Denudation Rates, Thermal Evolution, and Preservation of Blueschist Terrains

The erosion of low temperature, high pressure blueschist facies metamorphic rocks presents a number of problems. If their rate of erosion is not sufficiently rapid, then the rise in temperature caused by thermal relaxation and radioactive self heating is great enough to obliterate the original low temperature mineral assemblages. The problem has been investigated using a one dimensional, two layer model to mimic an accretionary prism overlying oceanic lithosphere. The results suggest that, if no erosion takes place, the maximum time that blueschist conditions may exist is about 26 m.y. if levels of radioactive self heating are high, and approximately 100 m.y. if levels are very low. If erosion takes place, the maximum temperature reached by an element of rock is at a higher level in the earth's crust than its initial depth of burial. Rocks may re-equilibrate at this temperature or be overprinted by incomplete reactions depending on the temperature rise and rocks involved. This could result in the geothermal profile of metamorphic rocks being concave to the temperature axis, in contrast to the convex shape of the original profile. The minimum erosion rate which would produce no overprint on original assemblages is estimated by us to be about 0.04 cm/year when radioactive self heating is negligible and rises to 0.14 cm/year for higher levels of self heating. Variations of erosion rates during unloading result in different thermal histories. An initial rapid rate of erosion which then slows is less likely to result in an overprint than the converse. Examination of estimates of erosion rates for the present suggests that normal fluvio-glacial erosion rates may be inadequate to successfully exhume blueschist assemblages without overprinting. Unloading may thus be tectonic or quasi-tectonic. This may explain the common association of blueschist blocks within olistostrome and melange like bodies.