The geometries and kinematics of the Ancona - M.Conero - Adriatic structures and the June-August 2013 seismic sequence

MENICHETTI, M.; ESTEBAN, F. D.

Milano

Encuentro; Riunione Anuala Gruppo Italiano Geologia Strutturale; 2013

The aim of this study is to provide a preliminary interpretation of the earthquake-fault associations in the Ancona - M.Conero Ð Adriatic seismic sequence area. The M. Conero anticline, where Meso - Cenozoic Umbria- Marche stratigraphic succession outcrops, is the easternmost structure.The area is characterized by an arcuate complex of fold-and-thrust structures with NW-SE axial directions; subvertical N-S trending strikeslip faults connect the thrust sheets and rotate the axial directions. These compressional structures are part of the most external Apennine thrust front of the Adriatic foreland.The Conero anticline is asymmetric with a subvertical NE limb, detached over a sole thrust rooted in the pre-Triassic basement. Shear zone related to the thrusts and strike-slip fault planes outcrop in different sectors of the structure with an offset of a few kilometers. The shear zones are characterized by cataclastic deformation, with calcite veins indicatingthe important role played by fluid flow. The kinematic analysis of these structures indicated a NE-SW shortening direction. An interpretation of the several seismic reflection profiles available (VIDEPI - http://unmig.sviluppoeconomico.gov.it/videpi/) has allowed to link andconstrain the geometries of the on/offshore structures in the whole Conero area. The deep structures observable in these seismic reflection profiles support the classic thin-skinned style interpretations adopted by several A. for the Northern Apennines.The M.Conero anticline overthrusts the external offshore syncline along a high angle surface for a few kilometers. The upper part of the thrust's staircase trajectory, splays into shallow minor structures where wrench and compressional structures involve Messinian and Plio-Pleistocene foredeep sediments. Towards the South, the anticline is affected by shallow low-angle crestal thrusts rooted in the Messinian and Plio-Pleistocene sediments. These compressional features involve the upper part of the Upper Pliocene and possibly the Quaternary stratigraphic sequence. Many of these structures seem to be associated with a shallow intrusive body, as also supported by magnetic and gravimetric data.The Adriatic thrust front is a complex structure formed during the evolution of the Northern Apennines in the latter stage of the Neogene compressive phase, involving the Plio-Pleistocene syntectonic sediments. The activity of the structures is well-documented by diffuse crustal seismicity from the Po valley to the Central sector of the Adriatic Sea.Historical seismicity shows that, at least in the upper crust, the compression is still active in different sectors of the compressive front. The 2012 Ferrara Arc earthquakes highlight the activity of these compressional structures in the northern sector of the Apennines.The Ancona 1972 long seismic swarm occurred in the upper crust (<15km deep) and the available focal mechanisms show a prevalent ENEWSW strike-slip component.The June- August 2013 seismic sequence also occurred in the upper crust (<15km), about 20 km SE of the 1972 events. The three main shocks, July 21 Mw 4.9 and D=10.1 km, July 21 Mw 4.2 and D=8.4 km and August 22 M 4.4 and D=7.9 km (from ISIDe database at http://iside.rm.ingv.it/iside/standard/index.jsp), occurred in the offshore M.Conero area and have focal mechanisms indicating both compressional and strike slip components in correspondence to activated structures. The integration of geological field and geophysical data has allowed to formulate a new picture of the tectonic framework of the area identifying the tectonically active structures.