Internal structure of the Adra Nappe (Alpujarride Complex, Betics, Spain)

Abstract

The Adra Nappe belongs to the Alpujarride Complex of the Betic Chain (southern Spain). This metamorphic nappe is of great interest to the understanding of the tectonic evolution of the Betic Chain since two episodes of thrusting inside the same Alpujarride Nappe can be differentiated. These two episodes have been produced in different physical ( P, T) conditions and with different directions of thrusting: the first one towards the northeast and the second towards the north-northwest. Furthermore, the plate tectonics reconstructions (Weijermars, 1987; Dewey, 1988) and geodynamic models (Andrieux et al., 1989; Platt and Vissers, 1989) published so far for the Betic Chain have not considered these two episodes of thrusting recognized in all nappes belonging to the Alpujarride Complex of the Betic Chain. In the area studied, to the south of Sierra Nevada, the Adra Nappe shows a marked structural variation, from an upper folded zone consisting of low-grade metamorphic rocks, to a mylonitic zone located in lower levels and characterized by medium to high-grade metamorphic materials. It has been observed that the emplacement of the Adra Nappe has involved two episodes. The first episode produces the formation of mylonitic rocks with a stretching lineation N45° E. The second one gives rise to gouges and fault breccias, and is bound to movements of the upper block in N10° W direction. From a regional point of view, in this sector the Adra Nappe occupies the upper tectonic position of the Alpujarride Complex. The extensional regime of the Adra Nappe appears to be intimately associated with the N–S convergence of Africa and Iberia from Oligocene times, whereas the NE-thrusting would reflect a previous strike-slip motion parallel to the trend of the Betic Chain, probably in an oblique convergence setting. These points concerning the two directions of movement must be considered for geological palaeoreconstructions of the Western Mediterranean area.