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Source of Oligocene to Pliocene Sedimentary Rocks in the Linxia Basin in Northeastern Tibet from Nd Isotopes: Implications for Tectonic Forcing of Climate

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Source of Oligocene to Pliocene Sedimentary Rocks in the Linxia Basin in Northeastern Tibet from Nd Isotopes: Implications for Tectonic Forcing of Climate Source of Oligocene to Pliocene sedimentary rocks in the Linxia basin in northeastern Tibet from Nd isotopes: Implications for tectonic forcing of climate Carmala N. Garzione† Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA, and Department of Geological Sciences and Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, Colorado 80309, USA Matt J. Ikari Asish R. Basu Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA ABSTRACT generate the εNd values observed in Linxia tion of the plateau margin was responsible for basin sedimentary rocks through mixing of the middle Miocene climate change observed We used Nd isotopes and trace element plutonic and metasedimentary sources. in the Linxia basin. data to determine the provenance of sedi- Rare earth element patterns suggest that mentary rocks in the Linxia basin, northeast- Cretaceous rocks were not a dominant Keywords: Nd isotopes, sedimentary prov- ern Tibet, whose Oligocene through Pliocene source of sediment. The Nd isotopic composi- enance, loess, Tibetan Plateau, paleoclimate, sedimentation history has been interpreted to tion and rare earth element pattern of Qua- unroofi ng. refl ect deposition in a fl exural basin associ- ternary loess are similar to older deposits in ated with contractional deformation along the Linxia basin and refl ect loess deposited INTRODUCTION the northeastern margin of the Tibetan elsewhere in the Loess plateau and the North Plateau. Paleozoic–early Mesozoic metasedi- Pacifi c (εNd = −8.6 to –10.5). In addition, the The unroofi ng histories of the margins of the mentary source rocks from the Kunlun- modern Daxia River, which drains the mar- Tibetan Plateau provide insight into the timing Qaidam and Songpan-Ganzi terranes have gin of the plateau today, transports clay and and mechanisms of outward growth of the pla- εNd values between −11.8 and −16.1, whereas silt with εNd values of –10.5 to –10.8 despite teau. The timing of surface uplift of the north- Paleozoic and Mesozoic plutonic source rocks the river’s source in more negative metasedi- eastern margin of the Tibetan Plateau is poorly that intrude the metasedimentary rocks mentary rocks of the Kunlun-Qaidam and understood because of the paucity of age con- have more positive εNd values between −3.6 Songpan-Ganzi terranes, which indicates straints on exhumation in this region. Cenozoic and −11.2. Cretaceous sedimentary source that the modern fi ne-grained sedimentary sedimentary basins along the northeastern mar- rocks display εNd values of −9.7 and –9.9 in budget is dominated by recent loess deposits. gin of Tibet contain records of both the unroof- the Maxian Shan, north of the Linxia basin, Considering the slow sedimentation rates in ing history of the margin of the plateau and the and –15.3 in the plateau margin south of the the Linxia basin, it is likely that loess sources evolution of central Asian climate. Potential basin. With εNd values that range between have contributed a signifi cant volume of fi ne- sources of sediment to these basins include −8.4 and −10.4 before ca. 15 Ma, and −6.2 grained sediment to this basin throughout its metasedimentary, plutonic, and sedimentary and −11.8 after ca. 14 Ma, sedimentary rocks history. An increase in the range of εNd values rocks exposed in the margin of the Tibetan Pla- of the Linxia basin are less negative than at ca. 14 Ma in the Linxia basin may refl ect teau as well as loess derived from central Asian metasedimentary rocks, which are dominant increased unroofi ng of the northeastern mar- deserts. Through an understanding of both the source rocks within the margin of the Tibetan gin of Tibet, which slightly preceded a change unroofi ng and climate histories in northeastern Plateau today. The relatively positive εNd val- in climate between ca. 13 and 12 Ma in the Tibet, we assessed the impact of the growth of ues of Linxia basin sedimentary rocks could Linxia basin. A 1.5‰ increase in baseline the plateau on the regional climate. refl ect several possible sources, including δ18O values of lacustrine carbonates has been Two types of data have been used to address (1) a mixture of plutonic and metasedimen- interpreted as the result of reorganization of the timing of Cenozoic deformation along the tary rocks within the northeastern margin atmospheric circulation and an increase in margin of the Tibetan Plateau: (1) subsidence of Tibet, (2) Cretaceous sedimentary rocks aridity on the northeastern margin of the histories from the records of sedimentation derived from the north, or (3) loess derived Tibetan Plateau, perhaps associated with (e.g., Bally et al., 1986; Métivier et al., 1998; from central Asian deserts. A mass balance the plateau having achieved an elevation Yin et al., 2002; Fang et al. 2003; Horton et calculation indicates that plutonic rocks suffi cient to block moisture from the Indian al., 2004), and (2) exhumation histories from are not volumetrically signifi cant enough to Ocean and/or Pacifi c Ocean. Similar timing thermochronologic studies of mountain ranges of exhumation and climate change suggests in the margins of the plateau (e.g., Jolivet et †E-mail: [email protected]. that northeastward and eastward propaga- al., 2001; Sobel et al., 2001; Kirby et al., 2002; GSA Bulletin; September/October 2005; v. 117; no. 9/10; p. 1156–1166; doi: 10.1130/B25743.1; 9 fi gures; 2 tables; Data Repository item 2005145. For permission to copy, contact [email protected] 1156 © 2005 Geological Society of America SOURCE OF OLIGOCENE TO PLIOCENE SEDIMENTARY ROCKS IN THE LINXIA BASIN, TIBET Clark et al., 2005). The earliest Cenozoic activ- signifi cance of loess as a source of fi ne-grained transgressive facies between the fossil localities ity on the northern margin of Tibet is recorded sediment to the basin. and paleomagnetic sections. The 4 m.y. hiatus by apatite fi ssion-track and 40Ar/39Ar cooling seems unlikely in the Maogou and Wangjiashan histories from rocks along the Altyn Tagh and GEOLOGIC SETTING sections, given that this interval is represented Kunlun faults, which suggest increased rates by apparently continuous lacustrine deposition of exhumation in late Eocene to Oligocene Linxia Basin in both sections and that both yield similar mag- time (Mock et al., 1999; Jolivet et al., 2001; netostratigraphic records. Yet we acknowledge Sobel et al., 2001). These ages are consistent The Linxia basin lies at the northeastern mar- that age constraints may contain errors of up to with other observations from the northwestern gin of the Tibetan Plateau in the eastern Kunlun- several million years resulting from uncertain- Qaidam basin, including growth strata observed Qaidam terrane, southwest of Lanzhou (Fig. 1). ties in the magnetostratigraphic and lithostrati- in seismic data (Bally et al., 1986); isopach data The basin is ~200 km long and ~75 km wide and graphic correlations. that show thick Oligocene deposition (Wang is elongate parallel to the fold-and-thrust belt on and Coward, 1990); and facies, paleocurrent, the northeastern edge of the Tibetan Plateau. Plateau Margin and provenance information that suggests posi- The basin is situated between two major left- tive relief in the Altyn Tagh (Hanson, 1998). lateral strike-slip faults, the Haiyuan fault to the Southwest of the Linxia basin, the north- Despite these older ages for initial exhumation northeast and the Kunlun fault to the southwest eastern margin of the Tibetan Plateau consists along the Altyn Tagh fault, Tertiary fault offsets (Fig. 1A and B). Movement along these faults of rocks of the eastern Kunlun-Qaidam terrane determined from sedimentary provenance rela- and along the Altyn Tagh fault to the northwest and the Songpan-Ganzi complex (e.g., Yin and tionships from the central and eastern parts of allows eastward transfer of material in India’s Harrison, 2000) (Fig. 1B). North of the suture, the fault have been interpreted as indicating a northward path into Eurasia and accommodates the Kunlun-Qaidam terrane consists of Devo- younger initial strike-slip motion of late Oli- some of the convergence between India and nian through Permian metasedimentary rocks gocene to early Miocene age (Yue et al., 2001, Eurasia. Shortening at bends and terminations in and plutons of early Paleozoic and late Paleo- 2004; Ritts et al., 2004). these strike-slip faults creates some of the minor zoic–early Mesozoic age. Devonian quartzites A second phase of exhumation in northern mountain ranges between them (e.g., Burchfi el and interbedded red and green phyllites were Tibet records higher rates in Miocene to Quater- et al., 1989; Tapponnier et al., 1990; Zhang et deposited in a terrestrial environment, which nary time. Apatite fi ssion-track ages, associated al., 1990, 1991). may have developed as a result of the collision with strike-slip deformation along the Altyn The Linxia basin has been interpreted as between the eastern Kunlun-Qaidam terrane and Tagh fault, show a Miocene-Pliocene increase a fl exural basin on the basis of its shape, ori- the southern Qilian Shan terrane to the north in exhumation rates in the Qilian Shan, Kun- entation, stratigraphy, and subsidence history (Hsu et al., 1995). Devonian to Early Carbon- lun Shan, and Altyn Tagh (Jolivet et al., 2001; (Fang et al., 2003). The stratigraphic thickness iferous quartzites and schists are interbedded George et al., 2001). In the southern Qaidam varies from >1200 m proximal to the plateau with metavolcanic rocks associated with granitic basin, an increase in sediment accumulation to <200 m near the southwestern fl ank of the intrusions (Yin et al., 1988; Huang et al., 1996). rates in middle to late Miocene time is consis- Maxian Shan (Fig.
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