New insights on the dynamics of the Sumatra and Mariana complexes inferred from the comparative analysis of gravity data and model predictions

Subduction is responsible for surface displacements and deep mass redistribution. This
rearrangement generates density anomalies in a wide spectrum of wavelengths which, in turn,
causes important anomalies in the Earth's gravity field that are visible as lineaments parallel to the
arc-trench systems. In these areas, when the traditional analysis of the deformation and stress
fields is combined with the analysis of the perturbation of the gravity field and its slow time
variation, new information on the background environment controlling the tectonic loading phase
can be disclosed.
Here we present the results of a comparative analysis between the geodetically retrieved
gravitational anomalies, based on the EIGEN-6C4 model, and those predicted by a 2D thermo-
chemical mechanical modeling of the Sumatra and Mariana complexes.
The 2D model accounts for a wide range of parameters, such as the convergence velocity, the
shallow dip angle, the different degrees of coupling between the facing plates. The marker in cell
technique is used to compositionally differentiate the system. Phase changes in the crust and in
the mantle and mantle hydration are also allowed. To be compliant with the geodetic EIGEN-6C4
gravity data, we define a model normal Earth considering the vertical density distribution at the
margins of the model domain, where the masses are not perturbed by the subduction process.
Model predictions are in good agreement with data, both in terms of wavelengths and magnitude
of the gravity anomalies measured in the surroundings of the Sumatra and Marina subductions.
Furthermore, our modeling supports that the differences in the style of the gravity anomaly
observed in the two areas are attributable to the different environments – ocean-ocean or ocean-
continental subduction – that drives a significantly different dynamic in the wedge area.