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Authors: Collettini, C.* 
Cardellini, C.* 
Chiodini, G.* 
De Paola, N.* 
Holdsworth, R. E.* 
Smith, S. A. F.* 
Title: Fault weakening due to CO2 degassing in the Northern Apennines: short- and long-term processes
Journal: Geological Society, London, Special Publications 
Series/Report no.: /299 (2008)
Publisher: Geological Society of London
Issue Date: 2008
DOI: 10.1144/SP299.11
Keywords: CO2 degassing
Northern Apennines
Subject Classification03. Hydrosphere::03.04. Chemical and biological::03.04.05. Gases 
04. Solid Earth::04.02. Exploration geophysics::04.02.01. Geochemical exploration 
Abstract: The influx of fluids into fault zones can trigger two main types of weakening process that operate over different timescales and facilitate fault movement and earthquake nucleation. Short- and long-term weakening mechanisms along faults require a continuous fluid supply near the base of the brittle crust, a condition satisfied in the extended/extending area of the Northern Apennines of Italy. Here carbon mass balance calculations, coupling aquifer geochemistry to isotopic and hydrological data, define the presence of a large flux (c. 12 160 t/day) of deep-seated CO2 centred in the extended sector of the area. In the currently active extending area, CO2 fluid overpressures at 85% of the lithostatic load have been documented in two deep (4–5 km) boreholes. In the long-term, field studies on an exhumed regional low-angle normal fault show that, during the entire fault history, fluids reacted with fine-grained cataclasites in the fault core to produce aggregates of weak, phyllosilicate-rich fault rocks that deform by fluid assisted frictional–viscous creep at sub-Byerlee friction values (m , 0.3). In the short term, fluids can be stored in structural traps, such as beneath mature faults, and stratigraphical traps such as Triassic evaporites. Both examples preserve evidence for multiple episodes of hydrofracturing induced by short-term cycles of fluid pressure build-up and release. Geochemical data on the regional-scale CO2 degassing process can therefore be related to field observations on fluid rock interactions to provide new insights into the deformation processes responsible for active seismicity in the Northern Apennines
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