IODP Proceedings    Volume contents     Search

doi:10.2204/iodp.proc.314315316.101.2009

Introduction

Integrated Ocean Drilling Program (IODP) Expeditions 314, 315, and 316 comprise the first of four stages of the multiyear project known as the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), an effort that is unique in the history of scientific ocean drilling. The first stage of NanTroSEIZE drilling operations was conducted by the new riser-capable drilling vessel D/V Chikyu from September 2007 to February 2008. These expeditions were unique in scientific ocean drilling history, not only because all three expeditions carried out drilling for common overarching goals along a single transect but also because logging and coring operations for the same sites took place on separate expeditions for logistical and operational reasons.

Comprehensive reporting of the operations and results from each site are contained in the individual expedition summaries and site-specific chapters in this volume. This short summary chapter is intended to tie together the observations from the three expeditions and synthesize them into a common set of observations for the three major regions of study along the NanTroSEIZE transect: the frontal thrust area, the shallow tip of the megasplay fault and associated out-of-sequence (OOS) thrust sheet, and the Kumano forearc basin region (Figs. F1, F2). It is an inherently limited selection of the reported data; for comprehensive information on findings at each site, see the individual site chapters in this volume. In particular, the site summary figures from each expedition that appear as the first figure in each site chapter in this volume are not reproduced here, and the reader is encouraged to consult them.

This volume is organized according to the three operational expeditions as they took place; therefore, nearly all of the individual sites have separate chapter in both the Expedition 314 section, which reports the results of dedicated logging-while-drilling (LWD) holes, and in either the Expedition 315 or 316 sections, which report the results from the nearby coring holes at the same site locations. In IODP terminology, an individual site may have numerous holes separated by a few tens of meters.

Background and objectives

NanTroSEIZE is the first attempt to drill into, sample, and instrument the seismogenic portion of a plate boundary fault (also known as a megathrust) within a subduction zone. Access to the interior of active faults where in situ processes can be monitored and pristine fault zone materials can be sampled is of fundamental importance to the understanding of earthquake mechanics. As the December 2004 Sumatra earthquake and Indian Ocean tsunami so tragically demonstrated, large subduction earthquakes represent one of the greatest natural hazards on the planet. Accordingly, drilling into and instrumenting an active interplate seismogenic zone was identified as a very high priority in the IODP Initial Science Plan (2001).

The fundamental goal of the NanTroSEIZE science plan (Tobin and Kinoshita, 2006a) is the creation of a distributed observatory spanning the updip limit of seismogenic and tsunamigenic behavior at a location where great subduction earthquakes occur, thus allowing us to observe the behavior and properties of subduction megathrusts in both aseismic and seismogenic regions of the megathrust system. This involves drilling of key elements of the active plate boundary system at several locations off the Kii Peninsula of Japan, from the shallow onset of the plate interface to depths where earthquakes occur (Figs. F1, F2); Stage 1 drilling is the first effort toward this goal. At this location, the plate interface and active megasplay faults implicated in causing tsunamis are accessible to drilling within the region of coseismic rupture in the 1944 Tonankai M 8.2 earthquake. The most ambitious objective is to access and instrument the Nankai plate interface within the seismogenic zone through deep riser drilling, now planned for a later stage. The science plan entails sampling and long-term instrumentation of the inputs to the subduction conveyor belt and a major OOS fault system that splays from the plate interface to the surface (termed the “megasplay”) and that may accommodate a major portion of coseismic and tsunamigenic slip and is thought to represent the main plate interface at a depth of ~6 km (Moore et al.).

The primary objectives of NanTroSEIZE Stage 1 were to essentially complete the riserless coring and logging component of operations along the NanTroSEIZE transect; that is, all objectives besides the two planned deep riser drilled sites into the plate boundary fault systems (Tobin and Kinoshita, 2006a, 2006b). These objectives included drilling six target sites, including:

  • The incoming sediment of Shikoku Basin and underlying oceanic crust (two sites),
  • The frontal thrust system at the toe of the accretionary wedge,
  • The mid-wedge megasplay fault system, and
  • Approximately 1000–1400 m deep holes at the two sites planned for later deep penetration of the seismogenic zone faults, one of which was to have a subseafloor observatory.

In 2006, planning for Stage 1 operations included the three Chikyu expeditions plus two others scheduled for drilling using the R/V JOIDES Resolution. Changes in the availability of the JOIDES Resolution led to cancellation of those two additional expeditions, and the objectives of drilling the input sites and installing a borehole observatory had to be deferred to a later stage of NanTroSEIZE efforts. The overall objectives of the three expeditions that were carried out were to sample and characterize the sediments and relatively shallow levels of the major fault systems targeted for deep drilling in NanTroSEIZE to characterize fault properties outside the seismogenic zone and to “pilot” the planned deep riser holes as a preparatory step toward borehole engineering for drilling to unprecedented depths (Tobin and Kinoshita, 2006a).

The three linked expeditions carried out in Stage 1 on the Chikyu were:

  • Expedition 314: LWD transect of the prism and forearc basin sites,
  • Expedition 315: coring and focused drilling of the megasplay fault thrust sheet and forearc basin, and
  • Expedition 316: coring and focused drilling of the shallow (<1000 meters below seafloor [mbsf]) portions of the frontal thrust and megasplay fault zone.

To varying degrees, operational conditions and time constraints caused modifications to the original plans for all of these expeditions, but the majority of objectives were successfully accomplished. Seventy-one scientists from 12 countries took part in the three expeditions as shipboard participants, and 33 holes were drilled at 8 sites to as deep as 1400 mbsf. Major accomplishments include:

  • Successful core sampling of the presumed aseismic portions of major faults and near-fault environment at both the megasplay fault system and the frontal thrust area;
  • In situ measurement of physical properties and borehole imaging with LWD instruments documenting present-day horizontal stress orientations through borehole breakout interpretation; and
  • Documentation of the lithology, structural features, and age of sediments in and below the thrust sheets, fault zones, slope cover, and forearc basin.

Age determination of the forearc basin fill, slope cover, and sections overridden by thrusts in particular provides a framework for understanding the materials and processes in the seismogenic zone by providing information on the history of its development and evolution.

Individual expedition objectives and Stage 1 strategy

The Stage 1 plan for these three expeditions separated the downhole logging and vertical seismic profile (VSP) objectives for the sites from the coring and other downhole tool measurements. In order to efficiently utilize the expensive LWD logging tools and to preview drilling conditions at each planned coring site all the dedicated LWD measurement holes were slated for Expedition 314. Then the same sites were to be revisited during Expeditions 315 and 316, with Expedition 315 focused on the thrust sheet overlying the prominent branch of the megasplay fault system (Fig. F3) at what became Site C0001 (planned for later deep riser drilling) and Expedition 316 focused on thrust fault and wall rock zone sampling at the megasplay fault tip and frontal thrust region (ultimately Sites C0004 and C0008 and Sites C0006 and C0007, respectively). Because of unforeseen drilling challenges and a resulting need to invoke contingency plans, not all sites that were logged were also cored, and vice versa.

Expedition 314

The original goals of Expedition 314 were to obtain a comprehensive suite of geophysical logs and other downhole measurements at sites (Figs. F2, F3) along the NanTroSEIZE transect using state-of the-art LWD technology. These six sites were designed to accomplish the principal goals of the NanTroSEIZE science plan, including documenting the material inputs to the subduction conveyor (fluid, solids, and heat), the properties of major thrust faults and their wall rocks at depths shallower than ~1.4 km, and the geology of the accretionary prism and overlying slope basin sediments (Tobin and Kinoshita, 2006a; Kinoshita et al., 2006). Four of the six planned sites were in fact drilled and logged during Expedition 314 in all three tectonic regions, but multiple attempts to find sites to drill into the megasplay fault zone resulted in six numbered sites drilled in total. Four of those were subsequently cored during Expeditions 315 and 316, as well as two additional sites that had not been logged during Expedition 314 (Sites C0007 and C0008).

Logging included the measurement of natural gamma radiation, azimuthal gamma ray density, neutron porosity, full waveform sonic velocity, azimuthal resistivity imaging, zero-offset VSP, ultrasonic caliper, and annular fluid pressure, though not all logs in this suite were collected at all sites. The overarching objective of the LWD program was to provide borehole data to be used in conjunction with cores to document the geology, physical properties, mechanical state, fluid content, and stress conditions at the drilling site locations. LWD operations were for the most part successful; however, mechanical failures caused loss of some logs at two sites, and the lost drill string with a full suite of LWD tools at Site C0003 meant that porosity and density data using the radioactive source tools could not be collected at subsequently drilled Sites C0004 and C0006.

Expedition 315

Expedition 315 was planned to focus all its efforts on riserless drilling, coring, downhole measurement, and casing installation over the upper 1000 m of Site C0001, planned as the pilot site for the first riser drilling in Stage 2. During Expedition 314 LWD was carried out successfully to 976 m LWD depth below seafloor (LSF), but difficult hole conditions were encountered, including an interval of high drill string torque caused by apparent borehole caving, enlargement, and partial collapse at ~460–540 m LSF, an interval that became known as “the sticky zone.” In attempts to core Site C0001 in multiple holes during Expedition 315, drilling did not advance beyond 458 m CSF. Coring results document the age and makeup of the deformed megasplay thrust sheet and overlying slope apron deposits, as well as structural features and inferences on evolution of the stress state. Installation of surface conductor casing and a 20 inch casing to 700 mbsf was a major objective in preparation for future riser drilling at this site. However, given the challenging drilling conditions and the strong Kuroshio Current, a mid-expedition decision was made to forego both casing and further attempts at deeper coring at this site, freeing up time for substantial coring operations at Site C0002 in the Kumano forearc basin (Fig. F3). In order to meet high-priority NanTroSEIZE objectives of sampling the lower Kumano Basin and underlying prism rocks, a hole was washed down to 500 mbsf and then cored from that depth to 1057 m CSF. In the waning days of the expedition, two additional holes cored the interval from 0 to 200 m CSF at the same site. Results indicate the nature and age of accretion of the prism rocks and the history of formation and filling of the forearc basin.

Expedition 316

Expedition 316 was designed to evaluate the deformation, inferred depth of detachment, structural partitioning, fault zone physical characteristics, and fluid flow at the frontal thrust and shallow portion of the megasplay fault system. To accomplish these objectives, drilling was conducted at two sites in the megasplay region, one within the fault zone (Site C0004) and one in the slope basin seaward of the megasplay (Site C0008). Two sites were drilled in the frontal thrust region (Sites C0006 and C0007). At Site C0006, several fault zones within the prism were penetrated before drilling was stopped short of the frontal thrust itself because of poor conditions in the borehole. The plate boundary frontal thrust was successfully drilled at Site C0007, and thrust fault material ranging from breccia to fault gouge was successfully recovered. The footwall proved to be remarkably coarse grained, as it was dominated by sand and gravel trench-axial channel facies deposit, and core recovery was very poor in this interval.