Stress fields and tectonic motions induced by time-varying density anomalies in the lithosphere

Abstract

Abstract We explore the extent to which a viscoelastic rheology of the lithosphere could affect the deviatoric stress and vertical deformation driven by deep seated, time-dependent density anomalies within the lithosphere. These sources can be associated with fast density instabilities of thermal, chemical or tectonic origin. In particular, we focus on the stress transfer from the ductile lower lithosphere to the elastic crust. The model is radially stratified and employs the Maxwell rheology. Stress transfer takes place from a wide region within the lower lithosphere to the thinner elastic top layer where it is amplified. The time history of the source and the lithospheric viscosity play a crucial role in determining the time evolution of the stress and deformation patterns. Attention is drawn to the temporal rates of maximum shear accumulation, which are interpreted as earthquake recurrence times. For realistic values of the model and source parameters, the increase of the deviatoric stress within the crust can be of a few bars on time-scales of 10 3 years, for lithospheric viscosities of around 10 22 to 10 23 Pa s −1 . Lower levels of stress variations are found in the viscoelastic lithosphere. These findings are consistent with the low levels of stress release and long recurrence times that characterize the intraplate seismicity, commonly attributed to density instabilities. As a specific application, we considered the tectonic subsidence of the Tyrrhenian basin. From the rate of subsidence derived from the ODP Leg 107 drilling campaign, we can estimate an upper bound for the lithospheric viscosity of around 5 × 10 22 Pa s −1 , which is appropriate for active tectonic regions.