Publication: Subsurface geometry and structural evolution of La Gomera island based on gravity data
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2011-02
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We hereby present a new Bouguer gravity map of the La Gomera island (Canarian Archipelago), which is analysed and interpreted by means of a 3-dimensional inversion, in order to contribute to the knowledge of the structural setting of this volcanic island and its evolutionary history. A land gravity data set covering the whole island of La Gomera is used in combination with offshore measurements to achieve a better determination of the gravity field in areas near the coasts. The study of this map let us to shed some light on the hypothesis established about the volcanism of this island. Moreover, it shows the information that is hidden from a geological surface exploration, modelling deep sections of the crust in La Gomera, which have been unknown until now. A first interpretation of the Bouguer gravity anomaly is achieved from 1) the residual gravity map calculated by removing a regional component and 2) the total horizontal gradient of the gravity. These residual and derivative maps allow us to identify the horizontal location and borders of the shallowest gravity sources. This provides a useful tool to study the structures associated to the latest periods of the volcanism in the area. Moreover, the information so obtained supports the hypothesis about the migration of volcanic activity towards the south of the island. Subsequently, an inversion process is carried out looking for the 3D-modelisation of the sources of the observed gravity field, which provides a comprehensive view of the structures in volcanic environments. The inversion technique used is based on a genetic algorithm (GA) applied upon a prismatic partition of the subsoil volume, and adopting a priori values of density contrast (positive and negative). The main advantage of this method is that let us to model deep and shallow bodies which exhibit very different geometries and density contrasts. So, results indicate that this inversion strategy can be very effective for characterization of volcanic structures, improving the information from previous geologic and volcanologic studies. The inversion model obtained shows correlation between several sources of the gravity field and the volcanic units associated with the growth of La Gomera Island. The main gravity source of this model is associated with the oldest unit, called the Basal Complex. This unit corresponds to the first submarine growth stage and it is modelled as the most important and deepest high density structure. According to previous geological studies, the following edifice (Old Edifice) was also submarine in its initial phases, later being represented by a wide basaltic shield volcano. The original location and morphology of this Old Edifice is deduced from the distribution of positive density contrasts that appears in the model. Moreover, other gravity field sources are identified and associated to several feeding systems of this stage of the volcanism in La Gomera. The shallowest sections of the model let us recognise the distribution of light material inside the Vallehermoso caldera, surrounded by high density structures. This gives us some insight into the internal structure and morphology of the caldera, pointing to a vertical collapse origin followed by erosion and other destructive processes. Finally, other conclusions are obtained from the correlation found between the sources of the gravity field and the migration of the volcanic activity towards the southern area of the island.
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