|
Irene Bianchi, Yale University (United States)
Jeffrey Park, Yale University (United States)
Nicola Piana Agostinetti, Istituto Nazionale de Geofisica s Vulcanologia (Italy)
Vadim Levin, Rutgers University (United States)
|
|
The seismic structure of a lithospheric collision zone plays a key role in modeling and testing different tectonic hypotheses, such as the rate and style of subduction. The Northern Apennines orogen (NA) of central Italy is a complex example of a convergent boundary between two continental lithospheres. Deformation in this area is thought to be related to the influence of a retreating slab beneath the Apennines, but is not understood in detail. A crucial issue is what causes surface extension within the orogen, particularly whether extension occurs as a steady-state process or represents a transitional process. A steady-state process is likely if a simple subduction geometry is revealed in seismic imaging; a transient process would be more likely if the seismic structure is complex. We analyze Receiver Functions (RF) using teleseismic data recorded at 48 temporary stations deployed during RETREAT experiment across NA chain. The large data-set (more than 12.000 teleseismic records) reveals new details of the seismic structure of the NA collision zone, suggesting the presence of downgoing lithosphere in the region where sub-crustal earthquake activity is sparse. In this study, we focus attention at a strategic depth range between 20 and 120 km (i.e. where the crust-mantle boundary and the supra-slab mantle wedge develops). We present our data-set in closely spaced profiles across and along the NA chain, to highlight the 3D structure of the subducting interfaces and the related anisotropy that develops as deformation proceeds. A shallow Moho is identified on the Tyrrhenian side of the orogen crest, beneath Tuscany. A deeper Moho is found on the Adriatic side of the orogen crest, and both seismic features associated with these "Moho"-like features overlap under the crest of the orogen. Starting from the orogen crest toward the Tyrrhenian side the deeper "Adriatic Moho" becomes a deep Ps converted phase with a moveout consistent with a dipping slab discontinuity. The Ps conversion amplitude depends strongly on the back-azimuth of the incoming P wave, suggesting anisotropy related to localized shear.
|
