Sunday, April 26, 2015

Geology of Lau

Today, another weather day, so we are on hold above ABE. There is a brisk wind, and the swell is building. So here is a bit about the geology of backarc basins.  Check the sidebar for more on basins, arcs etc.
A subduction zone is a place where physics, chemistry, and geology come alive in the best of ways. It is a place where a dense oceanic plate, often old and water-rich from its millions of years beneath the waters of the Pacific or other oceans, sinks beneath another younger or more buoyant plate. The dense plate is pushed from behind by a spreading center thousands of miles away, and pulled down by ultra-dense minerals and the sheer pressure of the depths to which it drops.
As the plate descends, the temperatures and pressures of the overlying mantle wedge can cause melting of the hydrated crustal slab. This melting forms magma, rich in incompatible elements such as sodium and potassium, which rises to the surface and erupts from the volcanic arc.
This arc grants the Lau Basin its geologic title of ‘back-arc basin’ – a descriptive name, meaning exactly as it says (see sidebar for more details). Behind the arc of subduction zone volcanoes, a center of spreading emerges and creates a basin. This tends to preferentially occur behind island arc systems, but may happen under continental crust as well – see the basin and range province in North America. One must also note that not all subduction zones form basins: indeed, some areas, like the central Andes, actually undergo compression behind the arc.
A back arc basin is an extensional province behind a subduction zone. At first thought it seems implausible to have an area of extension behind an area of convergent stresses, but consider: as a plate subducts, it creates a trench between the two plates. Over time, this trench begins to undergo ‘rollback’, which means it begins to move seaward as the overlying plate converges.  This rollback motion creates tensional forces, which pull at the mantle material beneath the overlying plate. These tensional forces meet tensional forces from the other direction, and thus the crust above begins to pull apart at a spreading center behind the arc. This is known as a back-arc basin.
Back-arc basins are curious in many ways: biologically, chemically, and physically. Some, like the Lau Basin, are extremely seismically active, meaning that earthquakes occur frequently. They tend to spread in an asymmetric manner compared to mid-ocean ridge spreading centers, meaning that some areas spread much faster or slower compared to others on the same center.
Each basin is a little different. The Lau Basin was actually formed from two separate spreading centers that joined together through an extensional transform zone. Currently, the spreading rate varies based on whether one is in the north or south of the center. The North is faster, up to 9 cm/year, compared to the 4 cm/year in the South. This rate means that the Lau Basin is a relatively fast spreading center.

Keep an eye on the blog for further information on the Lau Basin’s chemistry and biology as the cruise goes on!  
Contributed Morgan Haldeman

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