NanTroSEIZE Stage 1: First results of the Nankai Trough seismogenic zone IODP drilling project

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Tobin, Harold
Holding Date 27 September 2008

IODP Expeditions 314-316 took place over five months of continuous drilling operations in the off-Kumano region of the Nankai Trough subduction zone. This effort was both the maiden scientific voyage of the new drilling vessel Chikyu and the first leg in the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), a multi-year effort designed to investigate fault mechanics and seismogenesis along a subduction megathrust through direct sampling, in situ measurements, and long-term monitoring. During Expedition 314, (September - November 2007) our primary goals were to obtain a comprehensive suite of geophysical logs and other downhole measurements at several sites along a transect focused on the up-dip transition from seismic to aseismic fault behavior, using state-of-the-art logging-while-drilling (LWD) techniques and drilling to depths of 400 to 1400 m. Expeditions 315 and 316 were devoted to coring at these sites, plus two others within the same transect. Integrated results of all three expeditions shed light on the lithologies, physical properties, structural features, tectonic history, geochemical and biological systems, and present-day state of stress to 1.5 km below the seafloor.
In the Kumano region, the Nankai Trough forearc can be divided into (1) an inner wedge, comprised of a relatively older accretionary complex and overlying forearc basin that are hypothesized to lie over the up-dip end of the locked seismogenic megathrust; and (2) an "outer wedge" that is hypothesized as the active critical state accretionary wedge, with essentially aseismic mechanics. Seismically-imaged structural style and attributes vary markedly across this boundary. One highlight of the logs and cores is evidence for the orientation of present-day principal stresses from borehole breakouts, and paleostress from fractures and faults in cores. The orientation of the maximum horizontal stress axis (SHmax) from breakouts across the outer wedge is consistently perpendicular to the local strike of major structure, but is subparallel to strike in the forearc basin/inner wedge domain. These results are consistent with a compressional to transpressional stress state in the outer wedge, transitioning over a few kilometers maximum distance to an extensional stress state in the inner wedge. Additional key results, including internal structure of recently active faults, timing of development of the forearc basin, and interaction of syn-deformational slope deposits with recent thrust motion, will be discussed. These first expeditions in NanTroSEIZE produced exciting results in its own right and set the stage for coming deep drilling efforts to reach the seismogenic zone.