Magnetostratigraphy of the Xiaolongtan Formation Bearing

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Tectonophysics 655 (2015) 213–226

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Magnetostratigraphy of the Xiaolongtan Formation bearing Lufengpithecus keiyuanensis in Yunnan, southwestern China: Constraint on the initiation time of the southern segment of the Xianshuihe–Xiaojiang fault

Shihu Li a,⁎, Chenglong Deng a,⁎, Wei Dong b,LuSuna,SuzhenLiua, Huafeng Qin a,JiyunYinc, Xueping Ji d, Rixiang Zhu a a State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China b Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing 100044, China c Yunnan State Land Resources Vocational College, Kunming, Yunnan 650217, China d Department of Paleoanthropology, Yunnan Institute of Cultural Relics and Archeology, Kunming 650118, China article info abstract

Article history: The late Cenozoic extensional basins in Yunnan Province (southwestern China), which are kinematically linked Received 23 October 2014 with the regional strike-slip faults, can provide meaningful constraints on the fault activity history and tectonic Received in revised form 20 May 2015 evolution of the southeast margin of the Tibetan Plateau (SEMTP), and further on the geodynamic evolution of Accepted 2 June 2015 the Tibetan Plateau. However, this has been severely impeded by the lack of precise age constraints on the timing Available online 11 June 2015 of fault activity. To better constrain the timing of fault activity and the tectonic rotation of SEMTP, we undertook a high-resolution magnetostratigraphic study on the Xiaolongtan Formation in the Xiaolongtan Basin, which is lo- Keywords: – Magnetostratigraphy cated at the southern tip of the Xianshuihe Xiaojiang fault and is well-known by the presence of hominoid Xianshuihe–Xiaojiang fault Lufengpithecus keiyuanensis. Rock magnetic experiments indicate that magnetite is the main remanence carrier. Xiaolongtan Formation Correlation to the geomagnetic polarity timescale was achieved by combining magnetostratigraphic and bio- Lufengpithecus keiyuanensis stratigraphic data. Our correlation suggests that the Xiaolongtan Formation sedimentary sequence spans from Tectonic rotation Chron C5Ar.1r to Chron C5n.2n, which indicates that the age of the Xiaolongtan Formation ranges from Southeast margin of the Tibetan Plateau ~10 Ma to 12.7 Ma, and that the ages of the two sedimentary layers possibly bearing the hominoid L. keiyuanensis are ~11.6 Ma or ~12.5 Ma. The basal age of the sediments is 12.7 Ma, which indicates that the ac- tivation of the southern Xianshuihe–Xiaojiang fault was initiated at this time. The overall mean paleomagnetic

direction (D = 353.2°, I = 34.2°, α95 = 2.1°, n = 166) documents a counter-clockwise vertical axis rotation of −8 ± 3° with respect to Eurasia, which is the response to the activity of the left-lateral Xianshuihe–Xiaojiang Fault. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Ailao Shan-Red River, and Karakoram faults) accompanied by large- scale rotations (Leloup et al., 1995; Tapponnier et al., 1982, 1990). The As an important accommodation zone during post-collisional second model, however, suggests continuous deformation throughout intracontinental deformation between India and Eurasia, the Cenozoic the entire lithosphere, and the convergence of India–Eurasia collision tectonic deformation of the southeast margin of the Tibetan Plateau is dominated by broadly distributed crustal shortening and thickening (SEMTP) can provide meaningful constraints on the geodynamic evolu- within the plateau (England and Houseman, 1986; Houseman and tion of the Tibetan Plateau (Clark and Royden, 2000; Tapponnier et al., England, 1986). The key point of debate in the two models is whether 1990; Wang et al., 1998). Two end member models have been proposed the large-scale strike-slip faults play dominant roles in accommodating to interpret the crustal shortening caused by the India–Eurasia collision the convergence of the India–Eurasia collision or they are just by- since the early Cenozoic. The ﬁrst model emphasizes the role of strike- products of crustal thickening (Searle, 2006; Searle et al., 2011). There- slip faults, which suggests that the shortening is mostly accommodated fore, the history of fault activity and rotation pattern in SEMTP is crucial by rigid block lateral extrusion along large-scale faults (e.g., Altyn Tagh, to understanding the geodynamic evolution of the Tibetan Plateau. Paleomagnetism is one of the most valid tools to quantify the region- ⁎ Corresponding authors. al tectonic rotations. To constrain the rotation of SEMTP, extensive E-mail addresses: [email protected] (S. Li), [email protected] (C. Deng). paleomagnetic studies were undertaken in the last three decades

(e.g., Chen et al., 1995; Chi and Geissman, 2013; Huang and Opdyke, The left-lateral Xianshuihe–Xiaojiang fault is considered as a bound- 1993; Huang et al., 1992; Kondo et al., 2012; Li et al., 2013; Otofuji ary fault to accommodate the differential rotations around the eastern et al., 2010; Tong et al., 2013; Yang and Besse, 1993; Yang et al., 1995; Himalayan syntaxis (Schoenbohm et al., 2006b; Wang et al., 1998). It Zhu et al., 2008). However, most of these studies concentrated on the has a length of more than 1000 km, extending from eastern Tibet in Mesozoic, especially Cretaceous and early Cenozoic rocks (Li et al., the northwest to the south of Yunnan and terminating before reaching 2012). On the other hand, although geological studies have largely im- the Red River fault (Fig. 1). It is a single fault in the northwest segment proved our understanding of the slip history of a few faults, the timing (i.e., Xianshuihe fault), and branches to two faults in the central part of most faults in SEMTP remains unknown or controversial (Leloup (i.e., the Anninghe and Shimian faults). Further to the south, the et al., 2001; Wang et al., 1998, 2009). The lack of Cenozoic paleomagnet- Xianshuihe–Xiaojiang fault comprises at least ﬁve branching faults: ic studies and precise age constraints on the strike-slip faults prevents Yuanmou, Lufeng, Puduhe, Xiaojiang and Qujing faults from west to us from better understanding the effects of the India–Eurasia collision east (Fig. 1, Wang et al., 1998). The total offset along the Xianshuihe– on SEMTP; consequently, more studies are needed. Xiaojiang fault, which is about 78–100 km, is partitioned along the nu- Many strike-slip faults, such as the Red River, Xianshuihe–Xiaojiang merous branching faults or transferred to extension locally, resulting and Sagaing faults (Fig. 1), extended eastward from the interior of Tibet in a number of basins (Wang et al., 1998). Despite its importance in un- to Yunnan. Most of these faults have normal dip-slip components, derstanding the tectonic evolution of SEMTP, the initiation time of the resulting in a number of concomitant basins during the fault activity different segments of the Xianshuihe–Xiaojiang fault is still controver- (Fig. 1, Wang et al., 1998; Schoenbohm et al., 2006a; Wang et al., sial. Zircon U–Pb and 40Ar/39Ar dating in the northwest Xianshuihe– 2008). Thus, the sediments deposited in the fault-related sedimentary Xiaojiang fault suggested that the fault activity began at 12.8 Ma basins record the fault activity, basin inﬁlling and regional tectonic (Roger et al., 1995; Zhang et al., 2004b). Wang et al. (1998) suggested rotations. Therefore, detailed paleomagnetic studies on these Neogene that displacement of the Xianshuihe–Xiaojiang fault began at least at sedimentary basins, which can provide precise age constraints on the 4Ma.Wang et al. (2009) claimed that the Xianshuihe–Xiaojiang fault sediments and regional tectonic rotation, are essential. initiated at ~13 Ma on the Xianshuihe fault in the northwest segment

Fig. 1. Schematic tectonic map of Yunnan Province showing the major Cenozoic tectonic features, modified after YBGMR (1990). Red dots and numbers in parenthesis refer to the location and age of hominoids in Southeast Asia, respectively. ALSSZ: Ailao Shan–Red River Shear Zone; CSSZ: Chongshan Shear Zone; DCSSZ: Diancangshan Shear Zone; GLGSZ: Gaoligong Shear Zone; F1: Yuanmou fault; F2: Lufeng fault; F3: Puduhe fault; F4: Xiaojiang fault; F5: Qujing fault. S. Li et al. / Tectonophysics 655 (2015) 213–226 215 and at ~5 Ma on the Xiaojiang fault in the south. Zhu et al. (2008) pro- zones were transformed to a series of brittle faults during late Cenozoic posed that the Yuanmou fault also began at ~5 Ma. Therefore, more (e.g., the Red River fault and the Dali fault system), which may indicate studies on the timing of the Xianshuihe–Xiaojiang fault, especially for changes in the geodynamic evolution of the Tibetan Plateau (Wang the southern Xianshuihe–Xiaojiang fault, are needed. et al., 1998). The SEMTP is also a key area to improve our understanding of The Xiaolongtan Basin (103°10′–13′ E, 23°46′–49′ N), which is a Miocene hominoid evolution and its relationship with the tectonic pull-apart basin caused by the left lateral Qujing fault (Fig. 2b, Wang and climate change as many hominoid fossils were discovered in et al., 1998), is located 16 km northwest of Kaiyuan City in Yunnan SEMTP (Fig. 1). They are (1) Lufengpithecus chiangmuanensis from Province with an elevation of 1030–1110 m. The elliptical shaped Chiang Muan Basin, northern Thailand (Chaimanee et al., 2003), basin has an area of about 21 km2 and contains the Xiaolongtan coal (2) Khoratpithecus piriyai from Khorat in northeast Thailand mine where Cenozoic lignite is exposed. The Xiaolongtan Formation (Chaimanee et al., 2004), (3) Khoratpithecus ayeyarwadyensis from overlies the Triassic limestone with unconformity and is unconformably Myanmar (Jaeger et al., 2011), (4) Lufengpithecus keiyuanensis from overlain by the Plio-Pleistocene Hetou Formation (Fig. 2a, YBGMR, Xiaolongtan Basin (Woo, 1957; Zhang, 1987), (5) Lufengpithecus 1990). The unconformity of Cenozoic rocks on older strata is very com- lufengensis from Shihuiba in the Lufeng Basin (Woo et al., 1981), mon in Yunan Province. The Paleogene sediments are mainly distribut- (6) Lufengpithecus hudienensis from Xiaohe, Leilao and Zhupeng in the ed in the Lanping-Simao Basin and Chuxiong Basin (Fig. 1). They rest Yuanmou Basin (Jiang et al., 1987), (7) L. lufengensis from Shuitangba unconformably on Mesozoic terrestrial red beds, and locally they direct- in the Zhaotong Basin (Ji et al., 2013), and (8) the hominoid from Yangyi ly overlie the Proterozoic to Paleozoic rocks. The Neogene sediments are in the Baoshan Basin, which has not been formally described yet. mostly undeformed, coal-bearing and are widespread in Yunnan L. keiyuanensis is the earliest known hominoid in China. The general Province. They overlie an erosion surface with gentle relief which cuts complexion of L. keiyuanensis closely resembles those from the Chinji across all pre-Cenozoic rocks (Wang et al., 1998). and Nagri Formations of the Siwalik Group in Pakistan (Harrison et al., The Xiaolongtan Formation mainly comprises gray mudstones and 2002; Woo, 1957; Zhang, 1987) and has a close similarity with black lignite. The Hetou Formation is primarily fine-tomedium- L. chiangmuanensis (Chaimanee et al., 2003). Both the Sivapithecus and grained sands and silts interbedded with multi-layers of lignite Lufengpithecus were regarded as common ancestor of human and (Dong, 1987, 2001). However, the Hetou Formation is very limited in great apes. Therefore, the chronological sequence of hominoid horizons the Xiaolongtan Basin; in our studied section, it is limited to a few can provide important clues for understanding dispersal and evolution meters. Vertebrate fossils from the Xiaolongtan Formation include of early hominoids in eastern Asia and its relationship with coeval hom- L. keiyuanensis, Mustelidae indet., Castoridae gen. et sp. indet., inoids in Europe and Africa. The hominoids from Lufeng, Yuanmou and Tetralophodon xiaolohgtanensis, Gomphotherum cf. macrognathus, Zhaotong in Yunnan have been precisely dated by paleomagnetism, Zygolophodon chinjiensis, Tapirus cf. yunnanensis, Parachleuastochoerus however, the age of L. keiyuanensis remains highly debated. Various sinensis, Propotamochoerus parvulus, Hippopotamodon hyotherioides age estimates of the L. keiyuanensis by biochronology have been report- and Euprox sp. (Dong, 1987; Zhang, 1987; Pickford and Liu, 2001; ed: early Late Miocene (MN 9) or earliest Late Miocene, ca. 11 Ma (Dong, Dong and Qi, 2013). 1987; Pickford and Liu, 2001), or slightly older (MN 7 + 8) (Qiu and Qiu, The hominoid fossils in the Xiaolongtan Basin were first found in 1995), or a wider range from MN7 + 8 to MN10 (Dong and Qi, 2013). A 1956 (Woo, 1957). Five hominoid teeth, left and right lower premolars, magnetostratigraphic study was undertaken by Liang et al. (1994) to left and right lower second molars, and right lower third molar, belong- determine the age of L. keiyuanensis. However, their paleomagnetic ing to a single individual were recovered. Later that year, a lower right sampling intervals were too large to provide reliable results; only 37 dentition with five isolated cheek teeth from another individual was re- samples were collected in a 200-m section. The obtained age, ~8.3 Ma, covered from the same site (Woo, 1958). In 1980 and 1982, three isolat- was thought to be younger than that inferred from the mammalian ed molars and a maxillary fragment bearing 12 teeth were also faunas (Harrison et al., 2002). uncovered from the same site (Zhang, 1987). These fossils were first In this study, we conduct a high-resolution magnetostratigraphic in- assigned to Dryopithecus, Ramapithecus or Sivapithecus, and finally to vestigation on the Xiaolongtan Formation in the Xiaolongtan Basin Lufengpithecus, a newly described genus from the Lufeng hominoid in (Fig. 1), which is located at the southern tip of the Xianshuihe– the Lufeng Basin (Woo, 1987; Zhang, 1987; Harrison et al., 2002). Xiaojiang fault and is famous for the occurrence of L. keiyuanensis (Woo, 1957; Zhang, 1987). The aim of this study is to establish a chronology of the Xiaolongtan Formation that constrains the age of 2.2. Sampling L. keiyuanensis and the timing of fault motion along the southern Xianshuihe–Xiaojiang fault. The paleomagnetic results will be used to The section of this study is in the pit of the Xiaolongtan coal mine discuss the Neogene tectonic rotations of SEMTP. with a thickness of ~400 m. The lower part of the section is mainly black or black–brown thin to moderately stratified lignite interbedded 2. Geological setting and sampling with laminated black–gray peaty clay (Fig. 2e and f). The upper part of the section is gray–white moderately massive to massive mudstones in- 2.1. Geological setting terbedded with lignite at the base, which is characterized by conchoidal fractures and contains partial calcarenite bearing twigs, stems, leaf frag- The SEMTP is a fan-shaped area that broadens to the southeast and ments and invertebrates (Fig. 2c and d, Dong, 1987; Dong, 2001). The narrows to the northwest (Fig. 1, Yunnan Bureau of Geology and section shows a noticeable homoclinal tilts to the south–southeast, Mineral Resources (YBGMR), 1990). It is bounded by the Gaoligong– with a progressive shallowing of dip from ~31° to ~10° (Fig. 2). Sagaing fault to the west and the Xianshuihe–Xiaojiang fault to the The position of hominoid fossils in the Xiaolongtan Formation is un- east (Leloup et al., 1995; Wang et al., 1998). To accommodate the south- certain. Zhang (1987) suggested that the hominoid fossils were discov- eastward extrusion of Indochina caused by the India–Eurasia collision, ered in the upper part of the lignite, 20–30 m below the mudstone. the early Cenozoic deformation of SEMTP was dominated by large- Dong (1987) also stated that the fossils were in the upper part of the lig- scale ductile strike-slip shear faults (Fig. 1), such as the right-lateral nite, just below the mudstone. However, Jiyun Yin claimed that the Gaoligong Shear Zone (Wang et al., 2006), the left-lateral Chongshan hominoid fossils came from the lower part of the lignite, ~ 27 m in our and Ailao Shan–Red River Shear Zones (Akciz et al., 2008; Leloup et al., section, based on his communication with the chief geologist in the 1995; Zhang et al., 2010), and large-scale clockwise rotations (e. g., Xiaolongtan coal mine who discovered the hominoid fossils. Since we Yang and Besse, 1993; Li et al., 2012; Tong et al., 2013). The shear have no direct evidence to examine which view about the position of 216 S. Li et al. / Tectonophysics 655 (2015) 213–226

Fig. 2. (a) Cross section of the Xiaolongtan section showing the changing of bedding dips, and the contact relation with upper and lower beds. (b) Illustration showing the structure of the Xiaolongtan basin. The basin was formed as a pull-apart structure by a left step along the Qujing fault, modiﬁed after Wang et al. (1998). Note that the Nanpan River is offset left-laterally at least 3 km and perhaps as much as 8 km. (c) Photo showing the unconformity of the Xiaolongtan formation with the upper Hetou Formation. (d) Photo showing the gray–white massive marls. (e) Photo showing the black lignite interbedded with laminated black–gray peaty clay. The location of the person (red circle) is one of the possible sites of hominoid fossils. (d) Photo showing the unconformity of the Xiaolongtan formation with the underlying Triassic limestone. The locations of photos are marked in label (a). Persons (~1.7–1.8 m) are included for scale.

L. keiyuanensis is more reliable, both of the possible positions are pre- magnetic susceptibilities (χ–T curves), hysteresis loops, isothermal sented here. remanent magnetization (IRM) acquisition and DC field demagnetiza- Samples were collected in 2012 using a gasoline-powered drill. The tion of the saturation IRM (SIRM). χ–T measurements were performed sampling interval is typically 0.5−1 m for the upper part but is variable on a KLY-3 Kappabridge with a CS-3 furnace under argon atmosphere. in the lower part. A total of 300 samples were collected. All samples Hysteresis loops were measured using a Princeton MicroMag 3900 were oriented with a magnetic compass in the field and were cut into Vibrating Sample Magnetometer (VSM). The magnetic field was cycled cylindrical specimens (2.5 cm in diameter and 2 cm in height) in the between ±1.5 T. Saturation magnetization (Ms), saturation remanence laboratory. (Mrs), and coercivity (Bc) were determined after correction for para- magnetic contribution identified from the slope at high fields. Speci- 3. Methods mens were then demagnetized in alternating fields (AFs) of up to 1.5 T, and an SIRM was imparted from 0–1.5 T also using the MicroMag 3.1. Rock magnetic measurements 3900 VSM. The SIRM was then demagnetized in a stepwise backfield up to −1.0 T to obtain the coercivity of remanence (Bcr). The anisotropy of magnetic susceptibility (AMS) was measured using a KLY-MFK Kappabridge magnetic instrument with an applied 3.2. Demagnetization of the natural remanent magnetization (NRM) field of 300 A/m. Systematic rock magnetic experiments were conduct- ed on representative samples in order to determine the remanence car- Remanence was measured with a 2G Enterprises Model 760-R rier in the sediments, which include temperature-dependence of cryogenic magnetometer situated in a magnetically shielded room S. Li et al. / Tectonophysics 655 (2015) 213–226 217

(b300 nT). A total of 287 samples were subjected to progressive AF de- increase of magnetic susceptibility above ~400 °C with a hump at magnetization in a maximum ﬁeld of 60–70 mT with 5–10 mT intervals. ~500 °C followed by a quick decrease to zero may reﬂect the existence

Demagnetization results were evaluated by orthogonal vector diagrams of siderite (Pan et al., 2000). Tc of ~580 °C demonstrates the presence (Zijderveld, 1967), and the principal component directions were com- of magnetite. The great enhancement of susceptibility during cooling puted by least-squares ﬁts (Kirschvink, 1980). Data analysis was com- suggests the neoformation of magnetite during heating. Considering pleted using the PaleoMag software (Jones, 2002). The mean the possible complex mineral transformation during thermal treatment directions were computed using Fisher statistics (Fisher, 1953). and the soft coercivity, AF demagnetization is employed in this study.

4. Results 4.2. AMS

4.1. Rock magnetic results Fig. 4 shows equal area projections of the three principle susceptibil-

ity axes of AMS before and after tilt correction and bi-plots of PJ–Km Fig. 3 shows the representative rock magnetic results. All the hyster- (corrected degrees of anisotropy vs. mean magnetic susceptibility) esis loops are closed above fields of 300 mT (Fig. 3a), and the IRM and PJ–T (corrected degrees of anisotropy vs. shape parameter). In-situ acquired saturation below 100 mT (Fig. 3b), indicating the dominance coordinates, the maximum (K1) and medium (K2) principle anisotropy of low-coercivity minerals, such as magnetite, titanomagnetite and/or directions are mostly subhorizontal and well grouped in north–south maghemite. However, the χ–T curves show complex behaviors and east–west directions, respectively. The minimum principle anisot- (Fig. 3c). The magnetic susceptibility increases moderately from room ropy directions (K3) are nearly normal to the bedding. However, after temperature to ~300 °C, which is likely due to the gradual unblocking tilt correction, the K1 is distributed in a great circle with two distinct of fine-grained (near the superparamagnetic/single-domain (SP/SD) groups of lineations (N–WandS–W trends), suggesting a weakly west- boundary) ferrimagnetic particles (Deng et al., 2005). The abrupt ward dipping foliation; while the K3 is deflected from near bedding

Fig. 3. Rock magnetic properties of representative samples in the Xiaolongtan Basin. (a) Hysteresis loops; (b) IRM acquisition curves and DC ﬁeld demagnetization of the saturation IRM (cut at 0.2–0.5 T); (c) magnetic susceptibility versus temperature curves (χ–T). 218 S. Li et al. / Tectonophysics 655 (2015) 213–226

Fig. 4. (a) Equal-area, lower-hemisphere projection of anisotropy of the maximum (blue squares), medium (green triangles) and minimum (pink circles) susceptibility axes in geographic and stratigraphic coordinates. Biplots of the (b) PJ–Km (corrected AMS degree versus mean magnetic susceptibility), and (c) PJ–T (corrected degrees of anisotropy vs. shape parameter).

perpendicular to southeastward. On average, the declination of K3 gen- 60 mT with at least four continuous demagnetization points. A total of erally parallels the bedding dip direction, it inclined southeastward at 204 samples yield reliable ChRM directions. the bottom (the mean direction between 200 and 300 m is D/I = Based on the geocentric axial dipole model, the virtual geomagnetic

153.3°/78.7°, a95 = 5.3°) and eastward at the top (the mean direction pole (VGP) latitudes were calculated from the ChRM data to construct between 300 and 400 m is D/I = 83.7°/80.6°, a95 =1.5°).Thischaracter the magnetostratigraphy. As shown in Fig. 6, ten magnetozones were has been reported by Gilder et al. (2001) from Subei, NW China, which identified in the studied section: five with normal polarity (N1–N5) suggests that the original sedimentary fabric becomes progressively and five with reverse polarity (R1–R5). Each magnetozone was defined overprinted by tectonic strain during or after folding (Gilder et al., by at least three samples from different sedimentary horizons, except 2001). for the normal magnetozone N3 with only one sample.

5. Discussion 4.3. Paleomagnetic results 5.1. Paleohorizontal reconstruction Representative demagnetization diagrams are shown in Fig. 5. The NRM intensity of the Xiaolongtan samples ranges from 3 × 10−2 A/m The upward decreasing dip of the Xiaolongtan Formation suggests for the clay to 2 × 10−5 A/m for the lignites. Except for a few samples that the depositions are syntectonic sediments. The correlation between which show two components (Fig. 5e), most samples show a single the variation of K3 declinations and bedding dips after tilt-correction component and decay linearly toward the origin with increasing field suggests that the AMS was acquired in present stratigraphic geometry (e.g., Fig. 5c, g and h). After demagnetization at 60 mT, the intensity of during the deposition. Thus the paleohorizontal of the sediments is most samples decreases to less than 10% of NRM, which further indi- the present stratigraphy geometry. If this is the truth, we do not need cates that magnetite is the dominant remanence carrier. The success to make any bedding corrections on the paleomagnetic data. To confirm rate for clay in the upper part is 91% (165 out of 182). However, the suc- this, we calculated the mean direction of normal and reversal polarities cess rate for the lignites or peaty clay in the lower part is only 37% in both geography and stratigraphy coordinates (Fig. 7a and b). It seems (39 out of 105), which is mainly due to the weak magnetization in the quite clear that these two polarities are more symmetric in in-situ than lignites that cannot reliably record the ambient field. The characteristic in tilt-corrected coordinates (the angle between mean directions of nor- remanent magnetization (ChRM) was determined between 30 mT and mal and reverse polarities is 8.3° and 9.8° in in-situ and tilt-corrected S. Li et al. / Tectonophysics 655 (2015) 213–226 219

Fig. 5. Diagrams showing AF demagnetization of NRM and the corresponding orthogonal projections and intensity of magnetization versus AF field. The solid (open) circles denote the horizontal (vertical) planes. coordinates, respectively.). Moreover, the uncertainty of the overall remaining 166 specimens is 8.3°. The critical angle at 95% confidence mean direction is smaller in in-situ than in tilt-corrected coordinates level is 10.5°, suggesting that they pass the reversal test at 95% confi- (2.1° in in-situ and 2.2° in tilt-corrected coordinates). These evidences dence level with B classification (McFadden and McElhinny, 1990). support that the magnetic remanence is also acquired in present strati- Thus, we believe that the ChRM directions obtained in this study are ac- graphic geometry and no bedding corrections are needed. We thus use quired at the present stratigraphic geometry and primary. The overall the mean direction in geography coordinate to calculate the rotation of mean direction is D/I = 353.2°/34.2°, a95 =2.1°(Fig. 7a). the Xiaolongtan Basin. 5.3. Age constraints of the Xiaolongtan formation and associated hominoid 5.2. Reliability of the paleomagnetic directions L. keiyuanensis

The generally homoclinal structure of the sampled section prevents Except the noted L. keiyuanensis, a number of mammal fossils were us from conducting a paleomagnetic fold test with any statistical rigor. discovered in the Xiaolongtan Formation, that is, the Xiaolongtan However, the observed dual polarity does make a reversal test possible. fauna, including Mustelidae indet., Castoridae gen. et sp. indet., Since the magnetic remanence is acquired in the present stratigraphic Tetralophodon xiaolongtanensis, Gomphotherium cf. macrognathus, geometry, we performed reversal test in geography coordinates. After Zygolophodon chinjiensis, Tapirus cf. yunnanensis, Parachleuastochoerus rejecting 38 specimens with VGP latitudes less than 45° (open circles sinensis, Propotamochoerus parvulus, Hippopotamodon hyotherioides in Fig. 6d), which may signal transitional ﬁeld behavior, the angle be- and Euprox sp. These taxa represent six orders and eight families. tween mean directions of normal and reverse polarities of the Among these elements, proboscideans and suids are predominant 220 S. Li et al. / Tectonophysics 655 (2015) 213–226

Fig. 6. Lithology (a), and magnetostratigraphy of the Xiaolongtan section (b–e), and its correlation with the ATNTS2012 (f, Hilgen et al., 2012). VGP Lat = latitude of virtual geomagnetic pole; R = reverse polarity; N = normal polarity. Solid (open) circles represent VGP latitudes higher (lower) than 45°. Stars denote the two possible positions of hominoid.

(Dong and Qi, 2013). The presence of Gomphotherium cf. macrognathus similar to T. punjabiensis from the younger Dhok Pathan Formation and Zygolophodon chinjiensis indicates that the fauna is similar to that (Dong and Qi, 2013). The size and morphology of the teeth of of Chinji horizons of the Siwaliks, and that of Tetralophodon, L. keiyuanensis are also comparable to Sivapithecus (Woo, 1957). Thus Propotamochoerus and Hippopotamodon indicates its similarity to a the Xiaolongtan fauna is either equivalent to, or slightly younger than Siwalik Nagri assemblage. In addition, Tetralophodon xiaolongtanensis the Siwalik faunas (Dong, 1987), which is known as occurrence of appears morphologically more advanced than Chinji T. falconeri but Sivapithecus and has been paleomagnetically dated at 12.5 Ma S. Li et al. / Tectonophysics 655 (2015) 213–226 221

Fig. 7. Equal-area projection of ChRM directions before (a) and after (b) bedding correction. Solid squares and open triangles denote projections from lower and upper hemisphere, respectively. Variation of the Fisher mean declinations of the grouped 16 sites (c) in-situ coordinate and (d) tilt-corrected coordinate as a function of ages. The vertical bars represent the age intervals. The horizontal error bars are the 95% conﬁdence limit of the declinations. The dashed line shows the expected declination from the referenced European paleomagnetic pole of 10 Ma. The gray line indicates the tendency of the counter-clockwise rotation.

(Kappelman et al., 1991). Qiu and Qiu (1995) suggest that the Tuned Neogene Time Scale 2012 (ATNTS2012, Hilgen et al., 2012)of Xiaolongtan fauna represents the last stage of the Anchitherium fauna the late Miocene; and the two relatively long reverse polarity zones on the Eurasian continent, or approximately Zone MN 8 in the R2 and R3 to Chrons C5r.2r and C5r.3r, respectively (Fig. 6eandf). European mammal chronology. Their recent study further confirmed Thus, the magnetic polarity sequence can be best correlated to that the Xiaolongtan fauna can be correlated to Tungur (Inner Chrons C5n.2n–C5Ar.1r of the ATNTS2012, yielding a basal age of Mongolia, northern China) fauna, which is one of the most well- 12.7 Ma for the Xiaolongtan Formation. Although the quality of the known Miocene faunas in China and has been well dated to be about paleomagnetic data in the lower part of the section, especially be- 15−11 Ma (Qiu et al., 2013). In addition, the absence of Hipparion (ab- tween 75 and 200 m, is relatively poor due to the weak magnetiza- sent before 11 Ma, abundant thereafter) in the Xiaolongtan fauna also tion of lignite, the match of our polarity column to the ATNTS2012 suggests a middle Miocene age for the lower part of the Xiaolongtan (Hilgen et al., 2012) appears straightforward. The reliability of this Formation. correlation was further supported by the relatively constant sedi- Besides the mammal fossils, the Xiaolongtan Formation also has rich mentation rate (blue line in Fig. 8). If we assume that the accumula- plant fossils which are dominated by Fabaceae, Fagaceae, and Lauraceae tion is relatively stable during this period, the well correlation of N1– (Xia et al., 2009). The assemblage of plant genera suggested that the R2 to C5n.2n–C5r.2r yields a sedimentation rate of 11.7 cm/kyr. Xiaolongtan flora is younger than the Miocene Shuanghe flora but is Using this sedimentation rate, the basal age of the section is extrap- older than the late Miocene–Pliocene Sanying flora (YBGMR, 1990). olated to be 13.2 Ma, which is slightly older than 12.7 Ma. Therefore, Palynological study also indicated that the age of Xiaolongtan flora is we believed that our correlation is reliable, and the age of the middle-late Miocene (Wang, 1996). These lines of evidence indicate a Xiaolongtan Formation was paleomagnetically constrained to a latest Middle Miocene or earliest Late Miocene age for the Xiaolongtan span from ~12.7 Ma to ~10 Ma, representing a latest Middle Miocene Formation. to earliest Late Miocene age. Our magnetic polarity sequence is characterized by a long normal Based on this correlation, the hominoid L. keiyuanensis is placed near polarity in the upper part and is dominated by reverse polarity in the the C5r.3r–C5r.2n transition or at the end of the reverse Chron C5Ar.1r, lower part (Fig. 6e). Combined with biochronological constraints, we which were recently dated at 11.657 Ma or 12.474 Ma in the correlated the longest normal polarity zone N1 to long normal Chron ATNTS2012 (Hilgen et al., 2012); thus, the age of L. keiyuanensis is esti- C5n.2n, which is one of the most unique features of the Astronomically mated to be either ~11.6 Ma or ~12.5 Ma. 222 S. Li et al. / Tectonophysics 655 (2015) 213–226

Fig. 8. Depth versus age (black solid line) and sedimentation rate (black dashed line) plots of the Xiaolongtan Basin (with ages taken from the ATNTS2012 Hilgen et al., 2012). The blue line represents the average sedimentation rate deduced from the entire section. The red dashed line represents the extrapolation using the sedimentationratededucedfromN1–R2.

Among the hominoids in eastern and southern Asia (Fig. 1), starting age of activity of the Qujing fault. Previous studies have sug- Sivapithecus from Pakistan is ~12.5 Ma (Barry et al., 2002; Kappelman gested that the northern segment of the Xianshuihe–Xiaojiang fault et al., 1991), K. chiangmuanensis from Chiang Muan Basin, northern began at 12.8 Ma (Rogeretal.,1995;Zhangetal.,2004b), which is near- Thailand is 13.5–10 Ma (Benammi et al., 2004; Chaimanee et al., ly contemporaneous with the estimates of the initiation of the Qujing 2003), or 12.4–13 Ma (Suganuma et al., 2006) or 12.4–12.2 Ma fault at 12.7 Ma. The synchronous initiation time of the north and (Coster et al., 2010), K. piriyai from Khorat in northeast Thailand is south segments of the Xianshuihe–Xiaojiang fault probably suggests 9–7Ma(Chaimanee et al., 2004), K. ayeyarwadyensis from Myanmar is that the fault activated as a whole during the latest Middle Miocene. 10.4–8.8 Ma (Jaeger et al., 2011), L. lufengensis from Shihuiba in the However, it is also possible that different parts of the Xianshuihe– Lufeng Basin is 6.9–5.8 Ma (Flynn and Qi, 1982; Qi et al., 2006); Xiaojiang fault initiated at different times (Wang et al., 2009). The L. hudienensis in the Yuanmou Basin is 8.2–7.2 Ma (Ni and Qiu, 2002; Xianshuihe and Qujing faults initiated at ~13 Ma, while the Yuanmou Yue et al., 2004; Zhu et al., 2005), L. lufengensis in the Zhaotong Basin and Xiaojiang faults initiated at ~5 Ma (Wang et al., 2009; Zhu et al., is ~6.2 Ma (Ji et al., 2013). The age of the hominoid from Baoshan re- 2008). As the initiation time of other parts of the Xianshuihe– mains poorly constrained; however, it was considered to be much Xiaojiang fault (the Anninghe, Shimian, Lufeng and Puduhe faults) re- younger (3–5 Ma) than the others (Harrison et al., 2002). mains unknown, it is difficult to determine which one is more probable Our paleomagnetic results indicate that the age of L. keiyuanensis at present. Further studies about the timing of Anninghe, Shimian, is synchronous with or slightly later than Sivapithecus and Lufeng and Puduhe faults will help to resolve this confusion. K. chiangmuanensis, representing one of the earliest described hominoids in southeastern Asia. These age data suggest that once large- 5.5. Tectonic rotation of SEMTP bodied hominoids reached southern Asia, they spread rapidly and widely from southern Asia to southeastern Asia. The above chronological Ever since Tapponnier et al. (1982) proposed the lateral extrusion data of hominoids also indicate that the hominoids survived in south- model, the rotation pattern of SEMTP has been a contentious topic for eastern Asia during the period from middle Miocene to early Pliocene. earth scientists. The clockwise rotation has been confirmed on short Their survival may benefit from episodes uplift of the SEMTP, which iso- timescales by modern GPS observations (e.g., Gan et al., 2007; Zhang lated the hominoids geographically and ecologically (Harrison et al., et al., 2004a) and on long timescales by paleomagnetic studies (Chen 2002; Ji et al., 2013; Zhu et al., 2005). It could therefore be inferred et al., 1995; Kondo et al., 2012; Li et al., 2012; Otofuji et al., 2010; Tong that southeastern Asia was an important refugium for hominoids, et al., 2013; Yang and Besse, 1993; Yang et al., 1995). Geological studies which may be favored by the low paleoelevation (Hoke et al., 2014; have also suggested that the SEMTP has experienced large magnitude Jacques et al., 2014), and warm and humid climate during this period clockwise rotation (Schoenbohm et al., 2006b; Wang et al., 1998, (Biasatti et al., 2012; Jacques et al., 2011; Su et al., 2013a,b; Xia et al., 2009). Although the clockwise rotation of SEMTP has been identified 2009; Xing et al., 2012). by numerous paleomagnetic studies, the magnitude, spatial and tempo- ral distribution of rotation are still highly debated. Recent reviews of the 5.4. Initiation time of the Xianshuihe–Xiaojiang Fault available paleomagnetic data from SEMTP revealed a complex rotation pattern from anticlockwise to tens of degrees clockwise (Fig. 9, Li Based on a detailed geological field study, Wang et al. (1998) sug- et al., 2012; Chi and Geissman, 2013), which indicates that a complex in- gested that the Xiaolongtan Basin is a pull-apart basin resulting from ternal deformation occurred during the Cenozoic. the activity of the left-lateral Qujing fault (Fig. 2b), one branch of Compared with the expected direction derived from the 10 Ma Eur- the Xianshuihe–Xiaojiang fault. The upward decreasing dip of the asian apparent polar wander path (Torsvik et al., 2008), the overall Xiaolongtan Formation suggests that the depositions are syntectonic mean ChRM direction of the Xiaolongtan Basin (D/I = 353.2°/34.2°, sediments; consequently, the initial left lateral motion along the Qujing a95 = 2.1°, n = 166, Table 1) documents a significant (−8 ± 3°) fault and the formation of pull-apart basin was simultaneous. counter-clockwise vertical axis rotation. To identify the rotation history Our magnetostratigraphic results indicate that the basal age of the during the sedimentary deposition, we grouped the ChRM directions Xiaolongtan Formation is ~12.7 Ma, which can be regarded as the into 16 sites; each site contains at least 10 samples. Except one site at S. Li et al. / Tectonophysics 655 (2015) 213–226 223

Fig. 9. Observed relative rotations from currently available paleomagnetic results of the southeastern margin of the Tibetan plateau with respect to the Eurasian block as a function of ages. The labels are abbreviations of sampling localities. Vertical and horizontal thin lines represent 95% conﬁdence intervals in rotation and uncertainty in age or age span (see appendix for data details), respectively.

the lowest and two sites at ~ 11.1 Ma, the other 13 sites show a counter- the left-lateral Xianshuihe–Xiaojiang Fault, which is consistent with clockwise rotation with a rate of ~ 4°/Ma (Fig. 7c), suggesting that the general predictions of crustal behavior along intracontinental trans- basin experienced progressively counter-clockwise rotation during the forms that clockwise rotation along right-lateral fault and counter- sedimentary deposition. clockwise rotation along left-lateral fault. The available Neogene paleomagnetic data in SEMTP are all from magnetostratigraphy studies (Table 1). Benammi et al. (2002) reported 6. Conclusions a 13 ± 1.32° counter-clockwise rotation from Mae Moh Basin in northern Thailand. However, their result was later thought to be associated We have reported high-resolution magnetostratigraphic results with local tectonics (Coster et al., 2010), and no significant vertical from the Xiaolongtan Formation in the Xiaolongtan Basin, Yunnan axis rotation was detected by other paleomagnetic studies from the Province, southwest China. Rock magnetic experiments indicate that same area (Benammi et al., 2004; Coster et al., 2010; Suganuma et al., magnetite is the main remanence carrier of the sediments. Step-wise 2006). Zhu et al. (2008) reported a 12° clockwise rotation since AF demagnetization successfully isolated five normal and five 4.9 Ma in the Yuanmou Basin; however, the overall mean direction reverse polarity zones. The positive reversal test indicates that the did not suggest significant (0.2 ± 3.0°) rotation with respect to the Xiaolongtan Formation preserves the primary magnetization. Based on Eurasia reference pole. Our recent paleomagnetic studies from the Dali biochronological age constraints, the magnetostratigraphic column Basin suggested 4.4 ± 2.5° post-late Miocene rotation (Li et al., 2013). can be correlated with chrons from C5Ar.1r to C5n.2n. This correlation All the above paleomagnetic results suggest minor post-middle Mio- indicates that the Xiaolongtan Formation was accumulated from cene rotation. The significant counter-clockwise rotation observed in ~12.7 Ma to 10 Ma. The 12.7 Ma basal age of the Xiaolongtan Formation the Xiaolongtan Basin is not consistent with previous paleomagnetic indicates that the activity of the southern Xianshuihe–Xiaojiang fault data, it can therefore be a local tectonic that responds to the activity of was initiated at this time. The significant counter-clockwise rotation

Table 1 Neogene paleomagnetic results currently available from SEMTP. Lat. and Long. are latitude and longitude, respectively. The ages are from magnetostratigraphy results. D and I are declination and inclination, respectively. α95 is a radius of the cone of 95% conﬁdence for mean direction. n is the number of specimens. The reference pole is the 10 Ma Eurasia apparent polar wander path (Torsvik et al., 2008). The amount of rotation R and uncertainty △R are evaluated by using the method of Butler (1992).

Locality Lat.(°N) Long.(°E) Age (Ma) D (°) I (°) α95 (°) n R ± △R reference Mae Moh 18.3 99.7 13.5–12.1 358.3 22.2 4 86 3.0 ± 4.1 Benammi et al. (2002) Chiang Muan 18 100 13.5–10 5.7 33.6 10.3 21 4.4 ± 10.1 Benammi et al. (2004) Yuanmou 25.7 101.9 4.9–1.4 1.4 27.8 1.1 706 0.2 ± 3.0 Zhu et al. (2008) Mae Moh 18.3 99.7 14.1–12 5.4 26.5 7.1 65 4.1 ± 6.7 Coster et al. (2010) Myanmar 20.5 94.8 10.4–8.8 7.3 24 7 20 5.8 ± 6.5 Jaeger et al. (2011) Dali 26.3 100 7.6–1.8 5.6 27.9 1.7 421 4.2 ± 2.7 Li et al. (2013) Xiaolongtan 23.8 103.2 12.7–10 353.2 34.2 2.1 166 −8 ± 3 This study 224 S. Li et al. / Tectonophysics 655 (2015) 213–226 observed in the Xiaolongtan Basin is not consistent with the coeval pa- Chi, C.T., Dorobek, S.L., 2004. Cretaceous palaeomagnetism of Indochina and surrounding regions: Cenozoic tectonic implications. In: Malpas, J.F.C.J.N.A.J.R.A.J.C. (Ed.), Aspects leomagnetic data, which may be the response to local tectonics. Finally, of the Tectonic Evolution of China. Geol. Soc. Spec. Publ, pp. 273–287. the age of the L. keiyuanensis at ~11.6 Ma or ~12.5 Ma, which is contem- Chi, C.T., Geissman, J.W., 2013. A review of the paleomagnetic data from Cretaceous to poraneous with K. chiangmuanensis in northern Thailand and lower Tertiary rocks from Vietnam, Indochina and South China, and their implications for Cenozoic tectonism in Vietnam and adjacent areas. J. Geodyn. 69, 54–64. Sivapithecus in Pakistan, indicates that large-bodied hominoids spread Clark, M., Royden, L.H., 2000. Topographic ooze: building the eastern margin of Tibet by rapidly and widely from South Asia to Southeast Asia once they reached lower crustal flow. Geology 28, 703–706. eastern Asia. The relatively low elevation and the warm and humid cli- Coster, P., Benammi, M., Chaimanee, Y., Yamee, C., Chavasseau, O., Emonet, E.G., Jaeger, J.J., mate may provide a refuge for the survival of hominoids in Southeast 2010. A complete magnetic-polarity stratigraphy of the Miocene continental deposits of Mae Moh Basin, northern Thailand, and a reassessment of the age of hominoid- Asia during the middle Miocene to early Pliocene. bearing localities in northern Thailand. Geol. Soc. Am. Bull. 122, 1180–1191. Deng, C., Vidic, N.J., Verosub, K.L., Singer, M.J., Liu, Q., Shaw, J., Zhu, R., 2005. Mineral magnetic variation of the Jiaodao Chinese loess/paleosol sequence and its bearing Acknowledgments on long‐term climatic variability. J. Geophys. Res. 110. http://dx.doi.org/10.1029/ 2004JB003451. We thank Professors Tao Deng, Zhenyu Yang and Erchie Wang for Dong, W., 1987. Further investigations upon the age and characteristics of the Xiaolongtan fauna, Kaiyuan Co., Yunnan Province. Vertebrata PalAsiatica 25, the helpful discussions. Professor Laurent Jolivet, Yan Chen and an anon- 116–123. ymous reviewer are highly appreciated for their constructive comments Dong, W., 2001. Upper Cenozoic stratigraphy and paleoenvironment of Xiaolongtan on an earlier manuscript, which resulted in the great improvement of Basin, Kaiyuan, Yunnan Province. In: Deng, T., Wang, Y. (Eds.), Proceedings of the Eighth Annual Meeting of the Chinese Society of Vertebrate Paleontology. China this manuscript. Paleomagnetic and mineral magnetic measurements Ocean Prese, Beijing, pp. 91–100. were made in the Paleomagnetism and Geochronology Laboratory Dong, W., Qi, G., 2013. Hominoid-producing localities and biostratigraphy in Yunnan. In: (PGL), Beijing. Financial assistance was provided by the National Wang, X.M., Flynn, L.J., Fortelius, M. (Eds.), Fossil Mammals of Asia — Neogene Bio- stratigraphy and Chronology. Colombia University Press, New York, pp. 293–313. Natural Science Foundation of China grant 41404056, the National Key England, P., Houseman, G., 1986. Finite strain calculations of continental deformation.2. 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