Paleo-stress orientations from calcite twins in the North Pyrenean foreland, determined by the Etchecopar inverse method

Abstract

During the last decade, it has been shown that accurate analyses of calcite twinning in thin section may allow computation of the paleo-deviatoric stress tensor associated with this deformation. Following from the work of Larroque and Laurent (1988), and Lacombe et al. (1989), this is a new application of the Etchecopar inverse method. Weakly deformed limestones from about 150 to 200 km north of the Pyrenean chain (South France) were studied. The Etchecopar inverse method, which can be used on a microcomputer, is comparable to the method of analysis of striated fault planes proposed by Etchecopar et al. in 1981, as it determines the principal stress orientations and the stress ellipsoid shape ratio φ = (σ 2 − σ 3) (σ 1 − σ 3) . The Etchecopar method using calcite twins has been selected for this study because it is suitable for the determination of polyphase deformation in coarse-grained limestones (Lacombe et al. 1989). Ten samples of oolitic limestones, cemented with sparry calcite, from the Bajocian (170 Ma) and the Oligocene (30 Ma) were studied. For each of them, only one or two stress tensors were determined: the first is characterized by a principal compressive stress axis σ 1 which is horizontal and approximately N-S. Local departures from this orientation have been observed and are interpreted as local stress perturbations. For the second, which was defined in the Oligocene, but occurred only locally in the Jurassic limestones, the position of σ 1 is nearly vertical; the direction of extension σ 3 is either NE-SW or undefined into the horizontal plane ( σ 2 = σ 3); this radial extension is interpreted as the result of two or more superposed extensive deformation phases with vertical σ 1. The N-S compression was never found in the Oligocene limestones, and from its orientation, it is assumed to correspond to the Pyrenean deformation. These results are compared to the more complex deformation pattern obtained from classical microtectonical studies (Arthaud and Choukroune, 1972; Bonijoly and Blès, 1983; Granier and Blès, 1988; Blès et al., 1989). Only the major tectonic events are recorded by calcite twinning. Finally, the suitability of calcite twin analyses for the determination of complex polyphase deformation is discussed.