melange correlative to the famous Franciscan Melange.
Now the problem with melange (which, incidentally, is French for "mixture") is that it is a complete jumble of rock types that have no apparent business being together. For example, the blueschist under discussion is juxtaposed in many places against poorly sorted- and unmetamorphosed- marine sandstones. Try as they might, geologists could find no mappable patterns within melanges, and it was explained at length to me as a first year student in historical geology that this had been considered a hair-yankingly nasty problem. It turns out, an explanation had been worked out only a few years before I started my degree: as oceanic plate subducts, the upper portion of the plate and the overlying sediments can detach and mix into the accretionary wedge. Essentially, the accretionary wedge behaves as an enormous horizontal cement mixer, churning the pile, carrying shallower rocks down, and deeper rocks back up again. This explains both the lack of any consistent stratigraphy within melanges, as well as the chaotic mixture of apparently unrelated rock types.
Photo unmodified. March 9, 2012. FlashEarth location.
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