Magnetostratigraphy and Depositional History of the Miocene Wushan

Journal of Asian Earth Sciences 44 (2012) 189–202

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Journal of Asian Earth Sciences

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Magnetostratigraphy and depositional history of the Miocene Wushan basin on the NE Tibetan plateau, China: Implications for middle Miocene tectonics of the West Qinling fault zone ⇑ Wang Zhicai a,b, , Zhang Peizhen b, Carmala N. Garzione c, Richard O. Lease d,1, Zhang Guangliang b, Zheng Dewen b, Brian Hough c, Yuan Daoyang b, Li Chuanyou b, Liu Jianhui b, Wu Qinglong b a Institute of Earthquake Engineering, Shandong Earthquake Administration, Jinan 250014, China b Institute of Geology, China Earthquake Administration, National Laboratory of Earthquake Dynamics, Beijing 100029, China c Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627, USA d Department of Earth Science, University of California, Santa Barbara, CA 93106, USA article info abstract

Article history: Based on ﬁeld mapping, section measurement and magnetostratigraphy, 1700 m of sedimentary rocks Available online 19 July 2011 have accumulated in the Wushan basin between 16 Ma and 6 Ma. Basin geometry, sedimentation characteristics and the early syn-depositional deformation along the northern margin of the basin indi- Keywords: cate that formation of the Wushan basin was related to tectonic deformation along the West Qinling fault Middle Miocene zone during the middle Miocene. A series of basins of similar age to the Wushan basin were generated Wushan basin along and to the south of the West Qinling fault zone while basalts also erupted in this region at this time. West Qinling fault zone We suggest that the middle Miocene (16 Ma) may represent a change in kinematics and deformation Tibetan plateau style in the region along and to the south of the West Qinling fault zone. At this time, there was a tran- Magnetostratigraphy sition from NNE–SSW compressional deformation, that dominated the region since the late Paleogene, to the development of WNW–ESE and/or E–W trending strike-slip movement and associated transpressional and transtensional activity that continues today. The Miocene Wushan basin may have developed in association with transpression along the West Qinling fault zone. Whether this transition was related to the onset of strike-slip along the east Kunlun fault and related deformation transfer, lower crustal ﬂow, or removal of mantle lithosphere, the middle Miocene provides direct evidence for a change in the kine- matic style along the plateau margin. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction (Yin et al., 2007, 2008a,b; Yin and Harrison, 2000; Yin et al., 2002) based on a number of lines of evidence, including: (1) the exhuma- The northeastern Tibetan plateau, which is bounded to the tion rates along the East Kunlun fault and the Altyn Tagh fault accel- south by the east Kunlun fault, and to the north by the Altyn Tagh erated in Eocene to early Oligocene (40 ± 10 Ma) (Jolivet et al., 2001); and Qilian-Haiyuan faults (Fig. 1), is an actively deforming part of (2) crustal thickening occurred along the northern and southern the Tibetan plateau (Molnar and Tapponnier, 1975; Meyer et al., margin of the Qaidam basin at 49 Ma (Yin et al., 2002) or between 1998; Tapponnier et al., 2001; Yin and Harrison, 2000). This region 50 and 65 Ma (Yin et al., 2007, 2008a,b); (3) clockwise rotation of is characterized by a series of Cenozoic basins and bordering 24° occurred in Xining–Minhe basin at 41 Ma (Dupont-Nivet mountain ranges controlled or sliced by strike-slip faults and re- et al., 2004, 2008); and (4) reverse faulting and associated mountain lated thrust faults, together providing archives of tectonic defor- uplift in the West Qinling region initiated at 45–50 Ma (Clark et al., mation and environmental change. 2010). However, the tectonic deformation that ﬁnally established Studies of Cenozoic basins and mountain ranges indicate that the modern NE Tibetan plateau initiated in the late Miocene deformation of the NE Tibetan plateau began in the late Paleogene (8 Ma) (Song et al., 2001; Yuan et al., 2003; Zheng et al., 2003; Fang et al., 2003; Molnar, 2005; Zhang et al., 2006; Jiang et al., 2007; Lin ⇑ Corresponding author at: Institute of Earthquake Engineering, Shandong et al., 2010) and lasted until the Pliocene and Quaternary (Li et al., Earthquake Administration, Jinan 250014, China. Tel.: +86 13853166836; fax: +86 1979). In recent years, evidence for early and middle Miocene defor- 053188515212. mation has been identiﬁed in this region. For instance, the eastern E-mail address: [email protected] (Z. Wang). 1 Laji shan experienced thrusting and became the main source of sed- Present address: Institut für Geowissenschaft, Universität Tübingen, 72074 Tübingen, Germany. iments for the Xunhua basin to the south at 22 Ma (Lease et al.,

Fig. 1. Tertiary geological map highlighting faults of the West Qinling fault zone. Faults, WQNFZ: the West Qinling fault zone, RYSF: the Riyueshan fault, LJSF: the Lajishan fault, MXSF: the Maxianshan fault, DCMX: the Dangchang-Minxian fault, XLF: the Lixian fault. Basins, XN: Xining basin, LNT: Lintan basin, LNX: Linxia basin, XH: Xunhua basin, TB: tianshui basin, NY: Nanyang basin, NDS: Niudingshan basin, XL: Xihe–Lixian basin, GJ: Ganjia basin, TR: Tongren basin, GD: Guide basin. Small circles labeled with ‘‘T’’ point the location of thermochrological thansects (by Lease et al., 2011, and Clark et al., 2010). This ﬁgure is based on Geological map of 1:1500000 Tibetan plateau and revised according to ﬁeld observation. The inset shows the map location in which the Altyn Tagh fault (ATF), the east Kunlun fault (EKF), the West Qinling fault zone (WQNFZ), and the Haiyuan fault (HYF) are indicated. EQ: the East Qinling orogen, WQ: the West Qinling Orogen, QL: the Qilian Orogen, KL: the Kunlun Orogen, SPGZ: the Songpan-Ganzi terrane.

2011), the West Qinling range became a more pronounced source of terrane (Ratschbacher et al., 2003; Zhang et al., 2004; Zheng et al., sediment to the Linxia basin by 14 Ma (Garzione et al., 2005), and 2010)(Fig. 1). mountains to the south of the West Qinling fault zone show acceler- The West Qinling fault zone forms the northern boundary of the ated exhumation rates between 18 and 17 Ma (Clark et al., 2010). West Qinling orogenic belt and consists of several nearly parallel Therefore, the Cenozoic deformation history is rather complicated, faults (Fig. 1, Figs. 2 and 3). Thrust faults have developed on both and may consist of two or more stages (Harrison et al., 1992; sides of the fault zone (Figs. 2 and 3). Slip on the northern frontal Tapponnier et al., 2001; Yue et al., 2003). As for the kinematics of faults have thrusted pre-Cenozoic rocks and strata of the West major faults in this region, it seems that the strike-slip motion for Qinling northward and have formed the southern borders for major faults like the east Kunlun fault and the Haiyuan fault started several Cenozoic basins, such as Linxia, Xunhua, and Longxi basins, relatively late except for the long term left-lateral motion of the and the southern frontal faults dip northward and mark the north- Altyn Tagh fault (Yin and Harrison, 2000). The onset of strike-slip ern border of the Tange basin (Wang et al., 2006). An active, left- motion for the East Kunlun is at ca. 15 Ma (Jolivet et al., 2003), and lateral strike-slip fault is recognized in the middle part of the West the Haiyuan fault probably initiated in late Miocene or Pliocene time Qinling fault zone (Figs. 2 and 3), and its late Quaternary slip rate is (Burchfiel et al., 1991; Zhang et al., 1988, 1990). estimated to be 2 mm/yr (Li, 2005). However, this active strike- Based on the previous observations of long-term Cenozoic short- slip fault is not traceable to the west of the Taohe River (Fig. 1). ening in this region, this study focuses on the Miocene deformation To the north of the West Qinling fault zone, the Qilian block is a history of the West Qinling fault zone by exploiting the sedimenta- Paleozoic orogenic belt dominated by lower Proterozoic gneiss, tion history and magnetostratigraphy of the Wushan basin (Fig. 1). amphibolite, marble and schist and early Paleozoic metasediments The West Qiling fault zone is a significant structure on the NE that include phyllite, low-grade metamorphosed limestone and Tibetan plateau. It now marks part of the geomorphologic and siltstone. Most of the region is now covered by Cretaceous and topographic boundary of the NE margin of the Tibetan plateau Cenozoic red beds and loess, but early Paleozoic and Proterozoic and it also acts as the boundary for several Cenozoic basins rock and strata are commonly exposed in mountainous regions, (Fig. 1). Evidence of thrusting beginning in Paleogene time (Clark for instance, in the Laji Shan (Wang et al., 1997; Garzione et al., et al., 2010), together with documentation of active left-lateral 2005; Lease et al., 2007) (Fig. 1). To the south of the West Qinling strike-slip motion (Li, 2005) suggest that the West Qinling fault fault zone, Cenozoic, late Paleozoic, and Mesozoic strata are ex- zone has been active in the deformation process of the NE Tibetan posed, including Devonian, Carboniferous, Permian and Triassic plateau throughout Cenozoic time, and thus it played a role in the terrigenous metasediments and carbonates, such as sandstone, evolution of adjacent Cenozoic basins. However, the questions of siltstone, slate, and marine limestone (Fig. 1). The Cretaceous when strike-slip motion began on this fault and how it influenced deposits mainly consist of conglomerate, sandstone, and mudstone the generation and evolution of neighboring Cenozoic basins still that were deposited in an intra-continental extensional basin set- remain unanswered. Answers to these questions will help under- ting (Horton et al., 2004). Due to post-depositional erosion, Creta- stand the Cenozoic evolution of the NE Tibetan plateau and evalu- ceous red bed outcrops are sparse along the West Qinling fault ate related geodynamic mechanisms. zone. However, outcrops are well preserved in the Xining and Lanzhou basins. 2. Geological setting A number of Cenozoic sedimentary basins have developed in the region to the north of the West Qinling fault zone (Fig. 1), such The West Qinling orogenic belt separates the north China block as the Xunhua, Linxia, Longxi, and Guyuan basins. Thick Cenozoic and Qilian orogens from the south China block and Songpan-Ganzi red beds accumulated nearly continuously in these basins since Z. Wang et al. / Journal of Asian Earth Sciences 44 (2012) 189–202 191

Fig. 2. Tertiary geological map of the Wushan basin. Faults , GJSD: the Goujiahe-Sidian fault, LYF: the Leijiapo-Yanjiamen fault, WQNF: the active strike-slip fault of the West Qinling fault zone, ZJCP: the ZHangjiashan-Caopingshang fault, LGXF: the Laganxia, SDF: the Shandan fault, BLGF: the Bangluogou fault. For location, see Fig. 1. The locations of Figs. 3, 4a and 6 are labeled with lines, a rectangle and a small black star, respectively. at least late Oligocene until late Miocene or Pliocene time (Horton eastern Gansu province (Figs. 1 and 2). Red beds are distributed et al., 2004; Dupont-Nivet et al., 2004; Fang et al., 2003; Yue et al., along both the northern and southern sides of the Weihe and 2000; Shen et al., 2001; Jiang et al., 2007; Hough et al., 2010) and Zhanghe rivers and represent a complicated syncline with its were not internally deformed until the late Miocene (8 Ma) northern and southern limbs tilting toward the axis of the basin. (Zheng et al., 2006; Fang et al., 2003; Molnar, 2005; Yuan et al., A number of faults have developed in and along both sides of the 2003, 2007; Zhang et al., 2006; Lin et al., 2010). The north-trending basin (Figs. 2 and 3). The only active fault, a strike-slip fault in Jishi Shan did not appear as a barrier between the Xunhua basin the middle of the West Qinling fault zone, passes through the and the Linxia basin until the late Miocene (13 Ma) due to the middle of the basin and is associated with strong deformation, formation of new thrust faults along the eastern front of the Jishi tilting, or inverted folds in red beds. One of the normal faults, the Shan (Hough et al., 2010; Lease et al., 2011). Bangluogou fault, along with several minor ones along the middle There are also several Cenozoic basins on the southern side of southern border, separates the red beds from the Cretaceous and the West Qinling fault zone (Fig. 1), but their sedimentation history Carboniferous sandstone and siltstone to the south. The Leijiapo- varies. The Guide basin developed since late Paleogene time with Yanjiamen thrust fault marks part of the southern margin of the deposition continuing until Pliocene and Quaternary time. Tectonic basin in the west. It extends into the interior of the basin and deformation likely occurred in the early Miocene, creating an caused deformation of red beds. The WNW trending Zhangjia- unconformity in the Guide basin (Fang et al., 2005). The Tange ba- shan-Caopingshang fault on the northern margin of the basin in sin developed in the late Paleogene with accumulation of 800 m the west separates the Neogene strata of Wushan basin from of sandstone and conglomerate, but ceased deposition in Miocene Paleogene and Cretaceous rocks to the north. time (Wang et al., 2006). Other nearby basins did not develop until According to ﬁeld stratigraphic measurements and magnetos- Miocene time, like the Tongren, Ganjia, Xihe–Lixian and Wushan tratigraphy (Figs. 4 and 5), the red beds of the Wushan basin are basins. In this paper, we document that initial deposition in the all Miocene and are disconformably deposited over Paleozoic Wushan basin began in the middle Miocene (16 Ma). metasediments and some Mesozoic rock. Except for a growth unconformity near the base, strata are nearly continuously deposited. We measured 1400 m of Miocene strata that can be divided 3. Basin structure and stratigraphy into four units. Unit one (the lowest unit) is exposed along both the northern The Wushan basin is an elongate Cenozoic basin 120 km and southern margin of the basin. It consists mainly of red-brown long Â 30 km wide that lies along the West Qinling fault zone in conglomerate intercalated with gravel-bearing sandstone and 192 Z. Wang et al. / Journal of Asian Earth Sciences 44 (2012) 189–202

Fig. 3. Structural cross-sections through the West Qinling fault zone. The West Qinling fault zone (WQNFZ) consists of a group of faults. GJSD: the Goujiahe-Sidian fault, LYF: the Leijiapo-Yanjiamen fault, WQNF: the active strike-slip fault of the WQNFZ, ZJCP: the ZHangjiashan-Caopingshang fault, LGXF: the Laganxia fault, SDF: the Shandan fault. Strata: 1. Neogene, 2. Middle and upper Paleogene, 3. Cretaceous, 4. Paleozoic, 5. Basic intrusive, 6. Granite, 7. Mudstone, 8. sandstone 9. conglonmerate 10. sandy conglomerate 11. limestone. Simplified from Wang et al. (2007). See Fig. 2 for location. sandy mudstone along the northern margin of the basin (Fig. 4B), Sandstone and conglomerate are interpreted as channel deposits, such as the strata in section A-1 (Fig. 5) and the growth strata and mudstone reflects floodplain deposits, with paleosols develop- shown in Fig. 6. Approximately 300 m of conglomerate have been ing during times of inactivity on the floodplain. The total thickness measured in section A-1 along the northern margin (Fig. 5). of unit one is estimated at 300–600 m in the middle and southern Conglomerate is commonly massive to imbricated and clast- part of the basin. supported, with angular to subangular cobbles, pebbles, and some Unit two commonly crops out in the middle of the basin. The to- boulders. Individual conglomerate beds are tabular, tens of centi- tal thickness of this unit reaches up to 585 m in section A-2 meters to 2 m thick and extend laterally for up to several tens (Fig. 5). The lowest 50 m and upper 270 m are characterized of meters. Paleocurrent measurements in the imbricated conglom- by brown-red and violet calcareous mudstone intercalated with erate indicate generally southward paleoflow. We interpret this grey-green or blue mudstone and marl, thinly bedded gypsum, conglomerate to have been accumulated in a medial to distal and fine to very fine-grained sandstone (Fig. 7C and D). Mudstone alluvial fan setting within a braided stream sub-environment. layers are usually centimeters to tens of centimeters thick, and ex- In the center of the basin, the corresponding strata (the upper tend laterally for at least hundreds of meters as far as we could part of Unit one) consists of mudstone interbedded with granule trace them. Beds are commonly massive, tabular, and thinly bed- and/or pebble conglomerate and sandstone (Fig. 7A). We have ded, and extend laterally for tens of meters to more than 100 m. measured an 150 m thick exposure along the sub-section A of Mudstone layers are clay to silt-sized and are frequently massive A-2 in the middle of the basin (Fig. 5). Some conglomerate near to laminated. Siltstone beds often show ripple cross-lamination. the bottom of sub-section A is matrix supported, with angular We interpret that the lower and upper parts of unit two were and subangular granules, pebbles, and cobbles in a sandy to silty deposited in a shallow lacustrine setting, where marl and gypsum matrix. These conglomerate beds extend laterally for up to several reflect time periods of increased evaporation. Mudstone beds are tens of meters. The granule-sized conglomerate beds are tens of associated with suspension settling with cross-laminated siltstone centimeters to >1.5 m thick, lenticular, and display scouring at beds interpreted as being deposited by wave activity or possibly their base. Internally, the conglomerates show horizontal stratifi- unidirectional currents associated with either fluvial–deltaic activ- cation, trough cross-stratification, and/or tabular cross-stratifica- ity along the lake margin or storm activity. Massive, tabular sand- tion. Beds commonly show well-developed fining upward. The stone most likely reflects unconfined turbidity flows. The middle mudstone is typically massive brick red or red-brown and contains 265 m contains grey pebble conglomerate, sandstone, siltstone, floating sand, granules and/or pebbles. Brick red mudstone shows and red silty claystone. Conglomerate beds are commonly clast rare root traces and carbonate nodules, with diameters up to supported and consist mostly of pebbles and/or granules and con- 3 cm. Carbonate horizons frequently developed within the upper tain trough and tabular cross bedding, crude horizontal stratifica- portions of mudstone beds. Individual nodule layers are between tion, and scour and fill features. Individual beds are tens of 3 cm and 1 m thick (Fig. 7B). The entire succession fines up- centimeters to meters thick, lenticular, and extend laterally for me- ward. We interpret the sedimentary features described above as ters to several tens of meters. Mudstone beds are usually centime- associated with a meandering fluvial and floodplain environment. ters to tens of centimeters thick, and extend laterally for more than Z. Wang et al. / Journal of Asian Earth Sciences 44 (2012) 189–202 193

100 meters, as far as we could trace them. Mudstone beds are mas- and beds usually extend laterally for meters to tens of meters. Con- sive, horizontally laminated, and ripple cross-laminated in rare glomerate beds are often flanked by wedge-shaped sand bodies locations. Less common massive mudstone contains floating sand that are usually normally graded and horizontally laminated. or granule grains. Secondary gypsum veins cross cut this unit. Len- Ripple cross stratification is also common in these sandstones. ticular conglomerate and sandstone beds are inferred to reflect Mudstone beds are brick red or red-brown. Bedding thicknesses fluvial channel deposits, whereas mudstone reflects overbank range from 0.5 cm to 2.5 m, and beds are laterally continuous deposits. Unit three is exposed in the middle of the basin and con- except where they have been eroded by lenticular sandstone sists of 370 m of grey conglomerate and sandstone and brick red bodies. Internally, mudstone beds are usually massive, but also or red-brown silty mudstone (Fig. 4 and 5). The conglomerates show ripple bedforms on their upper surfaces. Carbonate nodules contain subangular clasts and are moderately sorted, with pebble are rare in this unit. Based on the lenticular nature of conglomerate to cobble clasts in a sand-sized matrix. Beds are lenticular and have and sandstone beds and sedimentary structures that indicate trac- an erosional base. Internally, conglomerate beds show rare trough tion transport, we interpret these deposits as fluvial in origin. cross stratification. More common are imbrications and large-scale, Alternating dips of bed-scale, low-angle cross strata are inferred low angle cross strata that show opposing dip directions. Bedding to reflect lateral accretion on migrating longitudinal bars suggest- thicknesses vary from several tens of centimeters to up to 3m, ing that the fluvial system was braided. Wedge-shaped sandstone

Fig. 4. Loction of paleomagnetic sampling section and synthetic cross section of the Wushan basin. (A) Sampling location. 1 . Quaternrary loess, 2. Quaternary alluvial deposits, 3. Neogene sedimentary rocks, 4. Paleozoic metasedimentary rocks, 5. Central active left-lateral strike-slip fault of the West Qinling fault zone, 6. Paleomagnetic sampling position. The black bars indicate samples that yield normal polarity and the white dots indicate samples of reversed polarity, 7. Orientation of Neogene layers. The short bar indicates dip direction of the layer, and the number is dip angle, 8. (A–F) The sampling sub-sections, 9. The arrows indicate locations and directions of translation. For location, see Fig. 2. (B) Schematic cross section. Unit 1 through Unit 4 are the four units of the Wushan Miocene deposits, Unit 2–1, Unit 2–2 and Unit 2–3 are the three parts of the Unit 2, for details see Fig. 5 and the text. F1 is an inferred fault of the Miocene West Qinling fault zone. F2 is the modern active strike-slip fault of the West Qinling fault zone. Bedding dip angles and dip directions are shown. 194 Z. Wang et al. / Journal of Asian Earth Sciences 44 (2012) 189–202 bodies that flank bars are likely sand wedge deposits laid down usually tens of centimeters to 6 m thick, lenticular, and extend during waning flow within fluvial channels between bars. Mud- laterally for meters to tens of meters. Normal grading is common, stone beds are interpreted as overbank deposits and weakly devel- from pebble and/or cobble conglomerate at the base of beds to oped paleosols. pebble bearing sandy mudstone or silty sandstone at the top of Unit four is mostly exposed in the middle of the basin in the beds. Sandstone is generally poorly to moderately sorted and con- core of the syncline (Figs. 4 and 5) and consists of 260 m of grey tains a large proportion of silt, granules and pebbles. Sandstone and brick-red conglomerate and brick-red or red-brown silty sand- beds commonly contain trough cross bedding, ripple cross lamina- stone and mudstone (Fig. 7E and F). The conglomerate beds are tions, and horizontal laminations. We interpret these conglomerate mostly massive, poorly to moderately sorted, clast-supported, with and sandstone-rich unit as deposits within a proximal braided riv- angular to subrounded cobbles and boulders. Sedimentary struc- er setting. Lenticular bedding and sedimentary structures associ- tures within conglomerate beds include crude horizontal stratifica- ated with traction transport suggest that these are deposits of tion, scour and fill textures and imbricated pebbles. Beds are channelized water flows. The distinct lack of mudstone in this unit

Fig. 5. Magnetostratigraphy and stratigraphic column of the Wushan basin. (A) Stratigraphic column of the Wushan basin. The upper section A-2 with paleomagnetic r measurement is composed of six sub-sections, A through F, the lower section A-1 is measured on the northern side of the basin. Section locations are marked in Fig. 4A, u through are the four units of the upper section A-2. Fossils and paleoflow directions are also shown. (B) VGP latitude of all the samples that show strong polarity, including those with low VGP latitudes. (C) Magnetostratigraphy of the Wushan basin, with 18 normal chrons and 17 reversed chrons. ÃAB, ÃBC, ÃCD, ÃDE, and ÃEF highlight the section translation points. ÃT shows the location of fossil tie points. (D) Part of the ATNTS2004 time scale (Lourens et al., 2005). Z. Wang et al. / Journal of Asian Earth Sciences 44 (2012) 189–202 195 indicates a lack of floodplain setting in this fluvial system, likely After the intensity of the natural remnant magnetization (NRM) associated with the coarse-grained nature of these deposits. was measured, the specimens were then immersed in liquid nitrogen and after the nitrogen evaporated completely and the speci- 4. Paleomagnetic sampling and measurement methods mens had returned to normal temperature, their low temperature magnetization (LT) was measured. Stepwise demagnetization was We took samples from 398 sites for paleomagnetic measurement conducted in an alternating field (AF) between 0 and 100 Oe, with along the valley to the north of Wushan (Fig. 4). Due to the cover of 25 Oe steps. After AF demagnetization, the specimens were sub- loess and vegetation, exposures were best along rivers and gullies jected to thermal demagnetization between 150 °C and 680 °C. where there had been focused erosion. Therefore, necessary The first batch of 398 specimens was divided into two groups translations resulted in a composite section that consists of seven (with even numbers and odd numbers). One group was thermally sub-sections, namely, sub-sections A–F (Figs. 4 and 5, Table 1 in Sup- demagnetized by 13 steps, the other by 9 or 10 steps. Over 90% plementary materials). Sub-section correlations were carried out specimens yielded good demagnetization results. Then, a second using distinctive, laterally continuous layers where available, as well batch of 50 specimens, from the initial set of the 398 sampling as geometric methods that take bedding orientation into account. localities was run for verification following the same procedure. The standard sampling interval is about 3 m. At each sampling site, three cylindrical specimens (398 Â 3 specimens in total), 5. NRM characteristics and magnetostratigraphic correlation 2.5 cm long and 2.5 cm in diameter, were taken with a portable, gasoline-powered drill. The attitude (azimuth of dip direction and The intensity of the natural remnant magnetization (NRM) is on dip angle of its axis) of each specimen and the stratum was mea- the order of 10À4 or 10À5 A/m. For most specimens, the NRM con- sured with a compass and an inclinometer, which were coupled sists of two components, usually a low temperature (secondary) to the sampling tube. The specimens were measured at either component, and a high temperature ChRM (often considered to the California Institute of Technology or Occidental College. Both be a primary NRM). The low temperature remnant magnetization laboratories have identical sample changer and magnetometer is clearly removed by LT (low temperature treatment in a LN2 bath set-ups. The magnetometers have a background noise of <1 pAm2 to remove potential multi-domain viscous magnetizations) and AF and are equipped with a vacuum pick-and-put, computer- (0–100 Oe). Below 200 °C thermal demagnetization most speci- controlled sample handling system. Remnant magnetization was mens show clear characteristic remnant magnetization directions measured using a 2G cryogenic magnetometer system housed in (ChRM) that decay towards the origin (Sup-Fig. 1). magnetically shielded l-metal rooms, following standard proce- Commonly, a secondary magnetic component aligned with the dures (e.g., Heermance et al., 2007). present-day magnetic field was found, but the ChRM commonly

Fig. 6. Miocene growth strata and growth unconformity in the northern margin of the Wushan basin. Growth strata and growth unconformity developed in conglomerates on the northern margin of the Wushan basin. Bed surfaces, the growth unconformity, dip azimuth and dip angle, and three minor fault surfaces are marked in the photo. For location see Figs. 2 and 4B. Photo taken looking toward the west. 196 Z. Wang et al. / Journal of Asian Earth Sciences 44 (2012) 189–202 decays linearly to the origin. For most specimens, demagnetization part of section A and sample W265 in the lower part of Sub- at 680 °C removed most of the remnant magnetization. The maxi- section B are normal suggests that no polarity zone nor a short mum unblocking temperature of 680 °C indicates that hematite reversed chron was lost between sub-sections A and B. Using a is the main magnetization carrier, but an accelerated decay of distinctive grey to pale green granule layer, sampling was the magnetization between 500 and 580 °C has been observed that transferred from sub-section B to C. Sample W297 in the upper indicates that magnetite is also an important carrier. part of sub-section B and sample W298 in the lower part of Paleomagnetic directions have been determined for all speci- sub-section C both indicate normal polarity. Combining field esti- mens by principal component analysis of the ChRM (Kirschvink, mation and magnetic polarity correlation, a gap of 16 m must 1980) as implemented in PaleoMag 3.1.0 b1 software (Jones, separate sub-section B and C, and we suspect that at least one 2002), and virtual geomagnetic poles (VGP) are also computed. short duration reversed polarity and a short normal polarity chron About 20% of the specimens are excluded in our final paleomag- have been missed due to this gap. netic analysis on the basis of three criteria: (1) ChRM directions Sub-section D is about 300 m to the north of sub-section C. Due could not be determined because of an ambiguous or noisy orthog- to the heavy loess cover, we cannot trace the layer or follow any onal demagnetization diagram; (2) maximum angular deviation distinct layers, but based on the attitude of strata, we used GPS (MAD) angles greater than 15° in the principal component analysis locations to ensure that we sampled an overlap between sub- of ChRM directions; or (3) virtual geomagnetic pole (VGP) latitude sections C and D. The observation that samples W391 to W393 values were less than 30°. in the upper part of section C are normal polarity and samples Following a distinctive green sand-mud-micrite layer, a trans- W394–W396 in the lower part of sub-section D are reverse polar- lation was made between sub-sections A and B, but due to the ity verifies that parts of a normal chron and a reversed chron were heavy cover of loess, an 5-m gap is estimated between sub- sampled in both sections, suggesting 20 m of overlap between sections A and B. The result that both sample W264 in the upper sub-sections C and D.

Fig. 7. (A) Granule conglomerate, sand, mudstone and paleosol sequence in unit one. (B) Carbonate nodules in unit one. (C) Brick-red, violet, and green/blue, mudstone, silty mudstone in the upper part of unit two. (D) Thin or laminated mudstone intercalated with thin gypsum layer in unit two. (E) Coarse sand, ﬁne sand and siltstone layers in unit three. (F) Alluvial conglomerate and muddy sandstone in unit four. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.) Z. Wang et al. / Journal of Asian Earth Sciences 44 (2012) 189–202 197

According to field observations, sub-section D can be translated A statistical assessment of the number of reversals expected to sub-section E with some overlap, and magnetic results demon- (42) given our sampling spacing for the periods 6–14.5 Ma, how- strate that this overlap may reach 30 m, and a reversed polarity ever, is greater than the actual number of reversals (35) that we de- and part of a normal polarity is sampled in each sub-section. tected (Johnson and McGee, 1983). This suggests that we have not According to field measurements with a Jacob staff and geomet- recovered the complete magnetostratigraphy. Furthermore, we do ric calculations, the gap between sub-sections E and F is about not use the more conservative criterion of defining each magnetoz- 20 m (±10 m). Except the upper 100 m of sub-section F, other one with two or more sites because the resulting magnetostratig- parts of the section are well exposed and sampled. Sample W459 raphy has even fewer reversals (31) than expected (42). The in the upper part of sub-section E shows normal polarity, and sam- potential cause of missing chrons within Wushan MPTS section ple W460 in the lower part of sub-section E shows reversed polar- may be the sampling gaps between sub-sections and erosional hia- ity. Thus, one or two polarity zones are missing in the 20 m tus.The correlation of the magnetostratigraphic polarity of the (±10 m) gap between sub-sections E and F or due to a hiatus in Wushan section with the ATNTS2004 timescale of Lourens et al. the section. (2005) is plausible because long duration normal and reversed Finally, 18 normal (N1–N18) and 17 reversed (R1–R17) magne- zones can be correlated to similar zones of the ATNTS2004 tozones (including the inferred R9) were determined and made (Fig. 5, Table 2 in Supplementary materials). However, there are into a single composite magnetostratigraphic column (Fig. 5). some minor differences (or uncertainties) between the Wushan Reversal boundaries were drawn at the midpoint between two polarity and the ATNTS2004. sites with opposite polarities. All inferred chrons are determined N12 is correlated with C5An.1n through C5An.2n. The possible by at least two samples with the exception of N3 (defined by missing polarity zone in N12 may be caused by the 5 m sampling W472 and W473, where the VGP of W472 < 30°), N10, N13 and gap between sub-sections A and B, though this would necessitate a N14 (one sample). If some single polarities are deleted, the general low rate of accumulation (50 m/Myr) for the duration of this gap. pattern of the Wushan magnetostratigraphy would not change N10 is correlated to C5r.2r-1, while Chrons between C5r.2r-1 and dramatically. C5n.2n may be lost due to the 16 m sampling gap (denoted as The paleomagnetic data passed the reversal test with a ‘‘B’’ R9). The reversed zone R8 may correspond with one of three excur- quality classification (Sup-Figs. 2a and 2b). The fold test analysis sions/crypto-chrons recently found in the long normal zone C5n.2n shows that the grouping of ChRM magnetic directions clusters (Evans et al., 2007). This correlation requires a high instantaneous most tightly near 81% untilting of bedding orientations (2 sigma rate of deposition (>4000 m/Myr), though it is only for a short confidence limits of 65–98% untilting; Sup-Fig. 3). Compared with 5000 year duration. an expected fold test result of 100% untilting if the magnetization C4Ar.2n, C4r.1n, and C3Br.1n are all short chrons and may have for all samples was acquired solely during deposition, our obtained been lost due to hiatuses. C4.2r-1 may have been lost due to the result of 81% untilting may suggest that some component of the 12 m gap in the lower part of sub-section E. Finally, the normal measured magnetization for some samples was acquired syn- chron C3Br.2n may have been skipped due to the 20 m gap be- deformation. Because deformation is quite complicated in this re- tween sub-sections E and F. gion, this 81% untilting result may have been caused by vertical Based on our correlation (Fig. 5), the top layer of the section is axis rotation, a plunging fold axis, or another structural/tectonic younger than 6.033 Ma (the upper age of C3An.1n). Here we tenta- feature. The Jackknife test (Tauxe and Gallet, 1991) gives tively assign it an age of 6 Ma ± 1 Ma. The bottom of the mea- J = À0.105 (Sup-Fig. 4), which falls within the recommended range sured section falls in the lower part of C5ADn between14.581 Ma of 0 to À0.5 for a robust magnetostratigraphic dataset. and 14.194 Ma, so its age is estimated to be 14.5 Ma. However, Although the fold test analysis yields only ‘‘81% untilting’’, the this is not the base of the Miocene basin. The lower part of the Mio- combination of the positive results from the reversal test, the cene sedimentation in Wushan basin is exposed in a 300 m long Jackknife test, and the general pattern of the magnetostratigraphy section located along the northern margin of the basin (Fig. 5), but still strongly suggest that these ChRM directions were acquired by its total thickness is still unknown and cannot be precisely corre- the sediment at or soon after deposition. lated to the measured section. In the region to the south of the According to the geologic relationship between the strata in measured section, most of the lower part of the basin is covered Wushan basin and that in Tange basin to the south, combined with by Quaternary sediments or is eroded by the Weihe River. Accord- their partial similarity with Cenozoic strata in Longxi and Linxia ing to the Cenozoic structure, the geometry of the strata in the ex- basin (Fang et al., 2003; Deng et al., 2004), we can confine the strata posed section, and the relation between red beds and the bedrock in Wushan basin to Miocene age. Additionally, a number of fossils, in the south margin of the basin, it is estimated that there may be including Stephanocemas sp. near sample W285 (425 m), 300 m of additional stratal thickness to the base of the measured Kubanochoerus sp. near W255, and Gomphotherium sp. near sample basin fill. Assuming the same sedimentation rate as in the mea- W245 (280 m), have been found in the lower part of the measured sured section, extrapolation to the base of 300 m of additional section (Fig. 5). These fossils are typically mid-middle Miocene deposits below the bottom of the measured section results in an (between 12 Ma and 19 Ma, personal communication with Zhang 16 Ma age for the base of the basin fill (Fig. 8). Given a 150 m Zhaoqun). Stephanocemas (Artiodactyla, Cervidae) are also found uncertainty for the thickness estimation, the age for the base of in Olongbuluk Fauna on Huaitoutala section of Qaidam basin and the basin fill would be 16 ± 1 Ma.When uncertainties are consid- are confined in this basin to be 12 to 13 Ma (Fang et al., 2007; Wang ered, an alternative correlation is presented (Sup-Fig. 5) in which et al., 2009). The Wushan fossil assemblage helps tie N11–N18 of the the 35 chrons of the Wushan section may be matched to chrons section to C5r.2n–C5ADn of mid-middle Miocene. C2n through C5An.1n of the ATNTS2004 timescale (Lourens et al., Our preferred correlation is presented in this paper. 35 magne- 2005). This correlation changes the age of the Wushan section to tozones are correlated to chrons C3An.1n through C5ADn of the be 1.8 Ma to 10 Ma. However, When N4–N9 (or N9 + N10) are ATNTS2004 timescale (Lourens et al., 2005)(Fig. 5). The correlated with C3n.1n C3An.2n, the matching of other magne- ATNTS2004 time scale (Lourens et al., 2005) is similar to the stan- tozones to GPTS is not straight-forward and incurs serious uncer- dard Geomagnetic Polarity Time Scales (GPTS) (Cande and Kent, tainties (Sup-Fig. 5). Additionally, this alternative correlation 1995), but with more details like normal chrons C4.2r-1 and requires larger fluctuations in sediment accumulation rates and C5r.2r-1 as well as a revised absolute chronology based on astro- does not satisfy the fossil age constraints (i.e. the alternative corre- nomical tuning. lation requires an age of 7–8.5 Ma for the fossils, which is unlikely 198 Z. Wang et al. / Journal of Asian Earth Sciences 44 (2012) 189–202

inception of the Wushan basin and the cessation of sedimentation in the Tange basin imply that another pulse of deformation occurred in the middle Miocene (16 Ma) along the West Qinling fault zone. Sedimentary facies, paleoflow directions, and stratal geometry in the Wushan basin provide insights into the nature of basin evolution. Red beds of the Wushan basin unconformably overlie pre-Neogene deposits, with growth strata and a growth unconformity near the base of the basin fill (Figs. 4–6). Therefore, the 16 ± 1 Ma age represents the onset of Wushan basin sedimentation and the age of syn-depositional southward contractional deformation that generated the Wushan basin. As we discussed above, the strata along the northern margin of the basin at the base of the basin fill consists mostly of conglomerate characterized by Fig. 8. Sediment accumulation curve for the Wushan basin. The average rate of angular pebbles, cobbles, and boulders (Figs. 4 and 5b) that have sediment accumulation is 160 m/Myr (ignoring compaction). The rate fluctuates undergone minimal transport from their source. Thus, we infer from 100 m/Myr to 200 m/Myr between 14.5 Ma and 6.5 Ma, and increases from 130 m/Myr to 400 m/Myr since 6.5 Ma. The age for the base of the that young relief, associated with slip on an active fault, provided Wushan basin is estimated by extrapolation. coarse sediment (e.g., McPherson et al., 1988) to the northern margin of the Wushan basin during the middle Miocene. Measure- ments of imbricated gravel indicate southward paleoflow (Fig. 5A), given known age ranges for these fossils in nearby basins and Asia consistent with derivation from the northern basin-bounding fault. as a whole). Other alternative correlations have also been consid- These observations suggest that the West Qinling fault zone was ered, but none satisfies as many magnetostratigraphic or other active during middle Miocene time and served as the northern requirements as our preferred correlation that we show in Fig. 5. boundary of the early Wushan basin. We evaluate the nature of deformation along the West Qinling 6. Discussion fault zone through observations of Wushan basin evolution and other activity along the fault zone at that time. Two possible 6.1. Cenozoic tectonics of the West Qinling fault zone end-member explanations can be offered for the formation of the Wushan basin (Fig. 9). The West Qingling fault zone follows a deeply rooted structure In the first end-member case, the West Qinling between the with a long, complicated history that dates back to at least Paleo- modern Wushan and Longxi basin, was deformed under NNE- zoic time (Zhang et al., 2004; Enkelmann et al., 2006). Its tectonic SSW compression that developed since the middle to late deformation is also complicated in the Cenozoic era. Paleogene, and one fault of the West Qinling fault zone acted as According to low-temperature thermochronometry, thrusting a northward vergent thrust that was rooted into the margin of (northward) began since late Paleogene time (45 Ma) along the the Tibetan plateau to the south. Accompanying the primary north West Qinling fault zone to the west of Taohe river, and caused ra- vergent thrust fault, a backthrust formed and caused subsidence to pid exhumation and emergence of the block to the south of the the south of the fault zone forming the Wushan basin (Fig. 9a). In West Qinling fault zone relative to that to the north (Clark et al., this case, the Longxi basin to the north of the fault zone would de- 2010)(Fig. 1). This deformation is regarded as the early response velop in association with the load of the Western Qinling thrust to the collision between the India and Eurasian continent (Clark wedge, and its subsidence should be greater than that in Wushan et al., 2010). Basins developed along the West Qinling fault zone basin. However, there is no evidence of accelerated subsidence since the late Paleogene. Deposition in the late Paleogene and/or an obvious increase in the rate of deposition in Longxi basin. Xunhua–Linxia basin was ongoing by 30 Ma (Fang et al., 2003). Furthermore, the Miocene strata in Longxi basin are much thinner Thrusting along the West Qinling fault was probably the main (max. 600 m) than its counterpart in Wushan basin (1700 m). source of flexural loading for the generation of the Xunhua–Linxia In the second end-member case, the Wushan basin developed basin (Fang et al., 2003). The Paleogene thrusting may have also due to transpressional deformation along the West Qinling fault deformed the eastern region along Wushan and Tianshui to gener- zone (Fig. 9b and c). As is observed in other strike slip settings, ate the Longxi and Tange basins to the north and south of the West the distribution and rate of subsidence can vary dramatically in Qinling fault zone respectively (Wang et al., 2006). Paleogene red sedimentary basins that form along these structures. Transpression beds of similar lithology are exposed in both the Longxi and Tange could result in rapid subsidence associated with shortening along basins (Figs. 2 and 3). Typical deposits include brick red sandstone the south side of the fault zone and cause thick deposits in the and conglomerate like that exposed in the Tala formation of Linxia Wushan basin, while generating lower sedimentation rates in the basin (Fang et al., 2003). Paleocurrent directions measured from Longxi basin to the north. imbricated conglomerate clasts are opposite for the Tange basin We prefer the second explanation based on several lines of evi- (southward) and Longxi basin (northward) (Wang, unpublished dence. Middle Miocene syn-depositional contraction has been data), which shows that the West Qinling belt between the Tange verified by the observation of growth strata and growth unconfo- and Longxi basins must have already been active by the late rmities, like those described above (Figs. 4, 5B, and 6). However, Paleogene. more evidence is needed to discern between transpressional fault- No Miocene deposits have been identified in the Tange basin, ing versus solely thrust faulting along the West Qinling fault zone whereas several hundred meters of red beds were deposited during in middle Miocene time. the Miocene in the Longxi basin. In sharp contrast, 1700 m of Although the current trace of the active strike-slip fault in the strata were accumulated in the neighboring Wushan basin since western part of the basin has not influenced past deposition in the middle Miocene (16 Ma ± 1 Ma). Thus, the history of the the Wushan basin, previously active faults along the margins of Wushan basin is different from that of the Tange and Longxi basin the basin that parallel the West Qinling fault developed in the and reflects relatively high rates of Miocene sedimentation proxi- early-middle Miocene and controlled the formation and deposition mal to the West Qinling fault zone (Fig. 3). The middle Miocene of the basin. The Zhangjiashan-Caopingshang fault on the northern Z. Wang et al. / Journal of Asian Earth Sciences 44 (2012) 189–202 199

Fig. 9. Models for the Miocene deformation and deposition in West Qinling region. (A) Model 1 – The Wushan basin was created by back thrusting along the WQNFZ, while the Longxi basin continued to develop as a ﬂexural basin caused by thrust wedge of the West Qinling range. (B) Model 2 – The Wushan basin was generated by transpressional deformation, that led to rapid subsidence and thick sedimentary deposits. (C) When transpressional and transtensional deformation occurred along the West Qinling fault zone in the middle Miocene, extensional environments were generated in the interior of the West Qinling range. Basalts were erupted. Some basins ceased deposition and new basins were formed, like the Wushan basin and Tianshui basin. Other basins show evidence of becoming partitioned at this time (e.g. Xunhua–Linxia basin). However, during this same period, the basins north of the West Qinling and the Laji Shan, such as the Xining, and Lanzhou basin show no evidence of being interrupted or internally deformed. WS: Wushan basin, TS: Tianshui basin, GD: Guide basin, TR: Tongren basin, XL: Xihe–Lixian basin.

margin of the basin in the west separates Miocene strata from sedimentation suggest that the formation and evolution of the Paleogene and Cretaceous rocks to the north (Fig. 2). It strikes Wushan basin was controlled by activities of the West Qinling fault roughly east-west, and dips nearly vertically, suggesting a strike- zone. We propose that the Wushan basin formed in a transpres- slip geometry. sional setting, and the Tianshui basin formed in a transtensional Not only did the Wushan basin develop along the modern West setting as a pull-apart basin related to the West Qinling fault zone Qinling fault zone, but the Tianshui basin to the east of Wushan (Fig. 9C). also shows a close relationship with the West Qinling fault zone (Fig. 1). According to a previous geological survey (RGTG, 1968) and our field observations, a currently active fault with both 6.2. The sedimentation history of the Wushan basin in middle Miocene strike-slip and normal displacement components (Li, 2005) makes time up the northern boundary of the Tianshui basin. The thickness of Miocene strata in Tianshui basin is greater than 1000 m, and this Approximately 1700 m of basin fill accumulated in the Wushan basin is similar to the Wushan basin in terms of lithostratigraphy. basin between 16 Ma ± 1 Ma and 6 Ma ± 1 Ma, consisting of a fin- With correlation to the red beds in the Yaodian section to the north ing upward succession from unit one to unit two, with a return of Tianshui, the Tianshui basin is inferred to be middle Miocene in to coarse sedimentation in units three and four. The average rate age, older than the 12.4 Ma age of the Yaodian section (Li et al., of sediment accumulation is 160 m/Myr (Fig. 8; ignoring compac- 2007). tion). This rate is about half of the average rate (4050 m/12.8 Myr) In the Wushan basin the elongate shape, relatively higher rate of Huitoutala section in eastern Qaidam basin (Fang et al., 2007). of sedimentation when compared with that in neighboring basins, However, it is higher than the average sedimentation rate of the syn-depositional deformation, and close relationship with the closer Maogou (350 m/17.06 Myr) and Wangjiashan sections West Qinling fault zone, as well as the middle Miocene onset of (900 m/15 Myr) in Linxia basin that lies on the northern side of 200 Z. Wang et al. / Journal of Asian Earth Sciences 44 (2012) 189–202 the West Qinling fault zone and about 150 km northwest of the ranges, and basalt eruption along or to the south of the West Wushan basin. Qinling fault zone during the middle Miocene (16–13 Ma) sug- The Miocene Wushan magnteostratigraphic section that we gest that the time period 16 Ma may mark a fundamental change measured began at 14.5 Ma. The sedimentation rate shows fluc- in the kinematics of the West Qinling range, when NNE–SSW tuations from 100 m/Myr to 200 m/Myr between 14.5 Ma contractional deformation ceased/weakened and E–W trending and 6.5 Ma (Fig. 8). The rate of sediment accumulation decreased strike-slip and transpression motion began. Small extensional in association with the return to coarse-grained deposition at basins and basaltic eruption were common during this time period 9.5 Ma (alluvial and braided stream conglomerate). The rate in- in the Tongren, Tianshui, Xihe–Linxian, Niudingshan, and Nanyang creased from 130 m/Myr to 400 m/Myr since 6.5 Ma in associ- basins, possibly associated with transtensional deformation along ation with deposition of poorly sorted sandstone intercalated with the fault. Likewise, rapid sedimentation in the Wushan, Linxia, conglomerate. We tentatively interpret the appearance of con- and Xunhua basins associated with contractional deformation sug- glomerate at 9.5 Ma to reflect renewed deformation along the gest regions of tranpressional deformation along the West Qinling West Qinling fault zone, with the sedimentation rate increase since fault zone. New basin development and basaltic eruptions in the 6.5 Ma signaling the climax of this deformation. This deformation middle Miocene in the region to the south of West Qinling fault is in concordance with the previously reported events since zone could be associated with the transfer of deformation from 9.5 Ma in the Linxia basin (Fang et al., 2003; Yuan et al., 2003; the East Kunlun strike-slip fault via a left step-over (Zhang et al., Zheng et al., 2003), in the LiuPan shan and neighboring basins 1995), or it may have been dynamically driven by lower crustal (Song et al., 2001; Jiang et al., 2007; Lin et al., 2010), and along flow (Royden et al., 1997; Beaumont et al., 2001; Clark and Royden, the entire Tibetan plateau, including its NE portion (Molnar et al., 2000; Enkelmann et al., 2006) or the removal of mantle lithosphere 2005, Zhang et al., 2006; Fang et al., 2007). (Molnar et al., 1993). Accompanying this change, the long-term slip rate along the Altyn Tagh fault decreased significantly to less than 6.3. Middle Miocene tectonic activity of the West Qinling range and its 10 mm/a since the end of the early Miocene (Yue et al., 2003). significance Therefore, the emergence of E–W trending strike-slip motion along the West Qinling fault zone may have resulted from the slowing of Numerous basins, besides the Wushan and Tianshui basin, the Altyn Tagh fault. Regardless of the precise mechanisms, the developed along the West Qinling range in Miocene time, including middle Miocene (16 Ma) event corresponds approximately with the Guide, Tongren, Ganjia, and Xihe–Lixian basins to the south of the slowing down of India’s convergence with Eurasia by more the West Qinling fault zone (Fig. 1). Another striking phenomenon than 40% between 20 and 10 Ma, and the onset of rapid outward in the West Qinling is the eruption of Miocene basic and ultrabasic growth along margins of Tibetan plateau (Molnar and Stock, 2009). igneous rocks (Fig. 1). In the Tongren basin, several hundred meters of sedimentary rock accumulated, with maximum thickness along its western margin, which is bounded by a normal fault. In 7. Conclusion the Xihe–Lixian basin, not only were several hundred meters of red beds deposited, but basalt, tuff, and volcaniclastic layers are (1) The Wushan basin is an elongate basin with deposits distrib- also commonly interbedded in the Miocene red beds. The volcanic uted in a narrow region along the West Qinling fault zone. eruption ages are concentrated between 14 and 7 Ma in and Approximately 1700 m of red beds were deposited between around this basin (Yu et al., 2001; Wang and Li, 2003), consistent 16 Ma (±1 Ma) and 6 Ma on the basis of magnetostratig- with the age of strata in the Wushan basin. In the Nanyang and raphy. The Miocene strata consist of a fining upward succes- Niudingshan basins near Dangchang, early-middle Miocene basalt sion from unit one to unit two, with a return (at 9.5 Ma) to (Horton et al., 2004) overlie or cut late Paleogene sandstone and coarse sedimentation in units three and four. The average conglomerate (Fig. 1). Furthermore, basaltic eruptions are also rate of sediment accumulation is 160 m/Myr (ignoring found in the western part of the West Qinling range (Zheng compaction). It shows minor fluctuations between 100 et al., 2010). The basaltic volcanic rocks in Duofutun region to m/Myr to 200 m/Myr between 14.5 Ma and 6.5 Ma. the southwest of the Tongren basin of Qinghai were erupted at Since 6.5 Ma, however, the sedimentation rate increases 14 Ma, based on zircon dating (Zheng et al., 2010). Accompanying from 130 m/Myr to 400 m/Myr in association with depo- the formation of new basins and widespread volcanic eruption in sition of poorly sorted sandstone intercalated with conglom- middle Miocene time, increased exhumation has also been docu- erate. This facies change since 9.5 Ma and sedimentation mented along the West Qinling fault zone. The West Qinling block rate increase since 6.5 Ma may be regarded as proxies for between Linxia and Xiahe underwent an increase in exhumation the previously reported late Miocene deformation event in rate in the early to middle Miocene time (18 Ma) (Clark et al., NE Tibetan plateau (Molnar et al., 2005; Zhang et al., 2006; 2010) that may be related to the faulting of the West Qinling fault Fang et al., 2007). zone. According to detrital thermochrological dating (Zheng et al., (2) The synclinal geometry, high rates of sedimentation, coarse-

2003) and eNd values (Garzione et al., 2005) in the Linxia basin, the grained nature of the deposits, and syn-depositional West Qinling range experienced increased erosional unroofing at deformation near the base of the Wushan section along its 14–15 Ma. The Laji shan underwent rapid exhumation during northern margin indicate that the Wushan basin was gener- the middle Miocene (since 16.2 Ma) (Dupont-Nivet et al., 2007). ated in association with tectonic deformation along the Recently, stable isotope records from the Neogene Xunhua and West Qinling fault zone. This fault zone was active as a Linxia basins also indicate that the Jishi Shan emerged as a thrust since the late Paleogene. The formation of the significant topographic barrier between 16 and 11 Ma (Hough Wushan basin (16 ± 1 Ma) and the contemporaneous et al., 2010). Finally, the magnetostratigraphy and sedimentary distribution of basin development and associated basaltic record of the Xunhua basin, as well as thermochronology from eruptions along the length of the West Qinling fault zone the Jishi Shan, indicate that the Jishi Shan range was deforming in the middle Miocene define the time as one of a fundamen- by 13 Ma associated with east–west contraction along the tal change in tectonic style along the West Qinling range eastern mountain front (Lease et al., 2011). from NNE–SSW contractional deformation to E–W trending In summary, rapid deposition and syndepositional deformation strike-slip deformation, with the Wushan basin likely of basins, deformation and exhumation of intervening mountain forming in a zone of transpression. Z. Wang et al. / Journal of Asian Earth Sciences 44 (2012) 189–202 201

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