Controles en el desarrollo y evolución del drenaje en zonas tectónicamente activas: el caso del río Mula (región de Murcia, SE España)

Resum

The affect and effect of climate and tectonics are often difficult to unravel in the sedimentary record of fluvial sequences. The Quaternary sequence of the Mula Basin (Murcia), located clase to the «Eastern Betics Shear Zone», records the dissectional-aggradational behaviour of a fluvial system which is mostly independent of eustatic and anthropogenic controls, but not of the neotectonic ones. The installation and evolution of the Mula fluvial system is recorded by the construction of 6 main fluvial terraces during 4 principal stages of drainage development: (1) Early aggradation (Early Pleistocene); (2) Dissection up to 50m, generating the Mula palaeovalley (Middle-late Pleistocene); (3) Large scale Late Pleistocene (Pre-Würm) aggradation infilling the palaeovalley of stage 2; and (4) Holocene dissection of sorne 36m down to the present day channel. It is evident that the development of these aggradational-dissectional sequences reflects the effects -base-leve! and sediment production variations- induced by the climatic changes occurred during the Quaternary. However the climatic trends only provide the overall background, on which was superimposed relevant modification by tectonic activity. Most of the Stage 3 large-scale aggradation was triggered by recent tectonic activity along the NE-SW strike-slip faults crossing the path of the Río Mula. Numerous evidences indicate that palaeoseismic activity along the Tollos-Rodeos Fault stimulated landslides, which impeded the fluvial system creating palustrine-type palaeoenvironments upstream of this fault. The Holocene dissection was stimulated by recoupling («vía» Headward erosion) of the Río Mula base-leve! with the Segura river. In particular this work enhances the value of combining geomorphological and sedimentological perspectives in this kind of studies and indicates the rapid response that a modest fluvial system can effect in response to externa! controls, as shows the 36m of incision over the Holocene. This process induced the generation of relevant headcuts along the longitudinal profile of the system, giving place to the spatial-temporal segmentation of the lower terrace levels.