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Pre-Arc, Near-Trench, Magmatism During Subduction Initiation at the Young (< 2 Ma) Matthew and Hunter Subduction Zone
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  • Martin Patriat,
  • Trevor Falloon,
  • Leonid Danyushevsky,
  • Julien Collot,
  • Marlon Jean,
  • Kaj Hoernle,
  • Folkmar Hauff,
  • Roland Maas,
  • Jon Woodhead,
  • Sandrin Feig
Martin Patriat

Corresponding Author:martin.patriat@ifremer.fr

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Trevor Falloon
Univ Tasmania
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Leonid Danyushevsky
CODES, University of Tasmania
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Julien Collot
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Marlon Jean
University of Alaska-Anchorage
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Kaj Hoernle
GEOMAR Helmholtz Centre for Ocean Research Kiel
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Folkmar Hauff
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Roland Maas
University of Melbourne
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Jon Woodhead
Univ Melbourne
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Sandrin Feig
University of Tasmania
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The Matthew and Hunter (M&H) area, in the South-West Pacific, was formerly interpreted as a transform boundary at the southern termination of the New Hebrides Subduction Zone. But new data collected during three voyages of RV Southern Surveyor (2004-2009), combined with detailed analysis of seismicity and GPS kinematics, shows it is a distinct subduction zone initiated only 2 Ma ago. In fact, M&H is the youngest known volcanically-active intra-oceanic subduction system. We demonstrate that the M&H subduction zone is a modern example of an immature subduction system at the particular stage of pre-arc, near-trench magmatism. It is not yet forming an arc but the proto-forearc. Indeed volcanism occurs much closer to the trench than volcanism at mature subduction zones. Also M&H hosts an exceptionally diverse range of magma compositions, which erupted contemporaneously and are spatially juxtaposed. Pb isotopic compositions and contents of LILE and REE indicate melting of upwelling asthenospheric mantle (Indian MORB) and subducted oceanic crust (Pacific MORB of the South Fiji Basin) and the mixing of these two components. It is worth noting that the present day proto-forearc of the M&H subduction zone corresponds to an area where highly contrasting terranes are juxtaposed: remnants of the old Vitiaz Arc crust, domains of classical backarc basin type oceanic accretion, and what we call Subduction Initiation Terranes (SITER). Such live observations of a growing forearc are rare. They should give insights into the study of fossil forearcs such as SSZ ophiolites but also the IBM forearc.