Holm, Robert J. (2013) Magmatic arcs of Papua New Guinea: insights into the late Cenozoic tectonic evolution of the northern Australian plate boundary. PhD thesis, James Cook University.

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Abstract

Papua New Guinea, as part of the southwest Pacific, lies in a complex tectonic setting of oblique convergence between the Pacific and Australian plate, and trapped between the converging Ontong Java Plateau of the Pacific plate and the Australian continent. Through studying the tectonic evolution of Papua New Guinea we can gain insight into how the region formed through time and explore the relationships between the driving forces of tectonics and the responses or feedbacks reflected in the geology. At present we lack the evidence to fully constrain the evolution of Papua New Guinea, however, through the study of arc magmatism I illustrate how we might approach defining a geodynamic framework for the region and unraveling the complex tectonic history.

In general, the deformation history of Papua New Guinea is well constrained but the same robust records do not exist for magmatism. To progress with unravelling the tectonic evolution of the region we require a time and cost effective means of robust age dating in young rocks. Investigation into the timing of two Quaternary magmatic occurrences by LA-ICP-MS U-Pb zircon dating compared with CA-TIMS and SHRIMP dating methods reveal good correlation and age agreement between dating methods. This is one of only few studies utilizing LA-ICP-MS U-Pb zircon dating for Quaternary ages. Uncertainty in such young rocks is relatively large due to potential matrix effects related to differential alpha dose accumulation and limitations on instrumental precision with an associated error threshold of 5% for a determined age. This makes LA-ICP-MS U-Pb dating a useful tool in regional studies of this nature.

The Late Cenozoic tectonic evolution of Papua New Guinea is set in motion with initial convergence at the Australia-Pacific plate boundary from 45 Ma. Combined U-Pb zircon geochronology and geochemical investigation into the evolution of the Melanesian arc from the Simuku Igneous Complex of West New Britain, Papua New Guinea reveals development of the embryonic island arc from at least 43 Ma. Progressive arc growth was punctuated by distant collision of the Ontong Java Plateau and subduction cessation from 26 Ma. This change in subduction dynamics is represented by emplacement of the adakitic Simuku Porphyry Complex between 24 and 20 Ma. Petrological and geochemical affinities highlight genetic differences between contemporaneous “normal” arc volcanics and adakite-like signatures of Cu-Mo mineralized porphyritic intrusives. Not only is this one of few studies into the geology of the Melanesian arc, it is also among the first to address the distant tectono-magmatic effects of major arc/forearc collision events and subduction cessation on magmatic arcs.

Understanding of the tectonic evolution of mainland Papua New Guinea throughout the Miocene is hampered by a lack of robust geological evidence. The Maramuni arc of Papua New Guinea represents the only continuous tectonic element throughout this dynamic period. I present the first detailed U-Pb geochronology and geochemical investigation of the Maramuni arc from intrusive rocks of the Kainantu region of the eastern Papuan Highlands. Arc magmatism related to north-dipping subduction at the Pocklington trough is punctuated by arrival of the Australian continent at approximately 12 Ma and growth of the New Guinea Orogen. Arc geochemistry from 12 Ma highlights a changing tectonic setting, marked by anomalous enrichments in high-field strength elements at 9 Ma. Crustal delamination from approximately 7 Ma is coincident with porphyry intrusion bearing similarities to adakitic rocks and renewed uplift of the New Guinea Orogen from 6 Ma. Although the fundamental controls on arc evolution are yet to be conclusively determined, this study contributes understanding of subduction dynamics in Papua New Guinea during the Miocene.

Papua New Guinea is a unique part of the planet in that Cenozoic orogenesis is superimposed on an earlier basement structure that has, in turn, influenced how the New Guinea Orogen has grown over time. Specifically, the underlying/underthrust Australian continental basement is characterized by a major accretionary boundary referred to as the Tasman Line separating a younger Paleozoic-Mesozoic Tasmanide orogenic belt from Precambrian cratonic rocks to the west. Using precise age dating of inherited zircon from Quaternary magmatic rocks in the overriding Cenozoic fold belt coupled with an evaluation of orogen-scale deformation and magmatic morphology, I show where the Tasman Line extends to the north beneath younger crust. This result is significant as it demonstrates that with the use of a well established technique combined with detailed structural and tectonic assessment, deep crustal boundaries can be traced in relatively young terrains.

The Bismarck Sea region of Papua New Guinea is marked by recent arc-continent collision giving rise to a highly dynamic tectonic environment. We present a new crustal and upper mantle architecture model for northeastern Papua New Guinea and western New Britain that reveals overprinting slab subduction and partial continental subduction. Earthquake hypocentre databases are combined with detailed topography and seafloor structure together with geology and regional-scale gravity in conjunction with an updated 3-D slab map to unravel this sub-surface structure. Subducting continental crust has resulted in a complex pattern of arc-related geochemical signatures from east to west along the Bismarck arc. In the east where the Solomon Sea plate is subducting beneath New Britain, the sedimentary component is low whereas in the west the arc volcanics exhibit a greater sedimentary component, consistent with subduction of Australian crustal sediments. As a result, a new plate reconstruction is provided for the region together with forward-looking reconstructions.

Overall the studies presented here reflect a region controlled by compounding major collision events resulting in increasing levels of tectonic complexity acting on decreasing spatial-scales. This thesis presents an insight into the tectonics of Papua New Guinea and significantly contributes to our understanding of the dynamic Australia-Pacific plate boundary zone, while also providing new tools that we can use to unravel complex tectonic scenarios at both the present day and at ancient plate boundary settings.

Item ID:	32125
Item Type:	Thesis (PhD)
Keywords:	Australia-Pacific plate boundary zone; geodynamic framework; Kainantu region; Maramuni arc; Papua New Guinea; tectonic evolution; tectonic history
Related URLs:	http://researchonline.jcu.edu.au/29388/ http://researchonline.jcu.edu.au/29377/
Additional Information:	For this thesis, Robert Holm received the Dean's Award for Excellence 2014. Publications arising from this thesis are available from the Related URLs field. The publications are: Chapter 3. Holm, Robert J., Spandler, Carl, and Richards, Simon W. (2013) Melanesian arc far-field response to collision of the Ontong Java Plateau: geochronology and petrogenesis of the Simuku Igneous Complex, New Britain, Papua New Guinea. Tectonophysics, 603. pp. 189-212. Chapter 6. Holm, R.J., and Richards, S.W. (2013) A re-evaluation of arc-continent collision and along-arc variation in the Bismarck Sea region, Papua New Guinea. Australian Journal of Earth Sciences, 60 (5). pp. 605-619.
Date Deposited:	30 Apr 2014 04:33
FoR Codes:	04 EARTH SCIENCES > 0403 Geology > 040313 Tectonics @ 33% 04 EARTH SCIENCES > 0403 Geology > 040303 Geochronology @ 34% 04 EARTH SCIENCES > 0403 Geology > 040304 Igneous and Metamorphic Petrology @ 33%
SEO Codes:	97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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