Fault kinematics and past plate motions at a convergent plate boundary: Tertiary Shimanto Belt, Southwest Japan

Abstract

Early- and late-stage, outcrop-scale faults have been documented in Paleogene strata of the Muroto Peninsula of southwest Japan. The former were active early in the consolidation history of the sediment and are crosscut by the latter, which were active after regional-scale imbrication, folding, and penetrative deformation (i.e., regional, spaced pressure solution cleavage). Orientation distributions of striae and kinematic axes indicate that the two generations of faults reflect distinct kinematic regimes. The orientations and cross-cutting relations indicate a counterclockwise (viewed downward) rotation of the principal shortening direction between early- and late-stage faulting. Existing fossil and radiometric age data suggest this transition occured during Eocene-Oligocene time. The fault kinematic data are interpreted to reflect a change in relative plate motions between Eurasia and an oceanic plate subducting beneath Eurasia in Paleogene time. These results, coupled with existing paleothermal and paleo-pressure data, are consistent with the presence of a spreading ridge in the western Pacific basin during this time.