Abstract

Most of the Cenozoic tectonic evolution of the New Guinea region is the result of obliquely convergent motion that led to an arc-continent collision between the Pacific and Australian plates. Detailed structural mapping was conducted along road exposures in the Ertsberg (Gunung Bijih) Mining District in the core of the Central Range in the western half of the island. Two distinct stages of deformation are recognized. The first stage took place between ca. 12 and ca. 4 Ma and generated kilometer-scale folds with subsidiary reverse/thrust faults and strike-slip tear faults. Regionally, this deformation records many tens of kilometers of shortening. The second stage began at ca. 4 Ma and generated five northwest-trending (∼300°) strike-slip fault zones up to a few tens of meters wide that are highly brecciated and contain numerous, mostly subparallel, planar faults. Each of these zones was a site of tens to a few hundred meters of left-lateral offset. Between these zones are domains containing three groups of planar strike-slip faults with orientations that are interpreted in terms of Riedel shears. (1) 040°–070° trending faults with left-lateral offset that plunge to the northeast (R shears), (2) 355°–015° trending faults with right-lateral offset that plunge to the N (R′-shears), and (3) 280°–300° trending faults with left-lateral offset that plunge to the northwest (D-shears). Subsidiary dip-slip faults (rakes > 45°) are associated with each group. Deformation in the district since ca. 4 Ma is best characterized as a left-lateral strike-slip fault system along an ∼300° azimuth.

The change from tens of kilometers of shortening deformation that caused large-scale folding to a few kilometers of left- lateral strike-slip faulting occurred during the latest stage of collisional orogenesis. Although strike-slip faulting was a minor late-stage process, it was of profound importance in generating pathways for magma ascent and permeability for the rise of mineralizing fluids. The Grasberg Igneous Complex, the host of a supergiant Cu-Au porphyry copper–type ore deposit, was emplaced into a 2-km-wide, left-stepping pull-apart.

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