The causal mechanism of the Sangihe Forearc Thrust, Molucca Sea, northeast Indonesia, from numerical simulation

Abstract

• Numerical simulation of forearc collision and thrusting. • Exploring the causal mechanism of the Sangihe Forearc thrust. • Plate boundary stress and arc volcanic loading are non-negligible factors on thrusting. • Causal mechanism difference in Sangihe Forearc thrust in north and south parts of Molucca Sea subduction zone . The Molucca Sea subduction zone contains an interesting geological phenomenon: the Sangihe Forearc Thrust (SFT). At 10 Ma, divergent double subduction (DDS) in the Molucca Sea was initiated, resulting in the convergence of the Sangihe and Halmahera forearcs, and their subsequent collision at 2 Ma. Associated with this collision, the Sangihe Forearc was thrust over the Halmahera Forearc. Currently, the causal mechanism associated with the SFT remains unknown. Here, we simulate the structural and morphological characteristics of forearc collision and thrusting within this DDS zone under different conditions, calculate the temperature structures and characteristics of magmatic activity under the Sangihe and Halmahera arcs, and explore the causal mechanism of the SFT. Our results demonstrate that plate boundary stress and volcanic loading are two non-negligible factors affecting the forearc thrust. We identify different causal mechanisms for the SFT in the northern and southern parts of the asymmetric DDS zone in the Molucca Sea. In the northern part of the DDS zone, the SFT is primarily caused by plate boundary stress, which is mainly generated by the southwestward subduction of the Philippine Sea Plate. In the southern part of the DDS zone, the SFT is mainly caused by the effects of differential volcanic loading. The effect of volcanic loading on the Halmahera Forearc is noticeably stronger than that on Sangihe Forearc, resulting in more severe vertical deformation and subsidence of the former. Finally, the less vertically-deformed Sangihe Forearc was thrust over the Halmahera Forearc under the action of horizontal arc-arc collision extrusion.