BLOCK ROTATION BY BASEMENT STRIKE-SLIP FAULTING IN THE SANCHU GRABEN,CENTRAL JAPAN

In the Kanto Mountains, central Japan, Cretaceous fore-arc basin sedim ents are sandwiched by basement rocks in a narrow zone traditionally c alled the Sanchu graben. In the axial part of the graben they are divi ded into numerous blocks by a series of subparallel sinistral strike-s lip oblique faults (block faults). These blocks have rotated clockwise together with block faults. Resulting clockwise deviation of strike o f the strata from the general trend increases gradually from the margi ns toward the axial part of the graben, where it attains 30-degrees. B lock faults are traced in part of the graben where the strata show thi s 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 under lying basement may have played the role of a detachment plane, and pre vented the fault from extending into the cover. The rotation of the ov erlying Cretaceous system is thus a superficial manifestation of under lying dextral strike-slip faulting.