Multichannel 2D seismic reflection study of Mid-Atlantic Ridge: The St. Paul fracture zone region

Abstract

The COLMEIA Project is an international collaboration between France and Brazil involving several universities and research institutes. One of the objectives of this project is to study the tectonic evolution and to understand the processes that result in the exhumation of the ultramafic body which composes the Sao Pedro e Sao Paulo archipelago, as wellasthe magmatism and tectonics of this part of the Mid-Atlantic Ridge, especially for the recent geologic time. The work is essential to understand the evolution of the Equatorial Atlantic Ocean. A fundamental part of thestudy targets the detailed analysis of sites that indicate recent off-axis deformation, since these faults could work as pathways for hydrothermal fluids, which is important to understand hydrothermalism in sediment environments. This paper presents the preliminary results of the analysis of the seismic data acquired during the COLMEIA cruise. The first step was to process the seismic lines in the north part of the area covered during the cruise, where the St. Peter peridotite ridge is located. The software used, Sispeed v5.5, was developed by the IFREMER and available for use at the laboratory. The seismic configuration on the COLMEIA cruise used 24 hydrophones and 2airguns. The value of 1500 m/s was assigned for the velocity of the sound wave on the water and sediments. After processing, interpretation was performed using a specific software called Kingdom 8.5. During the first part of the interpretation, the focus was to identify the most important seismic horizons, such as the basement and major seismic horizons inside the sediment infill. The seismic profiles showed a strongly rugged bathymetry. One can easily note that the area is full of geological depressions, representing fracture zones, forming small, elongated basins. These features, that during millions of years gathered sediments, most of them hemipelagic and volcanogenic, allow to infer the age of each sector of the study area and shows some deformation at their upper part. Some difficulties were faced, due to the strongly rugged bathymetry in the area, during the interpretation of the seismic profiles. Thus, some seismic attributes were used to find where is the true ocean floor. One method used was the envelope, since this seismic attribute represents the instantaneous energy of the signal and its magnitude is proportional to the reflection coefficient.