Constraining subducting slab viscosity with topography and gravity fields in free-surface mantle convection models

Abstract

Subducted slabs provide the primary driving force for mantle convection, and the slab strength directly controls the transfer of the forces between the slab and the lithospheric plates at the surface. The analysis of subducted slab viscosity structure has been one of the main concerns in geodynamics over the past few decades. Previous studies, using the topography, gravity or geoid as crucial observations, have provided some constraints on the viscosity of subduction plates (Bessat et al., 2020; Hager, 1984; Moresi & Gurnis, 1996). However, slab viscosity constrained by surface topography and gravity data is significantly lower than that suggested by mineral physics laboratory experiments. It is unclear whether the free-slip top boundary condition used in many previous studies affects the inverted slab viscosity with gravity or geoid data. In this study, we develop 2-D free-surface subduction models that can generate realistic topography by a modified "sticky-air" method using Underworld2 software (Moresi et al., 2019), and we compare the computed topography and gravity in our free-surface subduction models with observations to constrain the subducting slab viscosity. We investigate the influence of slab viscosity at the bending region and below the bending region on the topography and the gravity, respectively. Our model results support relatively weak slabs (20-120 times more viscous than the upper mantle) at the bending region, consistent with previous studies with a free-slip top boundary. The viscosity of the slab below the bending region barely affects the surface topography and gravity field, and both strong and weak slabs fit the observed topography and gravity field, suggesting that extra independent observations are needed to constrain the deep slab viscosity. Besides, in this study, we also find the comprehensive relations between subduction interface viscosity, surface topography and gravity anomaly, and trench motion. Models with trench advance have significantly low topography and gravity above the volcanic arc, contradicting subduction zone observations. Together with present trench motion observations and previous studies, we support the idea that the trench retreats under normal single-slab subduction conditions.   Bessat, A., Duretz, T., Hetényi, G., Pilet, S., & Schmalholz, S. M. (2020). Stress and deformation mechanisms at a subduction zone: insights from 2-D thermomechanical numerical modelling. Geophysical Journal International, 221(3), 1605–1625. https://doi.org/10.1093/gji/ggaa092 Hager, B. H. (1984). Subducted slabs and the geoid: Constraints on mantle rheology and flow. Journal of Geophysical Research: Solid Earth, 89(B7), 6003–6015. https://doi.org/10.1029/JB089iB07p06003 Moresi, L., & Gurnis, M. (1996). Constraints on the lateral strength of slabs from three-dimensional dynamic flow models. Earth and Planetary Science Letters, 138(1–4), 15–28. https://doi.org/10.1016/0012-821X(95)00221-W Moresi, L., Giordani, J., Mansour, J., Kaluza, O., Beucher, R., Farrington, R., et al. (2019, February 18). underworldcode/underworld2: v2.7.1b (Version v2.7.1b). Zenodo. https://doi.org/10.5281/ZENODO.257203