The structural evolution of folds in a deepwater fold and thrust belt – a case study from the Sabah continental margin offshore NW Borneo, SE Asia

The deepwater fold and thrust belt offshore Sabah, NW Borneo, is located on a continental margin that hosts a number of proven hydrocarbon accumulations. Reprocessed and depth-migrated regional 2D seismic data reveal the occurrence of a wide range of compressional, syn-depositional deformation features including large-scale thrust faults and prominent thrust-hangingwall anticlines. This paper focuses on the documentation and detailed description of the style of deepwater folds and the associated stratal patterns within a study area of ca. 3000 km2 offshore Sabah, NW Borneo. This region exhibits a variety of fold systems including (1) anticlines with large interlimb angles that exhibit no seafloor expression; (2) anticlines with medium interlimb angles, seafloor relief and a faulted anticline crest; (3) anticlines with small to medium interlimb angles, a seafloor expression and flanks associated with slides and slumps, and finally; (4) anticlines with small interlimb angles that are commonly buried by a thick sedimentary overburden and, in places, modified by superimposed syn-sedimentary, deltaic normal faults. If analysed section per section perpendicular to the strike of the fold and thrust belt, the widest anticlines occur in the most seaward parts of the fold belt near the present-day thrust front, whereas the narrower anticlines are located further landward. However, if viewed along the strike of the deepwater fold belt, there are significant variations in the geometry of laterally corresponding, contemporaneously developing fold systems. To explain these differences, factors such as rock inhomogeneities in the deformation wedge, variable surface slope, irregular basement topography, differential sediment input or the presence or absence of subsurface gas hydrates need to be taken into account. These factors vary significantly within the studied system, and are therefore difficult to generalise and transfer to other submarine fold and thrust belts.