ABSTRACT: Change in Structural Style from Thin-Skinned to Thick-Skinned Along the Strike of the Papuan Fold and thrust Belt, Papua New Guinea

HEBBERGER, JOHN J., JR., and JAMES C. PHELPS,* Chevron Overseas Petroleum Inc., San Ramon, CA

A prominent feature of the onshore Papuan fold and thrust belt is the gradual (150 km) change along strike from typical thin-skinned thrust-generated folds with 1-8 km spacing in the area of the Iagifu/Hedinia oil discoveries to 20-40 km wide thick-skinned (basement-involved) folds near the Irian Jaya border. This structural style change occurs in an area where no dramatic difference in the thickness or composition of the upper Jurassic to Tertiary sedimentary section is evident in either surface geology or well data.

Gravity data suggest that major changes in depth to granitic basement occur along strike. These changes are not expressed by surface structure and may be related to pre-existing shelf margin geometry. The change along strike from thin- to thick-skinned thrust belt geometry does not occur in the same area as the change in depth to basement, hence the two phenomena appear to be unrelated.

The change in structural style may be related to differing levels of detachment. In both areas two major detachment levels exist: one 15-20 km below the top of basement, and one near the base of the sedimentary section. In the thin-skinned portion of the Papuan fold and thrust belt most of the shortening is associated with the upper detachment and displacement within the basement is minor. In contrast, most of the shortening in the thick-skinned area occurs in a duplex geometry, with large displacement along the intrabasement detachment and the shallow detachment acting as a roof thrust.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)