GNGTS 2014 - Atti del 33° Convegno Nazionale

Gravity anomaly constraints for a crustal model of the Northern Apennines C. Girolami, C. Pauselli, M.R. Barchi Dep. of Physics and Geology, University of Perugia, Italy Introduction. In the last 50 years the structure of the deep crust and of the upper mantle of the northern Apennines was intensely investigated through different geophysical surveys, such as seismic refraction experiments (e.g. Scarascia et al. , 1994; Ponziani et al. , 1995), NVR deep seismic profiles (Pialli et al. , 1998; Pauselli et al. , 2006), analysis of the teleseismic waveforms (Piana Agostinetti et al. , 2002; Mele and Sandvol, 2003) and, more recently, passive seismic tomography (Cannarate et al. , 2013). Most of these surveys are focussed on a WSW-ENE trending transect, approximately following the alignment Grosseto-Perugia-Ancona. All these models agree on the general picture, highlighting the presence of a relatively thin Tyrrhenian crust, contrasting a thicker Adriatic crust, separated by a sharp Moho step beneath the Perugia area. In more detail, however, the model also shows significant differences in the attitude and depth of the two crustal domains: in particular the attitude of the Adriatic Moho [see Barchi et al. (2006) for a discussion]. Along the same transect, a complex geological and geophysical data-set has been progressively made available, mainly consisting of surface geology survey, commercial seismic profiles and well data, highlighting the details of the relatively shallow, upper crust structures (e.g. Pialli et al. , 1998; Brogi and Liotta, 2006). In this paper we present the preliminary results of a research project, aimed to propose new gravimetric modelling of both deep and surface structures of the northern Apennines. In the past, different Authors have already proposed gravimetric models of this region, at both regional (e.g. Larocchi et al. , 1998; Scarascia et al. , 1998; Marson et al. , 1998) and local (e.g. Di Filippo and Toro, 1980; Orlando et al. , 1991; Barberi et al. , 1994) scale. The novelty of our project is essentially based on: - the use gravimetric data as an independent data-set, aimed to compare the suitability of the different available crustal models; - the integration of both shallow and deep structures along regional transects. The studied area is located in the southern part of the northern Apennines, crossing Tuscany, Umbria and Marche regions. The northern Apennines consists of different tectonic units, pertaining to both oceanic (Thetys) and passive margine (Adria) environments. Top to bottom we find Liguride, Tuscan and Umbria-Marche units, covered by syn-orogenic units consisting of turbidites, deposited in foreland basins. The present-day crustal structure of the northern Apennines envisages two different crustal domains: a western Tyrrhenian domain, where extensional deformation destroyed the pre- existing compressional belt; and an eastern Adriatic domain where the compressional structures are still preserved. The upper crust of the Tyrrhenian domain is thinned by a set of east-dipping low-angle normal faults, driving the onset and evolution of the syn-tectonic hinterland basins. The age of the syn-rift deposits testifies the regular eastward migration of the extensional deformation. The shallow structures of the Adriatic domain correspond to the arc-shaped Umbria-Marche fold and thrust belt where the timing of deformation is marked by the onset and evolution of syntectonic foreland basins. The transition between these two domain is sharp and occurs along a narrow and arcuate strip that define the “transition zone” (Barchi, 2010). Characteristics of the Data-set and location of the transect. For this project we use a large data set, already published by ISPRA, in their “Carta gravimetrica digitale dell’Italia alla scala 1:250000” (Ferri et al. , 2006). The original data points were kindly made available by ENI. The data set is made of 50000 stations, which cover central Italy with an average spacing less than 1 km (Fig.1). Bouguer gravity anomaly was calculated using a constant density of 2.67 150 GNGTS 2014 S essione 3.2