Strike-slip faulting in the core of the Central Range of west New Guinea: Ertsberg Mining District, Indonesia Benyamin Sapiie² Mark Cloos³ Department of Geological Sciences, University of Texas, Austin, Texas 78712, USA ABSTRACT The change from tens of kilometers of major questions are addressed: (1) What are shortening deformation that caused large- the types, patterns, and distributions of Most of the Cenozoic tectonic evolution scale folding to a few kilometers of left- outcrop-scale structures? (2) What is the re- of the New Guinea region is the result of lateral strike-slip faulting occurred during lationship between deformation and the em- obliquely convergent motion that led to an the latest stage of collisional orogenesis. Al- placement of intrusive rocks, particularly the arc-continent collision between the Paci®c though strike-slip faulting was a minor late- Grasberg Igneous ComplexÐthe host of a su- and Australian plates. Detailed structural stage process, it was of profound impor- pergiant Cu-Au porphyry copper±type ore de- mapping was conducted along road expo- tance in generating pathways for magma posit. The data presented in this paper come sures in the Ertsberg (Gunung Bijih) Min- ascent and permeability for the rise of min- from detailed analysis of structures exposed ing District in the core of the Central Range eralizing ¯uids. The Grasberg Igneous along the 10-km-long Heavy Equipment Ac- in the western half of the island. Two dis- Complex, the host of a supergiant Cu-Au cess Trail (HEAT; called the HEAT Road here- tinct stages of deformation are recognized. porphyry copper±type ore deposit, was em- in) and the 5-km-long Grasberg mining access The ®rst stage took place between ca. 12 placed into a 2-km-wide, left-stepping road (called the Grasberg Road herein; Fig. 2). and ca. 4 Ma and generated kilometer-scale pull-apart. Much of the faulting is very recent because in folds with subsidiary reverse/thrust faults many places faults crosscut Pliocene and strike-slip tear faults. Regionally, this Keywords: New Guinea, Central Range, intrusions. deformation records many tens of kilome- strike-slip, porphyry copper. ters of shortening. The second stage began REGIONAL GEOLOGY at ca. 4 Ma and generated ®ve northwest- INTRODUCTION trending (;3008) strike-slip fault zones up The outline of the island of New Guinea has to a few tens of meters wide that are highly The Ertsberg (Gunung Bijih) Mining Dis- been described as similar to a bird ¯ying west- brecciated and contain numerous, mostly trict is located in the core of the Central Range ward with open mouth (Fig. 1). As a result, subparallel, planar faults. Each of these of west New Guinea, next to Puncak Jaya the island has been geographically divided zones was a site of tens to a few hundred (4884 m), the highest peak between the Him- into the Bird's Head, Neck, Body, and Tail meters of left-lateral offset. Between these alayas and the Andes (Fig. 1). The origin of regions. The central and largest part of the is- zones are domains containing three groups the island by continental drift was ®rst dis- land (the Bird's Body) can be divided into of planar strike-slip faults with orientations cussed by Wegener (1929), and it was consid- four lithotectonic provinces: the New Guinea that are interpreted in terms of Riedel ered a type locality for arc-continent collision foreland, the Central Range fold-and-thrust shears. (1) 0408±0708 trending faults with in the classic paper by Dewey and Bird belt, and a metamorphic belt with an overlying left-lateral offset that plunge to the north- (1970). Most of the Cenozoic tectonic evolu- ophiolite complex that is the forearc basement east (R shears), (2) 3558±0158 trending tion of New Guinea is the result of oblique of the accreted Melanesian island arc. faults with right-lateral offset that plunge convergence between the Australian and Pa- The New Guinea foreland (Arafura plat- to the N (R9-shears), and (3) 2808±3008 ci®c plates (Hamilton, 1979; Dow et al., form) is a swampy region of marine and non- trending faults with left-lateral offset that 1988). The Central Range is commonly de- marine Pliocene and Holocene siliciclastic plunge to the northwest (D-shears). Subsid- scribed as a fold-and-thrust belt, as spectacular sedimentary rocks that are underlain by Ce- iary dip-slip faults (rakes . 458) are asso- folding is evident on aerial photographs and nozoic carbonate and Mesozoic siliciclastic ciated with each group. Deformation in the satellite images (Fig. 2). No outcrop-scale strata deposited on the northern passive mar- district since ca. 4 Ma is best characterized structural analysis had been done in west New gin of Australia (Dow and Sukamto, 1984a, as a left-lateral strike-slip fault system Guinea until this investigation, which was 1984b; Pigram and Panggabean, 1984). The along an ;3008 azimuth. made possible by the construction of mining Central Range orogenic belt stretches 1300 roads. km from the Bird's Neck to the Bird's Tail. ²Present address: Departemen Teknik Geologi, One part of unraveling the tectonic evolu- The 150-km-wide belt has rugged topography Fakultas Teknologi Mineral, Institut Teknologi Ban- dung, Jl. Ganesha No. 10, Bandung 40132, tion of the western Central Range involves un- and numerous peaks over 3000 m in elevation. Indonesia. derstanding the relationships between defor- The Ruffaer metamorphic belt is a 1000-km- ³E-mail: [email protected]. mation, intrusion, and mineralization. Two long and 50-km-wide zone of deformed, low- GSA Bulletin; March/April 2004; v. 116; no. 3/4; p. 277±293; doi: 10.1130/B25319.1; 17 ®gures. For permission to copy, contact [email protected] q 2004 Geological Society of America 277 SAPIIE and CLOOS Figure 1. Lithotectonic map of the island of New Guinea. The Ertsberg (Gunung Bijih) Mining District is located within the P.T. Freeport Indonesia Contract-of-Work. RMBÐRuffaer metamorphic belt; IOBÐIrian ophiolite belt; GICÐGrasberg Igneous Complex; GRS Rd.ÐGrasberg Road; HEAT Rd.ÐHeavy Equipment Access Trail Road. temperature (,300 8C) metamorphic rocks that are bounded on the north by the Irian ophiolite belt and on the south by deformed, but unmetamorphosed, passive-margin strata (Dow et al., 1988; Granath and Argakoesoe- mah, 1989; Nash et al., 1993; Warren, 1995; Weiland, 1999). The Melanesian island-arc complex, built into the edge of the Paci®c plate, is poorly exposed. The details of the Cenozoic tectonic evo- lution in New Guinea are the subject of de- bate, as several kinematic models have been proposed. The most commonly published sce- nario is the subduction polarity reversal (``arc reversal'') model that entails movement of the Australian continental crust into a northward- dipping subduction zone, followed by colli- sional tectonism and initiation of southward subduction of the Paci®c plate at the New Guinea Trench (Dewey and Bird, 1970; Ham- ilton, 1979; Milsom, 1985; Dow et al., 1988). An alternative model proposed to explain re- lationships in eastern New Guinea postulates that the island is underlain by a doubly sub- ducted slab of oceanic lithosphere (``zipper- ing'' model), which would be the westward continuation of the Solomon Sea plate (Ripper Figure 2. Satellite imagery (SPOT panchromatic image acquired in 1987) of the mining and McCue, 1983; Cooper and Taylor, 1987). district, showing the location of the Ertsberg Mining District and the study area (inner Johnson et al. (1978) proposed a third model solid box). GICÐGrasberg Igneous Complex, EÐErtsberg intrusion, HEAT rd.ÐHeavy that is similar to the one involving northward- Equipment Access Trail Road, GRS Rd.ÐGrasberg Road (both are merely for mine dipping subduction of the Australian plate, but access). in their model, subduction reversal has not oc- 278 Geological Society of America Bulletin, March/April 2004 STRIKE-SLIP FAULTING, ERTSBERG MINING DISTRICT, INDONESIA Figure 3. Seismotectonic interpretation of New Guinea. Tectonic features: PTFBÐPapuan thrust-and-fold belt; RMFZÐRamu- Markham fault zone; BTFZÐBewani-Torricelli fault zone; MTFBÐMamberamo thrust-and-fold belt; SFZÐSorong fault zone; YFZÐ Yapen fault zone; RFZÐRansiki fault zone; TAFZÐTarera-Aiduna fault zone; WTÐWaipona Trough. After Sapiie et al. (1999). curred. In this scenario, the northward- western ends of the Central Range. In west GEOLOGY OF THE ERTSBERG subducted Australian lithosphere is thought to New Guinea, there are few events deeper than MINING DISTRICT be dipping vertically. 50 km, and there is remarkably little seismic- The accumulating record of seismicity of ity beneath where the Central Range eleva- Stratigraphy the region does not delineate the southward tions are higher than 1000 m. The overall seis- subduction of the Paci®c plate as envisioned micity indicates that most of the motion The stratigraphic units exposed in the min- by most workers. Abers and McCaffrey between the Australian and Paci®c plates is ing district are part of the Jurassic±Cretaceous (1988) analyzed the focal mechanisms of the accommodated by left-lateral slip along the Kembelangan Group and the Tertiary New 18 largest earthquakes from 1964 to 1985 Yapen and Bewani-Torricelli fault zones, Guinea Limestone Group (Fig. 4). On the ba- and found that more than half were roughly which are connected by a wide convergent sis of regional lithostratigraphy, the contact east-trending strike-slip movements, including bend beneath the Mamberamo region near the between the Kembelangan and New Guinea six of the eight largest events near the Central international border (Fig. 3; see also Punto- Limestone Groups is conformable and located Range. Sapiie et al. (1999) presented an anal- dewo et al., 1994). The Cendrawasih Bay area along the HEAT Road. Almost all of the sed- ysis of the distribution of moderate and larger is a wide extensional bend bounded to the imentary units along the HEAT Road dip earthquakes (M .