Source of Oligocene to Pliocene sedimentary rocks in the Linxia basin in northeastern Tibet from Nd isotopes: Implications for tectonic forcing of climate

Abstract

We used Nd isotopes and trace element data to determine the provenance of sedimentary rocks in the Linxia basin, northeastern Tibet, whose Oligocene through Pliocene sedimentation history has been interpreted to refl ect deposition in a fl exural basin associated with contractional deformation along the northeastern margin of the Tibetan Plateau. Paleozoic‐early Mesozoic metasedimentary source rocks from the KunlunQaidam and Songpan-Ganzi terranes have e Nd values between −11.8 and −16.1, whereas Paleozoic and Mesozoic plutonic source rocks that intrude the metasedimentary rocks have more positive e Nd values between −3.6 and −11.2. Cretaceous sedimentary source rocks display e Nd values of −9.7 and ‐9.9 in the Maxian Shan, north of the Linxia basin, and ‐15.3 in the plateau margin south of the basin. With e Nd values that range between −8.4 and −10.4 before ca. 15 Ma, and −6.2 and −11.8 after ca. 14 Ma, sedimentary rocks of the Linxia basin are less negative than metasedimentary rocks, which are dominant source rocks within the margin of the Tibetan Plateau today. The relatively positive e Nd values of Linxia basin sedimentary rocks could refl ect several possible sources, including (1) a mixture of plutonic and metasedimentary rocks within the northeastern margin of Tibet, (2) Cretaceous sedimentary rocks derived from the north, or (3) loess derived from central Asian deserts. A mass balance calculation indicates that plutonic rocks are not volumetrically signifi cant enough to generate the e Nd values observed in Linxia basin sedimentary rocks through mixing of plutonic and metasedimentary sources. Rare earth element patterns suggest that Cretaceous rocks were not a dominant source of sediment. The Nd isotopic composition and rare earth element pattern of Quaternary loess are similar to older deposits in the Linxia basin and refl ect loess deposited elsewhere in the Loess plateau and the North Pacifi c (e Nd = −8.6 to ‐10.5). In addition, the modern Daxia River, which drains the margin of the plateau today, transports clay and silt with e Nd values of ‐10.5 to ‐10.8 despite the river’s source in more negative metasedimentary rocks of the Kunlun-Qaidam and Songpan-Ganzi terranes, which indicates that the modern fi ne-grained sedimentary budget is dominated by recent loess deposits. Considering the slow sedimentation rates in the Linxia basin, it is likely that loess sources have contributed a signifi cant volume of fi grained sediment to this basin throughout its history. An increase in the range of e Nd values at ca. 14 Ma in the Linxia basin may refl ect increased unroofi ng of the northeastern margin of Tibet, which slightly preceded a change in climate between ca. 13 and 12 Ma in the Linxia basin. A 1.5‰ increase in baseline δ 18 O values of lacustrine carbonates has been interpreted as the result of reorganization of atmospheric circulation and an increase in aridity on the northeastern margin of the Tibetan Plateau, perhaps associated with the plateau having achieved an elevation suffi cient to block moisture from the Indian Ocean and/or Pacifi c Ocean. Similar timing of exhumation and climate change suggests that northeastward and eastward propagation of the plateau margin was responsible for the middle Miocene climate change observed in the Linxia basin.