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The Molucca Sea region is an active arc-arc collision zone whose structure and mantle dynamics are essential for reconstructing the tectonic evolution of Southeast Asia. The key to understanding the tectonic processes of the region is to clarify the morphology of the subducted slabs. However, geometric details of the subducted Molucca Sea slab and other slab segments under this region remain controversial due to the lack of high-resolution mantle tomography. In this study, we jointly invert a large number of local and teleseismic travel-time data recorded at 99 local seismic stations to determine a high-resolution model of P-wave mantle tomography beneath the Molucca Sea region. Our tomography reveals two aseismic slab segments in the deep upper mantle and the mantle transition zone, which may reflect the earlier subducted Molucca Sea slab. The total length of the Molucca Sea slab is estimated to be ∼1800 to 1900 km, being ∼300 km longer than previous estimates. • The Molucca Sea slab has experienced break-offs. • Aseismic slab fragments broken off from the early subducted Molucca Sea slab are revealed in the mantle transition zone. • The maximum length of the Molucca Sea slab is ∼1800 to 1900 km. [ABSTRACT FROM AUTHOR]

The modern volcanoes of Indonesia, represent the cycling of material and heat along the Southeast (SE) Asian subduction zone. This volcanic activity is well exemplified in the middle section of Java Island, which is found on the Sunda arc portion of the SE Asian convergent system. Central Java's volcanoes inherit their geochemistry from silicate melts that formed due to hydrous melting at a range of depths above their subducting slab: ∼136 to ∼272 km. It has been shown that across-arc variations in magmatic trace elements like boron (B) can shed light on the quantity and type of slab-derived fluid released in a forearc to back-arc sequence. In this study, we present major, trace and radiogenic chemistry data for five volcanoes arranged in a roughly N-S lineament across Central Java. We show that the observed chemical changes across the section of the Sunda arc can be explained by variations in different kinds of source from fluid mobile material being devolatilized across the arc. Our mixing models suggest that the magmas underlying these volcanoes see a decreasing ratio of sediment to altered oceanic crust (AOC) contributions from the volcanic front (Merapi volcano) toward the rear-arc (Muria). We estimate that a maximum ∼1% contribution of sediment-derived fluid is sufficient to explain the chemical systematics of this arc segment. Shallow slab dehydration, mostly from the sediment cover, at low temperature (at <60 km of slab depth) transfers the fluid high in B/Nb, Ba/Nb, 87Sr/86Sr and 207Pb/204Pb to the mantle beneath this region's arc front, inducing flux melting. Farther from the trench, the signature of the subducting oceanic plate increases and peaks at around ∼160–170 km of slab depth, carrying substantial slab liquids with moderate to low B/Nb and Ba/Nb with lower 87Sr/86Sr and 207Pb/204Pb to the mantle beneath middle- and rear-arc region. Beyond 200 km, in the rear-arc, evidence for slab liquid is low, but not non-existent. Unexplained Ba-enrichment in magmas from across the arc may account for the role of subduction serpentinite dehydration in the subduction channel or local crustal assimilation. This study provides a conceptual model for material transfer from the subducting slab in SE Asia. Our findings have important implications for the understanding of geochemical cycling in this region, as well as the impact of these cycles on present and past volcanism. • Slab liquid with different proportion of SED/AOC is evident across Central Java, high at arc front and low at the rear-arc • Shallow slab liquid is high in FME, 87Sr/86Sr, 207Pb/204Pb; deep slab liquid has moderate/low levels of those components • Elevated Ba/Nb across the arc magmas may hint to the importance of subducting serpentinite in subduction channel [ABSTRACT FROM AUTHOR]

Subducted slabs provide the primary driving force for mantle convection and plate tectonics. The strength of the slab directly controls the transfer of forces between it and the lithospheric plates at the surface. However, slab viscosity constrained by surface topography and gravity data is significantly lower than that suggested by mineral physics laboratory experiments. It is unclear whether the free-slip top boundary condition used by these studies affects the inverted slab viscosity. In this study, we constrain the subducting slab viscosity structure by comparing the computed topography and gravity in slab subduction models with a free surface with observations. The free-surface model results support relatively weak slabs (20–120 times more viscous than the upper mantle) at the bending region, consistent with previous studies with a free-slip top boundary. The viscosity of the slab below the bending region barely affects the surface topography and gravity field, and both strong and weak slabs fit the observed topography and gravity field, suggesting that extra independent observations are needed to constrain the deep slab viscosity. We investigate the comprehensive relations between subduction interface viscosity, surface topography and gravity anomaly, and trench motion. Models with trench advance have significantly low topography and gravity above the volcanic arc, contradicting subduction zone observations. Together with present trench motion observations and previous studies, we support the idea that the trench retreats under normal single-slab subduction conditions. • We constrain slab viscosity with topography and gravity anomalies in slab subduction numerical models with a free surface. • Model results support relatively weak slabs (20–120 times more viscous than the upper mantle) at the bending region. • Trench advance requires strong subduction interfaces, which result in significant arc depressions that contradict the observation. [ABSTRACT FROM AUTHOR]