Armand Galdeano

The Azores triple junction evolution since 10 Ma from an aeromagnetic survey of the Mid-Atlantic Ridge

Abstract In the past two decades several models have been presented to describe the evolution and the present structure of the Azores Triple Junction. These models were mainly based on morphological analysis of sea bottom topography, sparse magnetic profiling, sidescan sonar surveying over the plateaus and global plate kinematic considerations for the North Atlantic. In this paper we follow a different approach: from a detailed aeromagnetic survey covering both sides of the Mid-Atlantic Ridge between 37°N and 40°30′N the magnetic anomalies up to anomaly 5 are accurately identified, allowing careful modelling of the kinematics of this region for the past 10 Ma and thus establishing a coherent framework for the design of geophysical models for the Azores Triple Junction. The analysis of magnetic anomalies and the use of Fourier domain inversion techniques show that the ridge is made up of six segments, each one varying in length from 50 to 60 km. The more continuous section of the ridge can be defined from the first four northern segments, although the North Azores Fracture Zone right-offsets the ridge at 39°30′N, 29°40′W. The fifth and sixth segments are, respectively, right-offset by the Acor Fracture Zone (at 38°23′N, 30°15′W) and by the Princess Alice Fracture Zone (at 38°00′N, 30°50′W). Anomaly identifications using a two-dimensional model and plate tectonics reconstruction techniques allowed the calculation of rotation pole parameters. The results thus obtained reveal that, at least between anomalies 5 and 3 ( ∼ 10–3.85 Ma), the Azores displayed an independent motion relative to the neighbouring plates and after anomaly 2A (2.45 Ma) the Azores moved attached to the Eurasian plate. The triple junction (Azores-North America-Africa, or Eurasia-North America-Africa) moved northward from ∼ 38°00′N, 30°50′W to ∼ 38°20′N, 30°15′W (between 4 and 3A) and probably to 38°50′N, 30°00′W at anomaly 2A time. A detailed reconstruction model is presented.

Propagation of an accreting plate boundary: a discussion of new aeromagnetic data in the Gulf of Tadjurah and southern Afar

Abstract A detailed aeromagnetic survey of the Republic of Djibouti and immediate surroundings was performed in 1977. This paper summarizes the reduction techniques which are used in order to produce a magnetic anomaly map and discusses the accuracy of this map, which is presented as an insert at a scale of 1/250,000. Two distinct magnetic styles are recognized: linear anomalies with both large amplitude and short wavelength, considered to be typical of oceanic lithosphere, contrast with areas of lower-amplitude longer-wavelength anomalies, which are found mostly in the northern part of the survey. This quiet zone of subdued magnetic style is thought to have undergone major tectonic deformation in the last millions of years. The general morphology of magnetic anomalies is interpreted in terms of a propagating crack model, as proposed by Courtillot [23]. The crack propagates westwards at approximately 3 cm/yr and the crack tip is thought to lie close to Lake Asal, both on the basis of the magnetic data and of other geophysical evidence. The land section of the survey is a central topic of this paper and is interpreted in terms of the crack propagation model in the light of other available geological, geochemical and geophysical data.

Identification of sources of potential fields with the continuous wavelet transform: Complex wavelets and application to aeromagnetic profiles in French Guiana

A continuous wavelet technique has been recently introduced to analyze potential fields data. First, we summarize the theory, which primarily consists of interpreting potential fields via the properties of the upward continued derivative field. Using complex wavelets to analyze magnetic data gives an inverse scheme to find the depth and homogeneity degree of local homogeneous sources and the inclination of their magnetization vector. This is analytically applied on several local and extended synthetic magnetic sources. The application to other potential fields is also discussed. Then, profiles crossing dikes and faults are extracted from the recent high-resolution aeromagnetic survey of French Guiana and analyzed using complex one dimensional wavelets. Maps of estimated depth to sources and their magnetization inclination and homogeneity degree are proposed for a region between Cayenne and Kourou.

New paleomagnetic results from Cretaceous sediments near Lisboa (Portugal) and implications for the rotation of Iberia

Abstract Paleomagnetic study of outer platform sediments of Late Jurassic—Early Cretaceous age from Portugal (Lisboa area) leads to characteristic remanent magnetization directions for the Hauterivian-Barremian ( D = 317°, I = 58°) and the Aptian ( D = 344°, I = 47°). The absolute rotation of Iberia between 125 and 110 Ma is consequently of about 27° in an anticlockwise sense. A later rotation of about 14° follows the first one in the same sense from 110 Ma. The total absolute rotation of Iberia from the Hauterivian age determined by paleomagnetism is thus about 41°. The comparison of declinations measured with paleodeclinations calculated in Lisboa from the Eurasian and African poles of similar age indicates that the main phase of relative rotation of Iberia with respect to Eurasia occurs during the first stage of relative motion (i.e. Hauterivian to Aptian). The tectonic history of the Bay of Biscay is in good agreement with these two stages of rotation. The total relative rotation with respect to Europe is 34°. This value is consistent with results previously established by Van der Voo [2,3,25], but we have established that the main phase of rotation (30°) occurs in the Hauterivian-Aptian interval. The movement of Iberia is clearly linked to the movement of Africa in direction and age, but the amount of rotation of Iberia, during the Early Cretaceous, is much larger and implies that Iberia was not a real part of the African plate.

A new model for the evolution of the volcanic island of Réunion (Indian Ocean)

The island of Reunion has been studied using data from airborne and shipborne magnetic surveys. The subaerial history of Reunion spans the last 2.1 million years. The Brunhes-Matuyama geomagnetic reversal enables differentiation of volcanic rocks older and younger than 0.78 Ma. The lower submarine flanks are poorly magnetized and are interpreted as landslide deposits. The core of the island is composed of highly magnetized rocks. Piton des Neiges volcano is composed mostly of rocks older than 0.78 Ma. Only its western flank and central area include thick piles of younger rocks. Piton de la Fournaise is a highly and normally magnetized edifice, but its northern and eastern flanks are underlain at shallow depth by reversely magnetized formations. The latter are regarded as remnants of Les Alizes volcano, associated with the Grand-Brule hypovolcanic complex. At the Matuyama-Brunhes transition the island was composed of at least two main volcanoes (Piton des Neiges and Les Alizes) and perhaps also of a third old volcano (Takamaka) at the center-north of the island. Piton de la Fournaise grows on the flank of Les Alizes and is a relatively young focus of volcanism. These volcanoes have had successive phases of construction and destruction. The analysis of magnetic anomalies over Reunion was decisive in defining a new coherent model for the evolution of the island.

Application of artificial intelligence for Euler solutions clustering

Results of Euler deconvolution strongly depend on the selection of viable solutions. Synthetic calculations using multiple causative sources show that Euler solutions cluster in the vicinity of causative bodies even when they do not group densely about the perimeter of the bodies. We have developed a clustering technique to serve as a tool for selecting appropriate solutions. The clustering technique uses a methodology based on artificial intelligence, and it was originally designed to classify large data sets. It is based on a geometrical approach to study object concentration in a finite metric space of any dimension. The method uses a formal definition of cluster and includes free parameters that search for clusters of given properties. Tests on synthetic and real data showed that the clustering technique successfully outlines causative bodies more accurately than other methods used to discriminate Euler solutions. In complex field cases, such as the magnetic field in the Gulf of Saint Malo region (Brittany, France), the method provides dense clusters, which more clearly outline possible causative sources. In particular, it allows one to trace offshore the main inland tectonic structures and to study their interrelationships in the Gulf of Saint Malo. The clusters provide solutions associated with particular bodies, or parts of bodies, allowing the analysis of different clusters of Euler solutions separately. This may allow computation of average parameters for individual causative bodies. Those measurements of the anomalous field that yield clusters also form dense clusters themselves. Application of this clustering technique thus outlines areas where the influence of different causative sources is more prominent. This allows one to focus on these areas for more detailed study, using different window sizes, structural indices, etc.

Tectonic framework of the Azores Triple Junction

The lack of accurate and detailed magnetic information has, in the past, limited the development of well constrained models for the plate tectonic evolution of the Azores Triple Junction. An aeromagnetic survey, made possible by the existing airport facilities, has long been desired as it can provide high quality magnetic data, whose homogeneity and coherency is far better than those provided by classic marine surveys. The results presented in this paper concern only a part of the Aeromagnetic Survey conducted by the Portuguese Instituto Nacional de Meteorologia e Geofisica and already allow an improved definition of the basic tectonic boundaries at the central part of the Azores plateau.

An aeromagnetic survey of the southwest of the Western Mediterranean: Description and tectonic implications

Abstract We present an aeromagnetic survey performed in the summer of 1973 in the southwestern Mediterranean, over the Alboran Sea and part of the Algero-Provencal basin; data acquisition and reduction are described and maps of both the total field intensity and anomalies are given. The anomaly map is described in detail and its implications for the tectonic evolution of the two deep basins included in the surveyed area are discussed. We propose that both basins opened simultaneously along a NNW-SSE direction, and that the Alboran basin was later a site of compression, which led to the formation of the Alboran ridge.

Contribution de l'aeromagnetisme a l'etude du golfe de Valence (Mediterranee occidentale)

Abstract We present an aeromagnetic survey of the Gulf of Valencia and the Balearic Islands (western Mediterranean). A total field anomaly map and a map of the anomalies reduced to the pole have been obtained. From these maps, it is apparent that there are two regions of opposing magnetic style: the Balearic archipelago which is magnetically very smooth, and the north Balearic basin (or Gulf of Valencia) where anomalies are in places very intense. From a comparison of these two domains, we conclude that the Valencian basin was created during an extensional tectonic phase.

In-situ magnetostratigraphy: interpretation of magnetic logging in sediments

Abstract The interest of magnetic logging in different geological settings: oceanic crust, sedimentary basins and deep continental crust is discussed. In all these cases, a good estimate of both the magnetic susceptibility and the remanence of the surrounding rocks could yield invaluable information about the geological formations penetrated in boreholes. A calculation of all the components of the magnetic field produced in a borehole by horizontal magnetized layers is presented. The variations of these components are calculated along the axis and off axis through simple geological formations and particularly through a zone of transition of polarity of the earths magnetic field. Both susceptibility and remanence are involved in the interpretation of the field variations recorded when a magnetometer is passed through a borehole drilled in sediments; the separation of these variations is envisaged using magnetic field and susceptibility logs recorded simultaneously. We present an example of magnetic logging in sediments by the measurement of total field intensity. The results are compared with a model calculated from laboratory measurements of the natural remanent magnetization and magnetic susceptibility of cores.