Oceanic trenches mark where two converging tectonic plates meet and an oceanic plate sinks beneath (subducts) beneath the forearc of another plate.  The igneous rocks that make up convergent plate margins provide important scientific information about interactions between the two plates.  The igneous rocks of convergent plate margins near continents are commonly not available for geologic study because they are buried beneath thick sediments so sampling forearc igneous rocks requires studying forearcs and trenches that are far away from continents, such as the Mariana convergent margin in the Western Pacific. Study of oceanic forearcs and trenches requires special efforts because of the great depths involved. This grant funded a research cruise (32-day TN273 expedition aboard R/V Thomas Thompson: 22 December 2011 to 22 January 2012) to map and sample one of the most remarkable convergent margins on Earth, the Challenger Deep trench-forearc segment in the southernmost Marianas. During this grant period, we also participated in several cruises (2010, 2013, and 2014) with Japanese scientists to use manned submersible Shinkai 6500 to explore this region.  We focused our studies on a newly-discovered zone of forearc rifting in the easternmost part of the Challenger Deep trench-forearc segment, near Guam.  Forearc rifts develop at convergent margins where stresses associated with oceanward retreat of the subducting plate (slab rollback) causes extension in the overriding plate. These rifts form tectonic transects from the trench axis to the back-arc spreading center and overlie the shallowest part of the subducted Pacific plate.  Our work documented that this sector was the site of seafloor spreading as represented by depleted, low-K tholeiites with ~2 wt. % magmatic water. Such young volcanism so close to the trench indicates that mantle asthenosphere has been drawn in unusually close to the trench where flux melting from dewatering shallow slab fluids may have generated melts.  Because of the limited volume of asthenosphere that can be drawn into the rifts, their volcanic products may more directly reflect variations in slab fluxes and mantle melting associated with varying P-T conditions and breakdown of hydrous minerals in the underlying slab.  Our proposed work tested these hypotheses by mapping the distribution of igneous rocks in SEMFR and searched for evidence of active volcanism and hydrothermal activity. The TN273 expedition studied the southernmost Mariana convergent margin, including the Mariana Trough spreading ridge (aka Malano-Gadao Ridge or MGR), a poorly known arc volcanic complex to the SE known as the Fina-Nagu Volcanic Complex (FNVC), and the SE Mariana forearc rift (SEMFR), which lies between the Mariana Trench and FNVC.  A 19-member scientific team carried out the work, including graduate students from University of Texas at Dallas (UTD), University of Rhode Island (URI), and the University of Hawaii (UH) and one Japanese participant.  Fernando Martinez and Katherine Kelley served as co-chief scientists. In addition, we participated in JAMSTEC cruises usiing Shinkai 6500 to explore and sample farther west along the S Mariana forearc in 2014 (YK1413), 2013 (YK1308), and 2010 (YK1012).  This research grant provided a firm foundation for further studies devoted to understanding how the Challenger Deep forearc-trench segment formed and what it can tell us about how Plate Tectonics operates. Research funded under this grant also resulted in 16 scientific papers published in peer-reviewed scientific journals and one PhD dissertation.

Last Modified: 12/24/2014
Modified by: Robert J Stern