ORogenic cycle revised: Post-subduction tectonics at A contineNtal marGin

Project Information

ORANG

Grant agreement ID: 790203

DOI

10.3030/790203

Project closed

EC signature date 12 April 2018

Start date 27 March 2019

End date 5 May 2021

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 195 454,80

EU contribution

€ 195 454,80

Coordinated by

THE CHANCELLOR MASTERS AND SCHOLARS OF THE UNIVERSITY OF CAMBRIDGE
United Kingdom

Periodic Reporting for period 1 - ORANG (ORogenic cycle revised: Post-subduction tectonics at A contineNtal marGin)

Reporting period: 2019-03-27 to 2021-03-26

Summary of the context and overall objectives of the project

The primary aim of ORANG was to understand what happens to the continental lithosphere when subduction of one tectonic plate beneath another terminates. Over the course of geological time, this has been a wide-spread phenomenon, with global implications for the growth and evolution of continental plates, long term climate regulation and natural hazards such as volcanos and earthquakes. Yet it has rarely been studied, and the dynamic processes and structural changes that take place in the crust and mantle during and after the cessation of subduction are poorly understood.
One region of the world to have undergone two recent episodes of subduction termination is northern Borneo (eastern Malaysia), where subduction terminated in the last ~9 Ma. In order to directly address the knowledge gap in the subduction cycle, I proposed an observation-led, multi-disciplinary study that specifically targets a post-subduction setting. My results show how a downwelling lithospheric drip, developed after subduction termination, can stretch the crust in an adjacent mountain belt, causing lower crustal melting and possible exhumation of subcontinental material, which can explain intraplate volcanism, subsidence and subsequent uplift as seen in other areas of recent subduction termination.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

ORANG involved the acquisition of passive seismic data to illuminate the lithosphere and underlying mantle structure of northern Borneo, along with the integration of observational constraints into numerical simulations to fully unravel the multi-scale dynamical processes occurring in this complex tectonic setting. I participated in the installation, service and recovery of 46 seismic stations, deployed between March 2018 and January 2020. The dataset was augmented by 28 stations from the national seismic network run by MetMalaysia.
Relative arrival-time residuals from teleseismic P waves have been mapped as 3-D perturbations to obtain a tomographic image of the crust and upper mantle beneath the seismic stations.The P-wave tomographic model has several key implications for reconciling the geological and seismic observations with mantle structure. My results reveal a high-velocity perturbation subparallel to the western coastline of Sabah. This anomaly is clearly visible in map view at 300 km depth with a width of ~70 km, consistent with the thickness of oceanic lithosphere. I have associated this high-velocity anomaly with a remnant of the Proto-South China Sea slab, which ceased subducting beneath northern Borneo around 25 Ma ago.In addition, a vertical profile running from the Semporna Peninsula to Mt Kinabalu in the P-wave tomographic model reveals an elongated high-velocity anomaly extending from ~125-325 km depth. This anomaly does not show lateral continuity (as expected from a slab remnant of the Celebes Sea) and has a dip orientation broadly consistent with the current absolute plate motion orientation. Based upon the joint analysis of our new seismic results with the surface geological evidence, we interpret the northwest dipping high-velocity anomaly as a Rayleigh-Taylor gravitational instability that developed from the Sulu volcanic arc root, which I name the Semporna Drip. I believe the Semporna drip is responsible for thinning of the crust located beneath Mt Kinabalu (4100 m), which ultimately caused melting of the lower crust and emplacement of the mountain itself.
Seismic and geological information have been integrated in a new thermo-mechanical simulation, which validate this hypothesis.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

I have obtained the the first direct and complete set of observations derived from a post-subduction setting that has ever been made, and I believe it is of great importance to our understanding of how the Earth works. The dense instrumental coverage in northern Borneo has yielded a new and high quality passive-seismic dataset. This has allowed me to apply several methods to image in high detail the regional structure, thereby gaining a full picture of how the lithosphere and underlying upper mantle have responded to subduction termination. Additionally, I have reconciled the seismic images with time-dependent dynamical processes implemented in a new thermo-mechanical simulation. My seismic images reveal a lithospheric drip beneath northern Borneo, while my model predictions illustrate how this can be responsible for the puzzling surface features observed from surface geology information.
My model predictions illustrate how significant extension from a downwelling lithospheric drip can thin the crust in an adjacent orogenic belt, causing lower crustal melting and possible exhumation of subcontinental material, which can explain core-complex formations seen in other areas of recent subduction termination. This is a clear and fundamental discovery because it demonstrates how the subduction cycle can end. The development of a lithospheric drip is a short-lived process (~10 My from downwelling to detachment and disappearance in the lower mantle) and can be easily overlooked. However, it can produce considerable extension that is often, and perhaps wrongly, attributed to slab roll-back only.

Lower peak of Mt Kinabalu