Active tectonic deformation in the central-lower courses of Cai River, Khanh Hoa Province, Central Vietnam and its significance for the evolution of river and associated geological hazards 

Abstract The east-west trending middle and lower courses of the Cai River, central Vietnam, are underlain by a geologically complicated basement, which was strongly deformed by multiphase tectonic activities. Active tectonism is intensely, indicated by wide-spread brittle faulting, fracturing, significant uplifting or subsidence, and other tectono-geomorphological indicators, that systematically modify landscapes, drainage systems, and enhanced geological hazards such as landslides, riverbanks erosions, and coastal erosions. These, in conjunction with basement heterogeneity, have governed the evolution of the drainage network in the area. Active faulting forces the channels/currents to flow along the fault zones. The transecting of strike-slip faults across the flows leads to diversion of the flow orientation and formed complex flow patterns as well as pull-apart basins. Dip-slip motions of fault walls resulted in the formation of local uplift and/or subsidence blocks. In uplifted areas, the lowering of the base-level leads to incision and rejuvenation of the channels with a deep gorge, a step V-shape valley, and a widening of the beach. The subsidence led to a relative base-level rise, leading to lateral erosion, bankcuts and the windings of river course and formation of oxbow lake/lagoons, as well as erosion of the shoreline. In addition, uneven uplift and subsidence produce local tectonic tilting, resulting in the migration of flow towards the subsided areas. In this case, the pronounced uplifting of the south side of Cai River Valley has led to northeasterly tectonic tilting, which has forced the main flow and its mouth to migrated northeastward periodically, abandoned of older flows in the south and created an uneven north side-down river