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A.Triangle Zone Along the Active Thrust Front in Southern Irian Jaya, Indonesia
Excellent seismic and surface geology data from the central segment of the active New Guinea Fold and Thrust belt (Lat. 4–5°S, Long. 137–138°E) reveal a complex thrust front. The southward-propagating thrust belt results from oblique convergence of approximately 120 millimetres/year between the Australian and Pacific plates. Earthquakes and tilted, incised alluvial fans attest to active shortening along the thrust front. Industry interest in the area is stimulated by 340 million barrels oil and 1.8 trillion cubic feet proven recoverable reserves and substantial non-delineated gas and condensate reserves in Papua New Guinea.
Clastic foredeep fill along the ground surface permit acquisition of good seismic data, unlike the karstic topography in the Papuan segment of the thrust belt. However, seismic acquisition is still high cost due to difficult logistics in the remote jungle terrain.
The stratigraphy in the thrust front consists of a Mesozoic clastic sequence overlain by Miocene carbonate and marl in turn overlain by a thick Upper Miocene to recent clastic foredeep fill. Four detachment horizons in the Upper Cretaceous, middle Miocene and uppermost Miocene create a triangle zone duplex structure. The "upper detachment" or northward-directed back thrust lies at the uppermost Miocene level. The triangle zone is locally complicated by apparently contemporaneous southward-directed, cross-cutting emergent thrusts and shortening along deeper blind thrusts. Detailed relationships evident from surface geology and shallow levels of the seismic data indicate additional complicated interplays of south-verging emergent thrusts and the north-vergent backthrusts along the triangle zone with the active vergence direction changing along strike.
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