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Thermochronology and Tectonics of the Bird's Head Region, Irian Jaya: Apatite Fission Track Constraints
Apatite fission track thermochronology (AFTT) has been used to reconstruct the thermal and tectonic history of rocks from the Tosem Block in the northern Bird's Head region, the Tarof-2 well in the Bintuni Basin, and the eastern Lengguru Fold Belt.
The AFTT results from the Tosem Block suggest two distinct rapid cooling patterns. Dacitic and dioritic rocks cooled rapidly during the Early Miocene, probably following an episode of heating associated with hydrothermal fluids. In contrast, granite and syenite intrusions have experienced rapid Late Miocene cooling, probably associated with erosional denudation and uplift in response to obduction of the Tosem Block onto the northern Bird's Head microcontinent. The preservation of a central block of Early Miocene ages amongst the Late Miocene ages suggests that the central block was either overthrust from both sides in the Late Miocene or was dropped down as a keystone block in an extensional setting.
In the Tarof-2 well, data from Mesozoic samples deeper than ~1.5 km show that the sequences were partially annealed due to exposure to paleotemperatures of up to ~100°C. This suggests that generation of hydrocarbons from these rocks might have postdated the formation of structural traps beneath the Jurassic unconformity. Data from overlying Cenozoic rocks suggest they may have been heated to maximum paleotemperatures <85°C. Therefore, these rocks have not been exposed to temperatures necessary for hydrocarbon generation.
The AFTT data from the Upper Miocene to Pleistocene sequences in the eastern Lengguru Fold Belt (LFB) show that multiple cooling events occurred in the provenance terrains during the Tertiary. One episode of cooling occurred contemporaneously with Eocene-Early Miocene erosional denudation and uplift, perhaps in response to the Neogene Melanesian orogeny. A subsequent cooling episode took place in the Late Miocene, contemporaneous with deformation throughout New Guinea and possibly related to the opening of the Banda sea.
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