Posted by: paulgarner | January 12, 2010

Couple of items from Nature Geoscience

Another couple of papers caught my eye this week, both from the latest issue of Nature Geoscience:

First up is a paper by Vry et al. (2010) which indicates that the rapid exhumation of metasediments in orogenic belts might provide a rich and hitherto unrecognized source of hydrous fluids. They applied mineral-equilibria modelling to show that the rapid uplift and erosion of greywackes in an orogenic belt such as the Southern Alps of New Zealand could generate a constant supply of fluids, especially at temperatures below about 500 degrees C. This has significant implications because such fluids have the potential to enhance earthquake activity by weakening the rocks through which they migrate, as well as generating mineral deposits of various kinds.

In the second paper, van der Meer et al. (2010) used seismic tomography to identify 28 remnants of oceanic plates that were subducted into the lower mantle. Making the assumption that they sank vertically, they were then able to reconstruct the longitudinal position at which they were subducted. What they discovered was that the estimated locations of these palaeo-subduction zones were offset by about 18-20 degrees compared with accepted plate tectonic reconstructions. Interestingly, they found that all the slab remnants in the lower mantle were Permo-Triassic in age; none was Carboniferous or older. The authors interpreted that to mean that slab remnants only remain visible to seismic tomography for about 300 million years before assimilation. However, some creationists think that the phase of plate motions during the Palaeozoic may have been the result of an initial decoupling of the earth’s crust from the underlying mantle, rather than an episode of true plate tectonics. If so, that might also explain the apparent absence of pre-Permian slab remnants in the lower mantle.

References

Van der Meer D. G., Spakman W., van Hinsbergen D. J. J., Amaru M. L., Torsvik T. H. 2010. Towards absolute plate motions constrained by lower-mantle slab remnants. Nature Geoscience 3(1):36-40.

Vry J., Powell R., Golden K. M., Petersen K. 2010. The role of exhumation in metamorphic dehydration and fluid production. Nature Geoscience 3(1):31-35.

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Responses

  1. Evidence in the lithosphere for volcanism and subduction complexes go way back beyond the Phanerozoic. Present day oceanic crust generally is quite young going only back as far as the Mesozoic/end-Paleozoic for a good reason. The old oceanic crust is gone. Why complicate matters or invoke an earlier decoupling of the earth’s crust with the mantle ? It’s hard enough just getting your head around plate tectonics, but you must admit – it’s a pretty elegant robust paradigm.


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