Balancing Deformation in NW Borneo: Quantifying Plate-Scale Vs. Gravitational Tectonics in a Delta and Deepwater Fold-Thrust Belt System

Rosalind C. King¹, Guillaume Backé², Christopher Morley³, Richard Hillis¹, and Mark Tingay^2
1Australian School of Petroleum, University of Adelaide, Adelaide, SA, Australia
²School of Earth & Environmental Sciences, University of Adelaide, Adelaide, SA, Australia
³Geological and Geophysical Services, PTTEP, Bangkok, Thailand

Recent GPS measurements demonstrate that NW Borneo undergoes 4-6mm of plate-scale shortening a year, which is not accommodated by plate-scale structures. The only geological structure in NW Borneo described to accommodate on-going shortening is the Baram Delta and Deepwater Fold-Thrust Belt System (DDFTB) located on the outer shelf to basin floor. Delta toe fold-thrust belts are commonly thought to be caused by margin-normal compressional stresses generated by margin-parallel upslope gravitational extension.

The Baram DDFTB is divided into three neotectonic provinces: 1) an inactive onshore and inner shelf region of inversion superimposed on an older extensional deltaic province, 2) an outer shelf region of present-day deltaic extension and; 3) a compressional delta toe. The inner shelf compressional structures resulted from far-field stresses. However, it is uncertain whether compression in the delta toe area is purely driven by gravity (i.e. shortening is ≤ upslope extension) or if there is a component of regional shortening involved, which can only be demonstrated if shortening is > upslope extension.

In order to quantify the balance in the Baram DDFTB between the shortening related to the upslope gravitational extension and the shortening related to the regional geodynamic frame, we used a geomechanical program based on the Finite Element Method and conservation of mass and momentum, Dynel 2D. This enabled us to reconstruct the tectonic evolution of the delta and to demonstrate for the first time that ~10% of the total shortening observed in the delta toe does not balance against the active extension in the delta top. This ~10% of additional shortening is therefore attributed to plate-scale shortening across NW Borneo produced by far-field stresses, thus, demonstrating that the local thrusts in the delta toe accommodate the plate-scale shortening across NW Borneo.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery