Publication 88
The electrical structure of the Slave craton
Alan G. Jones, Pamela Lezaeta, Ian J. Ferguson, Alan D. Chave, Rob. L. Evans, Xavier Garcia, and Jessica Spratt
Abstract
The Slave craton in northwestern Canada, a relatively small Archean craton (600 km x 400 km), is ideal as a
natural laboratory for investigating the formation and evolution of Mesoarchean and Neoarchean sub-continental
lithospheric mantle (SCLM). Excellent outcrop and the discovery of economic diamondiferous kimberlite pipes in
the centre of the craton during the early 1990s have led to an unparalleled amount of geoscientific information
becoming available.
Over the last five years deep-probing electromagnetic surveys were conducted on the Slave, using the natural-source
magnetotelluric (MT) technique, as part of a variety of programs to study the craton and determine its regional-scale
electrical structure. Two of the four types of surveys involved novel MT data acquisition; one through frozen lakes
along ice roads during winter, and the second using ocean-bottom MT instrumentation deployed from float planes.
The primary initial objective of the MT surveys was to determine the geometry of the topography of the
lithosphere-asthenosphere boundary (LAB) across the Slave craton. However, the MT responses revealed, completely
serendipitously, a remarkable anomaly in electrical conductivity in the SCLM of the central Slave craton.
This Central Slave Mantle Conductor (CSMC) anomaly is modelled as a localized region of low resistivity (10-15 ohm.m)
beginning at depths of ~80-120 km and striking NE-SW. Where precisely located, it is spatially coincident with the
Eocene-aged kimberlite field in the central part of the craton (the so-called “Corridor of Hope”), and also with a
geochemically-defined ultra-depleted harzburgitic layer interpreted as oceanic or arc-related lithosphere emplaced
during early tectonism. The CSMC lies wholly within the NE-SW striking central zone defined by Grütter and
co-authors (1999) on the basis of garnet geochemistry (G10 vs. G9) populations.
Deep-probing MT data from the lake bottom instruments infer that the conductor has a total depth-integrated conductivity
(conductance) of the order of 2000 Siemens, which, given an internal resistivity of 10-15 W.m, implies a thickness of
20-30 km. Below the CSMC the electrical resistivity of the lithosphere increases by a factor of 3-5 to values of around
50 ohm.m. This change occurs at depths consistent with the graphite-diamond transition, which is taken as consistent
with a carbon interpretation for the CSMC.
Preliminary three-dimensional MT modelling supports the NE-SW striking geometry for the conductor, and also suggests a
NW dip. This geometry is taken as implying that the tectonic processes that emplaced this geophysical-geochemical
body are likely related to the subduction of a craton of unknown provenance from the SE (present day coordinates)
during 2630-2620 Ma. It suggests that the lithospheric stacking model of Helmstaedt and Schultze (1989) is likely correct
for the formation of the Slave’s current SCLM.
Source
Lithos, 71, 505-527, 2003.
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