Gravity anomalies and crustal signature of volcano-tectonic structures of Pico Island (Azores)

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Pico Island is the youngest (< 300 ka) island of the Azores archipelago, dominated on its western sector by the stratovolcano of Pico Mountain, which rises about 3.5 km above the surrounding ocean floor. The other two major morpho-tectonic units of the island include the shield volcano of Topo, and the "Achada Plateau," a 29 km long volcanic ridge, with a WNW-ESE trend. A gravimetric study on Pico Island, coupled with geological and volcanological data, was aimed towards a better understanding of its deep volcano-tectonic structures. From a total of 407 stations, a regional model of density contrasts was developed in order to reproduce the crustal structure of the island. In addition a local high-resolution model of the sub-surface structure of the westernmost part of the island is presented. Both models were obtained by means of an inversion global adjustment, on fixed density contrasts in order to determine the three-dimensional geometry of the sources of the anomalous gravity field. The most prominent anomalous gravity structure found in Pico Island is a body with positive density contrast located on the SE slopes of Topo volcano. This anomaly extends to depth of approximately 8 km (mean depth of 4.5 km) and has a volume of 310 km(3). This anomaly maybe best interpreted by the presence of a major intrusive system beneath Topo volcano and can be also correlated to gravitational collapses at the shield volcano. A second body of positive density contrast which appears to extend to 4-5 km depth is located near the town of Madalena. This anomaly may be correlated to a high-density anomaly recently detected in the NE sector of Faial Island. Smaller sized high-density bodies at shallow depths of around 1.3 km are located beneath Pico Mountain volcano and all together define a semi-circular arc with an average diameter of 8 km. We propose that this arcuate structure might outline a ring fracture associated with the collapse of the central sector of Pico Mountain volcano early in its growth. Low-density bodies are located on the NW flanks of Pico Mountain, along a linear ENE trending structure. These bodies extend to depths of 4 km and have a total volume of 69 km(3). They are interpreted as being associated with an "old" extensional fracture zone. Previous studies suggested the existence of a shallow and large magma reservoir (with volumes up to 1000 km(3)) beneath Pico Mountain volcano. In this study we have not found any evidence of the existence of such huge magma body. On the contrary, we propose that those discrete low-density bodies correspond to complex intersections of different sets of tectonic lineaments, along which magma tends to intrude, as shallow (2-5 km) and small (10-20 km(3)) magma pockets. (c) 2006 Elsevier B.V. All rights reserved.
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