CONTROLS ON FAN DEVELOPMENT - EVIDENCE FROM FAN MORPHOMETRY AND SEDIMENTOLOGY - SIERRA-NEVADA, SE SPAIN

We have studied the morphology and the morphometric relations between alluvial fans and drainage basins in a bajada system including more th an 20 coalescent fans developed since the Late Pleistocene as a result of the recent uplift of the Sierra Nevada (Betic Cordillera, Spain). Three allocycles of tectonic origin were recognised across which there is a clear evolution from debris flow to sheet flow dominated fans, i n connection with a decrease in the volume of fines available in the s ource areas. The larger volume of accommodation space created by highe r tectonic subsidence in the northern sector favoured vertical accumul ation of sediment, with the appearance of less elongated fans. In the rest of the system, where subsidence is less, more elongated fans appe ar, with the development of incised channels and irregular distributio n of sediment in depositional lobes. This involves the appearance of m arkedly asymmetric transverse profiles, as well as lengthy recurrence between the sedimentation events on particular sectors of the fans, wh ere headward-eroding gullies develop. The Lithology of the source area , where intensely fractured rocks are found, is responsible for an imp ortant sediment supply and a significant degree of clast sorting from the source area. Consequently, a weak longitudinal trend in particle s ize can be recognised, which influences the predominance of longitudin al constant-slope profiles and anomalous relations between both fan ar ea and fan slope and their drainage areas. Recent intense tectonic act ivity has caused the appearance of abnormally low slopes in upper sect ors of some catchments, where mass flows are trapped, and the fans pre sent a subsequently higher proportion of sheet flows. Recent piracy ph enomena can be recognised in some of the drainage basins, which indica te rejuvenation of the source area, in which case the fan presents a r apid increase in the proportion of mass flows with the development of a segmented longitudinal profile. Due to the recent nature of the proc esses, the morphometric relations in the fan fed by the basin affected by capture do not coincide with the other fans in the area. (C) 1997 Elsevier Science B.V.