Anthony Osei Tutu

As computational geophysicists I have interest in the dynamics of Earth's interior and the relation surface processes. My research focuses on understanding the various dynamic processes of the solid earth that influence the geoid, plate motions, intra-plate stresses and postglacial rebound. Other areas of interest include using numerical and observational constraints to relate changes in dynamic topography due to convection and surface mass variations to short and long-term sea-level change and coastal dynamics.

Topics

Geodynamics of lithosphere and mantle interactions and the effects on dynamic topography, stress field, plate motions and geoid.
Glacial Isostatic Adjustment (e.g., sea-level variations, crustal motion, time variable gravity).
Geodetic constraints on crustal/lithosphere.
Mantle rheology.
Topographic evolution over hot spots and plumes.

Current and ongoing research

1. Global Subduction influence on mantle viscosity

I developed local geoid kernels using spectral localization technique (Slepian basis functions) to infer regional depth-dependent viscosities for two unique regions of Earth's mantle with to constraint from GRACE static geoid.

Mantle structures such as subduction and slabs influence the Earth's 1D viscosity structure especially in top 800 km of the mantle. There is a distinct laterally viscosity difference between the two regions. The Pacific region with more slabs predicts strong mantel interface while the less/no-slabs region Atlantic shows low-viscosity transition zone. The origin of the distinct stiffness could be the presences of Majoritic garnet in the form of subducted oceanic crust, in the mantle transition zone.

(Submitted)

2. Regional mantle viscosity constraints using postglacial and mantle flow models

I employ Slepian Basis functions in regional mantle flow calculations for North America, combined with postglacial modeling in a joint inversion analysis to study local 1D viscosity structure. I use free-air gravity data and relative sea-level (RSL) in the Hudson Bay as constraints and considering ice melting history for the last 10,000 years in our postglacial model. By localizing for a continent-wide inversion as well as the east-west continental divide we can examine the lateral change in viscosity from east to west and the entire region. We inferred both absolute and relative viscosities.

Our show one order of magnitude viscosity jump at the 670 km with large seismic density scaling parameter 0.3 for the entire NA region. Large seismic conversion factor shows significant influence on the resulting viscosity. However, for the east-west divide local viscosities the scaling factor becomes less important for dictating the resulting upper mantle viscosity characteristics. Rather the respective local mantle heterogeneities dictate the characteristic of the upper mantle viscosity. We infer a high viscosity interface complimenting depth range of the eastern continental root. The corresponding relative sea level prediction for the Hudson Bay gave a better fit to the observed date compared to the western NA local inversion.

(Submitted. - Solid-Earth https://doi.org/10.5194/se-2021-151)

Past and Recent projects

3a. Plate boundary dynamics and plate motions

3b. Lithosphere net rotation

The strength at plate boundaries and the upper mantle viscosity affect the speed of lithosphere differently. To better quantify their tectonic controls, I used coupled lithosphere and mantle numerical method with seismic and petrological data to study global plate velocities. Since the mantle and the lithosphere deforms differently, I had to to account for a visco-elasto-plastic rheology within the crust and lithosphere coupled to a purely viscous mantle. I found that friction at plate boundaries, especially in subduction zones is very important to plate velocities. By testing different frictional values suggested by rock experiments and plate boundary heat flow data, I identified different regimes of plate motions from a stagnant plate regime to fast moving plates and an appropriate range of static frictional values at plate boundaries required to obtain the present-day velocities. (Osei Tutu et al., 2018)

Here I estimate the excitation of net rotation (NR) of the lithosphere by the LVVs, and the water content in the asthenosphere keeping plastic yielding at the plate boundaries constant. Modeled plate velocities with 1,000 H/10 Si in the asthenosphere exceed ∼ 20 cm/yr leading to the largest net rotation amplitude of up to ∼ 0.8349°/Ma and giving a global rms > 14 cm/yr for with μ = 0.01. A twofold decrease in the olivine water content (i.e. 500 H/10 Si) in the asthenospheric layer above 300 km depth, resulted in lower net rotations, similar to most of the observation‐based estimates of around 0.1‐0.2 /Ma, even for low coefficients of plate boundary friction μ = 0.03 or less. (Osei Tutu et al., 2018)

4a. Crust and lithosphere stress field

Here, I looked at the contributions of shallow and deep mantle heterogeneities to topography and crustal-lithospheric stresses to understand how thermal and dynamic effects within the lithosphere and mantle contribute to crustal stress and topography?

Using a coupled lithosphere and mantle model I found that lateral density heterogeneities within the lithosphere and asthenosphere have relatively limited contribution to the horizontal stress field. The study result suggest global stress state within the crust and lithosphere has deeper sources with about 80\% of the contributions coming from the convecting mantle. I compare the modeling results to limited observations from the World Stress Map, which shows great agreement in most regions. Large misfits were mostly concentrated in high topographic regions like the Andes and Tibet. (Osei Tutu et al., 2017)

4b. Dynamic topography

Density anomalies with the top 300 km of the mantle dominant dynamic topography anomalies. Based on the seismic model considered the resulting, dynamic topography is more variable and differs more strongly from published models. Dynamic topography also has a larger contribution at smaller scales. Some of these contributions can be related to subducted slabs or mantle plumes. Confirming previous results, I found that negative topography in cratons is too large, unless a correction for the depletion of cratonic lithosphere is considered. The best fit can be obtained, if the method of Cammarano et al. (2011) is used to convert seismic tomography models to temperature structures, by accounting for chemical depletion in cratons. The best agreement was obtained with residual topography on continents that considers crustal thickness variations based on CRUST1.0

(Osei Tutu et al., 2017)

5. Degree 2 amplitude of dynamic topography

We use mantle flow calculations based on seismic tomography model to reconcile the observed (Hoggard et al. 2016) and predicted dynamic topography. The study results show that while we can accurately compare the amplitudes power and patterns of predicted and observed dynamic topography at short wavelength, these is a greater disagreement at long wavelengths (i.e. l > 10,000 km). The disagreement is mainly associated with the two large, seismically slow regions in the lower mantle beneath Africa and the Pacific Ocean called the Large-low shear velocity provinces (LLSVPs). Predictive models suggest two large-scale uplifted regions above the “superplumes” that are barely seen in the observed topography. We suggest that this mismatch can only partly be reconciled by altering the seismic velocity to density conversion factor or by including the effects of lower mantle chemical heterogeneity(Steinberger et al., 2019).

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