Justo Cabrera

Changes in the tectonic regime above a subduction zone of Andean Type: The Andes of Peru and Bolivia during the Pliocene‐Pleistocene

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 two lithospheric plates, possibly due to a rupture of a long slab under its own weight. Other spatial changes in the stress pattern are related to the particular situation of the forearc, to the subduction of the buoyant Nazca ridge, and to the different dips of the slab. Extension in the High Andes is of small magnitude, of the order of 1% during the last 1–2 m.y.; in a few basins, it may have attained 40% during the Pliocene (≈5–3 m.y.).

Plio-Quaternary geodynamic evolution of a segment of the Peruvian Andean Cordillera located above the change in the subduction geometry: the Cuzco region

Abstract The Cuzco region, which is located above a change in subduction geometry, appears to be characterized by a variable Plio-Quaternary tectono-sedimentary evolution essentially located along the major fault system that separates the High Plateaux from the Eastern Cordillera. After the higher surface formation of the High Plateaux, a set of Neogene basins were filled by Miocene “ fluvio-torrential” series and by Plio-Pleistocene fluvio-lacustrine deposits. The Neogene series have been affected by compressional tectonic forces attributed to the Late Miocene. This compression is followed by roughly E-W trending syn-sedimentary extensional tectonics attributed to the Pliocene; it is related to reactivation of the pre-existing major faults, basin evolution, and volcanic activity concentrated along the faults. In the Early Pleistocene, fluvio-lacustrine deposits are affected by syn- and post-sedimentary compressional tectonism it is characterized by shortening that trends both N-S and E-W and produces folding and faulting of the sedimentary cover. Extensional tectonism trending roughly N-S has been taking place from the Middle Pleistocene to the Present; it is coeval with shoshonitic volcanic activity, and with sedimentation of fluvio-lacustrine terraces, torrential fans and moraines. Quaternary and active normal faults due to this tectonism, are located in a narrow zone more than 100 km-long between the High Plateaux and the Eastern Cordillera, and two 15 km-long fault sectors in the Eastern Cordillera. Characteristic Pleistocene scarps, 400 m or more high, are due to the cumulative normal offset, and there are also little scarps, with heights ranging between 2 and 20 m, which are related to Holocene fault reactivations. Recent fault reactivation on the Cuzco fault system, during the April 5, 1986 earthquake ( m b = 5.3), is due to the N-S trending extension. This state of stress, located at a mean elevation of roughly 3730 m, is generally homogeneous to different scales. The active Cuzco normal faults may be a consequence of adjustment between the compensated Western Cordillera and the undercompensated Eastern Cordillera, this latter being uplifted higher than its isostatic equilibrium due to compression acting on its eastern edge. The variation of the state of stress, during the Plio-Quaternary is in agreement with the variations of the compressional boundary forces. It may be explained by variation of the convergence rate or by the variation of pull-slab forces.