POST-MIOCENE DRAINAGE REORGANIZATION IN AN ACTIVE OROGEN, SIERRA NEVADA, BETIC CORDILLERA, SPAIN

Synorogenic and postorogenic sedimentation within the Betic Cordillera provide a record of aggradation and incision that has continued for >5 Ma. These strata record major base level changes of intermontane basins in thick sedimentary packages and large erosional unconformities linked to regional events that post-date widespread incision during the Messinian drawdown of the Mediterranean Sea. Preserved Pliocene and Quaternary fluvial strata were analyzed to understand recent drainage network configuration and rates of fluvial incision. Since the Pliocene the fluvial networks draining Sierra Nevada have fragmented into their modern configuration and continue to evolve. Analysis of imbricated clasts in dissected Pliocene fluvial deposits at 11 sites in the headwater of 3 modern drainages reconstruct a gently sloping (0.004˚) regional W-E longitudinal drainage bounded by Sierra Nevada to the north and coastal ranges to the south. This longitudinal network was aggradational during the Pliocene and encompassed a drainage area of at least 4 x 10³ km². This drainage network has since split into three modern incising drainages, which are steeply sloping (0.01-0.04˚), transverse to the orogen and are <2 x 10³ km² in size. Two separate drainage reorganization events occurred as the modern day Guadalfeo and Adra watersheds breached the southern coastal ranges and experienced a large change in local gradient. Topometric analysis of modern fluvial networks show that the landscape has experienced at least 2 significant base level changes (>100 m) since the Late Pliocene – Early Pleistocene that are driving modern fluvial reorganization. These base level changes are observed in all three modern watersheds although they are most clearly seen in the Andarax. The fragmenting of the regional fluvial network into its modern configuration is constrained by knickpoint celerity modeling using dated preserved surfaces. Pleistocene (<250 ka) incision rates of 0.8±0.2 km/Ma derived from OSL dating of fill terraces are similar to longer term published thermochronologic exhumation rates of Sierra Nevada (0.7+0.2, -0.3 km/Ma) suggesting uplift and erosion may be balanced on long timescales. However, short term incision rates in excess of 1.5 km/Ma between dated fill terraces implies rapid erosion can occur on glacial-interglacial timescales.