Figure 3 . The topography profiles of the Sula Deep (b), Tolo Trough (c), Cotobato Trench (e), and North Sulawesi trench (f) and the topographic results simulated by the spontaneous model (g) and induced model (h). Location of Celebes Sea (CS) and north Banda Sea (NBS) are shown in Figure 1a. The black vertical lines at 200 km in (b), (c), (e), (f) indicate the trench location. We put the deepest position of every topography profiles at this location. Because the proportion of horizontal axis and vertical axis is not practical, the angles in the figures (g) and (h) are only used to compare with each other, not the true angles.
4.2 Rotation of the North Arm of Sulawesi
The clockwise rotation of the North Arm of Sulawesi is also consistent with the spontaneous retreat of the North Sulawesi Trench. The negative buoyancy drags the slab down and promotes mantle flow, which determines trench retreat and deformation of the overriding plate (Alsaif et al., 2020). Seismic tomography shows that the Sangihe slab is subducting westward below the Celebes Sea plate (Hall and Spakman, 2015; Hayes et al., 2018). The subduction depth of the Sangihe slab increases from east to west; hence, the distance between the Celebes Sea plate and the Sangihe slab decreases from west to east, and the two plates are in contact on the eastern side of the Celebes Sea plate (Figure S1). Therefore, the Sangihe slab substantially reduces the space that is available for subduction on the eastern side of the Celebes Sea plate.
This is a complex 3-D situation, we can first simplify it to a two-dimensional model to simulate it. If we cut a section along the north-south direction (Figure S1). The Sangihe slab will be shown as a rectangular block below Celebes Sea on the profile. The distance between the top of the Sangihe slab and the earth’s surface on the west side is larger than that on the east side. So we can take the top of the Sangihe slab as the bottom of the model to build simplified models. We conduct experiments using the spontaneous subduction model; model depth is reduced from 660km to 400 km, 350 km, 300 km, 250 km and 200 km, respectively, while parameters and boundary conditions are held constant. We find that the reduction of the space available for subduction leads to the decrease of mantle convection velocity, which in turn reduces the trench retreat velocity (Figure 2d). These results are consistent with the hypothesis that the Sangihe slab reduces the space that is available for subduction, inhibiting spontaneous subduction and slowing down trench retreat on the eastern side (at about 123.5°E) of the Celebes Sea (“East” in Figure 2d). Although the length of the subducted slab is larger on the centre (at about 122°E) than the western side (at about 120.5°E) of Celebes Sea, there is always plenty of space for subduction on the western side of the Celebes Sea. As marked by “West” in Figure 2d, the slab reached to a depth of 150 km and the trench retreat speed increased to ~43 mm/yr during a short time. By contrast, space for subduction on the centre is always limited. As marked by “Centre” in Figure 2d, the trench retreat speed is only ~37 mm/yr, although its subducted slab has reached a depth of 260 km. Hence, trench retreat speed decreases from the western side to the eastern side of the Celebes Sea, resulting in the clockwise rotation of the North Arm of Sulawesi.
However, the 2D models ignored the effect of toroidal flow around the slab edge. Hence, we build a 3D model to simulate this complex natural 3D system (Figure 4). The 3D model consists of a continental plate as overriding plate and two oceanic subducted plates. The subduction direction of the two subduction plates is orthogonal. Model parameters and rheology are same with the 2D model. As we can see from the simulation result (Figure 4), the trench retreat speed simulated by 3D model is also roughly equal to the velocity of the North Arm of Sulawesi measured by GPS. According to the modelling results, we found that the subduction-driven mantle wedge flow is much faster than the toroidal flow around the slab edge. The mantle wedge flow of Sangihe slab is mainly parallel to the North Sulawesi Trench which has little effect on the subduction of Celebes Sea slab. The most important factor that affecting the subduct rate of Celebes Sea slab is also the available space for subduction. Therefore, we proposed that the reason for the clockwise rotation of the North Arm of Sulawesi at present is the presence of the Sangihe slab.