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, w hich consists of the cover rocks of the Iberian margin shortened to fo rm a thin-skinned fold and thrust belt in early to middle Miocene time s- Over much of its length the IEZB is roughly linear, trending approx imately 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 di stance as a gently-dipping thrust with displacement to the southeast o r south-southeast, oblique to its regional trend. There is no evidence of dextral strike-slip movement along the boundary. New micropalaeont ological studies of calcareous nannoplankton indicate that the Oligo-M iocene 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. Thrusti ng therefore occurred in this time interval. The IEZB is therefore unl ikely to have been the dextral boundary of a westward-moving Alboran m icroplate at this time, but was more likely to have been the locus of NW-directed dextrally oblique convergence.