Abstract Title: Signatures of Subduction: northern Borneo’s Complex, Seismically Active Mantle Transition Zone Imaged with P-to-S Receiver Functions

Abstract Submitted to: STUDY OF THE EARTH’S DEEP INTERIOR

Abstract Text:

Subduction termination remains a poorly understood tectonic phase, and Sabah, Malaysia (in northern Borneo) is a prime location for its study, having experienced various subduction events throughout the past 50 Ma and beyond. We investigate the impact of these events via P-to-S receiver function (RF) imaging of Sabah’s mantle transition zone (MTZ), the region bounded by olivine phase transitions at approximately 410 km (d410) and 660 km (d660) depths, which is sensitive to subduction-induced changes in temperature and mineralogy. The RFs, computed from high resolution passive seismic data from the northern Borneo Orogeny Seismic Survey (nBOSS), MetMalaysia and KalNet arrays, and publicly available data obtained via IRIS, reveal considerable MTZ complexity. Notably, for RFs produced with Gaussian pulse widths >0.4 Hz, the d410 appears fragmented across much of Borneo, and the d660 broadens from ~100 km to ~200 km in eastern Sabah, with pulse width 0.8 Hz revealing the d660 “splitting” into two distinct discontinuities in eastern Sabah.
In and around Sabah, deep seismicity - a phenomenon usually reserved for active subduction zones - has been observed over the past four years. Sabah's d660 complexity is co-located with three deep focus earthquakes straddling Sabah’s western coastline. Based on this, and velocity anomalies in tomographic models, we hypothesise that the seismicity and complexity are a result of of slab material settling at the base of the MTZ due to buoyancy forces or increased viscosity, with this material introducing non-olivine mineralogy and providing an interface for seismicity. We further hypothesise that the d410 was disrupted by the descent of the slab material. We explore these hypotheses via synthetic modelling of the MTZ, introducing hypothetical slab fragments and mineralogically induced velocity discontinuities (e.g., discontinuities corresponding to non-olivine phase transitions 600-700 km depth) informed by regional reconstructions and tomographic models.