Mountain Building or Backarc Extension in Ocean‐Continent Subduction Systems: A Function of Backarc Lithospheric Strength and Absolute Plate Velocities

Abstract The crustal structure of overriding plates in subduction settings around the world varies between a wide range of deformation styles, ranging from extensional structures and backarc opening as in the Tonga or Hellenic subduction zone to large, plateau‐like orogens such as the central Andes. Both end‐member types have been intensively studied over the last decades, and several hypotheses have been proposed to explain their characteristics. Here we model ocean‐continent collision using high‐resolution, upper mantle scale plane‐strain thermo‐mechanical models, accounting for phase changes of rocks that enter the eclogite stability field and the phase transition at the 660 km mantle discontinuity. We test model sensitivity to varying plate velocities and backarc lithospheric strength as the main variables affecting the strain regime of the overriding plate in subduction zones. With our small set of variables, we reproduce both overriding plate extension and shortening and provide insight into the dynamics behind those processes. We find that absolute plate velocities determine the possible strain regimes in the overriding plate, where overriding plate movement toward the trench inhibits backarc extension and promotes overriding plate shortening. Additionally, a weak and removed backarc lithospheric mantle is required for backarc extension and facilitates overriding plate shortening. Comparison of the models with natural subduction systems, specifically the Andes and Hellenic subduction zones, corroborates that lithospheric removal and absolute plate velocities guide overriding plate deformation.