Cenozoic Dynamic Topography of Madagascar

S N Stephenson,D Seward,P W Ball,N J White,A Carter,M Klöcking

doi:10.1029/2020gc009624

Abstract

AbstractIt has been proposed that Oligo‐Miocene regional uplift of Madagascar was generated and is maintained by mantle dynamical processes. Expressions of regional uplift include flat‐lying Upper Cretaceous‐Paleogene marine limestones that crop out at elevations of hundreds of meters along the western seaboard and emergent Quaternary coral‐rich terraces that rim the coastline. Here, we explore the history of subcrustal topographic support through a combined analysis of four sets of observational constraints. First, we exploit published receiver function estimates of crustal thickness and spectral admittance between gravity and topography. An admittance value of ∼+40 ± 10 mGal km−1 at wavelengths >500 km implies that ∼1 km of topography is supported by subcrustal processes. Secondly, new apatite fission‐track and helium measurements from 18 basement samples are inverted, constraining temperature and denudation histories. Results suggest that 0.5–1.6 km of regional uplift occurred after ∼30 Ma. Thirdly, we calculate a history of regional uplift by minimizing the misfit between observed and calculated longitudinal river profiles. Results suggest that topography was generated during Neogene times. Finally, inverse modeling of rare earth element concentrations in Neogene mafic rocks indicates that melting of the asthenospheric source occurred at depths of ≤65 km with potential temperatures of 1300–1370 °C. Melting occurred at higher temperatures beneath Réunion Island and northern Madagascar and at lower temperatures beneath the Comores and southern Madagascar. These inferences are consistent with shear wave velocities obtained from tomographic models. We conclude that Madagascar is underlain by thinned lithospheric mantle and that a thermal anomaly lies within an asthenospheric layer beneath northern Madagascar.

Highlights

IntroductionThe southern Indian ocean has basin-and-swell bathymetry, a long-wavelength positive free-air gravity anomaly, and anomalously slow shear wave velocities within both the upper and lowermost mantle (Figures 1a and 1b; Becker et al, 2009; Celli, Lebedev, Schaeffer, & Gaina, 2020; Celli, Lebedev, Schaeffer, & Ravenna, 2020; Förste et al, 2012; Mazzullo et al, 2017; Ritsema et al, 2011; Schaeffer & Lebedev, 2013; Tapley et al, 2005)
Thermochronologic measurements imply that significant escarpment retreat occurred during Neogene times which is consistent with the cumulative uplift history calculated by calibrated inverse modeling of an
We suggest that regional uplift commenced in southern Madagascar at 30–40 Ma and spread into the central and northern parts of the island

Summary

Introduction

The southern Indian ocean has basin-and-swell bathymetry, a long-wavelength positive free-air gravity anomaly, and anomalously slow shear wave velocities within both the upper and lowermost mantle (Figures 1a and 1b; Becker et al, 2009; Celli, Lebedev, Schaeffer, & Gaina, 2020; Celli, Lebedev, Schaeffer, & Ravenna, 2020; Förste et al, 2012; Mazzullo et al, 2017; Ritsema et al, 2011; Schaeffer & Lebedev, 2013; Tapley et al, 2005). The spatial and temporal history of regional uplift together with the nature of the processes that drive this uplift are poorly understood

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