Neogene Magnetostratigraphy of the Xunhua and Hualong Basins, NE Tibetan Plateau

The Xunhua and Hualong intermontane basins in NE Tibet contain strata recording Mio-Pliocene surface uplift, deformation, and lateral expansion of the Tibetan Plateau. The timing and nature of broad surface uplift of the Tibetan Plateau remains uncertain. Competing interpretations argue for synchronous uplift of the entire plateau, pulsed plateau uplift, and incremental outward and upward plateau growth. Even within any given sector of the plateau, controversy prevails over whether the entire region rose synchronously or whether the growth of individual ranges and ponding of sediment in intervening basins led to incremental expansion and upward plateau growth. To address this question we aim to combine information from basin stratigraphy and provenance, chronology of deposition, stable isotopes, structural geometry, and cooling histories of bounding ranges to develop a more coherent synthesis of plateau growth in this region. The Xunhua and Hualong basins lie along the NE margin of the Tibetan Plateau, a broad, eastward-sloping topographic ramp punctuated by individual mountain ranges: locally, the W. Qinling Shan to the S, the Laji Shan to the N, the Jishi Shan to the E, and the Gongbo and Zamazari Shan to the W. The youngest (U-Th)/He cooling ages from these ranges are Late Miocene, suggesting accelerated rock uplift, erosion, and cooling at this time. Additionally, the Linxia basin to the E and the Guide basin to the W exhibit increased sediment- accumulation rates and grain size in the Late Miocene (Fang et al., 2003; 2005). Both the Xunhua and Hualong basins contain a coarsening-upward sequence of fine-grained, gypsiferous, lacustrine mudstone at the base that grade up-section into fluvial sandstone and siltstone (including numerous paleosols), with conglomerate capping the sequence. The homogeneity of the oldest strata in the Xunhua and Hualong basins suggests that the basins were initially contiguous. However, at present the significantly folded Hualong section is separated from the relatively undeformed Xunhua section by frontal thrust faults of the Jishi Shan as deformation of this range steps basin-ward. Furthermore, preliminary work indicates multiple local ranges began shedding sediment into the basins in Late Mio-Pliocene time. Here we present new magnetostratigraphic age constraints from a section in each basin, with the Hualong section located 20 km NE of the Xunhua section. The Xunhua section is ~950 m thick and contains >200 sampling sites, whereas the Hualong section is ~1450 m thick and contains >500 sampling sites. Characteristic remanent directions were isolated by progressively demagnetizing samples through a low- temperature step, four AF steps, and 8-16 thermal steps. Preliminary correlation with the GPTS has the sections span the middle to at least latest Miocene. High-resolution magnetostratigraphy of the Xunhua and Hualong basins will allow us to define sediment accumulation rates and identify how major facies changes relate to local deformation and uplift events. By providing a detailed reconstruction of basin development, range growth, and coeval environmental change in one area, this work represents a step in addressing the relationship between plateau growth and climate change.