On the paralellism of bedding and cleavage in deformed rocks from the internal zone of the Betric Cordilleras-S.E. Spain

Abstract

Bedding-parallel foliations may cause serious problems for structural geologists who try to unravel the sequence of deformational events in polyphase deformed rocks. In many cases it may be difficult to prove whether an early bedding-parallel foliation represents an inherited sedimentary fabric or has formed during an early phase of deformation. In the past there have been such problems in several tectonic units in the Internal Zone of the Betic Cordilleras of S.E. Spain. This paper describes the microstructure of early foliations in low grade slates and metasandstones from the Almagro and Almanzora Units in the Sierra de Almagro and upper greenschist facies micaschists from the Variegato Unit in the N.E. Sierra de los Filabres (province of Almeria). It is shown that the dominant planar structure in the Almagro and Almanzora Units represents the first tectonic cleavage (St) that has been formed in these rocks. The foliation is axial planar to first phase folds and shows different stages of cleavage development from initial sedimentary microstructures to the fully developed slaty cleavage fabric. In upper greenschist facies micaschists of the Variegato Unit there is a preferred orientation of phyllosilicates parallel to bedding. In contrast to previous interpretations it is argued that this fabric does not represent an inherited sedimentary fabric. Pressure solution processes were involved in cleavage formation indicating non-hydrostatic stress conditions during tectonic deformation. Metamorphic conditions-culminating in upper greenschist facies grade after formation of the early foliations-can not be explained by contact metamorphism during burial underneath a thick pile of undisturbed sediments. They indicate that the rocks have been transported deep into the crust during a tectonic event prior to the peak of metamorphism.