Structural controls on basin evolution in the Kutai Basin and Mekassar Straits.
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 lurassicCretaceous
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 halfgraben.
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 enechelon
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.