Isotopic mapping reveals the location of crustal fragments along a long-lived convergent plate boundary

Highlights

• We present new SrNd isotopic data for magmatic rocks from NW New Guinea.

• Three different isotopic signatures can be linked to distinct crustal fragments.

• Isotopic mapping reveals the extent of these fragments across a poorly exposed region.

• We determine the northern-most extent of Australian continental crust.

• These data will better inform new tectonic reconstructions of this convergent margin.

Abstract

New Guinea has acted as the boundary between the Australian and Pacific plates for hundreds of millions of years. Strike-slip movement and arc–continent collisions along this boundary during the Cenozoic have shuffled rocks of different age and composition in a series of terranes along the plate boundary making mapping them a considerable challenge. Here we report results of SrNd isotopic data obtained from rock samples from western New Guinea that are representative of the different terranes. These isotopic data reveal the crustal affinity of the terranes and we have used these data to map their spatial distribution. The isotopic data show three distinct crustal domains underlying western New Guinea; Palaeozic–Mesozoic Australian continental crust (⁸⁷Sr/⁸⁶Sr = 0.719594 to 0.710921; ε_Nd = −13.85 to 1.373); thinned transitional crust intruded by Miocene–Pleistocene magmatic rocks (⁸⁷Sr/⁸⁶Sr = 0.706524 to 0.704019; ε_Nd = 6.67 to 2.13); and accreted island arc crust (⁸⁷Sr/⁸⁶Sr = 0.704053 to 0.703759; ε_Nd = 6.63 to 4.97). These data, together with crustal contamination models, indicate that the northern-most extent of Australian continental crust exists beneath the northern-most section of western New Guinea. We also combined our isotopic data with existing data across New Guinea and used these to develop an isotopic map that shows the position of the ancient Australian–Pacific Plate boundary, producing results that are also consistent with broad-scale seismic tomography imagery. Our findings provide a framework for mapping other plate boundaries, particularly ancient systems where only fragmentary data exist.

Introduction

Convergent plate boundaries are complicated zones where slices of the Earth's crust of different age and composition (‘terranes’) are often juxtaposed. Unravelling the history of these zones is key to understanding what the Earth looked like in the past. New Guinea is recognised as one of the world's youngest arc–continent collisional orogens (Ali and Hall, 1995; Baldwin et al., 2012; Davies, 2012; Hall, 2002; Hill and Hall, 2003; Holm et al., 2016), but has acted as a plate boundary to the northern margin of the Australian Plate since the Triassic (Hill and Hall, 2003; Jost et al., 2018). If we can understand the tectonic evolution of New Guinea in its current setting, then we can use this knowledge to better understand other much older plate boundary systems.

The island of New Guinea marks the meeting point of the Australian, Philippine Sea, and Caroline tectonic plates (Bird, 2003; Hall and Spakman, 2003; Milsom et al., 1992; Tregoning and Gorbatov, 2004). The interaction of these plates during the Cenozoic saw the production of subduction-related volcanism, the collision and accretion of island arc volcanoes, as well as extensive strike-slip faulting. All of this tectonic activity has resulted in the island's current geological configuration, that is a complicated region of juxtaposed sections of oceanic and continental crust. The complicated tectonic history, together with large areas of difficult to access terrain (e.g., mountains, dense tropical rainforest, vegetated wetlands) has meant it has been difficult to identify the location of ancient plate boundaries.

Australian continental crust was initially proposed to extend no further north than the New Guinea Fold and Thrust belt in eastern New Guinea (Abers and McCaffrey, 1988). Recent studies from eastern New Guinea, however, show that underthrust Australian continental crust may continue north beneath the New Guinea Mobile Belt and be in contact with oceanic or island arc crust of the Marum Ophiolite and Adelbert–Finisterre–Huon Block along the Ramu–Markham Fault (Crowhurst et al., 1996). Isotopic studies from central New Guinea indicate that ancient Archaean or Proterozoic Australian continental crust extends north beneath the Central Range to at least the 4th parallel South (based on the location of crustally contaminated Pliocene–Pleistocene igneous rocks) (Housh and McMahon, 2000).

In addition, limited geophysical data exist in this region of the world. This means it is difficult to map the variation of crustal thicknesses across New Guinea. Those data that do exist (e.g., seismic tomography) have typically been applied to only eastern New Guinea (Abers, 1991; Abers and McCaffrey, 1988; Abers and Roecker, 1991) or used to image subducted slabs deeper in the mantle (Hall and Spakman, 2003), while other work has focussed on looking at upper crustal structures that may control the location of ore deposits (White et al., 2014) – however, all of this work has relied on predominately coarse resolution data. It therefore presents a significant challenge as to how one could map the crustal structure of New Guinea.

Recent Nd isotopic studies of the Australian continent have been used to isotopically map different geological domains, from Archaean–Proterozoic cratons to Palaeozoic arc and orogenic terranes (Champion, 2013; Champion and Huston, 2016; Mole et al., 2015). Such work has also been applied in the Himalaya (Ahmad et al., 2000; Richards et al., 2005), the western United States (Bennett and DePaolo, 1987), the Central Asian Orogenic Belt (Wang et al., 2009; Wang et al., 2015), and Antarctica (Borg and DePaolo, 1994), providing insight into the age and isotopic signature of juxtaposed terranes and the underlying crust. Here we take a similar approach using ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd whole-rock isotopic data from a series of Devonian–Carboniferous and Permian–Triassic basement rock granitoids, metamorphosed Mesozoic passive margin sedimentary rocks, Oligocene oceanic island arcs, and Miocene–Pleistocene magmatic rocks from NW New Guinea. Using these isotopic data, we define isotopic signatures for and map different crustal domains across the region. We then compared our data with existing isotopic data available from other parts of New Guinea to map the northern-most extent of Australian continental crust and other crustal fragments.