Catchment lithology as a major control on alluvial megafan development, Kohrud Mountain range, central Iran

• Published in: Earth surface processes and landforms Vol. 37; no. 7; pp. 726 - 740
• Main Author: Arzani, Nasser
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 15-06-2012; Wiley
• Subjects: alluvial megafans; Asia; Bgi / Prodig; central Iran; climate; geomorphology; Iran; source area; Southwest Asia; tectonism; Iran; Central Iran; Sedimentology; Climatic variability; Alluvial cone; Base level; Mountain; Lithology; Watershed; Arid area; Weathering; Holocene; Tectonics; Pleistocene
• ISSN: 0197-9337; 1096-9837
• DOI: 10.1002/esp.3194

ABSTRACT
The relative importance of tectonics, climate, base level and source lithology as primary factors on alluvial‐fan evolution, fan morphology and sedimentary style remain in question. This study examines the role of catchment lithology on development and evolution of alluvial megafans (>30 km in length), along the flanks of the Kohrud Mountain range, NE Esfahan, central Iran. These fans toe out at axial basin river and playa‐fringe sediments towards the centre of basin and tectonics, climatic change and base‐level fluctuations, were consistent for their development. They formed in a tectonically active basin, under arid to semiarid climate and a long term (Plio‐Pleistocene to Recent) change from wetter to drier conditions. The key differences between two of these fans, Soh and Zefreh fans, along the west and south flanks of this mountain range, is that their catchments are underlain by dissimilar bedrock types. The source‐area lithologies of the Soh and Zefreh fans are in sedimentary and igneous terrains, respectively, and these fans developed their geometry mainly in response to different weathering intensities of their catchment bedrock lithologies. Fan surface mapping (based on 1/50000 topographic maps, satellite images, and fieldwork), reveals that the geomorphic evolution of these fans differs in that the relatively large‐scale incision and through trenching of the Soh fan is absent in the Zefreh fan. Whereas the limited sediment supply of the Soh fan has resulted in a deep incised channel, the Zefreh fan has remained aggradational with little or no trenching into proximal to medial fan surface due to its catchment bedrock geology, composed mainly by physically weathered volcaniclastic lithology and characterized by high sediment supply for delivery during episodic flash floods. Sediment supply, which is mainly a function of climate and source lithology, is a dominant driver behind the development of fan sequences in alluvial megafans. Copyright © 2012 John Wiley & Sons, Ltd.