HR: 1340h
AN: T53C-1443    [Abstracts]
TI: Switches in the Stress Field Induced by Fluid Overpressures Below a Low-Permeability Fault Zone
AU: * Collettini, C
EM: colle@unipg.it
AF: Gruppo di Geologia Strutturale e Geofisica, Department of Earth Sciences, University of Perugia, Piazza dell'Universita', 1, Perugia, 06100 Italy
AU: De Paola, N
EM: depaola@unipg.it
AF: Gruppo di Geologia Strutturale e Geofisica, Department of Earth Sciences, University of Perugia, Piazza dell'Universita', 1, Perugia, 06100 Italy
AU: Goulty, N R
EM: n.r.goulty@durham.ac.uk
AF: Department of Earth Sciences, University of Durham, South Road, Durham, DH1 3LE United Kingdom
AB: The geological structure of Elba comprises thrust sheets stacked during the Apennine orogeny that are cross-cut by later Neogene extensional faults. The Zuccale fault is a gently east-dipping normal fault that offsets part of the thrust stack eastwards. Stratigraphic separations imply an offset of 7-8 km and exhumation of 3-6 km. A complex hydrofracture system exposed in the footwall of Palaeozoic schists and Triassic sediments at Punta di Zuccale consists of three vein sets: two vertical sets trending N-S and E-W and one sub-horizontal. The veins show a crack-and-seal texture and are characterized by mutual crosscutting relationships. The regional stress field throughout the period when the Zuccale fault was active, evidenced by numerous kinematic indicators, was extensional with the minimum principal stress oriented E-W, consistent with the N-S trending set of vertical hydrofractures. We interpret the vertical E-W trending and sub-horizontal hydrofractures as the result of overpressure development, due to exhumation, below the low-permeability, phyllosilicate-rich fault core. In our proposed mechanical model, exhumation reduced the vertical stress and also the horizontal stresses which responded poroelastically. The N-S trending vertical hydrofractures formed when the pore pressure exceeded the minimum principal stress by the tensile strength of the rock. Continued exhumation further reduced the vertical and N-S principal stresses, whilst the E-W principal stress remained equal to the pore pressure, until the pore pressure exceeded the N-S principal stress by the tensile strength of the rock, causing the E-W hydrofractures to form. Yet more exhumation further reduced the vertical stress, while the horizontal stresses remained equal to the pore pressure, until the pore pressure exceeded the vertical stress by the tensile strength of the rock, causing the sub-horizontal hydrofractures to form. The reduced normal effective stress across the fault encouraged fault slip with release of pore water and reduction in overpressure. Continued exhumation allowed the cycle to be repeated, with fractures re-opening and extending due to local variations in the stress tensor.
DE: 8010 Fractures and faults
DE: 8020 Mechanics, theory, and modeling
DE: 8045 Role of fluids
DE: 8164 Stresses: crust and lithosphere
SC: Tectonophysics [T]
MN: Fall Meeting 2005