Satellite image and outcrop-based structural mapping of Brunei and the contiguous region reveals a new fault, the Brunei Bay Fault

Abstract

The cause of tectonic subsidence and uplift in the Brunei Bay region of Borneo Island, SE Asia, has remained one of the critical quests for researchers. Previous works have demonstrated that major rivers, including the Padas, Limbang, and Trusan, were captured during the bay formation; however, the causative tectonic event(s) and structures have remained unresolved. The Island scale structural mapping of Brunei and contagious areas using satellite imagery was used to map the structures and interpret the deformation history. The mapping was supplemented with outcrop mapping at selected regions in the Temburong area, which is critical to answering the questions on the existence of Brunei Bay and its tectonic history. The results suggest that faults control the geometry of folds with open synclines and narrow anticlines, and the spacing between two faults decides the width of the synclines. The larger spacing has produced wider synclines, such as the Belait Syncline. The dominant shale lithology was observed and mapped in Labu Syncline, where some outcrops are of slate-grade metamorphism. The Brunei Bay Fault, mapped as a major >80 km long NNW-SSE trending fault, was discovered using satellite, field and bathymetric data. The fault is interpreted to represent a significant crustal-scale left-lateral strike-slip fault that could have been the extension of the Red River-Ailao Shan (RRAS) fault before the opening of the South China Sea. The fault could be the onshore expression of the West Baram Line. Other significant faults mapped are the Batang Trusan Fault (BTF) and the Batang Mitus Fault (BMF), of which the latter is the youngest.