Influence of the thermal state of the overriding plate on subduction dynamics and slab geometry

Significant variability in slab dip can be found between different subduction zones, or along a single subduction zone. The effects on subduction dynamics and slab geometry of a number of factors such as trench motion, the presence of a low viscosity mantle wedge and slab rheology have been previously investigated. In contrast, the influence of the thermal thickness of the overriding plate on slab dip has not been studied before, even though the thermal thickness is expected to significantly influence the suction force acting on the slab due to corner flow in the mantle wedge due to both increased viscosity of a cooler mantle wedge and narrowing of the wedge corner. The main purpose of this study is to perform a systematic analysis of the influence of the thermal state of both the overriding and subducting plates on the balance between gravitational and hydrodynamic torques, and therefore on slab dip. We present the results of non-Newtonian numerical thermo-mechanical modeling of subduction, where we vary the age of both the overriding and subducting plates in order to test its effect on the slab dip at different depth ranges. We find that colder overriding plates result in shallower slab dips, as a result of the increased suction force in the mantle wedge. The influence of the thermal state of the overriding plate on slab dip is shown here to be more important than that of the age of subducting lithosphere. Modeling results are qualitatively compared to the large dip variability of the Cocos slab including a flat-slab segment. We suggest that this variability is likely related to the change of the thermal state of the overriding plates, with flat subduction occurring under cold lithosphere in southwestern Mexico and steep subduction under the warmer lithosphere of the northwestern Caribbean plate.