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ABSTRACT: Structure and Thermochronology of the Irian Jaya Fold Belt, Indonesia

Kendrick, R. D., K. C. Hill, and P. B. O'Sullivan , Australian Geodynamics Cooperative Research Centre, Melbourne, Australia

New Guinea, the NE margin of the Australian plate, underwent Neogene to Recent collision with Paleogene island arcs, resulting in uplift to elevations >5km in the Irian Jaya Fold Belt (IJFB) of central New Guinea. The fold belt to the east and west is lower, shows less basement involvement and contains giant gas and oil deposits. Despite dense jungle cover and difficult karst terrain in the IJFB, interpretations based on recent Geosec structural modelling and new constraints from fission track thermochronology define the timing and style and hydrocarbon prospectivity of the region.

An older, structurally higher, thin-skinned thrust system records the initiation of orogeny in the northern IJFB between ~12-5 Ma. A Pliocene-Recent, structurally lower, thick-skinned thrust system is interpreted to have produced the rapid and recent uplift of the IJFB, by transporting the ~200 km long Mapenduma Thrust sheet to the surface. The hanging wall geometry of the Mapenduma Thrust is essentially a deeply eroded fault-bend fold above an inverted pre-existing extensional fault. These reactivated extensional faults probably sole into a regional decollement at depths >20km, based on cross sections and the relatively low (greenschist) metamorphic grade in basement. Minimum shortening of ~53 km was accommodated on Late Proterozoic and basement decollements contributing to total regional shortening across the IJFB of 78-116 km (44-49%).

Hydrocarbon potential in the IJFB is demonstrated by oil discoveries at Cross Catalina and Kau-2. The active petroleum system is likely sourced from Kimmeridgian shales, as in the prolific Papuan Basin and Northwest Shelf.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia