The effective elastic lithosphere under the Cook-Austral and Society islands

Abstract

We have determined the elastic thickness T e of the oceanic lithosphere along two volcanic chains of the South Central Pacific: Cook-Austral and Society islands. We used a three-dimensional spatial method to model the lithospheric flexure assuming a continuous elastic plate. The model was constrained by geoid height data from the SEASAT satellite. Along the Cook-Austral chain the elastic thickness increases westward, from 2–4 km at McDonald hot spot to ∼ 14 km at Rarotonga. At McDonald seamount, however, the data are better explained by a local compensation model. The observed trend shows an increase of T e with age of plate at loading time. However, the elastic layer under the Cook-Austral appears systematically thinner by several kilometers than expected for “normal” seafloor, suggesting that substantial thermal thinning has taken place in this region. Considering the apparent thermal age of the plate instead of crustal age improves noticeably the results. Along the Society chain T e varies from 20 km under Tahiti to 13 km under Maupiti which is located ∼ 500 km westward. When plotting together the Society and Cook-Austral T e results versus age of load, we notice that within the first five million years after loading, T e decreases significantly while tending rapidly to an equilibrium value. This may be interpreted as the effect of initial stress relaxation which occurs just after loading inside the lower lithosphere and suggests that the presently measured elastic thickness under the very young Tahiti load (∼ 0.8 Ma) is not yet the equilibrium thickness.