The Structure and Kinematics of the Nankai Trough Accretionary Prism, Japan

Subduction zone frontal prisms are among the most tectonically dynamic environments on Earth. 3D seismic data acquired as a part of the Nankai Trough Seismogenic Zone Experiment provides a unique opportunity to study accretionary prism processes. My research focuses on three aspects of the structure of the outer wedge fold and thrust belt.;Based on the geometry of thrust sheets in the outer wedge, the décollement has ramped up and then back down within the incoming sedimentary section. This has resulted in an unusually thick underthrust sediment volume, which has caused uplift and mass wasting ∼30 km landward of the deformation front. I hypothesize that changes in the depth of the décollement are due to the subduction of a basement high beneath the outer wedge ∼25 km to the east of the study area. This suggests that basement features may be associated with anomalously thick underthrust sediments for a significant distance along-strike.;Bed-length balancing suggests that it has accommodated 99±10 km of shortening in the last ∼2.3 myr, 34±17 km of which has been accommodated by a zone of out-of-sequence thrusting. Kinematic modeling of syn-kinematic sediments in the forearc basin indicates that the landward-most structure in the out-of-sequence thrust zone has accommodated 14.6±1.8 km of shortening. Additionally, our results demonstrate that uplift of the forearc and activity on the landward out-of-sequence thrust must have began at ∼0.9 Ma and continued until sometime younger than 0.5 Ma.;Finally, I use arrays of small (<100 m throw) faults imaged by 3D seismic to constrain the stress state variability throughout the outer wedge. Based on the distribution of strike-slip and normal faults, most of the wedge is in transpressional failure, while normal faults are restricted to discrete regions. Furthermore, normal faulting is coeval with strike slip faulting. This suggests that the temporal variability in the stress state is larger than the difference between the two smallest principal stresses. Therefore, the observed normal faults are a result of local perturbations to the transpressional stress state, rather than collapse of the outer wedge, as was previously suggested.