June 5, 2011

Geology Word of the Week: A is for Accretionary Wedge

Posted by Evelyn Mervine

Illustration of a convergent plate boundary. I’ve added a red arrow pointing out the
location of the accretionary wedge. Illustration from TASA graphics and taken from here.
Click to view larger.

def. Accretionary Wedge (aka Accretionary Prism, Subduction Complex):
A wedge- or prism-shaped mass of sediments and rock fragments which has accumulated where a downgoing oceanic plate meets an overriding plate (either oceanic or continental) at a subduction zone. The sediment is generally marine sediment that has been scraped off of the downgoing plate by the overriding plate. However, sediment from the overriding plate can also contribute to the accretionary wedge. Fragments of rock from the colliding tectonic plates can also accumulate in an accretionary wedge.  The sedimentary rocks which form at accretionary wedges are deformed, faulted, poorly-sorted mixtures which are often referred to as “melange” (which means “mixture” in French).

Since I’m hosting this month’s Accretionary Wedge Geoblog Carnival and I’m at the letter A in my second geologist’s alphabet, I thought it would be fitting for “accretionary wedge” to be featured as this week’s geology word (phrase) of the week.

An accretionary wedge is basically a hodge-podge collection of various sediments and rocks, scraped up and squished together where two tectonic plates collide and one plate subducts underneath another. The downgoing plate is always an oceanic plate (continental plates don’t really subduct as continental crust is too buoyant), but the overriding plate can be either another oceanic plate (such as the Japan subduction zone) or a continental plate (such as the Cascades subduction zone). The sediments in an accretionary wedge are mostly marine sediments scraped off of the downgoing oceanic plate. Most of the marine sediments on the oceanic plate actually subduct down into the mantle. However, some of the marine sediments pile up and are accumulated into a wedge or prism-shaped pile of sediments where the downgoing plate meets the overriding plate. This scraped-off marine sediment is mixed with other material such as sediments weathered/transported from the overriding plate and fragments of rock broken off of the colliding tectonic plates.

Because the sediments are primarily scraped off of the downgoing plate, accretionary wedges actually accrete new material primarily on the bottom of the wedge. This means the younger sedimentary rocks in an accretionary wedge are generally on the bottom, which is topsy-turvy to the classic Law of Superposition in geology.

The primary rock type which forms at accretionary wedges is a jumbled, fractured sedimentary rock known as melange. I’m not sure why– I guess French sounds smarter and more scientific?

Geologist Donald Prothero describes melange wonderfully in his textbook Interpreting the Stratigraphic Record:

“The most characteristic rock type of the accretionary wedge is melange (French, “mixture”), a mass of chaotically mixed, brecciated blocks in a highly sheared matrix. This deformation and pervasive shearing and brecciation are due to the tremendous compressional and shear forces generated by the downgoing slab [aka tectonic plate]. Melange is so mixed that it shows no stratigraphic continuity or sequence, and blocks and boulders from everywhere are mixed together. Some are exotic blocks from terranes no longer present in the vicinity.”  

Does anyone have any good pictures of melange rocks? If so, post a link below in a comment or send me the pics by email (see sidebar), and I’ll add them to the post. 

I think that Accretionary Wedge is a great name for a Geoblog Carnival, which is a jumbled mixture of blog posts just as a real accretionary wedge is a jumbled mixture of sediments and rocks.

Prothero, Donald. Interpreting the Stratigraphic Record. New York: W.H. Freeman & Company, 1990.