Title

The relation between the rock record and changing plate motions at a convergent margin: The tertiary Shimanto belt of SW Japan

Date of Completion

January 1998

Abstract

The goal of this research is to better understand the relation between plate kinematics and structural fabrics at the Paleogene convergent plate margin of SW Japan. Relating plate kinematics to ancient deformation fabrics at convergent margins has historically been difficult for two reasons. First, much of the record of convergence is subducted and second, the perception that deformation partitioning often renders structural fabrics too complex to be related to past plate motions has deterred detailed investigation. The research presented here exploits the Paleogene Shimanto belt of SW Japan because the deformation history of these rocks is relatively straightforward and evidence for deformation partitioning is limited.^ Chapter one focuses on the role of diagenesis in controlling the deformation of poorly consolidated sediments which are commonly found at active convergent margins. Examination of disaggregated sandstone blocks in an ancient mud diapir suggests that redox potential gradients drove diagenetic reactions which contributed to the development of overpressures in the blocks. The envisioned process provides a mechanism for the maintenance of elevated pore-fluid pressures in mud diapirs and suggests a role for syn-deformation diagenesis in controlling deformation during the formation of tectonic melange. ^ Chapter two presents relative age relations, kinematic data and pressure-temperature constraints for outcrop-scale brittle faults in Paleogene strata. Cross-cutting relations reveal populations of early- and late-stage faults, and kinematic analyses indicate that these two populations reflect different kinematic regimes in the history of the accretionary prism. Microthermometric and microchemical analyses of fluid inclusions in syn-kinematic quartz suggest that late-stage faulting was accompanied by elevated geothermal gradients, which suggests the subduction of relatively young crust. ^ Chapter three reexamines plate configurations and kinematics in the west-central Pacific basin during the Paleogene. The kinematic data presented in Chapter two provide new constraints on relative motion between the Kula and Eurasia plates. The results of this reexamination have implications for the formation of the Philippine Sea plate, and help account for (1) thermal and age-lithology data from SW Japan that suggest the subduction of young crust and (2) evidence for a transition from sinistral to dextral obliquity along the Tertiary SW Japan margin. ^