Numerical simulation and dynamic analysis of ocean-continent coupling effects in Molucca Sea area, Indonesia

Abstract

The Molucca Sea area, situated on the southwestern side of the Philippine Sea in eastern Indonesia, is critical research area for the study of the ocean-continent coupling effect and subduction dynamic system of the western Pacific. Despite its significance as a research hotspot, several research gaps exist in this region. We aim to address two outstanding scientific issues: the cause of the Sangihe Forearc Thrust (SFT) in Molucca Sea, and the mechanism of volcanic discontinuity and migration of Halmahera arc. Numerical simulation is utilized to analyze these issues. For the SFT in Molucca Sea, our results show that plate boundary stress and volcanic loading are two critical factors affecting forearc thrusting during asymmetric divergent double subduction (DDS). In the northern part of the DDS in Molucca Sea, the SFT is primarily caused by plate boundary stress. This stress is mainly generated by the southwestward subduction of the Philippine Sea Plate. In contrast, the SFT in the southern part of the DDS is mainly caused by the effects of differential volcanic loading. The effect of volcanic loading on the Halmahera forearc is considerably stronger than that on the Sangihe forearc, resulting in more severe vertical deformation and subsidence of the former. Consequently, the Sangihe forearc, which exhibits less vertical deformation, is thrust over the Halmahera forearc. For the mechanism of volcanic arc migration in Halmahera, our results show that the dehydration depth of subduction slab and the temperature structure of mantle wedge are closely linked to the rates of subducting or overriding plates. A lower rate of subducting plate or a higher rate of overriding plate is favorable for arc magmatism. Changes in the rates of subducting and overriding plates are identified as the cause of the magmatic activity interruption and volcanic migration of Halmahera arc after the Middle Miocene. During the Miocene-Pliocene period, the rate of subducting plate was lower than the westward convergence rate of the eastern microplate, which created a high-temperature zone favorable for arc magmatism. However, starting in the Middle Pliocene, the rate of subducting plate became close to the westward convergence rate of the eastern microplate fragments due to the tilted subduction of the Philippine Plate, which was unfavorable for arc magmatism. This led to the interruption of volcanic activity and the westward migration of the volcanic arc. In the Holocene, the westward migration of Halmahera arc was blocked, leading to an accelerated convergence of the eastern microplate. As a result, the volcanic activity of Halmahera arc resumed in a new location with a high-temperature zone favorable for arc magmatism.