Crustal setting of the Apennines from joint inversion of seismic tomography and magnetic anomaly data: Evidence from L'Aquila fault zone (Italy)

Abstract

High-resolution tomography from the 2009 L'Aquila extensional seismic sequence has shown that the Mw 6.1 main shock and most of the aftershocks occurred within a high velocity body (6.6≤Vp≤6.8 Km/s), located between depths of 3 and 12 km. The nature of the high Vp-body has remained speculative because the Vp velocities are compatible with different lithologies: exhumed mafic lower crustal rocks, hydrated mantle rocks (serpentinites), or dolomites. We used 3D magnetic anomaly modelling to investigate the nature of the L'Aquila body and the deep crustal setting in the central Apennines. The modelling does not support the presence of mafic rocks with a 30-50% serpentinization degree. Accordingly, we conclude that the high Vp-body may represent non-magnetic upper Triassic and possibly lower Liassic dolomites that have been drilled in neighbouring wells for 2-4 km. This conclusion is also consistent with the lack of a coherent gravity anomaly for the body. We speculate that ultra-thick Triassic dolomites may have been deposited in syntectonic wedges formed at the northern margin of the Ionian Sea, where oceanic spreading occurred in mid-late Triassic times.