Dynamic interaction of cold anomalies with the mid-ocean ridge flow field and its implications for the Australian–Antarctic Discordance

Abstract

Negative thermal anomalies beneath a mid-ocean ridge are dynamically isolated from the ambient upwelling and diverging flow field in the asthenosphere whose viscosity is on the order of 5×1019 Pa s or less. This study examines on what condition a near-ridge cold anomaly ascends with the upwelling passive flow and spread off-axis. Three-dimensional numerical modeling demonstrates that, for a given magnitude of the cold anomaly, the viscosity of the asthenosphere, the spreading rate and the interference from continental rifting are the dominant controlling factors to the ascent/descent of the anomaly. To overcome the weight of such an anomaly and couple it with the upwelling, either the spreading rate or the asthenospheric viscosity has to be high. In a low viscosity asthenosphere, the cold anomaly also ascends during the early stage of continental rifting due to the enhanced upwelling induced by the thick continental lithosphere. The dynamic interaction between the cold anomaly and the ambient flow renders a transient nature of the subsidence of the seafloor, which may lead to exaggerated temperature variation estimated by using a conduction model alone. The scenarios examined are employed to place a constraint on dynamic models recently proposed for the Australian–Antarctic Discordance, in which the source of the negative anomaly is hypothesized to be deeply rooted in the upper mantle. With the asthenospheric viscosity less than 1020 Pa s, the upwelling of the cooler material from great depths, which causes a significant topographic low at the Discordance, is made possible only by rifting of the Australian continent off the Gondwanaland.