From subduction to arc-continent collision: Geodynamic modeling of strain partitioning and mountain building in the Indonesia Archipelago

The arc-continental collision in the eastern Indonesia archipelago began 3-5 million years ago. This process has been continuously building one of the youngest orogenic belts on Earth. Both GPS measurement and geological observation indicated westward weakening of crustal shortening in this region, correlating with the transition from continent-arc collision in the Timor trough to oceanic slab-arc convergence in the Java trench. We have developed a finite element model to investigate how the continent-ocean floor transition on the subducting Australian plate and rheologic heterogeneity of the Sunda region have controlled strain partitioning and mountain building in the overriding plate. To simulate the progressive oblique collision between the Indo- Australian plate and Southeast Asian plate, we constructed the model in 3D with nonlinear viscous rheology that also includes fault zones and rheological heterogeneity in both horizontal and vertical direction. Topographic loading is considered, and tectonic loading is calculated from the velocity boundary condition based on either Nuvel-1A or GPS measurements. Our model showed direct link between plate coupling and crustal shortening in the upper plate. When the coupling is weak, deformation concentrates near the trench. As coupling increases, more plate convergence is accommodated by crustal shorting in the upper plate. The initiation of the continent-arc collision in the eastern Timor could explain the cease of oceanic floor spreading in the south Banda basin and the development of back arc thrust. Slipping of the back arc thrusts played a critical role in uplift and tilting of the upper plate in the double-vergent Timor islands. Within a reasonable range of rheological parameters, our model is able to predict surface velocity, uplift rate, and stress states consistent with the GPS data, geological observation and earthquake mechanism solutions. Results of this model provide some useful insights into the evolution of this young orogenic belt.