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Evolution of the Tonga-Kermadec forearc in response to seamount subduction

FUNNELL, MATTHEW,JAMES (2017) Evolution of the Tonga-Kermadec forearc in response to seamount subduction. Doctoral thesis, Durham University.

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Subducting plate characteristics are recorded as temporal variations in overriding plate deformation.
Downgoing bathymetric anomalies superimpose enhanced tectonic erosion on pre-existing forearc crustal and sedimentary structural variations.
Existing models of inherited forearc crustal structure along the Tonga-Kermadec subduction system are simplistic and neglect along-strike variability, which has resulted in a limited understanding of margin evolution and Louisville Ridge seamount chain (LRSC) subduction.

In this study, robustly tested, velocity-depth and density-depth models are synthesised with existing data from the Tonga-Kermadec margin to reveal along-strike variations in the subducting and overriding plate structure.
In regions north and south of the point of LRSC subduction, the incoming Pacific plate displays $>$2 km-throw bend faults and reduced seismic velocity throughout the crust and upper mantle by $\sim$1.0 km s\textsuperscript{-1} and $\sim$0.5 km s\textsuperscript{-1}, respectively.
Around the LRSC-trench intersection, the trench axial depth decreases by 4 km and normal fault throw is reduced to $<$1 km, suggesting the seamounts reduce subducting plate deformation.
The forearc structure is dominated by the extinct ($\sim$51 Ma) Tonga arc, defined by a high velocity (7.0-7.4 km s\textsuperscript{-1}) and density (3.30 $\pm$ 0.10 g cm\textsuperscript{-3}) lower crustal anomaly.
Increases in Tonga-Kermadec forearc crustal thickness from 12 to $>$18 km over 300 km along-strike are coincident with variations in bathymetry and free-air gravity anomaly that reveal a broader trend of northward-increasing crustal thickness between 18$\degree$S and 32$\degree$S, predating LRSC subduction at the margin.
Beyond this region, the overriding crust formed as the south Fiji Basin opened $\sim$35 Ma.
Within this framework of existing crustal structure, LRSC subduction promotes erosion of the overriding crust, forming a steep unstable lower-trench slope.
Following seamount subduction, trench-slope stability is re-established by the collapse of the extinct Tonga arc, suggesting that seamount subduction commenced at 22$\degree$S along the margin.

Item Type:Thesis (Doctoral)
Award:Doctor of Philosophy
Faculty and Department:Faculty of Science > Earth Sciences, Department of
Thesis Date:2017
Copyright:Copyright of this thesis is held by the author
Deposited On:09 Nov 2017 14:10

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