FW: Dynamics of continental accretion

We are interested in collision processes in which subduction stops and restarts, therefore it is important to distinguish between the positive buoyancy associated with basaltic oceanic crust at shallow depths, and the strong negative buoyancy associated with oceanic crust which has transformed to eclogite after having been subducted. We assume that the oceanic crust transforms instantaneously and completely to eclogite at a depth of 150 km (density 3,500 kg m−3 at the surface temperature). This gives an average density of the subducted slab below 150 km which is consistent with the values assumed in the previous models33<http://www.nature.com/nature/journal/v508/n7495/full/nature13033.html#ref33>, 34<http://www.nature.com/nature/journal/v508/n7495/full/nature13033.html#ref34> on which we build, but allows the oceanic crust to contribute positive buoyancy when caught up in the collision zone.

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Posted on: Sunday, 23 March 2014 11:00 AM
Author: L. Moresi
Subject: Dynamics of continental accretion


Dynamics of continental accretion

Nature 508, 7495 (2014). doi:10.1038/nature13033<http://dx.doi.org/10.1038/nature13033>

Authors: L. Moresi, P. G. Betts, M. S. Miller & R. A. Cayley

Subduction zones become congested when they try to consume buoyant, exotic crust. The accretionary mountain belts (orogens) that form at these convergent plate margins have been the principal sites of lateral continental growth through Earth's history. Modern examples of accretionary margins are the North American Cordilleras and southwest Pacific subduction zones. The geologic record contains abundant accretionary orogens, such as the Tasmanides, along the eastern margin of the supercontinent Gondwana, and the Altaïdes, which formed on the southern margin of Laurasia. In modern and ancient examples of long-lived accretionary orogens, the overriding plate is subjected to episodes of crustal extension and back-arc basin development, often related to subduction rollback and transient episodes of orogenesis and crustal shortening, coincident with accretion of exotic crust. Here we present three-dimensional dynamic models that show how accretionary margins evolve from the initial collision, through a period of plate margin instability, to re-establishment of a stable convergent margin. The models illustrate how significant curvature of the orogenic system develops, as well as the mechanism for tectonic escape of the back-arc region. The complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction. We find geological and geophysical evidence for this process in the Tasmanides of eastern Australia, and infer that this is a recurrent and global phenomenon.



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