Retrodicting flow of the early Cenozoic mantle: perspectives from an adjoint modelling approach

Convection in the mantle provides the primary forces that shape the long wavelength structure of the Earth's surface
through dynamic topography. These forces have long been known as the cause of key events in the Cenozoic era: the
termination of large-scale marine inundation in North America in the Palaeocene, the late Tertiary rise of Africa
relative to other continents and the long-wavelength tilting of Australia since the late Cretaceous. It is an
overarching goal in geodynamics to construct reliable models that can retrodict (make predictions about the past)
these key events correctly. This year marks the 20th anniversary since the introduction of adjoint modelling as a
powerful method to retrodict mantle flow. Using the adjoint method, various datasets are assimilated to optimize
dynamic earth models by deriving the necessary gradient information. Here we explore a suite of eight high-resolution
(about 670 million finite elements), compressible, global mantle flow retrodictions going back to 50 Ma. Our
retrodictions involve the dynamic effects from an upper mantle low-viscosity zone, assimilate a past plate-motion
model for the tangential surface velocity field, probe the influence of two different present-day mantle state
estimates derived from seismic tomography, and acknowledge the rheological uncertainties of dynamic Earth models
by taking in four different realizations for the radial mantle viscosity profile, two of which were published
previously. The retrodictions show for the first time that key Cenozoic events emerge jointly as part of global
Cenozoic mantle flow histories. We show that the retrodicted mantle flow histories are sensitive to the present-day
mantle state estimate and the rheological properties of the Earth model, meaning that this input information is
testable with inferences gleaned from the geological record. Retrodictions allow one to track material back in
time from any given sampling location, making them potentially useful, for example, to geochemical studies. Our
results call for improved estimates of non-isostatic vertical motion of the Earth’s surface — provided, for
instance, by basin analysis, seismic stratigraphy, landform studies, thermochronological data or the sedimentation
record — to constrain the recent mantle flow history and suggest that mantle flow retrodictions may yield synergies
across different Earth science disciplines.