Structural variability of the Tonga-Kermadec forearc characterized using robustly constrained geophysical data

Subducting bathymetric anomalies enhance erosion of the overriding forearc crust. The deformation associated with this process is superimposed on pre-existing variable crustal and sedimentary structures developed as a subduction system evolves. Recent attempts to determine the effect and timescale of Louisville Ridge seamount subduction on the Tonga-Kermadec forearc have been limited by simplistic models of inherited overriding crustal structure that neglect along-strike variability. Synthesis of new robustly tested seismic velocity and density models with existing data sets from the region, highlight along-strike variations in the structure of the Tonga-Kermadec sub-ducting and overriding plates. As the subducting plate undergoes bend-faulting and hydration throughout the trench-outer rise region, observed oceanic upper-and mid-crustal velocities are reduced by similar to 1.0 kms(-1) and upper mantle velocities by similar to 0.5 kms(-1). In the vicinity of the Louisville Ridge Seamount Chain (LRSC), the trench shallows by 4 km and normal fault throw is reduced by > 1 km, suggesting that the subduction of seamounts reduces plate deformation. We find that the extinct Eocene frontal arc, defined by a high velocity (7.0-7.4 kms(-1)) and density (3.2 g cm(-3)) lower-crustal anomaly, increases in thickness by similar to 6 km, from 12 to > 18 km, over 300 km laterally along the Tonga-Kermadec forearc. Coincident variations in bathymetry and free-air gravity anomaly indicate a regional trend of northward-increasing crustal thickness that predates LRSC subduction, and highlight the present-day extent of the Eocene arc between 32. S and similar to 18. S. Within this framework of existing forearc crustal structure, the subduction of seamounts of the LRSC promotes erosion of the overriding crust, forming steep, gravitationally unstable, lower-trench slopes. Trench-slope stability is most likely re-established by the collapse of the mid-trench slope and the trenchward side of the extinct Eocene arc, which, within the framework of forearc characterization, implies seamount subduction commenced at similar to 22. S.