Cenozoic sediment flux in the Qaidam Basin, northern Tibetan Plateau, and implications with regional tectonics and climate

Accepted Manuscript Cenozoic sediment flux in the Qaidam Basin, northern Tibetan Plateau, and implications with regional tectonics and climate

Jing Bao, Yadong Wang, Chunhui Song, Ying Feng, Chunhua Hu, Sirui Zhong, Jiwei Yang PII: DOI: Reference:

S0921-8181(16)30252-1 doi: 10.1016/j.gloplacha.2017.03.006 GLOBAL 2571

To appear in:

Global and Planetary Change

Received date: Revised date: Accepted date:

29 June 2016 23 March 2017 24 March 2017

Please cite this article as: Jing Bao, Yadong Wang, Chunhui Song, Ying Feng, Chunhua Hu, Sirui Zhong, Jiwei Yang , Cenozoic sediment flux in the Qaidam Basin, northern Tibetan Plateau, and implications with regional tectonics and climate. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Global(2017), doi: 10.1016/j.gloplacha.2017.03.006

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ACCEPTED MANUSCRIPT Cenozoic sediment flux in the Qaidam Basin, northern Tibetan Plateau, and implications with regional tectonics and climate Jing Baoa, Yadong Wangb, Chunhui Songa, Ying Fenga, Chunhua Hua, Sirui Zhonga, Jiwei Yangc School of Earth Sciences & Key Laboratory of Mineral resources in Western China (Gansu Province),

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Key Laboratory of Petroleum Resources, Gansu Province/Key Laboratory of Petroleum Resources

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b

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Lanzhou University, Lanzhou 730000, China

Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou, China Gansu province nuclear geological brigade 212, Wuwei 733000, China

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*Corresponding author: [email protected] (C. Song) Abstract

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As the largest Mesozoic-Cenozoic terrestrial intermountain basin in the northern

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Tibetan Plateau, the Qaidam Basin is an ideal basin to examine the influences of regional tectonics and climate on sediment flux. Research conducted over the last two

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decades has provided abundant information about paleoclimatology and tectonic

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histories. In this study, we used the restoration of seven balanced cross-sections and compiled thickness data of ten outcrop sections and four boreholes to reconstruct the basin boundaries, develop isopach maps, and calculate the sediment flux in the Qaidam Basin. Our results show that the sediment flux in the Qaidam Basin increased gradually between 53.5 and 35.5 Ma, decreased to its lowest value from 35.5 to 22 Ma, increased between 22 and 2.5 Ma, and then increased dramatically after 2.5 Ma. By comparing the changes in the sediment flux with our reconstructed shortening rate in 1 1.

ACCEPTED MANUSCRIPT the Qaidam Basin, and the records of regional tectonic events and regional and global climate changes, we suggest that the gradual increase in the sediment flux from 53.5 to 40.5 Ma was controlled by both the tectonic uplift of the Tibetan Plateau and the relatively warm and humid climate, and that the high sediment flux from 40.5 to 35.5

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Ma was mainly controlled by tectonics. The low sediment flux from 35.5 to 22 Ma

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was a response to the relatively cold and arid climate in a stable tectonic setting. The

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relatively high sediment flux between 22 and 15.3 Ma was related to tectonic activity and the warm and humid climate. The intense tectonic uplift of the northern Tibetan

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Plateau and the frequent climate oscillations after 15.3 Ma, particularly the

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glacial-interglacial cycles after 2.5 Ma, caused the high sediment flux after 15.3 Ma

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and the dramatic increase after 2.5 Ma, respectively.

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Keywords: Cenozoic; Qaidam Basin; Balanced cross-section; Sediment flux;

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Tectonic uplift; Climate change

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1. Introduction

Continental weathering and denudation are the primary ways by which the solid Earth interacts with the atmosphere, hydrosphere and biosphere and are the main drivers of landform evolution. Continental weathering and denudation are influenced by both climate changes and tectonic uplift (e.g., Wobus et al., 2003; Molnar, 2004). In Asia, the relationships between continental denudation, climate change, and the uplift of the Tibetan Plateau are long-standing research topics (e.g., Clift et al., 2006, 2 1.

ACCEPTED MANUSCRIPT 2009, 2010; Nie et al., 2015). In the late 1980s, researchers found that chemical weathering of silicate rocks in the Himalaya can consume atmospheric CO2 and cause global climate change (Raymo et al., 1988, 1992; Berner, 1997). Nie et al. (2015) inferred from sedimentary records in the Chinese Loess Plateau and western Mu Us

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desert that denudation in the NE Tibetan Plateau increased during the Pliocene, and

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further suggested that the increased denudation may have driven Pliocene global

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climate cooling. However, considerable controversy remains regarding the main controlling factors of weathering and denudation in the Himalaya-Tibetan Plateau. For

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examples, Burbank et al. (2003) suggested that Quaternary denudation of the

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Himalayas was not controlled by the monsoon intensity, whereas Clift et al. (2008, 2010) concluded that the erosion rate of the Tibetan Plateau, based on sedimentary

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record in the marine basins in southeastern and eastern Asia, gradually increased after

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the Oligocene and peaked in the middle Miocene mainly in response to global climate change. Therefore, significant uncertainty exists about whether tectonic uplift of the

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Tibetan Plateau or global climate change enhanced denudation (Burbank et al., 2003;

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Wobus et al., 2003; Dadson et al., 2003; Zhang et al., 2001; Molnar, 2004). In this paper, we provide sedimentary records from the interior of the Tibetan Plateau to elucidate this debate. The Qaidam Basin is an ideal area to study the relationships between denudation, deposition, tectonic uplift, and climatic change during the Cenozoic (Metivier et al., 1999; Xia et al., 2001; Molnar, 2005; Fang et al., 2007; Song et al., 2014). In this study, we identified the Qaidam Basin has been an intermountain basin since the 3 1.

ACCEPTED MANUSCRIPT Mesozoic, thus the sediment flux in the Qaidam Basin should reflect the denudation of the basin-bounding mountains. To determine changes in the denudation rate through time, we first determined the changes in sedimentation amounts through time using new delineated basin boundaries and new isopach maps. We then calculated the

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sediment flux by dividing the sedimentation amounts with the durations of deposition.

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Our isopach maps and basin boundaries were reconstructed based on seven restored

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cross-sections and strata thickness data compiled from ten measured sections and four boreholes. By comparing the sediment flux with previously documented regional

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tectonic events and climate changes, we discuss the key factors that controlled the

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sediment flux and denudation rate around the basin and provide evidence for the relationships between denudation, climate, and tectonics in the northern Tibetan

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Plateau.

2. Geologic setting and stratigraphy

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The Qaidam Basin covers an area of 12.1×104 km2 and is the largest intermountain

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basin in the northern Tibetan Plateau (Huang et al., 1996; Xia et al., 2001; Molnar, 2005; Fang et al., 2007). The elevations of the Qaidam Basin range from 3 to 3.5 km and decrease gradually from the northwest to the southeast, and the elevations of the surrounding mountains are 4-5 km above sea level (Huang et al., 1996; Xia et al., 2001). The basin is bounded by the Altyn Tagh to the northwest, the Kunlun Mountains to the south, the Qilian Mountains to the northeast, and the Ela Shan Mountains to the east (Fig. 1). The Qaidam Basin can be divided into four tectonic 4 1.

ACCEPTED MANUSCRIPT units based on changes in topography, including the western Qaidam uplift, the Yiliping depression, the northern Qaidam uplift, and the Sanhu depression (Fig. 2). The western uplift contains three tectonic subunits, including the north Kunlun fault terrace, the Mangya depression, and the Dafengshan bulge. The northern Qaidam

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uplift is composed of the Sai-kun fault depression, the Yuka-Hongshan fault

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depression, the Mahai bulge, and the Delingha fault depression. The Sanhu depression

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consists of the north slope, the Sanhu sag, and the south slope (Qinghai Petroleum

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Administrative Bureau, 1990).

Cenozoic strata are well exposed in the Qaidam Basin and can be divided into

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seven formations, including, from bottom to top, the Lulehe Formation (Fm.), the Xia Ganchaigou Fm., the Shang Ganchaigou Fm., the Xia Youshashan Fm., the Shang

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Youshashan Fm., the Shizigou Fm., and the Qigequan Fm. (Huang et al., 1996; Xia et al., 2001). The Xia Ganchaigou Fm. is further divided into the Lower and Upper

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members (Huang et al., 1996; Xia et al., 2001). The age of these strata have been previously constrained by ostracods, fossil mammals, lithostratigraphic correlations

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and magnetostratigraphy (Yang et al., 1992; Zhang, 2006, 2013; Sun et al., 2005b; Fang et al., 2007; Wang et al., 2007; Lu and Xiong, 2009; Ke et al., 2013; Chang et al., 2015). The age constraints have considerable uncertainties for two reasons. The first is the identification of the boundaries between different formations based on lithostratigraphy, and the second is the lateral variation of lithofacies (Fang et al., 2016). One of the examples is that the age of the top of the Shang Ganchaigou Fm. is

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ACCEPTED MANUSCRIPT determined to be ∼22 Ma (Lu and Xiong, 2009; Chang et al., 2015), while Sun et al. (2005b) assigned the top to be 26.5 Ma. While we cannot fully assess the uncertainties, we used the current magnetostratigraphic ages that have been accepted by most researchers (Sun et al., 2005b; Fang et al., 2007; Lu and Xiong, 2009; Yin et al., 2008;

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Mao et al., 2014; Wang et al., 2015; Chang et al., 2015). The uncertainty of ages, such

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as the case of the Shang Ganchaigou Fm., has small influence on the overall pattern of

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the calculated sediment flux. We assigned the age of the Lulehe Fm. is 53.5-43.8 Ma (Zhang, 2006; Ke et al., 2013; Sun et al., 2005b), the age of the Lower Xia

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Ganchaigou Fm. is 43.8-40.5 Ma (Zhang, 2006; Sun et al., 2005b), the age of the

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Upper Xia Ganchaigou Fm. is 40.5-35.5 Ma (Sun et al., 2005b; Lu and Xiong, 2009), the age of the Shang Ganchaigou Fm. is 35.5-22 Ma (Sun et al., 2005b; Lu and Xiong,

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2009; Chang et al., 2015), the age of the Xia Youshashan Fm. is 22-15.3 Ma (Fang et

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al., 2007; Lu and Xiong, 2009; Chang et al., 2015), the age of the Shang Youshashan Fm. is 15.3-8.1 Ma (Fang et al., 2007; Lu and Xiong, 2009), the age of the Shizigou

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Fm. is 8.1-2.5 Ma (Fang et al., 2007; Lu and Xiong, 2009), and the age of the

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Qigequan Fm. is 2.5-0 Ma (Fang et al., 2007).

3. Methodology Isopach maps of the Cenozoic strata are the basis for calculating the sediment flux. Previously published isopach maps were constructed using the present-day basin boundaries (Huang et al., 1996; Yin et al., 2008; Meng and Fang, 2008). Because the 6 1.

ACCEPTED MANUSCRIPT size of the basin has decreased over time due to crustal shortening, these isopach maps cannot be used to calculate the sediment flux. We first reconstructed the basin boundaries using the amounts of shortening determined from balanced cross-sections. We then reconstructed new isopach maps using strata thickness data from measured

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sections, seismic profiles, and borehole. Finally, we calculated the sediment flux by

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analyzing the new isopach maps.

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3.1 Restoration of balanced cross-sections

The method of balanced cross-section calculations in the Qaidam Basin were

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first systematically constructed by Dahlstrom (1969). Strata deformation and thrust

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and growth faulting can be restored using several computer programs, such as 2Dmove, Geosec, and Locace. The primary restoration methods include forward

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modeling, reconstruction, and inverse modeling (Liang et al., 2002; Wang et al.,

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2012). In this paper, the reconstruction method is adopted to restore the deformed cross-sections to their original horizontal states. Prior to restoring these cross-sections,

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the eroded strata were restored. The estimates for the eroded strata were based on the

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assumptions that the erosion has occurred mainly since the Quaternary and that the eroded strata in each stratigraphic unit follows the thickness trend (Liang et al., 2002; Zhou et al., 2006). After importing seven 2D seismic sections into the 2Dmove software and tracing the planes and faults, the sections were restored by decompaction, fault restoration and layer flattening. The simple shear method was adopted for normal faults, and the fault-parallel flow method was adopted for reverse faults. Each section was restored from top to bottom, formation by formation. 7 1.

ACCEPTED MANUSCRIPT In this study, the geographical locations of the mountains is assumed to remain unchanged, and sections 1, 2 and 3 were restored toward the south side of the basin using the Altyn Tagh Mountains as the reference line (Lou et al., 2016). Cross-sections 4, 5, 6 and 7 were drawn in stable tectonic regions, such as the

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Yiliping depression in the western basin and the Sanhu depression in the

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central-eastern basin. Thus we used fixed positions in stable tectonic regions as the

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reference positions and stretched the strata to the opposite sides (Lou et al., 2016). Fig. 4 demonstrates how section 5 was restored to its originally horizontal state through

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time. Based on the current and restored lengths and the age of deposition of each

were obtained (Fig. 5a-h, Table 1).

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3.2 Plotting isopach maps

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stratigraphic unit, the amount, ratio, and rate of shortening of each seismic profile

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In this step, the isopach map boundaries were reconstructed by incorporating previously obtained shortening amounts from the balanced cross-sections for different

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stages. The main data source for the isopach maps of the Cenozoic Qaidam Basin was

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Huang et al. (1996), who compiled information from more than 1000 drill holes and a dense network of seismic profiles. The stratal thicknesses of ten measured sections (Table 2), seven seismic profiles and four boreholes were used to modify the isopach maps where appropriate (Fig. 6). 3.3 Calculation of sediment flux The Qaidam Basin has been an endorheic drainage basin after the early Paleogene, and the Cenozoic sediments in the Qaidam Basin were derived mainly 8 1.

ACCEPTED MANUSCRIPT from the surrounding mountains (Huang et al., 1996; Liu et al., 1998; Shi et al., 2001; Yin et al., 2002; Zhuang et al., 2011b; Bush et al., 2016). Based on the principle of mass conservation, the total amount of sediment in the basin is equal to the total amount of denudation from the surrounding mountains (Wang and Liu, 1993;

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Metivier et al., 1998, 1999). Sediment flux is defined as the mass of sediment

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deposited per unit time per unit area (Einsele, 1992; Yan et al., 2010). The formula for

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calculating the sediment flux is as follows: SF= V• ρ• (1-Φ) / (S• Δt)

(1)

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where SF [t Ma-1 m-2] is the sediment flux, V [m3] is the total volume of the

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basin fill, ρ [t m-3] is the density of the basin fill, Φ [%] is the porosity of the basin fill, S [m2] is the area of the basin fill, and Δt [Ma] is the duration of deposition (Einsele,

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1992; Yan et al., 2010). We calculated the average density of the sedimentary rock (ρ)

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using the absolute age of each formation in the empirical equation lg t = -7.533+3.846ρ (Pang, 1988) (Table 3). The rock porosity (Φ) was calculated by

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considering the lithology, depth and duration of burial (Jaeger, 1972; Liu et al.,

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2007b). The lithology of most of the stratigraphic units and the thickness of each formation in the Qaidam Basin vary laterally, which made the estimation of the porosity difficult. Carbonate cements that formed after the deposition of the silicilastic grains (Zhu, 2008; Shu, 2010) may have altered the original porosity. Thus, the empirical equation and standard curve for porosity (e.g., Liu et al., 2007b) may not be suitable for estimating porosity in the entire basin. In this study, we collected the porosity of each formation in the southwestern, northern and southeastern regions of 9 1.

ACCEPTED MANUSCRIPT the basin (Table A. Supplementary material) and used the average value of the porosity from these regions to represent the porosity of each formation. We imported the reconstructed isopach maps into MapGIS software. Using regional topological reconstruction, we obtained the areas (m2) of the basin for

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different time period. Finally, we multiplied the area of each region by the strata

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thickness to determine the volume of each region and added the volumes of the

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regions to obtain the total volume (m3) for each time period.

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4. Results

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4.1 Shortening amount and rate

The shortening amount, ratio and rate in the Qaidam Basin varied significantly

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over time and space (Fig. 5). The cumulative shortening ratio (Fig. 5a-g) and average

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shortening rate (Fig. 5h) were high from 43.8 to 35.5 Ma and from 15.3 to 0 Ma, and were low between 53.5 and 43.8 Ma and between 35.5 and 15.3 Ma. From 43.8 to

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40.5 Ma, the shortening rates of sections 1-6 and the average shortening rate increased

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suddenly (Fig. 5g). From 40.5 to 35.5 Ma, the shortening rates of sections 2 and 4-7 and the average shortening rate remained relatively high (Fig. 5b, 5d-g). The shortening rates of all of the sections were lowest from 35.5 to 15.3 Ma, and highest from 2.5 to 0 Ma. 4.2 Cenozoic strata thickness The Qaidam Basin expanded eastward and southward through time (Fig. 6). From 53.5 to 35.5 Ma, the depositional area increased from 109,439 km2 in the 10 1.

ACCEPTED MANUSCRIPT Lulehe Fm. to 142,194 km2 in the Upper Xiaganchaigou Fm. (Table 3). From 35.5 to 15.3 Ma, the southern boundaries retreated to the north, and the depositional area decreased. Since 15.3 Ma, the basin area has decreased gradually, and the sedimentation center has migrated to the east parallel to the Eastern Kunlun

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Mountains.

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4.3 Sediment flux in the Qaidam Basin

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The sediment flux in the Qaidam Basin increased gradually from 53.5 to 35.5 Ma, and the flux changed from 126.6 t Ma-1 m-2 in the Lulehe Fm. to 339.1 t Ma-1 m-2 in

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the Upper Xia Ganchaigou Fm. (Fig. 7, Table 3). The sediment flux decreased

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considerably to its lowest value of 96.7 t Ma-1 m-2 between 35.5 and 22 Ma. The sediment flux increased gradually from 22 to 2.5 Ma and dramatically after 2.5 Ma to

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637.5 t Ma-1 m-2. Because the Qigequan Fm. was partially eroded during the

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Quaternary and because we used the present thickness to calculate the sediment flux, the actual sediment flux during the deposition of the Qigequan Fm. is likely larger

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than the amount reported in this paper.

5. Discussion

Increases in denudation rates have been suggested to be caused by tectonic uplift and climate change (Xia, 1995; Molnar, 2004). Climate can influence denudation rates in two ways. A warm and humid climate generally favors intense weathering and denudation (Xia, 1995; White and Blum, 1995; Edmond and Huh, 1997; West et al., 2005; Clift et al., 2008), and the oscillation between cold-dry and warm-wet climates 11 1.

ACCEPTED MANUSCRIPT may incise and denude surfaces more rapidly than stable climates of any type (Zhang et al., 2001; Molnar, 2004). In particular, high-frequency climate oscillations between two extremes cause more denudation than low-frequency oscillations (Zhang et al., 2001; Molnar, 2004). Therefore, the relationships between denudation, tectonic uplift

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and climatic change are complex.

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To understand the roles that climate and tectonics played in changing the

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sediment flux in the Qaidam Basin, we compared the changes of sediment flux in the Qaidam Basin with changes in the shortening rate in the Qaidam Basin, regional

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tectonic events, and regional and global climate proxies (Fig. 8). The regional tectonic

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events were summarized from sedimentological, structural, and thermochronologic studies that suggest deformations in the Qilian, Altyn Tagh, and Kunlun Mountains.

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The regional climate proxies include pollen records from the Dahonggou section in

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the northeastern Qaidam Basin (Song et al., 2013) and the KC-1 and SG-3 cores in the western Qaidam Basin (Miao et al., 2013), gypsum and halite compositions and illite

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crystallinity (Wang et al., 2013), the chemical index of alteration (CIA) and the

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chemical index of weathering (CIW, Song et al., 2013) in the Dahonggou section, and the SO42− and Cl− geochemical records from the late Cenozoic sedimentary sequence (ca. 15.3 to 1.8 Ma) of the Huaitoutala section in the northeastern Qaidam Basin (Ying et al., 2016). We used the δ18O values of benthic foraminifera (Zachos et al., 2001, 2008) as a record of the global climate. 5.1 Tectonic and climate controls from 53.5 to 40.5 Ma The sediment flux from 53.5 to 43.8 Ma was relatively low, and the flux 12 1.

ACCEPTED MANUSCRIPT increased slightly from 43.8 to 40.5 Ma (Fig. 7). We suggest that both regional tectonics and climate change controlled the denudation in the source terrenes and contributed to the sediment flux, but tectonics may have caused the slight increase from 43.8 to 40.5 Ma (Fig. 8). A summary of the tectonic deformation in the Qaidam

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Basin and the surrounding mountains shows that the region experienced crustal

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shortening and thickening during the late Paleocene-Eocene. Structural analysis

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suggests that the dominant structure in the northern Qaidam Basin started to develop in the Paleocene-early Eocene (Yin et al, 2008). The sedimentary facies, paleocurrent

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directions, and provenance analysis of the Lulehe and Dahonggou sections in the

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northern Qaidam Basin suggest exhumation of the Kunlun Mountains during the late Paleocene-early Eocene (Yin et al., 2002) and major fault movement and crustal

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shortening in the southern Qilian region and northern Qaidam Basin between 53.5 and

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40.5 Ma in response to the collision between the Indian and Eurasian plates (Yin et al., 2002; Zhuang et al., 2011b; Bush et al., 2016). The increase of sediment flux in the

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Qaidam Basin from 43.8 to 40.5 Ma was associated with the increase of the average

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shortening rate to 120.25 cm ka-1, which suggests increases in crustal shortening and plateau uplift. This period of an increase in the shortening rate was also documented in a previous study that examined the shortening rate based on the restoration of six other balanced cross-sections in the Qaidam Basin (Zhou et al., 2006). Therefore, tectonic movements contributed to the relatively high sediment flux during this period, and the exhumation of the Qilian Mountains may have contributed to the increase in sediment flux between 43.8 and 40.5 Ma. 13 1.

ACCEPTED MANUSCRIPT By comparing the Cenozoic sediment flux in the Qaidam Basin with climatic proxies, we found that the relatively warm and humid regional climate also influenced on the sediment flux between 53.5 and 43.8 but was not the major cause of the increase of sediment flux from 43.8 to 40.5 Ma (Fig. 8). Montmorillonites became the

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primary clay minerals in the Dahonggou section between 48 and 40.5 Ma, which

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suggests that the regional climate became warm and humid (Song et al., 2013). The

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warm and humid climate is also supported by the increase of illite crystallinity (Wang et al., 2013) and the decrease of gypsum and halite abundances between 48 and 40.5

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Ma (Wang et al., 2013) in the Dahonggou section. The pollen assemblages during this

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period mainly consisted of subtropical broad forests, thermophilic plants and few xerophilous plants (Fig. 8f-h; Song et al., 2013), and the CIA and CIW values (Fig. 8k,

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l) in the Dahonggou section were relatively high (Song et al., 2013), both of which

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suggest that the regional climate was somewhat warm and humid. The warm regional climate is consistent with the warm global climate that is recorded in benthic

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foraminifera δ18O values during the early-middle Eocene (Fig. 8o; Zachos et al., 2001,

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2008). Although the relatively warm and humid climate affected the sediment flux between 53.3 and 40.5 Ma, both regional and global climate proxies do not show changes to a more humid and warmer climate, or frequent climate oscillations that favor intense denudation from 43.8 to 40.5 Ma. 5.2 Tectonics played a major role from 40.5 to 35.5 Ma The sediment flux continued to increase between 40.5 and 35.5 Ma (Fig. 7), and we suggest that the increase was mainly a result of tectonics (Fig. 8). Although the 14 1.

ACCEPTED MANUSCRIPT average shortening rate in the Qaidam Basin decreased slightly, two lines of evidence suggest that the surrounding mountains experienced intense exhumation during this period. Thermochronologic studies have revealed that the Altyn Tagh was exhumed during the late Eocene-early Oligocene (Jolivert et al., 2001; Liu et al., 2007a) and

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that the eastern Kunlun Mountains were exhumed during the late Eocene (Jolivert et

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al., 2001; Clark et al., 2010). Sediment derived from the Kunlun Mountains

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dominated the deposition in the Qaidam Basin during this period (Bush et al., 2016). By reviewing climate records, we found that the change to a drier and colder

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climate did not favor the increase of denudation during this period. The pollen record

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in the Dahonggou section suggests that the regional climate became relatively cold and arid after 40 Ma (Fig. 8f-h; Song et al., 2013). The illite abundance (Fig. 8i) and

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CIA and CIW values (Fig. 8k, l; Song et al., 2013) in the Dahonggou section all

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decreased after 40 Ma, which supports the change to a more arid and dry climate. Moreover, palynological assemblages in the western Qaidam, Hoh Xil, Xining,

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Jiuquan and Hetao basins all indicate a relatively arid climate during the late Eocene

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in the northern and central Tibetan Plateau under the influences of a northwestern Chinese subtropical high and Tibetan uplift (Zhu et al., 1985; Wang et al., 1999; Fu et al., 1994; Dupont-Nivet et al., 2008; Miao et al., 2008, 2010, 2016). The benthic foraminifera δ18O values increased, which suggests that the global climate became cold during this period (Fig. 8o; Zachos et al., 2001, 2008). Therefore, we suggest that climate change did not cause the increase of sediment flux in the Qaidam Basin from 40.5 to 35.5 Ma. 15 1.

ACCEPTED MANUSCRIPT 5.3 Tectonic and climate controls from 35.5 to 22 Ma The sediment flux in the Qaidam Basin reached its minimum in the Cenozoic between 35.5 and 22 Ma (Fig. 7), and we suggest that both tectonics and climate change contributed to the low sediment flux (Fig. 8). The average shortening rate also

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reached its minimum during the Cenozoic, possibly in response to the weak

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collisional stress between the Indian and Eurasian plates (Zhu et al., 2006; Lu et al.,

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2009; Zhuang et al., 2011b). The Altyn Tagh sinistral strike-slip fault initially fractured, and the crustal shortening rate was slow in the northern Qaidam Basin and

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southern Qilian region during this period (Meng and Fang, 2008; Zhuang et al.,

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2011b). The tectonic activity in the southern depression of the Qaidam Basin also weakened between 35.5 and 26.5 Ma (Fig. 8c-e; Zhu et al., 2006). These two lines of

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evidence suggest that the tectonic setting of the Qaidam Basin was stable during this

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time.

Our summary of climate records suggests that the cold and dry climate also

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contributed to the low sediment flux during this period. Pollen record in the

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Dahonggou section shows that the proportion of broadleaf forests decreased, thermophilus plants disappeared, and the proportion of xerophilous plants increased during this period (Fig. 8f-h; Song et al., 2013). The illite content remained low (Fig. 8i), and the CIA and CIW values were relatively low from 35.5 to 27 Ma (Fig. 8k, l; Song et al., 2013), which suggests that the regional climate became dry and cold. This aridification was also documented in the sedimentary record in the Xining Basin, and the timing of the initiation was placed at the Eocene-Oligocene boundary 16 1.

ACCEPTED MANUSCRIPT (Dupont-Nivet et al., 2007). Benthic foraminifera δ18O values show that the global climate cooled dramatically at the Eocene-Oligocene boundary in association with Antarctic glaciation and the cold climate remained during the early Oligocene (Fig. 8o; Zachos et al., 2001, 2008). Therefore, the cold and dry climate during this period also

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contributed to the lowest sediment flux in the Qaidam Basin.

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5.4 Tectonic and climate controls after 22 Ma

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The sediment flux increased from 22 to 2.5 Ma and became significantly high after 2.5 Ma (Fig. 7). We suggest that both tectonics and climate change contributed

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to the high sediment flux after 22 Ma (Fig. 8). The average shortening rate in the

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Qaidam Basin increased gradually between 22 and 2.5 Ma, which suggests gradual increases in crustal shortening and thickening (Fig. 8b). The increase in sediment flux

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is consistent with the acceleration of the sedimentation rate at 14.7 Ma, 8.1 Ma, and

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3.6 Ma in the Huaitoutala section in the eastern Qaidam Basin (Fang et al., 2007) and the gradual increase in the sedimentation rate in the Tiejianggou section in the

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northern Tibetan Plateau (Sun et al., 2005a). Our summary of the tectonic events in

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the northern Tibetan Plateau suggests that an increase in the intensity of tectonic activity after 22 Ma contributed to the increase of the sediment flux. The presence of growth strata in the southwestern Qaidam Basin suggests that the Qimen Tagh was uplifted in the early Miocene and that the Eastern Kunlun Mountains were uplifted in the mid-Miocene (Mao et al., 2014). Thermochronologic and magnetostratigraphic evidence from the northern Qaidam Basin suggest widespread contractional deformation in the northern Qaidam Basin and large magnitude strike-slip motion 17 1.

ACCEPTED MANUSCRIPT along the Altyn Tagh Fault since the Miocene (Sun et al., 2005a; Ritts et al., 2008; Lu et al., 2009, 2012, 2014; 2015). Sedimentary facies, paleocurrent directions and depositional pattern analysis revealed that the crustal deformation increased, and the thickness of the strata increased rapidly during the Miocene (Zhuang et al., 2011b).

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Angular unconformities formed between the Shang Youshashan and Shizigou Fms.

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and between the Shizigou and Qigequan Fms. in the Honggouzi section in the western

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Qaidam Basin (Song et al., 2014), which indicate intense tectonic activity. Our isopach patterns show a shift of the depocenter to the front of the Eastern Kunlun

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Mountains, which suggests uplift of the Eastern Kunlun Mountains after 15.3 Ma (Fig.

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6g). Finally, several published thermochronologic studies have suggested that the Altyn Tagh, Eastern Kunlun and Qilian Mountains experienced rapid exhumation at

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~10 Ma (Fig. 8c-e; Jolivet et al., 1999, 2001; Liu et al., 2007a; Zhang et al., 2012a;

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Duvall et al., 2013; Kirby et al., 2002; Zheng et al., 2010). The dramatic increase in the sediment flux after 2.5 Ma is consistent with the

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dramatic increases in the average shortening rate in the Qaidam Basin (Fig. 8) and the

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sedimentation rates in the Huaitoutala and Qigequan sections (Fang et al., 2007; Zhang et al., 2013). The changes in the sedimentation rate and the development of unconformities in the Qigequan section in the western Qaidam Basin suggest four phases of tectonic uplift in the Altyn Tagh, which occurred at approximately 3.6 Ma, 2.5 Ma, 1.1 Ma and 0.8 Ma (Zhang et al., 2013). However, another study, suggested that rapid uplift of the northern Tibetan Plateau occurred after 8 Ma, with five large-scale rapid uplift events between 3.6 and 0.15 Ma (Fig. 8c-e; Li et al., 2014). 18 1.

ACCEPTED MANUSCRIPT The dramatic increase in the sediment flux is also consistent with the increase in the sediment grain size in the Qigequan Fm. in most regions of the Qaidam Basin, which has been suggested to be a result of tectonic uplift (Zhang et al., 2006, 2013; Fang et al., 2007; Song et al., 2014). Additionally, the vicinity of the Tian Shan experienced

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rapid uplift since ~ 3 Ma (Bullen et al., 2001, 2003; Zhang et al., 2014a). Therefore,

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the acceleration in the sediment flux after 2.5 Ma was associated with intense tectonic

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activity in the northern Tibetan Plateau.

Our summary of climate records suggests that the warm and humid climate during

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the early-middle Miocene also contributed to the high sediment flux between 22 and

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15.3 Ma. From 27 to 18 Ma, gypsum and halite completely vanished, the illite content became higher than in the Oligocene (Fig. 8i, j; Wang et al., 2013), and the CIA and

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CIW values were relatively high (Fig. 8k, l; Song et al., 2013) in the Dahonggou

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section, which suggests that the regional climate became warm and humid. The pollen assemblages in the Qaidam Basin (Wang et al., 1999) and in the high-elevation

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foreland basin of the Tianshan range (Sun and Zhang, 2008) both show that Betula

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and thermophilic taxa were common and that the proportions of conifers and xerophilous plants were relatively low from 18 to 15 Ma (Wang et al., 1999; Sun and Zhang, 2008), which indicate a warm and humid early Miocene regional climate. The warm and humid regional climate is generally consistent with the global climate change. The benthic foraminifera δ18O values decreased between 26 and 15 Ma, and the Earth entered the early Miocene warm period (Fig. 8o; Zachos et al., 2001, 2008). Therefore, the warm and humid climate may have led to an abrupt increase in the 19 1.

ACCEPTED MANUSCRIPT sediment flux. Our summary of the climate records suggests that the high-frequency climate oscillations after 15.3 Ma, and particularly the glacial-interglacial oscillations after ~2.5 Ma, contributed to the high sediment flux after 15.3 Ma. The pollen records from

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the KC-1 and SG-3 cores indicate that xerophilous plants have increased steadily

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since 14 Ma in the Qaidam Basin, while arbor plants have decreased (Fig. f-h; Miao et

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al., 2013). The gypsum and halite contents gradually increased in the Dahonggou section since 13.5 Ma, and the illite crystallinity was relatively low (Fig. 8i, j), which

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indicate that the climate steadily shifted to cold and dry (Wang et al., 2013). The

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geochemical records of SO42− and Cl− in the Huaitoutala section have gradually increased since 13 Ma (Fig. 8m, n; Ying et al., 2016) and the amounts of SO42− and

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Cl− in the Honggouzi section has also increased since 11.1 Ma (Song et al., 2014),

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which suggest that the regional climate in the Qaidam Basin has become drier since ~12 Ma. The carbonate δ18O and δ13C values from the Huaitoutala section increased

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after ~12 Ma, which suggests that the aridity in Central Asia intensified at ~12 Ma

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(Zhuang et al., 2011a). The change to a cold and dry regional climate after 15.3Ma is consistent with the global climate pattern. The benthic foraminifera δ18O values have increased gradually since ~15 Ma, which suggests that the global climate became cold after ~15 Ma (Fig. 8o; Zachos et al., 2001, 2008). The variations of the δ18O values show that both the amplitude and frequency of global climate oscillations appear to have increased since ~15 Ma, and the climate appears to have been more variable between 3 and 4 Ma and between 1 and 1.5 Ma in response to the glaciation in the 20 1.

ACCEPTED MANUSCRIPT northern hemisphere and glacial-interglacial cycles (Fig. 8o; Zachos et al., 2001, 2008). The cold climate associated with glaciation after ~3-4 Ma has been suggested to have accounted for the increase in the global sediment accumulation rate beginning at 3–4 Ma (Hays et al., 1976; Heinrich, 1988; Zhang et al., 2001; Molnar, 2004; Clift,

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2010). Therefore, we suggest that the increases in the magnitude and frequency of

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global climate changes contributed to the high sediment flux in the Qaidam Basin

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after the middle Miocene.

In summary, it is not possible to separate the competing influences of climate and

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tectonic uplift on the changes in the sediment flux in the Qaidam Basin. By comparing

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the sediment flux with the average shortening rate in the Qaidam Basin, tectonic events in the northern Tibetan Plateau, and regional and global climate proxies, we

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suggest that the intense tectonic uplift and relatively warm and humid climate

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between 53.5 and 40.5 Ma caused a relatively high sediment flux, the tectonic uplift from 40.5 to 35.5 Ma further increased the sediment flux, the stable tectonic setting

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and the cold and dry climate between 35.5 and 22 Ma decreased the sediment flux,

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both intense tectonic uplift and the relatively warm and humid climate during 22-15.3 Ma caused a high sediment flux; and both tectonic uplift and frequent climate oscillations after 15.3 Ma, particularly the glacial-interglacial cycles after 2.5 Ma, caused the high sediment flux after 15.3 Ma and dramatic increase after 2.5 Ma. 6. Conclusions In this study, we used the restoration of seven N-S balanced cross-sections and strata thickness data compiled from ten outcrop sections and four boreholes to 21 1.

ACCEPTED MANUSCRIPT reconstruct basin boundaries and create isopach maps for different periods of deposition in the Qaidam Basin during the Cenozoic. The new isopach maps were then used to calculate the sediment flux in the Qaidam Basin. Because the basin is the largest closed Cenozoic basin in the northern Tibetan Plateau, the sediment flux is

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considered to be equal to the amount of denudation in the basin-bounding mountains.

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Our results show that the sediment flux in the Qaidam Basin increased gradually

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between 53.5 and 35.5 Ma, decreased to its lowest value from 35.5 to 22 Ma, increased between 22 and 2.5 Ma, and dramatically increased after 2.5 Ma. By

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comparing these results with the reconstructed shortening rate in the Qaidam Basin,

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the regional tectonics in history of the northern Tibetan Plateau and the regional and global climate, we found that the Cenozoic sediment flux in the Qaidam Basin was

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controlled by both climate and tectonics. From 53.5 to 40.5 Ma, the gradual increase

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in the sediment flux was controlled by both intense tectonic activity and a relatively warm-humid climate, and the continued increase from 40.5 to 35.5 Ma was mainly

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controlled by tectonics. The sediment flux was the lowest from 35.3 to 22 Ma due to

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both a stable tectonic setting and a relatively dry and cold climate. The increase in the sediment flux from 22 to 15.3 Ma was controlled by both tectonic uplift and a relatively warm-humid climate. Since 15.3 Ma especially after 2.5 Ma, both tectonic uplift and frequent climate oscillations caused a high sediment flux. Acknowledgements We would like to thank the editor Sierd Cloetingh and other anonymous reviewers for their valuable comments. This work was co-supported by the (973) 22 1.

ACCEPTED MANUSCRIPT National Basic Research Program of China (2013CB956403), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant no. XDB03020402), the NSFC grants (41272128, 41330745, 40902015) and the Fundamental Research Funds for the Central Universities the Fundamental Research Funds for the Central

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Universities (2022016zr0198). Many thanks are given to T. Zhang, Q.Q. Meng, B.S.

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Li, P.J. He, H. Ying, C. Liu, L.F. Ma and J.Q. Pan for their valuable field and

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laboratory assistance.

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Figure and Table Captions: Fig. 1 Location of the Qaidam Basin and its surrounding mountains in the northern Tibetan Plateau.

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Fig. 2 Structural features within the Qaidam Basin (modified from Zhang, 2006) and the locations of the seven seismic profiles, ten measured sections, and four boreholes. QGQ, XCG, HGZ, HSG, YH, LM, LLH, DHG, HTTL and NG represent the Qigequan,

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Xichagou, Honggouzi, Hongsanhan, Yahu, Lake Mahai, Lulehe, Dahonggou,

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Huaitoutala and Naoge sections, respectively.

Fig. 3 Cross-sections constructed from depth-migrated seismic profiles. Section 1 is

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quoted from Zhang et al. (2012c); sections 2, 3, 4, and 5 are quoted from Zhou et al.

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(2006); and section 6 is quoted from Liu et al. (2008).

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Fig. 4 Sequential restorations of section 5. The location of this profile is shown in Fig.

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Fig. 5 Cumulative shortening ratio versus time for the seven 2D seismic profiles in the

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Qaidam Basin (a-g), and the average shortening rate (h). Data are summarized in Table 1.

Fig. 6 Reconstructed isopach maps of the Qaidam Basin.

Fig. 7 Changes in the sediment flux in the Qaidam Basin.

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ACCEPTED MANUSCRIPT Fig. 8 Sediment flux of the Qaidam Basin is compared with tectonic events and major climate changes during the Cenozoic. (A) Sediment flux in the Qaidam Basin versus time (a, this study). (B) Tectonic events in and around the Qaidam Basin during the Cenozoic, including average shorting rate in the Qaidam Basin (b, this study); tectonic

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events in the Qilian Mountains (c), Altyn-Tagh Mountains (d) and Kunlun Fault (e),

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respectively. Each pentagon represents a tectonic event, which includes onset of thrust 33,84

Yin et al., 2008), apatite

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faulting (28,32Yin et al., 2002; 4Tapponnier et al., 2001;

fission track cooling ages, (U-Th)/He ages and thermal models from the northeastern Jolivet et al.,2001; 81Jolivet et al., 2003;

Zheng et al., 2010; 89Clark et al., 2010; 87Mock et al., 1999;

72,78

40,61

Zhang et al., 2012a;

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7

6,36

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Tibetan Plateau (68Jolivet et al., 1999;

Duvall et al., 2013; 85Liu et al., 2005; 42,70Liu et al., 2007a; 51Yue et al., 2003; 19Xu

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et al., 2015; 21George et al., 2001; 73Kirby et al., 2002; 80Yuan et al., 2006; 45Wang et

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al., 2006; 82Cheng et al., 2015; 35Wang et al., 2015; 8Wang et al., 2016a; 17Wang et al., 2016b; Qi et al., 2016), changes in sedimentary characteristics and rock magnetism of

Chen et al., 2004;

1,20

Fang et al., 2007;

66

Chang et al., 2012; 9Fang et al., 2013;

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46,60

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sediments on the northeastern margin of the Tibetan Plateau (76Zheng et al., 2000;

62

Sun et al., 2005a; 27Sun et al., 2005b; 57Ritts et al., 2008; 47Ritts et al., 2004;

et al., 2009; 2Li et al., 2014; 56,79Chang et al., 2015; 48,55Wu et al., 2012;

52

10,24

Lu

Yue et al.,

2001; 43Hanson, 1999; 44Meng et al., 2001; 22,37Zhuang et al., 2011b; 11,14Zhang et al., 2012b; 16,26,75

12,71

Wang et al., 2003;

Craddock et al., 2011;

31,92

3,88

Metivier et al., 1998;

Bush et al., 2016;

65

77

Zhang et al., 2013;

Zhang et al., 2016), seismic

profile studies, growth strata and unconformity studies (39Bally et al., 1986; 38Wang et 42 1.

ACCEPTED MANUSCRIPT al., 1990; 83Mao et al., 2014; 64,77Zhang et al., 2013), vertical-axis rotation (50,58Lu et al., 2014; 5,15Lu et al., 2012), stable isotopes (13,74Yuan et al., 2011; 23Garzione et al., 2005) and others (86Wang et al., 2014; 18,25Lu et al., 2015; 54Cowgill et al., 2000). (C) Regional and global climate changes during the Cenozoic, including pollen records in

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the Dahonggou section (f, Song et al., 2013) and KC-1 and SG-3 core (g, Miao et al.,

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2013), and thermophilic elements and Xerophilous elements (h, Miao et al., 2013;

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Song et al., 2013); bulk clay mineral contents of the sediments, including illite crystallinity (i) and content of gypsum and halite (j) in the Dahonggou section (Wang

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et al., 2013); chemical weathering indices CIA (k) and CIW (l) in the Dahonggou

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section (Song et al., 2013); geochemical proxy records for soluble anions SO42- (m) and Cl- (n) in the Huaitoutala section (Ying et al., 2016); and global climate record

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based on δ18O values of benthic foraminifera (o, Zachos et al. 2001, 2008). (D)

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Summary of major controlling factors on the sediment flux in the Qaidam Basin.

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Table 1. Shortening amount, ratio, and rate deduced from seven cross-sections (see

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Figure 2 for locations).

Table 2. Compiled thickness data of different measured sections and cores in the Cenozoic Qaidam Basin (see Figure 2 for locations of the sections and cores).

Table 3. Sediment flux results for different periods of deposition in the Qaidam Basin.

43 1.

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Fig. 2

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Fig. 5

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Table 1. Average Total

Total

shortening

shortening

(km)

ratio (%)

Shortening

Cross-section

53.5-43.

rate

number

43.8-40.

8

(cm/ka)

5

40.5-35.

22-15.

15.3-8.

8.1-2.

3

1

5

(Ma)

(Ma)

(Ma)

35.5-22 5

2.5-0

(Ma) (Ma)

17

(Ma)

21

17

38

47

2

111

7

56

6

434

2

37

25

69

27

97

85

19

48

2

112

576

3

35

23

66

71

59

18

18

4.5

50

60

503

4

59

23

111

91

235

142

2

106

137

116

486

5

33

15.

62

56

140

132

16

3

75

57

225

6

29

15

54

34

69

108

36

37

68

50

112

7

12

23

27

22

28

8

19

32

19

71

9

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Formation

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1

D

(Ma)

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(Ma)

Shortening rate (cm/ka)

LLH

Table 2. Thickness（m）

LM

DHG

XCG

NG

HTTL

HGZ

Qigequan

QGQ

YH

>304

248

>501

>430

HSH

SG-1

SG-1b

SG-3

~900

~100

~500

>600

KC-1

Shizigou

800

>400

1000

>400

>457

1750

434

>700

Shangyoushansha

750

600

1000

1238

1738

2250

396

2005

Xiayoushashan

450

1240

1220

1341

874

>200

>730

Shangganchaigou

700

620

1390

1467

Xiaganchaigou

1240

640

1100

Lulehe

1058

980

500

396 368

Note: Data for LLH and XCG sections are from Zhang (2006); Data for the QGQ section is from Zhang et al. (2013); DHG section (Lu and Xiong, 2009); HTTL section (Fang et al., 2007); HGZ section (Song et al., 2014); HSH section (Sun et al., 2005b); LM section (Zhuang et al., 2011b); KC-1

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Table 3. Age

Area

Volume

Formation (km )

3

(km )

density

porosity

(g/cm3)

(%)

Sediment flux (t*Ma-1/m2)

(Ma)

Qigequan

2.5-0

82048.64

85731.9

2.5

2.05

25.6

Shizigou

8.1-2.5

102665.6

71612.3

5.6

2.13

Shangyoushashan

15.3-8.1

124792.4

122988.2

7.2

2.23

Xiayoushashan

22-15.3

129047.3

113224.2

6.7

2.29

Shangganchaigou

35.5-22

131569.5

82864.63

13.5

2.34

11.4

96.7

Upper Xiaganchaigou

40.5-35.5

142194

114302

5

2.37

11

339.1

Lower xiaganchaigou

43.8-40.5

100446.2

36979.18

3.3

2.38

11.9

233.9

Lulehe

53.5-43.8

109439.3

62579.75

9.7

2.4

10.5

126.6

637.5

18.5

216.2

16.8

254

15.4

253.7

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Average

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(Ma)

2

Average Duration

ACCEPTED MANUSCRIPT Highlight: 1. Cenozoic sediment flux and shortening rates of the Qaidam Basin were studied. 2. Two relatively rapid shortening stages occurred between 43.8-35.5 Ma and 15.3-0 Ma.

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3. Sediment flux was relatively high during late Eocene and since middle Miocene.

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4. Tectonic activity and climate change both influenced sediment flux.

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