Abstract

More than 2000 km of good quality, post-1980 marine seismic data, acquired offshore from Kerema, allow perception of the structural style and evolution of the frontal (external) part of the Papuan Fold Belt. Previous interpretations of the sub-surface structure of the Papuan Fold Belt have been based on outcrop evidence, where thrust and reverse faults are documented as having dominantly southward (external) vergence. The offshore seismic data reveal that southward-dipping (interior-verging) back- thrusts are important structures, especially as constructive elements in large culminations. The backthrusts have been instrumental in raising the target section within reach of the drill in large anticlinal fold traps.

Marine seismic data provide insight which is critical of ascertaining the geometry of deep structures, and for determining optimum locations for drilling. Additionally, concepts derived from perception of offshore structures might be applied in future interpretations of the onshore part of the fold belt, where seismic acquisition is made difficult by rugged terrain.

Folds and faults of the frontal part of the Papuan Fold Belt are detached along a nearly horizontal deep, supracrustal level (probably in ductile Mesozoic shales). The thick assemblage of detached cover rocks is internally delaminated and laterally compressed at shallower levels by structural thickening of ductile zones, and by exterior-verging and interior-verging thrusts that break the limbs of asymmetric folds. The stratigraphic and structural geometries demonstrates that compression and tightening of the fold belt migrated progressively southwestward in episodic fashion: successively younger stratigraphic sequence are less tightly folded than their predecessors.