Comment [on “Brittle failure in the upper mantle during extension of continental lithosphere” by Dale S. Sawyer]

In an earlier paper I concluded that for slow enough strain rates (< 10- s-), cooling during extension, and the consequent increase in the strength of the lithosphere, could provide a limit to extension. In contrast, Sawyer [1985] contends that when brittle deformation of the lithosphere is considered, the correct conclusion should be that the rheological properties of the lithosphere cannot limit its extension under any circumstances. The purpose of this comment is to point out that Sawyer's conclusion is based on the results of calculations using a single, probably inappropriately cool, continental geotherm and that it is not justifiable to generalize these results to all reasonable continental geotherms.