Abstract: Basin Development and Inversion on an Oblique-Slip Convergent Margin: Nias Island, Western Indonesia
A. Samuel, N. A. Harbury, M. E. Jones
The island of Nias forms part of an emergent, nonvolcanic outer-arc ridge and marks the western margin of the Sunda fore-arc basin of northwestern Sumatra. Four tectono-stratigraphic provinces are identified on Nias and are separated by major faults, with original throws of up to 5 km. These faults controlled basin and subbasinal development. Two periods of basin inversion with reactivation of the normal and transtensional faults can be demonstrated.
The first inversion event at 20 Ma resulted in local, but important, unconformity in western subbasins and is recorded by apatite fission track data. Rapid uplift caused overpressuring of basal sections, which behaved diapirically leading to formation of melanges. During the Pliocene, all major extensional faults were reactivated with further widespread basin inversion. This second phase of uplift resulted in remobilization of melanges and further diapirism with associated active mud volcanism. Basement material has been incorporated into some of the melanges as it has been thrusted over rising diapirs.
Field observations of structures indicate original extension followed by later compressional deformation. Abundant small-scale normal faults in the basin-fill sequences, with throws of up to 10 m, resulted from Oligocene and Miocene extension. Subsequent compressional features include fold and reverse fault structures, which are variably developed in the five tectono-stratigraphic provinces. Vergence is generally to the northeast. Lineament studies identify right-lateral and antithetic left-lateral faults and indicate a right-lateral stress regime operated during compression. These faults have been important, in conjunction with the basin-bounding faults, in controlling deformation styles.
Sedimentation in extensional and pull-apart basins began in the Oligocene, with the deep-marine deposits characterized by massive micaceous sandstones deposited on a basement ophiolite complex. The thick sequences passed up into lower Miocene conglomerates, turbiditic sandstones, and mudstones. Distinct detrital components provide evidence for active tectonism at the subbasin margins. Contemporaneous shallow marine carbonates accumulated on highs produced by along-strike, fault-controlled topographic variations in basinal development, while rapid uplift in other areas led to local unconformities. By the middle Miocene, reefal and carbonate shoal deposition became widespread, although some subbasins remained areas of deep-marine sedimentation. Late Miocene subsidence followed with vari d sedimentation in the subbasin. Major reverse movement on the basin-bounding faults during the Pliocene led to widespread uplift and erosion, with subsequent deposition of shelfal limestones and clastics derived from the uplifted areas in the fore arc.
Although important differences exist in the stratigraphic development of separate subbasins exposed on Nias, there is an underlying similarity in the structural evolution through the Tertiary. Moreover, the early evolution can be matched with that of the offshore areas of the Sunda fore arc.
AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994