Brittle–ductile shear zone evolution and fault initiation in limestones, Monte Cugnone (Lucania), southern Apennines, Italy
Published:January 01, 2004
S. Mazzoli, C. Invernizzi, L. Marchegiani, L. Mattioni, G. Cello, 2004. "Brittle–ductile shear zone evolution and fault initiation in limestones, Monte Cugnone (Lucania), southern Apennines, Italy", Flow Processes in Faults and Shear Zones, G. I. Alsop, R. E. Holdsworth, K. J. W. McCaffrey, M. Hand
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The processes of Brittle–ductile shear zone evolution and fault initiation by the coalescence of en echelon arrays of tensile cracks are quantitatively analysed in terms of displacement and temperature conditions at which they took place in very low-grade, well bedded micritic limestones from the southern Apennines, Italy. Three different types of structures are distinguished: (i) conjugate arrays of en echelon, calcite-filled tension gashes, showing extensional shear offsets; (ii) en echelon vein arrays showing incipient development of discontinuous shear-parallel fractures cutting through the tension gashes; and (iii) faulted vein arrays, in which vein array breaching by a continuous, discrete normal fault has occurred. Fluid inclusion microthermometry from vein calcite sampled from the different sets of structures (i) to (iii) above indicates that environmental conditions remained roughly constant during the different stages of vein array evolution and fault development, with average homogenization temperatures from primary fluid inclusions being in the range 130–140°C. Our results show how displacement accumulation and shear strain essentially control vein array evolution by rotation of en echelon tension gashes, fracture linkage and, eventually, fault nucleation, at approximately constant temperature.
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Flow Processes in Faults and Shear Zones
Faults and their deeper level equivalents, shear zones, are localized regions of intense deformation within the Earth. They are recognized at all scales from micro to plate boundary, and are important examples of the nature of heterogeneous deformation in natural rocks. Faults and shear zones are significant as they profoundly influence the location, architecture and evolution of a broad range of geological phenomenao The topography and bathymetry of the Earth’s surface is marked by mountain belts and sedimentary basins that are controlled by faults and shear zoneso In addition, faults and shear zones control fluid migration and transport including hydrothermal and hydrocarbon systems. Once faults and shear zones are established, they are often long-lived features prone to multiple reactivation over very large time-scales. This collection of papers addresses lithospheric deformation and the rheology of shear zones, together with processes of partitioning and the unravelling of fault and shear zone histories.