Paleomagnetism and anhysteretic remanent magnetization as a tool for time correlation and tectonic studies

We present paleo- and rock magnetic results from three Cenozoic magnetostratigraphic sections in central Asia. Two (Chong Kyzyl-Suu and Jeti Oguz) are ca. 1 km thick parallel sections composed of continental sediments deposited at the northern flank of the Tianshan mountains in Kyrgyzstan. The Xishuigou (2179 m thick) section is located on the northern flank of the Kunlun Mountains in western China. We employed our recently developed automated system (SushiBar) based on a three-axis superconducting magnetometer and a custom-made coil to measure the grain-size spectra, anisotropy and viscous decay of anhysteretic remanent magnetization (ARM), as well as a conventional stepwise demagnetization on the samples. Both magnetostratigraphy and characteristic changes in rock magnetic parameters help establish the relative and absolute time correlation of the Kyrgyz sections with each other and with the geomagnetic polarity time scale. Our preferred, albeit tentative, magnetostratigraphic correlation with the geomagnetic polarity time scale suggests absolute deposition ages range from 25.5 to 13.2 Ma, with a fairly constant sedimentation rate of 10 cm per ka. Principal anisotropy axis directions from the lowest switching fields correlate well with principal axis directions from anisotropy of magnetic susceptibility. Principal axis directions defined at higher switching fields systematically deviate from those at lower switching fields, perhaps defining the fabric of the remanence carrying grains. The ferromagnetic fabric, derived from the anisotropy of ARM for both Kyrgyz sections, shows a well developed tectonic fabric with sub-horizontal maximum axis that parallel the strike directions, orthogonal to the local maximum strain direction. ARM data for the Xishuigou section shows a sedimentary fabric with a small but increasing tectonic overprint down-section.