The role of transfer structures on volcanic activity at Campi Flegrei (Southern Italy)

The Tyrrhenian margin of Central Italy underwent extension during Pliocene and Quaternary. Extension occurred mainly through NW-SE normal faults, bordering a sequence of Plio-Quaternary basins. These basins are offset by coeval NE-SW faults, which show strike-slip and normal motions and have been interpreted as transfer faults. Plio-Quaternary volcanic activity along the margin occurred along a NW-SE belt, systematically in correspondence with NE-SW transverse systems. The Campi Flegrei Volcanic District (CFVD), on the Southern Tyrrhenian margin, consists of an active NE-SW volcanic ridge developed along NE-SW fractures. We performed a structural field analysis with analogue and mechanical models to investigate the role of transverse structures upon volcanism at Campi Flegrei. Field analysis at Campi Flegrei recognized NE-SW and, to a lesser extent, coeval NW-SE active fractures. Analogue experiments have simulated the development of transfer fault systems in brittle extensional domains. The experiments show that subvertical transfer faults connect offset adjacent normal faults dipping 60 degrees. The mechanical model is based on the stress equations in uniaxial lithostatic conditions and absence of regional stresses. It shows how pre-existing subvertical fractures require the smallest magmatic pressures to be penetrated. For a given magmatic pressure, subvertical fractures might be penetrated more deeply, tapping more easily primitive magmas. These results suggest that the CFVD is located along a NE-SW transfer zone connecting NW-SE regional normal faults. Volcanic activity along such NE-SW trend would be induced by the subvertical dip of the transfer faults. The subvertical dip of transfer faults also suggests an explanation for the emission of the more primitive products along NE-SW systems at Campi Flegrei. These considerations find a wider application on the remaining volcanic districts of the margin, located in the same overall structural setting. (C) 1999 Elsevier Science B.V. All rights reserved.