Cretaceous paleomagnetic and detrital zircon UPb geochronological results from the Tethyan Himalaya: Constraints on the Neo-Tethys evolution
Introduction
The evolution of the Neo-Tethys Ocean involved various types of tectonic events, such as continental breakup, convergence, collision, and accretion. These geological events have had long-term influences on global paleogeography, paleoenvironments, paleoclimate, and the evolution of life (Raymo and Ruddiman, 1992; Yin and Harrison, 2000; Chatterjee et al., 2013; Guo et al., 2021; Zhu et al., 2022). One of the most prominent events, convergence between India and Asia, not only led to the closure of the Neo-Tethys Ocean but also produced the world's largest and highest plateau, the Tibetan Plateau. Despite the large numbers of geological, geochemical, and geophysical investigations conducted on the Tibetan Plateau and its adjacent areas (Hu et al., 2016), several issues related to the evolution of the Neo-Tethys Ocean, especially the size of Greater India and the India-Asia collision process, are still hotly debated.
The size of Greater India is not well constrained; although complementary methods have been used, these approaches have yielded large differences ranging from ~3400 km to only a few hundred kilometers (Powell et al., 1988; Ali and Aitchison, 2005; Yi et al., 2011; van Hinsbergen et al., 2011, van Hinsbergen et al., 2019; Meng et al., 2020; Bian et al., 2021; Dannemann et al., 2022). These conspicuous inconsistencies in estimates of the size of Greater India are associated with different India-Asia collision processes: single-stage, two-stage, or multi-stage collision. The single-stage collision models primarily consist of a large Greater India continent-continent collision model (Ingalls et al., 2016; Meng et al., 2020) and a relatively small Greater India continent-continent collision model (Le Pichon et al., 1992; Yang et al., 2019). Two-stage collision models mainly include the Greater India Basin and dual continent-continent collision model (van Hinsbergen et al., 2012), the North India Sea and dual continent-continent collision model (Yuan et al., 2021), a model of first collision of an island arc with India (Aitchison et al., 2007) or with Asia (Pusok and Stegman, 2020) followed by the final collision, and India-Xigaze arc collision and closure of the Xigaze backarc basin (Kapp and DeCelles, 2019). The multi-stage collision model suggests an initial collision between the Tethyan Himalaya or Tibetan Himalaya (the Tethyan Himalaya plus the Greater Himalaya) and Trans-Tethyan subduction system (TTSS), followed by the Tethyan Himalaya (or Tibetan Himalaya)/TTSS-Asia collision, and finally by the India-Tethyan Himalaya (or Tibetan Himalaya) collision (Jadoon et al., 2022; Yuan et al., 2022). One focus of the debate surrounding the collision process is whether the Tethyan Himalaya (or Tibetan Himalaya) rifted away from the Indian craton. If so, when did the rifting occur? This has triggered heated discussion in subsequent studies (Wu et al., 2014; Hu et al., 2016; Searle, 2019; Yuan et al., 2021, Yuan et al., 2022).
Paleomagnetism is one of the principal methods used to quantify plate paleogeography, and is thus essential to constrain the kinematic processes of plate movement (Chen et al., 1993; Sun et al., 2012; Lippert et al., 2014; Yan et al., 2016; Ma et al., 2019; Song et al., 2020; Dannemann et al., 2022). Because the Tethyan Himalaya was located at the northern margin of India (Fig. 1a), reliable Cretaceous paleomagnetic data from the Tethyan Himalaya play an important role in constraining the evolution of the Neo-Tethys Ocean. Three robust paleomagnetic results from the Early Cretaceous volcanic rocks of the southeastern Tethyan Himalaya (Yang et al., 2015b; Ma et al., 2016; Bian et al., 2019), which meet all seven quality criteria proposed by Van der Voo (1990), show minor north-south differences (~70–530 km) of the southeastern Tethyan Himalaya relative to the Indian craton, suggesting a relatively small Greater India (Yang et al., 2019). However, four reliable paleomagnetic results from Late Cretaceous and Paleocene limestone of the southeastern Tethyan Himalaya (Patzelt et al., 1996; Yi et al., 2011), demonstrate large north-south differences (~1500–1900 km) of the southeastern Tethyan Himalaya relative to the Indian craton, implying a much larger Greater India. Because of this difference, some researchers have suggested that the Tibetan Himalaya rifted from the Indian craton after the Early Cretaceous, causing the above-mentioned two-stage collision, with a first collision of the Tibetan Tethyan and Asia at ca. 50 Ma and a final collision of the Indian craton and the Tibetan Tethyan at ca. 25–20 Ma (van Hinsbergen et al., 2012). However, the lack of paleomagnetic data of late Early and Late Cretaceous age in the Tethyan Himalaya makes this two-stage collision model uncertain from a paleomagnetic point of view.
Recently, Qin et al. (2019) reported a Cretaceous (ca. 120.8–93.9 Ma) paleopole (25.0°N, 285.7°E, dp/dm = 4.8°/7.1°) from purplish-red cherts of the Gyabula Formation in the northern Tethyan Himalaya, interpreted that no wide oceanic basin was present between the Tethyan Himalaya and the Indian craton during the Cretaceous, and favored the traditional single-stage collision model. Meng et al. (2020) obtained a Maastrichtian paleopole (−33.4°N, 5.7°E, dp/dm = 1.6°/3.2°) and an Albian paleopole (27.2°N, 231.3°E, dp/dm = 4.8°/7.7°) from limestone of the Zongshan Formation in the western Tethyan Himalaya, indicating that Greater India extended ~2700 km farther north at the longitude of 79.6°E during the Cretaceous, but they still proposed a single-stage collision model. Yuan et al., 2021, Yuan et al., 2022 reported a Late Cretaceous (76.2–74.0 Ma) paleopole (40.8°N, 256.3°E, A95 = 1.8°) from the Cailangba section and an updated Paleocene (62.5–59.2 Ma) paleopole (74.8°N, 263.4°E, A95 = 1.9°) from the Sangdanlin and Mubala sections in the northern Tethyan Himalaya, which indicated that Greater India extended no more than 1000 km before 75 Ma. They proposed that the North India Sea (between India and the Tibetan Himalaya) formed from ca. 75 Ma until ca. 53–47 Ma, with a triple-stage India-Asia collision model involving the first arc-continent collision at ca. 64 Ma between the Tibetan Himalaya and the TTSZ, followed by continent-continent collision at ca. 61 Ma between the Tibetan Himalaya/TTSZ and Asia, and finally continent-continent collision at ca. 53–47 Ma between India and the Tibetan Himalaya (Yuan et al., 2022). Because of the apparent inconsistencies in the existing Cretaceous paleomagnetic datasets from the Tethyan Himalaya (Qin et al., 2019; Meng et al., 2020; Yuan et al., 2021, Yuan et al., 2022), the scarcity of results from the late Early and Late Cretaceous, and widespread remagnetization (Tong et al., 2008; Appel et al., 2012; Huang et al., 2017; Dannemann et al., 2022), it remains difficult to accurately constrain the precollisional paleolatitude of the Tethyan Himalaya. Therefore, additional reliable Cretaceous paleomagnetic data from the Tethyan Himalaya, especially from late Early and Late Cretaceous, are required to settle these controversies.
In this study, we present a combined rock magnetic, paleomagnetic, petrographic, and detrital zircon UPb geochronological study on the Gyabula Formation in the Zhongba area of the northern Tethyan Himalaya (Fig. 1b). The new paleomagnetic results improve the basis to constrain the Cretaceous paleogeographic position of the northern Tethyan Himalaya, to estimate the northern extent of Greater India, and further provide a better understanding of the evolution of the Neo-Tethys Ocean.
Section snippets
Geological background and sampling
The Himalayas lie between the Indian craton to the south and the Lhasa terrane to the north and consist of four major units: the Sub-Himalaya, the Lesser Himalaya, the Greater Himalaya, and the Tethyan Himalaya from south to north (Fig. 1a). These tectonic units are separated by the Main Frontal Thrust, the Main Boundary Thrust, the Main Central Thrust, the South Tibetan Detachment System, and the Indus-Yarlung Tsangpo suture zone from south to north. The Tethyan Himalaya mainly consists of
Methods
All of the core samples were collected in the field using a gasoline-powered drill. Cores were oriented by magnetic compass and sometimes by sun compass when the weather permitted. The average absolute differences between these two oriented results were <2°, indicating that magnetic disturbance can be ignored in the sampling area.
Oriented cores with 2.5 cm in diameter were trimmed to standard cylindrical specimens 2.2-cm in length. With the goal of determining the magnetic remanence carriers,
Zircon UPb analytical results
Zircon grains were rounded and subrounded (~150–70 μm long, ~80–40 μm wide) with clear oscillatory zoning in most grains (Fig. 3a). The Th/U ratios of most of the zircons were higher than the range for metamorphic zircons (generally <0.1), except for one zircon that had a value of 0.08 (Table S1). These features indicate that the dated zircon grains were of primary magmatic origin. One hundred and one zircon grains were analyzed, and 60 available ages were obtained. Among the 60 available ages,
Reliability of the paleomagnetic data
Tectonic strain not only causes the folding of strata but also may affect the direction of remanence (Graham, 1949; Kodama, 2012). Therefore, it is important to test whether the remanence directions were affected by tectonic strain when paleomagnetic data are used for paleogeographic reconstructions (Zhao et al., 2013, Zhao et al., 2014; Huang et al., 2019; Bian et al., 2020). The strain state of the rocks can be estimated using AMS. AMS analysis revealed that the studied Gyabula Formation
Conclusions
We present reliable mid-Cretaceous paleomagnetic results from the purplish-red cherts of the Gyabula Formation in the northern Tethyan Himalaya. Our new results, together with reliable Cretaceous paleomagnetic data from the Tethyan Himalaya, Lhasa terrane, and Indian craton, led us to draw the following conclusions:
- 1.
The influence of tectonic strains (stage 4) on remanence directions of the Gyabula Formation cherts in the Zhongba area can be ignored.
- 2.
The revised GPTS within the CNS, together with
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We thank Liviu Matenco and Weitao Wang for editorial handling, as well as Erwin Appel and an anonymous reviewer for their constructive comments and suggestions. We are grateful for discussions with Chenglong Deng, Huafeng Qin, Shihu Li, Jie Yuan and Hanbiao Xian, and sincerely thank Shuangchi Liu, Kuang He, Zhongshan Shen, Min Zhang, Yifei Hou, Yong Yao, and Jin Deng for assistance with the field and laboratory work. This research was supported by the National Natural Science Foundation of China
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