Structure and Origin of the Crozet Plateau and Conrad Rise, SW Indian Ocean: Insights from Crustal Thickness Mapping Using 3-D Satellite Gravity Inversion

We determine Moho depth and crustal thickness for the SW Indian Ocean, using a gravity inversion method which incorporates a lithosphere thermal gravity anomaly correction, in order to investigate the structure and origin of the Crozet Plateau and the Conrad Rise. Data used in the gravity inversion are bathymetry, free-air gravity anomaly and sediment thickness from Smith and Sandwell (2008), Sandwell and Smith (2008) and Laske and Masters (1997) respectively. The gravity inversion method, which is carried out in the 3D spectral domain and predicts Moho depth, incorporates a lithosphere thermal gravity anomaly correction. Lithosphere thermal model re-equilibration (cooling) times, used to calculate the lithosphere thermal gravity anomaly correction, are conditioned by ocean isochron information (Mueller et al. 2008), and continental rifting and breakup ages. The continental lithosphere thinning distribution, used to define the initial thermal model temperature perturbation are derived from the gravity inversion and use no a priori isochron information; as a consequence the gravity inversion method provides a prediction of OCT location which is independent of ocean isochron information. Crustal thickness under the Crozet Plateau, Conrad Rise and Madagascar Ridge are predicted to exceed 25 km. The Crozet Plateau and Conrad Rise are separated by an abandoned sea-floor spreading centre active between 83 and 55 Ma. The Crozet Plateau and the Madagascar Ridge are separated by the currently active South West Indian ocean Ridge (SWIR) which formed at ~50 Ma. Superposition of illuminated satellite gravity data onto crustal thickness maps from gravity inversion provides improved determination of pre-breakup conjugacy and sea-floor spreading trajectory within the SW Indian Ocean. Plate reconstructions of present day crustal thickness shows that at ~ 83 Ma, the Crozet Plateau, Conrad Rise and Madagascar Ridge formed a single entity which was subsequently fragmented and separated by rifting and sea-floor spreading. Comparison of Moho depths determined from gravity inversion and seismic refraction suggests that crustal basement densities for Crozet Plateau, Conrad Rise and Madagascar Ridge may be significantly greater than those for adjacent unequivocal oceanic crust within the Indian Ocean. Average crustal basement densities under Crozet Plateau, Conrad Rise and Madagascar Ridge may be as large as 3000 kg/m3 or more, and suggest that they are underlain by anomalously thick oceanic crust rather than continental fragments, a conclusion consistent with their seismic velocity structure. We propose that the Crozet Plateau, Conrad Rise and Madagascar Ridge, together with the southern portion of the Mascarene and Chagos Plateaus, were part of a single large oceanic plateau prior to 83 Ma, formed as an oceanic large igneous province during the early evolution of the Indian Ocean.