Migrated Teleseismic P Wave Receiver Functions Reveal Intricate Post‐Subduction Crust and Upper Mantle Structure of Northern Borneo (Sabah), Malaysia

Abstract

Abstract 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: (a) westward Triassic Palaeo‐Pacific (∼240–200 Ma); (b) southward Cretaceous proto‐South China Sea (PSCS) (92–86 Ma); (c) southward Late Eocene to Early Miocene (∼40–20 Ma) Proto‐South China Sea; (d) northwestward Celebes Sea (∼17–9 Ma); and (e) 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 4,095 m high Mt. Kinabalu granite pluton. We utilize the northern Borneo Orogeny Seismic Survey seismological data set 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. Plain Language Summary Borneo in southeast Asia has experienced a long history of subduction tectonics, where one tectonic plate descends beneath another. Each subduction event changes or adds to the structure of the crust and upper mantle, so we used a seismology imaging technique called receiver functions to make deep images under Sabah in northern Borneo to understand how subduction interacted and formed the present‐day topographical and geological features. Receiver functions are made from seismometer recordings of large distant earthquakes that pass through the Earth and collect information about different geological layers and the boundaries between them. Our new images show a very complex crustal structure that changes in different parts of Sabah, and reveal part of an old subducted oceanic tectonic plate stuck under the crust at 45–55 km depth. Interpretations of the new images help us understand Sabah geology and what happens when several subduction events affect the same region. Key Points P‐wave receiver function discontinuities beneath Sabah reveal complex crust/upper mantle structures in a post‐subduction tectonic setting A westward‐dipping 75–100 km wide discontinuity at 45–55 km depth is interpreted as the oceanic crust part of an underthrust slab remnant Major lateral and vertical changes in crustal and upper mantle structure record multiple subduction events