ON THE USE OF LOW-TEMPERATURE THERMOCHRONOLOGY TO STUDY ANCIENT EXHUMATION PROCESSES

Low-temperature thermochronology methods such as fission-track analysis and U-Th/He dating are commonly used to study exhumation of upper crustal rocks in active mountain belts. Rock exhumation in the upper crust occurs due to structural and/or erosional denudation, which are caused by tectonic processes and climate driven surface processes and the interaction of both.

In contrast, studies of rock exhumation in ancient orogens mostly employ higher temperature dating techniques (e.g. Ar/Ar, U/Pb) resulting in exhumation models that explain the motion of rocks from deep structural levels into the middle crust. The use of low-temperature systems to study the processes that brought material to the upper crust and surface during orogeny is challenged due to the post-orogenic geologic history of the region. Due to that geological history, the bedrock cooling ages of ancient orogens mostly record the post-orogenic cooling history. The same is the case for the cooling-ages of synorogenic sediments, which mostly record the thermal history of the basin that experienced heating due to further subsidence and burial followed by uplift and basin inversion. However, in some cases the orogenic cooling history may be preserved over hundred of million years and can be studies using low-temperature thermochronology. In these cases, we are able to derive first order interpretations about exhumation processes that can be used to speculate about paleo-climate, paleo-geography and ancient surface processes.

Several cases are presented where low-temperature thermochronology can be used to obtain information about upper crustal exhumation processes in ancient orogens. These results are compared with studies in active mountain belts and the limitations of the methods are discussed and possible sampling and analytical strategies are suggested.