Paleomagnetism of Borneo

The paleomagnetism of Borneo remains controversial, although the prepondera nce of results, both from the island itself and from the surrounding region s, suggest that counterclockwise (CCW) rotation has taken place. CCW rotati ons are seen in minor intrusions in Sarawak, Sabah and Kalimantan, which in crease systematically with the age of the intrusion to a maximum value of 5 1.8 degrees +/- 3.7 degrees. The rotation can be no older than 25 Ma, which is the age of the intrusion showing the maximum rotation. The rotation app ears to have neared completion by 10 Ma. Similar CCW rotations are seen in sites from Peninsular Malaysia through Borneo, to Sulawesi, the Celebes Sea and Palawan in the Philippines, but the ages of these rotations are: for t he most part, unknown. In Mesozoic rocks in Kalimantan and Sarawak, a stron ger declination rotation of nearly 90 degrees CCW is recorded at seven site s, including sites which pass fold and reversal tests, This strong rotation is no older than youngest Cretaceous, and although seen over a wide region in Borneo, it is nor seen in Peninsular Malaysia, nor in the Celebes Sea o r Palawan, where only the weaker CCW rotation is seen. The widespread occur rence of this strong rotation in Western Borneo suggests that it is essenti ally a rigid plate, or microplate rotation, and not a series of local rotat ions caused by distributed shear in limited deformation zones. The rotation of Borneo appears to be a consequence of convergence between the Australia n and Eurasian plates, which is accommodated by subduction along the northw est margin of Borneo, (C) 1999 Elsevier Science Ltd. All rights reserved.