Relations entre kink-bands decametriques et fractures de socle dans l'Hercynien des Monts d'Ougarta (Sahara Occidental, Algerie)

Abstract

The palaeozoic stratas of the Ougarta Variscan aulacogen are affected by kneefold-style deformations, or “kink-bands”, of decametric width. The geometric parameters have been measured, for 100 of these structures, most of which are reverse kink-bands, some of them associated in conjugate fold pairs. From the observed data (Appendix I), the following conclusions may be drawn: 1. (1) The mechanism of kinking must have been by flexural slip along the strata in the kink-band. 2. (2) There is no tendency for the kink-plane to bisect the kink-fold. 3. (3) Where the kink-planes are steeply dipping, the kink-band exhibits contraction normal to these planes; it is dilated where the kink-planes are gently dipping. Thus, we can say that the maximum compressive stress was flat-lying, at a low angle to the kinked stratas. 4. (4) The analysis of the conjugate fold pairs enables us to compute the approximate of the three principal axes of the stress which generated the kink-bands. The localization of kink-bands near flexures of the same tectonic style leads us to admit a genetical connection between both types of accidents. Field observations, related to regional geological evidence, enable us to admit that the formation of flexures and their associated kink-bands has been induced by the reactivation in a reverse-fault way of large fractures of the Precambrian basement, during a compressive episode of the Ougarta trench, at the end of the Variscan orogeny.