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1 Laboratoire de Géophysique, Tectonique et Sédimentologie, c.060, Université Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 05, France
2 Present address: TOTAL, CSTJF, Avenue Larribau, 64018 Pau Cedex, France (e-mail: christopher.wibberley{at}total.com)
3 Department Interpretation Structurale, TotalFinaElf, Avenue Larribau, 64018 Pau, France
Excellent quarry exposures have been studied to examine the controls on the growth of fault networks in Cretaceous high-porosity sands. An inverse correlation is found at any one locality between the frequency of faults of an earlier tectonic event and the frequency of later faults. The early faults are cataclastic deformation bands with displacements typically up to 300 mm, and have thicknesses approaching their displacements. Later faults are also deformation bands except where present within a high-frequency array of earlier faults, where they are typically clustered high-displacement ultracataclasite zones that are narrower (smaller width/displacement ratios) than for the deformation band faults. A mechanical model using critical state soil mechanics explains the observed distributions and fault zone characteristics in terms of strength changes in the deforming sand unit and the stress path by which the material is subjected to ‘clastic’–plastic yielding. Localized faulting by constant-volume cataclastic flow at the critical state line will result in deviatoric stress reduction as Coulomb plasticity softening occurs within the fault zone. Elastic unloading of the walls will suppress the continued formation of deformation bands. The point at which the stress state reaches the critical state line, governed by the stress state and position of the ‘clastic’–plastic yield envelope, is therefore crucial in controlling the final distribution of deformation bands and larger faults in the system. Within this framework, the field and microstructural data suggest that earlier deformation became distributed by hardening processes such as compaction and grain-size reduction, resulting in a higher bulk yield strength. In a later tectonic event, the unit behaves in a stronger manner and deformation quickly localizes by fault zone softening processes into fewer fault zones that individually grow larger.
