Isotopic and palaeomagnetic evidence for rates of cooling, uplift and erosion

Abstract

SummaryThe use of isotopic and magnetic blocking temperatures to infer cooling history is reviewed in the light of kinetic models of the blocking process. Published isotopic estimates of cooling and uplift (erosion) rates are based upon (a) vertical gradients of fission track ages, (b) age differences amongst isotopic-mineral systems with different blocking temperatures and (c) kinetic analysis of 39Ar-40 Ar step-heating experiments. The palaeomagnetic approach to cooling history depends upon thermal demagnetization experiments upon individual samples, from which different blocking temperatures can be assigned to different directions of magnetization having known age relationships. Successive magnetic reversals might in principle be used but only in areas of rapid postorogenic cooling. Where cooling is very slow it may be possible to use apparent polar wander paths to infer cooling rates, as in the mid-Proterozoic of West Greenland. Conversely, in the Grenville Province distinctive components of magnetization have been dated by reference to a cooling history based upon argon-39. In Scotland a study of imposed magnetizations in thermal aureoles of Tertiary dykes provided estimates of ambient temperature at the times at which they were intruded. Cooling (erosion) rates so derived vary bimodally from ~ 30°C (~ 1 km) per million years over periods of ~ 2 x 107 years immediately following orogeny, to ~ 1°C (~ 30 m) per million years in cratonic areas over periods of ~ 2 x 108 years, and broadly agree with sedimentological estimates.