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.