Magnetochronology of the Upper Cenozoic strata in the Southwestern Chinese Tian Shan: rates of Pleistocene folding and thrusting

The southwestern Chinese Tian Shan of Central Asia is an actively deforming part of the Indian^Asian collision system. Paleomagnetic investigations of two Plio^Pleistocene terrestrial successions provide the first detailed magnetostratigraphy for the upper Cenozoic strata in this region. Paleomagnetic samples were collected from 358 sites within the Atushi Formation and Xiyu Formation across the Atushi^Talanghe anticline near Atushi. Thermal demagnetization behavior, reversal and fold tests of paleomagnetic stability indicate that the characteristic remanence directions were acquired before tilting and folding of the strata. A composite magnetostratigraphic section for this sequence correlates with the Mammoth subchron to the Jaramillo subchron, between 3.3 and 1.07 Myr, of the geomagnetic polarity timescale of Cande and Kent [J. Geophys. Res. 100 (1995) 6093^6095]. The mean declination (349 þ 33) in the Boguzihe section indicates a counterclockwise vertical-axis rotation (311 þ 23) during the past 1.4 Myr. The conglomeratic Xiyu Formation is time-transgressive along its progradational contact with the underlying Atushi Formation across a map distance of V6 km, the basal contact ranges from less than 1.0 Myr in the Ganhangou section to 1.9 Myr in the Boguzihe section to the southwest, and to V2.8 Myr on the northwest limb of the anticline. Northeastward lateral propagation and growth of the Atushi^Talanghe anticline initiated at V1.4 Myr in the Boguzihe, and V1.2 Myr in the Ganhangou. The cross section in the Boguzihe provides a conservative (maximum) estimate of shortening rate of V3.3 (4.4) mm/yr and uplift rate of V3 mm/yr. This rate represents about half of the geodetically determined shortening rate between the northern Tarim Basin and the Kyrgyz Tian Shan [Wang et al., Acta Seismol. Sin. 22 (2000) 263^270; Reiger et al., Earth Planet. Sci. Lett. 191 (2001) 157^165]. During fold growth over the past 1.4 Myr, the crest of the Atushi^Talanghe anticline eroded at an average rate of 2.6^2.7 mm/yr. ß 2001 Published by Elsevier Science B.V. 0012-821X / 01 / $ ^ see front matter ß 2001 Published by Elsevier Science B.V. PII: S 0 0 1 2 8 2 1 X ( 0 1 ) 0 0 5 7 9 9 * Corresponding author. Tel. : +86-10-62009149; Fax: +86-10-62009003. E-mail addresses: [email protected] (J. Chen), [email protected] (D.W. Burbank), [email protected] (K.M. Scharer), [email protected] (E. Sobel), [email protected] (C. Rubin). EPSL 6075 11-12-01 Earth and Planetary Science Letters 6075 (2001) 1^19 www.elsevier.com/locate/epsl