The convergent margins of the western Pacific are marked by widespread tectonic erosion of the overriding plate by the subducting oceanic lithosphere. At Ocean Drilling Program Site 841 on the mid-trench slope of the Tonga forearc, rates of tectonic trenchward tilting and subsidence of the basement appear to be linked, supporting models of basal erosion focused near the trench. Rates of subsidence significantly exceed those predicted from plate cooling models and require crustal thinning to account for the difference in this extensional environment. Estimated extension at Site 841 is 280%. Tilting and subsidence rates peaked ca. 32–34 Ma, just after spreading in the South Fiji Basin at 35 Ma. However, the trenchward direction of tilt and higher extension on the trench slope than on the Tonga platform suggest a link to tectonic erosion near the trench rather than extension focused in the arc edifice. Since this time, ca. 135 km has been eroded from the edge of the plate, of which 80 km is related to the ongoing collision of the Louisville Ridge. A probable paleo–collision event at 16 Ma caused the forearc at Site 841 to be uplifted and tilted arcward, forming a major unconformity due to mass wasting. Steady-state erosion averages <1.5 km/m.y. There is no compelling evidence to suggest loss of several hundred kilometers of arc crust since the Eocene and thus no reason to suspect that subduction initiation and related magmatism may have predated the ca. 45 Ma boninitic magmatic activity.

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