An ancient exhumed analogue of microseismically active weak low-angle normal faults: the Zuccale detachment, Elba

C. Collettini1, M. R. Barchi1 and R. E. Holdsworth2

1Dipartimento di Scienze della Terra Università degli Studi di Perugia, Italy

2Reactivation Research Group, Dept. of Geological Sciences, University of Durham, DH1 3LE, UK

colle@unipg.it

Seismic reflection profiles and detailed seismogenic surveys reveal the presence of a regional low-angle (~ 20°) normal fault (LANF) in the Umbria region, the seismically active area of the Northern Apennines in Italy. The upper Pliocene-Quaternary displacement of this fault is 2 km suggesting a long-term slip rate in the region of 1 mm/yr. No historical or instrumental moderate to large earthquakes are associated with this fault and the observed level of microseismicity suggests that a significant component of the fault displacement must be accommodated aseismically. The active fault lies in an area characterised by high fluxes of mantle-derived CO2 and produces abundant microseismicity, M<3, at depths between 3-14 km under a regional stress field characterised by a vertical s1 that lies at high angles to the LANF. This relationship suggests that the active fault may be weak, but the slip behaviour and possible weakening mechanisms operating along the fault cannot be observed directly and are therefore uncertain.

One way of addressing this problem is to study older LANFs – ideally those belonging to the same system - where deeper fault segments representative of those currently buried at depth in Umbria are now exposed at the surface. Fortunately, this is possible in the Northern Apennines where, during the past 18 Ma, the locus of extensional tectonics has moved from the Tyrrhenian sea eastward into the Northern Apennines. As a result, low-angle east-dipping normal faults now exhumed in the Tyrrhenian islands and Tuscany, although active deformation continues to the E in the main Apenninic chain.

Here we present the initial results of a detailed study of a regional exhumed LANF – the Zuccale detachment - cropping out in the Isle of Elba, North Tyrrhenian sea. The fault emplaces upper Cretaceous flysch over Palaeozoic basement schists, dips shallowly E (5-15°) and has an estimated down-dip displacement of 7 km. The fault geometry and kinematics have been investigated at Punta di Zuccale by constructing detailed structural maps, and structural logs of fault rock distribution and textures. The fault is characterised by extremely several metres of foliated carbonate-rich mylonite that appear to be derived from shearing of vein material emplaced into the fault zone during early hydrofracturing events. Variably sheared sets of later cross-cutting veins are abundant both in the fault zones and adjacent wall rocks where they exhibit clear crack-seal textures and local vuggy infills indicating that fluid overpressures were sufficient to sustain fluid filled cavities, at least during the later stages of fault movement. Our preliminary observations point to a long history of fluid-overpressure, hydrofracture and embrittlement along the Zuccale detachment. The mylonitization of the early vein material conceivably enhances the impermeable character of the fault zone and suggest a slip model in which aseismic movements occur on a slow (slip rate < 1 mm/yr) weak viscous fault interdispersed with small seismic ruptures due to short-lived cyclic build-ups in fluid overpressure followed by hydrofracture and veining.