Integrated geophysical approach for the imaging of the porto azzurro (Elba, Italy) plutons and the associated structures

Category Geophysic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Siniscalchi, Agata; Diaferia, Ida; Liuni, Maria Pina; Loddo, Mariano; Magrی, Cosimo; Moretti, Pierpaolo; Schiavone, Domenico; Tripaldi, Simona
Holding Date 04 October 2008

Elba island, located in the Northern Tyrrhenian Sea, is considered the innermost outcrop of the Northern Appennine orogenetic belt. It is well known for its FE-ore bodies, connected with intrusion of plutons but not all is well known about relationships between complex tectonic evolution and emplacement of magmatic bodies during Mio-Pliocene.
The main goal of the research was to contribute to the geometric typifying of magmatic bodies of this area by means of definition of detailed resistivity and density models.
During this fieldwork Electrical Resistivity Tomographies (ERT), a Magnetotelluric (MT) profile and gravimetry surveys were carried out in Eastern Elba island were the Porto Azzurro pluton outcrops.
Geoelectric survey consisted of 3 profile long 800-900 m executed using different combination on each profile of Dipole-Dipole, Wenner-Schlumberger, Pole-Dipole and Pole-Pole arrays. The first is sensitive to lateral discontinuities; the second to horizontal ones and the last two allow to investigate greater depth in regard to the first two array. This acquisition strategy allows to enhance the resolution power thanks to the higher spatial sensitivity. After checking data goodness, pseudosections were subjected to 2D inversion using RES2DINV software based on the algorithm proposed by Loke (1997). For each profile, a comparison was made between the inversion result of single array data set and the one cohomprensive of multiple arrays data set. This last provides a resistivity image more suitable to the structural reconstruction.
MT profile consisted of 10 remote referenced soundings. The estimate of transfer function have covered an interval of period between 0.008-100 s although, because of the coastal effect, the inversion of data was carried out until a period of 10 s. An estimate of the electric strike versus period was carried out too by using WAL’s decomposition scheme (Weaver et al., 2000). The decomposed MT data were inverted using the regularized algorithm of Rodi and Mackie (2001). Gravimetry survey consisted of 66 stations disposed along 4 profiles. These are disposed in E-W, N-S directions and contain stations with an average distance of 100m. Bouguer anonaly profiles were inverted by a 2D1/2 code.
By considering the whole results, both geophysical parameters (resistivity and density) show a good sensitivity for the characterization of the different lythotype. In particular, the recovered resistivity and density contrasts well image the thermo-metamorphic aureoles around the intrusion of the Porto Azzurro pluton. Moreover the high resolution resistivity image clearly define the geometrical orientation in depth of all the involved features.