庄内堆積盆地東部における伏在断層の成長に伴う活しゅう曲の変形過程

Abstract

Detailed morphotectonic and geological research shows the the Quaternary surface deformation processes associated with the thrusting in the eastern Shonai basin, northeast Japan.The Neogene and Quaternary structure in the eastern Shonai basin is dominated by a series of major north-south trending folds and thrust belts which include the Aosawa trend, the Sakata trend, and the Oishi trend from east (mountain-side) to the west (basin central-side). The Aosawa trend, which bounded the Shonai basin and the Dewa Hills, began to move in the late Pliocene. The Sakata trend and the Oishi trend, which are located at the eastern margin of the Shonai alluvial lowland, began to move in the Early Pleistocene and the Middle Pleistocene, respectively with the frontal migration of thrusting. These structures are interpreted to be fault propagation folds on a east dipping detachment fault.On the Aosawa trend, the late Pleistocene terraces are tilted with little faulting overlying the late Pliocene to middle Quaternary reverse faults. On the Sakata trend, the late middle Pleistocene deformation is similar to the underlying Neogene structure, which is characterized by asymmetrical folds with thrusts. On the Oishi trend, the late Quaternary terraces deformed as asymmetrical folding with a narrow flexure zone on the western (thrust tip side) limb overlying symmetrical early to middle Quaternary folds. These morphological figures show the general deformation processes of each structure in the eastern Shonai fold and thrust zone as follows.·In the early stage, the tip of the concealed thrust elongates to the surface, so the figurefor surface deformation changes from symmetrical folding to asymmetrical folding with a narrow flexure zone on the thrust tip side limb.·In the middle stage, the fault tip reaches near the surface, with continuing uniformdeformation composed of asymmetrical folding with surface faulting.·In the later stage, the thrusting on the fault near the surface is decreased, and gentletilting becomes to predominate on the surface.A simple dislocation model suggests that the elongation rate of the fault tip of the thrust underlying the Oishi trend is about 1 mm/yr in the late Pleistocene. This rate is similar to its vertical displacement rate.