Numerical Modeling of Mantle Flow Beneath Madagascar to Constrain Upper Mantle Rheology Beneath Continental Regions

Abstract

Over the past few decades, azimuthal seismic anisotropy measurements have been widely used proxy to study past and present‐day deformation of the lithosphere and to characterize convection in the mantle. Beneath continental regions, distinguishing between shallow and deep sources of anisotropy remains difficult due to poor depth constraints of measurements and a lack of regional‐scale geodynamic modeling. Here, we constrain the sources of seismic anisotropy beneath Madagascar where a complex pattern cannot be explained by a single process such as absolute plate motion, global mantle flow, or geology. We test the hypotheses that either Edge‐Driven Convection (EDC) or mantle flow derived from mantle wind interactions with lithospheric topography is the dominant source of anisotropy beneath Madagascar. We, therefore, simulate two sets of mantle convection models using regional‐scale 3‐D computational modeling. We then calculate Lattice Preferred Orientation that develops along pathlines of the mantle flow models and use them to calculate synthetic splitting parameters. Comparison of predicted with observed seismic anisotropy shows a good fit in northern and southern Madagascar for the EDC model, but the mantle wind case only fits well in northern Madagascar. This result suggests the dominant control of the measured anisotropy may be from EDC, but the role of localized fossil anisotropy in narrow shear zones cannot be ruled out in southern Madagascar. Our results suggest that the asthenosphere beneath northern and southern Madagascar is dominated by dislocation creep. Dislocation creep rheology may be dominant in the upper asthenosphere beneath other regions of continental lithosphere.

Cenozoic Dynamic Topography of Madagascar

“…For example, Rocco et al (2017) infer a local maximum lithospheric thickness of 60 km based upon the absence of garnet-bearing peridotitic xenoliths from magmatic rocks in northern Madagascar. This value is ∼30 km thinner than that calculated by Priestley and M c Kenzie (2013), but in closer agreement with the results of Rajaonarison et al (2020) and Hoggard et al (2020), who calculate thicknesses of <80 km beneath the northern and central highlands. Note that these three studies exploited different surface wave tomographic models to obtain lithospheric thickness estimates.…”

AFRP20: New P‐Wavespeed Model for the African Mantle Reveals Two Whole‐Mantle Plumes Below East Africa and Neoproterozoic Modification of the Tanzania Craton

“…Adjacent to the central Madagascan volcanic province, a circular fast wavespeed anomaly exists ( δV P ≥1.5%), centered at ∼19°S, 48°E. This feature extends to the northeast with depth and is located beneath thicker lithosphere (∼120 km) than is present below adjacent magmatic provinces (Rajaonarison et al., 2020). Resolution tests (Figure demonstrate that this feature is not likely an inversion artifact, although anomaly amplitude appears to be strongly influenced by the crustal correction (Figure ).…”

Lithospheric Control of Melt Generation Beneath the Rungwe Volcanic Province, East Africa: Implications for a Plume Source

“…Partial melt in the model is tracked through the compositional field called "porosity." The viscosity at each quadrature point is calculated from the effective value of the viscosity of the compositional fields weighted by the volume fraction of each composition at the same location (Figures 4a and 4b; Rajaonarison et al, 2020).…”