2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 74-6
Presentation Time: 2:45 PM


MAZZOLI, Stefano1, AMOROSO, Ortensia2, ASCIONE, Alessandra1, BIGI, Sabina3, CIOTOLI, Giancarlo4, PARENTE, Mariano1, RUGGIERO, Livio3, TARTARELLO, Maria Chiara3 and ZOLLO, Aldo5, (1)Department of Earth Sciences, Environment and Georesources (DiSTAR), University of Naples ‘Federico II’, Largo San Marcellino 10, Naples, 80138, Italy, (2)AMRA Scarl, Naples, Italy, (3)Department of Earth Sciences, University of Rome Sapienza, Rome, Italy, (4)Institute of Environmental Geology and Geoengineering, National Research Council of Italy, Rome, Italy, (5)Department of Physics, University of Naples ‘Federico II’, Naples, Italy, stefano.mazzoli@unina.it

The southern Apennines fold and thrust belt has been undergoing post-orogenic extension since ca. 400 kyr. Effective decoupling between deep and shallow structural levels is related to the strong rheological contrast produced by a fluid-saturated, clay-rich mélange zone interposed between buried autochthonous carbonates – continuous with those exposed in the Apulian foreland – and the allochthonous units. This mélange zone also acts as a seal preventing the migration of deep-seated aqueous fluids – as well as oil in the Basilicata region, which hosts the largest Europe’s onshore oil fields – towards the surface. The presence of water reservoirs below the mélange zone is shown by a high VP/VS ratio – which is a proxy for densely fractured, fluid-saturated crustal volumes – recorded by seismic tomography within the buried autochthonous carbonates and the top part of the underlying basement. These crustal volumes, in which background seismicity is remarkably concentrated – hence the term ‘earthquake reservoir’ – are fed by fluids rising along the major active faults and are laterally bounded by the faults themselves, which typically act as barriers for cross-fault fluid flow and as conduits for along-fault flow due to the high permeability of their damage zones. High pore fluid pressures, decreasing the yield stress, are recorded by low stress-drop values associated with the earthquakes. On the other hand, the mountain belt is characterized by substantial gas flow to the surface, recorded as both distributed soil gas emissions and vigorous gas vents. We measured a CO2 flux up to 34000 g/m-2 per day at a gas vent, as well as large amounts of He (up to 52 ppm), Rn (up to 228 kBq/m3) and CH4 (up to 5000 ppm). Overpressured CO2 has been proposed as triggering normal fault earthquakes in the Apennines. However the continuous, huge soil gas emission suggests that CO2, although locally accumulated in gas caps at the top of buried carbonate culminations and sealed by the mélange, is not effectively trapped at deeper seismogenic levels, which are instead saturated with aqueous fluids. Fault lubrication processes are driven by water pressurization along mature, inherited and multiply reactivated fault zones, producing a substantial decrease of dynamic friction marked by relatively low mean seismic radiation efficiency.