Uplift rate of Kitadaito Jima Island on the lithospheric forebulge of the Philippine Sea Plate

Abstract

Estimates of uplift rates for lithospheric forebulges are needed to understand exact plate motions at plate convergence zones and to delineate the fate of coral reefs atop the forebulges. A carbonate island on a lithospheric forebulge can provide excellent materials for estimating uplift rate because carbonate sediments can be dated and their paleo-water depth determined. We estimated the uplift rate of Kitadaito Jima Island, a carbonate island on the lithospheric forebulge of the Philippine Sea Plate that is subducting beneath the Eurasian Plate. Marine skeletal sands containing pebble-sized bioclasts and filling the concavity of an intertidal erosional notch at an elevation of ~ 71 m were found near the top of this island. Strontium isotope ages ranging from 1.78 to 2.01 Ma with an average of 1.89 Ma (standard deviation = 0.07 Ma) were obtained from these deposits. As global sea level at 1.89 Ma was ~ 21 m lower than the present, the mean uplift rate was estimated at ~ 49 m/million years (Myr) with a compounded uncertainty of ± 2.6 m/Myr. This rate is comparable to, or up to ~ 140 m/Myr less than, rates reported from other Indo-Pacific carbonate islands in similar tectonic settings. This study illustrates how contemporary Sr isotope age models and careful considerations of limestone sample depositional depths can yield more accurate and precise uplift rates of modern forearc bulges than possible in many older studies.