PALEOMAGNETIC DATA OF LATE CRETACEOUS ROCKS FROM SUMBA, INDONESIA - THE ROTATION OF THE SUMBA CONTINENTAL FRAGMENT AND ITS RELATION WITH EASTERN SUNDALAND

Sumba island forms part of a continental fragment, located near the tr ansition of the Sunda Are to the Banda Are. It lies within the forearc region, between the active volcanic are to the north and the Java Tre nch to the south. Palaeomagnetic studies of Cretaceous (late Albian-ea rly Campanian) Lasipu sediments revealed a mean characteristic remanen ce (ChRM) direction with D = 42.5 degrees, I = -23.0 degrees, and alph a(95) = 6.1 degrees, indicating a palaeolatitude of 12 degrees S. This ChRM is, most likely, a secondary magnetization, possibly caused by t he intrusion of the 65-Ma-old Tanadaro granodiorite. This granodiorite gave a mean ChRM direction with D = 44.7 degrees, I = -16.3 degrees, and alpha(95) = 12.2 degrees, pointing to a palaeolatitude of 8.3 degr ees S. Eastern Sundaland with Borneo, west and south Sulawesi, and Sum ba formed one continental unit in the late Mesozoic, most likely attac hed to the southeast Asian mainland. Borneo and west and south Sulawes i underwent large counterclockwise (CCW) rotations since the Jurassic with similar to 45 degrees during the Cretaceous, and similar to 45 de grees during the Palaeogene. The Sumba microcontinent, most likely, be came detached from eastern Sundaland soon after deposition of the Lasi pu sediments. Palaeomagnetic data show that Sumba underwent subsequent clockwise (CW) rotations of up to 96 degrees:53 degrees between 82 an d 65 Ma, and 38 degrees between 65 and 37 Ma. Since the late Eocene, o nly small rotations occurred. The data indicate that eastern Sundaland , including Sumba, remained close to the equator since the Jurassic. C W rotations occurred in Sundaland both in the north (Indochina) and in the west (Sibumasu) as a consequence of the India-Eurasia collision. The same sense of rotation is seen further east in Sulawesi's East Arm and the Philippine Sea plate. Eastern Sundaland (Borneo and west Sula wesi) with CCW rotations is being trapped between these CW rotating pl ates.