Variations in magmatism and the state of tectonic compensation of the Mariana subduction system

We investigate the variations in the state of isostatic compensation of the Mariana subduction system. We first calculate and analyse the gravity-derived crustal thickness and non-isostatic topography of this region. The Mariana back-arc spreading centre and West Mariana Ridge are both close to Airy isostatic compensation. In contrast, the Mariana Trench axis is associated with a clearly defined zone of negative non-isostatic topography and free-air gravity anomaly (FAA), which have been shown to be caused by the flexural bending of the subducting Pacific Plate. The new analyses reveal that the southern Mariana Arc at 12.5 degrees-16.7 degrees N is associated with positive non-isostatic topography and a local zone of positive FAA, which we interpret to be the result of accretion of young volcanism on a relatively strong lithospheric plate and/or supported by the tectonic stresses induced by the interaction of the subducting Pacific Plate with the upper Mariana micro-plate.