Exploring Upper-Mantle Dynamics using Global Adjoint Convection Models

Abstract:

Seismic tomography provides important insight into the structure and dynamics of Earth’s interior. Recent global and regional seismic tomography models revealed prominent slower-than-ambient seismic velocity anomalies in the upper mantle far from mid-ocean ridges. The origin and temporal evolution of these slow seismic anomalies are subject to ongoing debates. In models assuming a purely thermal origin, these slow anomalies are usually attributed to mantle plumes. In contrast, others prefer the compositional origin, which relates the slow anomalies to volatiles, melts or basaltic minerals. In theory, these different hypotheses will generate different mantle flow fields due to their different dynamic behaviors. We propose to better understand the nature of these slow anomalies by investigating their geodynamic properties (e.g. density and viscosity) and the resultant effects on mantle flow and surface observables.

In practice, we will construct global adjoint mantle convection models based on present-day seismic tomography, through which we investigate the temporal evolution of mantle flow by assuming different dynamic properties of these seismic anomalies. We further constrain the models’ dynamics by comparing both calculated depth-dependent seismic anisotropy and SKS splitting with observations. Through these exercises, we hope to better understand the density and viscosity of these mantle slow anomalies, and to explore their spatial-temporal relationship with end-member tectonic scenarios (mantle plumes, subducted slabs, or delaminated materials). We will also evaluate the mantle-flow-generated dynamic topography with a particular focus on the contribution from upper mantle slow anomalies. Traditionally, dynamic topography is considered a long-wavelength signal originated mostly from large-scale mantle convection, but some recent studies challenged this view by suggesting that there is prominent short-wavelength dynamic topography supported by small-scale upper-mantle and/or lithospheric processes. By constraining the dynamics of upper mantle seismic anomalies, which play an important role in controlling mantle flow and short-wavelength dynamic topography, we hope to shed some lights on the origin of mantle heterogeneities and their temporal evolution.