Geophysical Investigation of the Tonga-Kermadec Subduction System: Do Inherited and Seamount Subduction-Influenced Lateral Variations Control Long-Term Crustal Structure?
Abstract
Over the last decade, a number of geophysical and geological sampling studies have been conducted at the Tonga-Kermadec subduction zone (TKSZ) and its intersection with the Louisville Ridge seamount chain (LRSC). These studies investigated not only the structural and geochemical variations throughout the subduction system, but also how they relate to local and regional effects of seamount chain collision, and its consequences for subduction erosion and seismic coupling. We synthesise the results of multiple seismic profiles across the TKSZ to develop an overview of subduction zone structural variability before, during and after LRSC subduction, showing how seamount subduction deforms the forearc, and how the overriding plate subsequently recovers.
Generally, the crustal cross-sectional area of the Tonga-Kermadec forearc decreases from ~3000 km2 to <2000 km2 over a distance of ~300 km from north-south along the system, with the variation accommodated by crustal thinning of ~5 km, primarily concentrated in the lower crust. This variation, which extends beyond the present-day point of seamount subduction, characterizes the inherited structure of the forearc formed during subduction initiation ~52 Ma. The seamounts of the LRSC have been proposed to comprise a spatially discrete high (>6 km s-1) seismic velocity core reaching to shallow depths sub-seabed. In addition, each edifice is structurally supported by a thickened crust extending up to 50 km across-chain. Subduction related bend faulting is reduced with proximity to the seamounts due to their greater structural robustness, moderating fluid flow and lessening the effect of serpentinization-related velocity reduction in the upper mantle by 0.5 km s-1. LRSC subduction causes forearc uplift and erosion, with maximum forearc deformation manifesting ~1 Ma after collision. Following seamount subduction-induced deformation, the inner and mid-forearc slowly recover, while the apparent loss of upper forearc material demarks the extent of seamount subduction over the last 5 Ma. Although this work provides an improved understanding of TKSZ dynamics, seamounts within the subduction channel are not robustly imaged, resulting in uncertainty as to seamount chain orientation beneath the forearc and whether they remain intact as the down-going plate subducts.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T21B..01F
- Keywords:
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- 8145 Physics of magma and magma bodies;
- TECTONOPHYSICS;
- 8170 Subduction zone processes;
- TECTONOPHYSICS;
- 8178 Tectonics and magmatism;
- TECTONOPHYSICS;
- 8439 Physics and chemistry of magma bodies;
- VOLCANOLOGY