Post-subduction tectonics induced by extension from a lithospheric drip

Abstract

Subduction termination leads to complex tectonic and geological activity, with the observational record often including clear evidence for exhumation, anomalous magmatism and topographic subsidence, followed by rapid uplift. However, the mechanism(s) driving these responses remain enigmatic and cannot be reconciled with our current understanding of post-subduction tectonics. A prime example of recent subduction termination can be found in northern Borneo (Malaysia), where subduction ceased in the late Miocene (at ~9 Ma). Here we use recently acquired passive seismic data to image, at unprecedented resolution (~35 km), a sub-vertical lithospheric drip, inferred to have developed as a Rayleigh-Taylor gravitational instability from the root of a volcanic arc. We use thermo-mechanical simulations to reconcile these images with time-dependent dynamical processes within the crust and underlying mantle following subduction termination. Our model predictions illustrate how substantial extension from a lithospheric drip can thin the crust in an adjacent orogenic belt, facilitating lower-crustal melting and possible exhumation of sub-continental material, as is observed. These discoveries provide evidence for extension-driven melting of the lower crust, exhumation, core-complex formation and orogeny that also may occur in other areas of recent subduction termination.