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Fluid flow in cataclastic thrust fault zones in sandstones, Sub-Andean Zone, southern Bolivia |
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| Authors: | Pierre Labaume Simon M F Sheppard Isabelle Moretti |
| Institution: | a Laboratoire de Géophysique Interne et Tectonophysique, Université Joseph Fourier–CNRS, BP 53, 38041 Grenoble Cedex 9, France b Laboratoire de Sciences de la Terre, Ecole Normale Supérieure de Lyon–CNRS, 46 Allée d'Italie, 69364 Lyon Cedex 07, France c Insitut Français de Pétrole, 1–4 avenue de Bois-Préau, 92852 Rueil–Malmaison, France |
| Abstract: | The Bolivian Sub-Andean Zone (SAZ) corresponds to a Neogene thrust system that affects an about 10-km thick Palaeozoic to Neogene siliciclastic succession. The analysis of macro and microstructures and cement distribution in thrust fault zones shows that they are sealed by quartz at depths > 3 km, due to local silica transfer by pressure-solution/precipitation activated at temperatures >70–90 °C. At shallower depths, faults have remained open and could be preferential drains for lateral flow of carbonate-bearing fluids, as shown by the occurrence of carbonate cements in fractures and their host-sandstone. Due to decreasing burial, resulting from foothill erosion during fault activity, critically buried fault segments can be affected by non-quartz-sealed structures that post-date initial quartz-sealed structures. The integration of textural, fluid inclusion and isotopic data shows that carbonates precipitated at shallow depth (<3 km), low temperature (<80 °C) and relatively late during the thrusting history. Isotopic data also show that precipitation occurred from the mixing of gravity-driven meteoric water with deeper formation water bearing carbonate carbon derived from the maturation of hydrocarbon source rocks (Silurian and Devonian shales). The combined microstructural and isotopic analyses indicate that: (i) fluid flow in fault zones often occurred with successive pulses derived from different or evolving sources and probably related to episodic fault activity, and (ii) at a large-scale, the faults have a low transverse permeability and they separate thrust sheets with different fluid histories. |
| Keywords: | Fault zone Cataclasis Cement Fluid flow Carbon and oxygen isotopes Bolivia |
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