S U M M A R YWe 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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Azorean lava-tubes and volcanic pits adequately sampled for arthropod fauna were evaluated for species diversity and rarity. An iterative partial multiple regression analysis was performed to produce a multi-criteria index (Importance Value for Cave Conservation, IV-CC) incorporating arthropod species diversity indices but also including indices qualifying cave geological and management features (e.g., the diversity of geological structures, threats, accessibility). Additionally, we calculated complementarity solutions (irreplaceability and Fraction-of-Spare measures) for each cave with different targets per species, i.e., the minimum number of caves needed for each species to be represented either once or twice. Our results clearly show that to preserve all troglobiont arthropods endemic to the Azores, it is crucial to protect several caves per island. As many as 10 and 15 caves are needed to include one or two occurrences, respectively, per species.

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The Ferraria thermal water emerges at the sea level in the Ferraria lava delta (western edge of S. Miguel Island, Azores) with temperature of ca. 60°C and pH varying between 5.4 and 6.2. It is of sodium chloride type, resulting from ca. 50% seawater mixing with an acid brackish, at &100°C, denoting the presence of significant CO 2 (g) and the progress of water-rock interactions in open system conditions. The thermal Na-Cl water is strongly enriched with Sr and Mn and, comparatively, has low concentrations in Al, Fe and As. These elements are removed from the solution as critical conditions for the formation of several neo-formed mineral phases are gradually attained. Thermodynamic equilibrium calculations are consistent with this interpretation, showing that the thermal fluid can precipitate Fe 3? -(hydr-)oxides, kaolinite and noncrystalline silica. Wells logging show fracture planes and pores fully/partly filled up with polyphase botryoidal aggregates mostly composed of goethite ? ferrihydrite and displaying variable adsorbed contents of Si, P and As. These neo-formed phases result from the pristine fluid oxidation due to seawater mixing; its precipitation is easily affected by pH and redox variations of the brackish, due to volcanic gases pressure alterations, and fluid pressure or flowvelocity oscillation in the fractured aquifer.