Testing a Pseudotachylyte Geothermometer

University Of Kentucky Research Foundation, Lexington KY

Investigators

Abstract

0003482 O'Hara A simple new geothermometer for frictionally melted rocks (so called pseudotachylytes) that allows estimate of ambient country rock temperature during melting is being tested on a fossil intracontinental pseudotachylyte-bearing fault zone the (Homestake shear zone, Colorado) and ultradeep pseudotachylytes from western Norway. Temperature (and hence depth) estimates on frictional melting along fault zones give a three dimensional picture of the rupture geometry. Frictional melting appears to be one of the few indicators of ancient seismic activity in the crust and the depth of frictional melting is thought to correspond to the seismic regime. The recognition that some pseudotachylytes are overprinted by ductile fabrics, together with the discovery of ultra-deep pseudotachylytes, suggests that frictional melting is not confined solely to the brittle upper crust. Information on the depth distribution of frictional melting places important constraints on the rheological behavior of the crust and its potential to behave seismically. Frictional melting during faulting can be regarded as an adiabatic heat pump in which the fault does work at the ambient country rock temperature. Some of this work is converted to heat producing a melt at a higher temperature. The efficiency of the conversion process depends solely on temperature difference between the heat reservoirs. If an estimate of efficiency can be made and the temperature of one of the heat reservoirs is assumed, the temperature of the other reservoir can be calculated. It is shown here that the ratio of frictional melt volume to mechanical wear volume in natural pseudotachylytes is a proxy for the ratio of thermal to mechanical energy during faulting, and that this ratio is independent of fault area, displacement, stress and mineralogy. Thermodynamic arguments indicate it depends solely on temperature. Preliminary data from four different pseudotachylyte localities, which are not overprinted by ductile fabrics, yield reasonable ambient temperature estimates of 132-484oC, corresponding to the upper and middle crust. The geothermometer yields a country rock temperature of 627- 827oC for eclogite-facies pseudotachylytes from western Norway, consistent with an independent estimate of 670oC. The geothermometer appears to be applicable throughout the entire crust.