Evolución tectónica de las sierras de Tejeda y Almijara: colapso extensional y exhumación de áreas metamórficas en el dominio de Alborán (Cordilleras Béticas)

Resumo

The aim of this paper is to describe the tectonic evolution of the Tejeda-Almijara mountain range (Alpujárride Complex, Central Betics). The importance of the Alpujárride lithostratigraphic type sequence in order to determine the tectonic organization of the Complex is discussed and the main mineralogical assemblages which characterize the metamorphism of the Alpujárride units are also analyzed. Detailed structural analysis at different scales has revealed the existence in the region of a sequence of deformation events. Up to four tectonic foliations (S1-S4), developed at different times during the Paleogene and Neogene and related with larger structures, have been distinguished; nevertheless not all the foliations mentioned are equally represented in the region. In some cases the P-T conditions existing during the formation of the structures can be determined. S1 surfaces can only be observed as an internal foliation within porphyroblasts and it is related with high-pressure metamorphic conditions. S2 is generally the principal foliation observed in rocks and developed under medium grade metamorphic conditions; it is a mylonitic (and transposition) type foliation which was related with isoclinal folds which were tightened during the onset of the extensional collapse. S3 is a crenulation foliation associated to kilometric size recumbent folds which produced important inversions of the lithostratigraphic sequence; S3 developed below 5 kbar pressure conditions. This was followed by local development of extensional crenulation cleavage associated with Neogene extensional detachment faults which bound the tectonic units. S4: crenulation foliation which is related with open to close upright folds developed, probably, during the Late Miocene. Boundaries between tectonic units are interpreted as low-angle normal faults which were active in different time periods during the Neogene and some of them even more recently. The transport direction of the Axarquía Extensional System is roughly NE-SW. Other older faults, e.g. the Tejeda Extensional Fault and some extensional detachment faults in northern Sierra Tejeda, also show NE-SW and ENE-WSW extension direction. Due to the extensional fault systems important tectonic omission developed, the result being that the thickness of the Alpujárride units which overlie the Tejeda unit (and even the thickness of the overlying Maláguide Complex) have been strongly reduced. The studied area is a locus where the northern basement of the Alborán Sea crops out. This is related with the Neogene exhumation of a metamorphic terrain which shows characteristic features of a highly extended region. In such basement, large recumbent folds with overturned limbs several kilometres in length are preserved. Some of the folds have been dismembered and masked by development of extensional detachment faults as those described in this paper. Finally, these faults were folded in Late Miocene time, more or less at the same time the general emersion of the region took place.