On a type classification of lower crustal layers under Precambrian regions

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 Omegam); Type II - 'Intermediate' - high compressional wave velocity (either fixed V _p=7.0km s ^-1 or transitional V _p= 6.7??7.3km s ^-1) and a moderate resistivity (100-300Omegam); Type III - 'Low' - a low shear wave velocity layer (Lv _sL), high Poisson's ratio (>0.30) and low electrical resistivity (10-50Omegam). 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 too small. -Author