BRGM

Geophysical modelling of the Catalan magnetic anomaly: study of the crustal structure of the Gulf of Lion margin - Case Study

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Courtesy of BRGM

As part of a research project (the OROGEN project, carried out jointly with TOTAL and INSU-CNRS), BRGM has drawn on datasets from the French Gravimetric Database and the French Airborne Geophysical Database to study and model the geological structure that causes the Catalan magnetic anomaly.

The need

One of the main aims of the OROGEN research project is to improve knowledge of the processes involved in the life cycle of a mountain chain, from its formation to its dislocation during rifting phases, and places particular emphasis on the role of transfer areas. At its eastern end, the Pyrenean mountain range ends abruptly and gives way to the passive margin of the Gulf of Lion. One of the key questions that remains to be answered concerns the nature of the transition between the Pyrenees and the Gulf of Lions margin.

The results

The work carried out, based on the use of data archived in BRGM's geophysical databases, supplemented by data from other organisations, made it possible to produce:

  • the Bouguer anomaly map of the Gulf of Lions, calculated from a compilation of seabed data acquired by the Compagnie Générale de Géophysique (CGG) in 1996, land-based data from the French Gravimetry Database, and the database of the BGI (Bureau Gravimétrique International), for the Spanish side. Data from multiple sea-going campaigns carried out by IFREMER (Institut Français de Recherche pour l'Exploitation de la Mer) and a 1996 ELF exploratory campaign have been added to extend the dataset to include the ocean domain.
  • the magnetic anomaly map reduced to the poles, calculated from an airborne campaign (CGG, 1965), available at BRGM in paper format, digitised as part of the project for enriching the national airborne geophysics database, and then reprocessed for project purposes. This map shows a very strong positive anomaly, called the Catalan anomaly, which, although known for many years, remained very poorly understood.

Using the results

The interpretation of the geophysical maps revealed the main structures of the Gulf of Lion while more detailed work was carried out on the Catalan anomaly. Different geological structures were modelled in 2D (location of the profile: see Figure 1). The gravimetric and magnetic effects calculated for each of these models were then compared with the observed data. Among all the geological hypotheses considered, the one that best fits the geophysical data associates the Catalan anomaly with the presence of a gabbroic magmatic intrusion or subplating.

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