Development of large north-facing folds and their relation to crustal extension in the Alboran domain (Alpujarras region, Betic Cordilleras, Spain)

Detailed research in the eastern part of the Alpujarras region, Betic Cordilleras, has revealed the existence of large recumbent fold structures involving lithological sequences previously considered to belong to different tectonic units. We have reconstructed a large, complex fold structure (which can be followed axially for over 50 km) based on a careful revision of the boundaries between lithological formations, a structural analysis of the carbonate and metapelite sequences, and the use of mineralogical techniques such as measurements of d(001) basal spacing of phengite, paragonite and chlorite, determinations of the illite 'crystallinity' index (IC) and b(0) parameters of mica and chlorite. Structural field data described in this paper include gently dipping axial plane cleavage surfaces, common occurrence of gently dipping boudinage and pinch-and-swell structures and associated sub-orthogonal stretching lineation, similarity in orientation of this stretching lineation with the slip direction of the low-angle normal faults, and close association between north-facing recumbent folds and top-to-the-north low-angle normal faults (the slip direction of the faults being broadly perpendicular to the fold axes). These features clearly indicate that extensional processes played a decisive role in the generation of the Alpujarras large recumbent folds. The development of these recumbent folds probably occurred during the Early to Middle Miocene extensional tectonics that affected the Betic Cordilleras. The formation of large-scale folds in association with normal faulting is consistent with the P-T paths and geochronological data established for the Alpujarride region. (C) 1998 Elsevier Science B.V. All rights reserved.