Gravity Rate of Change Due to Slow Tectonics: Insights from Numerical Modeling

Gravity anomalies caused by tectonics are commonly assumed to be static, based on the argument that the motions are slow enough for the induced mass changes over time to be negligible. We exploit this concept in the context of rifting and subduction by showing that the horizontal motions of density contrasts occurring at active and passive margins are responsible for sizable amounts of gravity rate of change. These findings are obtained via 2D finite element modeling of the two tectonic mechanisms in a vertical cross-section perpendicular to the ocean–continent transition as well as through evaluating the time-dependent gravity disturbance at a reference height caused by mass readjustment underneath. This disturbance originates from deep-seated changing density anomalies and dynamic topography with respect to a reference normal Earth. The gravity rate of change is proven to scale linearly with extensional and trench migration velocity; the peak-to-peak values between the largest maxima and minima are 0.08 $\mathsf{\mu }$Gal/yr and 0.21 $\mathsf{\mu }$Gal/yr, for a velocity of 1 cm/yr. For both tectonic mechanisms, the dominant positive rate of change is due to the horizontal motion of a density contrast of about 300–400 kg/m${}^3$. We also consider the role of dynamic topography in comparison to that of deep-seated changing density anomalies.