Lithospheric thickness

These arguments can be refined. In Appendix B, we calculate the mantle heat flow as a function of lithosphere thickness and show how it depends on the surface heat flow. Pressure and temperature estimates from mantle xenoliths may be combined to determine a best-fit geotherm consistent with heat transport by conduction. Mantle heat flow estimates obtained in this manner are in the following ranges 7-15 mW beneath the Fennoscandian Shield (Kukkonen and Peltonen, 1999), 17-25 mwm for the Kalahari craton. South Africa (Rudnick and Nyblade, 1999), and... 

In stable continents, the cmstal contribution is obtained by removing the mantle heat flow from the surface heat flow. The mantle heat flow is constrained by the heat flow and heat production data and by the required consistency of thermal data with lithospheric thickness from seismology and xenolith studies. Results from heat flow studies yield an average heat production of 0.77 0.08 pW m for the Precambrian cmst and 1.03 0.08 pW m for the Phanerozoic. The global value for each age group lumps together different types of cmstal structures. 

APPENDIX B MANTLE HEAT FLOW, MOHO TEMPERATURE AND LITHOSPHERE THICKNESS... 

This shows that, for constant lithospheric thickness and basal temperamre, an increase in surface heat flow can come only from crustal heat... 

Gupta M. L., Sundar A., and Sharma S. R. (1991) Heat flow and heat generation in the Archean Dharwar cratons and implications for the southern Indian Shield geotherm and lithospheric thickness. Tectonophysics 194, 107—122. 

Jones M. Q. W. (1988) Heat flow in the Witwatersrand Basin and environs and its significance for the South African shield geotherm and lithosphere thickness. J. Geophys. Res. 93, 3243-3260. 

Figure 3 Schematic diagram showing how original lithospheric thickness and mantle potential temperature affect the amount of melt produced (melt thickness) and how these factors relate to continental flood basalts (CFB), volcanic rifted margins (VRM), off-ridge and ridge-centered oceanic plateaus (OP), and midocean ridges (MOR).

Seismic studies of cratonal lithosphere at both the global and local scale indicate that Archaean cratons are sites of unusually thick, cold lithosphere, which may have mantle roots extending to depths of200-400 km compared with normal continental lithosphere, which is less than 150 km thick (e.g. van der Lee Nolet 1997 Ritsema et al. 1998 Ritsema van Heijst 2000 James Fouch 2002). Heat-flow data place some bounds on lithospheric thickness, but regional comparisons require major assumptions as to the lower-crustal contribution to radioactive heating. 

Larger cratons will be more susceptible to rifting as a result of basal drag at the LAB. Variations in lithospheric thickness within a large craton may lead to ponding of plume material, generating extensional body forces within the cratonic interior. 

Between the oceanic lithosphere and asthenosphere is a TBL, about 80 km thick, in which there is small-scale convection (Fig. 3.10). This small-scale convection removes material from the base of the conductively cooling layer and replaces it with hotter material and so maintains a constant lithospheric thickness (White, 1988). 

A related debate focused on heat flow data from different regions of the continental crust. Nyblade and Pollack (1993) showed that average heat flow measurements in Archaean cratons are lower than those for Proterozoic cratons. This observation has, however, been interpreted in two quite different ways. On the one hand, it has been argued that cratons of different age have different bulk compositions, and so have different concentrations of heat producing elements (U, Th and K), hence different levels of heat production. Alternatively, the observed differences in heat flow do not derive from the crust but reflect different lithospheric thicknesses between Proterozoic and Archaean cratons reflecting different mantle heat flow contributions (Rudnick et al., 1998 Nyblade, 1999). 

Fig. 3 (Upper) Map of the UMISM model of the Weichselian ice sheet at the last glacial maximiun (LGM) 18,400 years BP (Schmidt et al. 2014). Endglacial faults indicated with red lines. (Lower) Modelled glacially induced maximum horizontal stress at LGM (Lund and Schmidt 2011). The model has elastic lithospheric thickness 120 km, upper mantle viscosity 5 10 Pa s and lower mantle viscosity 3 10 ...

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