HR: 0800h
AN: U11A-0821    [Abstracts]
TI: Swath bathymetric transect across the Sunda trench in relation with the December 26, 2004 earthquake: First tectonic results of the R/V Marion Dufresne cruise
AU: Rangin, C
EM: rangin@cdf.u-3mrs.fr
AF: CNRS-Cerege-College de France, BP 80, Europole de l'Arbois, Aix en Provence, 13545 France
AU: * Sibuet, J
EM: jcsibuet@ifremer.fr
AF: Ifremer Centre de Brest, B.P. 70, Plouzane, 29280 France
AB: We present here swath bathymetric coupled with 3.5 kHz echo-sounder data acquired during the 'Sumatra Aftershocks' cruise performed on the French R/V Marion Dufresne during summer 2005 . We discuss the fabric of the Sunda wedge across a transect that extends along the Indonesia-Indian boundary from the trench to the offshore trace of the Semangko fault. The studied area that belongs to the Burmese sliver plate is located at the northern tip of Sumatra Island where the trench swings northwards and is indented by the NS trending transform fault ridges of the lower plate, tectonically reactivated along the diffuse Indian/Australian plate boundary. Obliquity of the plate convergence increases rapidly eastwards inducing wrenching of the Outer Ridge and diffuse partitioning into the wedge. A large part of the oblique motion is accommodated along Sumatra Fault large and deep pull-apart basins. The Outer Ridge is relatively shallow and extends in the Nicobar and Simelue islands. In the studied area, it is bounded north-eastwards by the partly active NE verging back-thrust of the wedge and fresh dextral strike slip faults. This dextrally wrenched ridge is bounded south-westwards by an active tectonic zone imaged on 3.5 kHz data that affects the uppermost piggy back basin of the wedge. This active deformation is interpreted as the emergence of a major splay fault that affects the wedge. This fault could be the place where a large part of the December 26th earthquake thrusting has been accommodated. The upper part of the wedge is marked by anticlines and tight synclines crosscut by a linear fabric that can be interpreted as the trace of strike slip faults (N110°E and N000). Dominant N000 trending structures might be the imprint of the transform fault fabric of the lower plate. The frontal part of the wedge is outlined by a series of sigmoidal folds affected by a complex story of superposed slope instabilities, submarine landslides, debris flows and rotated folds. Only part of these submarine collapses could be related to the last Sumatra earthquake as indicated by heat flow and piezometer results obtained on some of the scars. Additional data were collected along the southernmost extension of the West Andaman Fault. There, we have evidenced some dextral restraining bends and small pull apart structures that connect the narrow wedge developed offshore Simelue Island Outer Ridge and its considerably wider counterpart along the main transect. The studied area is imaging the progressive, but relatively fast transition between slightly oblique to hyper oblique convergence along the Sunda trench. However, during the Dec. 26th earthquake only the co-seismic motion perpendicular to the trench was evidenced.
DE: 8104 Continental margins: convergent
SC: Union [U]
MN: Fall Meeting 2005