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

Abstract

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 1.0 km s and 0.5 km s, 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 (51 Ma) Tonga arc, defined by a high velocity (7.0-7.4 km s) and density (3.30 0.10 g cm) 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 18S and 32S, predating LRSC subduction at the margin.
Beyond this region, the overriding crust formed as the south Fiji Basin opened 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 22S along the margin.