Anisotropy of magnetic susceptibility of the Jingou River section: Implications for late Cenozoic uplift of the Tian Shan

Better constraints on the uplift history of Tian Shan will increase our understanding of both mountain building processes and deformation patterns related to the India-Asian collision. Anisotropy of magnetic susceptibility (AMS) holds great potential to track variations in tectonic strain, which is ultimately responsible for mountain uplift. Here we present an AMS record, spanning the interval of 28-4 Ma, from the fluviolucastrine Jingou River section in the Tian Shan foreland. Rock magnetic measurements show that the AMS are dominated by antiferromagnetic hematite and some paramagnetic minerals, both of which have a crystallographic preferred orientation, and therefore the AMS sequence may be applied to reconstruct tectonic strain. Although the principal minimum (K-min) direction is nearly perpendicular to the bedding plane, indicative of a predominantly sedimentary fabric for the section, the tightly grouped principal maximum (K-max) parallel to the fold axis suggests that the section was subject to an embryonic deformation at least since the Late Oligocene. The K-max direction is most likely associated with the N-S strain caused by the India-Asian collision. During the intervals of 23.3-20.0 Ma and 16.5-14.0 Ma, the strain markedly increased as evidenced by more tightly grouped K-max directions and K-min largely distributed within a clear N-S girdle. These two intervals are accompanied by changes in sedimentary facies (higher energy conditions and doubled sedimentation rates), and are interpreted as the periods when the Tian Shan experienced significant uplift. A conceptual model is tentatively proposed to reconcile various timings for the Tian Shan uplift.