Tectonic origin and evolution of a transverse drainage: the Rio Almanzora, Betic Cordillera, Southeast Spain

The Rio Almanzora forms one of the larger drainage systems within the Betic Cordillera, Southeast Spain. In its distal reaches, prior to joining the Mediterranean Sea, the Rio Almanzora cuts across a major topographic barrier, the Sierra Almagro, forming drainage that is transverse to the regional geological structure. The long-term drainage evolution of the Rio Almanzora and its creation as a transverse drainage across the Sierra Almagro has been examined by reconstruction of an evolving basin-scale drainage network using a combination of the geological and geomorphological record. The early stages of drainage evolution (Late Miocene-Pliocene and Plio-Pleistocene) reveals the emergence of marine basins to the north and south of the Sierra Almagro and the development of unconnected drainage systems. This emergence records the development of the proto Rio Almanzora on the south side of the Sierra Almagro as a prograding fan-delta and alluvial fan system. Expansion of this drainage network via incision and headward erosion across the Sierra Almagro took place during the Pleistocene. During the Early-?Mid-Pleistocene, the proto Rio Almanzora became fully transverse across the Sierra Almagro and linked the endorheic Almanzora/Huercal-Overa basin with the Vera basin. A tectonically induced lowering of regional base level is proposed for the incision, headward erosion and drainage network expansion that has resulted in the creation of the Rio Almanzora as a transverse drainage. Differential uplift between the Huercal-Overa/Almanzora and Vera basins has resulted in southwards tilting and the creation of a regional gradient. The fluvial response to tilting was for incision into the steepened regional surface gradient via an increase in stream power. The incision, headward erosion and drainage network expansion by the proto Rio Almanzora and its creation as a transverse drainage appears to be part of a regional response by fluvial systems to differential uplift recorded throughout the Betic Cordillera of Southeast Spain during the Plio-Pleistocene. (C) 2002 Elsevier Science B.V. All rights reserved.