Spatial variations in effective elastic thickness in the Western Pacific Ocean and their implications for Mesozoic volcanism

Abstract

We have used free-air gravity anomaly and bathymetric data, together with a moving window admittance technique, to determine the spatial variation in oceanic elastic thickness, T e , in the Western Pacific ocean. Synthetic tests using representative seamounts show that T e can be recovered to an accuracy of ± 5 km for plates up to 30 km thick, with increased accuracy of ± 3 km for T e ≤ 20 km. The Western Pacific has a T e range of 0–50 km, with a mean of 9.4 km and a standard deviation of 6.8 km. The T e structure of the region is dominated by relatively high T e over the Hawaiian-Emperor Seamount Chain, intermediate values over the Marshall Islands, Gilbert Ridge, and Marcus-Wake Guyots, and low values over the Line Islands, Mid-Pacific Mountains, Caroline Islands, Shatsky Rise, Hess Rise, and Musician Seamounts. Plots of T e at sites with radiometric ages suggest that T e is to first order controlled by the age of the lithosphere at the time of loading. In areas that backtrack into the South Pacific Isotopic and Thermal Anomaly (SOPITA), T e may be as low as the depth to the 180 ± 120 °C isotherm at least locally. In the northern part of the study area including the Hawaiian-Emperor Seamount Chain, T e correlates with the depth to 310 ± 120 °C. These best-fitting isotherms imply peak rates of volcanism during 100–120 Ma (Early Cretaceous) and 140–150 Ma (Late Jurassic). The corresponding addition of 8 × 10 6 km 3 and 4 × 10 6 km 3 of volcanic material to the surface of the oceanic crust would result in long-term sea-level rises of 20 m and 10 m respectively. The Late Jurassic volcanic event, like the later Early Cretaceous event, appears to have influenced the tectonic evolution of the Pacific plate convergent boundaries, resulting in increased volcanism and orogenesis.