Abstract

The results of detailed structural mapping in the Central Range of Irian Jaya has provided new information concerning deformation and the Cenozoic structural evolution of this SSE trending fold belt.

More than 11,000 structural data from faults, slickensides, veins, folds and foliations have been collected from the Gunung Bijih (Ertsberg) Mining district and in particular along the Gunung Bijih Mining Access road (GBMA). The overall bedding orientation shows similar pattern throughout all rock formations from pre-Cambrian to Miocene strata which is consistent with overall trend of folding both outcrop scale and km-scale folds in the district. The density and distribution of structural data varies greatly along the road reflecting strain distribution and lithologic variation. From the results it can be concluded that two stages of deformation took place.

The first stage took place between ~12 Ma and ~4 Ma. It was characterized by the development of en echelon, km-scale NW-trending folds with open symmetrical and chevron geometries. Comparatively few minor reverse or thrust faults have been observed. Slickensides due to bedding-plane slip are common on both limbs of folds indicating flexural slip was an important mechanism of deformation. Regionally this deformation records tens of kilometers of shortening.

The second stage of deformation began at ~4 Ma age and was dominated by strike-slip faulting. Some strike-slip faults formed along pre-existing weaknesses such as bedding surfaces and two high angle reverse faults. This change in deformation style is related to a change in relative plate motion between the Indo-Australian and Pacific Plates at ~4 Ma and to the probable, but short-lived, existence of the Caroline Plate directly north of the New Guinea from ~4 Ma to 2 Ma.