EVIDENCE FOR VARIOUS STAGES OF COLLAPSE IN THE SOUTHEAST ASIAN FOLD-THRUST BELT: BORNEO, PAPUA AND SOUTH CHINA SEA

Erosion in active orogens involves massive superficial landslides. It is also observed that extension due to bulk forces or to subsequent rifting is the fate of mountain ranges. However, the mechanisms of extension and the rapidity at which compression shifts to extension are not well understood. For example, in the late Mesozoic/early Cenozoic, the South China Sea margin and also rifts related to the Indian indenter-related strike-slip faults overprinted an Andean-type margin with high elevation as seen from the amount of erosion. The presence of a series of Cretaceous molasse basins affected by Tertiary extension indicates that the two mechanisms may be responsible for an abnormally long period of extension. If clear detachments and domes are imaged by seismic data, the Yenshanian Orogeny structures have totally been overprinted in the large zone of extension which led to ocean spreading in the South China Sea.

Western Papua shows an earlier stage of extensional collapse although contractional deformation still dominates in the region. It is a very recent orogenic wedge where the front has ceased its activity 2 m.y. ago, whereas the back-side of the internal zones is the site of a localized and seismically active extension. There, metamorphic rocks (gneisses dated around 5 Ma) are exposed in an elongated dome. The exhumation of these rocks is associated with migmatization, which post-dates the HP event of the wedge. However, the stretching lineations in the metamorphic units are at right angle from the active T axis of the active extension, suggesting that the final stage may be a gravitational event.

The present-day evolution of NW Borneo may be interpreted in relation to an incipient collapse. The construction of the intra-oceanic wedge started in the Eocene but the topography was acquired by the middle Miocene when the South China Sea margin entered the subduction. It was followed by important gravity tectonics at the front of the range. This phenomenon is related to the subduction of a crustal high that induced erosion and delta deposition and subsequent gravity tectonics. However, the shallow thin-skinned extension cannot account for the motion of the front of the topographic wedge detected by GPS data. We present a new model involving a collapse at the scale of the Crocker range but toward the front of the FTB.