Doubly Vergent Accretionary Wedge Active Tectonics in the Sumatra Subduction Zone

American Geophysical Union, Fall Meeting 2011, abstract #T21B-2356

Doubly Vergent Accretionary Wedge Active Tectonics in the Sumatra Subduction Zone

Mukti, M.M.*, Singh, S.C.*, Hananto, N.D*., Permana, H.**, Deighton, I.***
* Geosciences marines, IPGP, France
** Geotek – LIPI
*** TGS

Abstract
The Sumatra subduction zone is a classical example of oblique subduction where the slip is partitioned between an orthogonal component along the megathrust and an arc parallel component along the Sumatran fault. It was previously suggested that the part of the arc parallel motion is accommodated along the Mentawai strike-slip fault. Using high-quality industry seismic reflection profiles spaced at 20 km spacing, we find that the Mentawai Fault Zone is not a strike-slip fault, but it is actually a backthrust system developed along the boundary between the retro accretionary wedge and the continental backstop. The active deformation along the western margin of the present day Mentawai forearc basin takes place along NW-SE trending parallel convex eastward anticlinal ridges. Beneath the anticlinal ridges, the deformation zone exhibits (1) main backthrust, as the upward continuation of the Mentawai Backthrust in forearc basin, (2) seaward verging imbricated thrusts developed in the accretionary wedge and (3) the landward verging thrust developed in the forearc basin. The Mentawai Backthrust developed in NW-SE direction along the eastern edge of the present outer arc islands, beneath the forearc basin sediments. This backthrust system formed as a long-lived boundary between the accretionary wedge and the continental backstop. This system was initiated by the translation and back-rotation of the closely spaced seaward-verging imbricated thrust, and developed through the continuous growth of the accretionary wedge. The compressional-dominated phase, active since the early Pliocene, initiated the Mentawai backthrust reactivation and deformed the basin fill sediments. To the west of the Main Backthrust, the deformation is interpreted to result from the back-rotation of the seaward verging imbricated thrusts in the accretionary wedge. Later, to the east of the Main backthrust, fault-related folding developed within sedimentary sequences and progressively moved to the east. The landward verging Mentawai Backthrust zone characterizes the major structural style in the Sumatra subduction system, and together with the Frontal Thrusts, formed the doubly vergent active tectonics in the Sumatra subduction system, which seems to be valid along much of the Andaman-Sumatra-Java margin.

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