# The Tectonic Responses in the Overriding Plate During the Tethyan Convergence and the Progressive Extrusion Process in Sundaland: Insights From the Wuliangshan Massif, SE Tibetan Plateau

Qinying Wu, Wei Lin, Lingtong Meng, Yin Wang, Fei Liu, Yang Chu, Wei Wei, Michel Faure, Vuong van Nguyen, Hoai Luong Thi Thu, et al.

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► To cite this version:

Qinying Wu, Wei Lin, Lingtong Meng, Yin Wang, Fei Liu, et al. The Tectonic Responses in the Overriding Plate During the Tethyan Convergence and the Progressive Extrusion Process in Sundaland: Insights From the Wuliangshan Massif, SE Tibetan Plateau. Geochemistry, Geophysics, Geosystems, 2025, 26, 10.1029/2025gc012371. insu-05161816

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# Geochemistry, Geophysics, Geosystems®

## RESEARCH ARTICLE

10.1029/2025GC012371

**Special Collection:**

Tethyan Evolution and its Effect on the Earth Habitability during the Phanerozoic Eon

**Key Points:**

* The tectonic response of the overriding plate to the Mesozoic Sibumasu-Indochina collision and the Cenozoic India-Eurasia collision

* The Permian-Triassic tectonic processes of extension and contraction occurred within a brief interval in a back-arc tectonic setting

* A Cenozoic two-stage extrusion model of the Sundaland has been proposed instead of the previous one-stage extrusion

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

W. Lin,
linwei@mail.iggcas.ac.cn

**Citation:**

Wu, Q., Lin, W., Meng, L., Wang, Y., Liu, F., Chu, Y., et al. (2025). The tectonic responses in the overriding plate during the Tethyan convergence and the progressive extrusion process in Sundaland: Insights from the Wuliangshan Massif, SE Tibetan Plateau. *Geochemistry, Geophysics, Geosystems, 26*, e2025GC012371. https://doi.org/10.1029/2025GC012371

Received 23 APR 2025

Accepted 20 JUN 2025

**Author Contributions:**

**Conceptualization:** Wei Lin

**Data curation:** Qinying Wu, Lingtong Meng, Yin Wang, Fei Liu, Hoai Luong Thi Thu

© 2025 The Author(s). Geochemistry, Geophysics, Geosystems published by Wiley Periodicals LLC on behalf of American Geophysical Union. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

## The Tectonic Responses in the Overriding Plate During the Tethyan Convergence and the Progressive Extrusion Process in Sundaland: Insights From the Wuliangshan Massif, SE Tibetan Plateau

<figure><img src="image_3.png" /></figure>

Qinying Wu¹,², Wei Lin¹,², Lingtong Meng¹,², Yin Wang³, Fei Liu⁴, Yang Chu¹,², Wei Wei¹,², Michel Faure⁵, Vuong Van Nguyen⁶, Hoai Luong Thi Thu⁶, Chao Song¹,², and Zhao Guo¹,²

¹State Key Laboratory of Lithospheric and Environmental Coevolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China, ²College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing, China, ³Geosteering & Logging Research Institute, Sinopec Matrix Corporation, Qingdao, China, ⁴Wuhan Center of China Geological Survey (Central South China Innovation Center for Geosciences), Wuhan, China, ⁵Institut des Sciences de la Terre d'Orléans, UMR CNRS 7327, Université d'Orléans, Orléans Cedex 2, France, ⁶University of Science, Vietnam National University, Hanoi, Viet Nam

**Abstract** The overriding plate commonly accumulates less strain than the subducted plate during continental collision. Analyzing plate interactions from the perspective of the overriding plate provides important insights into orogenic dynamics. In the southeastern Tibetan Plateau, the Wuliangshan massif, as a key region of the overriding plate, records tectonic events related to the closure of both the Paleo-Tethys and Neo-Tethys. In this study, we present detailed structural analysis, Titanium-in-quartz geothermometry, and zircon U-Pb and Hf isotopic data. These results indicate that the Wuliangshan massif experienced rapid subsidence and deposition within a short period from 260 to 250 Ma in a back-arc tectonic setting. The polyphase deformation events were recognized in the Wuliangshan massif. The ductile syn-metamorphism D₁ event, characterized by a top-to-the-SE ductile shearing under middle-to high-temperature conditions, represents the tectonic response to the collision between the Sibumasu block and the Indochina block occurred at ca. 250–246 Ma. The D₂ deformation is expressed by E-directed thrusts and E-verging folds coeval with a slaty cleavage. The D₃ deformation is marked by SW-directed thrusts and SW-verging folds. The D₂ and D₃ events correspond to positive flower structures related to transpression along the Chongshan-Lancang River fault and the Ailaoshan-Red River fault, respectively. The D₂-related Chongshan-Lancang River fault accommodated the southward extrusion of the western part of Sundaland occurred at 32–27 Ma. The southeastward extrusion of the entire Sundaland was accommodated by the D₃-related Ailaoshan-Red River fault at 27–23 Ma.

**Plain Language Summary** A comprehensive understanding of the tectonics developed in the southeastern Tibetan Plateau is crucial for reconstructing the tectonic and geodynamic evolution of Southeast Asia. However, the ancient orogens (Paleozoic or Mesozoic) underwent significant modification due to large-scale strike-slip deformation resulting from the lateral extrusion of the Sundaland during the late stages of the Cenozoic India-Eurasia collision. This study reveals that the response of the overriding plate to the closure of the Paleo-Tethys underwent a series of tectonic processes, including back-arc extension, metamorphism, and contraction, which occurred within a relatively short period of about 260–246 Ma. More importantly, we report transpressional tectonics that constrain the tectonic extrusion process of the Sundaland. Our structural analysis of polyphase deformation indicates a more complex extrusion model, with the west Sundaland extruding earlier to the south, followed by the southeastward extrusion of the entire Sundaland.

## 1. Introduction

In the process of continental collision, it is common that the overriding plate accumulates less strain compared to the subducted plate (Grool et al., 2018; Schäfer et al., 2000). The numerical and analog models demonstrate that strain resulting from continental collision is primarily localized within the subducted plate, with limited transmission of stress to the overriding plate (Willett et al., 1993). The deformation related to continental collision is
**Formal analysis:** Qinying Wu, Lingtong Meng, Yin Wang, Fei Liu, Wei Wei

**Funding acquisition:** Wei Lin

**Investigation:** Qinying Wu, Wei Lin, Lingtong Meng, Yin Wang, Fei Liu, Yang Chu, Wei Wei, Michel Faure, Vuong Van Nguyen, Chao Song, Zhao Guo

**Methodology:** Wei Lin

**Project administration:** Wei Lin

**Resources:** Wei Lin

**Supervision:** Wei Lin, Yang Chu, Wei Wei, Michel Faure

**Validation:** Michel Faure

**Visualization:** Qinying Wu, Wei Lin

**Writing – original draft:** Qinying Wu

**Writing – review & editing:** Wei Lin, Lingtong Meng, Yin Wang, Fei Liu, Yang Chu, Wei Wei, Michel Faure, Vuong Van Nguyen, Hoai Luong Thi Thu

mainly focused on the subducted plate forming large-scale folds, thrusts, and ductile shearing as demonstrated by the Western Alps (e.g., Bellahsen et al., 2014; Bellanger et al., 2014; Leloup et al., 2005; Platt et al., 1989), the Zagros (Agard et al., 2005; Alavi, 1994, 2007; McQuarrie, 2004), the Himalaya (DeCelles et al., 1998; Ratschbacher et al., 1994; Yin, 2010), and the Ailaoshan-Song Ma-Song Chay (Faure et al., 2014; Wang, Lin, et al., 2022). However, understanding plate interactions from the perspective of the overriding plate can provide important insights into orogen dynamics. For example, exhumation tectonics of the ultrahigh-pressure metamorphic rocks have significantly modified the geometry of the Sulu-Dabie orogenic belt (Faure et al., 2003). In contrast, the overriding North China plate, located farther from the orogenic belt and less affected by exhumation tectonics, has preserved contractional structures related to the plate convergence process (Meng & Lin, 2021; Ren et al., 2020; Ren & Lin, 2019).

The Tibetan Plateau was created by the overriding plate response to the Cenozoic continental collision between the Indian and Eurasian plates (e.g., Yin & Harrison, 2000). Two competing geodynamic models, including the lower crustal flow model (Clark & Royden, 2000; Royden et al., 1997, 2008) and the collision-extrusion model (Tapponnier et al., 1982, 1986), have been proposed to explain the tectonic extrusion process of the Sundaland and South China blocks, as well as the Cenozoic intracontinental deformation in the southeastern Tibetan Plateau. The hypothesis of lateral extrusion of the Sundaland was supported by the sinistral strike-slip shearing of the Ailaoshan-Red River Fault (ARF) in the east, and the dextral strike-slip shearing of the Gaoligong Fault (GLF) and the Sagaing Fault in the west (Anczkiewicz et al., 2007; Bertrand et al., 1999, 2001; Cao, Liu, et al., 2011; Leloup et al., 1995; Tapponnier et al., 1982, 1990; Zhang, Zhang, Zhong, et al., 2012). Paleomagnetic data further demonstrated that the extrusion of the Sundaland was accommodated by a clockwise rotation (Chen et al., 1995; Funahara et al., 1993; Yang & Besse, 1993). However, the sinistral strike-slip shearing of the Chongshan-Lancang River fault (CLF), lying between the Sibumasu and Indochina blocks, was identified as playing an important role in accommodating the clockwise rotation and southeastward extrusion of Sundaland (Akciz et al., 2008; Chen et al., 2022; Li et al., 2023; Wang & Burchfiel, 1997; Wang et al., 2006; Wang, Wang, et al., 2022; Zhang et al., 2010; Zhang, Zhang, Chang, et al., 2012). In the collision-extrusion model, the southeastward displacement of Sundaland relative to the South China block has been estimated as 700 ± 200 km (for review, see Leloup et al., 1995). However, the sinistral strike-slip shearing of the CLF and ARF at the boundary of the Indochina block would primarily result in block rotation rather than large-scale extrusion. Previous geodynamic models have been primarily derived from the geometry, kinematics, deformation timing, and emplacement processes of large-scale strike-slip faults such as the ARF, CLF, and GLF (e.g., Akciz et al., 2008; Leloup et al., 1995; Tapponnier et al., 1990; Wang et al., 2006; Zhang et al., 2010; Zhang, Zhang, Zhong, et al., 2012). However, the intracontinental deformation of the blocks during the extrusion process remains unclear. The most crucial point is how the crustal deformation accommodated the tectonic extrusion of the Sundaland.

The southeastern Tibetan Plateau records both the tectonic response to the closure of the Paleo-Tethys Ocean and large-scale strike-slip deformation associated with the lateral extrusion of Sundaland during the Cenozoic indentation of India into Eurasia, making it a key region for understanding the tectonic evolution of Southeast Asia (Figure 1a; Deng et al., 2014, 2018; Leloup et al., 1995; Lacassin et al., 1996, 1997; Metcalfe, 2013, 2021; Tapponnier et al., 1990; Wang et al., 2018; Wang, Wang, et al., 2022; Zhong, 1998). The Changning-Menglian (Trans-Mekong) orogenic belt was formed by the eastward subduction of the Sibumasu block beneath the Indochina block during the Middle-Late Triassic (Figure 1b; Huang & Chen, 1987; Metcalfe, 2013, 2021; Wang et al., 2018; Zhong, 1998). The tectonic evolution of the Paleo-Tethys, from its opening to closure and subsequent collision, has been studied extensively in recent decades (Deng et al., 2018; Fan et al., 2015; Liu et al., 1991, 1993; Wang, Liu, Schertl, et al., 2019; Wang, Liu, Li, et al., 2019; Zhong, 1998). However, the architecture, deformation, related metamorphism, and even the geodynamics for the Early Triassic contractional deformation associated with orogenic processes are still unsettled (Fan et al., 2015; Zhong, 1998). Meanwhile, significant Cenozoic intracontinental deformation has been superimposed on the primary spatial relationships of the pre-Tertiary tectonics in the Changning-Menglian orogenic belt, making it arduous to decipher the Triassic tectonic events (Wang & Burchfiel, 1997; Zhong, 1998).

The Wuliangshan massif, located between the ARF and the CLF, belongs to the overriding plate of the Changning-Menglian orogenic belt (Figure 1b). It records the Early Mesozoic and Cenozoic polyphase deformation events, providing an ideal place for understanding the tectonic response of the overriding plate to the Mesozoic Sibumasu-Indochina collision and Cenozoic India-Eurasia collision. However, the geometry,
<figure><img src="image_4.png" /><figcaption>**Figure 1.** (a) Tectonic outline of East Asia (adapted from Lin et al., 2008). (b) Tectonic map of the southeast Tibetan Plateau (adapted from YBGMR, 1990). ARF: Ailaoshan-Red River Fault; CLF: Chongshan-Lancang River Fault; GLF: Gaoligong Fault.</figcaption></figure>

kinematics, and deformation timing within this massif remain limited. These results may provide new insights into important issues such as plate coupling mechanisms during continental subduction and strain partitioning within extruded blocks. In this work, based on the field observation, we present a new structural analysis, Titanium-in-quartz geothermometry, and detrital zircon U-Pb geochronological and geochemical results. By integrating previously published structural, petrological, and geochronological data with our new results, we reconstruct the tectonic evolution of the Changning-Menglian orogenic belt and the Cenozoic progressive extrusion process of the Sundaland.

## 2. Geological Background

In the southeast Tibetan Plateau, the Changning-Menglian orogenic belt is regarded as the result of the closure of the main Paleo-Tethys Ocean, which recorded a prolonged history of subduction, back-arc extension, and continental collision during the Late Paleozoic to Early Mesozoic (Huang & Chen, 1987; Metcalfe, 2013, 2021; Wang et al., 2018; Zhong, 1998). The Changning-Menglian suture zone represents a main branch of the Paleo-Tethys and separates the Sibumasu block from the Indochina block (Figure 1b; Metcalfe, 2013, 2021; Wang et al., 2018). The cherts within this zone contain pelagic radiolarians that range in age from Middle Devonian to
Middle Triassic (Liu et al., 1991, 1993; Metcalfe, 2013). Amphibolite from the ophiolitic mélange yields zircon U-Pb ages ranging from 349 to 331 Ma (Duan et al., 2006). The Lancang Group is dominated by blueschists, eclogites, and greenschist-facies metasedimentary rocks (Fan et al., 2015; Wang, Liu, Li, et al., 2019; Zhong, 1998). The ⁴⁰Ar/³⁹Ar dating of glaucophane and phengite from blueschists have yielded HP metamorphic ages of 243–229 Ma (Fan et al., 2015; Wang, Liu, Schertl, et al., 2019). The Sibumasu block exhibits Late Carboniferous-Early Permian stratigraphic and paleontological affinities to cold-water high-latitude Gondwana (Fang et al., 1994; Jin, 2002; Metcalfe, 2002; Zhong, 1998). On the contrary, the Indochina block is characterized by the Paleozoic siliciclastic rocks and limestone with warm-water flora and fauna, which are significantly different from the Sibumasu block (Wu et al., 1995; YBGMR-Yunnan, 1990; Zhong, 1998).

In the west of the Indochina block, the occurrence of the Late Carboniferous to Early Triassic intermediate-felsic and mafic-ultramafic plutons is interpreted as parts of the Yunxian-Menghai magmatic arc, which formed due to the eastward subduction of the Paleo-Tethys oceanic lithosphere beneath the Indochina block (Figure 1b; Deng et al., 2014, 2018; Jian et al., 2009; Li et al., 2012). At the same time, back-arc spreading and rifting led to the fragmentation of the Indochina block and developed the Jinggu-Jinghong back-arc basin (Feng et al., 2002; Feng & Liu, 1993; Hennig et al., 2009; Sone & Metcalfe, 2008; Zhai et al., 2019; Zhu et al., 2021). This event is well documented by the occurrence of peridotite, cumulate gabbro, basalt, and radiolarian chert (Feng et al., 2002; Hennig et al., 2009; Zhai et al., 2019; Zhu et al., 2021). Gabbroic rocks yield SHRIMP zircon U-Pb age of 313–305 Ma with positive ε$_{Nd}$(t) values (+4.5 to +6.7) and zircon ε$_{Hf}$(t) values (+12.4 to +14.3), which are interpreted as being emplaced in a Late Carboniferous back-arc short-lived ocean basin (Zhai et al., 2019). Radiolaria from chert associated with tholeiitic basalts yield Middle to Late Permian ages (Feng et al., 2002). The Jinggu-Jinghong back-arc basin extends southward to the Nan-Uttaradit back-arc basin in Thailand, which is characterized by the occurrence of gabbro, tholeiitic metabasalt, andesite, and radiolarian chert (Metcalfe, 2021; Yang et al., 2016). Meta-gabbro and meta-basalt, dated at 311 ± 10 Ma and 316 ± 3 Ma by LA-ICP-MS U-Pb method on zircon, respectively, indicate that the Nan-Uttaradit basin already existed in the Late Carboniferous (Yang et al., 2016).

The pre-Middle Triassic strata are unconformably overlain by the upper Triassic coarse-grained continental red beds (YBGMR-Yunnan, 1990; Zhong, 1998). The Lincang batholith belongs to peraluminous S-type granite and yields zircon U-Pb age ranging from 230 to 200 Ma (Cong et al., 2020; Dong et al., 2013; Kong et al., 2012; Zhao et al., 2021). The architecture of the Changning-Menglian orogenic belt is sealed by the Lincang batholith, which is regarded as a post-collisional pluton (Deng et al., 2018; Peng et al., 2013). The Early Triassic hiatus may relate to the collision between the Sibumasu and Indochina blocks (Zhong, 1998).

A significant intracontinental deformation was accommodated by continental-scale strike-slip faulting that developed in the southeast Tibetan Plateau, triggered by the collision between the Indian and Eurasian plates in the Cenozoic (Leloup et al., 1995; Tapponnier et al., 1982, 1990; Wang & Burchfiel, 1997; Yin, 2010; Zhang, Zhang, Zhong, et al., 2012; Zhang, Zhang, Chang, et al., 2012). Three major strike-slip faults in the Sundaland, from east to west, are the ARF, the CLF, and the GLF (Figure 1b). The ARF has recorded a large-scale sinistral strike-slip displacement assumed to be more than 500 km and reworked by a dextral strike-slip (Red River Fault) and normal fault that began at 5 Ma (Jolivet et al., 2001; Leloup et al., 1993, 1995; Tapponnier et al., 1986). The ⁴⁰Ar/³⁹Ar dating of syn-kinematic minerals from the ARF is clustered at ca. 28–17 Ma (Cao, Neubauer, et al., 2011; Chen et al., 2015; Harrison et al., 1996; Leloup et al., 2001; Liu et al., 2013; Wang et al., 1998, 2000). U-Pb radiometric studies on leucogranitic intrusions have yielded an age span of 30–20 Ma (Cao, Liu, et al., 2011; Chen et al., 2017, 2018; Gilley et al., 2003; He et al., 2019, 2020; Liu et al., 2013, 2015a, 2015b, 2019; Sassier et al., 2009; Schärer et al., 1990, 1994; Tang, Liu, et al., 2013). The CLF extends from the Chongshan range to the eastern margin of the Lincang batholith (Figure 1b; Wang, Wang, et al., 2022). Numerous kinematic indicators, based on macroscopic and microscopic observations, consistently record a sinistral ductile shearing (Akciz et al., 2008; Li et al., 2023; Wang & Burchfiel, 1997; Wang et al., 2006; Wang, Wang, et al., 2022). However, Zhang et al. (2010) argued that the northern segments of the CLF exhibit a dextral-dominated strike-slip shearing, while sinistral shearing occurs in the southern segments. U-Pb dating of syn-shearing leucogranite veins and ⁴⁰Ar/³⁹Ar ages of muscovite and biotite within mylonites suggested that large-scale sinistral shearing occurred between ca. 33 and 19 Ma (Akciz et al., 2008; Chen et al., 2022; Li et al., 2023; Wang et al., 2006; Wang, Wang, et al., 2022; Zhang et al., 2010; Zhang, Zhang, Chang, et al., 2012). The GLF was interpreted as the western boundary of the extruded Sundaland (Wang & Burchfiel, 1997; Wang et al., 2006; Zhang, Zhang, Zhong, et al., 2012). In contrast to the sinistral ARF and CLF, kinematic indicators along the GLF show consistent
<figure><img src="image_5.png" /><figcaption>**Figure 2.** (a) Simplified geological map of the Wuliangshan massif (adapted from YBGMR, 1990). Equal-area lower hemisphere projection of structural elements: (b) bedding and cleavage poles, (c) foliation poles, (d) mineral and stretching lineation, and (e) bedding and cleavage poles. All structural elements are acquired by field work. Zircon U-Pb age is from (Liu et al., 2017).</figcaption></figure>

evidence of dextral ductile shearing (Wang & Burchfiel, 1997; Wang et al., 2006; Zhang, Zhang, Zhong, et al., 2012). The $^{40}$Ar/$^{39}$Ar dating of amphibole revealed that dextral strike-slip shearing along the GLF might have begun at 32–30 Ma (Tang et al., 2020; Wang et al., 2006). Amphibole, muscovite, and biotite from mylonites have yielded $^{40}$Ar/$^{39}$Ar ages span of ca. 32–11 Ma (Eroğlu et al., 2013; Lin et al., 2009; Tang et al., 2020; Wang et al., 2013; Zhang, Zhang, Zhong, et al., 2012).

The Wuliangshan massif, situated between the ARF and the CLF, comprises unmetamorphosed Permian to Cretaceous volcanic and sedimentary rocks, along with the metamorphosed Wuliangshan metamorphic complex (Figure 2a). From bottom to top, the Wuliangshan metamorphic complex is divided into three units (Figure 3).
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Geological cross-sections across the Wuliangshan massif show the bulk geometry and polyphase deformation (location in Figure 2).</figcaption></figure>

The lower unit is composed of epidote-biotite schist, quartzite, meta-sandstone, marble, and biotite-quartz schist. Garnet, epidote, amphibole, and albite indicate that this lower unit underwent epidote-amphibolite facies metamorphism (Hou et al., 2013). The middle unit comprises sericite schist, quartzite, and calcareous meta-sandstone with quartz, biotite, and muscovite assemblages corresponding to a greenschist facies metamorphism (Hou et al., 2013). The upper unit is represented by calcareous slate, marble, muddy limestone, and phyllite with sericite, albite, chlorite, and quartz assemblages corresponding to a low-greenschist facies metamorphism (Zhang, 2019). West of the Wuliangshan metamorphic complex, the Middle to Upper Permian strata are primarily composed of schistose sandstone and slate (Figure 2a). Along the western margin of the Wuliangshan metamorphic complex, the volcanic sequences include the Middle Triassic rhyolite (Manghuai Formation) and Late Triassic basalt (Xiaodingxi and Manghuihe formations). The pre-Triassic sedimentary sequences are unconformably overlain by the Jurassic-Early Cenozoic continental red beds.

# 3. Structural Analysis of the Wuliangshan Massif

## 3.1. Bulk Architecture of the Wuliangshan Massif

Based on our field observations, the bulk architecture of the Wuliangshan metamorphic complex is dominated by a NNW-SSE trending antiform that records heterogeneous polyphase deformation events (Figure 2a). The metasedimentary rock of the Wuliangshan metamorphic complex has undergone ductile deformation, characterized by penetrative foliation that has been folded into an antiform (Figures 2c and 3). The NW-SE trending mineral and stretching lineations, marked by elongated sericite, biotite, and quartz grains, are subparallel to the long axis of the antiform (Figures 2a and 2d). To the south of Mengpian, the Wuliangshan metamorphic complex is unconformably overlain by the Middle Jurassic conglomerate and sandstone (Figures 2a and 3e). The biotite monzogranite and granodiorite intrude into the Wuliangshan metamorphic complex (Figure 2a). In the Anzhao area, the biotite monzogranite appears massive and undeformed, with LA-ICP-MS zircon U-Pb ages of 246.4 ± 2.6 Ma (Figure 2a; Liu et al., 2017). Likewise, in the Shaluo area, the undeformed granodiorite has been dated at 245.7 ± 3.6 Ma using the LA-ICP-MS zircon U-Pb method (Figure 2a; Liu et al., 2017).
The Wuliangshan metamorphic complex is bounded to the southwest by a NE-dipping reverse fault, which places mica-schists above the Cretaceous sedimentary rocks and older strata (Figures 2a and 3c). To the southwest of the Wuliangshan metamorphic complex, the Triassic to Cretaceous strata are characterized by E-verging, N-S trending folds with a W-dipping slaty cleavage (Figures 2a, 3c, and 3d). In the northeast of the study area, a NE-dipping fault separates the underlying Wuliangshan metamorphic complex from the overlying Late Triassic or Jurassic sedimentary rocks (Figures 2a, 3a, and 3b). The Late Triassic to Cretaceous strata were deformed by NW-SE trending folds and reverse faults (Figures 3a and 3b). Further to the northeast, the Ailaoshan complex, consisting of gneiss, amphibolite, quartzite, and mica schist, overthrusts southwestward onto the Permian sedimentary rocks (Figure 2a; Tapponnier et al., 1990; Wang & Burchfiel, 1997; Wang, Lin, et al., 2022; Liu, Lin, et al., 2023). Based on our structural analysis of the Wuliangshan massif at macro-, meso-, and microstructural scales, we identified three stages of deformation (referred to as D₁, D₂, and D₃ in the following).

## 3.2. Early Mesozoic Ductile Deformation D₁ in the Wuliangshan Massif

The D₁ event represents the ductile deformation in the Wuliangshan massif, characterized by the development of pervasively foliated rocks in the Wuliangshan metamorphic complex. The foliation (S₁) contains a NW-SE trending mineral and stretching lineation (L₁). In the northern part of the Wuliangshan metamorphic complex, S₁ dips to the SW with low dip angles on the southwestern flank, and changes to NE dips with moderate to high dip angles on the northeastern flank, forming an antiform (Figures 2a and 3a). However, in the central and southern parts of the Wuliangshan metamorphic complex, S₁ dips steeply to the southwest on the southwestern flank, and gently to the northeast on the northeastern flank, forming the asymmetric antiformal structure (Figures 3b–3d). Although the strike of S₁ is reworked by the late tectonic events, the L₁ remains consistently oriented NW-SE. The average trend of L₁ is around 160°E (Figure 2d). In the lower unit of the Wuliangshan metamorphic complex, S₁ dips at a low angle in different directions. The NW-SE trending L₁ is marked by the alignments of quartz, muscovite, and biotite (Figure 4a). In the outcrops, isoclinal folds with axes parallel to L₁ affect quartz veins (Figure S1a in Supporting Information S1). Along the direction of L₁, elongated and asymmetric felsic veins unambiguously indicate a top-to-the-SE sense of shear (Figure 4b). In the middle unit of the Wuliangshan metamorphic complex, the foliation also contains the NW-SE trending L₁ expressed by the preferred orientation of biotite and quartz. The top-to-the-SE shearing is documented by the sigma-type quartz ribbons (Figure 4c). In the upper unit of the Wuliangshan metamorphic complex, the L₁ is represented by the preferred orientation of quartz. In the outcrop, felsic veins are also elongated along the direction of the lineation (Figure S1b in Supporting Information S1). Sigmoidal mica fish arranged along shear bands, σ-type porphyroblast systems manifest an unambiguous top-to-the-SE shear sense along the direction of L₁ at the microscopic observation of oriented thin sections (Figures 4d and S1c in Supporting Information S1). All the kinematic indicators related to the D₁ deformation show a top-to-the-SE sense of shear. Notably, the isotropic biotite monzogranite composed of K-feldspar, plagioclase, and biotite intrudes into the Wuliangshan metamorphic complex (Figures 2a and S1d in Supporting Information S1). To the south of Mengpian, the penetrative S₁ and L₁ related to the D₁ ductile deformation are never observed in the overlying Jurassic conglomerate and sandstone.

It is worth noting that to the west of the Wuliangshan metamorphic complex, similar brittle-ductile deformation is also observed in the Middle to Upper Permian slate and schistose sandstone (Figure 2a). For example, in the Manghuai area, the Permian rocks exhibit a pervasive schistosity that contains recrystallized sericite and chlorite. On the schistosity, NW-SE trending semi-penetrative lineations are defined by oriented chlorite and quartz (Figure S1e in Supporting Information S1). Along with the lineation, top-to-the-SE shearing can be inferred from the foliated cataclasite (Figure 4e). In the thin section, similar kinematics is shown by a sigma-type quartz porphyroclast (Figure 4f).

## 3.3. Evidence for the D₂ Deformation

To the east of the CLF, although the D₂ deformation was overprinted by the D₃ deformation, it can still be identified in the field. The D₂ deformation, extending from the Manghuai to Jingfu area, is well recorded in the Late Permian to Cretaceous strata (Figure 2a). The Late Permian to Cretaceous strata are deformed by nearly N-S trending folds and W-dipping faults (Figures 3c and 3d). The W-dipping slaty cleavage (S₂) related to E-verging folds is widely developed in these incompetent layers (Figure 2b). In the western part of the Wuliangshan metamorphic complex, the superimposition relationship between the D₁ and D₂ events can be observed. The
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Field photographs and microphotographs related to the D₁ deformation. (a) NW-SE trending mineral and stretching lineations expressed by quartz, muscovite, and biotite in the meta-sandstone (YZ139: 24.54281°N, 100.5662°E). (b) Biotite and quartz are oriented along the NW-SE direction, with quartz exhibiting plastic deformation. Sigmoidal felsic veins showing top-to-the-SE shearing (CM1163: 24.4456°N, 100.6031°E). (c) Sigma-type quartz ribbons showing top-to-the-SE shearing (SV154: 24.3258°N, 100.6718°E). (d) Sigma-type quartz porphyroclast in the quartz schist (CM640: 24.7803°N, 100.5164°E). (e) The Middle Permian foliated cataclasite exhibits a top-to-the-SE sense of shear (CM660: 24.5317°N, 100.4878°E). (f) Sigma-type quartz porphyroclast in the Middle Permian pelitic schist (CM660: 24.5317°N, 100.4878°E).</figcaption></figure>

bedding-parallel quartz veins are folded, and the foliation S₁ of the Wuliangshan metamorphic complex is overprinted by SW-dipping cleavage S₂ (Figure S1f in Supporting Information S1).

In the Manghuai area, an eastward-directed décollement places Middle Triassic volcanic rocks above the Upper Permian sandstone (Figures 2a and 3c). Near the décollement, the Middle Triassic volcanic rocks are intensely sheared, with sigmoidal lenticular quartz veins indicating a top-to-the-E shearing (Figures 3c and 5a). Meanwhile, the Upper Permian schistose sandstone (S₁) was overprinted by the D₂ deformation, forming the E-verging asymmetric folds with continuous cleavage S₂ (Figures 3c and 5b).
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Field photographs related to the D₂ and D₃ deformations. (a) Sigmoidal lenticular quartz in Middle Triassic volcanic rocks indicates a top-to-the-E shearing (CM664: 24.5099°N, 100.4667°E). (b) E-verging folds with W-dipping continuous cleavage (S₂) are developed within Late Permian schistose sandstone (S₁) (CM662:24.5114°N, 100.4689°E). (c) The relationship between the bedding (S₀) and cleavage (S₂) in Cretaceous siltstone indicates an E-verging fold (CM1132: 24.3258°N, 100.8003°E). (d) The relationship between the bedding (S₀) and cleavage (S₂) in Cretaceous sandstone indicates an E-verging fold (LM254: 24.4233°N, 100.7921°E). (e) The relationship between the bedding and foreset laminae indicates that the Cretaceous strata have been overturned to the SW (LM297: 24.7157°N, 100.8026°E). (f) The angular relationship between bedding (S₀) and cleavage (S₃) in the Jurassic sandstone indicates a SW-verging fold (LM292: 24.6991°N, 100.6435°E).</figcaption></figure>

To the west of the Jingfu area, a high-angle W-dipping or sub-vertical cleavage is well developed in the Late Triassic volcanic rocks (Figure S1g in Supporting Information S1). In the hinge zones of the fold, the bedding (S₀) in the Late Triassic tuff was replaced by continuous cleavage (S₂) dipping to the west at various angles. However, the relationship between bedding and cleavage is well documented in the competent Jurassic sandstone (Figure 3d). In several outcrops, the relationship between the bedding with a moderately dipping angle and steeply dipping spaced cleavage is indicative of upright folds.
To the east of the Wuliangshan metamorphic complex, near the Wenbo area, a W-dipping thrust places the Wuliangshan metamorphic complex over the Jurassic and Cretaceous strata (Figures 3c and 3d). Adjacent to the thrust, the Jurassic and Cretaceous strata are overturned to the east (Figures 3c and 3d). The bedding (S₀) is generally parallel to SW-dipping continuous cleavage (S₂). More to the east, the Cretaceous sandstone was folded with a W-dipping spaced cleavage (Figure 3d). Fold hinges are rarely observed there, but the relationships between bedding and cleavage indicate an eastward overturning (Figures 5c and 5d).

### 3.4. Evidence for the D₃ Deformation

Southwest of the ARF, the D₃ deformation extends from the Xinjie to Longjie area and is expressed by NW-SE trending folds and NE-dipping faults (Figure 2a). Associated with the folding, a NE-dipping spaced cleavage (S₃) is recognized in fold hinge zones (Figure 2e). The D₃ deformation overprints the earlier D₁ and D₂ structures and is characterized by a series of NE-dipping thrusts developed within strata that have been modified by both D₁ and D₂ deformations (Figure 3).

In the northeast of the study area, adjacent to the ARF, the Late Triassic to Cretaceous strata are deformed by NW-SE trending tight folds (Figures 3a and 3b). The NE-dipping continuous cleavage is well-developed in these incompetent layers (Figure S1h in Supporting Information S1). In the competent Cretaceous sandstone strata, the relationship between the bedding and foreset laminae indicates that the strata have been overturned to the SW (Figure 5e). A NE-dipping thrust places the cleavage-developed Late Triassic rocks southwestward over the Jurassic or Cretaceous strata (Figures 3a and 3b). The Jurassic to Cretaceous strata are represented by gently inclined folds overturned to the southwest (Figures 3a and 3b). The cleavage in these strata is generally less penetrative than those adjacent to the ARF. The angular relationship between steeply dipping bedding and moderately dipping spaced cleavage in the Jurassic sandstone indicates a SW-verging fold (Figure 5f). Further to the SW, a southwestward-directed décollement places the Late Triassic to Jurassic strata above the Wuliangshan metamorphic complex (Figures 3a and 3b).

Near the Jingfu area, the Wuliangshan metamorphic complex thrusted southwestward over the Jurassic to Cretaceous strata (Figures 3c and 3d). Further to the SW, the Jurassic to Cretaceous strata are transported southwestward to overlie the Permian strata by décollement, indicating the D₃ deformation continuity to the SW (Figure 3c).

## 4. Estimation for the D₁ Deformation Temperature

Eight samples, including meta-sandstone (CM646, SV138, CM1141), biotite-rich quartz schist (CM640, CM645), and metavolcanic rocks (CM1165, CM1172, SV141), were selected from the Wuliangshan metamorphic complex for quartz c-axis fabric and Titanium-in-quartz geothermometric analysis to evaluate the D₁ deformation temperature. These samples were cut perpendicular to foliation and parallel to lineation to make XZ thin sections. Microphotographs of the representative samples are shown in Figure S2 in Supporting Information S1, and the quartz c-axis fabric and Titanium-in-quartz results are presented in Figure 6.

### 4.1. Quartz c-Axis Fabrics

Samples from the Wuliangshan metamorphic complex are characterized by middle-to high-temperature deformation microstructures. Quartz grains are plastically deformed, showing undulose extinction, dynamic recrystallization, and ribbon structures. Quartz grains form rectangular polycrystalline ribbons with their long axis parallel to the foliation (Figure S2A in Supporting Information S1). Recrystallized quartz aggregates exhibit sutured and serrated grain boundaries and commonly undulose extinction, indicative of dynamic recrystallization activated by grain boundary migration (Figures S2b–S2d in Supporting Information S1; Stipp et al., 2002). In some samples, the new grains of recrystallized quartz have an average size of 50 µm, characteristic of subgrain rotation recrystallization (Figures S2e and S2f in Supporting Information S1; Stipp et al., 2002). Subrounded feldspars exhibit serrated grain boundaries and fine recrystallized grains characteristic of bulging recrystallization (Figures S2d and S2e in Supporting Information S1).

Quartz c-axis fabric analysis was conducted using a universal stage. Three samples (CM1165, SV138, SV141) exhibit maxima distributed asymmetrically around the Y-axis attributed to the prism <a> slip at high temperature (Figure 6; Toy et al., 2008). The quartz c-axis fabrics of the samples CM645, CM646, and CM1172 display
<figure><img src="image_9.png" /><figcaption>**Figure 6.** The Lattice Preferred Orientation of the quartz c-axis and Titanium-in-quartz results for the Wuliangshan metamorphic complex.</figcaption></figure>

asymmetrical maxima distributed around the Y-axis, with a subordinate cluster located between the Z-axis and the Y-axis (Figure 6). These quartz c-axis fabrics are interpreted as the activity of the dominant prism <a> slip with subsidiary rhomb <a> slip (Toy et al., 2008). Samples CM640 and CM1141 exhibit the asymmetrical maxima in-between the Z-axis and Y-axis or close to the Y-axis, indicating a combination of rhomb <a> and slip at medium-temperature (Figure 6; Toy et al., 2008).

To summarize, our samples from the Wuliangshan metamorphic complex exhibit the quartz deformation mechanisms dominated by grain boundary migration and incipient subgrain rotation, indicative of middle-to high-temperature (450–550°C) deformation (e.g., Stipp et al., 2002). The quartz c-axis patterns of the deformed rocks are defined by prism <a> slip with subordinate rhomb <a> slip component, suggesting corresponding to a deformation temperature of 450–550°C (Passchier & Trouw, 2005; Stipp et al., 2002). Furthermore, the asymmetrical quartz c-axis patterns indicate gliding planes compatible with a to-to-the-SE shearing (Figure 6).

## 4.2. Titanium-in-Quartz Approach

Titanium-in-quartz geothermometry has become an increasingly common tool for evaluating the deformation temperature based on the re-equilibration of Ti concentration during quartz dynamic recrystallization (e.g., Bestmann & Pennacchioni, 2015; Cavalcante et al., 2022; Kohn & Northrup, 2009; Liu et al., 2021; Nachlas et al., 2014; Taylor et al., 2023). Here, we apply the Titanium-in-quartz method to estimate the D₁ deformation temperatures. All the analyzed quartz grains show evidence of intracrystalline deformation, such as lobate contacts, undulose extinction, dynamically recrystallized grains, and quartz ribbons. Detailed analytical methods are described in Text S1 in Supporting Information S1. A summary of quartz Ti concentrations and calculated temperatures is presented in Table 1, and the complete Titanium-in-quartz results are provided in Table S1.

Samples CM640, CM645, CM646, CM1141, CM1165, CM1172, SV138, and SV141 yield average Ti concentrations (ppm) of 11.6, 16.6, 22.1, 5.4, 23.5, 23.1, 5.3, and 9.2, respectively (Table 1). Correspondingly, average temperatures calculated using Thomas, Bruce Watson, Spear, Shemella, Nayak, et al. (2010) for these
<table><caption>**Table 1**
Detailed Information on Selected Samples for Titanium-in-Quartz Analysis</caption><thead><tr><th rowspan="2">Sample</th><th rowspan="2">GPS location</th><th rowspan="2">Lithofacies</th><th colspan="2">Quartz Ti (ppm)</th><th colspan="3">Temperature (°C)</th></tr><tr><th>Median</th><th>Mean</th><th>Median</th><th>Mean</th><th>1σ SD</th></tr></thead><tbody><tr><td>CM640</td><td>100.516389°E<br>24.780278°N</td><td>Biotite-quartz schist</td><td>6.0</td><td>11.6</td><td>440</td><td>456</td><td>63</td></tr><tr><td>CM645</td><td>100.490833°E<br>24.704722°N</td><td>Biotite-quartz schist</td><td>10.0</td><td>16.6</td><td>472</td><td>484</td><td>63</td></tr><tr><td>CM646</td><td>100.493333°E<br>24.69°N</td><td>Meta-sandstone</td><td>17.0</td><td>22.1</td><td>509</td><td>516</td><td>48</td></tr><tr><td>CM1141</td><td>100.704722°E<br>24.297222°N</td><td>Meta-sandstone</td><td>4.0</td><td>5.4</td><td>416</td><td>429</td><td>26</td></tr><tr><td>CM1165</td><td>100.620556°E<br>24.457222°N</td><td>Metavolcanic rock</td><td>21.5</td><td>23.5</td><td>526</td><td>528</td><td>32</td></tr><tr><td>CM1172</td><td>100.575278°E<br>24.4825°N</td><td>Metavolcanic rock</td><td>19.0</td><td>23.1</td><td>517</td><td>526</td><td>34</td></tr><tr><td>SV138</td><td>100.538925°E<br>24.601692°N</td><td>Meta-sandstone</td><td>5.0</td><td>5.3</td><td>429</td><td>430</td><td>19</td></tr><tr><td>SV141</td><td>100.571644°E<br>24.504128°N</td><td>Metavolcanic rock</td><td>8.5</td><td>9.2</td><td>461</td><td>464</td><td>20</td></tr></tbody></table>

Note. 1σ SD: 1σ standard deviation of the data set.

samples were 465, 484, 516, 429, 528, 526, 430, and 464°C, respectively (Figure S3 in Supporting Information S1 and Table 1). The Titanium-in-quartz results of the eight analyzed samples show average temperatures ranging from 429 to 528°C, which is consistent with the estimated temperatures based on our structural observations and quartz c-axis fabrics analysis (Figure 6).

# 5. Geochronological Constraints

Three samples (CM645, CM1141, CM1166) were selected for detrital zircon U-Pb dating and Hf isotope analysis to determine the depositional age and provenance of the protoliths of the Wuliangshan metamorphic complex.

## 5.1. Methodology

Analytical methods of the zircon U-Pb dating and Hf isotopes are presented in Text S2 and S3 in Supporting Information S1, respectively.

## 5.2. Sample Descriptions

CM645 is a gray-colored, fine-to coarse-grained biotite quartz schist from the lower unit of the Wuliangshan metamorphic complex. The D₁ ductile deformation is well-marked in this sample. In the field, the rock has experienced an intensive shearing indicated by a pervasive foliation (Figure S4a in Supporting Information S1). At the microscope scale, the foliation is expressed by elongated quartz grains and oriented biotite (Figure S4b in Supporting Information S1). The major rock components are quartz (80%-90%), biotite (5%-10%), and minor lithic grains.

CM1141 is a meta-sandstone collected from the middle unit of the Wuliangshan metamorphic complex. In the outcrop, the rock is light gray in color, fine-grained, and exhibits unambiguous foliation (Figure S4c in Supporting Information S1). The lineation is constituted by the alignment of quartz and biotite. In the thin section, the preferred alignment of quartz and biotite constitutes the foliation (Figure S4d in Supporting Information S1). The major rock components are quartz (70%-80%), biotite (10%-20%), and minor lithic grains.
CM1166 is a meta-sandstone collected from the lower unit of the Wuliangshan metamorphic complex. In the outcrop, the rock recorded intense shearing indicated by a pervasive foliation. The lineation is expressed by the alignment of the quartz and muscovite (Figure S4e in Supporting Information S1). In the thin section, the foliation is defined by oriented muscovite and elongated quartz (Figure S4f in Supporting Information S1). The major rock components are quartz (75%-85%), biotite (5%-10%), and minor lithic grains.

### 5.3. Zircon U-Pb and Lu-Hf Analytical Results

Representative CL images of the dated zircons, spot ages, and $ε_{Hf}(t)$ are shown in Figure S5 in Supporting Information S1. Zircons from the three metasedimentary rock samples exhibit a wide range in size from 50 to 250 µm with aspect ratios of 1:1 to 4:1. Most zircon grains are transparent, and euhedral to subhedral prismatic in shape, implying a relatively short distance transport. Most zircon grains display obvious oscillatory zoning under CL images with high Th/U ratios (>0.1), being suggestive of a magmatic origin (Hoskin & Ireland, 2000). Some grains exhibit bright homogeneous structures with low Th/U ratios (<0.1), which are interpreted as a metamorphic origin. Lu-Hf isotopes were acquired on the same spots or the same growth domains of zircon grains with concordant U-Pb age. The concordant age obtained from the same zircon grain was used for calculating the initial Hf isotopic ratios and crustal model ages. The U-Pb ages and $ε_{Hf}(t)$ values are presented in Figures 8 and S6 in Supporting Information S1. All the LA-ICP-MS U-Pb and Lu-Hf results are listed in Supplementary Tables S2 and S3, respectively.

CM645: The detrital zircon grains are 50–200 µm in length with aspect ratios of 1:1 to 3:1. The detrital zircons mostly have prismatic shapes, whereas a few others show rounded morphology. Th/U ratios exhibit a wide range of 0.03–1.47. Except for one grain, most grains of Th/U ratios are higher than 0.1. Combined with the CL images, these zircon grains manifest oscillatory zoning indicative of a magmatic origin. One hundred analytical spots were measured, and 92 analyses yielded concordant ages ranging from 2681 to 198 Ma. Three major groups occur at 350–200 Ma, 500–350 Ma, and 1,900–1,700 Ma with peaks at ~260 Ma, ~425 Ma, and ~1,850 Ma (Figure 7a). Three subordinate age peaks at ~750 Ma, ~960 Ma, and ~2,400 Ma. Thirty-two dated zircon grains were analyzed for Hf isotopes with $ε_{Hf}(t)$ values varying from −20.8 to +10.2. The 310–250 Ma zircon grains have both positive and negative $ε_{Hf}(t)$ values between −16.0 and +10.2, yielding TC DM model ages ranging from 2280 to 616 Ma.

CM1141: The detrital zircon grains range from 100 to 250 µm in length with length-width ratios of 1:1 to 4:1. The zircon grains exhibit subhedral morphology or a prismatic shape. Most of the analyzed grains display high Th/U values ranging from 0.01 to 1.37, but only six grains have lower Th/U of 0.01–0.09. The CL images of the zircon grain manifest oscillatory zonation with high Th/U ratios (>0.1), implying a magmatic origin. A total of 100 analytical spots were conducted on zircon grains, and 93 analyses yielded concordant U-Pb ages ranging from 2621 to 160 Ma. These concordant ages are clustered in three age groups, which include three major age populations of 350–200 Ma, 500–350 Ma, and 2,100–1,700 Ma, corresponding to three peaks at ~250 Ma, ~450 Ma, ~1,860 Ma, and three subordinate age populations peaking at 780 Ma, ~1,180 Ma, and ~2,500 Ma (Figure 7b). Forty dated zircon grains were analyzed for Hf isotopes with $ε_{Hf}(t)$ values varying from −20.0 to +10.7. The 310–250 Ma zircon grains have both positive and negative $ε_{Hf}(t)$ values between −19.5 and +5.5, and display $T_{DM}^C$ model ages ranging from 2516 to 926 Ma.

CM1166: The detrital zircon grains range from 50 to 150 µm in length with length-width ratios of 1:1 to 3:1. The zircon grains exhibit sub-rounded to sub-angular shapes. Most of the analyzed grains display high Th/U values ranging from 0.1 to 4.05. The CL images of the zircon grains exhibit oscillatory zonation with Th/U ratios (>0.1), indicating a magmatic origin. A total of 70 analytical spots were conducted on zircon grains, and 56 analyses yielded concordant U-Pb ages ranging from 3,603 to 220 Ma. Three major groups occur at 360–210 Ma, 530–360 Ma, and 830–700 Ma with peaks at ~250 Ma, ~420 Ma, and ~750 Ma (Figure 7c). Two subordinate age peaks at ~980 Ma, and ~1,865 Ma. Twenty dated zircon grains were analyzed for Hf isotopes with $ε_{Hf}(t)$ values varying from −22.58 to +4.33.

## 6. Discussion

### 6.1. Depositional Age and Provenance of the Protoliths of the Wuliangshan Metamorphic Complex

The Wuliangshan metamorphic complex was situated within the Indochina block and was involved in the D₁ ductile deformation. Constraining its depositional age and conducting provenance analysis is crucial for understanding the tectonic response of the overriding plate during the closure of the Paleo-Tethys Ocean. The
<figure><img src="image_10.png" /><figcaption>**Figure 7.** (a–c) Probability diagrams of U-Pb ages of the detrital zircons from the Wuliangshan metamorphic complex (this study); (d–f) Synthesis and comparison of probability diagrams for U-Pb ages of the detrital zircons in the different units of the Wuliangshan metamorphic complex (Data from this study and Xing et al., 2016).</figcaption></figure>

depositional age of the protoliths of the Wuliangshan metamorphic complex has been variously interpreted as Cambrian or Early Paleozoic (YBGMR-Weishan, 1975), Middle Carboniferous to Early Permian (Li, 2015), Permian (YBGMR-Fengqing, 2006), and Lower Cretaceous (YBGMR-Yongping, 1979). In a recent study, Xing et al. (2016) reported detrital zircon U-Pb ages from metasedimentary rock of the Wuliangshan metamorphic complex with the youngest age peak at 247 Ma. The depositional age of the protoliths of the Wuliangshan metamorphic complex remains unclear.

In this study, the U-Pb zircon dating of three samples from the Wuliangshan metamorphic complex provides further information to constrain the maximum depositional age of the metasedimentary rocks. The Wuliangshan metamorphic complex is unconformably overlain by Middle Jurassic conglomerate and sandstone, and intruded by unmetamorphosed and undeformed granitic plutons (Figure 2a). These granitic plutons yielded zircon U-Pb ages of 246.4–245.7 Ma (Liu et al., 2017). This indicates that the protoliths of the Wuliangshan metamorphic complex were deposited before the Early Triassic. Our sample contains the five youngest zircon grains yielding
<caption>**Table 2**</caption>

Summary of Four Measures of the Youngest Age of Detrital Zircon From the Wuliangshan Metamorphic Complex in This Study and the Results From Xing et al. (2016)

<table><thead><tr><th></th><th>Sample</th><th>GPS location</th><th>Lithofacies</th><th>YSG</th><th>YPP</th><th>YClσ (2+)<sup>a</sup></th><th>YC2σ (3+)<sup>a</sup></th></tr></thead><tbody><tr><td>Upper unit</td><td>09YN-3</td><td></td><td>Phyllite</td><td>255 ± 5</td><td>260</td><td>264.7 ± 5.8(10)</td><td>264.7 ± 5.8(10)</td></tr><tr><td>Middle unit</td><td>CM1141</td><td>100.704722°E<br>24.297222°N</td><td>meta-sandstone</td><td>160.3 ± 5.2</td><td>250</td><td>181.6 ± 3.8(2)</td><td>240.8 ± 5.5(24)</td></tr><tr><td></td><td>09YN-4</td><td></td><td>Calcareous sandstone</td><td>241 ± 8</td><td>247</td><td>252 ± 6.3(9)</td><td>257.1 ± 6.5(11)</td></tr><tr><td></td><td>09YN-6</td><td></td><td>Calcareous phyllite</td><td>243 ± 6</td><td>269</td><td>257.4 ± 5.4(13)</td><td>266.4 ± 5.8(20)</td></tr><tr><td>Lower unit</td><td>CM645</td><td>100.490833°E<br>24.704722°N</td><td>biotite quartz schist</td><td>197.7 ± 2.4</td><td>260</td><td>198.8 ± 2.4(2)</td><td>262.9 ± 3.3(13)</td></tr><tr><td></td><td>CM1166</td><td>100.623889°E<br>24.456389°N</td><td>meta-sandstone</td><td>220.1 ± 3.0</td><td>250</td><td>245.3 ± 4.3(5)</td><td>245.3 ± 4.3(5)</td></tr><tr><td></td><td>09YN-8</td><td></td><td>Calcareous sandstone</td><td>248 ± 8</td><td>259</td><td>260.3 ± 8.3(3)</td><td>269.8 ± 8.5(4)</td></tr></tbody></table>

Note. YSG: youngest single grain age; YPP: youngest graphical age peak; YClσ (2+): youngest 1σ grain cluster; YC2σ (3+): youngest 2σ grain cluster. Bold are samples from this work. <sup>a</sup>Numbers of grain ages in clusters in parentheses.

concordant U-Pb ages ranging from 199 to 160 Ma (Tables 2 and S2 in Supporting Information S1). Although the youngest single-zircon age can constrain the maximum depositional age, it is inconsistent with our field observations. We argue that these younger zircon ages (199–160 Ma) do not represent the depositional age of the protoliths of the Wuliangshan metamorphic complex. Moreover, Jurassic magmatic rocks had not been reported in the Indochina block.

Based on the metamorphic facies of the Wuliangshan metamorphic complex, we divided our data, along with previous detrital zircon works, into three metamorphic units: lower, middle, and upper (Table 2; Xing et al., 2016). The lower unit (CM645, CM1166, 09YN-8) is expressed by a significant youngest age group peak at 260 Ma, 250 Ma, and 259 Ma, respectively (Figure 7d and Table 2). It can be inferred that the maximum depositional age of the lower unit ranges from 260 to 250 Ma. The middle unit (CM1141, 09YN-4, 09YN-6) has the youngest age group peaks at 250 Ma, 247 Ma, and 269 Ma, respectively (Figure 7e and Table 2). It can be inferred that the maximum depositional age of the middle unit ranges from 269 to 247 Ma. The upper unit (09YN-3) exhibits the youngest peak located around 260 Ma, which indicates that the maximum depositional age does not exceed 260 Ma (Figure 7f and Table 2). In fact, the age distribution spectra of the three units do not show significant differences (Figures 7d–7f). These three units exhibit roughly identical maximum depositional age, indicating contemporaneous and rapid sedimentation. This means that from the view of the protoliths, these three units are deposited simultaneously or quasi-simultaneously. The maximum depositional age of the Wuliangshan metamorphic complex could be considered to be between 260 and 250 Ma (Late Permian).

Detrital zircon U-Pb age spectra and isotopic Lu-Hf are useful tools for investigating the provenance of these sedimentary rocks (Cawood et al., 2007; Dickinson & Gehrels, 2009; Griffin et al., 2000). Combined with the previous work, all the detrital zircon analytical results from the Wuliangshan metamorphic complex display three major age peaks at ~260, ~440, and ~1,850 Ma, and three subordinate age populations peak at ~780, ~960, and ~2,450 Ma (Figure 8a). Detrital zircon with U-Pb ages ranging from 310 to 250 Ma are euhedral to subhedral in shape and exhibit oscillatory zoning with Th/U ratios between 0.03 and 1.47, indicating a magmatic origin and short-distance transportation from their source regions. These detrital zircons of 310–250 Ma display a wide range of $\epsilon_{Hf}(t)$ values with 70% negative and 30% positive $\epsilon_{Hf}(t)$ values, which is indicative of the magma source from the mixing of significantly enriched crustal and minor depleted material (Figure 8c). The Late Carboniferous to Middle Triassic (310–240 Ma) magmatic rocks are widely exposed in the western margin of the Indochina block, such as the granodiorite (exposed in the Linchang batholith, 261–252 Ma; Deng et al., 2018; Liu, Yang, et al., 2023), the Banpo mafic-ultramafic complex (312–285 Ma; Jian et al., 2009; Zhai et al., 2019), the Jinghong granodiorite intrusion (284–282 Ma; Hennig et al., 2009), and the Damenglong diorite and granodiorite pluton (273–252 Ma; Wang et al., 2021). Therefore, we suggest that the 310–250 Ma detrital zircons were likely derived from the nearby Late Carboniferous to Middle Triassic plutons and volcanic rocks. The detrital zircon age peak at 450 Ma is the predominant age population in the sedimentary cover of the Indochina block (Arboit et al., 2016;
<figure><img src="image_11.png" /><figcaption>**Figure 8.** (a) Compilation of detrital zircon age data from the Wuliangshan metamorphic complex (Data from this study and Xing et al., 2016). (b) Compilation of detrital zircon age data for the Permian sedimentary rocks from the Indochina block (Data from Arboit et al., 2016; Xia et al., 2019). (c) Plot of ε<sub>Hf</sub>(t) value versus age for the detrital zircons from the Wuliangshan metamorphic complex (Data from this study and Xing et al., 2016). (d) Plot of ε<sub>Hf</sub>(t) value versus age for the detrital zircons from the Permian sedimentary rocks in the Indochina block (Data from Arboit et al., 2016; Xia et al., 2019).</figcaption></figure>

Wang et al., 2014; Xia et al., 2019). This age spectrum is consistent with data from the Wuliangshan metamorphic complex (Figure 8). The ε<sub>Hf</sub>(t) values of the 500–400 Ma detrital zircons from the Wuliangshan metamorphic complex range from –20 to +10.7 and mainly consist of negative ε<sub>Hf</sub>(t) values (Figure 8c). The Late Cambrian to Ordovician magmatism is well recorded on the western margin of the Indochina block, as an example, the Dazhonghe volcanic rock (429–418 Ma; Liu et al., 2018; Mao et al., 2012), the 476.3 ± 2.8 Ma granitoids in Lincang batholith (Xiao et al., 2018), and the 429 ± 3 Ma dacite in Dapingzhang (Lehmann et al., 2013). The 500–400 Ma age group with the peak at around 440 Ma may come from the proximal magmatic rocks or recycled from the pre-Permian sedimentary rocks in the Indochina block. The Wuliangshan metamorphic complex and the Indochina block share similar detrital zircon age spectra, characterized by prominent peaks at 260–250 and 500–400 Ma (Figure S7 in Supporting Information S1). In contrast, the Sibumasu block and the Lancang Group exhibit distinct age spectra with prominent Pan-African (600–500 Ma) peaks (Figure S7 in Supporting Information S1). Thus, the provenance for the protoliths of the Wuliangshan metamorphic complex was the Late Carboniferous to Permian arc-related volcanics together with a significant detrital signal from the Indochina block.

## 6.2. Geometry, Kinematics, and Timing of the D₁ Deformation

Our structural analysis reveals that the Wuliangshan massif underwent polyphase deformation. The kinematics related to the deformation events with the available radiometric ages are shown in Figure 9 and Table 3. The D₁ event is widely developed in the Wuliangshan metamorphic complex and is expressed by a pervasive foliation and a NW-SE trending mineral and stretching lineation L₁ associated with a top-to-the-SE sense of shear. Middle-to high-temperature ductile deformation is expressed by quartz polycrystalline ribbons, subgrain rotation recrystallization, grain boundary migration recrystallization, and feldspar bulging recrystallization (Figure S2 in Supporting Information S1). This is also supported by the quartz c-axis fabrics, which are characterized by prism
<figure><img src="image_12.png" /><figcaption>**Figure 9.** Structural map of the Wuliangshan metamorphic complex showing the available radiometric data. Ages are defined using the dating methods (Zr: Zircon U-Pb age; K-Ar: K-Ar age). Data are from: (1) Heppe et al., 2007; (2) Hennig et al., 2009; (3) Wang et al., 2010; (4) Yang et al., 2014; (5) Liu, Huang, et al., 2015; (6) Xing et al., 2016; (7) Liu et al., 2017; (8) Yan et al., 2019; (9) Cong et al., 2020.</figcaption></figure>

<a> slip with subordinate rhomb <a> slip component (Figure 6). The Titanium-in-quartz results of the eight analyzed samples yield average temperatures ranging from 429 to 528°C (Figure S3 in Supporting Information S1 and Table 1). Therefore, our fieldwork, microscopic observation, quartz c-axis fabrics, and Titanium-in-quartz geothermometry reveal a consistent top-to-the-SE shearing under medium-to high-temperature (450–550°C) conditions.

The D₁ event was previously interpreted as Cenozoic intracontinental NNW-SSE sinistral ductile strike-slip deformation that developed in the central part of the Wuliangshan metamorphic complex (Hou et al., 2013). However, in the field, the foliation forms an antiform with a flat-lying or slightly tilted attitude in the central part of the Wuliangshan metamorphic complex, which is inconsistent with the geometry of a simple ductile sinistral strike-slip fault (Figure 3). Although such a domal structure has been suggested to occur along the ARF (e.g., Anczkiewicz et al., 2007; Leloup et al., 2001), from the viewpoint of the kinematics, the consistent top-to-the SE sense of shear observed in both limbs of the Wuliangshan metamorphic complex makes this interpretation unlikely. Therefore, we argue that the Wuliangshan metamorphic complex experienced a top-to-the SE shearing, which was reworked by the late tectonic events, resulting in the geometry of antiform (Figure 3).

The age of D₁ deformation is not well constrained yet. The only low-temperature geochronological data comes from phyllites in the upper unit of the Wuliangshan metamorphic complex, which yields a K-Ar age of 17.4 ± 1 Ma interpreted as a cooling age (Heppe et al., 2007). Chen et al. (2022) argued that the top-to-the-SE shearing in the Wuliangshan metamorphic complex might be a Cenozoic event related to the formation of the A-
<table><caption>**Table 3**
Summary of Characteristics of the Three Deformation Events in the Wuliangshan Massif</caption><thead><tr><td>Deformation event</td><td>Geometry</td><td>Kinematics</td><td>Deformation timing</td><td>Tectonic interpretation</td></tr></thead><tbody><tr><td>D<sub>3</sub></td><td>NW-SE trending folds with NE-dipping spaced cleavage, and NE-dipping faults</td><td>SW-directed thrusts and SW-verging folds</td><td>27–23 Ma</td><td>Transpressional tectonics related to the ARF</td></tr><tr><td>D<sub>2</sub></td><td>N-S trending folds with W-dipping continuous cleavage, and W-dipping faults</td><td>E-directed thrusts and E-verging folds</td><td>32–27 Ma</td><td>Transpressional tectonics related to the CLF</td></tr><tr><td>D<sub>1</sub></td><td>Foliations forms an asymmetric antiformal structure with NW-SE trending mineral and stretching lineations</td><td>Top-to-the SE</td><td>250–246 Ma</td><td>Syn-collision compression</td></tr></tbody></table>

type Chongshan dome. The D₁ event occurred after the deposition of the protoliths of the Wuliangshan metamorphic complex, around 250 Ma. The Middle Triassic volcanic rocks, dated at 239.3 ± 1.2 Ma by the LA-ICP-MS zircon U-Pb method (Figure 9; Cong et al., 2020), are not involved in this D₁ deformation. Moreover, the undeformed granitic plutons that intruded into the Wuliangshan metamorphic complex yield LA-ICP-MS zircon U-Pb ages of 246–245 Ma (Liu et al., 2017). The D₁ deformation should have occurred within a short time span between 250 and 246 Ma. Therefore, we consider that the D₁ event was a Mesozoic rather than Cenozoic strike-slip shearing with a possible décollement (Chen et al., 2022; Hou et al., 2013; Wang & Burchfiel, 1997).

## 6.3. The Tectonic Response in the Overriding Plate During Paleo-Tethys Closure

The Changning-Menglian Ocean is regarded as representing the relic of the main Paleo-Tethys Ocean, separating the Indochina block to the east and the Sibumasu block to the west (Liu et al., 1993; Metcalfe, 2013, 2021; Wang et al., 2018; Zhong, 1998). The closure of this part of the Paleo-Tethys was accommodated by the eastward subduction of the Sibumasu block beneath the Indochina block (Wang et al., 2018; Zhong, 1998). The subduction of the Paleo-Tethys Ocean triggered back-arc extension along the western margin of the Indochina block, leading to the formation of the Jinggu-Jinghong back-arc basin (Hennig et al., 2009; Sone & Metcalfe, 2008; Wang et al., 2018). The back-arc basin preserves a complete Penrose-type ophiolite sequence, including serpentinized peridotite, cumulate gabbros, pillow basalts, and plagiogranite (Zhai et al., 2019). Gabbro with an N-MORB geochemical signature yields a U-Pb zircon age of 313–293 Ma (Zhai et al., 2019; Zhu et al., 2021). The Late Carboniferous to Late Permian sedimentary environment gradually deepens in agreement with a back-arc basin setting (Feng et al., 2002; Jia, 1995). The Wuliangshan massif is located on the northern prolong of the Jinggu-Jinghong back-arc basin (Figure 1b). Our detrital zircon data show that the protoliths of the Wuliangshan metamorphic complex experienced rapid subsidence within a short period from 260 to 250 Ma (Figure 7). The Late Carboniferous to Permian arc-related volcanics provided the primary provenance for the protoliths of the Wuliangshan metamorphic complex. Quartz c-axis fabrics and Titanium-in-quartz geothermometry suggest that the deformation temperature of the Wuliangshan metamorphic complex ranged from 450 to 550°C (Figure 6). Assuming a standard geothermal gradient of 30°C/km, the sedimentation depth of the protoliths is estimated at approximately 15–18 km. This indicates that the Wuliangshan massif experienced rifting and rapid sedimentation during 260–250 Ma, reflecting the extensional tectonics of the back-arc basin.

In southwestern Yunnan, evidence for the Triassic collision between the Sibumasu and Indochina blocks includes the following. (a) Deng et al. (2018) identified in the composite Lincang Batholith three magmatic episodes corresponding to subduction (before ~252 Ma), syn-collisional (250–237 Ma), and post-collisional (235–203 Ma) stages. (b) Evidence of Triassic HP metamorphism has been identified in the Changning-Menglian orogenic belt (Fan et al., 2015; Wang, Liu, Schertl, et al., 2019; Wang, Liu, Li, et al., 2019). Metamorphic zircons from eclogites yield HP metamorphic ages at 246–245 Ma (Wang, Liu, Li, et al., 2019). $^{40}$Ar/$^{39}$Ar dating of phengite and glaucophane from blueschists yields prograde-to-peak metamorphic ages of 243–229 Ma (Fan et al., 2015; Wang, Liu, Schertl, et al., 2019). These ages record an Early to Middle Triassic subduction-collision event associated with the closure of the Paleo-Tethys Ocean. (c) Extensive crustal shortening, coeval with greenschist-to amphibolite-facies metamorphism, occurred along the Changning-Menglian orogenic belt in the Triassic, forming penetrative foliation and shortening structure (Wang, Wang, et al., 2022; Zhong, 1998). (d) In the Changning-Menglian suture zone, the Middle Triassic strata contain the radiolarite, whereas Late Triassic strata have changed to the continental sedimentation (Feng et al., 2002).
The Wuliangshan massif preserves a significant Early Triassic ductile deformation expressed by antiformal structures and NW-SE trending linear structures in geometry (Figure 3). From a regional tectonic perspective, several contractional deformations have been documented in the overriding plate (i.e., Indochina block), which are considered to be the tectonic response to the collision between the Sibumasu and Indochina blocks. The Triassic (S)E-directed thrusts and (S)E-verging asymmetric folds were recognized in the east of the Lincang batholith (Zhong, 1998). In the west margin of the Indochina block, deformation in the Lancang group is characterized by Top-to-the-(S)E ductile shearing and shortening structures such as folds and thrusts (Cui et al., 2023; Wang, Wang, et al., 2022). Mica $^{40}$Ar/$^{39}$Ar dating constrains the compressional deformation to 251 to 230 Ma (Cui et al., 2023; Wang, Wang, et al., 2022). These structures have been interpreted as the result of nearly E-W-directed shortening during the continental collision between the Sibumasu and Indochina blocks. Therefore, we argue that the D$_{1}$ top-to-the-SE shearing that occurred in the Wuliangshan massif at 250–246 Ma as a SE-directed back-thrusting developed in the Indochina block. During this stage, significant crustal shortening occurred within the Indochina block, resulting in top-to-the-(S)E ductile shearing and shortening structures at various scales.

6.4. Cenozoic Deformation and Its Tectonic Significance

The structural analysis allows us to separate two Cenozoic events in the Wuliangshan massif. In the east of the CLF, the D₂ deformation is mainly developed within the Permian to Cretaceous strata (Figure 9). The E-verging, N-S trending folds with W-dipping slaty cleavage (S₂) indicate that the D₂ deformation was formed under a near E-W shortening. The CLF extends from the Chongshan range to the eastern margin of the Lincang batholith (Figure 1b; Wang, Wang, et al., 2022). Based on macroscopic and microscopic observations, the CLF along the eastern margin of the Lincang batholith consistently records a sinistral ductile shearing (Wang, Wang, et al., 2022). Previous studies suggest that the sinistral transpression along the CLF was contemporaneous with E-W-directed contractional structures such as folds, cleavages, and thrusts (Zhang et al., 2009, 2010). Therefore, we interpret the geometry and kinematics of the D₂ deformation as a “positive flower structure” (e.g., Harding, 1985; Sylvester, 1988; Woodcock & Rickards, 2003) related to the Cenozoic transpression along the CLF (Figures 2a, 3, and 9). As the last tectonic event, the D₃ determined the final architecture of the Wuliangshan massif (Figure 3). The D₃ event corresponds to NW-SE trending folds and NE-dipping faults developed in the northeastern part of the study area (Figure 9). The SW-verging folding and thrusting indicate that the D₃ deformation occurred under roughly NE-SW shortening (Figures 3 and 9). Previous studies have interpreted this thin-skinned shortening as a positive flower structure related to the ARF induced by the sinistral transpression (Liu, Lin, et al., 2023; Wang, Lin, et al., 2022). We follow this interpretation and consider that D₃ deformation resulted from transpressional tectonics along the ARF.

It is difficult to determine the deformation timing of upper crustal level structures using geochronological measurements. Since the D₂ and D₃ events are associated with the positive flower structures formed by the transpression of the CLF and the ARF, respectively, we infer the deformation timing of these events based on the thermochronological data from the CLF and the ARF. According to previous thermochronological data from the CLF and the ARF (Akciz et al., 2008; Cao, Liu, et al., 2011; Cao, Liu, et al., 2011; Chen et al., 2015; Gilley et al., 2003; Li et al., 2023; Liu, Chen, et al., 2015, 2019; Schärer et al., 1990, 1994; Wang et al., 1998, 2006; Zhang et al., 2010), the sinistral shearing along the CLF and the ARF occurred in the Oligocene-Miocene. However, since the D₃ event overprints the D₂ event, this suggests that the formation of the positive flower structure associated with transpression along the CLF occurred earlier than that related to the ARF (Figures 3c and 3d). A compilation of the thermochronological data from the CLF and the ARF provides an opportunity to constrain the timing of transpressional tectonics (Figures 10, S8 in Supporting Information S1, and Table S4). The statistics on the amphibole, biotite, and muscovite $^{40}$Ar/$^{39}$Ar age from the ARF exhibit an age peak at 27, 24, and 23 Ma, respectively (Figure 10d). A synthesis of $^{40}$Ar/$^{39}$Ar age from the ARF suggests that the metamorphic rocks may have experienced a fast cooling event at 27–23 Ma. The zircon U-Pb ages of syn-shearing leucogranite veins along the ARF range from 30 to 22 Ma with a peak around 27 Ma (Figure 10a). We suggest that the rapid cooling event at 27–23 Ma reflects the exhumation of the metamorphic rocks and may represent the deformation timing of the positive flower structure due to the sinistral transpression along the ARF. This interpretation is consistent with field observations, where the Cretaceous and Paleogene strata were involved in the folding, while the Neogene fluviatile and lacustrine deposits unconformably overlie the pre-Neogene deformed strata (Figures 3b and 3d). However, the statistics on the muscovite and biotite $^{40}$Ar/$^{39}$Ar age from the CLF exhibit an age peak at 31 Ma
<figure><img src="image_13.png" /><figcaption>**Figure 10.** Kernel Density Estimate diagrams of zircon U-Pb ages and amphibole, muscovite, and biotite <sup>40</sup>Ar/<sup>39</sup>Ar ages are presented for the ARF, the CLF, and the GLF, respectively. N is defined as the number of samples. Zircon U-Pb ages and <sup>40</sup>Ar/<sup>39</sup>Ar ages of the ARF are compiled from (Cao, Liu, et al., 2011, Cao, Neubauer, et al., 2011; Chen et al., 2015, 2017, 2018; Guo et al., 2016; Harrison et al., 1996; He et al., 2019, 2020; Leloup et al., 1993, 2001; Li et al., 2009; Liu et al., 2013, 2015a, 2015b, 2019; Qi et al., 2014; Tang, Liu, et al., 2013; Wang et al., 1998, 2000). Zircon U-Pb ages and <sup>40</sup>Ar/<sup>39</sup>Ar ages of the CLF are compiled from (Akciz et al., 2008; Chen et al., 2022; Ji et al., 2015; Li et al., 2023; Tang, Yin, et al., 2013; Wang et al., 2006; Wang, Wang, et al., 2022; Zhang et al., 2010; Zhang, Zhang, Chang, et al., 2012). Zircon U-Pb ages and <sup>40</sup>Ar/<sup>39</sup>Ar ages of the GLF are compiled from (Bertrand et al., 1999, 2001; Dong et al., 2021; Eroğlu et al., 2013; Lin et al., 2009; Liu, Wang, et al., 2015; Song et al., 2010; Tang et al., 2016, 2020; Wang et al., 2006, 2013; Zhang, Zhang, Zhong, et al., 2012).</figcaption></figure>

and 27 Ma, respectively (Figure 10e). The statistical analysis of muscovite and biotite <sup>40</sup>Ar/<sup>39</sup>Ar age indicates that a rapid cooling event occurred from 31 Ma to 27 Ma. Syn-shearing leucogranite veins along the CLF yield U-Pb ages at 35–18 Ma with a peak at 32 Ma (Figure 10b). The more likely interpretation is that rapid cooling at 32–27 Ma reflects the exhumation of the metamorphic rocks resulting from the sinistral transpression along the CLF. Although mica <sup>40</sup>Ar/<sup>39</sup>Ar ages along CLF show prominent age peaks at 17 Ma, our field observations indicate that the sinistral transpression along the CLF occurred earlier than that of the ARF. Therefore, we tend to place the deformation time of the D<sub>2</sub> and D<sub>3</sub> events at 32–27 Ma and 27–23 Ma, respectively.
The continental collision between the Indian and Eurasian plates since the Eocene has produced significant crustal thickening and E- to SE-ward extrusion of the continental blocks from the Tibetan Plateau to the southeast (Jiao et al., 2023; Lacassin et al., 1996, 1997; Leloup et al., 1995; Royden et al., 2008; Tapponnier et al., 1982, 1990; Yin, 2010; Zhang, Zhang, Zhong, et al., 2012; Zhang, Zhang, Chang, et al., 2012). The lateral escape of the Sundaland was considered to be accommodated by the sinistral ARF in the east, and the dextral GLF in the west (e.g., Tapponnier et al., 1982 and references therein). Tapponnier et al. (1982) proposed the collision-extrusion model as a mechanism to explain the development of large-scale strike-slip fault systems in Southeast Asia. The model emphasizes that the extruded Sundaland behaves as one single rigid block, with deformation localized along major lithosphere-scale strike-slip faults and little internal deformation within the block. Some researchers argue the existence of a thin, viscous lower crust that was decoupled from the upper-middle crust and the mantle (Clark & Royden, 2000; Royden et al., 1997, 2008). A thin, viscous lower crust was flowing out of the Tibetan Plateau induced by gravitational loading, resulting in crustal deformation and outward expansion of the Tibetan Plateau (Clark & Royden, 2000; Royden et al., 1997, 2008). Based on this hypothesis, it is proposed that strike-slip faults are confined to the upper-middle crust above the lower crustal domain where horizontal shear zones predominate (Chen et al., 2022; Jolivet et al., 2001; Wang & Burchfiel, 1997; Zhang, Zhang, Zhong, et al., 2012). However, large-scale N-S to NW-SE trending folds and thrusts have developed within the Lanping-Simao basin (Wang & Burchfiel, 1997; Zhang et al., 2010). The presence of large-scale contractional structures within the interior of the block is incompatible with the predictions of both the collision-extrusion model and the lower crustal flow model. To address this, Wang and Burchfiel (1997) proposed a block rotation model to explain the large-scale contractional structures within Sundaland. The Sundaland did not extrude as a single block but can be subdivided into several crustal fragments bounded by strike-slip faults, with each crustal fragment undergoing differential rotation and internal deformation (Wang & Burchfiel, 1997). In this model, the contractional structures within the block are predicted to exhibit a consistent structural trend. However, our field observations and laboratory analyses identify two distinct phases of Cenozoic positive flower structures related to the ARF and CLF within the Wuliangshan massif. The earlier N-S trending contractional structures ($D_2$) are overprinted by later NW-SE trending ones ($D_3$), indicating a shift in the tectonic regime.

The CLF divides the Sundaland into two parts, namely, the western and eastern blocks (Figure 1a; Akciz et al., 2008; Wang et al., 2006; Wang, Wang, et al., 2022; Zhang, Zhang, Chang, et al., 2012). Previous studies have suggested that the CLF, together with the ARF and the GLF, played an important role in accommodating the southeastward extrusion of the Sundaland (Akciz et al., 2008; Chen et al., 2022; Wang et al., 2006; Wang, Wang, et al., 2022; Zhang et al., 2010). Some workers argue that the faster extrusion of the west Sundaland relative to the eastern one may be responsible for the sinistral shearing along the CLF (Akciz et al., 2008; Wang et al., 2006). In the Wuliangshan massif, our structural analysis reveals Cenozoic $D_2$ and $D_3$ deformations. The early positive flower structure associated with the CLF was reworked by the transpression of the ARF, indicating that the CLF accommodated the block extrusion earlier than the ARF. Thermochronological data from previous studies indicate that dextral shearing along the GLF occurred between ca. 33 and 12 Ma (Figure 10 and Table S4). Large-scale extrusion of blocks is commonly accompanied by intense strike-slip shearing and intracontinental deformation (Ratschbacher et al., 1991; Wang & Burchfiel, 1997). Based on our understanding of regional tectonic evolution, these two events could be interpreted as a two-stage extrusion process of the Sundaland related to the CLF and the ARF (Figure 11). The $D_2$-related CLF, as an earlier event, was likely the eastern extrusion boundary of the west Sundaland. The CLF, coupled with the GLF, accommodated the extrusion of the west Sundaland at 32–27 Ma (Figure 11a). During this stage, a series of N-S trending positive flower structures, such as E-directed thrusts and E-verging folds, formed as a result of sinistral transpression along the CLF. Subsequently, the Sundaland as a whole block (Sibumasu and Indochina blocks) was extruded to the southeast by the $D_3$-related ARF and the GLF at 27–23 Ma (Figure 11b). The development of NW-SE trending contractional structures, including SW-directed thrusts and SW-verging folds, is attributed to sinistral transpression along the ARF. This model is at variance with the single extrusion model (Tapponnier et al., 1982). The progressive extrusion of the west and east Sundaland may account for the large-scale contractional deformation observed within the Indochina block during the Cenozoic.

## 6.5. Tectonic Implications for the Indochina Block

Our structural analysis enables us to recognize the bulk architecture and polyphase deformation of the Wuliangshan massif. Combined with the available geological data, these new results allow us to discuss the
<figure><img src="image_14.png" /><figcaption>**Figure 11.** Tectonic reconstructions illustrating the two-stage extrusion history of Sundaland correspond to the continental collision between the Indian and Eurasian plates (adapted from Royden et al., 2008). WCF: Wang Chao Fault.</figcaption></figure>

tectonic evolution of the Indochina block during the Mesozoic Sibumasu-Indochina collision and the Cenozoic India-Eurasia collision (Figure 12).

The subduction of the Changning-Menglian Ocean was initiated in the Late Carboniferous and led to the development of the Yunxian-Menghai arc documented in the western margin of the Indochina block (Deng et al., 2014, 2018). The mafic-ultramafic intrusions exposed in Banpo, Nanlinshan, and Damenglong areas yield zircon U-Pb ages at 312–285 Ma, 298–282 Ma, and 273–252 Ma, respectively (Hennig et al., 2009; Jian et al., 2009; Li et al., 2012; Wang et al., 2021; Zhai et al., 2019). These intrusions exhibit positive $\epsilon_{Hf}(t)$ and $\epsilon_{Nd}(t)$, and whole rock major and trace elements correspond to the geochemical signature of an island arc magmatism (Jian et al., 2009; Li et al., 2012; Wang et al., 2021). In the Lincang batholith, granodiorite pluton roof pendants yield zircon U-Pb ages of 261–260 Ma (Deng et al., 2018; Liu, Yang, et al., 2023). They are represented by high-K calc-alkaline series and I-type granite and fall into the arc or syn-collisional fields (Deng et al., 2018; Liu, Yang, et al., 2023). Meanwhile, a back-arc basin (Jinggu-Jinghong back-arc basin) developed to the east of the Yunxian-Menghai arc during the Early Permian (Figure 12a; Hennig et al., 2009; Sone & Metcalfe, 2008; Wang et al., 2018). The Wuliangshan massif, located in the northern part of the Jinggu-Jinghong back-arc basin, underwent back-arc extension during the Late Permian. At this stage, the protoliths of the Wuliangshan
<figure><img src="image_15.png" /><figcaption>**Figure 12.** Schematic reconstruction of the geodynamic evolution of the Indochina block from the Late Carboniferous to the Oligocene.</figcaption></figure>
metamorphic complex were deposited as arc-related volcanic rocks and large amounts of debris were supplied from the Indochina block (Figure 12b).

During the Early Triassic (250–246 Ma), the progressive closure of the Changning-Menglian Ocean resulted in the collision between the Sibumasu and the Indochina blocks, causing the top-to-the-SE shearing recorded in the Indochina block (Figure 12c). This event caused a widespread absence of Early Triassic sediments in the study area. The Cenozoic CLF and the ARF sinistral strike-slip faults accommodate the extrusion of Sundaland during the India-Eurasia collision (Tapponnier et al., 1990; Zhang et al., 2010). The D₂ deformation is represented by the well-developed E-verging folds with penetrative S₂ cleavage related to the transpression of the CLF at 32–27 Ma in the Wuliangshan massif (Figure 12d). The D₃ deformation is characterized by SW-directed folds and thrusts related to the transpression of the ARF at 27–23 Ma (Figure 12e).

## 7. Conclusion

In the southeast Tibetan Plateau, the Wuliangshan massif exhibits the overriding plate in response to both the Mesozoic Sibumasu-Indochina collision and the Cenozoic India-Eurasia collision. The Wuliangshan massif was involved in NW-SE contractional tectonics between 250 and 246 Ma as a response to the overriding plate, providing constraints on the timing and process of the collision between the Sibumasu and Indochina blocks. This massif also records two phases of positive flower structures related to Cenozoic large-scale strike-slip faulting. Such transpressional tectonics accommodate the two-stage extrusion of the Sundaland during the Cenozoic collision between the Indian and Eurasian plates. The structural analysis of the Wuliangshan massif provides a case study for deformation in the overriding plate, thereby enhancing our understanding of the tectonic evolution and geodynamics of continental convergence. In this study, we use the two-stage extrusion model to explain the extrusion process of Sundaland. However, it should be noted that this represents a preliminary analysis based on the structural analysis of transpressional tectonics. In the future, more issues such as the southern prolongation of the CLF, the mechanism, and the geodynamic process need to be considered.

## Data Availability Statement

All data are available at the following link https://doi.org/10.6084/m9.figshare.24872034.v5 (Wu & Lin, 2025).

## Acknowledgments

We acknowledge Editor Prof. Paul D. Asimow, Associate Editor, Prof. Chen WU, and an anonymous reviewer for their constructive comments that helped to improve this manuscript. This study was funded by the National Natural Science Foundation of China (U2344213), Deep Earth Probe and Mineral Resources Exploration-National Science and Technology Major Project (2024ZD1001103), State Key Laboratory of Lithospheric and Environmental Coevolution, Institute of Geology and Geophysics, Chinese Academy of Sciences (SKL-Z202205).

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