On the generation of pre-trap hot spot tracks: history of La Réunion hot spot
Abstract
Recent observations made on La Réunion hot spot (Mahoney et al. EPSL 2002) documented the existence of a Réunion hotspot track before the Deccan traps. Based on the location, ages, and geochemical signatures of marine volcanic rocks preserved in the South Tethian suture zone of Pakistan, they suggested that the Réunion hotspot was active off northwestern Greater India well before the emplacement, far to the South, of the Deccan flood basalts. However, in the classical "Plume's head and tail" framework, the Deccan traps, emplaced 65-66 Myr ago, would represent the signature of the plume head reaching the lithosphere. A pre-trap track, i.e. a pre-head event, remains therefore to be explained. We show here that it could be generated by the filtering effect of the 660-km depth endothermic phase transition, which is expected to delay upwelling plumes and modify their morphology. We investigated experimentally the interaction of thermal starting plumes with a density boundary by using simultaneous visualizations of temperature, composition and velocity fields. In a tank initially stratified with two viscous fluids with different densities and viscosities, a thermal starting plume was generated by using a circular plate heater at the bottom of the tank. Its interaction mode with the inner interface depends on the local buoyancy number (BL: the ratio of the stabilizing chemical buoyancy to the plume thermal buoyancy at the interface), the Rayleigh number (Ra), the viscosity contrast between the chemical layers (γ), and the local viscosity ratio of the ambient material to the plume head (γp). For BL < 0.6, the "Pass-through mode" develops, whereby a large volume of the lower material rises through the upper layer and reaches the top surface, since the plume head has a large thermal buoyancy compared to the stabilizing density contrast between the two layers. When BL > 0.6, the "Rebirth mode" occurs, where the thermal plume ponds and spreads under the chemical boundary and secondary thermal plumes are generated from the interface. Depending on the magnitude of BL, these plumes can entrain upwards by viscous coupling a significant amount of lower layer material. Scaling laws are determined for the onset of secondary plumes, their number and wavelength, the amount of entrainment and the topography of the interface. Based on our experimental results, we suggest that a large-hot thermochemical plume was first delayed under the 660-km endothermic phase transition, such that a secondary plume was generated in the upper mantle, causing the pre-Deccan traps activities. According to our scaling laws, the onset time of the secondary plume is about 10 Myr. That means the hot-large plume was delayed at least 45 Myr under the 660-km phase transition.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMDI53A2369K
- Keywords:
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- 8121 TECTONOPHYSICS / Dynamics: convection currents;
- and mantle plumes;
- 8137 TECTONOPHYSICS / Hotspots;
- large igneous provinces;
- and flood basalt volcanism