Magnetostratigraphy of the Northern Tian Shan foreland, Taxi He section, China

Abstract

TheTian Shan range formed in the late Cenozoic in response to the northward propagation of deformation related to the India^Eurasia continental collision. Precise timing of theTian Shan uplift is required to understand possible mechanisms of continental lithosphere deformation and interactions between climate, tectonism and erosion.Here, we provide magnetostratigraphic age control on the northern ChineseTian Shan foreland successions. A thorough rock magnetic analysis identifies haematite- and magnetite-bearing alluvial fan deposits in the upper portion of the sampled strata as more reliable palaeomagnetic recorders than magnetite-bearing £uvial and lacustrine deposits that are often maghaemitized in the lower part of the record. As a result, a robust correlation to the geomagnetic polarity time scale is obtained from6 to 2Ma while a tentative correlation is proposed from6 to 16Ma. Sediment accumulation rates increase from155 to 260mMyr-1 at 3.9 ± 0.3Ma.This change coincides with a gradual lithologic transition from £uvial (sandstonedominated) to alluvial fan (conglomerate-dominated) deposits that likely records an approaching erosional source related to tectonically increased subsidence rather than diierential compaction. Clear evidence for growth strata starting at an estimated age of ~2Ma provides a minimum age for folding.These results are compared with previous magneotstratigraphic studies from the same and other sections of the northernTian Shan foreland basin fill, thus enabling a critical assessment of the reliability of magnetostratigraphic dating and the significance of sediment accumulation rate variations with respect to facies variations and growth strata. Our results in theTaxiHe section provide a sequence of events that is consistentwith enhanced tectonic forcing starting at ~4Ma, although a climatic contribution must be considered given the close relationship of these ages with the Pliocene climate deterioration.