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UBC Theses and Dissertations

Inversion of magnetotelluric impedances from above young lithosphere Whittall, Kenneth Patrick


Ocean bottom magnetometer data from a site on the Pacific plate above 3 my old lithosphere are inverted for electrical conductivity as a function of depth. Magnetotelluric impedances are calculated by the vertical gradient method using the fields at the OBM in conjuction with those measured at the Victoria Geomagnetic Observatory. The approximations involved are examined. Winnowing criteria are proposed which isolate those impedances compatible with all the model and source field assumptions. These then define the best possible data set. A number of inversion algorithms are applied to the data and a wide range of acceptable conductivity profiles are constructed. All profiles exhibit a uniform, relatively high conductivity of about 0.2 S/m from the surface down to a depth of 100 km. Exact and approximate bounds on the conductance are calculated in an effort to quantify the non-uniqueness of the diverse conductivity models. Profiles with a minimum of structure are used to calculate the partial melting and temperature variations beneath the 3 my old site. All results are compared with three other magnetotelluric analyses above 1, 30 and 72 my old lithosphere. The 3 my old datum is discordant and does not fit the trends interpreted from the other three studies.

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