TRACKING THE EVOLUTION OF CONVERGENT MOUNTAIN BELTS WITH DETRITAL GEO-THERMOCHRONOLOGY

Age dating of detrital grains from sedimentary basins adjacent to convergent mountain belts has become one of the primary tools in recent years for studying orogenic evolution. Cooling ages of detrital grains, determined with Ar/Ar, fission-track and (U-Th)/He analysis, provide a wealth of information that not only allows calculating exhumation rates of bedrock in the past, but may also permit reconstructing the thermal history of the sedimentary basin itself. Some of the advantages of the detrital approach lie in the fact that it can be easily combined with other sediment petrological or geochemical techniques to better constrain sediment provenance. Therefore, a general trend in the past few years has been the combination of different techniques on the same samples or on the same grains. For example, recent studies of detrital zircon in the Siwaliks Formation in western and central Nepal successfully applied fission-track and U/Pb double-dating on single detrital zircons to reconstruct sediment provenance and the cooling history of the sediment source areas in the Higher and Lesser Himalayas, while apatite fission-track ages of the same samples were used to gain insights into the thermal history of the foreland basin itself. A new study in the European Alps uses the fission-track and U/Pb double-dating approach to distinguish between grains that were derived from Oligocene volcanic sources, from those grains that were derived from exhumation of deep-seated metamorphic rocks, because they both may have similar ~30 Ma cooling ages. In addition, it is possible to take this approach one step further by adding (U-Th)/He analysis to fission-track and U/Pb dating on single grains. This triple-dating technique of detrital zircons may be useful in certain cases when it is necessary to constrain sediment provenance as well as the cooling path (e.g. fast versus slow cooling) of the same grains, while they were still part of the upper crust.