## 2 Geologic Setting  
The intermittent and migratory nature of volcanic activity of the Halmahera arc is intricately linked to the retreat of the Halmahera subduction zone [8]. According to the Molucca Sea Plate evolution model [9; 11; 12; 17; 18; 19], the Molucca Sea Plate, an oceanic plate that separated from the Philippine Sea Plate approximately 15 Ma, has descended into the mantle as a result of double subduction, convergence, and arc-arc collisions on both sides. Eastward subduction beneath Halmahera initiated approximately 11 million years ago (Ma) followed by a westward retreat of the subduction zone at a rate of 1-10 cm/a. Around 2 Ma, volcanic arcs on the eastern and western peripheries of the Molucca Sea began to collide. Analysis of seismic data suggests that during the late Pliocene or Pleistocene, a period of reduced volcanic and subduction activity occurred [12]. Geological investigations on Halmahera Island identify a brief interruption in volcanic activity during the Pleistocene (Figure 1a). The age distribution of the Neogene volcanic arc (Figure 1b) reveals significant details about the evolution of volcanic activity in the Halmahera region. According to [PERSON] and [PERSON] [11], volcanic activity in the Halmahera arc commenced around 11.5 Ma on Obi Island. The volcanic activity experienced interruptions between 11 and 9.5 Ma, followed by a northward migration starting around 9 Ma. Comparing the oldest volcanic age on Obi Island (11.5 Ma) with that of the central region of Halmahera Island (7.7 Ma), the northward migration of volcanic activity occurred at an estimated rate of approximately 6.5 cm/a [13]. Obi Island did not witness volcanic activity before 8 Ma, whereas volcanic rocks in Halmahera Island range in age from 8 Ma to 2 Ma [11]. Around 2 Ma, volcanic activity associated with the Halmahera arc was interrupted once again, followed by a westward migration of the frontal volcanic arc, leading to the onset of Quaternary volcanic activity (Figure 1a). Examining Halmahera subduction and volcanic activity involves the analysis of two key elements: the stratigraphic history and the crust-mantle structure of the arc. [PERSON] [1] originally suggested that the western part was a magmatic arc, whereas the eastern part was defined by a melange zone. The western region of Halmahera Island features an active volcanic arc, with its basement mainly covered by Neogene sedimentary and pyroclastic rocks whose exact lithology and age are uncertain [13; 19]. This suggests that the present volcanic basement in western Halmahera originated from the Late Oligocene to Early Miocene period, following the erosion of the Late Cretaceous to Early Tertiary volcanic arc [20]. Conversely, the eastern basement of Halmahera Island is composed of ophiolite formations mixed with Mesozoic and Eocene sedimentary rocks. Since the Middle Oligocene, this sequence has been overlain unconformably by sedimentary and volcanic deposits [21]. During the early Oligocene, the eastern basement of Halmahera underwent uplift and substantial erosion, followed by gradual subsidence in the middle to late Oligocene [1]. In the Miocene, subsidence progressed from east to west, with the Pliocene seeing an increase in volcanic activity in the eastern region affected by the western volcanic arc [11]. By the Middle Pleistocene, intense deformation led to significant tilting of boundary blocks, extensive folding, and thrusting along both the eastern and western margins of Halmahera Island. The pre-Neogene geology of the Halmahera region consists of a basement of ophiolitic and arc-related rocks overlain by and imbricated with arc volcanic, plutonic, and sedimentary rocks of the late Mesozoic to Eocene age [11; 15]. Following an Eocene uncomformity (possibly linked with the Pacific plate reorganization at this time) a Late Eocene arc developed with associated basaltic pillow lavas and volcaniclastic turbidites; this arc ceased activity in the earliest Miocene and its termination is marked by a regional uncomformity.