A LATE OLIGOCENE TECTONO-VOLCANIC EVENT IN EAST KALIMANTAN AND THE IMPLICATIONS FOR TECTONICS AND SEDIMENTATION IN BORNEO

New apatite fission-track data, K-Ar radiometric ages, and major and t race element geochemistry from the northern part of the Kutai Basin, B orneo document a major regional tectono-volcanic episode. Apatite fiss ion-track data from late Cretaceous sandstones indicate a period of ra pid cooling in the late Oligocene. Cooling, initiated by a rapid tempe rature drop of over 40 degrees C within c. 2 Ma might have been caused by increased denudation associated with tectonic movements. Rapid coo ling is also concomitant with a phase of igneous activity. New K-Ar ra diometric ages of andesitic-dacitic high-level intrusive rocks and ass ociated volcanic rocks give ages of 23-18 Ma, which correlate with the regionally extensive late Oligocene-Miocene Sintang Intrusives Suite. These andesitic-dacitic high-level intrusive rocks also have geochemi cal characteristics of are-related rocks. The sedimentary response to this tectono-volcanic event is the eastwards shift of the western basi n margin and the inception of deltaic deposition around the newly re-d efined basin margins. Up to 9 km of sediment was deposited in the basi n during Miocene delta progradation, covering thick sequences of Palae ogene bathyal shales, providing an excellent decollement surface for l ater inversion. Elsewhere in Borneo and adjacent areas of SE Asia, maj or events recorded in the late Oligocene to early Miocene include majo r thrust imbrication and volcanic are activity, the cause of which is unclear: Possibilities include the collision of Australia with the Phi lippine Sea Plate, the counter-clockwise rotation of Borneo in the Neo gene and the initial impingement of blocks of South China origin with northern Borneo-south Palawan.