Plain Language Summary
Transition of a passive margin into a subduction zone is the key process
of the Earth. However, our understanding for the dynamic mechanism of
this process is still ambiguous. North Sulawesi Trench provides unique
opportunities to study it. Since the North Sulawesi Trench is located in
an area where the plates converge, its formation and retreat have been
thought to have been driven by the collision of the surrounding plates.
However, our simulations suggest that the subduction initiation was
caused by the compression of other plates, but the trench retreat is a
consequence of the negative buoyancy of the subducted plate itself.
Mantle convection driven by subducted slab plays an important role in
trench retreat and overriding plate deformation. The rotation of North
Arm of Sulawesi is also not due to the collision of the surrounding
plates, but because the trench retreat more slowly on its eastern side
than on its western side. The reason for the different retreating speed
is that the subduction space on the east side of the trench is reduced
by the Sangihe slab.
1 Introduction
In the Wilson cycle theory, the oceanic plate formed by seafloor
spreading subducts beneath the continental margin. Numerous young
subduction zones were formed during the Cenozoic, but there have been
few reports of subduction initiation at passive continental margins
(Stern, 2004; Stern and Gerya, 2018). Subduction initiation at passive
continental margins remains a very popular hypothesis that is supported
by some numerical modelling studies (eg. Marques et al., 2013; Nikolaeva
et al., 2010, 2011; Rey et al., 2014; Zhong and Li, 2019). However, in
the absence of geological evidence, it remains a subject of constant
research and debate (Leng and Gurnis, 2015; Mueller and Phillips, 1991;
Stern and Gerya, 2018).
The North Sulawesi Subduction Zone (NSZZ) is located at the convergence
of the Sunda, Australia, and Philippine Sea plates and only about 500 km
long (Figure 1a). Magnetic anomalies and plate reconstruction indicate
that the spreading of the Celebes Sea basin occurred along a
north-northwest to south-southeast axis between approximately 48 and 40
Ma (Gaina and Müller, 2007; Hall, 2012; Rangin and Silver, 1991;
Weissel, 1980). The southward subduction of the Celebes Sea plate began
in approximately 9–5 Ma (Hall, 2012; Lai et al., 2021). The maximum
depth of the subducted plate is about 260 km (Figure 1b). It has been
overlooked as an excellent location for the study of the transition from
passive to active margin (Hall, 2019).
Trench migration is closely correlated with overriding plate deformation
and lateral migration of the subducted slab (Funiciello et al., 2008;
Holt et al., 2015; Schellart, 2008). The North Sulawesi Trench has
retreated since the post-Miocene. Now, the overriding plate–North Arm
of Sulawesi is rotating clockwise around its eastern end (Socquet et
al., 2006) (Figure 1a). The conventional view is that
the collision of the surrounding
plates led to the formation of the NSSZ and the retreat of the trench
(Kopp et al., 1999; Silver et al., 1983). However, the Sunda plate is
moving slowly towards the southeast, while the Australia plate (in the
south) and the Philippine Sea plate (in the northeast) are moving
rapidly towards the northeast and the northwest, respectively (Figure
1a). If the movement of these three plates was the dominant mechanism
underlying the retreat of the North Sulawesi Trench, the North Arm of
Sulawesi would have been more prone to counterclockwise. Thus, the
trench retreat and rotation mechanism of the North Sulawesi Trench
remains unclear.
In this study, we conduct a series of thermomechanical model simulations
to investigate the initial subduction and trench retreat mechanisms of
the NSSZ, which is representative of the transition of a passive margin
into a subduction zone. Variable boundary conditions are applied,
representing different dynamic mechanism. Trench retreat is driven by
either overriding plate or subducted plate, which results in different
regime diagrams. The model results are compared to the observations in
North Sulawesi. The mechanism of model that best fit the observations is
the subduction and trench movement mechanism of the NSSZ.