PLATE COUPLING VS. STRENGTH IN CONTROLLING OROGENIC STRUCTURE

Collisional orogens show two major differences in basic structure: the degree to which the orogen is singly vs. doubly vergent and the distribution of deformation between the overriding and subducting continents. Two factors appear to control these 1^st order characteristics: 1) the strength of the overriding plate relative to the subducting plate, which is controlled by the compositional and thermal structure of the lithosphere, and 2) plate-boundary coupling, which is the long-term strength of the plate-boundary interface (i.e., the time-averaged strength of the subduction-zone megathrust and deeper plastic shear zone). Both factors modulate the fraction of total plate convergence that is localized in the overriding plate. Boundaries with weak overriding vs. subducting plates localize a larger percentage of the total convergence in the overriding plate than those with relatively strong overriding plates. Likewise, high coupling increases stress transmission into the overriding plate. Thus, different combinations of relative strength and coupling can yield the same fraction of total shortening in the overriding plate. However, we propose that temporal variations in the fraction of total convergence that is absorbed in the overriding plate reflect temporal changes in the relative importance of plate strength vs. coupling. For example, during the transition from subduction to collision, plate-boundary coupling systematically increases as buoyant material progressively enters the subduction zone, decreasing the dip of the subduction interface. The progressive increase in coupling during the subduction-collision transition predicts progressively greater localization of strain in the overriding plate as collision proceeds. Thus, we predict a positive, linear correlation between total plate convergence and the fraction of total shortening within the overriding plate. The slope of this relationship depends on the contrast between the strength of the overriding plate and coupling: an orogen with high plate strength relative to coupling will yield a shallower slope (i.e., a lower fraction of total shortening in the overriding plate for any given amount of convergence) than one with low plate strength relative to coupling. Observations from natural orogens appear consistent with these predictions.