Late cretaceous to early tertiary structural elements of west Kalimantan

Recent geological and geophysical mapping in West Kalimantan has allowed the definition of three domains with distinct geological and geophysical characteristics which were imprinted on the region by Early Cretaceous to Eocene convergent tectonic events. Subduction related granitoids intruded low-grade regional metamorphic rocks during the Early Cretaceous forming the Schwaner Mountains domain extending east-west across the southern and central part of West Kalimantan. This is flanked in the northwest by a complex sequence of sedimentary and volcanic rocks ranging in age from Late Carboniferous to Cretaceous which comprise the Northwest Kalimantan domain. Deformed Cretaceous flysch deposits east of the Northwest Kalimantan domain form an accretionary complex representing the southern continuation of the extensive, dominantly Tertiary accretionary wedge in Sarawak. The boundary between the Cretaceous accretionary domain and rocks of the Northwest Kalimantan domain is a fault which we interpret as a transform that marked the western limit of Late Cretaceous southward subduction beneath West Kalimantan. Growth of the accretionary complex resulted in uplift of a ridge of melange and flysch deposits, on which a half graben extensional basin formed and accumulated thick lacustrine deposits. A sedimentary basin developed between continental rocks to the south and the emergent ridge comprising accretionary complex rocks and outer shelf deposits to the north, in the position of a forearc basin. As accretion proceeded, the locus of underthrusting migrated northward and a second melange ridge and sedimentary basin developed farther to the north. The evidence for accretionary processes and the association of these with sedimentary basin formation, supports the hypothesis that an ancient, south-dipping subduction zone existed in the eastern part of West Kalimantan during Late Cretaceous to Early Tertiary times.