Thermal rheological structure analysis of crust and upper mantle in Sulawesi based on joint tomography of surface wave and gravity data

Abstract

Sulawesi Island is situated to the east of Kalimantan Island, composed of four branches. Throughout its tectonic evolution, influenced by both the Sulawesi Sea and the Banda Sea, Sulawesi Island involves the continental crustal fragment displacement and reorganization. Therefore, it is pivotal for studying geodynamic problems of ocean-continent coupling and subduction rollback. In this paper, combining dispersion data extracted from seismic surface wave and ambient noise with the satellite gravity anomaly data, we conducted a joint tomographic inversion to obtain a three-dimensional VS velocity model around Sulawesi. Combined with heat flow data, we derived a temperature structure and calculated the thickness and viscosity variation characteristics of the thermal rheological boundary layer at the bottom of the lithosphere by the thermal rheological method. The results suggest that: 1) The thickness of the rheological boundary layer in the southeastern part of Sulawesi Island, closer to the Banda Sea, is greater than that in the northern part, closer to the Sulawesi Sea. The different deformation rate, driven by lateral variations in rheological structure within the lithosphere, may control the continental crustal fragment displacement and reorganization in the Cenozoic, resulting in the maximum northward velocity in the eastern part of the East Branch of Sulawesi Island and the second in the western part of the North Branch of Sulawesi Island. 2) The low VS anomalous velocity zone, corresponding to the overlying crust of the subduction retreating plate of the North Sulawesi trench, likely represents a fluid-rich weak plate, leading to the extensional environment that contributes to the trench retreat. During the retreat, the area between the North Branch of Sulawesi Island and the Sulawesi subduction exhibits different rheological strengths under the influence of the surrounding tectonic stress field, resulting in the discontinuous Sulawesi subduction rollback.