antofagasta earthquake (M w = 8.0) defining the seismogenic zone.

Description: The seismic reflection profile is the convolution of the seismic acquisition impulse response and the target impedance contrasts. In the ocean, these contrasts are mainly determined by the widths and gradients of the temperature transitions between the different water masses. Hence seismic oceanography profiles are sensitive to the frequency bandwidth of the seismic acquisition system. We tested a novel seismic source that allowed us to simultaneously profile the ocean with differing impulse responses. We show that frequencies ∼20 Hz are best to delineate large impedance contrasts that occur over a vertical scale of several tens of meters whereas frequencies ∼80 Hz image the boundaries of layers of around 10 m. We demonstrate a towed acquisition system that can operate from a research vessel to give a bandwidth from 10 to 120 Hz that could, if required, be modified to provide frequencies up to 200 Hz.

Description: The Cocos and Malpelo Volcanic Ridges are blocks of thickened oceanic crust thought to be the result of the interaction between the Galapagos hot spot and the Cocos‐Nazca Spreading Center during the last 20 m.y. In this work we investigate the seismic structure of these two aseismic ridges along three wide‐angle transects acquired during the Panama basin and Galapagos plume—New Investigations of Intraplate magmatism (PAGANINI)‐1999 experiment. A two‐dimensional velocity field with the Moho geometry is obtained using joint refraction/reflection travel time tomography, and the uncertainty and robustness of the results are estimated by performing a Monte Carlo‐type analysis. Our results show that the maximum crustal thickness along these profiles ranges from ∼16.5 km (southern Cocos) to ∼19 km (northern Cocos and Malpelo). Oceanic layer 2 thickness is quite uniform regardless of total crustal thickness variations; crustal thickening is mainly accommodated by layer 3. These observations are shown to be consistent with gravity data. The variation of layer 3 velocities is similar along all profiles, being lower where crust is thicker. This leads to an overall anticorrelation between crustal thickness and bulk lower crustal velocity. Since this anticorrelation is contrary to crustal thickening resulting from passive upwelling of abnormally hot mantle, it is necessary to consider active upwelling components and/or some compositional heterogeneities in the mantle source. The NW limit of the Malpelo Ridge shows a dramatic crustal thinning and displays high lower crustal velocities and a poorly defined crust‐mantle boundary, suggesting that differential motion along the Coiba transform fault probably separated Regina and Malpelo Ridges.

Description: The West Iberia margin is the focus of intense research since the 1980s, with some of the most exemplary geophysical cross-sections and drilling expeditions. Those data sets have been used to create conceptual models of rifting used as a template to interpret margins worldwide. We present two collocated ∼350 km long lines of multi-channel seismic (MCS) streamer data and wide-angle seismic (WAS) data collected across the Tagus Abyssal Plain (TAP). We use travel-times of first arrivals identified at WAS and reflected seismic phases identified at both WAS and MCS records to jointly invert for the P wave velocity (Vp) distribution and the geometry of a sediment unconformity, the top of the basement, and the Moho boundary. The Vp model shows that the TAP basement is more complex than previously inferred, presenting abrupt boundaries between five domains. Domain I under the foot of the slope and Domain III under the abyssal plain display Vp values and gradients of thin continental crust. In between, Domain II displays a steep Vp gradient and high Vp values at shallow depth that support that basement is made of exhumed partly serpentinized mantle. Domain IV and Domain V, further oceanward, have oceanic crust Vp structure. The new results support an unanticipated complex rift history during the initial separation of Iberia and America. We propose a geodynamic scenario characterized by two phases of extension separated by a jump of the locus of extension, caused by the northward propagation of the oceanic spreading center during the J-anomaly formation, which terminated continental rifting.