7Accretionary wedge and outer ridge 99

Nin

etye

ast R

idge

90°E 100°E 110°E

10°N

10°S

0°

Bengal Fan(with depthcontours)

AndamanSea M

alaysia

Java

Sumatra

slopebasins

Mentawai

Siberut

strike-slip fault

active volcano

outer ridge

subduction zone8 cm/yr

4000 m

3500 m

4500 m

7 cm/yr

Indo-AustralianPlate

EurasianPlate

SundaShelf

Java Sea

Sahul Shelf

Australia

Borneo Sulawesi

Nicobar Islands

Andaman Islands

location ofcross section

volcanic arcforearcbasin

outerridgedeep sea

trench

Sumatra

T i m o r Tr e n c h

Men t aw a i

R i d ge

J a v a

R i d g eT i m o r R i d g e

J a v a

Tr e n c hT i m o r Tr e n c h

Men t aw a i

R i d ge

J a v a

R i d g eT i m o r R i d g e

J a v a

Tr e n c h

4200m

5210m 1790m

6070m

7450m

3560m

5270m

7360m

4200m

5210m 1790m

6070m

7450m

3560m

5270m

7360m

Fig. 7.9 Map showing contrasting plate-tectonic conditions along the Sunda Arc. In front of Sumatra, sediment of the thick Bengal fan are scraped-off and incorporated into the accretionary wedge. This causes the outer ridge to emerge from the sea at this location ( Mentawai Ridge – see insert). In front of Java, the deep sea trench and the outer

ridge are signifi cantly deeper. In front of Australia, the continental crust of the Sahul shelf is being subducted beneath the Sunda Arc; this causes a particularly strong uplift of the outer ridge (Timor Ridge) and marks the initial stage of orogenesis.

The accretionary wedge of the Sunda Arc

The Sunda Arc provides an instructive example demonstrat-ing how the shape of the accretionary wedge depends on the amount of sediments transported into the subduction zone (Fig. 7.9). Fueled by an enormous monsoon-controlled supply of sedimentary material, the Ganges and Brahmaputra rivers constructed the huge several-kilometer-thick submarine Ben-gal fan (Fig. 4.16) that extends to the southernmost point of Sumatra. The sedimentary fan lies on the Indo-Australian Plate and is transported towards the NNE where it is being subducted in the Sunda Arc. Therefore, the deep sea trench along the northwestern part of this island-arc system is mostly masked by the high sedimentation rate while the outer ridge, built by the tectonic stacking of fan-supplied sediments, is emergent several hundred meters above sea level. It forms islands such

as the Andaman and Nicobar Islands and the Mentawai Ridge in front of Sumatra. Farther SW away from the infl uence of the Bengal fan and adjacent to Java, the trench is distinctive with depths of almost 7500 m. Accordingly, the outer ridge is not well expressed and lies mostly below a water depth of 2000 m.

Along Sumatra, the slope of the accretionary wedge falling from the outer ridge to the deep sea trench typically displays distinctive subdivisions. Here active thrusts produce elongate fl at areas and depressions that are called slope basins (Fig. 7.9, insert). They are common along Mentawai Ridge where some are exposed above sea level. During their complex history, they acted as sediment traps that recorded the uplift history of the outer ridge. Analyses of the microfauna indicate uplift from deep water to shallow water conditions. The youngest sedimen-tary rocks include the formation of reefs in very shallow water, followed by uplift above sea level (Moore et al., 1980).

and various rocks transported into the deep sea trench are juxtaposed within the tectonic zone of friction between the two plates where all compo-nents are mixed by the strong tectonic movement. Pieces of all magnitudes (up to kilometers in size) of disrupted competent rocks like basalt and

indurated sedimentary rocks within a soft matrix of clay or slate comprise a tectonic mélange. Parts of this mélange may have been mixed initially by sedimentary and later by tectonic processes (sedimentary-tectonic mélange; Fig. 7.11) although the processes can be diffi cult to distinguish. If

Licensed to jason patton<Jason-Patton@redwoods.edu>