*Revised manuscript with no changes marked Click here to view linked References 1 Melanesian back-arc basin and arc development: constraints from the eastern Coral 1 2 2 Sea 3 4 5 1* 2 1 3 4 6 3 Maria Seton , Nick Mortimer , Simon Williams , Patrick Quilty , Phil Gans , Sebastien 7 8 4 Meffre3,5, Steven Micklethwaite6a, Sabin Zahirovic1, Jarrod Moore1, Kara J. Matthews1b 9 10 11 1 12 5 EarthByte Group, School of Geosciences, The University of Sydney, NSW 2006, Australia 13 14 15 6 2GNS Science, Private Bag 1930, Dunedin, 9054, New Zealand 16 17 18 3 19 7 Discipline of Earth Sciences, University of Tasmania, Private Bag 79, Hobart, TAS, 7001, 20 21 8 Australia 22 23 24 4 25 9 Department of Earth Science, UC Santa Barbara, Santa Barbara, CA, 93106-9630, USA 26 27 28 10 5ARC Centre of Excellence in Ore Deposits, University of Tasmania, Private Bag 79, Hobart, 29 30 31 11 TAS, 7001, Australia 32 33 34 12 6Centre for Exploration Targeting, University of Western Australia, Perth, 6009, Australia 35 36 37 * 38 13 Corresponding author. e-mail: [email protected] 39 40 41 14 a Present address: School of Earth, Atmosphere and Environment, Monash University, 42 43 44 15 Melbourne, VIC, 3800, Australia 45 46 47 16 b Present address: Department of Earth Sciences, University of Oxford, South Parks Road, 48 49 17 50 Oxford OX1 3AN, UK 51 52 53 18 54 55 56 19 57 58 59 60 20 Submitted to Gondwana Research 61 62 63 64 1 65 21 1 2 3 22 4 5 6 7 23 Abstract 8 9 10 24 The eastern Coral Sea is a poorly explored area at the north-eastern corner of the Australian 11 12 13 25 Tectonic Plate, where interaction between the Pacific and Australian plate boundaries, and 14 15 26 accretion of the world’s largest submarine plateau - the Ontong Java Plateau - has resulted in 16 17 18 27 a complex assemblage of back-arc basins, island arcs, continental plateaus and volcanic 19 20 28 products. This study combines new and existing magnetic anomaly profiles, seafloor fabric 21 22 23 29 from swath bathymetry data, Ar-Ar dating of E-MORB basalts, palaeontological dating of 24 25 30 carbonate sediments, and plate modelling from the eastern Coral Sea. Our results constrain 26 27 31 28 commencement of the opening of the Santa Cruz Basin and South Rennell Trough to c. 48 29 30 32 Ma and termination at 25-28 Ma. Simultaneous opening of the Melanesian Basin/Solomon 31 32 33 Sea further north suggests that a single > 2,000 km long back-arc basin, with at least one 33 34 35 34 triple junction existed landward of the Melanesian subduction zone from Eocene-Oligocene 36 37 35 times. The cessation of spreading corresponds with a reorganization of the plate boundaries 38 39 40 36 in the area and the proposed initial soft collision of the Ontong Java Plateau. The correlation 41 42 37 between back-arc basin cessation and a widespread plate reorganization event suggests that 43 44 38 45 back-arc basins may be used as markers for both local and global plate boundary changes. 46 47 48 39 Keywords 49 50 51 40 52 Eastern Coral Sea; Melanesia; SW Pacific; back-arc basin; subduction 53 54 55 41 1. Introduction 56 57 58 59 60 61 62 63 64 2 65 42 The Coral Sea lies in the north-eastern corner of the Australian Tectonic Plate, in a pivotal 1 2 43 position at the juncture between two of the world’s most tectonically complex regions: the 3 4 5 44 SW Pacific and SE Asia (Fig. 1). While the origin and evolution of the western part of the 6 7 45 Coral Sea, including the Queensland and Marion Plateaus and Coral Sea Basin is relatively 8 9 10 46 well-known (Falvey and Taylor, 1974; Gaina et al., 1999; Weissel and Watts, 1979), the 11 12 47 eastern Coral Sea, with its mosaic of ridges, plateaus and basins has a largely unknown 13 14 48 15 history. Previous plate tectonic reconstructions seeking to explain the origin and evolution of 16 17 49 the submarine features in the eastern Coral Sea have been limited. The area is either largely 18 19 50 ignored in regional plate tectonic models (Hall, 2002; Sdrolias et al., 2003; Weissel and 20 21 22 51 Watts, 1979), or overly simplified with inferred ages for many of the features (Schellart et al., 23 24 52 2006; Whattam et al., 2008; Yan and Kroenke, 1993). These poorly constrained models 25 26 27 53 reflect the scarcity of marine geophysical data and geological samples from the area, the 28 29 54 inherent complexity of the many ridges and basins, and substantial volcanic and tectonic 30 31 32 55 overprinting. Such a significant gap in our understanding of the tectonic development of the 33 34 56 Coral Sea is unsatisfactory particularly for an area that provides plate boundary continuity 35 36 57 37 between the SW Pacific and SE Asia along the western Pacific boundary, and thus is critical 38 39 58 for plate reconstructions of both areas. In addition, this area lies adjacent to the North 40 41 59 Solomon subduction zone and Ontong Java Plateau, the site of a major plateau accretion and 42 43 44 60 subduction event, which is believed to have occurred either in the Oligocene (Musgrave, 45 46 61 2013), Miocene (Petterson et al., 1999) or Pliocene (Cowley et al., 2004). This collision 47 48 49 62 induced a subduction polarity reversal in the Pliocene (Cooper and Taylor, 50 51 63 1985). Understanding the evolution of the submarine features in the eastern Coral Sea may 52 53 64 54 provide insights into the timing and processes of the Ontong Java Plateau collision and help 55 56 65 identify the effects of a major collision event on the over-riding plate. 57 58 59 60 61 62 63 64 3 65 66 We present the results of a recent research voyage (SS2012_V06) on RV Southern Surveyor 1 2 67 to the eastern Coral Sea in October-November, 2012 where 13,600 km2 of swath bathymetry 3 4 5 68 data, 6,200 km of magnetic and 6,800 km of gravity data were collected, together with 6 7 69 igneous and sedimentary samples from 14 seafloor sites (Fig. 1). Swath bathymetry and 8 9 10 70 magnetic anomaly data are used to model the timing and orientation of seafloor spreading in 11 12 71 the Santa Cruz Basin. Geochemical analysis and Ar-Ar dating of recovered igneous samples 13 14 72 15 together with paleontological age constraints from sedimentary samples at the dredge site 16 17 73 locations are used to relate spreading in the Santa Cruz Basin to activity along the South 18 19 74 Rennell Trough and the igneous activity along the Rennell Island Ridge. Combining the 20 21 22 75 results of our study with recently dated dredge samples from earlier ORSTOM voyages 23 24 76 (Mortimer et al., 2014), we develop plate tectonic reconstructions of the eastern Coral Sea 25 26 27 77 since the Cretaceous underpinned by age constraints from both marine geophysical data and 28 29 78 geological samples. Our plate reconstructions include a network of continuously closing 30 31 32 79 plate boundaries (Gurnis et al., 2012), which can be used to provide plate boundary 33 34 80 continuity between the SW Pacific and SE Asia during this time period. 35 36 37 81 2. Regional tectonic framework 38 39 40 41 82 The SW Pacific is characterised by a series of marginal basins, submerged continental slivers 42 43 83 44 and back-arc—arc—forearc complexes largely controlled by the interaction of the Australian 45 46 84 and Pacific Tectonic Plates since the Mesozoic (Crawford et al., 2003; Schellart et al., 2006; 47 48 85 Sdrolias et al., 2003; Whattam et al., 2008; Yan and Kroenke, 1993) (Fig. 1). In contrast, SE 49 50 51 86 Asia is an amalgamation of accretionary continental fragments, exotic terranes and intra- 52 53 87 oceanic island arcs, formed largely from the long-term interaction between the Australian, 54 55 56 88 Pacific and Eurasian plates and Gondwana-derived terrane accretion (Acharyya, 1998; 57 58 89 Golonka, 2004; Hall, 2002; Metcalfe, 2009; Stampfli and Borel, 2002; Zahirovic et al., 59 60 61 62 63 64 4 65 90 2014). These two tectonically complex regions are connected through the “Melanesian 1 2 91 Borderlands” region, which includes the Coral and Solomon Seas. 3 4 5 6 92 The Coral Sea is bounded by the passive margin of eastern Australian in the west, the 7 8 93 inactive Pocklington Trough and Papuan Peninsula to the north, the active South 9 10 11 94 Solomon/San Cristobal and New Hebrides Trenches to the northeast and east, and the Lord 12 13 95 Howe Rise/New Caledonia region to the south (Fig. 1). It consists of the relatively well- 14 15 16 96 explored Coral Sea Basin and Queensland and Marion plateaus in the western part and a 17 18 97 complex arrangement of poorly explored and under-sampled submarine plateaus, linear 19 20 98 21 depressions, elongated ridges, oceanic basins and seamounts in the eastern part, including the 22 23 99 Rennell Basin, Rennell Island Ridge, South Rennell Trough, West Torres Plateau and Santa 24 25 100 Cruz and d’Entrecasteaux Basins (Fig. 1). 26 27 28 29 101 Many of the features in the area are named after Rennell Island, which lies in the eastern 30 31 102 Coral Sea at 160°E, 11.5°S (Fig.