Relation between alternations of uplift and subsidence revealed by L ate C enozoic fluvial sequences and physical properties of the continental crust

Reversals in vertical crustal motion, alternations between uplift and subsidence over time scales of hundreds of thousands of years or more, have been identified in L ate C enozoic fluvial sequences in many regions worldwide. They form a class of fluvial archive that is distinct from the extreme stability observed in A rchaean cratons and the monotonic uplift or subsidence that is widely observed in other regions. Such alternations between uplift and subsidence are characteristic of regions of E arly or M iddle P roterozoic crust, where the initial crustal consolidation included the development of a thick ‘root’ of mafic material at the base of the crust; the present study focuses on localities with this crustal type in the USA and eastern E urope. It has previously been suggested on the basis of uplift modelling that this style of crustal behaviour occurs only in regions where the mobile lower‐crustal layer is relatively thin. This study supports this conclusion on the basis of independent geothermal calculations, which indicate that such alternations between uplift and subsidence occur where the mobile lower‐crustal layer is ≤∼7 km thick. An understanding of this phenomenon, in relation to the understanding of vertical crustal motions induced by surface processes (and thus by climate change) in general, therefore requires analysis of the properties and dynamics of the mobile lower‐crustal layer; detailed analysis of fluvial sequences thus contributes unique information in this area.