Block rotation by basement strike-slip faulting in the Sanchu graben, central Japan

Abstract

In the Kanto Mountains, central Japan, Cretaceous fore-arc basin sediments are sandwiched by basement rocks in a narrow zone traditionally called the Sanchu graben. In the axial part of the graben they are divided into numerous blocks by a series of subparallel sinistral strike-slip oblique faults (block faults). These blocks have rotated clockwise together with block faults. Resulting clockwise deviation of strike of the strata from the general trend increases gradually from the margins toward the axial part of the graben, where it attains 30°. Block faults are traced in part of the graben where the strata show this clockwise deviation of strike. The structural evidence and the high length/width ratio of the rotated domain suggest that a major dextral strike-slip fault lies beneath the Cretaceous cover of the axial part. The unconformable boundary between the Cretaceous cover and the underlying basement may have played the role of a detachment plane, and prevented the fault from extending into the cover. The rotation of the overlying Cretaceous system is thus a superficial manifestation of underlying dextral strike-slip faulting.