Geophys. zyxwvutsr J. R. astr. zyxwvu SOC. zyxwvuts (1979) 56,239-240 zyxwv Letter to the editors Authors’ reply to ‘A comment on ‘Frictional heating on a fault zone with finite thickness’ zyx ’ R. K. Cardwell, D. S. Chinn, G. F. Moore and D. L. Turcotte Department of Geological Sciences. CorneU University, Zthaca, New York 14853, zyxwvu USA Received 1978 June 12 The principal points made by Sibson (1978) is that the production of pseudotachylytes and mylonites do not occur synchronously. He argues that mylonites are produced at high pressures and temperatures by plastic flow whereas pseudotachylytes are produced by frictional heating during shallow brittle fracture. We have concluded that the present geological evidence on this point is ambiguous. Our interpretation that pseudotachylyte was generated under high-temperature conditions was based partly on the detailed field and laboratory study of Philpotts (1964). He states on p. 1030, ‘the rocks from which the pseudotachylytes were formed must have been at a temperature of at least 400°C. and therefore the amount of frictional heating necessary to cause melting must have been quite small’. Similarly, Park (1961) in his discussion of pseudotachylytes in Scotland states. The significant common factor [of all known occurrences of pseudotachylytes] is great depth at the time of formation’. Until it is demonstrated that these authors misinterpreted the field relations, generation of pseudotachylyte at high temperatures cannot be ruled out. Sibson argues that pseudotachylyte forms by melting under dry conditions within the top 10 km of the crust. Because of his assumption of faulting under dry conditions, he is forced to propose melting temperatures of 950-1 100°C. However, other workers (e.g. Ermanovics, Helmstead & Plant 1972), believe that melting has occurred with the presenceof water, and Wallace (1976) proposes that fusion along a fault zone in New Zealand should have occurred at about 750°C at shallow depths (2-7 km). This temperature of fusion is significantly lower than that proposed by Sibson. If we use Wallace’s number as the minimum tempera- ture required for melting then, even using Sibson’s figure of 250°C for ambient temperatures, melting may occur during temperature increases of 500°C. Brittle faulting under dry conditions presents a number of significant problems. The coefficient of dry friction is well known and leads to high stresses on faults at relatively shallow depths. Stick-slip behaviour requires strain softening or a more complex rheology . Griggs & Baker (1969) have shown ductile flow is consistent with stick-slip faulting. Mylonites may in fact be associated with this type of ductile faulting and the pseudo- tachylytes may be the result of the unsteady heating during the faulting process. References Ermanovics, I. F., Helmstead, H. & Plant, A., 1972. An occurrence of archaen pseudotachylyte from southeastern Manitoba, Can. J. Earth Sci., 9, 257-265. Downloaded from https://academic.oup.com/gji/article/56/1/239/822049 by guest on 27 March 2022