Gravimetric determination of an intrusive complex under the Island of Faial (Azores): some methodological improvements |
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Authors: | Antonio G Camacho J Carlos Nunes Esther Ortiz Zilda França Ricardo Vieira |
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Institution: | Instituto de Astronomía y Geodesia (CSIC-UCM), Fac. CC. Matemáticas. Ciudad Universitaria, Madrid 28040, Spain. E-mail:;Universidade dos Açores –Departamento de Geociências, Rua da Mãe de Deus, Apartado 1422, 9501-801 Ponta Delgada, Azores, Portugal |
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Abstract: | We present some improvements of a gravity inversion method to determine the geometry of the anomalous bodies for priori density contrasts. The 3-D method is based on an exploratory process applied, not for the global model, but for the steps of a growth approach. The (positive and/or negative) anomalous structure is described by successive aggregation of cells, while its corresponding gravity field remains nearly proportional to the observed one. Moreover, a simple (e.g. linear) regional trend can be simultaneously adjusted. The corresponding program is applied to new gravity data on the volcanic island of Faial (Azores archipelago). The inversion approach shows a subsurface anomalous structure for the island, the main feature being an elongated high-density body. The body is interpreted as a compact sheeted dyke swarm, emplaced along Faial-Pico Fracture Zone, a leaky transform structure that forms the current boundary between Eurasian and African plates in the Azores area. The new results in this paper are (1) a Bouguer gravity anomaly map, (2) several improvements in the inversion process (robust process, optimal balance fitness/model magnitude), (3) a new gravimetric method for estimating the mean terrain density, (4) a 3-D model for subsurface mass anomalies in Faial and (5) some interpretative conclusions about a main intrusive complex detected under the island as a wall-like structure extending from a depth of 0.5 to 6 km b.s.l., with a N100°E trend and corresponding to an early fissural volcanic episode controlled by the regional tectonics. |
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Keywords: | Faial (Azores) gravity anomalies inverse problem volcanic structure |
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