On a Type Classification of Lower Crustal Layers under Precambrian Regions
Alan G. Jones
Various parameters pertinent to the lower crustal layer
under Precambrian regions are listed for locations where seismic,
and geomagnetic or geoelectric, studies have been undertaken.
The parameters define three distinct types of lower crustal layer
with certain dominant characteristics: Type I - "Normal" - typical
continental seismic parameters and a high electrical resistivity
(10^3 - 10^4 ohm.m); Type II - "Intermediate" - high compressional
wave velocity (either fixed Vp=7.0 km/s or transitional Vp=
6.7 - 7.3 km/s) and a moderate resistivity (100-300 ohm.m); Type
III - "Low" - a low shear wave velocity layer (LVsL), high
Poisson's ratio (>0.30) and low electrical resistivity (10-50 ohm.m).
Possible conditions and rock types, existing at the P-T environment
of the lower crust and which could account for the observations,
are suggested. The zoning of Canada into types implies
that Type II layers are shield 'edge" effects, and that inability to
observe what is regarded as the final stage of development of
a shield region under certain shields may be due to their being
Journal of Geophysics (Zeitschrift fuer Geophysik), 49, 226-233.
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Alan G Jones / 28 February 2010 /