The underplate/underthrust/relamination tango
At the Geological Society of America meeting in Vancouver and was mildly amused at the latest name game, the identification of “relamination.” Aside from being a very ugly word (it is an attempt to play off of “delamination,” itself not the height of elegance and rarely applied properly–it should refer to a peeling away of material), are we playing with some new ideas?
Mostly, no. The term is not that new (Brad Hacker was using it in 2008, for instance) but seems to have reached that magical point (as delamination did awhile back) to become a geological meme. It appears to have been inspired by observations of ultrahigh pressure (UHP) minerals (like diamond) in “crustal” rocks, suggesting subduction of continental crust to great depths, though by what seems to be the touchstone paper by Hacker et al., in 2011 the requirement for UHP minerals was gone (though still a prominent observation). Anyways, the idea of crustal material descending to some great depth in a subduction zone results in the material, being less dense than the surrounding mantle, then rising up as a diapir to join the base of the continental crust, where it might well be incorporated into melt that becomes granitic plutons and silica-rich volcanic rocks. If in this process the silica-rich component is separated from the silica-poor component, it would arguably qualify as a new way to extract continental crust from the mantle.
However, much of the application of the term seems simply a way to apply a new buzzword to an old issue: how much continental material that is subducted is simply added to the upper plate?
If you look at the diagram above, what exactly is the difference between (A) and imbrication of subducted sediments under the crust? (B) and imbricating arc crust? (C) and imbricating and telescoping the upper plate? (D) and doubling a continent? Here’s the deal: the difference (at least so far as GG can tell at present) is entirely the mobilization and motion through the upper mantle of the material. So identification of high-pressure assemblages bears on “relamination”, but identification of the source of silicic input is not itself remotely diagnostic. Yet all the talks GG saw were dealing with the latter and not the former. Was the material seemingly from the accretionary complex really relaminated? Or was it simply a series of structural slivers at the base of the crust?
Look, its not like the crustal recycling/creation problem was irrelevant; it doesn’t need to misapply or appropriate a buzzword. Let’s try to speak of things without imputing something we have no information about.
(Fixed a partial sentence and a typo, 4/27/15)