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Mesozoic and Tertiary Tectonics of Irian Jaya: Evidence for Non-Rotation of Kepala Burung
Hydrocarbon exploration in Irian Jaya, which initially targeted near surface Late Miocene reef plays, has evolved to exploration targets in Jurassic-Cretaceous clastics which were deposited along the rifting margins of the Australian continent. Tectono-sedimentary relationships play a critical part of the play assessment. Rifting, initiated in the Early Jurassic, included two systems: 1) A Northern Rift is interpreted as a major transform boundary, the location of which is approximated by the present day location of the NW striking New Guinea Trench and the E-W striking Papuan Foldbelt. The Kepala Burung (Bird's Head) northern margin, is a part of the Northern Rift. 2) The NE striking Australian NW Shelf Rift continues to an area between Tanimbar and Aru islands where propagation of the extension faulting effectively terminated against a postulated northwest striking transform fault. The failed portion of the NW Shelf rifting, northeast of the transform fault, intersected Northern Rift faulting at the present location of the Lengguru Foldbelt. Northern Rift faulting aligned with Paleozoic (and possibly older) structural trends. However, NW Shelf Rift faulting appears to strike roughly perpendicular to pre-Mesozoic structural trends.
The Miocene (25 Ma) collision of the Pacific and Australia plates resulted in obduction of ophiolite and melange against the Northern Rift transform faults, including at Kepala Burung. Continuing collision of the plates resulted in forming the New Guinea Trench, and subduction in the Trench elevated the continent northern margin. Thrust and wrench faulting, driven by subduction in the New Guinea Trench, are pervasively present across Papua New Guinea and Irian Jaya, including Kepala Burung. This Neogene faulting has been divided into three distinct groups: Direwo-Nawa, Aiduna, and Yapen-Ransiki. Each of these fault groups developed unique SW directed thrusting plus left and right lateral shear faulting (W and SE, respectively), which are predictable from a simple strain ellipse. Faults of the Direwo Nawa Group follow tectonic trends established by the Northern Rift, and faults of the Aiduna Group reoccupy tectonic trends established by the NW Shelf Rift. Faults of the Yapen-Ransiki Group do not seem to have Jurassic rift precedents, but parallel structural trends of northern Australia and offshore southern Irian Jaya which appear to be of pre-Mesozoic age. The New Guinea Trench consists of alternate strike segments which correspond to the three fault groups, suggesting that pre-Tertiary tectonic trends have provided a means of differential movement on the New Guinea Trench.
Structural complexities were introduced along the NW Shelf Rift with the Banda Arc collision at about 10-5Ma. The Banda Arc collision thrust portions of the rifted Australian plate, including Seram Island, eastward across the boundary transform fault with crustal loading inducing a foreland basin with the Kumawa Onin-Misool Arch forming on the far side.
Early Jurassic rift trends are continuous across Irian Jaya. The predominant Neogene deformation is related to Australian Pacific plate collision. Results of the collision are principally left lateral movement (westward) along wrench faulting, limited movement occurring along right lateral wrench (SE strike) faulting, and southward thrust transport and folding. This continuity of rift and collision related structural trends across Irian Jaya indicates that there has been no significant rotation of Kepala Burung since the Jurassic.
Exploration risk for Jurassic hydrocarbon reservoir targets may be reduced by identifying major structural trends, and particularly trend intersections, which have had a longterm presence and limited disruption.
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