Sumatra Trench

It extends over 5,000 km (3,100 mi) long, starting from Myanmar in the northwest and ending at Sumba Island in the southeast.

The trench slope is now being considered as a result of folds and faults in the area which were originated from the local seafloor deformation.

[3] Seismic profiles at Sumatra Trench shows that, the down-going Indo-Australian Plate has its slip vectors rotate to a NE direction.

Within the Sumatra Fault Zone lies the majority of the right-lateral stress from the relative motion between the Indo-Australian and Eurasian plates.

[1] The inner part of the prism forms a NW-SE arc ridge offshore Sumatra with Enggano Islands being its highest point.

A distinctive strip-thrust fold locates on the western boundary of the accretionary wedge occurs on south of Enggano Island.

The scarce but still existing missing of part of the arc ridge offshore is result from extensional tectonic activities together with compression partitioning.

[1] All the basement can be easily realized based on strong multiple reflections from the seafloor except some places in the north part because of the existing of some down faulted continental blocks.

In the Central Sumatra region, a wide basement high, which has connections with the N–S trending fracture zone, is being subducted and causing the generation of variations in sediment thickness across the oceanic plate.

Changes in prism morphology southward are gradual and an alternative boundary can be determined at 4–5°S, at this area the seafloor hardness is decreasing and deformation front heads towards sea.

But there are some exceptions exist in nearby regions like Andaman where the eastern flank of the Ninety East Ridge can result in the reducing of the sediment thickness.

[10] Sediment thickness along the Sumatra Trench varies significantly, which is the result of fracture zones and fossil ridges' influence on basement topography.

The thickness is a function of oceanic basement topography in some extent and this feature is extremely distinctive in the part between Simeulue and Siberut Islands (Central Sumatra region 2°N to 3°S) along the trench.

On the other hand, the narrowing and steepening of the prism and toe vergence together often correlate with reduced thickness on Central Sumatra region.

As an example of mixed and variable convergence, Central Sumatra region (sediment thickness is ~1–3 km) may has basement topography as a more dominant factor.

In the north Sumatra region, where sediment is thick and prism wedge is wide, the dip is always shallow and landward vergence in toe is ubiquitous.