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Quantification of lithospheric thinning and enhanced melt production

Figure 4.11 Indicating the degree of pressure-release melt, by instantaneous adiabatic decompression of asthenospheric mantle, for different thicknesses of lithospheric thinning, where the thinning is represented by B. (a) is after McKenzie and Bickle (1988) and (b) is after White (1992). The difference in these diagrams derives from using the entropy change at melting of 250 J (kg K-1) in (a) and 400 J (kg K-1) in (b). Note the different values of B required to 'produce' oceanic crust.

Figure 4.11 Indicating the degree of pressure-release melt, by instantaneous adiabatic decompression of asthenospheric mantle, for different thicknesses of lithospheric thinning, where the thinning is represented by B. (a) is after McKenzie and Bickle (1988) and (b) is after White (1992). The difference in these diagrams derives from using the entropy change at melting of 250 J (kg K-1) in (a) and 400 J (kg K-1) in (b). Note the different values of B required to 'produce' oceanic crust.

that the instantaneous stretching/thinning model gives rise to melting, the precise thickness of melt that will be produced at any given temperature for some specified value of B is somewhat contentious. However, what is even more important is the fact that plate tectonic events occur slowly.

In the Great Basin, where B is about 2, the stretching strahvrates are extremely small, so that even the development of B=2 has taken tens of millions of years. To attain B=50, the stretching would need to progress for 25 times longer. Clearly, plate tectonic controlled natural lithospheric spreading or thinning is very far from instantaneous. Hence, the thicknesses of melting for values of B shown in Figure 4.11a and Figure 4.11b, would not be realised as the result of slow stretching and thinning.

McKenzie and Bickle have stated that the B values 'with a factor of 2' are typical of those found in intracontinental sedimentary basins which have not subsequently developed into ocean basins. A factor of

B=5 or 6, they suggest, is roughly the point at which stretched continental crust breaks to fully igneous oceanic crust: and the factor of B=50 (or, possibly B=20, following White) is representative of the conditions that give rise to total continental rifting. If one translates a B=50, this means that it is necessary to envisage an initial 100 km square section of continental lithosphere to be instantaneously converted into a section 5000 km wide by 2 km deep. Such an instantaneous change in geometry of lithosphere by lateral stretching is physically impossible.

From our simple fracture-based model of graben development, we suggest that the value of B is more likely to be 1.14 rather than a value of 2 cited by these authors. If we assume a square section of lithosphere of initial length 100 km, this means we would expect the development of a major graben with a depth of 6 km to develop if the lateral spreading is 7 km, or at most 14 km. If the value of B were 2.0, this would mean that lateral extension has been 100 km. Thus, the lateral extension, which our brittle model requires, is about an order of magnitude smaller than that required for the McKenzie and Bickle or White model.

If, as we contend, the McKenzie B model leads to about one order of magnitude exaggeration as regards the development of major graben basins in continental lithosphere, it is likely that a value of B=5 (i.e. 500 km of extension), which these authors required to cause the opening of oceans, is also heir to a comparable degree of exaggeration.

These large values of B are important to the cited authors, for only with such magnitudes of (instantaneous) lateral stretching is it possible, by their mechanism, to generate the volume of magma necessary to give rise to LIPs with a volume of basalt in the range of 106-7 km3.

It is emphasised that the concepts proposed by these authors, namely that of instantaneous thinning of continental lithosphere cannot be explained by any conventional plate tectonics model. Consequently, they cannot justify the cited values of B. These values have been specified because they are what are required to generate the observed, or estimated, volume of melt. It is apposite, therefore, to consider how continental extension and lithospheric thinning may develop.

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