Characterization of the Evolution of a Foreland Basin Active Margin from Sedimentary Records and AFT Analysis on Detrital Apatites : Example of the NE Tian Shan Foothills
Résumé
The present study aims to assess how detrital sediments deposited in mountain foothills record the exhumation and erosion of a range. The high topography of the Tian Shan range is mostly due to the Tertiary reactivation of the chain. The northeastern Tian Shan foothills present unique conditions of preservation and exposure of the foreland synorogenic sedimentary series. The latter are continuously exposed in two sections, which are entirely constrained by magnetostratigraphical studies. We analysed these series (facies, depositional environments, paleocurrent directions and petrological compositions) over two several kilometres long sections. The sedimentary environments evolve through time and space between anoxic lakes and braided alluvial fans with paleocurrent directions shifting from E-W to N-S. This evolution is diachronous from E to W over the two sections. AFT analyses performed on the Tertiary sections provide indication on the amount of erosion in the range, the evolution of the sediment sources and the recycling of sediments. Five AFT age groups are defined: > 200 Ma, 200-150 Ma, 150-50 Ma, 50-40 Ma and < 40 Ma corresponding to major events of the range history (Mesozoic tectonics between 200 and 150 Ma, low tectonic activity and peneplanation between 150 and 50 Ma, onset of the India-Asia collision between 50 and 40 Ma, ongoing deformation phase since 40 Ma). AFT ages belonging to the final (<40 Ma) age class appear in the sediments between 6.7 and 5.4 Ma immediately prior the major environmental change between the Dushanze alluvial plain and the Xiyu alluvial fan. This correlation is interpreted in terms of proximity of the source area (avoiding mixing between the newly eroded "young" apatites and older, recycled crystals), maturity of the relief (exhumation in the range is sufficient to expose completely reset sources) and/or increase of the erosion rate in the source area. However, nearly no age corresponding to the supposed age of the topography (i.e. 10 Ma) are found indicating that exhumation in the range is still not sufficient to expose large portions of completely reset sources. The constant occurrence of a large proportion of AFT ages within the 50-150 Ma and older classes demonstrate that while some old sources are still preserved in the range, sediment recycling inside the foothills series must be strong. This should be taken into account when translating sedimentation rates into erosion rates at the source.