Neotectonics of the Southern Sumatran Forearc

Proceedings, Indonesian Petroleum Association
Thirty-Sixth Annual Convention & Exhibition, May 2012

Neotectonics of the Southern Sumatran Forearc

M. Ma’ruf Mukti*, Satish C. Singh*, R. Moeremans*, N. D. Hananto**, H. Permana**,
I. Deighton***
* IPG Paris
** Geotek – LIPI
*** TGS

Abstract
The structural features in the Mentawai forearc have been interpreted as products of the compression of the accretionary wedge and forearc basin sediments with high-quality industry-standard seismic reflection data. Compressional phases since the Late Miocene initiated (a) the landward vergent fold-thrust belt in the Mentawai Fault Zone (MFZ), (b) reactivation of seaward vergent imbricated thrusts in the retro-accretionary wedge, and (c) uplift of the accretionary wedge and some parts of the forearc basin. The landward vergent fold-thrusts formed as forward-breaking structures toward the northeast. These structures were disrupted by mud diapirism since the Pliocene. The landward vergent fold-thrust belt is rooted at depth to the Main Backthrust that lies at the boundary between the accretionary wedge and continental backstop. The alignment of the fold-thrust belt is controlled by the geometry and position of continental backstop. The seaward vergent imbricated thrusts were contracted and deformed sediments in the overlying piggy-back basin, indicating the shortening of the accretionary wedge since early Middle Miocene. In the northwest, the MFZ is developed on the western margin of the present forearc basin, whilst in the southeast it is near the present basin depocenters. The accretionary wedge formed as a narrower zone in the northwest area. The slope of the Pliocene sediments is highly rotated in the northwestern area. Those features suggest that the northwest part of the accretionary wedge underwent higher compression and tilting. The location of the higher uplifted forearc area coincides with the locus of impingement of the Investigator Ridge. The compression of the forearc is suggested to be controlled by the combination of the geometry and position of the continental backstop, and the subducting bathymetric high in the oceanic plate. These factors govern the neotectonic features in the southern Sumatran forearc.

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