Paleomagnetism of Borneo

Abstract

The paleomagnetism of Borneo remains controversial, although the preponderance of results, both from the island itself and from the surrounding regions, suggest that counterclockwise (CCW) rotation has taken place. CCW rotations are seen in minor intrusions in Sarawak, Sabah and Kalimantan, which increase systematically with the age of the intrusion to a maximum value of 51.8°± 3.7°. The rotation can be no older than 25 Ma, which is the age of the intrusion showing the maximum rotation. The rotation appears 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 the most part, unknown. In Mesozoic rocks in Kalimantan and Sarawak, a stronger declination rotation of nearly 90°CCW is recorded at seven sites, 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 not seen in Peninsular Malaysia, nor in the Celebes Sea or Palawan, where only the weaker CCW rotation is seen. The widespread occurrence of this strong rotation in Western Borneo suggests that it is essentially a rigid plate, or microplate rotation, and not a series of local rotations caused by distributed shear in limited deformation zones. The rotation of Borneo appears to be a consequence of convergence between the Australian and Eurasian plates, which is accommodated by subduction along the northwest margin of Borneo.