Kutai Basin, Borneo
controls on the basin evolution of the Kutai Basin and Makassar
The Kutai basin is an inverted extensional basin located onshore Borneo, to the west of the Makassar Straits. Basin initiation had occurred by the middle Eocene and was contemporaneous with oblique oceanic spreading in the Makassar Straits. Field studies, geophysical data and computer modelling elucidate the structural evolution of the rift basin.
Vein sets, C-S fabrics and chevron folds were studied in the Jurassic-Cretaceous basement and NW–SE and NE–SW trending fractures were reactivated during the Tertiary controlling the basin architecture. Seismic profiles across both the northern and southern margins of the Kutai Basin show inverted middle Eocene half-graben. These were infilled by syn-rift successions that demonstrate considerable lateral and vertical facies variations. Provenance studies of syn-rift sediments suggest differing source areas for individual half-graben. Offsets of middle Eocene carbonate horizons and thickening of syn-tectonic units seen on seismic sections, indicate late Oligocene extension on NW–SE trending en-echelon extensional faults. Middle Miocene inversion was concentrated on east-facing half-graben and asymmetric inversion anticlines are found on both the northern and southern margins of the Kutai basin. Structural data from these anticlines suggest a shortening direction oriented 290°-310°. The presence of dominantly WNW vergent thrusts indicates compression from the ESE.
During the middle Eocene graben-bounding faults initially formed offset en-echelon arrays trending N–S, NNE–SSW and NE–SW characteristic of oblique extension of a basement fabric. Reactivation of these structures led to breaching of relay ramps and ‘hard-linkage’ of faults. The orientation of middle Eocene dykes and faults show the extension direction was oriented WNW–ESE.
A reorientation of the stress direction during the late Oligocene was inferred from extension on en-echelon NW–SE trending faults. A rotation of the extension direction by between 45 and 60° counter-clockwise is suggested by fault and vein orientations. Neogene micro-continental collisions with the margins of northern and eastern Sundaland strongly influenced the inversion of the basin. Inversion was focused in the weak attenuated crust underlying the Kutai basin and adjacent basins, whereas the stronger oceanic crust underlying the northern Makassar Straits acted as a passive conduit for compressional stresses. This study has implications for the development of rift basins associated with oceanic spreading and for regional tectonics.