3 Model Results
3.1 Parameters that affect spontaneous subduction
Since the subduction initiation of NSSZ occurred at 5–9 Ma and the
maximum depth of the subducted slab is about 260 km at present. The
simulation ends when the subducted slab reaches a depth of 260 km or the
time reaches 10 myr. If the oceanic slab does not exceed a depth of 150
km within 10 myr, we denot it by “No” (Figur 2b and Table 1). If the
maximum depth of the subducted slab is deeper than 150 km but shallower
than 260 km (Figure 2b), we list the depth. For the experiments where
the slab reached 260 km depth within 10 myr, we list this time (Figure
2b).
Experiments show transition from fast to slow subduction with increasing
thickness of the continental lithosphere. Spontaneous subduction
initiation is almost impossible to occur with 70 km or more thick
continental plate (Experiment 9–12, 21–24). The effect of density
contrast is opposite to that of the thickness of the continental plate.
A larger density contrast promotes spontaneous subduction initiation.
The ocean-continent boundary angle has a nonlinear effect on the
spontaneous subduction initiation. Either too large (60°) or too small
(15°) angle are all adverse to spontaneous subduction initiation. The
thickness and density of the continental lithospheric mantle and the
ocean-continent boundary angle
all affect whether subduction
initiation occurs spontaneously. It suggests that the force due to the
density difference between the continental and oceanic plate is the main
force leading to the spontaneous subduction initiation.
3.2 Trench retreat velocity in spontaneous and induced model experiments
Trench location and speed are inferred from the location and speed of
the edge of the continental crust that is the farthest forward in the
model. In all spontaneous and induced model experiments, trench retreat
speed increases and then decreases during the first 1–2 myr (Figure
2c). At the beginning of the simulation period, the density difference
between the continental and oceanic crusts gives rise to lateral
stresses that push the continental crust towards the ocean; as the
difference between the stresses decreases, trench retreat speed
decreases gradually. This process is active in approximately 2 myr.
After the first 2 myr in the spontaneous model (Figure 2c), subduction
depth increases, the trench retreat velocity increases correspondingly.
The increasing begins slowly, then more and more rapidly. It is because
as the oceanic plate sinks, there are more and more eclogite-face
oceanic crust to increase the negative buoyancy of the subducted slab.
In the induced model, the trench retreat speed increasing slowly, then
becoming almost constant. The final speed is slightly greater than the
specified velocity V in.
3.3 Deformation and heat flow of the overriding plate
The deformation of the overriding plate varies in the different
subduction models. In the spontaneous subduction model, the rollback of
the subducted plate obviously causes stretching of the overriding plate.
The crustal thickness decreased from 30 km to about 23 km (eg.
Experiment 2, 3). In the induced subduction model, the overriding plate
thrusts over the subducted plate, and the overriding plate is slightly
thinned (Experiment 25–30). The latest published geological profile of
the North Arm of Sulawesi indicates lithospheric
significant extension between the
Late Miocene and Pliocene (Nugraha et al., 2022, Advokaat et al., 2017),
which is consistent with the results of the spontaneous subduction
model.
The heat flow of the overriding plate is closely related to the
lithospheric thickness either in spontaneous or induced model
experiments. It is opposite to the continental lithospheric thickness.
The values of heat flow listed in Table 1 is the calculated surface heat
flow which lies at a distance of 100
km above the top of the subducted slab. It corresponds to the location
of the maximum heat flow (68 mW/m2) on the North Arm
of Sulawesi (Figrue 1b). In all the experiments, the model with 75 km
thick continental lithosphere has the closest value to that observation.
The heat flow values in models have a 70 km or thinner lithosphere are
all greater than the maximum observed values.
4 Discussion
4.1 Implications for transition of passive margins into subduction zones
The mechanism of subduction initiation at passive margins have been
studied by several numerical simulations (Faccenna et al., 1999; Goren
et al., 2008; Marques et al., 2014; Turcotte et al., 1977; Zhou et al.,
2020). Our intention is not repeat other studies and prove the
feasibility of spontaneous
subduction initiation at passive
margins, but to estimate the subduction mechanism through the
constraints of the geological case–NSSZ. The spontaneous and induced
subduction models represent the extreme cases of subduction driven by
horizontal far-field forces and of subduction driven by the negative
buoyancy of the down-going plate itself. Examples of completely
spontaneous or induced mechanisms are difficult to reproduce the
evolution of NSSZ—subduction depth of 260 km after approximately 5–9
myr, 68 mW/m2 heat flow of the overriding plate and
obvious extended continental crust. Because the mainly force that cause
the oceanic plate to bend and sink in spontaneous subduction initiation
is lateral stress, which is caused by the density difference between the
continental plate and the oceanic plate (Nikolaeva et al., 2010). A
thick crust and hot lithosphere are necessary to produce the
sufficiently large lateral stress (Nikolaeva et al., 2010). According to
the present crustal structure and heat flow of North Arm of Sulawesi,
spontaneous subduction initiation
is unlike to occur in the NSSZ. Therefore,
an external force is necessary to
push the continental plate towards the oceanic plate in the early stage
of subduction initiation. For the NSSZ, the external
force may originate mainly from
the collision of the surrounding plates.
Because since the Cenozoic,
Sulawesi has been at the center of the convergence of surrounding plates
(Hall, 2012). This induced mechanism dominates the early stage of
subduction initiation, which takes about 2 myr. Once the oceanic crust
reaches the depths of ~60 km where eclogite
transformation can occur, the main force controlling the subduction
begin to change from the horizontal external force to the vertical
negative buoyancy of the
subducted slab. The subsequent subduction, trench retreat and extension
of continental crust are all mainly depend on the negative buoyancy of
the subducted slab, as exhibitions in spontaneous model experiments
(Table1 and Figure 2). In addition, adjacent areas in the same tectonic
setting can also provide insight into the subduction process.
The Sula Deep, Tolo Trough and Cotobato Trench located near the North
Sulawesi Trench (Figure 3a, d) are considered to examples of three
stages in the subduction initiation. Sula Deep and Tolo Trough is
suggested to record the earliest stages and second stage in subduction
development, respectively (Hall, 2019). Cotobato Trench is interpreted
as the third stage where a seismically defined slab can be recognized
(Hall, 2019). The North Sulawesi Trench represents the final stage,
which indicates that subduction initiation is end and a mature
subduction zone has been formed. From the topography of these trenches
(Figure 3), we can see that each trench has a different landward slope.
The areas (Sula Deep and Tolo Trough) in early stage of subduction
initiation, has a very steep landward slope (Figure 3b, c). As the
subduction development (Cotobato Trench and North Sulawesi Trench), the
landward slope changes from steep to gentle (Figure 3e, f).
There are obvious differences in landward slope angles vary in different
mechanisms models. In the spontaneous numerical model, the slope
gradually smooth with the subduction development (Figure 3g). While the
slope in induced model is steeper than that in spontaneous model and the
slope can be maintained at a steeper state under the push of far field
forces (Figure 3h). Combining the steeper slope in the Sula Deep and
Tolo Trough and gentle slope in Cotobato Trench and North Sulawesi
Trench, we could also suggest the subduction slab controlled the retreat
of the trench.