Gravity signature of crustal subduction inferred from numerical modelling

Gravitational signatures of subduction are a major feature of the Earth's gravity pattern, being visible as lineaments parallel to the arc-trench systems. The novelty of our study resides in the simulation of this subduction fingerprint via a newly developed dynamic subduction model based on the finite-element approach, which includes the compositional stratification of the lithosphere into a light upper crust and a dense lithospheric mantle. Crustal material sinks in the mantle in proximity to the top of the slab due to the viscous dragging forces and off-scraping of the overriding plate. We show that the sinking of light crustal material from both the subducting and overriding plates to depths of the order of 200 km may be responsible for a trough of about 100 MGal extending 100-200 km in a direction perpendicular to the trench. This anomaly is superimposed on a global positive gravity anomaly due to thermal contraction of the mantle-lithosphere system. The negative gravity anomaly due to compositional stratification of the crust has comparable magnitude and horizontal extent of that due to the negative dynamic topography of the trench. The gravity patterns from compositional stratification of the crust and from dynamic trench topography are evaluated for different tectonic subduction styles related to active convergence, with and without roll-back. In general, we found that case without roll-back produces a broader and smoother dynamic topography gravity anomaly than with roll-back, while the gravity signature from compositional stratification is less sensitive to the style of subduction.