Figure 1 . Structure-tectonic maps of the Celebes Sea and
surrounding area. (a) Location of the study area—Celebes Sea
(CS)—within the wider Southeast Asian region, including the Sulu Sea
(SS), the South China Sea (SCS), the north Banda Sea (NBS) and the south
Banda Sea (SBS). Thick lines indicate subduction zones; triangles on the
lines indicate the down-dip direction. Plate motions are indicated by
the arrows (Mustafar et al., 2017; Kreemer et al., 2014). Red triangles
indicate volcanoes (Global Volcanism Program, 2013). (b) Regional
topographic map. The yellow hollow circles are heat flow measurements
(Fuchs et al., 2021). The pink dashed lines show the major magnetic
anomalies (Weissel, 1980). The irregular shape with transparent colour
gradient represents the subducted plate; colours indicate depth. The
dotted lines with triangles indicate trench retreat. Events in the EHB
catalogue (Engdahl et al., 2020) are represented by black circles.
Model setup
We used the geodynamic code ASPECT (Advanced Solver for Problems in
Earth’s ConvecTion, version 2.3.0-pre) to setup a series of 2D
subduction models. The initial model
consists of oceanic plate and
continental plate on either side (Figure 2a). According to the
evolutionary history of NSSZ, the age of oceanic plate is taken to be 40
Ma. The oceanic lithosphere geotherm corresponds to a half-space cooling
model for a seafloor age of 40 Ma. The thickness of oceanic plate is
therefore set to 82 km. The oceanic crust consists of two layers and is
10 km thick in total. The
composition for the mantle part of the oceanic lithosphere and for the
asthenosphere are taken to be identical. The density of them is both set
to 3300 kg/m3. The continental plate consists of upper
crust, lower crust and lithospheric mantle. The continental crust is set
to 30 km based on the current crustal thickness of Sulawesi (Fauzi et
al., 2021). Because the initial lithospheric thickness, density of
lithospheric mantle and ocean-continent transition angle have never been
definitely settled, we vary the values of these three parameters to
investigate their effect on the spontaneous subduction (Table 1).
Temperature structure of the continental plate is controlled by the
lithospheric thickness and is defined by a steady state profile from 0 ℃
at the surface to 1300℃ at the base of lithosphere (Figure 2a).
Spontaneous and induced
subductions are modelled by controlling the boundary conditions and
density differences between oceanic plate and asthenosphere.
The spontaneous models
(Experiment 1~24) have all free slip boundaries (Figure
2a) and the Gabbro-Eclogite phase
transition for the oceanic crust was implemented. Density of oceanic
crust increases from 3000 kg/m3 to 3450
kg/m3 at pressures of ~1.9 GPa
(~60 km depth). In the induced model (Experiment
25~30), horizontal far field forces are represented by
specifying a convergence velocity (V in) on one
side of the overriding plate; the Gabbro-Eclogite phase transition for
the oceanic crust was not executed.