Collision, mélange and circular basins in north Borneo: a genetic link?

Abstract Attention has been given to the relevance of chaotic rock units including tectonic, sedimentary, and diapiric melanges in the evolution of subduction-related accretionary complexes but they have often been perceived as relicts in suture zones. Northwest Borneo (West Sabah) was evidently a subduction complex which experienced collision caused by the Proto-South China Sea, and a consequence of post-collision intrusion currently expressed as a 4000-metre high mountain, together with gravity tectonics offshore of Palawan and Borneo. The continuous deformation of the accretionary wedge from the collision to the post-collision stages is, however, a matter of contention, along with the timing as subduction ceased. This study integrates the seismic profiles, field observation and topographic interpretation to show that the backstop is composed of ultramafic rocks and a vast amount of severely sheared pillow basalts and radiolarite. Wide shear-bands also exhibit metric phacoid-shaped blocks of basalt and radiolarite. Successor basins cover these collision features to the west, and through a system of circular basins overlying a shale matrix, engulfing metric blocks of basalts, radiolarite and sandstone. The chaotic rock unit was occasionally brought to the surface and the overlying strata are deformed as circular synclines and anticlines. We, therefore, suspect that the mechanism of exposing the melange involved a diapiric process that pushed this low viscosity material upward. Accordingly, the entire process illustrates a tectonic evolution involving gravity tectonics from the end of the collision to the post-collision setting of northern Borneo.