Ailao Shan-Red River Shear Zone Research Articles

Thermal and Physical Properties of Barrovian Metamorphic Sequence Rocks in the Ailao Shan‐Red River Shear Zone, and Implications for Crustal Channel Flow

AbstractThe collisional history between Greater India and the Eurasian plate has been well constrained by the study of exhumed Barrovian metamorphic sequence (BMS) rocks in the Himalayan Range. However, in the southeastern Tibetan Plateau, the collisional records have been obscured by intense, regional‐scale strike‐slip overprinting and recrystallization. Here, in BMS rocks from the Ailao Shan–Red River shear zone (ARSZ), we report the first discovery of a >250 km long, high‐pressure (high‐P) granulite belt (>1.0 GPa), identified by the presence of relict kyanite and associated decompression reaction textures. Petrological phase equilibrium modeling showed that exposed micaschists in the region represent exhumed middle crust (20–25 km, 600–670°C), while the high‐P granulite rocks are remnants of thickened lower crust (45–55 km, 800–850°C). This indicates that the northeast edge of the ARSZ experienced an additional ∼25 km of uplift and exhumation compared to the southwest side, facilitated by brittle thrusting/imbrication along the Ailao Shan fault (micaschists) and ductile extrusion along the Red River fault (granulite). Geochronological study shows that the upper portion of the BMS preserves older metamorphic ages (52–34 Ma) than the lower portion (32–29 Ma), which was attributed to spatial variation in cooling rates. Using calculated P–T–t–d paths, we also examined variation in density and seismic wave speeds for BMS in the ARSZ. Our data correlate with fieldwork conducted elsewhere within the Himalayan Range indicating that the kyanite to sillimanite transition zone may serve as a “cap” for the horizontal migration of melt within the lower crust.

Petrogenesis and Tectonic Implications of the Oligocene Dalongtan Shoshonitic Syenite Porphyry in Central Yunnan, Southeastern Tibetan Plateau: Constraints from Geochronology, Geochemistry and Sr-Nd-Hf Isotopes

Shoshonitic rocks are widely distributed in post-collisional settings and provide key information on deep geodynamic mechanisms and magmatic evolution. In this paper, we present petrographic, zircon U-Pb age-related, trace elemental, Hf isotopic, bulk-rock elemental, and Sr-Nd isotopic data of the Dalongtan shoshonitic syenite porphyries (DSSPs) in central Yunnan, southeastern Tibet. The DSSPs formed at 33.2 ± 0.3 Ma in a post-collisional setting. They define linear trends on Harker diagrams, and they display similar trace element patterns and enriched bulk-rock Sr-Nd isotopes [(87Sr/86Sr)i = 0.70964–0.70968, εNd(t) = −12.9 to −12.7] and zircon Hf isotopes (εHf(t) = −15.7 to −13.1) to the coeval mantle-derived potassic mafic rocks. This suggests that the DSSPs were fractionated from the lithospheric mantle-derived mafic magmas. The DSSPs, along with the coeval felsic and mafic magmatic rocks (37.2–32.3 Ma), exhibit a planar distribution on the SE Tibet and predate the left-lateral shearing of the Ailaoshan–Red River shear zone (ARSZ) (32–22 Ma), suggesting that there are no genetic relationships between them. The DSSPs have geochemical characteristics similar to those of A-type granites, with high total alkalinity (10.39–11.17 wt.%), HFSE concentrations (Zr + Nb + Ce + Y = 890.2–1054.3 ppm), Ga/Al ratios (10,000 × Ga/Al = 2.95–3.46), whole-rock zircon saturation temperatures (906–947 °C), and oxygen fugacity (ΔFMQ = +3.30–+4.65), indicating that they are products of the high-temperature melting of the lithosphere as a result of asthenosphere upwelling in extensional settings. Based on our data and regional observations, it is proposed that the generation of the DSSPs may be linked to the convective thinning of the thickened lithospheric mantle following the India–Asia collision.

Detailed crustal deformation in the Ailaoshan Orogenic Belt revealed by receiver functions from a dense array

The Ailaoshan Orogenic Belt (AOB), located at the southeastern boundary of the Tibetan Plateau, is an ideal place for investigating the mechanisms of lateral growth of Tibet. Using the data recorded by a dense seismic array across the Ailaoshan belt, we investigate the detailed lateral variations of crustal anisotropy on the basis of Pms phase of receiver functions. Remarkable crustal anisotropy is observed throughout this study region with a mean delay time of 0.33 ± 0.19 s, indicating the anisotropy primarily originates in the middle-lower crust. The fast directions beneath the AOB including the Ailaoshan-Red River shear zone (ARRSZ) and its western low-grade metamorphic unit generally align with the NW-SE strike of ARRSZ. The weak anisotropy in the South China Block (SCB) argues that the block is relatively stable, with limited internal deformation. Meanwhile, the anisotropy beneath the western boundary of the SCB is strong, and the N-S oriented fast direction is influenced by both the crustal stress and Xiaojiang Fault. Combining the high Vp/Vs and significant lateral variations of crustal anisotropy parameters, we suggest that the strike-slip motion along the ARRSZ induces the partial melting and pronounced anisotropy in the middle-lower crust of AOB, without the presence of crustal flow. The differences between crustal and mantle anisotropy indicate crust-mantle decoupling deformation of the AOB, supporting the block extrusion model occurring only in the crustal scale as the primary deformation pattern.

Tectonic evolution of the Ailaoshan–Red River shear zone and its relationship with the growth of the southeastern Tibetan Plateau

The Ailaoshan–Red River shear zone (ARSZ), which is ∼ 1000 km long and one of the largest boundary strike-slip fault zones in the southeastern margin of the Tibetan Plateau, provides valuable insights into the growth process of the southeastern Tibetan Plateau since the India-Asia collision. During the Late Miocene to Early Pliocene, the ARSZ underwent a kinematic transition from left-lateral strike-slip to right-lateral strike-slip motion. To reveal the mechanism of this kinematic transition and its relationship with the growth process of the southeastern Tibetan Plateau during the Cenozoic, we conducted paleomagnetic and tectonic deformation analyses on both sides of the ARSZ. The results show that the formation of the S-shaped bending and kinematic transition of the ARSZ were controlled by the changes in clockwise rotation rates of the Chuandian Fragment and the Indochina Block. This suggests that since the Oligocene, the growth of the southeastern Tibetan Plateau progressively extended outward from the southern margin of the Lhasa Terrane to the southeastern margin of the Tibetan Plateau, resulting in the sequential initiation of the clockwise rotation of the Shan-Thai Block, Indochina Block, and Chuandian Fragment since the Late Oligocene, and in variations in the rotational rates of these blocks. The different rotational movement of these blocks controlled the tectonic evolution of strike-slip fault systems on the southeastern margin of the Tibetan Plateau.

Eocene–Miocene tectonic reworking of the southeastern Tibetan Plateau: Geochronological and geochemical records from granitoids along the Ailao Shan–Red River Shear Zone

The convergence of the Indian Plate and Eurasia shaped the Earth’s broadest and highest-elevation collisional system and topographic growth, which is not simply illustrated by ongoing contractional deformation. The Cenozoic magmatism along the Ailao Shan-Red River Shear Zone (ARSZ), linking surface geology with deep lithospheric processes, provides additional information regarding the age and progressive deformation of the India-Asia collision. In this study, we present new zircon U–Pb geochronological, elemental geochemical and zircon Hf isotopic data for the Eocene–Miocene granitoid to resolve the southeastern Tibetan Plateau deformation history. The Jinping granitoids in the southern ARSZ were dated at 37.5–33.6 Ma. They show similar geochemical characteristics (whole-rock major oxides, trace element, Sr–Nd–Pb isotopic and zircon in-situ Hf isotopic data) with respect to coeval potassic granitoids in NW Yunnan, suggesting their origination from partial melting of thickened lower crust with additional contribution from enriched mantle-derived magmas. Leucogranite samples within the ARSZ yielded distinctive zircon U–Pb ages of 28.3 Ma and 18.5 Ma. Geochemical features suggest that they were derived from a heterogeneous crustal source consisting of metasediments and metabasites. Our results, along with those previous studies, suggest that the widespread potassic magmatism in the central and southeastern Tibetan Plateau is related to lithospheric mantle upwelling, which resulted in the partial melting of the thickened lower crust and a short-lived crustal extension during a period of decreased convergence rate. Subsequently, the orogenic-scale sinistral strike-slip movement provided shearing heat for generating leucogranites within the shear zones during the late Oligocene–Miocene. The plateau-wide shift in stress reorganization and tectonic reworking led to the lateral extrusion of the SE Tibetan Plateau and its attainment of modern high elevation.

Origin and tectonic environment of the Longbohe Fe-Cu-(LREE) deposit in the Ailao Shan–Red River shear zone, Southwest China: a Neoproterozoic subduction-related IOCG deposit?

Origin and tectonic environment of the Longbohe Fe-Cu-(LREE) deposit in the Ailao Shan–Red River shear zone, Southwest China: a Neoproterozoic subduction-related IOCG deposit?

Magmatic-hydrothermal evolution and tectonic implications of granitic pegmatites from the Ailao Shan-Red River shear zone in Southeast Asia

Magmatic-hydrothermal evolution and tectonic implications of granitic pegmatites from the Ailao Shan-Red River shear zone in Southeast Asia

Development of Amphibole Crystal Preferred Orientations (CPOs) and Their Effects on Seismic Anisotropy in Deformed Amphibolites

AbstractCrystal preferred orientations and seismic anisotropy of amphibole play a crucial role in fingerprinting the rheological and physical properties of the deep crust. This study presents typical naturally deformed (banded, mylonitic, and ultramylonitic) amphibolites from the Ailao Shan‐Red River shear zone (ASRR‐SZ), southwestern China. In the banded amphibolite, coarse amphibole grains are highly elongated by dislocation creep and develop type I fabric of amphibole. In the mylonitic amphibolite, amphibole grains are characterized by porphyroclasts with core‐mantle structure and fine‐grained amphiboles in the matrix. Subgrain rotation recrystallization dominantly contribute to the grain size reduction process and display type III fabric of amphibole. In the ultramylonitic amphibolite, the fine‐grained amphiboles are mainly deformed by mechanical rotation of prismatic grains and develop type IV amphibole fabric. The seismic anisotropies are calculated from crystal preferred orientation (CPO) of naturally deformed amphiboles and model fabric. They both display that the transition of amphibole fabric from type III through type I to type IV also comes along with the transition of AVp patterns from Vp(X) ∼ Vp(Y) > Vp(Z) through Vp(X) > Vp(Y) > Vp(Z) to Vp(X) > Vp(Y) ∼ Vp(Z). Under the same texture strength, the strongest seismic anisotropy (both AVp and Max. AVs) occurs in the type III fabric of amphibole, the weakest seismic anisotropy is related to type IV fabric and a medium seismic anisotropy in type I fabric. This work indicates that the CPO types of amphiboles need be carefully considered in interpreting observed seismic anisotropy at the deep crustal level.

Depositional environments of the Miocene sediments in northern Song Hong basin

Northern Song Hong Tertiary Sedimentary basin is a classic case study of a pull-apart basin in southeast Asia, whose formation was controlled by the India-Eurasia collision, sinistral and dextral strike-slip motion of the Ailao Shan-Red River Shear Zone and Opening of the East Vietnam Sea during the Cenozoic. Unlike the central and southern parts of the basin, the northern Song Hong Basin experienced a very strong inversion during the Late Miocene. This rapid uplift of the region has led to significantly change in lithofacies and sedimentary environments, which are now still poorly understood. This uncertainty is considered one of the main challingings in prediction of the non-structural traps in the region. The recent results derived from well logging and 2D/3D seismic interpretation allowed us to define the Miocene formation in northern Song Hong basin, which are subdivided into three substrata, namely: the Lower, Middle and Upper stratum, which are characterized by typical characteristics of lithology and depositional environments. The Lower Miocene formation is dominated by deltaic environment at the bottom, transitioning to the overlying shelf environment. Lithology of the section varies from coarse-grained sediment (sandstone) to fine grained material such as shale and mudstone upward; The Middle Miocene stratum demonstrate sandier, coalic materials of the delta plain and delta front environments intercalated with swampy shale. In contrast, the Upper Miocene section is characterized by more fluvial and nearshore elements. It is illustrated by presence of the channel-filled sand bodies and mouth/longshore sand bars. These sand bodies demonstrate good porosity and horizontal permeability, which are considered to be good potential reservoir for both structural and non-structural traps in the Miocene formation.

Meso-cenozoic tectonic evolution of the ziyun-luodian fault in SW China

Regional-scale fault systems are important in adjusting intracontinental deformation. Closure of the Paleo-Tethys, Paleo-Pacific subduction, and India-continent-continent collision have all interacted to shape the Meso-Cenozoic tectonics of South China. The Ziyun-Luodian fault (ZLF) straddles across the Guizhou region in southwestern China. New petrographic and structural studies were conducted to reveal the structural evolution of the ZLF and regional deformation. At least three distinct deformation events are identified within the fault zone: During the Indosinian orogeny, dextral thrusting occurred along the ZLF, accompanied by moderate regional uplift in Guizhou, as evidenced by the Upper Triassic-Lower Jurassic disconformity. The main tectonic framework across Guizhou was established during the Yanshanian orogeny. Driven by the west-dipping Paleo-Pacific subduction (Middle Jurassic-Early Cretaceous), sinistral transpression occurred along the ZLF and NE-to N-S-trending fold belts in central-eastern Guizhou, and the NW-striking folds west of the ZLF were developed in under nearly E-W-directed compression. In the late Cenozoic, the ZLF may have undergone sinistral strike-slip movement, similar to the sinistral Ailaoshan-Red River shear zone. The Ziyun-Luodian fault plays an important role in accommodating the Meso-Cenozoic regional deformation in SW South China.

A shift of mantle sources for the post-collisional lavas and tectonic links with synchronous deformation in the SE Tibetan Plateau

Strong post-collisional deformation and contemporaneous magmatic flare-ups during the Cenozoic indicate large-scale continental lithospheric modification in the southeastern (SE) Tibetan Plateau. However, the relationship between regional deformation and magmatism is poorly understood. In this paper, we report for the first time the presence of 32–17 Ma mafic rocks within the Ailaoshan-Red River shear zone (ARSZ) and provide new insights into the correlation between deformation and magmatism. Using 369 Cenozoic mafic and felsic rocks (this study and literature data) in and around the ARSZ, we document a continuous magmatic record that delineates a consecutive depletion of their mantle sources through time, which is correlated with the development of regional deformation. The spatial distributions, rock associations, and geochemical features of these rocks indicate a three-stage magmatic evolution in the SE Tibetan Plateau. During the earliest episode (40–32 Ma), the strong emplacement of both mafic and felsic rocks shows a planar-shaped geographic distribution. The second episode (32–17 Ma) is characteristic of voluminous mylonitized granites and minor basaltic rocks generally along the ARSZ. The latest episode (<17 Ma) magmatic rocks are sporadically exposed with dominant OIB-type basaltic rocks and minor A-type granites. From the earliest to the latest episodes, the mafic rocks exhibit progressive depletion in SrNd isotopic compositions, with initial Sr isotopic ratios and εNd(t) values from 0.70509 to 0.71151 and − 12.5 ~ −0.3 to 0.70313–0.70734 and − 3.9 ~ +8.1, respectively. These data, together with the covariations in the (Nb/La)PM and Zr/Nb ratios, signify a dramatic mantle-source shift from enriched to depleted through time for these Cenozoic mafic rocks. Mafic rocks from the earliest episode were generated from an enriched lithospheric mantle through convective thinning. In contrast, the OIB-type basalts from the latest episode were pure asthenospheric magma from decompression melting. Therefore, we propose an interdependent evolution between the shift of mantle sources and the development of synchronous deformation in and around the ARSZ. Our discovery of 32–17 Ma mafic rocks stands as the first direct evidence for the key role of the synchronous development of the lithospheric-scale ARSZ in the shift of mantle sources, asthenospheric melt transportation, and heating-induced crustal magma generation.

Crustal and Uppermost Mantle Heterogeneities Across the Ailaoshan Red River Shear Zone, SE Tibet: Implications for Cenozoic Magmatic Activity

AbstractThe Ailaoshan Red River shear zone (ARSZ) was formed in the Mesozoic as a suture zone between the Indochina block and the Yangtze craton. Since the Cenozoic, block extrusion due to the Indo‐Asian collision has reactivated the fault zone and caused large‐scale shearing. Affected by the Cenozoic orogeny, a large volume of magmatic and metamorphic rocks developed in the ARSZ, forming many orogenic gold deposits. However, the source and the geodynamic process of these magmatic activities are still unclear. To gain a basic understanding of the subsurface magmatic activity, we deployed a dense array of 24 broadband seismic stations across the Daping and Chang'an gold deposits at the southern end of the ARSZ. Receiver function analysis, common conversion point stacking, and a joint inversion of receiver functions and surface wave dispersions are performed to image the detailed structure of the crust and uppermost mantle. Low‐velocity zones in the mid‐lower crust and thinned lithosphere (∼70 km) are imaged under the ARSZ. The observed subsurface structures are verified by 3D numerical modeling with the SEM‐FK method. We speculate that the mantle upwelling caused by lithospheric delamination has provided the main source of the mantle component in the magmatic rocks since ∼35 Ma; afterward, high temperatures produced partial melting in the lower crust, which was emplaced along active shear zones.

Chronological and Geochemical Study of the Cenozoic Potassic Felsic Igneous Rocks in Western Yunnan, SE Tibet: Implications for their Tectonic Mechanisms

AbstractWhether there is a relationship between widespread Cenozoic igneous rocks and the activated Ailao Shan–Red River shear zone (ASRR) in western Yunnan, SE Tibet, remains highly controversial. New geochronological, and geochemical data derived from Cenozoic igneous rocks both within and far from the ASRR offers answers. Although these coeval felsic rocks (36.9–33.5 Ma) are enriched in large‐ion lithophile elements relative to high field strength elements, they also display some special geochemical features, such as high Y and Yb values of syenitic porphyries, which contrast with the high Sr and low Y contents of monzoporphyries and granitic porphyries. The syenitic porphyries exhibit geochemical characteristics similar to those of coeval mafic magmas, e.g., high K2O, Y, and Yb contents, indicating that the former were probably generated by fractional crystallization of the latter. However, the high Sr/Y (56.1–109) monzoporphyries and granitic porphyries, combined with their low MgO, and Cr (or Ni) contents, suggest that they were mainly derived from partial melting of a thickened mafic lower crust. Given that these Cenozoic magmatic events exhibiting planar distributions occurred outside of the activation of the ASRR (31–20 Ma), then they were not controlled by the latter. Instead, they were tied to convective removal of a thickened lower lithospheric mantle in western Yunnan during the Paleogene.

Crustal anisotropy and deformation of the southeastern Tibetan Plateau revealed by seismic anisotropy of mylonitic amphibolites

The composition and deformation patterns in the deep crust of southeastern Tibet are remain controversial. In this study, we present microstructures, fabric and seismic properties of amphibolites from the Ailao Shan-Red River shear zone, western Yunnan, China. The amphibolites display a distinct mylonitic foliation, and the amphibole grains show low intracrystalline deformation, slight compositional zoning but strong crystallographic preferred orientations (CPOs). We explain these CPOs could have formed by rigid body rotation during deformation accommodated by diffusion creep and grain boundary sliding. The amphibolites show strong seismic anisotropies (AVp = 4.6–9.3% and Max. AVs = 3.2–9.0%). Rock recipe modelling results indicate the seismic anisotropies of the mylonitic amphibolites are dominated by the content of biotite and amphibole. The delay time (0.28–1.01 s, average of 0.58 s) calculated for a 40 km-thick amphibolite layer is consistent with the observed delay time (0.52 s) of Pms wave splitting in this region, which indicates a thick amphibolitic layer in the crust of the southeastern Tibetan Plateau. The observed Pms wave splitting, extensive fold deformation and low velocity zones (LVZs) in the Diancang Shan (DCS) massif indicate two anisotropic layers in the crust. The upper layer related to frozen fabrics of amphibolites, which deformed during compressional folding and strike-slip shear. The lower layer related to the alignment of amphibole and melt pocket induced by present-day channel flow. The observed Pms wave splitting in the Ailao Shan (ALS) massif is attributed to amphibolites with vertical foliation and horizontal lineation which are generated by compressional folding and strike-slip shearing.

Rapid Exhumation Processes of the Gaoligong Mountain Range in the Southeastern Margin of the Qinghai–Tibet Plateau Since the Late Cenozoic

Three continent-scale ductile shear zones trending N-S are distributed in the southeast margin of the Qinghai–Tibet Plateau. Their tectonic deformation and exhumation histories are of great significance to understanding the orogenic processes within the continent and the growth as well as expansion mechanism of the plateau. The Gaoligong shear zone (GLGSZ) is the westernmost zone near the boundary of the Indian subduction plate and is less well studied than the Ailaoshan–Red River shear zone (ASRRSZ) in the east. In this study, low-temperature thermochronological methods including apatite (U-Th)/He (AHe), zircon (U-Th)/He (ZHe), and apatite fission track (AFT) were used to date the vertical profile samples from the Gaoligong Mountain to understand the exhumation processes that occurred in the late Cenozoic. Our results show that the GLGSZ has experienced two stages of rapid exhumation events since the late Cenozoic: in the middle Miocene (∼14.5 Ma) and early Pleistocene (∼2.9 Ma). Based on our data, we divided the late Cenozoic tectonic deformation and exhumation processes in the Gaoligong Mountain area into two stages: 1) From the middle to the late Miocene, large-scale regional dextral strike-slip movements and lateral compressions controlled the ductile shear zone and continuously denuded the mountain surface; 2) in the Pleistocene, rapid river erosion and undercutting caused by fluctuations of the monsoon system, together with the continuous activity of brittle faults, drove the latest rapid exhumations. A comparison of the thermochronological data of the different areas along the Gaoligong Mountain shows that the exhumation rate in its northern transect is significantly higher and the time of the onset of exhumation is earlier than that in the southern transect. These results indicate that the deformation processes began in the north and continued southwards and controlled the geomorphological characteristics of the Gaoligong Mountain, whose elevation is higher in the northern part than in the southern part.

Crush-origin pseudotachylyte in granitic mylonites of continental exhumed Ailaoshan-Red River shear zone southeastern Asia

Tectonic pseudotachylytes have provided valuable insights into the evolution of seismic activity during crustal deformation. However, the origin and formation processes of pseudotachylytes are still debated. Thin pseudotachylyte veins were first recognized within the granitic mylonites of the continental exhumed Ailaoshan-Red River strike-slip shear zone, southeastern Asia, which record the deformation characteristics from ductile (mylonite) to brittle deformation (ultracataclasite). Mylonitization resulted in the recrystallization of quartz aggregates/ribbons, fine-grained plagioclase, and the concentration of biotite into biotite-rich foliation planes. The pseudotachylyte veins have various angles oblique to the host rock foliation, which record superimposition of the latter brittle deformation onto former ductile fabrics. Observations of the microstructure and quartz clast size distribution (obey power-law distribution) suggest that crushing was the dominant process in the formation of pseudotachylyte in the samples. Plagioclase within the pseudotachylyte vein presents a complex texture of high-strength nanocrystalline to partly amorphous material. Detailed microstructure observation revealed that amorphous materials form by a solid-state amorphization process and is different from friction melts. The changes in composition between plagioclase clasts and amorphous materials (enriched in Ca, but depleted in Na) provide evidence of mechanochemical effect that accompanies fault motion and comminution process. When a sufficient amount of amorphous material is formed, it has great significance in fault weakening through the coseismic slip of the strain partition.

Tectonic and Magmatic-Hydrothermal Evolution Of Granitic Pegmatites from the Ailao Shan-Red River Shear Zone in Southeast Asia

Tectonic and Magmatic-Hydrothermal Evolution Of Granitic Pegmatites from the Ailao Shan-Red River Shear Zone in Southeast Asia

Cenozoic rifting and inversion of Beibuwan Basin and its linkage with the strike-slip movement along the Ailao Shan-Red River Shear Zone

ABSTRACT The Cenozoic Beibuwan Basin is located along the eastern margin of the Ailao Shan Red River Shear Zone (ASRRSZ). The rifting and inversion processes of this basin provide important clues to the mechanism by which extrusion tectonics drive continental rifting and inversion development. However, the rifting and inversion of this basin remain ambiguous. Here, we employ seismic data, information about the time and displacement of faulting, syn-rift fault and isopach maps to analyse the Cenozoic rifting and inversion of Beibuwan Basin. Our results demonstrate that three stages of rifting and inversion occurred in this basin: (1) During the Palaeocene initial rifting, a series of NE-striking isolated half-grabens formed, which indicated that NW-SE extension dominated in this basin, and the southward slab-pull of Proto-South China Sea (SCS) was the likely cause of the extension. (2) During the Eocene rifting stage, besides significant motion on NE-striking faults, a small number of E-striking faults started to become active, which indicates that the extensional stress field shifted from NW-SE to N-S. (3) During the Oligocene rifting stage, the E-striking faults intensified, and controlled the distribution of the sediment depocentres, which implies that N-S extension dominated the deformation during this stage. The left-lateral strike-slip movement of the ASRRSZ and the slab-pull of the Proto-SCS are suggested to explain the extension stress field shift from NW-SE to N-S during the Eocene to Oligocene. (4) End-Oligocene, end-Miocene and end-Pliocene inversions have been recognized in this basin. The regional end-Oligocene inversion was stronger in the western area than the eastern area of the basin, which suggests that the compression caused by the left-lateral shearing of the ASRRSZ gradually decreased towards the east. In addition, the localized end-Miocene and end-Pliocene inversions occurred in the Haizhong sag.

Deformation and Sedimentary Responses to Top‐to‐North Shear Along the Range Front of the Big Bend of the Ailao Shan–Red River Shear Zone, SE Edge of the Tibetan Plateau, and Its Tectonic Implications

AbstractUnderstanding the mountain–basin coupling relationship is fundamental to placing constraints on the tectonic evolution of the Ailao Shan mylonite shear zone, the key feature accommodating relative movement between the Tibetan Plateau and SE Asia, because a contemporary basin, namely Red River, bounds its middle segment on the northeast, along which the shear zone is bent from northwest–southeast to roughly east–west (The Big Bend). The basin comprises two units: the Mubang Breccia and the Lengdun Conglomerate of Early Oligocene and Late Oligocene–Early Miocene age, respectively. This study reveals evidence indicating that the Wubang Breccia marks a high‐strain zone, resulting from top‐to‐north shear (range‐front detachment (RFD), along which the mylonite on the footwall experienced northward bending in the form of creeping. Moreover, the Lengdun Conglomerate on the hanging wall was deposited as growth strata, overlying a thrust belt to the north. The latter marks the southern rim of the Yangtze block, composed of extensional fault blocks, whose northward displacement along the toe of the RFD was synchronized with the north–south extension across the Red River basin. The spatial and temporal relationships between the Red River basin and the Ailao Shan shear zone indicate that basin formation was controlled by the change in geometry of the shear zone. The Red River basin can be viewed as an extensional step‐over in the left‐lateral strike‐slip field, in which all sedimentary and deformation processes were controlled by the extension and subsequent topographic spreading, accommodated by the RFD. This indicates that the extensional crustal materials, including both Langdun Conglomerate and underlying extensional fault blocks, were all shed from the top of the Ailao Shan mylonite belt. The cause of bending of the shear zone is attributed to the northward movement of India with respect to South China and the bending process came to an end as the left‐lateral movement ceased.

High-pressure granulite-facies metamorphism and anatexis of deep continental crust: New insights from the Cenozoic Ailao Shan–Red River shear zone, Southeast Asia

Studies of crustal anatexis have provided valuable insights into the evolution of metamorphism, deformation, and tectonic processes at convergent plate margins during the orogeny. The transition of metatexite to diatexite migmatites records crucial information about the tectonothermal evolution and rheology of the deep crust. Along the Ailao Shan–Red River shear zone, metatexite migmatites, diatexite migmatites, and leucogranites are widely distributed within the upper amphibolite and granulite facies zones of the Diancang Shan metamorphic complex. The high-pressure granulite-facies metamorphism, with the mineral assemblage comprising garnet + kyanite + K–feldspar + plagioclase + biotite + quartz + melt, is first recognized from the patch of metatexite migmatites in the complex. Detailed petrographic evidence and thermodynamic modeling revealed that the migmatite underwent nearly isothermal decompression metamorphism, presenting a clockwise P–T path. The peak metamorphic P–T conditions are constrained at approximately 11 kbar and 810 °C using thermodynamic modeling, and the amount of melt generated during heating is up to 18 mol%. The extraction and segregation of the melts are evidenced by the presence of leucosomes within the migmatites and leucogranite dikes, which record the melt flow network through the crust. Zircon and monazite geochronology of migmatites record the timing of the melting episode that began at approximately 36 Ma and lasted until 20 Ma. All these results are consistent with orogenic crust thickening accompanied by pervasive anatexis during the Late Eocene to the Early Oligocene in the Ailao Shan–Red River shear zone. Combined with the available data related to the other continental–exhumed shear zone, we propose that the crustal anatexis had a significant effect on the thermal state of deep–seated shear zones, thus controlling the rheological behavior of the lithosphere and playing an essential role in the initial localization of shearing in the lower crust.