CONDUCTIVITY DISCONTINUITIES IN THE UPPER-MANTLE BENEATH A STABLE CRATON

We present evidence for approximate collocation of seismic and electri cal transitions in the upper mantle. More than two years of very long period magnetotelluric (MT) data were recorded at a lakebottom observa tory in the central Canadian Shield. After processing to contend with non-stationary source effects, and removal of galvanic distortion, the underlying structure is ID for periods of one hour to four days. The response was extended to periods of 100 days by appending Geomagnetic Depth Sounding data to the MT curves. Minimum structure linearised inv ersion, nonlinear extremal inversion, and a new genetic algorithm for nonlinear hypothesis testing, reveal discrete jumps in conductivity at depths near the major upper mantle seismic discontinuities. The jumps occur over limited depth ranges.