Details

• Dr. Alan G. Jones, Dublin Institute for Advanced Studies, Dublin, Irlanda
• Date: Jun, 18, 2015 12:00 am
• Place: Sala d’Actes del Institut de Ciències de la Terra Jaume Almera(ICTJA)
• Location: C/ Solé i Sabarís s/n, Barcelona
• Contact: Pilar Queralt (UB)

Abstract

The Earth is a single thermodynamic system that we sense through geophysical and geochemical means yielding disparate data that all too often we interpret one subset of in complete ignorance, even disregard, of the others resulting in a plethora of models. These models are sometimes complementary to one another, and other times are in conflict. Yet there is only one Earth! Deep-probing electromagnetic studies using magnetotellurics (MT) can add considerably to our knowledge of the lithosphere and sub-lithospheric mantle due to their sensitivity to vertical and lateral variations in electrical conductivity. Conductivity of bulk silicate rocks is exquisitely sensitive to thermal variations and to water content. It is greatly enhanced through the presence of an interconnected conducting phase, such as metasediments or fluids (saline or partial melts). Combined with other parameters, knowledge of conductivity significantly constrains acceptable model space.I will focus mainly on Southern Africa as an excellent natural laboratory for combining MT data with seismic (Rayleigh) data as well as ensuring consistency with the geoid and surface heat flow (SHF). In particular, I will look at the African Superswell. The deep mantle African Superswell is thought to cause up to 500 m of the uplift of the Southern African Plateau. I investigate this phenomenon through stochastic thermo-chemical inversion modelling of the geoid, surface heat flow, Rayleigh dispersion curves and MT data, in a manner that is fully petrologically-consistent. I show that a single layer lithosphere can fit most of the data, but not the MT responses. I further show that modelling the seismic data alone, without the constraint of requiring reasonable oxide chemistry or of fitting the geoid, permits wildly acceptable elevations and with very poorly-defined lithosphere-asthenosphere boundary (LAB). I parameterise the lithosphere into three layers, and bound the permitted oxide chemistry of each layer consistent with known chemical layering. I find acceptable models, from 5 million tested, that fit all responses and yield a posterior elevation PDF distributions centred on 800 m, suggesting dynamic support from the lower mantle of some 500 m. I will show application of the method to other areas, such as the Rae Craton west of Hudson Bay in Canada.