Wednesday, September 8, 2010

Mission complete

As of September 3, 2010, our survey of Shatsky Rise was completed, and we started transit to Honolulu. Because of two medical diversions, we couldn't finish everything we planned, but given the science days we ended up with, what we were able to achieve can be called a great success. The following is the executive summary of the cruise report, which we are currently trying to write up before the end of the cruise:

"R/V Marcus G. Langseth MGL1004 formed the major data acquisition phase of the NSF-funded project, "Geophysical Constraints on Mechanisms of Ocean Plateau Formation from Shatsky Rise, Northwest Pacific" (OCE-0926611). Deciphering the origins of large oceanic plateaus is a critical element for understanding mantle dynamics and its relation to terrestrial magmatism, and Shatsky Rise was chosen as a high-priority target because it provides a unique tectonic setting to distinguish between various models proposed for the formation of oceanic plateaus. The purpose of this survey was to provide critical missing information on (1) the thickness, velocity structure, and composition of the Shatsky Rise crust, and (2) the history of magmatic emplacement and later tectonic development of the Rise. This was planned to be achieved by acquiring seismic data along two refraction lines over the Tamu Massif, which represents the early, most voluminous phase of the Rise construction, and over 3,000 km of reflection lines covering both the Tamu and Ori Massifs, the latter of which corresponds to the intermediate phase of the plateau evolution.

The cruise was unfortunately hampered by two medical diversions, which took ~16 days in total, and even with a seven-day extension provided by NSF, the survey had to be scaled down to focus on the southern part, leaving the northern part to be completed in another cruise tentatively scheduled for spring 2012. The southern part includes all of refraction lines (yellow lines in the map) and around 1800 km of reflection transects (red lines), all on the Tamu Massif. The work remaining to be done includes the rest of reflection transects (dotted red lines), which extend from the northern flank of the Tamu Massif to the center of the Ori Massif.

The Langseth fired over 47,000 shots from its 36-gun tuned airgun source into an array of seismic receivers: the Langseth's 6-km-long multichannel streamer and 28 Woods Hole Oceanographic Institution ocean-bottom seismometers (OBSs). As far as the southern part of the survey is concerned, the operational goals of the experiment were achieved in full. All of 28 OBSs deployed (shown as circles) were recovered successfully, and all instruments returned high-quality data. Multichannel seismic (MCS) profiling was also conducted with no major issues, yielding high-quality reflection data. Migrated brute stacks of all MCS lines were produced during the cruise, exhibiting intriguing intrabasement reflectors as well as revealing the true lateral extent of Shatsky Rise. OBS data show spectacular wide-angle refraction and reflection arrivals with the source-receiver distance often exceeding 200 km. The data collected during this experiment are sufficient to accurately determine the entire crustal structure of the Tamu Massif and will provide key information on the early magmatic construction of Shatsky Rise. By combining with future seismic data from the northern part of the survey, this information will provide an important tectonic framework for synthesizing existing geological, geophysical, and geochemical data and for resolving the formation mechanism of this large igneous province."

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