High Resolution Seismic Imaging of Protothrusts at the Nankai Trough Subduction Zone

Protothrust zones (PTZs) are areas of incipient thrust faulting between the deformation front and the frontal thrust at subduction zones. Although they are assumed to control the development of new frontal thrusts, the resolution of seismic images has hindered the study of their role in subduction accretion along most subduction margins. New high-resolution seismic reflection surveys provide clear images of the protothrusts and enables detailed structural analysis of the PTZ along the Nankai Trough, which is key to understanding the formation of future frontal thrusts. Seventeen multichannel seismic reflection lines were collected across the Nankai Trough trench axis between Cape Ashizuri and Cape Muroto using a 1200m long hydrophone cable and a 380 in³ cluster gun array for high resolution imaging. We processed the lines using pre-stack depth migration to maximize the structural interpretability of the profiles. Preliminary results reveal closely-spaced, small-offset thrust faults in the trench fill sediment package that form a protothrust zone that varies in size and characteristics along strike. The thrusts are predominantly landward dipping; however, some seaward-dipping thrusts that usually intersect the landward dipping faults are seen in some profiles. The protothrust spacing generally decreases landward, which could be an indication of strain localization consistent with incipient development of new thrusts seaward of the frontal thrust. Protothrusts are also seen landward of the frontal thrust in the imbricated thrust packages, which is to be expected if the frontal thrust migrates seaward, but this is rarely seen in previous seismic studies. Additionally, the high-resolution images have revealed small-offset normal faults in the subducting hemipelagic sediment that were previously below seismic resolution. Thus, our high resolution seismic profiles have revealed faults that were previously 'sub-seismic' both within the PTZ and imbricate zone as well as within the subducting sediment. Future study of the kinematics of these 'sub-seismic' faults will further our understanding of the formation of future frontal thrusts.