The application of Curie depth, effective elastic thickness, seismic tomography and seismicity in regional exploration area selection

Abstract

When selecting geological provinces prospective for giant ore deposits knowledge of the location of deep lithospheric structures and craton margins, and an understanding of the temperature distribution of the Earth, are all important. Curie depth, effective elastic thickness, seismic tomography and seismicity are techniques with the potential to provide this information. Curie depth depends on the geothermal gradient and may, in principle, be derived from aeromagnetic data. However, it is concluded here that such Curie depth maps may have little value in many shield areas. Effective elastic thickness (TE) is a measure of the strength of the lithosphere. Methods for estimating TE are described and, as an example, a new TE map of Brazil is presented. TE gradients correlate well with province boundaries and seismicity, implying that they map major lithospheric structures. Seismic tomography uses the travel times of earthquake P- and S-waves to map velocity variations within the Earth. Many new tomographic velocity models are now becoming available, with resolution <100km in some cases. It appears that temperature is the major control on velocity, at least in the upper mantle. Global seismicity information may be analysed in terms of the distribution of earthquakes, their magnitudes and their focal depths. A correlation between seismicity and large mineral deposits has been observed in certain regions, which motivates the use of this data. Possible reasons for the correlation are discussed here. Since the different maps - TE, seismic velocity and seismicity - respond to different physical properties (or the same physical properties in different ways), it is most beneficial to interpret all three together.