HR: 09:20h
AN: T31G-06    [Abstracts]
TI: Docked Or Accreted Indian Ocean Fracture Ridges Along The Sumatra Subduction Zone Northern Tip
AU: * Rangin, C
EM: rangin@cdf.u-3mrs.fr
AU: le Pichon, X
EM: lepichon@cdf.u-3mrs.fr
AF: Xavier le Pichon, college de France europole de l'arbois BP 80, Aix en Provence, 13545, France, Metropolitan
AU: Lin, J
EM: lin@cdf.u-3mrs.fr
AU: Maury, T
EM: maury@cdf.u-3mrs.fr
AU: Jean Claude, S
EM: jcsibuet@ifremer.fr
AB: Detailed multibeam mapping coupled with echo sounder data collected offshore the northern tip of Sumatra over the rupture area of the Dec. 26 2004 Mw 9.3 earthquake during the Sumatra aftershock cruise reveal dominant sub-meridian dextral wrenching at the western termination of the Sunda subduction zone. N10°W trending dextral wrench faults with both west and east vergency affect the wedge that overlies in this area the northern prolongation of the NS trending oceanic fracture zone ridges system that absorbs the Indian/Australian plates relative motion between the 90° E and 92°E ridges. Three main N10°W dextral wrenched fault zones with a discrete westward vergency were continuously mapped on the basis of bathymetry and low frequency sounder profiles over 150km across the wedge. The Upper Splay Fault located immediately west of the Inner Ridge (backstop or innermost wedge?) extends from south of Simelue/Nias Island to the vicinity of the Nicobar Islands to the north. The N/S Median Splay Fault within the core of the wedge is the site of the most intense strike-slip deformation evidenced by flower structures and micro-seismicity (OBS). Piggy-back basins are dextrally wrenched as shown by echo sounder data. The westernmost half of the wedge is dominated by NNW elongated pop-up structures with both west and east verging thrusts. Many of these are interpreted by us as oceanic ridges deformation in progress. We propose that the structural fabric of the wedge is due to the interaction of the 90°-92°E oceanic fracture ridges system with the Sumatra backstop and inner wedge. The ridges deformed at depth by NS left-lateral strike-slip faulting due to the distributed Indian/Australian motion dominate the fabric of the hyper- oblique convergent zone. As a consequence, if the ridges are not yet incorporated within the wedge, as we believe, the inter-plate boundary that is well defined by after-slip seismicity near 40-50km depth below the Aceh basin and is known to extend westward with a gentle dip under the upper wedge, does not extend below the outer oceanic fractures system that forms the backbone of the lower wedge. It would have to steepen upward to merge into the MSF in the median part of the wedge. Thus, in our preferred interpretation, the inter-plate boundary does not extend below the lower part of the wedge that is essentially constituted by deformed but still not accreted oceanic fracture volcanic ridges and the steep dip of the decollement east of the MSF below the shallow upper wedge may have played a major role in the generation of the Sumatra Tsunami. Alternatively one would have to assume that the ridges are already incorporated within the wedge which we have at this stage no evidence for. We show that this type of interaction of the submeridian Indian ocean fracture ridges with the subduction zone extends far north into Myanmar and Bangladesh along the NS trending Andaman trench.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8123 Dynamics: seismotectonics
DE: 8150 Plate boundary: general (3040)
DE: 8155 Plate motions: general (3040)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting