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.