Effect of a retreating subduction zone on deformation in simple regions of plate convergence

P. Waschbusch, C. Beaumont
Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, B3H 4J1

Abstract:

Plane strain, two-dimensional, finite element models are used to calculate deformation patterns within model mountain belt systems where subduction zone retreat (the process by which space is created between the subducting ("pro") and overriding ("retro") lithospheres either by motion of one or both) is active. The model uses Coulomb plastic and thermally activated power law viscous rheologies. Overall deformation is determined by balancing the internal strength of the model layer against the sum of applied boundary stresses plus the gravitational stress induced by mass redistribution within the deforming model. The effect of incorporating retreat in the subduction setting diffuses deformation, as the moving (retreating) subduction zone deforms a larger cross-sectional area of material for a shorter period of time than equivalent models without subduction zone retreat. Material is deformed while within the subduction zone, but once the material has passed completely through the moving subduction zone it is passively appended onto the retrolithosphere and is no longer deformed. Thus, relative to a convergent setting without subduction zone retreat, the retreating convergent zone has an areally extensive zone of simple deformation. A deformation factor,, is derived which characterizes the deformation (shortening) the material experienced within the subduction zone. Variable is the ratio of convergence rate (rate at which material enters the subduction zone) to subduction rate (rate at which material is subducted). Variables and f, the fraction of the layer that is subducted, together determine the dominant style of deformation in the orogenic zone. Subduction zone retreat is shown to have significant and predictable effects on the model morphology (e.g., basin or mountain formation) and on the model geology (e.g., its presence suggests lower topography, lower metamorphic grade rocks at the surface, and simpler deformational structures), which agree with observations.

AGU Index Terms: 8150 Plate boundary-general; 8120 Dynamics of lithosphere and mantle-general; 8102 Continental contractional orogenic belts
Keywords/Free Terms: Subduction, Retreat, Tectonics

JGR-Solid Earth 96JB02482
Vol. 101 , No. B12 , p. 28,133