Abstract: New Insights into the Inversion History of the West Natuna Basin

D. C. Ginger, J. Pothecary, R. J. Hedley

Late Eocene to mid-Oligocene transtensional rifting created a complex network of graben in the West Natuna and Malay basins. From the earliest Miocene, the grabens were inverted to form folds and wrench zones as a result of a right-lateral stress regime.

The nature of the inversion is strongly controlled by the orientation of underlying rift faults with respect to the principal stress, sigma 1. Rift basins with a strike oriented at a high angle to the principal stress form folds through reactivation of graben-bounding faults. In

these rifts the synrift graben fill is inverted over the graben footwall, often along a fault with a convex upward geometry.

The magnitude of inversion is closely correlated to the heave of the initial extensional faults; large extensional faults often have large inversion folds associated with them and vice versa. Within any one graben, inversion appears to commence at younger ages away from these large faults. The mechanisms of inversion fold development have been investigated using detailed interpretations of modern seismic data and a section balancing and restoration computer software package. Results of this work are presented in support of the conclusions documented in this paper. The original grabens were formed through extension of basement equivalent to ß = 1.05 to 1.30. In most grabens, at least some of the extension was removed by the subsequent inversion. Amounts of shortening range from 2 to 18%, equivalent to removal of between 40 and 100% of the original graben extension.

Grabens that strike at lower angles to ^sgr₁ form en echelon folds with a significant wrench component, although reactivation of graben bounding faults does not necessarily occur. Fault systems that strike at very low angles to ^sgr₁, for right-lateral wrench zones, usually expressed as a series of north-south-trending extensional faults. Estimated displacement along these wrench zones is relatively small, ranging from 2 to 5 km offset.

West Natuna inversion structures were formed by a series of discrete inversion events beginning in the earliest Miocene; however, each graben has a unique inversion history. Significant events have been dated at 23, 22, 18.5, and approximately 12 Ma, although other events are evident from seismic data. In general, inversion occurred earliest in the eastern parts of the West Natuna basin and progressively spread westward. The termination of inversion appears to follow a similar trend.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994