Superimposed deformations and vergence of lower Tertiary sediments near Tatau, Sarawak

Good, and often extensive outcrops along the trunk road in the vicinity of Tatau, Central Sarawak, exhibit a variety of complex structures. The older rocks are typical turbidites of Late Eocene to Early Oligocene age that in the Arip area are also associated with products of explosive volcanism. These are unconformably separated from mid-Oligocene to mid-Miocene sediments of neritic, but mostly of littoral character. The younger rocks are much less deformed than the former. The turbidites possess wide zones of slumping and upon these nondiastrophic structures were superimposed tectonic structures. Mainly two tectonic vergences were found, an earlier vergence northward that is represented by large-scale recumbent folds and a later west vergence that produced overturned folds of moderate dimensions. We interpret the lithology, soft-sediment and tectonic deformations of the Early Tertiary sediments in the Tatau area to have taken place in afore-arc basin on the ocean side of a magmatic arc. The superimposed west upon north vergence may have been formed by warping about the Tatau area of an initially east-west elongated fore-arc basin. The regional structural trends support this interpretation. As an extrapolation of the interpreted island-arc setting for the Tatau area, some 200 to 250 km north of the Arip area is probably the buried SW-extension of the Palawan subduction zone. Activity on this particular extension began in the Late Eocene, while the Palawan suture proper only started in mid-Oligocene time