Thursday, May 15, 2014

Geo 730: May 15, Day 501: (Blue) Schist Happens

Trying to get close-ups is problematic... I seem to have a difficult time getting the plane of focus actually aligned with the rock surface. The mica, likely mostly chlorite, is nice and sharp, but the glaucophane (dark blue) and pyrite (gold) are badly fuzzed out. That said, this is one of those amazing oddball rocks I love so much, because its mere existence tells an incredible story. Blueschist is composed of a suite of minerals that indicate extremely high pressure, but comparatively low temperature. Until the advent of plate tectonics theory, which came into its prime about 50 years ago, and broad acceptance and utility over the last 40 years or so, this rock was enigmatic in terms of its genesis. With the realization that subduction could transport great slabs of cold oceanic crust down into the mantle relatively rapidly- that is, carried into much higher pressure environments before heating up to expected higher ambient temperatures for those depths- came the understanding of how such a rock could form. The jetty here at Bandon was quarried from a larger pod of blueschist in the marine terrace on the Coquille River's south side. That pod, in turn, is a part of the Otter Point Formation, a classic 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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