Abstract

H. Austrheim & A. K. Engrik writes: Recent attempts to model continent collision zones have focused attention on processes in, and the petrophysical properties of, crustal root zones. In particular the density changes during the formation of eclogites and their back reaction to amphibolites and granulites have been used to explain various aspects of collision orogens such as seismic velocity, crustal thickness, orogenic uplift and exposure of high P-rocks (Dewey et al. 1993). We are happy to see that Ryan & Dewey (1997) now also introduce the rheological properties of an eclogitized crust into their models and point to the importance this may have for the opening and closing of oceans (the Wilson cycle). Ryan & Dewey (1997) use observations from the Bergen Arcs of Western Norway as evidence of low shear-strength in an eclogitized crust. We agree that the Bergen example shows that eclogites, at some stage in their evolution, had much lower strength than their granulite facies protolith (see discussions by Austrheim 1991,, 1994). A similar situation can also be observed in the Sunnfjord area, Western Gneiss Region (Norway), where granulites of mafic and leucocratic composition form lenses in an eclogite-facies melange-like rock (Austrheim et al. 1997). The geodynamic significance of the weakening must, however, depend on its cause and the duration of the weakening. The possible reasons for this weakening were discussed by Klaper (1990) and Austrheim (1991).While Ryan & Dewey (1997) model weakening of orogenic lithosphere as a function of heat production

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