The structure of the orogenic belt is directly related to the understanding of the orogeny process and the orogenic method, which restricts the changes of tectonic landforms and climatic-environmental changes. Therefore, the structural anatomy of the orogenic belt occupies a central position in the study of geotectonics.
The Himalayan orogenic Belt, located in the southern part of the Tibetan Plateau, between Eurasia and The Indian continent (Figure 1), records information on the tectonic evolution of the Neo-Tethys Ocean and is involved in shaping Cenozoic tectonic-environmental change in East Asia. There are currently three end-element models of the properties of Himalayan blocks: (1) the Tethys Himalayas are the northern rim of the Indian continent, which was cleaved from the Eastern Gondwana continent in the Early Cretaceous (Yin and Harrison, 2000) ;(2) Based on paleomagnetic data, it is speculated that the Tethys Himalayas are 118 Ma or 75 Ma micro-blocks that split from the northern edge of the Indian continent (van Hinsbergen et al., 2012; Yuan et al., 2021) ;(3) Tethys Himalayan as an augmentation wedge (Xiao Wenjiao et al., 2017). However, existing models still do not fully explain the following phenomena: (a) ~130Ma The Brahmaputra SSZ type aophyllite-horned amphibolite and Kohistan-Ladakh magmatic belts represent intraocean subduction of the Neoteth Ocean in the Early Cretaceous; (b) the development of early Cretaceous extrusion structures in the northeastern Himalayas, during which the Indian continent disintegrated to East Gondwana, whose northern edge was passive; (c) the northeastern Himalayas were attached to the Eastern Gondwana continent during the Triassic and Jurassic periods, while the negative εNd (t) and εHf(t) values indicate the developmental Precambrian substrate of the region. The focus of the above problems remains on the structure of the Himalayan orogenic Belt. A large number of Early Cretaceous veins have developed in the northeastern Himalayas (Figure 2a), which can be used to define the tectonic deformation era in the region, providing a possibility for studying the mesozoic tectonic evolution of the East Tethys tectonic domain. Zhang Ji'en and Xiao Wenjiao, State Key Laboratory of Lithosphere Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, cooperated to study the relationship between folds and rock veins in this area, defined the extrusion structure of the Early Cretaceous, combined with the late Jurassic stretch structure, and proposed the multi-island ocean pattern of the Eastern Tethys Himalayan Tectonic Belt in the late Mesozoic Era.
Figure 1 (a) Tectonic diagram of Southeast Asia; note the development of the late Jurassic crust in the eastern Indian Ocean; the age of clastic zircon and Hf isotopic characteristics of the contemporaneous stratigraphic sandstones of the Himalayan Triassic Langerian Group and the North Carnarvon Basin in the northwestern edge of Australia; the age of clastic zircon in Jurassic quartzites can be compared with the northern edge of the Indian continent, indicating that the Himalayas are related to East Gondwana. The pink body marked in the figure represents the Argoland mass group split from the Eastern Gondwana continent by the Late Jurassic, and (b) A brief tectonic map of the Himalayas and Indo-Burma showing the outcrops of the Apophyllite and the Early Cretaceous Horn amphibolite
Figure 2 (a) Geological map of the Longzi region in the northeastern Himalayas; (b) Fold production in the Triassic, Jurassic and Early Cretaceous strata of the study area, showing polar southward inverted folds
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Early Cretaceous extrusion structure
The strata of the triassic (Figure 3a-b), Jurassic (Figure 3c), and Early Cretaceous (Figure 3d-e) that emerge from the northeastern Himalayas all have symmetrical and asymmetrical folds (including oblique and inverted folds) (Figure 2b), with the main body of the fold axial surface leaning northward and the hub dominated by east-west orientation (Figure 2b). Field observations show that the folds in the formation are often cut through by ~130 Ma veins (Figures 3a-d), indicating that these folds formed before the intrusion of the veins. Considering that the youngest folded stratum cut by the rock vein is the Early Cretaceous Carbra Formation, the extrusion structure represented by the folds in the northeastern Himalayas is defined as developing in the Early Cretaceous. At the same time, the inverted folds of the axial surface toward the north (Figure 2b) indicate that the northeastern Himalayas suffered from a simple shear pointing southward at the top in the Early Cretaceous. For ease of reference, the geological bodies that developed the Early Cretaceous extrusion structure were named "Longzi blocks" (Figure 4).
It has been thought that the Indian continent split off from the eastern gondwana continent in the Early Cretaceous (Yin and Harrison, 2000), and the northern edge of the Indian continent was the edge of the passive continent, developing stretched structures. The longzi block develops a squeeze structure, indicating that it does not belong to the northern edge of the Indian continent at this time.
Figure 3 Developing structures in the northeastern Himalayas. (a-b) Folds and intrusive basal veins in the Triassic formation; note that the veins in figure a are only inclined northward at a low angle and not folds; (f) Late Jurassic basal-superstition rocks exposed from the Triassic strata of the Kara Pass, which were eroded into serpentine rocks; locally, the bedrocks and strata showed an intrusive contact relationship
Figure 4 Age comparison of geological and tectonic phenomena in the Himalayas and Southeast Asia, noting the tectonic events corresponding to these phenomena during the Late Jurassic and Early Cretaceous periods
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Late Jurassic stretch structure
There are three outcrops in the Longzi block (Figure 2a), of which the bedrocks in the Kara Pass outcrop have an intrusive contact relationship with the Triassic strata (Figure 3f), indicating that these magmatic rocks are intrusive rocks. The bedrock has the geochemical characteristics of alkaline rocks, with a belch zircon age of 152.8Ma, which is a late Jurassic alkaline intrusive rock. The results of others have shown that there are also 148-145 Ma of bedrock veins in the area, corresponding to the change of geochemical characteristics from OIB to MORB type (Figure 4). At the same time, ammonite data in the western section of the Longzi block in Nielamu and adjacent Areas of the Late Jurassic strata show that the sedimentary water bodies are gradually deepening (Yin Jiarun and Wan Xiaoqiao, 1996), and angular disunity surfaces are developed (Garzanti, 1999). These data indicate that the Longzi mass underwent stretching tectonics in the Late Jurassic (Garzanti, 1999; Yin Jiarun and Wan Xiaoqiao, 1996).
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Structural evolution
Cai et al. (2016) and Wang et al. (2016) Studied the clastic zircons and their Hf isotopes of sandstones of the Triassic Langjet Group of Longzi blocks, and concluded that they are similar to the North Carnarvon Basin in the northwestern rim of Australia; the quartz conglomerates of the Late Jurassic Vime Formation are similar to the age characteristics of the clastic zircons at the northern edge of the Indian continent (Pan Wanying, 2018). Since the Indian continent and the Australian continent had not yet split at this time to form the eastern gondonwana continent, the Longzi block was part of the northern edge of the eastern gondwana continent before the Late Jurassic (>155Ma) (Figure 5a).
Figure 5 Tectonic evolution patterns of the northeastern edge of the Himalayas: (a) Tectonic patterns before 155Ma show that the Tectonic blocks are attached to the Eastern Gondwana continent; (b) during the Late Jurassic Period, the Eastern Gondwana continent splits out the Argoland block group, including the Longzi block, and expands to form the (northern) Indian Ocean crust; (c) during the Early Cretaceous period, the East Gondwana region developed a multi-island ocean tectonic pattern, and the Longzi block collided with the inner arc of the Zedan Ocean, triggering a new subduction in the Neoteth Ocean. Metamorphic base plate and SSZ type snake green rock develop along the Brahmaputra Tectonic Zone
By 155-152Ma, the bulge block develops and stretches. At this point, the northwestern edge of Australia, to which the longon block is attached, is cleaved, and the Argoland block group is separated (Figure 1a, Figure 4). The Longzi block, which is part of the Argoland block group, is also cleaved from the eastern gondwana continent (Figure 5b). The expanding ocean basin included the eastern Indian Ocean and the northern Indian Ocean between the Longzi mass and the Indian continent, and the Himalayas began to develop a multi-island ocean pattern.
By the 136-132Ma period, the longzi block collided with the Trans-Tethyan intra-ocean arc, resulting in the development of a large number of polarized south-pointing folds in the longzi block; the collision triggered the development of a new subduction zone on the north side of the intraypian arc, and the development of a pre-Cretaceous metamorphic substrate and SSZ-type snake greenstone along the Brahmaputra Tectonic Belt. Paleomagnetic data show that the longson blocks of this period are still at the same latitude as the Indian continent, and it is speculated that they are now divided by the north Indian Ocean in a directional direction (Figure 5c).
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Further study the content
This study preliminarily established a multi-island ocean tectonic model that has developed since the Mesozoic era in the Himalayas, which means that multiple ocean basins have developed in this area. The closure of the Neo-Tethys Ocean forms the Brahmaputra Suture Belt, and the location of the Suture Belt after the closure of the Northern Indian Ocean has not been studied (Figure 5c), which is the focus of future research. In addition, the timing of the closure of the multi-island ocean is also crucial, because the late closing time is the starting point of the squeeze and collision between the Indian continent and Eurasia, and it is the key to understanding the relationship between structure and environmental change.
Research results published in the international academic journal JGR: Solid Earth (Zhang Jien*, Xiao Wenjiao, John Wakabayashi, Brian F. Windley, Han Chunming. A fragment of Argoland from East Gondwana in the NE Himalaya [J]. Journal of Geophysical Research: Solid Earth, 2022, 127: e2021JB022631. DOI: 10.1029/2021JB022631）。 The research was jointly funded by the National Natural Science Foundation of China (41888101, 42172247) and the Strategic Pilot Science and Technology Project of the Chinese Academy of Sciences (Category B) (XDB18030103).
Editor: Chen Feifei
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