This paper adresses the changes in the tectonic regime in the Peruvian and Bolivian Andes that have occurred since the upper Miocene when the present‐day elevation of the Cordillera above sea level has been almost reached. The stress patterns are deduced essentially from a field study of fault kinematics and a numerical inversion of the slip vector data measured on the fault planes. The Cuzco fault system in southern Peru is chosen as an example to illustrate the methodology used. In this region, striations on both active and Holocene faults are in agreement with a N‐S extension. But faults affecting early Pleistocene deposits exhibit two families of striations. The younger results from the previous N‐S extension: the older, involving reverse motions, results from either an E‐W or a N‐S compression. Faults affecting Pliocene formations often show an oldest family of striations resulting from a NE‐SW or an E‐W trending extension. Thus three tectonic regimes are demonstrated which are also supported by regional unconformities and sedimentological data: (1) a Pliocene extensional regime, (2) a lower Pleistocene compressional regime, and (3) a mid‐Pleistocene‐present‐day extensional regime. Similar analyses conducted in the Pacific and sub‐Andean lowlands allow sketching of the successive Pliocene‐Pleistocene stress patterns in the Central Andes. The Quaternary and present‐day stress pattern is characterized by a N‐S extension in the High Andes and in the Pacific lowlands and by an E‐W compression in the sub‐Andean lowlands and at the contact between the Nazca and South American plates. This stress pattern is interpreted at a large wavelength (>100 km) as an effect of compensated topography. This model supposes that the vertical lithospheric stress, σzz, increases with the topography, the crustal thickness, and the low‐density mantle beneath and that the lithospheric maximum (compressional) horizontal stress σHmax, trending E‐W roughly parallel to the convergence, is fairly constant. On both edges of the Andes, the tectonics being compressional, σzz is σ3 and σHmax is σ1. In the High Andes, σzz becomes σ1, then the E‐W trending σHmax is σ2 and σHmin trending N‐S is σ3, allowing extension to occur in this direction. The Pliocene stress pattern was characterized by a NE‐SW or an E‐W trending extension in the High Andes, in the Pacific lowlands, and possibly in the sub‐Andean lowlands. This stress pattern was clearly different from the present‐day one because the E‐W trending stress was σHmin. This required a weak push or, eventually, tractional boundary forces acting on the South American lithosphere. It is suggested that this might result from a strong slab pull due to a long, steeply dipping slab which decreased the value of the σxx stress transmitted to the overriding plate. The early Pleistocene state of stress was compressional. Since the elevation of the Andes had not markedly decreased during this period, this required an increase of the E‐W trending stress value. This resulted from a strong coupling between the...

International audienceSpectral induced polarization or complex conductivity is a promising electric method in hydrogeophysics because of its sensitivity to water saturation, permeability, and particle size distribution (PSD). However, the physical and chemical mechanisms that generate the low-frequency complex conductivity of clays are still debated. To explain these mechanisms, the complex conductivity of kaolinite, smectite, and clay-sand mixtures was measured in the frequency range 1.4 mHz-12 kHz with various clay contents (100%, 20%, 5%, and 1% in volume of the clay-sand mixture) and salinities (distilled water, 0.1 g L-1, 1 g L-1, and 10 g L-1 of NaCl in solution). The results indicated the strong impact of the cation exchange capacity of smectite upon the complex conductivity of the material. The quadrature conductivity increased steadily with the clay content and was fairly independent of the pore fluid salinity. A mechanistic induced polarization model was also developed. It combined a Donnan equilibrium model of the surface electrochemical properties of clays and sand, a conduction model of the Stern and diffuse layers, a polarization model of the Stern layer, and a macroscopic conductivity model based on the differential effective medium theory. It also included the effect of the PSD. Our complex conductivity model predicted very well the experimental data, except for very low frequencies (<0.1 Hz) at which membrane polarization may dominate the observed respons

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In the Andes of central Peru that are situated above a flat dipping slab, analysis of both structural data collected in the field and available focal mechanisms show that the Quaternary and Recent state of stress is characterized by N‐S trending extension in the Western Cordillera and along the Pacific Coast, whereas E‐W trending compression prevails in the sub‐Andean Lowlands and at the contact between the Nazca and South American plates. The Eastern Cordillera deformations suggest an intermediate state of stress (i.e., strike‐slip faulting) characterized by E‐W trending compression and N‐S trending extension. Thus major fault zones striking NW‐SE, i.e., parallel to the central Peruvian Andes, are normal with a sinistral component in the Western Cordillera (e.g., Cordillera Bianca) and reverse sinistral in the Eastern Cordillera (e.g., Cordillera de Huaytapallana). This state of stress displays similarities with that of southern Peru and may be also interpreted as an effect of compensated high topography. In this model the vertical stress σzz increases with the topography and σHmax is considered as fairly constant, trending E‐W roughly parallel to the convergence. On both edges of the Andes, tectonics being compressional, σzz is σ3 and σHmax is σ1; in the High Andes, σzz becomes σ1, then σHmax is σ2, allowing σHmin striking N‐S to be σ3. Along the Pacific Coast, tectonics is not compressional as expected, and a nearly neutral state of stress may be due to a topographical effect related to the nearby deep Peru‐Chile trench. However, a conspicuous difference occurs in the Andes of central Peru in respect to the southern Peru Andes: compressional strike‐slip tectonics is observed in the Eastern Cordillera. Indeed, the mean elevation of the Eastern Cordillera is 3.7 km, whereas that of the Western Cordillera is 4.2 km, allowing an intermediate state of stress to occur. Therefore in central Peru a lower mean Andean topography and a stronger coupling consequence of the flat subduction seems to explain that compressional tectonics is distributed in a more widespread area than in southern Peru. Superficial deformations related to the subduction of the aseismic Nazca ridge are restricted to the coastal domain opposite to this ridge. They support the fact that submarine topography of large extension may subduct without producing compressional deformation in the overriding plate which can be reminiscent of a collisional setting.

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