Science Background

So as far as scientific research cruises go, this one is a little more traditional than the previous one. Instead of being funded by a seafloor mining company, this cruise is funded by the National Science Foundation (NSF). My advisor, who is the chief scientist for this cruise as well, applied for grant funding for this project, which is paying for the ship time as well as my meager salary. The cost of a ship per day is on the order of $25,000-$30,000 (up to $50,000 for certain ships), so research cruises are definitely not cheap. The area that we are working in is the southern Mariana Trough, which is ~100 miles southwest of Guam. This is another backarc basin environment, similar to the Lau basin (for a description of what a backarc basin is, see my previous post, "Science Background" on 11/11/11, or a really old post from the first cruise, "The Science of it all," although I misspoke in that post saying that the subducting plate melts to create the arc volcanoes. This is not true. Water released from the subducting slab causes the mantle above to melt, the slab appears to not melt much at all and can still be seen in mantle imagery (tomography) sometimes as deep as the core-mantle boundary.)

The Mariana Trough is the southern portion of the Izu-Bonin-Mariana Arc system, which is a subduction zone that stretches all the way up to Japan, and is the geologic feature along which the recent devastating earthquake/tsunami in northern Japan occurred. I don't know quite as much about the complex history of this area, but the basic situation is that the Pacific plate is subducting under the Phillipine Sea plate (a small plate sandwiched between the Pacific and Eurasian plates) in a roughly W-NW direction, creating a chain of arc volcanoes and a well developed backarc basin seafloor spreading system. For a more detailed but not-too-technical description of this area, check out the wikipedia page: http://en.wikipedia.org/wiki/Izu-Bonin-Mariana_Arc. This area has been active for much longer than the Lau basin (~50 My instead of only 5-6 My for the Lau basin), and so has a longer and more complex history. One interesting thing here is that there has been two episodes of arc rifting, creating two backarc basins. The first episode separated the Palau-Kyushu Ridge on the west side of the system from the Izu-Bonin-Mariana arc, leaving behind the extinct Parece-Vela and Shikoku basins (see the map below). Then in the southern portion of the system, a more recent episode of arc rifting has formed the much smaller and still active Mariana Trough, which is the region that we are focused on. At the southern end of this trough, along the trench where the two plates converge, is the famous "Challenger Deep," the deepest part of the ocean in the world at almost 11,000 meters, although it is still possible that a slightly deeper trench may yet be found.

Map of the entire Izu-Bonin-Mariana (IBM) Arc system. We're looking at the southern Mariana Trough west of Guam and the region between Guam and the trench.

As you can see from the maps, the trench runs roughly N-S for most of its length, but along the southernmost portion of the system it makes a sharp bend and becomes almost E-W. The geometry of the subducting slab here is very complex and not entirely known. There appears to be a tear in the slab near the SW corner and while for the N-S portion of the system the slab is subducting in a W-NW direction under the basin, in the southernmost E-W portion, a small chunk of slab appears to be subducting in a more northerly direction. This piece of slab is plunging almost vertically down into the mantle, which is part of the explanation for why the Challenger Deep is so deep here. But, to make it more complex, while this chunk of slab is subducting toward the north, because it is still attached to the Pacific plate, it is also moving W-NW. This creates strike-slip motion along this E-W trending part of the trench, the Pacific plate is moving toward the W, and the Mariana Arc is moving toward the E as seafloor spreading continues in the Mariana Trough. There is another short wikipedia article on the Mariana Trough for those who want a little more info: http://en.wikipedia.org/wiki/Mariana_Trough. Friction along this boundary causes the overlying plate to rift apart, creating widening cracks that stretch from the trench to the backarc spreading center.

Bathymetry map of the Mariana Trough. We are mapping the area W and SW of Guam. Unfortunately the spreading center is not marked here, but it is ~1/2 way between the ridge that Guam and Saipan are on and the West Mariana Ridge. You can see in the north that the ridges have not yet split apart and are still a continuous volcanic arc. The dark blue crescent-shaped feature is the trench itself, you can see how it changes from ~N-S to ~E-W.

These rifts are the primary focus of our mapping and sampling, and have never been examined in much detail before. They also will be a major focus of my PhD work, the data we are collecting during this cruise will be the primary data set for most of my PhD. These rifts are called the Southeast Mariana Forearc Rifts (SEMFR), and there are two primary ones that we will look at. Unfortunately they are too small to be seen on the maps I've posted here, I'll try to get some more zoomed in maps later. Our survey will be divided into three legs, two of them along the two rifts, and a third along the spreading center N of these rifts. Less detailed mapping of this region has shown that there is active volcanism along parts of these rifts. As the crust is stretched and pulled apart, the mantle rises to fill the space and melts because of the reduced pressure, creating volcanic eruptions along these rifts, which is roughly similar to what happens along spreading centers. These rifts are unique because the slab is much shallower underneath them, starting at a few km depth near the trench and increasing to a few 100 km by the time you reach the spreading center. Normally, the first volcanism that occurs is along the volcanic arc (the row of circular red/yellow features that run along the west side of Guam and Saipan on the map above), where the slab is ~100+ km deep in the mantle. Between the arc and the trench (the forearc), the mantle typically does not melt, and even if it does, the crust is too thick for the melt to penetrate. But because of these rifts opening up in this region, the water-rich melt rising in the mantle has a window to get out onto the seafloor. Also, because the slab is so shallow and there is so much water being released from it, the mantle is relatively cool, so you don't get "normal" types of lavas that you would see along a spreading center or one of the arc volcanoes. I'll give some more details on this once we actually get some samples of this stuff, but one of the things I expect to see is serpentinite. Serpentinite is what is formed when the mantle (which is largely a rock called peridotite) reacts with water and incorporates it into the crystal structure (it is ~40% water by volume), and this can actually erupt onto the seafloor as serpentinite "mud". This requires relatively cool temperatures and lots of water, which is what you find in the shallow part of a subduction zone. Serpentinite is the state rock of California, and is normally found within the oceanic crust where water has reacted with the mantle. In California, it was scraped off onto the continent from the top of the subducting Farallon plate, it was not actually erupted onto the seafloor. It is very rare for it to be erupted onto the seafloor, so I am excited to see some samples of that. I think that covers the basic scientific background, I'll get into what we're doing and the instruments we're using in the next post. I tried to make this explanation accessible to non-geologists, but it is rather difficult, so if anything is confusing, let me know in a comment.