Complex Morphology of Subducted Lithosphere in the Mantle below the Molucca Collision Zone from Non-linear Seismic Tomography

Abstract

Abstract.Results of seismic studies presented in previous publications depict two opposing subducted oceanic lithospheric slabs under the Molucca region. This unique structure is related to the arc-arc collision between the Halmahera and Sangihe arcs. Recently, we have revisited the complex subduction zone structure by employing a non-linear tomographic imaging technique in which 3-D ray tracing has been implemented. We have used P-as well as S-wave arrival times from carefully reprocessed global data set. The results provide some improvements in the positioning of wave-speed anomalies. Consistent with earlier results, the new P-wave model depicts the two opposing subducted slabs of the Molucca Sea plate. The intriguing new observation is that the westward dipping slab appears to penetrate into the lower mantle by taking the form of folded slab. We envisage that the folding behavior may have been caused by the shift of the whole subduction system in the Molucca region toward the Eurasian continent due to the westward thrust of the Pacific plate combined with the large left-lateral movement of the Sorong fault. The inversion of traveltime residuals of direct S phases strongly confirms the new observation.

Travel Time Tomography Beneath the Molucca Sea Collision Zone Based on Local and Regional BMKG Networks Using a 3-D Model

“…Previous studies of P-wave tomography have also identified a double subduction of the Molucca Sea Plate to depths of ~600 -650 km subducting to the west, and depths of ~250 -400 km subducting to the east. (Puspito et al 1993;Widiyantoro and Hilst 1997;Hafkenscheid et al 2001;Widiyantoro 2003;Amaru 2007;Fan and Zhao 2018). The arch of double subduction in this area is in the uppermost mantle layer with a depth of ~50 km, which is consistent with the geometry of the slab2 model from Hayes et al (2018).…”

“…Meanwhile, the slab that subducts to the east under the Halmahera Arc reaches a depth of 400 km, which is conformable with the results of the study by Widiyantoro and Hilst (1997). This research was later updated by Widiyantoro (2003) who shows that the subducted slab to the west reaches the lower mantle with a folded-looking slab. Huang et al (2015) also detected a high-velocity zone in the form of bipolar subduction.…”

“…These results are consistent with those of Chou et al (2009), Zhao (2015) and Zenonos et al (2019Zenonos et al ( , 2020 who conclude that high-Vp, high-Vs, and low-Vp/Vs can be associated with slab subduction. Previous studies of P-wave tomography also identified a double subduction of the Molucca Sea Plate (Puspito et al 1993;Widiyantoro and Hilst 1997;Hafkenscheid et al 2001;Widiyantoro 2003;Amaru 2007;Fan and Zhao 2018) in which the plate subducting to the west could reach the mantle transition layer at a depth of ~600 km. According to Fan & Zhao (2018), the Molucca Sea Plate is sinking deeper in the north than in the south.…”