Abstract

Structural style, thermo-mechanic state and seismogenic role of the Adriatic thrust: data and hints from an overview of the CROP 03 seismic profile and seismological data. Controversial geological interpretations of the Punta Ala-Gabicce CROP 03 near vertical seismic reflection profile are given in the literature (Barchi et alii, 1998; Decandia et alii, 1998; Finetti et alii, 2001). For the Apennine-outer Marche portion of the seismic line, the interpretations mainly differ in suggesting a thick-skinned or a thin-skinned style of the deformation. By re-analysing the line reflectors, comparing the previous interpretations and also considering information from the closely parallel Piombino-Ancona seismic refraction profile (DSS' 78, Ponziani et alii, 1995), a revised geometry is proposed. This is substantially coherent with the thick-skinned model and points out the presence of a SW-dipping thrust zone, not previously identified, here named the Adriatic Thrust (TA). The TA is the outermost thrust at the front of the Northern Apennines fold-and-thrust belt system. It emerges along the Po Plain-Adriatic front, cross-cuts the entire crust and dislocates its base in a position external with respect to the lower crust discontinuities shown in previous papers. In order to evaluate a possible seismogenic role of the TA, the historical and instrumental seismicity occurring in the coastal Adriatic and Marche-Romagna pede-Apennine areas is analysed. The down-dip geometry of the TA is compared with the depth-distribution of well-located minor seismic events (data from Collettini et alii, 1997; Boncio et alii, 1999 and 2000) occurring within a half-width of 30 km along the trace of the DSS'78 profile. The earthquake distribution shows that the TA is presently undergoing deformation. The brittle deformation is concentrated preferentially within the upper crust (<10 km) and within the uppermost lower crust (15 to 25 km). Less frequent earthquakes also occur at larger depths, specifically close to the crust-mantle boundary (35 to 45 km) and within the mantle lithosphere (55 to 70 km). Down to a depth of about 45-50 km, the TA down-dip mechanical layering, here reconstructed by yield strength envelope, fits the earthquake distribution. At larger depths, the deformation mechanism is dominated by plastic flow which is expected to be almost aseismic. Nevertheless, a strong correlation between the subcrustal hypocenter pattern and the deep geometry of the TA shear zone is observed to a depth of about 70 km. This suggests that minor infrequent ruptures may take place within the Adriatic continental lithosphere that is deforming primarily by plastic processes. The brittle failure of the continental lithosphere may be favoured by localized focusing of shear strain along the TA in association with the strong enrichment of the mantle of Italy in hydroxyl-CO <sub>2</sub> rich volatiles. In conclusion, the TA is interpreted as a still active SW-dipping lithospheric-scale thrust zone sited at the outer front of the Northern Apennines. It is responsible for both the shallow seismicity of the Marche coastal and adjacent off-shore region and for the intermediate-depth seismicity beneath the Apennines. Thus, the latter is not controlled by subduction, as commonly stated, but by reverse faulting of the continental lithosphere.

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