Subduction without volcanic arc magma: Insights from two young subduction zones in the western Pacific

Missing volcanic arcs in young subduction zones are commonly attributed to an insufficient subduction depth of the downgoing slab. The North Sulawesi subduction zone (NSS) in the western Pacific is deeper than the average volcanic sub -arc depth; however, no corresponding volcanoes or magmas are along this arc. To investigate this apparent contradiction, we conducted a comparative analysis of the NSS with the middle section of the Philippine subduction zone (MPS) examining the plate flexure, slab dehydration, and mantle wedge thermal structure. Based on our results, the subducting plates of the NSS exhibit a shallow maximum depth of the yield zone, indicating a lower water content. Additionally, the dehydration depth in the NSS system is shallower compared to MPS. The wedge temperature corresponding to this dehydration depth is exceptionally low, which restricts the production of a significant amount of arc magma. Therefore, we propose that the absence of arc magma in the NSS could be explained by the little water influx and the shallow water dehydration of the subducting slab. Our findings also suggest that, when the angle between a young slab and the overriding plate is small, a long arc magma history will not be observed during early subduction stage.