Abstract
At convergent plate boundaries, the properties of the actual plate contact are important for the overall dynamics. Convergent plate boundaries both mechanically decouple and link tectonic plates and accommodate large amounts of strain. We investigate two fundamental physical states of the subduction contact: one based on a fault and the
... read more
other based on a subduction channel. Using a finite element method, we determine the specific signatures of both states of the subduction contact. Based on a consistent comparison of fault and channel numerical models, we find that the nature of the plate contact is one of the controlling factors in developing or not of back-arc extension. To obtain back-arc extension, roll-back is required as an underling geodynamical process, but it is not always a sufficient condition. In part of this thesis, we investigate how the plate contact affects the initial stage of continental collision. For the same rheological properties and driving forces, varying the nature of the plate contact leads to three types of responses. The presence of a subduction channel promotes coherent and, when the boundary conditions allow it, plate-like subduction of the continental margin. In models with a subduction fault, coherent subduction of the incoming continental lithosphere occurs when the colliding passive margin has a gentle slope. The approaching continental sliver starts to subduct and the subduction is characterized by a non-plate-like behavior, slower subduction velocity than in channel models and strong slab deformation. If the continental margin is steep and the strength of the incoming continental crust is high, fault models result in locking of the trench, eventually leading to slab break-off. If the crustal strength is relatively low, shear delamination of part of the crust is expected. In the channel model this type of delamination never occurs. We conclude that initial stages of continental collision are strongly affected by whether the subduction contact is a fault or a channel. Neither the slab pull magnitude nor the tectonic setting is very important to the overall geodynamics at this stage. The plate contact type, the slope of the incoming passive margin and the rheology of the continent, controls whether the incoming crust subducts entirely, separates partially or entirely from the lithospheric mantle, or blocks the trench, likely leading to slab break-off. We identify four classes of subduction zones. The first two classes follow directly from our numerical experiments in combination with a re-analysis of published observations. In class 1 subduction zones are characterized by a plate contact that is largely fault like with an accretionary margin. In class 2 the plate contacts are largely channel-type and have an erosive margin. The third class, where the plate contact is a channel entirely, consists of accretionary margins with a high sediment supply. Subduction zones of class 4, mostly characterized by an erosive convergent margin (northern Chili, Peru, Honshu and Kuril), are more complicated; however, they can be explained by incorporating regional observations.
show less
