THE ELECTRIC MOHO

Alan G. Jones(1), Ian J. Ferguson(2)
(1) Geological Survey of Canada
    615 Booth St., Ottawa, Ontario, K1A 0E9, Canada
(2) Department of Geological Sciences
    University of Manitoba
    Winnipeg Manitoba, R3T 2N2, Canada
ajones@cg.nrcan.gc.ca 

Since Mohovicic discovered a dramatic increase in compressional 
seismic velocity from 5.68 km/s to 7.75 km/s at a depth of 54 km 
beneath the Kulpa Valley in Croatia, the "Moho" has become arguably 
the most important seismic horizon in the Earth in its role as 
defining the crust-mantle boundary.  It is now known to be a 
ubiquitous feature of the Earth, and is usually assumed to separate 
lower crustal mafic rocks from upper mantle ultramafic rocks. 
Electromagnetic experiments conducted to date have failed to 
demonstrate a convincing change in electrical conductivity at the base 
of the crust. Here we report on the interpretation of magnetotelluric 
data from the southwestern edge of the Slave craton which show an 
unequivocal change at the Moho, the seismically-defined base of the 
crust. This change is a conductivity increase with depth, contrary to 
expectations, and requires a conducting phase in the upper mantle 
beneath the Slave craton.