Variations in Earth's 1D viscosity structure in different tectonic regimes

Abstract

Earth’s long-wavelength geoid provides insights into the thermal, structural, and compositional evolution of the mantle. Historically, most estimates of mantle viscosity using the long-wavelength geoid have considered radial variations with depth in a symmetric Earth. Global estimates of this kind suggest an increase in viscosity from the upper mantle to lower mantle of roughly 2 -- 3 orders of magnitude. Using a spatio-spectral localization technique with the geoid, here we estimate a series of locally constrained viscosity-depth profiles covering two unique regions, the Pacific and Atlantic hemispheres, which show distinct rheological properties. The Pacific region exhibits the conventional Earth's 1D rheology with a factor of roughly 80-100 increase in viscosity occurring at transition zone depths (400 - 800 km). The Atlantic region in contrast does not show significant viscosity jumps with depth, and instead has a near uniform viscosity in the top 1000~km. The inferred viscosity variations between our two regions could be due to the prevalence of present-day subduction in the Pacific and the infrequence of slabs in the Atlantic, combined with a possible hydrated transition zone and mid-mantle of the Atlantic region by ancient subduction during recent supercontinent cycles. Rigid slab material within the top 800 km, with about 90\% Majoritic garnet in the form of subducted oceanic crust, coupled with unique regional mantle structures, may be generating a strong transition zone viscosity interface for the Pacific region. These effective lateral variations in mantle viscosity could play a role in the observed deformation differences between the Pacific and Atlantic hemispheres.