The tectonic evolution of Sulawesi is shaped by complex subduction processes, yet the geometry and extent of its slabs remain debated. Using teleseismic P-wave tomography, we present a new 3-D model of the lithosphere and underlying upper mantle beneath Sulawesi, based on passive seismic data from 89 seismic stations. Our results reveal at least three distinct slabs beneath North Sulawesi: a south-dipping Celebes Sea slab, a westward-subducting Sangihe slab, and a north-dipping structure that likely represents the Sula slab. Beyond the subduction system, we identify a prominent low-velocity anomaly beneath the East Arm of Sulawesi, extending from ~200 to ~500 km depth, which we interpret as partial melting in the mantle transition zone. This process may be driving localized volcanism in the Gulf of Tomini. These findings provide new constraints on subduction dynamics and highlight the role of deep mantle processes in driving surface magmatism.

The Semail ophiolite and the UAE-Oman mountain range offer a rare window into continental subduction and exhumation. Despite almost half a century of conflicting studies, the geometry and number of subductions that characterised the formation of the ophiolite and subsequent obduction remain debated. Competing tectonic models range from one subduction to the NE that started with ophiolite and sole formation at $\sim$96-95 Ma, multiple metamorphic events in conjunction with opposite subduction polarity involving prograde metamorphism at $\sim$110-130 Ma followed by retrograde metamorphism at $\sim$80 Ma, and the juxtaposition of two subduction zones. This study investigates the tectonic evolution of the Semail ophiolite, and the subduction polarity structurally beneath it using 3-D P- and S-wave tomographic models integrated with global tomographic datasets, plate reconstruction frameworks, and surface geology.   The results reveal a northeast-dipping high-velocity anomaly extending 250 km beneath the metamorphic sole exposed in the northern mountain range, and 150 km beneath Jebel Akhdar, which I interpret as remnants of the Neo-Tethyan slab attached to the Arabian continental margin. These observations offer a plausible explanation for the contrasting exhumation conditions between the northern and southern segments of the mountain range. In the southern region, evidence of possible slab detachment is consistent with the exhumation of high-pressure rocks, such as those exposed in the Saih Hatat Dome. This study challenges earlier hypotheses of southwest-directed subduction and strongly supports a single northeast-dipping subduction system, providing new insights into continental subduction, obduction, exhumation, and the creation of orogenic belts.

Borneo is a key piece of the regional tectonic puzzle of southeast Asia and a unique location to study subduction termination, particularly in the north. The tectonic history of Sabah in northern Borneo includes elements of five different subduction events: 1) westward Triassic Palaeo-Pacific (~240-200 Ma); 2) southward Cretaceous Proto-South China Sea (92-86 Ma); 3) southward Late Eocene to Early Miocene (~40-20 Ma) Proto-South China Sea; 4) northwestward Celebes Sea (~17-9 Ma); and 5) southeastward Middle Miocene Sulu Sea (~15-10 Ma) subduction. Anti-clockwise (~45°) rotation of Borneo and Neogene (~23-5 Ma) uplift and subsidence events preceded Late Miocene-Early Pliocene exhumation and emplacement (7.8-7.2 Ma) of the 4095 m high Mt. Kinabalu granite pluton. We utilize the northern Borneo Orogeny Seismic Survey (nBOSS) seismological dataset to calculate teleseismic receiver functions and constrain crust and upper mantle discontinuities beneath Sabah using common conversion point migration imaging. Mid-crustal and Moho discontinuities occur beneath most of Sabah at 22-25 and 30-40 km depth, respectively, but major lateral variations delineate lithospheric boundaries that appear associated with surface ophiolites, Mt. Kinabalu granite, and sedimentary depocenters. We show evidence for an uppermost mantle slab remnant beneath Sabah in the form of a gently westward-dipping, 75-100 km wide, discontinuity at 45-55 km depth, likely composed of oceanic crust, its lithosphere detached, and associated with Celebes Sea subduction. Sabah geology, tectonics, uplift and subsidence are reexamined in light of the new lithosphere images, revealing Sabah and post-subduction tectonics in unprecedented detail.

Subduction termination remains poorly understood. Sabah, Malaysia (in northern Borneo) is a prime study location due to its subduction-rich history. We investigate the impact of subduction on Sabah’s mantle transition zone via P-to-S receiver function imaging. Receiver functions are computed using high resolution passive seismic data from the northern Borneo Orogeny Seismic Survey (nBOSS), MetMalaysia and KalNet arrays. Stacking of ~1800 receiver functions reveals considerable complexity, including a low amplitude 410 km discontinuity in north-western Sabah and two distinct discontinuities at depths ~675 km and ~735 km in eastern Sabah. Since 2019, three deep (>600 km) earthquakes have occurred near these discontinuities. We attribute these observations to subducted Proto-South China Sea slab material settling at the base of the mantle transition zone, introducing cool, hydrous oceanic material with a high-basalt fraction to the mantle, thus providing an interface for deep seismicity, and facilitating garnet phase transitions at 600-700 km depth.