Pulsed growth of the West Qinling at ~30Ma in northeastern Tibet: Evidence from Lanzhou Basin magnetostratigraphy and provenance

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1 PUBLICATIONS RESEARCH ARTICLE Key Points: Lanzhou Basin preserves a record extending from basin formation at ~47 Ma until ~15 Ma The basin initiated as a topographically enclosed depression at ~47 Ma resulting from right-lateral transtensional deformation The pulsed uplift of the West Qinling that occurred at ~30 Ma suggests outward growth of the Tibetan Plateau Supporting Information: Supporting Information S1 Table S1 Table S2 Table S3 Data Set S1 Correspondence to: W. Wang, taotaowang@126.com Citation: Wang, W., P. Zhang, C. Liu, D. Zheng, J. Yu, W. Zheng, Y. Wang, H. Zhang, and X. Chen (2016), Pulsed growth of the West Qinling at ~30 Ma in northeastern Tibet: Evidence from Lanzhou Basin magnetostratigraphy and provenance, J. Geophys. Res. Solid Earth, 121, , doi:. Received 16 JUN 2016 Accepted 2 NOV 2016 Accepted article online 5 NOV 2016 Published online 18 NOV American Geophysical Union. All Rights Reserved. Pulsed growth of the West Qinling at ~30Ma in northeastern Tibet: Evidence from Lanzhou Basin magnetostratigraphy and provenance Weitao Wang 1, Peizhen Zhang 1,2, Caicai Liu 1, Dewen Zheng 1, Jingxing Yu 1, Wenjun Zheng 1,2, Yizhou Wang 1, Huiping Zhang 1, and Xiuyan Chen 3 1 State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing, China, 2 School of Earth Science and Geological Engineering, Sun Yan-Sen University, Guangzhou, China, 3 Research Institute of Petroleum Exploration and Development, Beijing, China Abstract The development of Cenozoic basins in the northeast margin of the Tibetan Plateau is central to understanding the dynamics of plateau growth. Here we present a magnetostratigraphy from the Lanzhou Basin, dating the terrestrial deposits from the Eocene (~47 Ma) to the middle Miocene (~15 Ma). The stratigraphic observation, palocurrent, and sediment provenance analysis suggest that the Lanzhou Basin (subbasin of the Longzhong Basin) probably initiated as a topographically enclosed depression during Eocene to early Oligocene (~47 30 Ma). We suspect that right-lateral transtensional deformation inherited from the Cretaceous may result in formation of the Lanzhou Basin at the Eocene. Subsequently, changes in paleocurrent, sandstone and conglomerate compositions and detrital zircon provenance reflect the pulsed growth of the West Qinling at ~30 Ma, which triggered not only the formation of new flexural subsidence to the north of the West Qinling, but also renewed subsidence of Lanzhou Basin into the broad foreland basin system. We compare this growth history with major NE Tibet deformation and suggest that it may result from northeastward extrusion of the Tibetan Plateau due to the onset of Altyn Tagh Fault activity at Oligocene. 1. Introduction The most impressive geologic feature of the northeastern Tibetan Plateau, which is defined by the East Kunlun Fault to the south, the Altyn Tagh Fault to the west, and the Haiyuan Fault to the north and east, is numerous Cenozoic sedimentary basins separated by a series of WNW or NW trending ranges (Figure 1a). These basins and ranges documented shortening associated with outward growth of the Tibetan Plateau during the Cenozoic [e.g., Clark et al., 2010; Dupont-Nivet et al., 2004; Molnar, 2005; Yuan et al., 2013]. Early studies show that the northeastern Tibetan Plateau was a rising plateau in the late Miocene to Pleistocene; deformation progressively propagated from an initially elevated plateau in the south to the northern parts of the Tibetan Plateau, either steadily [England and Houseman, 1986] or in steps [Meyer et al., 1998; Tapponnier et al., 2001]. However, recent studies have yielded evidence that the deformation and surface uplift in the northeastern Tibetan Plateau may have started much earlier, such as Paleocene to Eocene deformation within the Qaidam Basin [Yin et al., 2008a; Zhuang et al., 2011], the Qilian Shan [Yin et al., 2008b], West Qinling [Clark et al., 2010; Duvall et al., 2011], and the Longzhong Basin [Dai et al., 2006; Dupont-Nivet et al., 2004]. This led many researchers to consider that the deformation in the modern northeastern margin of the Tibetan Plateau initiated shortly after the onset of the Indo-Asian collision. Terrestrial deposits within Cenozoic basins provide important constraints on Tibetan Plateau growth. For example, the suggestions of early Tertiary deformation in the northern Tibetan Plateau [Clark et al., 2010; Yin et al., 2008a] imply the onset of flexural basins in the region as early as ~50 Ma; on the contrary, the northward stepwise deformation pattern suggests gradually younger generation of basins from south to north [Métivier et al., 1998; Meyer et al., 1998]. Thus, the depositional history of sedimentary basins in the northeastern Tibetan Plateau may serve as potential tester for distinguishing different deformational models that predict the growth of regionally extensive plateaus. Although many studies have been conducted on the basins in the northeastern Tibetan Plateau [Dai et al., 2006; Dupont-Nivet et al., 2004; Fang et al., 2003, 2005, 2007; Horton et al., 2004; Lease et al., 2012; Lu and Xiong, 2009; Wang et al., 2011, 2013, 2016; Xiao et al., 2012; Yin et al., 2008a, 2008b], the Eocene to Miocene history of basin evolution is also subject of debate. Some authors WANG ET AL. CENOZOIC GROWTH OF THE WEST QINLING 7754

2 Figure 1. (a) Shaded relief map of the northeast Tibetan Plateau and its adjacent Songpan-Ganzi terrane showing distribution of basins, ranges, and major active faults in the region. Inset map shows the location of the Figure 1a. AFT (A in the inset map), Altyn Tagh Fault; EKLF (K), East Kunlun fault; HYF (H), Haiyuan fault; WQLF, West Qinling Fault; LMF, Longmen Shan fault; XB, Xorkol Basin; QB, Qaidam Basin; SB, Subei Basin; GB, Gonghe Basin, LMS, Longmen Shan. (b) Generalized geological map of the West Qinling, East Qilian Shan and the Longzhong Basin with locations of magnetostratigraphic Xianshuihe section (stars), detrital zircon samples from the East Qilian Shan and West Qinling, and subbasins of the Longzhong Basin. WQLF, West Qinling Fault; HYF, Haiyuan Fault. argue for basin formation in the early Cenozoic as extensional pull-apart basins [Liu et al., 2015; Zhang et al., 2016], whereas others suggest that a broad foreland basin system covered the NE Tibetan Plateau during the early Cenozoic [Clark et al., 2010], and this basin was separated by activity of folds and faults in the middle Miocene [e.g., Lease et al., 2007, 2012; Wang et al., 2013; Yuan et al., 2013]. To better constrain the Cenozoic basin evolution and growth of the NE Tibetan Plateau requires well-dated long and continuous basin records. The Lanzhou Basin is a subbasin of the Longzhong Basin within the northeastern Tibetan Plateau (Figure 1b). Incision by the Yellow River and its tributaries has exposed the Paleocene to Quaternary sequence with numerous index fossils. Recently, the magnetostratigraphy and paleoenvironment have been studied WANG ET AL. CENOZOIC GROWTH OF THE WEST QINLING 7755

3 Figure 2. Geological map of the Lanzhou Basin modified from GBGMR [1965, 1989]. Shown are the distributions of three Cenozoic stratigraphic units (Xiliugou, Yehucheng, and Xianshuihe Formations), two Quaternary subdivisions (Q3 and Q4), and magnetostratigraphic sections (XSH, Xianshuihe section; FHS, Fenghuangshan section). [Qiu et al., 2001; Sun et al., 2011; Yue et al., 2001; Zhang et al., 2014]. However these studies have mainly focused on aeolian accumulation and regional aridification [Sun et al., 2011; Zhang et al., 2014], with little attention to the provenance and sedimentary facies changes in the Lanzhou Basin that may result from the growth of the northeastern Tibet. In this paper, we present a detailed magnetostratigraphy as well as provenance analyses of the Eocene to Miocene deposits preserved in the basin to evaluate the basin evolution. The evolution history of the Lanzhou Basin offers insights into tectonic deformation and growth of ranges in the northeastern Tibetan Plateau. 2. Geological Setting The Longzhong Basin is a fault-bounded basin, located in the northeast part of the topographic front on the northeastern Tibetan Plateau. The basin is bounded by the West Qinling Fault (WQLF) to the south, the Haiyuan Fault (HYF) to the west and north, and the Liupanshan Fault (LPSF) to the east (Figure 1a). The WQLF, extending roughly E-W for ~500 km, is a northward thrust fault with left-lateral strike-slip component [Li et al., 2007; Z. Wang et al., 2012]. The HYF is a major left-lateral strike-slip fault that runs ~1000 km (Figure 1a). To the east, the LPSF is a N-S trending thrust fault, which is oriented east tip of the HYF and is thought to accommodate left slip on the HYF [Zhang et al., 1991]. Topographically, the West Qinling in the south, East Qilian Shan in the west, and north and Liupan Shan in the east defined the extent of the Longzhong Basin. Late Cenozoic folding and thrusting along the Laji-Jishi Shan, Maxian Shan, and Liupan Shan generated a series of narrow ranges ( m), which divide the Longzhong Basin into several subbasins, such as the Xunhua Basin, Linxia Basin, Xining Basin, and Lanzhou Basin sitting at m (Figure 1b). As a subdepression lying in the central part of the Longzhong Basin, the Cenozoic Lanzhou Basin is characterized by thick, red mudstone, gypsum and sandstone beds. Cenozoic terrestrial strata mainly outcrop to the northwest of Lanzhou City, covering an area of ~300 km 2. These strata were deformed into a syncline with Neogene in the core and Paleogene at the eastern and western limbs (Figure 2). The axis of the syncline trends northwest, parallel to the eastward thrusting Zhuanglang fault, suggesting that formation of the Lanzhou syncline might be closely related to activation of the Zhuanglang thrust fault. In the Dahong Shan of the western Lanzhou Basin, Cenozoic rocks unconformably overlie Lower Cretaceous dark red WANG ET AL. CENOZOIC GROWTH OF THE WEST QINLING 7756

4 Figure 3. Lithostratigraphic correlations of the Xianshuihe and Fenghuashan section in the Lanzhou Basin and the paleocurrents as well as clastic compositions of the sandstones and conglomerates from the Xianshuihe section (Q: quartz; F: feldspar; Ls: sedimentary rock fragments; Lg: granite rock clasts; Lm: limestone fragments or clasts). mudstones and sandstones (Figure 2). Farther to the west and north of the Lanzhou Basin, the East Qilian Shan consists largely of Paleozoic sedimentary rocks and Lower Paleozoic plutons, which are interpreted to relate to Early to Middle Paleozoic subduction and collision between the Kunlun-Qaidam block and North China Craton [Xu et al., 2006, 2010]. South of the basin, the West Qinling are dominated by Triassic submarine fan deposits and Permo-Triassic plutons that are bounded by the WQLF [Li et al., 2014]. WANG ET AL. CENOZOIC GROWTH OF THE WEST QINLING 7757

5 Figure 4. Photographs showing the sedimentary characteristics in the Xianshuihe section. (a) Well-sorted large-scale lowangle crossbedded sandstones in the lower Xiliugou Formation. The view is ~8 m in width. (b) Gypsiferous sandstone with numerous gypsum nodules and veins in the base of the Yehucheng Formation. The view is ~15 m in width. (c) Gypsummudstone cyclic alternations in the Yehucheng Formation. The dark red protruding beds are gypsum layers, and the red cavernous beds are mudstones in Figure 4c. (d) Beds of euhedral gypsum crystals showing rosette-like structures. (e) Laminated fine-grained sandstone interbedded with very thin gypsum beds in upper part of the Yehucheng Formation. (f) Thick red laminated mudstone in the lower Xianshuihe Formation. 3. Lanzhou Basin Stratigraphy Cenozoic strata in the Lanzhou Basin are dominated by sandstone, mudstone, and thick-bedded gypsum or muddy gypsum layers that were divided into four lithostratigraphic units: the Xiliugou, Yehucheng, Xianshuihe, and Linxia Formations based on lithofacies associations, depositional contacts, and fossil assemblages [Qiu et al., 2001; Yue et al., 2001]. The Xianshuihe section (XSH), located ~ 45 km northwest of the Lanzhou City, is one of the most continuous sections in the basin, containing the Xiliugou, Yehucheng, and Xianshuihe Formations. These stratigraphic units can be correlated well with other stratigraphic sections in the Lanzhou Basin through lithostratigraphic correlation (Figure 3). In the Xianshuihe section, a total thickness of 187 m of Xiliugou Formation is mainly composed of red colored, well-sorted, coarse- to fine-grained sandstones. These sandstone beds unconformably overlie the Lower Cretaceous Hekou Group. Individual beds in the sandstones are usually several meters to tens of meters thick and are laterally continuous over hundreds of meters. Sedimentary structures including large-scale, lowangle cross-stratification, planar crossbedding and horizontal bedding appear primarily in the lower part of the Xiliugou Formation (Figure 4a), whereas massive sandstone beds are common in the upper part of the Formation. Mudstones are extremely rare in this formation (Figure 3). These lithological features allow basin-wide correlation of the Xiliugou Formation. Based on the dominant sandstone lithology without mudstone, erosive bedding contacts, and macroform migration indicated by crossbedding, the depositional environment of the Xiliugou Formation is interpreted as a sandy braided river system [Cant and Walker, 1978; Miall, 1996; Ritts et al., 2004b]. WANG ET AL. CENOZOIC GROWTH OF THE WEST QINLING 7758

6 The paleocurrent orientations that were primarily determined from the cross-stratification and clast imbrications within the Xianshuihe section were measured to restore the direction of sediment transport into the basin. All of the paleocurrent data are corrected by the titled stratigraphic beds. For the sandstone-rich Xiliugou Formation, two sets of the crossbeddings are counted and received 14 and 18 data, respectively (Figure 3). Largely east-southeast directed cross stratifications in these two sets indicate that braided river system developed in the Xiliugou Formation flows toward the east or southeast. The Yehucheng Formation conformably overlies the underlying Xiliugou Formation in the Xianshuihe section. This stratigraphic unit includes a 123 m thick sandstone-rich interval in the lower part and a 365 m thick gypsum-rich interval in the upper part (Figure 3). The lower sandstone-rich interval is composed mostly of reddish sandstone, gypsiferous sandstone, mudstone, and gypsum beds. The sandstone beds are meters thick with tabular shapes and well-developed horizontal beddings. The gypsiferous sandstone beds occur in the base of this interval and largely are fine-grained sandstones with a large amount of gypsum nodules and veins (Figure 4b). The nodules are a few centimeters to tens of centimeters in size, whereas veins, millimeters to centimeters thick, are either parallel to or crosscut the sandstone beddings (Figure 4b). The sandstone lithofacies interval is separated from the gypsiferous sandstone lithofacies interval by ~10 m thick reddish laminated mudstones that are intercalated with centimeter-thick gypsum beds. The upper gypsum-rich interval of the Yehucheng Formation consists of regularly alternating of extensive laterally continuous red to dark red mudstone beds and green to gray gypsum layers (Figure 4c). Red mudstones are tens of centimeters to meters thick and are tabular. Gypsum layers show various shapes, such as tabular, nodular or alabastrine, massive, or laminar, with thickness ranging from centimeters to meters (Figures 4d and 4e). Horizontal laminations and occasionally small-scale ripple marks can be observed in the mudstone and fine-grained sandstone beds. Based on the occurrences of thick gypsiferous sandstone, thick sandstone in the lower part, and gypsum and mudstone in the upper part of the Yehucheng Formation, we infer that the Yehucheng Formation represents fluvial deposition in an arid setting gradually evolving into a shallow lacustrine environment with playa-like conditions [Dupont-Nivet et al., 2007; Schreiber and Tabakh, 2000]. Paleocurrent indicators obtained from cross stratification in sandstone in the lower part of the Yehucheng Formation show east directed flow (Figure 3). This is similar to flow direction of the underlying Xiliugou Formation. Thus, sandstones in the lower Yehucheng Formation seem to also be deposited by braided river system that derived sediments from the west or northwest and delivered them to the Lanzhou region. We did not obtain any paleocurrent data in the upper part of the Yehucheng Formation, due to lack of the crossstratification or imbricated clast in the mudstone beds and gypsum layers. The upper 830 m thick strata in our Xianshuihe section are divided into the Xianshuihe Formation, which also serves as the type section of Xianshuihe Formation. This stratigraphic unit is dominated by red mudstones in the lower part and sandstone interbedded with the mudstone beds in the upper part (Figure 3). The mudstones in the lower part are meters to tens of meters thick, massive or laminated, and laterally continuous for hundreds of meters (Figure 4f). The dominant mudstones with rare occurrence of coarse-grained sediments in the lower part suggest that they were probably deposited in a lacustrine environment. Many sandstone beds appear in the middle and upper parts of the Xianshuihe Formation. These sandstone beds are tens of centimeters to meters thick, and tabular or lenticular with erosional bases. Channel scours that exhibit tens of centimeters of relief are common. These lines of evidence suggest that the middle and upper parts of the Xianshuihe Formation were deposited in a fluvial environment [Miall, 1996]. Paleocurrents of the Xianshuihe Formation received from five sets in the middle and upper parts of this stratigraphic unit (Figure 3). All of these paleocurrent indicators show a north directed flow suggesting that main provenances were located to the south, which is distinct from that of the lower Yehucheng Formation (Figure 3). The Linxia Formation only crops out to the south of the Lanzhou Basin and consists of red clay, gray to yellowish sandstone and conglomerates. Zhang et al. [2014] interpreted the Linxia Formation as deposits of alluvialfluvial system. 4. Sampling and Measuring To assess the evolution of the Lanzhou Basin and its adjacent ranges, we carried out magnetostratigraphy study, Quantitative clastic petrography of sandstones and conglomerates as well as detrital zircon WANG ET AL. CENOZOIC GROWTH OF THE WEST QINLING 7759

7 provenance analysis. The magnetostratigraphy was conducted to set up a more accurate chronology of the Xianshuihe section; the sandstone and conglomerate compositions and detrital zircon U-Pb dating were used to trace potential source regions. Collection of magnetostratigraphic samples was started from the middle part of the Xiliugou Formation ( N and E) and ended in the upper part of the Xianshuihe Formation ( N and E). Because the lowermost (~60 m thick) Xiliugou Formation includes severely weathered coarse-grained sandstone and the uppermost (200 m thick) Xianshuihe Formation are mostly covered by loess, no oriented samples were taken from these stratigraphic levels. The magnetostratigraphic samples were collected by an electrically driven drill. At least three cores per stratigraphic horizon were drilled with a stratigraphic interval of typically 2 m. In total, 1893 core were collected from 631 horizons. All the drill cores were oriented using magnetic compass that was corrected to 1.7 for local magnetic declination anomaly. Field drill core samples were cut into standard specimens of 2 cm in length. One set of samples (631) was subjected to progressive thermal demagnetization in a TD-48 thermal demagnetizer. For thermal demagnetization, the following stepwise heating routine was performed: (1) 20 C, 100 C, and 200 C in the lower temperature; (2) 50 C steps from 250 C to 500 C; (3) 30 C/20 C steps from 500 C to 620 C; and (4) 10 C steps between 620 C and 680 C. Magnetizations were measured by a 2G Enterprises Model 760, three-axis, cryogenic magnetometer shielded in field-free space (<300 nt), at the Paleomagnetism Laboratory of the Institute of Geology and Geophysics, Chinese Academy of Sciences. All of the original paleomagnetic data are presented in Data Set S1 in the supporting information. Six sandstone samples were collected from the magnetostratigraphic Xianshuihe section for Quantitative clastic petrography analyses. The exact locations of these samples are shown in Figures 3 and 7. Thin sections were made for clast counting. In addition, two conglomerate clast count data sets were obtained from the upper part of the Xianshuihe section for provenance analyses (Figure 3). Seven sandstone samples were also collected from the Xianshuihe section for detrital zircon U-Pb dating. More than 5 kg of sandstones for each of the seven samples were taken from a single outcrop. Zircon crystals were extracted from sandstone samples following standard water table, magnetic and density separation procedures. Approximately 100 grains were randomly dated for each of the samples. Zircon U-Pb dating was carried out at the laboratory of the Institute of Geology and Geophysics, Chinese Academy of Sciences, by an Agilent 7500a inductively coupled plasma mass spectrometer (ICP-MS) equipped with a 193 nm laser excitation system. The laser ablation spot size is approximately 50 μm; the Harvard zircon 91500, GJ-1, and silicate glass NIST 610 were used as standard. Details of the analytical procedures were previously described by Xie et al. [2008]. The 207 Pb/ 206 Pb and 206 Pb/ 238 U ratios were calculated using GLITTER version 4.0 software (Macquarie University) and then corrected using zircon as an external standard with a recommended 206 Pb/ 238 U age of Ma [Wiedenbeck et al., 2004]. The 207 Pb/ 235 U ratio was calculated from values of 207 Pb/ 206 Pb and 206 Pb/ 238 U( 235 U= 238 U/137.88). For grains younger than 1000 Ma, we used the 206 Pb/ 238 U ages, where for grains older than 1000 Ma, 207 Pb/ 206 Pb ages were chosen, following Gehrels et al. [2006]. Zircon ages with 25% discordance or 10% reverse discordance were excluded. All of the 680 results that yield isotopic data with acceptable discordance and uncertainty are in listed in supporting information Table S1. 5. Paleomagnetic Results 5.1. Demagnetization and Tests For the magnetostratigraphic specimens from the section, the intensity of the natural remanent magnetization (NRM) was typically on the order of 10 2 A/m, with a range of A/m. Progressive thermal demagnetization successfully resolved multiple components of magnetization. Figure 5 shows representative demagnetization diagrams, which were evaluated using orthogonal vector diagrams [Zijderveld, 1967] and stereographic projections. Most samples show a low- and a high-temperature component (Figure 5). The low-temperature component was typically removed at C, but sometimes not until 450 C. We interpreted this low-temperature component as removal of a secondary remanent magnetization. The high-temperature component decays toward the origin, typically exhibits stable behavior between 450 and 670 C and is interpreted to reflect the characteristic remanent magnetization (ChRM). Complete WANG ET AL. CENOZOIC GROWTH OF THE WEST QINLING 7760

8 Figure 5. Orthogonal (Zijderveld) vector plots of the representative thermal demagnetization behaviors of specimens from the Xianshuihe section. Hollow (solid) circles show the declination (inclination) within the orthogonal demagnetization diagrams. NRM is the natural remanent magnetization before demagnetization, and the numbers mark the temperature steps of demagnetization. unblocking of the high-temperature component by 650 C 670 C indicates that hematite is the carrier of the magnetization in the section, but the presence of magnetite is suggested as well by an accelerated decay of the magnetization around 580 C (Figure 5). For most samples throughout the stratigraphic section, there is no significant difference in remanence direction, when it is defined by the C or C parts of the unblocking temperature spectra. This suggests that both magnetic carriers recorded the same paleomagnetic field when their remanences became fixed in the rock. WANG ET AL. CENOZOIC GROWTH OF THE WEST QINLING 7761

9 ChRM directions were determined by principal component analysis [Kirschvink, 1980] as implemented in Pmag 3.1 b2 [Jones, 2002]. At least four points, but typically five to eight points from the stable hightemperature component (>450 C), were chosen to calculate the ChRM direction. ChRM directions with maximum angular deviation (MAD) above 15 were rejected from further analyses, thus discarding 169 (27%) of the measured specimens. Virtual geomagnetic poles (VGPs) were then calculated based on the magnetic declination and inclination. Samples with VGPs < 30 were not used to define the magnetic polarity stratigraphy for the Xianshuihe section. This eliminated 29 sampled horizons. Finally, a total of 433 (69%) samples were accepted to construct the magnetic polarity sequence of the Xianshuihe section. The accepted ChRM directions and VGPs are listed in Table S2 in the supporting information. Of the 433 samples, 264 are of normal polarity and 169 are of reverse polarity. The mean directions for normal and reversal polarity poles are D g = 334, I g = 55 with k g = 7.6, α95 = 3.4, and D g = 156.4, I g = 62.7 with k g = 6.5, α95 = 4.6 in geographic coordinates and D s = 353.3, I s = 35.7 with k s =8, α95 = 3.3, and D s = 181.1, I s = 40 with k s = 7.9, α95 = 4.1 in tilt-corrected coordinates, respectively (Figure 6a and 6b), where D is declination, I is inclination, k is the precision parameter, and α95 is the radius that the mean direction lies within 95% confidence [Fisher, 1953]. The reversal test is negative at the 95% confidence level [McFadden and McElhinny, 1990], because the mean declination of normal polarity directions is biased toward west directions for 7. Considering recent local magnetic declination is also biased toward west, the negative reversal test is likely due to an unremoved recent field overprint. Because no samples were collected from the north limb of the syncline in the Xianshuihe region, the simple fold test of McElhinny [1964] was performed. We divided 433 ChRM directions from the Xianshuihe magnetostratigraphic section into eight groups, each one with thickness of m (Table 1). Before tilt adjustment, the overall mean direction for eight groups is D = 333, I = 59, k = 34.9, and α95 = 9.2 (Figure 6c), and the mean direction is significantly improved by tilt adjustment (D = 356.5, I = 37.3, k = 130.1, and α95 = 4.5 after tilt adjustment, Figure 6d). Almost 4 times increase in precision parameter defines a positive fold test for the Xianshuihe magnetostratigraphic section Magnetostratigraphic Correlation The detailed magnetic polarity stratigraphy was constructed according to the VGP latitudes from the Xianshuihe paleomagnetic data set (Figure 7). We identified 25 pairs of normal and reversed magnetozones, marked as N1-N25 and R1-R25, respectively. All of the magnetozones are represented by two or more specimens that display the same polarity. Eight single specimen intervals are shown with a half-bar in Figure 7, but not used to define polarity epochs (Figure 7). Correlation of the magnetostratigraphy to the geomagnetic polarity time scale (GPTS2012) is facilitated by several mammal fossil faunas discovered in the Lanzhou Basin. First, as mentioned above, the Quantougou local fauna (Table S3 in the supporting information) found in the top of the Xianshuihe section (Figure 7) contains small mammals, which are similar with that of Middle Miocene Tunggurianl fauna [Qiu et al., 2001]. The age of the Quantougou local fauna therefore is estimated to be similar or slightly older than the Tung-gur fauna and would be ~14 Ma [Qiu et al., 2001]. Second, there are four more faunas recognized from the Xianshuihe Formation over the basin (Table S3), indicating that the Xianshuihe Formation was deposited between the Oligocene to middle Miocene in age [Qiu et al., 2001]. Third, some edentulous mandibles and foot bones of a small-sized amynodont (probably Cadurcodon) have been found in the gypsiferous mudstone of the Yehucheng Formation near the Yehucheng village [Qiu et al., 2001], about 10 km southwest of our section. These findings suggest that at least part of the Yehucheng Formation is late Eocene in age (Table S3). Based on the fossil age constraints and spacings of observed polarity intervals, we anchor the magnetostratgraphy of the Xianshuihe section to the GPTS [Hilgen et al., 2012; Vandenberghe et al., 2012]. As show in Figure 7, three moderate long normal intervals N1-N3 are separated by two very short reverse magnetozones in the uppermost part of the Xianshuihe Formation. Considering the middle Miocene Quantougou fauna discovered in the strata above this stratigraphic level, the most reasonable correlation for this polarity pattern is provided by the three closely spaced normal chrons C5Cn.1n, C5Cn.2n, and C5Cn.3n of the GPTS. Then, the normal interval N4 and two tightly spaced relatively long normal polarity zones N5 and N6 can be readily correlated to chrons C5Dn-C6n. Below N7, local magnetozones (R8 R13) show a distinctive interval pattern, which is dominated by reversed polarity with frequent occurrence of short normal polarity zones, e.g., N8, N9, N10, N11, and N12. This pattern of magnetozones appears to be correlated with chrons C6An.1n to WANG ET AL. CENOZOIC GROWTH OF THE WEST QINLING 7762

10 Figure 6. Equal-area plots of accepted ChRMs (433 sites) from the Xianshuihe section in (a) geographic and (b) tilt-corrected coordinates, and eight site-mean directions (Table 1) of ChRM from the section (c) before and (d) after tilt adjustment. In the Figures 6a and 6b, hollow (solid) circles plot in the lower (upper) hemisphere. In Figures 6c and 6d, rectangle symbols show eight site-mean directions and red stars indicate mean directions. C6Cn.3n of the GPTS (Figure 7). Further below, the magnetozones of the lower part of the Xianshuihe Formation between ~700 and 885 m (N13 N18) are characterized by three relatively long normal polarity zones (N13, N15, and N18) interrupted by four shorter events (N14, N16, N17, and N19). The best fit to these distinctive magnetozones is sequences C7n C12n of the GPTS, if the early Oligocene Nanpoping fauna in the lower part of the Xianshuihe Formation is taken into consideration. The magnetozones of the upper part of the Yehucheng Formation are characterized by a moderately long normal interval N21 and a remarkably long normal polarity zone N22 (Figure 7). Although there are no corresponding long normal chrons to match to magnetozones N21 and N22, we correlate these two intervals to C15n-C16.2n and C17n.1n-C18n.2n, which are dominated by normal polarity [Vandenberghe et al., 2012], Table 1. Eight Site-Mean Paleomagnetic Directions From This Study a Site Thickness (m) S d n D g (deg) I g (deg) D s (deg) I s (deg) k α95 (deg) a Abbreviations are n, number of samples; S, strike; d, dip; D, magnetic declination; I, magnetic inclination; g, geographic coordinates; s, stratigraphic coordinates; k, precision parameter; α95, radius of the cone in which the mean direction lies within 95% confidence. WANG ET AL. CENOZOIC GROWTH OF THE WEST QINLING 7763

11 Figure 7. Magnetostratigraphy of the Xianshuihe section and its correlations with GPTS2012 [Hilgen et al., 2012; Vandenberghe et al., 2012]. The normal (reversed) polarity intervals are labeled N1 to N25 (R1 to R25). The half-width symbols represent questionable polarity epochs that were only defined by a single specimen. WANG ET AL. CENOZOIC GROWTH OF THE WEST QINLING 7764

12 considering the upper Eocene fossils reported from the Yehucheng Formation [Qiu et al., 2001]. In the nearby Xining Basin, a similar pattern of long normal polarity zones from the Mahalagou Formation (stratigraphically equivalent with the Yehucheng Formation) was correlated with the same GPTS intervals [Dai et al., 2006]. Given this correlation, the disappearance of gypsum beds (also the boundary of the Yuhucheng and overlying Xianshuihe Formations) in the Xianshuihe section is estimated to be at 33.2 Ma, which correlates well to the stratigraphy of the Xining Basin [Dai et al., 2006; Dupont-Nivet et al., 2007; Xiao et al., 2012]. Accordingly, the magnetozones of N20 and N23-N24 can be readily matched with C13n and C19n-C20n, respectively (Figure 7). In the Xiliugou Formation, there is a remarkably long reversed interval (R25) and a short normal interval (N25). Since no fossil mammals have been found in this formation over the basin, the R25-N25 intervals are tentatively correlated to the C20r and C21n of the GPTS. The correlations between our magnetostratigraphy and the GPTS2012 indicate that the sampled Xianshuihe section spans the time period between ~46 Ma and ~15 Ma. Based on these correlations, we calculate the sediment accumulation rates of the section. A plot of thickness versus age (Figure 8) shows four times sediment accumulation rate variations: (1) a constant high rate of 5.8 cm/kyr from ~46.0 to ~39 Ma, (2) lowest rate of 2.0 cm/kyr between ~39 and ~29 Ma, (3) ~3.0 cm/kyr during ~29 and 19 Ma, and (4) the highest rate of 8 cm/kyr from 19 to 15 Ma. These sediment accumulation rate variations are consistent with grain size distributions that coarse-grained sedimentary rock should reflect a relatively higher sediment accumulation rate. Along the Xianshuihe section, additional ~60 m coarse-grained sandstones in the lowermost part of the Xiliugou Formation were not dated. Extrapolating 60 m downward into the base of the Xiliugou Formation at the accumulation rate of 6 cm/kyr suggests initiation of the Xiliugou Formation at ~ 47 Ma, and this stratigraphic unit continues to perhaps 42.5 Ma. The complete Yehucheng Formation is determined to deposit between 42.5 and 33.2 Ma and the Xianshuihe Formation from 33.2 to <15 Ma. 6. Provenance Analysis Results 6.1. Compositions of Sandstones and Conglomerates The grain compositions of six sandstones and two conglomerates are shown in Figure 3. Figure 9 shows the representative photomicrographs of sandstone samples. Sandstone samples (C1 C4) were collected from the pre-30 Ma strata according to the magnetostratigraphy (Figure 7). These four sandstones are dominated by quartz (vary between 75% and 83%), feldspar (12 15%), and sedimentary lithic grains (5 10%). Sandstones (C5 and C6) collected from the post-27 Ma strata are dominated by quartz (61 68%), sedimentary lithic grains (22 28%), and feldspar (9 12%). The sedimentary lithic grains in the two post-27 Ma sandstones include mudstone and shale and limestone (Figure 9). The major change of compositions of pre-30 Ma and post- 27 Ma sandstones is marked by increasing in sedimentary lithic grains. This inference is also supported by the clast counting of gravels in the upper part of the section. Gravels within the upper part of the Xianshuihe Formation (Figure 3) are composed of white quartz (27 35%), gray to dark green sandstone and mudstone (30 34%), granite (13 16%), feldspar (9 12%), and limestone (5 10%) Detrital Zircon U-Pb Age Distributions Source Region Zircon U-Pb Ages Around the Longzhong Basin (here referred to as the Lanzhou subbasin), the West Qinling to the south and the East Qilian Shan terrane in the north and west serve as the two most straightforward source regions. Recently, numerous detrital zircon U-Pb ages from the West Qinling and East Qilian Shan have been reported [Chen et al., 2008; Lease et al., 2007, 2012; Liu et al., 2015; Xu et al., 2010; Yuan and Yang, 2015]. We compiled the published detrital zircon U-Pb age distributions from the West Qinling and the East Qilian Shan. Detrital zircons from the West Qinling are dominated by Ma ages with broader additional ages of Ma, Ma, and Ma (Figures 10a 10d) Lease et al. [2007, 2012] suggest that zircons with U-Pb ages of Ma are probably derived from the Permo-Triassic plutons and Triassic sedimentary rocks that are cropped out widely in the West Qinling (Figure 1b). The Ma age population is rarely seen in the East Qilian Shan rocks [Chen et al., 2008; Xu et al., 2010; Yuan and Yang, 2015], which is instead characterized by a prominent age population between 400 and 480 Ma (Figures 10l 10n). The rest of the East Qilian Shan age spectrum is scattered, with smaller peaks between 800 and 1000 Ma, 1600 and WANG ET AL. CENOZOIC GROWTH OF THE WEST QINLING 7765

13 Figure 8. Stratigraphic height as a function of magnetostratigraphic age in the Xianshuihe section (age data are based on the correlations of Figure 7) Ma, and 2400 and 2600 Ma. Although both the West Qinling and East Qilian Shan contain ages between 1800 and 2000 Ma, and 2300 and 2500Ma, they can be distinguished according to younger zircon ages ( Ma) Detrital Zircons U-Pb Ages From the Xianshuihe Section According to the magnetostratigraphy, the depositional ages of seven detrital zircon samples (LC1-LC7) from the Xianshuihe section are ~45 Ma, ~43 Ma, ~39.5 Ma, ~30 Ma, ~27 Ma, ~20 Ma, and ~16 Ma, respectively (Figures 10e 10k). The accepted zircon U-Pb ages were plotted on age-relative probability diagrams derived from the probability density function [Ludwig, 2003]. Among the measured samples, four pre-30 Ma samples (LC1 LC4) share similar age spectra (Figures 10h 10k), which mainly include the following four intervals: (1) (18 24%), (2) Ma (20 30%), (3) Ma (15 25%), and (4) Ma (20 25%) (Figures 10h 10k). A small age interval of Ma is also present and accounts for ~5% of the total dated grains in these four samples (Figures 10h 10k). A most striking characteristic of our four pre-30 Ma sample zircon age spectra is that zircons are similarly distributed in four populations of Ma, Ma, Ma, and Ma. Compared with potential source regions, the proportion of Ma population interval is much lower than that of the West Qinling; the proportion of Ma zircons is lower than that of East Qilian Shan, but much Figure 9. Photomicrographs of sandstones from the Xianshuihe section. The stratigraphic positions are shown in Figures 3 and 7. C1 and C4 sandstones are collected from the Xiliugou Formation and Yehucheng Formation, respectively, C5 and C6 are from Xianshuihe Formation (Q: quartz; F: feldspar; Ls: sedimentary rock fragment). WANG ET AL. CENOZOIC GROWTH OF THE WEST QINLING 7766

14 Figure 10. Detrital zircon U-Pb age probability distributions from source terranes in West Qinling [Lease et al., 2012], East Qilian Shan [Chen et al., 2008; Xu et al., 2010; Yuan and Yang, 2015], and zircon age distributions from magnetostratigraphic horizons within the Xianshuihe section (locations of samples are shown in Figure 7). Radioisotope database is presented in Table S1. Detrital zircon U-Pb age spectra are depicted as age probability density functions (thick lines) and age histograms. WANG ET AL. CENOZOIC GROWTH OF THE WEST QINLING 7767

15 higher than that of the West Qinling; the proportion of Ma zircons is also slightly lower than that of East Qilian Shan. These observations suggest that zircons in these four samples were mixed detritus derived from both the West Qinling and East Qilian Shan. A sediment-mixing model modified from Amidon et al. [2005] was used to estimate the detrital zircon contribution from each of the two possible sources. It is estimated that ~60% of the detrital zircons of these four samples were derived from the East Qilian Shan, and ~40% from the West Qinling. Compared with the previous four samples, the detrital zircon age spectrum of sample LC5 (26.5 Ma, middle Xianshuihe Formation, Figure 7) sees a significant increase in the Ma age population, which accounts for 56% of all the dated grains in the sample (Table S1 and Figure 10g). Ages older than 500 Ma mainly cluster in two intervals: Ma and Ma, which together account for 31%. Five grains yield ages at Ma (6%) and just two individual ages exist in Ma (2%). This age spectrum is very similar to that of the West Qinling terrane (Figure 10h), suggesting that zircons of LC5 were nearly all derived from the West Qinling source. Samples LC6 (20.3 Ma) and LC7 (16.2 Ma) were collected from the upper part of the Xianshuihe Formation. Zircon ages between 230 and 320 Ma, with peak at 260 Ma account for ~40% of all the zircon populations (Figures 10e and 10f). The remaining ages are clustered in the Ma, Ma, and Ma ranges (Figures 10e and 10f) The proportion of Ma zircons in both samples slightly increase to ~11%, which can be either derived from the West Qinling or East Qilian Shan. Although the proportion of Ma zircons is mildly higher than that of West Qinling, the dominance of Ma population and lack of Ma zircons strongly suggest that the strata dated to Ma in the Lanzhou Basin were derived primarily from the West Qinling source. Detrital zircon age distributions (Figure 10) from the Xianshuihe magnetostratigraphic section in north of the Lanzhou Basin thus show a major provenance change after ~30 Ma, with an enhanced contribution of West Qinling zircons by ~27 Ma. This strengthening of the West Qinling as a source area for Lanzhou Basin is sustained throughout the upper Xianshuihe section to ~15 Ma. 7. Discussion 7.1. Basin Evolution in the Northeastern Tibetan Plateau Our new paleomagnetic data supported by mammalian fossils from the Lanzhou Basin indicate that the west part of the Longzhong Basin began to receive deposits since at least ~47 Ma, which is comparable to the nearby Xining Basin [Dai et al., 2006]. The onset of the accumulation in the Lanzhou-Xining region was previous attributed to part of a broad foreland subsidence of the Longzhong related to activity along the West Qinling Fault (WQLF), the primary structure that bounds the southern flank of purported foreland sediments [Clark et al., 2010]. However, the distribution of the Eocene to lower Oligocene sediments in the west Longzhong Basin seems to contradict the prediction of initial accumulation in the west Longzhong Basin as foreland basin related to the thrusting along the WQLF. Our results from the Lanzhou Basin and magnetostratigraphic study from the Xining Basin [Dai et al., 2006] show that m thick of Eocene to lower Oligocene strata (~47 30 Ma) occurred across the Lanzhou- Xining region. These Eocene to early Oligocene strata may pinch out in the north of the Maxian Shan-Laji Shan region (Figure 11a). This observation is also supported by the sedimentary records in the basins, which lie to the south of the Maxian Shan-Laji Shan and north of the WQLF. Basins located in the south of the Lanzhou-Xining region include Linxia Basin, Xunhua Basin, and Guide Basin (Figure 1b). Basal Cenozoic sediments in these basins date to ~30 Ma [Fang et al., 2003, 2005; Hough et al., 2011; Lease et al., 2012] indicating that the southern Longzhong Basin lacks the Eocene to lower Oligocene sediments. Similarly, the north region of the Lanzhou-Xining area is also devoid of Eocene to lower Oligocene deposits [Gansu Bureau of Geology and Mineral Resources (GBGMR), 1989]. These patterns suggest that in the Eocene to early Oligocene, the Lanzhou-Xining region was an east-west elongated depression located in the central part of the Longzhong Basin, far away from the present-day boundaries of the WQLF in the south and the Haiyuan Fault (HYF) in the north (Figure 11a). On the basis of the Eocene to Oligocene strata distribution, we argue that the basin in the Lanzhou-Xining region born as a topographically enclosed catchment instead of a broad flexural subsidence. During the early WANG ET AL. CENOZOIC GROWTH OF THE WEST QINLING 7768

16 Figure 11. Cenozoic evolution of the Longzhong Basin and its boundary fault activation. (a) The basin probably initiated as a topographically enclosed catchment, which restrict the sediments in the central part of the basin (Lanzhou-Xining region) during the Eocene to early Oligocene. (b) Late Oligocene to middle Miocene tectonic and depositional configuration of the Longzhong Basin. The northward thrusting of the WQLF and uplift of the West Qinling drove the formation of the broad foreland basin at late Oligocene. Eocene, the growth of the West Qinling may have created a relatively high topography in the south. Rivers draining the West Qinling in the south and the preexisting highland of the East Qilian Shan in the north merged somewhere and then flowed toward the east and delivered the sediments to the Lanzhou-Xining region (Figure 11a). This model may explain not only the very low accumulation rate of 2 6 cm/kyr in the Lanzhou Basin (Figure 8) and of ~2 cm/kyr in the Xining Basin [Dai et al., 2006] but also the eastward paleocurrents and mixed sources supplying sediments to the basins. Published thermochronological data also support this hypothesis for basin formation in the Lanzhou-Xining region. Evidence from the apatite (U-Th)/He dating from Archean to Triassic age plutonic rocks in the West Qinling indicates initial uplift of the West Qinling at Ma [Clark et al., 2010]. The apatite fission track study in the East Qilian Shan shows that eastern Qilian Shan experienced rapid initial cooling in the Late Cretaceous and quasi isothermal quiescence during ~ 80 to 24 Ma [Pan et al., 2013], suggesting that highlands of the East Qilian Shan existed in early Cenozoic time following Cretaceous uplift. A significant reorganization of the basin-type and regional tectonic deformation at Ma is revealed by the changes of the paleocurrent directions, compositions of grains or clasts in sandstones and conglomerates and detrital zircon records from the Lanzhou Basin. Since ~27 Ma, paleocurrent orientations changed from the eastward direction to the northward directions; sediment lithic grains in the sandstones sharply increased; detrital zircon age populations showed similar distribution patterns of rocks in West Qinling, rather than rocks in the East Qilian Shan. These lines of evidence suggest that the mixed sources were replaced by the West Qinling to supply materials to the Lanzhou Basin. We interpreted these changes to reflect a phase of WANG ET AL. CENOZOIC GROWTH OF THE WEST QINLING 7769

17 pulsed growth of the West Qinling since ~30 Ma. Following the accelerated uplift of the West Qinling, new subsidence was generated in the north of West Qinling closely adjacent to the WQLF, such as Linxia Basin and Xunhua Basin. Both of the basins initiated at ~29 Ma with a northward thinning upper Oligocene to Quaternary sediments [Fang et al., 2003; Lease et al., 2012] suggest that the newly formed basins are flexural basins created by topographic loads of the West Qinling. These flexural basins extended north to connect with preexisting Lanzhou and Xining Basins forming broad Longzhong Basin under compressional and transpressional geological settings (Figure 11b) Implications for Deformation of the Tibetan Plateau A clear knowledge of the chronological, stratigraphic, and provenance evolution of the Lanzhou Basin presented in this study has significant implications for understanding of the Cenozoic tectonic history of the northeastern Tibetan Plateau. As we mentioned in previous section, the Longzhong Basin may initiate as an intermontane basin. Possible mechanisms that drove the basin formation in the Eocene include (1) significant thrusting of the WQLF induced the subsidence [Clark et al., 2010; Duvall et al., 2011], (2) postrift thermal subsidence [Horton et al., 2004], and (3) right-lateral transtension along the WQLF and HYF triggered the subsidence [Wang et al., 2013]. First model linked the thrusting of the WQLF and formation of the Longzhong Basin to deformation associated with the India-Asia collision. Following this logic, previous studies inferred that crust shortening and thickening in the northeastern Tibetan Plateau was underway shortly after the India-Asia collision. Although this hypothesis is supported by cooling of the West Qinling since ~45 50 Ma [Clark et al., 2010] and activation of the WQLF at ~50 Ma according to fault gouge age [Duvall et al., 2011], it is difficult to interpret the Paleogene clockwise rotation of Lanzhou-Xining region [Dupont-Nivet et al., 2004] and distributions of the Eocene to lower Oligocene strata in the basin [Fang et al., 2003; Lease et al., 2012]. The second model indicates that the initiation of crustal shortening and related range uplift did not occur around the Longzhong Basin before Ma [Horton et al., 2004]. This prediction seems inconsistent with observations of the Eocene uplift of the West Qinling and motivation of the WQLF [Clark et al., 2010; Duvall et al., 2011]. Thus, we suggest that the early Cenozoic development of the Longzhong Basin may be related to the right-lateral transtensional deformation along the HYF and the WQLF (Figure 12a). The right-lateral transtension in the study region probably inherited from the Cretaceous deformational pattern. This inference is supported by two observations: (1) structure studies in the Qinling indicate a component of rightlateral transtension on E-W striking faults during the late Cretaceous and early Cenozoic [Ratschbacher et al., 2003] and (2) a rapid uplift of the East Qilian Shan during Late Cretaceous was followed by a lower uplift rate in the early Cenozoic time [Pan et al., 2013]. On the other hand, distributed right-lateral shear would also explain reconstructed fault activity in the basin margins [Liu et al., 2015; Wang et al., 2013] and vertical axes rotations [Dupont-Nivet et al., 2004]. It should be note that we did not observe the Eocene to early Oligocene rotations over the Lanzhou Basin based on the Xianshuihe paleomagnetic data. This may be partially attributed to that the Eocene to lower Oligocene sediments within the Xianshuihe section are dominated by sandstones and gypsum beds. The erratic demagnetization directions from the sandstones and gypsums hindered the calculations of the vertical axes rotations. Although the West Qinling Fault (WQLF) might have been active since ~50 Ma [Clark et al., 2010; Duvall et al., 2011], uplift and, by inference, shortening around the Longzhong Basin and topographic growth of the West Qinling happened in Oligocene time, i.e., since 30 Ma. Such an Oligocene onset of crustal thickening and uplift in the West Qinling in the northeastern Tibetan Plateau is consistent with numerous lines of evidence. For example, the ~30 Ma formations of the Linxia Basin and Xunhua Basin in the north flank of the WQLF with wedge-shaped package (northward thinning) of sediments [Fang et al., 2003; Lease et al., 2012] suggest the inception of flexural subsidence of the Longzhong Basin and growth of the West Qinling. Topographically, the West Qinling extends westward to connect with the East Kunlun Shan (Figure 1a). Thermochronologic data from the East Kunlun Shan reveal that significant cooling and exhumation along the range started around ~30 Ma [Clark et al., 2010; Jolivet et al., 2001; Mock et al., 1999] suggesting a phase of rapid growth of the East Kunlun Shan at 30 Ma. Cooling ages of crystalline massifs in the Longmen Shan, which is located in the east margin of the Tibetan Plateau (Figure 1a), reveal a significant exhumation beginning around Ma [E. Wang et al., 2012]. Moreover, the Altyn Tagh Fault (AFT) was inferred to WANG ET AL. CENOZOIC GROWTH OF THE WEST QINLING 7770