Constrains on the structure of Maio Island (Cape Verde) by a three-dimensional gravity model: Imaging partially exhumed magma chambers

We propose a 3-D gravity model for the volcanic structure of the island of Maio (Cape Verde archipelago) with the objective of solving some open questions concerning the geometry and depth of the intrusive Central Igneous Complex. A gravity survey was made covering almost the entire surface of the island. The gravity data was inverted through a non-linear 3-D approach which provided a model constructed in a random growth process. The residual Bouguer gravity field shows a single positive anomaly presenting an elliptic shape with a NWSE trending long axis. This Bouguer gravity anomaly is slightly off-centred with the island but its outline is concordant with the surface exposure of the Central Igneous Complex. The gravimetric modelling shows a high-density volume whose centre of mass is about 4500 m deep. With increasing depth, and despite the restricted gravimetric resolution, the horizontal sections of the model suggest the presence of two distinct bodies, whose relative position accounts for the elongated shape of the high positive Bouguer gravity anomaly. These bodies are interpreted as magma chambers whose coeval volcanic counterparts are no longer preserved. The orientation defined by the two bodies is similar to that of other structures known in the southern group of the Cape Verde islands, thus suggesting a possible structural control constraining the location of the plutonic intrusions.

Deep crustal structure across a young passive margin from wide-angle and reflection seismic data (The Sardinia Experiment) - I. Gulf of Lion's margin

The conjugate margins system of the Gulf of Lion and West Sardinia (GLWS) represents a unique natural laboratory for addressing fundamental questions about rifting due to its landlocked situation, its youth, its thick sedimentary layers, including prominent palaeo-marker such as the MSC event, and the amount of available data and multidisciplinary studies. The main goals of the SARDINIA experiment, were to (i) investigate the deep structure of the entire system within the two conjugate margins: the Gulf of Lion and West Sardinia, (ii) characterize the nature of the crust, and (iii) define the geometry of the basin and provide important constrains on its genesis. This paper presents the results of P-wave velocity modelling on three coincident near-vertical reflection multi-channel seismic (MCS) and wide-angle seismic profiles acquired in the Gulf of Lion, to a depth of 35 km. A companion paper [part II Afilhado et al., 2015] addresses the results of two other SARDINIA profiles located on the oriental conjugate West Sardinian margin. Forward wide-angle modelling of both data sets confirms that the margin is characterised by three distinct domains following the onshore unthinned, 33 km-thick continental crust domain: Domain I is bounded by two necking zones, where the crust thins respectively from 30 to 20 and from 20 to 7 km over a width of about 170 km; the outermost necking is imprinted by the well-known T-reflector at its crustal base; Domain II is characterised by a 7 km-thick crust with anomalous velocities ranging from 6 to 7.5 km/s; it represents the transition between the thinned continental crust (Domain I) and a very thin (only 4-5 km) "atypical" oceanic crust (Domain III). In Domain II, the hypothesis of the presence of exhumed mantle is falsified by our results: this domain may likely consist of a thin exhumed lower continental crust overlying a heterogeneous, intruded lower layer. Moreover, despite the difference in their magnetic signatures, Domains II and III present the very similar seismic velocities profiles, and we discuss the possibility of a connection between these two different domains.

Deep crustal structure across a young passive margin from wide-angle and reflection seismic data (The Sardinia Experiment) - II. Sardinia's margin

Geophysical data acquired on the conjugate margins system of the Gulf of Lion and West Sardinia (GLWS) is unique in its ability to address fundamental questions about rifting (i.e. crustal thinning, the nature of the continent-ocean transition zone, the style of rifting and subsequent evolution, and the connection between deep and surface processes). While the Gulf of Lion (GoL) was the site of several deep seismic experiments, which occurred before the SARDINIA Experiment (ESP and ECORS Experiments in 1981 and 1988 respectively), the crustal structure of the West Sardinia margin remains unknown. This paper describes the first modeling of wide-angle and near-vertical reflection multi-channel seismic (MCS) profiles crossing the West Sardinia margin, in the Mediterranean Sea. The profiles were acquired, together with the exact conjugate of the profiles crossing the GoL, during the SARDINIA experiment in December 2006 with the French R/V L'Atalante. Forward wide-angle modeling of both data sets (wide-angle and multi-channel seismic) confirms that the margin is characterized by three distinct domains following the onshore unthinned, 26 km-thick continental crust : Domain V, where the crust thins from 26 to 6 km in a width of about 75 km; Domain IV where the basement is characterized by high velocity gradients and lower crustal seismic velocities from 6.8 to 7.25 km/s, which are atypical for either crustal or upper mantle material, and Domain III composed of "atypical" oceanic crust.The structure observed on the West Sardinian margin presents a distribution of seismic velocities that is symmetrical with those observed on the Gulf of Lion's side, except for the dimension of each domain and with respect to the initiation of seafloor spreading. This result does not support the hypothesis of simple shear mechanism operating along a lithospheric detachment during the formation of the Liguro-Provencal basin.

Deep structure of the Santos basin-São Paulo plateau system, SE Brazil

The structure and nature of the crust underlying the Santos Basin-São Paulo Plateau System (SSPS), in the SE Brazilian margin, are discussed based on five wide-angle seismic profiles acquired during the Santos Basin (SanBa) experiment in 2011. Velocity models allow us to precisely divide the SSPS in six domains from unthinned continental crust (Domain CC) to normal oceanic crust (Domain OC). A seventh domain (Domain D), a triangular shape region in the SE of the SSPS, is discussed by Klingelhoefer et al. (2014). Beneath the continental shelf, a similar to 100km wide necking zone (Domain N) is imaged where the continental crust thins abruptly from similar to 40km to less than 15km. Toward the ocean, most of the SSPS (Domains A and C) shows velocity ranges, velocity gradients, and a Moho interface characteristic of the thinned continental crust. The central domain (Domain B) has, however, a very heterogeneous structure. While its southwestern part still exhibits extremely thinned (7km) continental crust, its northeastern part depicts a 2-4km thick upper layer (6.0-6.5km/s) overlying an anomalous velocity layer (7.0-7.8km/s) and no evidence of a Moho interface. This structure is interpreted as atypical oceanic crust, exhumed lower crust, or upper continental crust intruded by mafic material, overlying either altered mantle in the first two cases or intruded lower continental crust in the last case. The deep structure and v-shaped segmentation of the SSPS confirm that an initial episode of rifting occurred there obliquely to the general opening direction of the South Atlantic Central Segment.

Earthquakes in western Iberia: improving the understanding of lithospheric deformation in a slowly deforming region

Mainland Portugal, on the southwestern edge of the European continent, is located directly north of the boundary between the Eurasian and Nubian plates. It lies in a region of slow lithospheric deformation (< 5 mm yr(-1)), which has generated some of the largest earthquakes in Europe, both intraplate (mainland) and interplate (offshore). Some offshore earthquakes are nucleated on old and cold lithospheric mantle, at depths down to 60 km. The seismicity of mainland Portugal and its adjacent offshore has been repeatedly classified as diffuse. In this paper, we analyse the instrumental earthquake catalogue for western Iberia, which covers the period between 1961 and 2013. Between 2010 and 2012, the catalogue was enriched with data from dense broad-band deployments. We show that although the plate boundary south of Portugal is diffuse, in that deformation is accommodated along several distributed faults rather than along one long linear plate boundary, the seismicity itself is not diffuse. Rather, when located using high-quality data, earthquakes collapse into well-defined clusters and lineations. We identify and characterize the most outstanding clusters and lineations of epicentres and correlate them with geophysical and tectonic features (historical seismicity, topography, geologically mapped faults, Moho depth, free-air gravity, magnetic anomalies and geotectonic units). Both onshore and offshore, clusters and lineations of earthquakes are aligned preferentially NNE-SSW and WNW-ESE. Cumulative seismic moment and epicentre density decrease from south to north, with increasing distance from the plate boundary. Only few earthquake lineations coincide with geologically mapped faults. Clusters and lineations that do not match geologically mapped faults may correspond to previously unmapped faults (e.g. blind faults), rheological boundaries or distributed fracturing inside blocks that are more brittle and therefore break more easily than neighbour blocks. The seismicity map of western Iberia presented in this article opens important questions concerning the regional seismotectonics. This work shows that the study of low-magnitude earthquakes using dense seismic deployments is a powerful tool to study lithospheric deformation in slowly deforming regions, such as western Iberia, where high-magnitude earthquakes occur with long recurrence intervals.

Magma flow pattern in dykes of the Azores revealed by ansiotropy of magnetic susceptibility

The localization of magma melting areas at the lithosphere bottom in extensional volcanic domains is poorly understood. Large polygenetic volcanoes of long duration and their associated magma chambers suggest that melting at depth may be focused at specific points within the mantle. To validate the hypothesis that the magma feeding a mafic crust, comes from permanent localized crustal reservoirs, it is necessary to map the fossilized magma flow within the crustal planar intrusions. Using the AMS, we obtain magmatic flow vectors from 34 alkaline basaltic dykes from São Jorge, São Miguel and Santa Maria islands in the Azores Archipelago, a hot-spot related triple junction. The dykes contain titanomagnetite showing a wide spectrum of solid solution ranging from Ti-rich to Ti-poor compositions with vestiges of maghemitization. Most of the dykes exhibit a normal magnetic fabric. The orientation of the magnetic lineation k1 axis is more variable than that of the k3 axis, which is generally well grouped. The dykes of São Jorge and São Miguel show a predominance of subhorizontal magmatic flows. In Santa Maria the deduced flow pattern is less systematic changing from subhorizontal in the southern part of the island to oblique in north. These results suggest that the ascent of magma beneath the islands of Azores is predominantly over localized melting sources and then collected within shallow magma chambers. According to this concept, dykes in the upper levels of the crust propagate laterally away from these magma chambers thus feeding the lava flows observed at the surface.