Structural controls on the evolution of the Kutai Basin, East Kalimantan

The Kutai Basin formed in the middle Eocene as a result of extension linked to the opening of the Makassar Straits and Philippine Sea. Seismic profile s across the northern margin of the Kutai Basin show inverted middle Eocene half-graben oriented NNE-SSW and N-S. Field observations, geophysical data and computer modelling elucidate the evolution of one such inversion fold, NW-SE and NE-SW trending fractures and vein sets in the Cretaceous basemen t have been reactivated during the Tertiary. Offset of middle Eocene carbon ate horizons and rapid syn-tectonic thickening of Upper Oligocene sediments on seismic sections indicate Late Oligocene extension on NW-SE trending en -echelon extensional faults. Early middle Miocene (N7-N8) inversion was con centrated on east-facing half-graben and asymmetric inversion anticlines ar e found on both northern and southern margins of the basin, Slicken-fibre m easurements indicate a shortening direction oriented 290 degrees-310 degree s. NE-SW faults were reactivated with a dominantly dextral transpressional sense of displacement, Faults oriented NW-SE were reactivated with both sin istral and dextral senses of movement, leading to the offset of fold axes a bove basement faults. The presence of dominantly WNW vergent thrusts indica tes likely compression from the ESE. Initial extension during the middle Eo cene was accommodated on NNE-SSW, N-S and NE-SW trending faults. Renewed ex tension on NW-SE trending faults during the late Oligocene occurred under a different kinematic regime, indicating a rotation of the extension directi on by between 45 degrees and 90 degrees, Miocene collisions with the margin s of northern and eastern Sundaland triggered the punctuated inversion of t he basin. Inversion was concentrated in the weak continental crust underlyi ng both the Kutai Basin and various Tertiary basins in Sulawesi whereas the stronger oceanic crust, or attenuated continental crust, underlying the Ma kassar Straits, acted as a passive conduit for compressional stresses. (C) 1999 Elsevier Science Ltd. All rights reserved.