December 19, 2007
The Japan Agency for Marine-Earth Science and Technology
The Japan Agency for Marine-Earth Science and Technology (JAMSTEC; Mr. Yasuhiro Kato, President) is pleased to announce that the Deep-sea Drilling Vessel Chikyu has just completed IODP*1 expedition, "Expedition 315" of the "Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE)" as of December 18th.
The Expedition 315 has completed 33 days mission carrying out scientific drilling in the Kumano basin off the Kii Peninsula from November 16th to December 18th. The main scientific objective of this expedition was to obtain detail profile of upper limit of plate boundary at shallow part of the Nankai Trough Seismogenic Zone by the means of drilling and collecting core samples. Due to difficult operational conditions mainly related to strong Kuroshio current, the well head installation for the future riser drilling has been postponed.
Dr. Juichiro Ashi (Associate Professor, Graduate School of Frontier Sciences, the University of Tokyo, Japan) and Dr. Siegfried Lallemant (Professor, Earth Sciences Department, Cergy-Pontoise University, France) are the Co-Chief Scientists, while the science party comprises 25 onboard scientists from 6 countries.
At the 2 sites (Fig.1) which are accretionary prism*2 of Nankai Trough (Site C0001) and landward side of Kumano forearc basin*3 (Site C0002), drilling operations were conducted at 8 holes and core samples were successfully collected down to 458 meters and 1,057 meters below seafloor (mbsf), respectively.
Site C0001 targeted a small slope basin*4 overlying the accretionary prism, previously identified on 3D seismic reflection profiles obtained during a preliminary site survey. Samples were collected from the 200 meter thick recent slope basin and the underlying accretionary complex down to a final depth of 458 meters below seafloor. At Site C0002, the objective was to drill through the thick Kumano forearc basin composed of alternating layers of sandstone and mudstone, and core samples have been successfully collected down to 1,057 meters below seafloor, reaching the main body of the accretionary prism.
The data collected at this site will provide important new constraints for the scientific models but also critical information for the engineers who are preparing the future deep riser drilling at this site.
Many small faults were described and measured in the samples obtained during this expedition both by visual observation and by nondestructive 3 D structural analyses with a CT scan (Fig.2). The results confirm the present stress condition revealed by Logging While Drilling (LWD) during the previous Expedition 314 and provide new insights in the temporal changes of the stress field.
In addition, new age determinations obtained by combining micropaleontology and paleomagnetic studies were critical to understand the evolution of this part of the Nankai subduction complex though time and geological history of the seismogenic zone.
During the coring operation, good quality data of borehole temperature were obtained by a sensor embedded to core sampling system. Those allowed us to evaluate the thermal gradient in the upper layers. These new data used with thermal conductivities measured on the cores will result in more accurate estimates of the heat flow and thus in a better knowledge of the temperatures at greater depths near the seismogenic zone.
Core samples and data collected and analyzed during this expedition were highly valuable for a better understanding of the seismic processes in the accretionary prism off Kumano basin. In order to achieve this goal, a large number of samples will be dedicated to intensive mechanical studies conducted by both onboard and shore based scientists in their institutes after the cruise.
The next NanTroSEIZE Expedition 316 which immediately began since December 19th. Science party, comprises 26 scientists from 10 countries, with Co-Chief Scientists Dr. Gaku Kimura (University of Tokyo) and Dr. Elizabeth Screaton (University of Florida). This expedition will obtain samples down to 1,000 meters below seafloor during a survey focusing on mechanical and hydrogeological conditions of the two most prominent faults of the accretionary complex: the frontal thrust and the main splay fault. The Chikyu will return to the Port of Shingu on February 5th, 2008.
Fig.1 Survey Area Red stars indicate
drilling sites of Expedition 315
Fig.2: X ray CT scan image of collected core sample
Many small faults were identified by displacement of fractures (vertical lines). 3D analyses of data reveal detailed changes of stress fields recorded on strata. (Left: X ray CT scan images. Colors mainly indicate density differences. Right: Sketch showing fractures (gray thick lines) and small faults)
Pic.1: Scientists picking up samples to be study in the laboratory on Chikyu
(Photographed on November 29, 2007)
|Drilling site:C0001(Proposed site:NT2-03)|
|North Latitude:33°14.0’ East Longitude:136°42.0’|
|Core sampling depth
|Summary of Result|
|E||2188.5m||0m to 118.1m||Collected samples by Hydraulic Piston Core Sampling System(HPCS)|
|F||2187.5m||108.0m to 229.8m
229.8m to 248.8m
|Collected samples by Hydraulic Piston Core Sampling System(HPCS) and Extended Shoe Coring System (ESCS)|
|G||2187.0m||-||Abandoned drilling since ROV cable tangled to drill pipe right after starting core sampling by Rotary Core Barrel Sampling System (RCB).|
|H||2197.0m||240.0m to 458.0m||Collected samples by Rotary Core Barrel Sampling System (RCB). Skip coring the difficult portion and tried to collect samples from 600meters below seafloor or deeper, borehole wall was not stable. Moved to next site.|
|I||2198.5m||-||Tried to collect samples by Rotary Core Barrel Sampling System (RCB) from 600mbsf or deeper, but borehole wall was not stable. Moved to next site.|
|Drilling site:C0002(Proposed site:NT3-01)|
|North Latitude:33°18.0’ East Longitude:136°38.0’|
|Core sampling depth
|Summary of Result|
|B||1937.5m||475.5m to 1057.0m||Collected samples by Rotary Core Barrel Sampling System (RCB). Abandoned further coring since borehole wall conditions never get any better despite many efforts.|
|C||1937.1m||0m to 13.77m||Collected samples by Hydraulic Piston Core Sampling System (HPCS).|
|D||1937.1m||0m to 204.0m||Collected samples by Hydraulic Piston Core Sampling System (HPCS). Evaluated geological intensity of shallow portion for future riser drilling.|
Applied to very soft formations. Stub edgy tip like knife into layers using hydraulic pressure, and collect core without rotate drill bit.
Applied to soft formation that is difficult to collect. Tip of the system can adjust to layers intensity using spring power.
Applied to layers of middle to high intensity. Collect samples while rotating drill bit and scraping layers. Good for consolidated layers with less ruptures.
Zone at where only drilling was operated (no core sampling) in accordance with scientific objectives.