Regional conductivity structure of the island of Newfoundland

Gary McNeice, Memorial University of Newfoundland, St. John's
              and Geological Survey of Canada, Ottawa (now at Phoenix
              Geophysics, Scarborough)
Alan G. Jones, Geological Survey of Canada, Ottawa
Toivo Korja, Oulu University and Geological Survey of Canada, Ottawa
Jim Craven, Geological Survey of Canada, Ottawa
James A. Wright, Memorial University of Newfoundland, St. John's, and
Robert G. Ellis, University of British Columbia, Vancouver (now at
                 BHP Minerals, Golden, Colorado)

Data from 77 magnetotelluric (MT) soundings across the Appalachian orogen in
Newfoundland, Canada, are analysed to determine the conductivity structure of 
major tectonic boundaries of the orogen and the regional conductivity 
structure. An extended Groom­Bailey tensor decomposition analyses is developed 
and employed to remove galvanic distortion effects produced by the complex near 
surface geology of Newfoundland. In the proposed decomposition analysis a 
global minimum is sought to determine the most appropriate strike direction and 
telluric distortion parameters for a range of frequencies and a set of sites. 
The recovered 2D responses are representative of regional­ scale structures and 
2D inversion algorithms can be employed to recover the regional conductivity 
structure. The regional conductivity structure of the orogen suggests that the 
Baie Verte Line, Red Indian Line and Dover fault ­ Hermitage flexure represent 
significant crustal tectonic boundaries. The conductivity structure changes 
both across and along strike in Newfoundland. The magnetotelluric data do not 
support a near two­dimensional model of three lower crustal blocks but indicate 
a more complex three­dimensional geometry in southern Newfoundland.  The Baie 
Verte Line is imaged as an upper­ middle crustal boundary separating the 
deformed, overthrust passive margin of Laurentia (Humber zone) and resistive 
arc terrain of the Notre Dame subzone. The Red Indian Line marks the boundary 
between the Notre Dame and Exploits subzones of the Dunnage zone and is roughly 
coincident with a change in lower crustal conductivity. The Dover is imaged as 
a near vertical boundary between upper­middle crusts of contrasting electrical 
properties, extending to depth of at least 20 km.  The sharp change in upper­ 
middle crustal conductivity observed in the magnetotelluric data suggest that 
orogen parallel motions were important in the development of many of the major 
boundaries of the orogen. The data support a three part division of the lower 
crust. 

Session 9: Local and regional electromagnetic studies