Abstract
The project Retreating-trench, extension, and accretion tectonics, RETREAT, is a multidisciplinary study of the Northern Apennines (earth.geology.yale.edu/RETREAT/), funded by the United States National Science Foundation (NSF) in collaboration with the Italian Istituto Nazionale di Geofisica e Vulcanologia (INGV) and the Grant Agency of the Czech Academy of Sciences (GAAV). The main goal of RETREAT is to develop a self-consistent dynamic model of syn-convergent extension, using the Northern Apennines as a natural laboratory. In the context of this project a passive seismological experiment was deployed in the fall of 2003 for a period of three years. RETREAT seismologists aim to develop a comprehensive understanding of the deep structure beneath the Northern Apennines, with particular attention on inferring likely patterns of mantle flow. Specific objectives of the project are the crustal and lithospheric thicknesses, the location and geometry of the Adriatic slab, and the distribution of seismic anisotropy laterally and vertically in the lithosphere and asthenosphere. The project is collecting teleseismic and regional earthquake data for 3 years. This contribution describes the RETREAT seismic deployment and reports on key results from the first year of the deployment. We confirm some prior findings regarding the seismic structure of Central Italy, but our observations also highlight the complexity of the Northern Apennines subduction system.
Highlights
IntroductionSyn-orogenic and late orogenic extension has been recognized in many convergent orogens, including the Himalayas (Burchfiel et al, 1992; Molnar et al, 1993), the Cyclades of the Aegean (Lister et al, 1984), the Hellenic subduction wedge (Jolivet et al, 1996), the Central Range of
Such anisotropy suggests strongly sheared rock beneath the nominal Moho. If this interpretation is maintained after analysis of the larger RETREAT data set, the SCUR Receiver Functions (RFs) may support the suggestion of Lucente and Speranza (2001) that the retreating slab is locked at the Alpine-Apennines transition in Liguria, but that detachment of the lithosphere beneath a 30-km orogenic crust has begun to occur
The seismic stations deployed in the RETREAT project are collecting waveforms which densely sample the Northern Apennines subduction system
Summary
Syn-orogenic and late orogenic extension has been recognized in many convergent orogens, including the Himalayas (Burchfiel et al, 1992; Molnar et al, 1993), the Cyclades of the Aegean (Lister et al, 1984), the Hellenic subduction wedge (Jolivet et al, 1996), the Central Range of. Margheriti et al (2003) reports a shift in shear-wave splitting behaviour south of this transition, with the locus of orogen-parallel fastpolarization shifting toward the Tyrrhenian Sea. The absence of fast velocity anomaly in the Central Apennines may be interpreted as a tear in the slab (Lucente and Speranza, 2001). Tomographic images show low velocity in the uppermost mantle beneath the Apennines crest. This might be interpreted as asthenospheric wedge above the slab (Di Stefano et al, 1999; Piromallo and Morelli, 2003). Anisotropy beneath Italy has been identified and studied by several techniques, both with P and S phases (Babuska and Plomerová, 1992; Mele et al, 1998; Plomerová et al, 1998; Margheriti et al, 2003; Civello and Margheriti, 2004; Plomerová et al, 2006). Some RETREAT data confirm prior findings but other data deny some of the starting hypotheses about the Northern Apennines subduction system
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