THE INTERNAL EXTERNAL ZONE BOUNDARY IN THE EASTERN BETIC CORDILLERA, SE SPAIN

The Internal-External Zone Boundary (IEZB) in the Betic Cordillera of southern Spain separates the Internal Zone, which was deformed and in part metamorphosed before the early Miocene, from the External Zone, which consists of the cover rocks of the Iberian margin shortened to form a thin-skinned fold and thrust belt in early to middle Miocene times- Over much of its length the IEZB is roughly linear, trending approximately 070-degrees. It has been referred to as a major dextral strike-slip zone, and has even been considered as the northern boundary of a westward moving Alboran microplate. Field and kinematic data from the eastern Betic Cordillera show that the IEZB crops out over a 60 km distance as a gently-dipping thrust with displacement to the southeast or south-southeast, oblique to its regional trend. There is no evidence of dextral strike-slip movement along the boundary. New micropalaeontological studies of calcareous nannoplankton indicate that the Oligo-Miocene basin along the boundary was the site of continuous deposition until the beginning of the middle Miocene. The thrust at the IEZB cuts early Miocene rocks, and is overlapped by the middle Miocene. Thrusting therefore occurred in this time interval. The IEZB is therefore unlikely to have been the dextral boundary of a westward-moving Alboran microplate at this time, but was more likely to have been the locus of NW-directed dextrally oblique convergence.