Role of convergence obliquity and inheritance on sliver tectonics: Insights from 3-D subduction experiments
Résumé
The subduction dynamics and deformation style in the framework of subduction zones have been classically understood under a vision of trench-orthogonal plate convergence. We perform 3-D upper mantle-scale analog models to further understand the dynamics of subduction systems and overriding plate strain distribution under varying convergence angles and in the presence or absence of intraplate inherited weak zones. The laboratory experiments show that whatever the obliquity of convergence, the subduction dynamics is largely influenced by the interaction of the slab with the 660 km discontinuity. The slab geometry alternates between stages of slab shallowing and steepening, which are accompanied by alternating periods of shortening and stretching (or moderate shortening) of the overriding plate, respectively. As convergence departs from being orthogonal and in the absence of an intraplate inherited weak zone, strain within the overriding plate shifts from pure shear to sub-simple shear. When a lithospheric-scale weak zone is introduced along the forearc-arc interface and under trench-oblique convergence, the strain is partitioned and the forearc develops sliver motion - and independent deformation - from the rest of the overriding plate. The forearc evolution is characterized by a three-step history: (i) detachment and oceanward motion, (ii) advancing motion toward the continent, and, (iii) accretion that can ultimately lead to underthrusting of the forearc. Decreasing slab dip increases the interplate force, resulting in larger stresses applied on the intraplate fault separating the forearc sliver from the continent. Hence, models show that because larger compression increases the coupling between the forearc and the plate, it does not favor sliver motion.
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