Two-Stage Evolution of the Altyn Tagh Fault System during the Tertiary: Constraints from Heavy Mineral Chemistry in Sediments of the Northwestern Qaidam Basin, Western China

Abstract

The tectonic evolution of the Altyn Tagh Fault (ATF) remains controversial during the Tertiary. Qaidam Basin is the largest and highest plateau inland basin inside the Tibet Plateau. Sediments in the basin provide sedimentary records of the evolution history of its surrounding orogens, such as the ATF, located on the northwest margin of the Qaidam Basin. Comprehensive analyses of sandstone petrography, heavy mineral assemblages, and mineral geochemistry were adopted to effectively indicate the tectonic evolution history of ATF. The result indicates that the sediments in a wide range of the northwestern Qaidam Basin (e.g., the Xichagou section, the Yueyashan section) were derived from the Altyn Mountains. The increasing immaturity of sediments, increased denudation, and sedimentation processes from the early-middle Miocene to the Pliocene can be explained by the active tectonic setting of the ATF. During the early Miocene (ca. 22 Ma), there was an abrupt change in the heavy mineral composition of sediments in the northwestern Qaidam Basin. This change may be attributed to the large-scale slip motion along the ATF. Therefore, ~22 Ma is the key transforming period of the ATF system. On the foundation of the above, we suggest a two-stage evolution model of the ATF during the Tertiary: (1) From the late Eocene to the Oligocene, the tectonic setting of the ATF was relatively calm; (2) During the early Miocene period, the ATF underwent large-scale tectonic activation. It is likely to be a strike-slip tectonic activity, accompanied by an uplift of the Altyn Mountains. The active tectonic setting of the ATF was sustained after the Miocene.