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

The 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.


Introduction
High-strain continental-scale shear zones are typically characterized by low-strength lithosphere, and show notable vertical and horizontal changes in rock types and mineral assemblages according to variations in deformation-related temperature (T ) and pressure (P) conditions (Bürgmann & Dresen, 2008;Cao & Neubauer, 2016;Ceccato et al., 2020;Jamieson & Beaumont, 2011).Exhumed Barrovian metamorphic sequence (BMS) rocks in tectonically active orogenic-scale shear zones can record physico-chemical variations that occur between brittle rocks in the shallow crust, through the brittle-ductile transition, to the highly deformable and partially melted lower crust (Masters & Ague, 2005;Palin et al., 2018).BMS units in the Himalaya have been used to constrain the thermal evolution and convergence processes during collision between Greater India with Eurasia since c.50 Ma (Anczkiewicz et al., 2014;Burg & Moulas, 2022;Carosi et al., 2010;Jessup et al., 2016;Khanal et al., 2021;Mottram et al., 2014;Smit et al., 2014;St-Onge et al., 2013;Tewari et al., 2021).In the southeastern Tibetan Plateau (SETP), the flank of the Himalaya orogenic belt, a set of NW-SE-trending ductile shear zones accommodated ∼30%-40% of this deformation and crustal shortening by allowing ductile extrusion of continental crust towards the south east (Leloup et al., 1995(Leloup et al., , 2001;;Replumaz & Tapponnier, 2003).Activity along such megashear systems was accompanied by partial melting of middle-lower crustal rocks, which promoted their exhumation to the surface.Studying these deformed high-grade units therefore allows unique insight into the complicated dynamic process of the SETP as well as a more complete understanding of collisional orogeny in general (Ji et al., 2020;Jolivet et al., 2001;Nantasin et al., 2012;Wang et al., 2022).
Crustal channel flow (Bai et al., 2010;Bao et al., 2015;Liu et al., 2014), rigid block extrusion (Tapponnier et al., 1982(Tapponnier et al., , 2001) ) and successive thrusting (Cao et al., 2019(Cao et al., , 2020;;Ge et al., 2023;Pitard et al., 2021;Zhu et al., 2021) models have been proposed to account for crustal deformation within the lateral Himalayan orogenic belt.In these models, major fault systems in Southeast Asia are reported to have played a key role in controlling deep crustal ductile deformation and/or exhumation.Modern geophysical observations show low velocities, high conductivities and quality factors (Q-values) under high-slip-rate fault systems, which have been accounted for by middle-lower crustal channel flow (Bai et al., 2010;Bao et al., 2015;Liu et al., 2014).Additionally, paleomagnetic analysis has been used to show that many micro-blocks, such as the Indochina and Chuandian terranes, have experienced clockwise rotations and extrusion since the early Cenozoic (Li et al., 2017(Li et al., , 2020)).Recently, a crustal stacking model was proposed through systematic investigation of fault-and-thrust systems in this region (Cao et al., 2019(Cao et al., , 2020;;Zhu et al., 2021) to argue that channel flow is not necessary to explain these geophysical characteristics (Ge et al., 2023;Pitard et al., 2021).In the SETP, several shear zones reported to have experienced extremely large lateral displacements (>500 km), high slip rates, and expose high-grade metamorphic rocks, such as the Ailao Shan-Red River shear zone (ARSZ), can be studied to test the veracity of these competing models (He et al., 2021;Liu et al., 2014).Moreover, the ARSZ is distinct from other strike-slip faults within the Tibetan Plateau due to its shear sense having changed through time, and its strike-slip movement having resulted in the uplift and exhumation of the deep crust (Wang et al., 2021).
Medium-pressure BMS rocks in these shear zones represent exhumed middle-lower crust, which is the same structural level (20-40 km depth) along which channel flow has been reported along fault systems in Southeast Tibet (Bai et al., 2010;Bao et al., 2015).Furthermore, lateral ductile flow can produce a horizontal micaceous fabric in metapelitic units and induce pronounced radial anisotropy (Huang et al., 2007).These features are prominent in the upper levels of the BMS units in these shear zones, making them ideal samples to study such deformational phenomena (Fan et al., 2021).Many BMS units in shear zones in Southeast Tibet have experienced extensive recrystallization and syn-shear magmatic intrusion, which has overprinted pre-deformational metamorphic records (Gilley et al., 2003;Leloup & Kienast, 1993).For instance, much study of units in the ARSZ has focused on horizontal deformation/extrusion processes (Cao et al., 2010;Leloup et al., 1995Leloup et al., , 2001;;Liu et al., 2012;Searle et al., 2010;Tang et al., 2013;Wang et al., 2016b;Wu et al., 2017), with less attention having been given to spatial and thermal variations in the properties of distinct metamorphic units at various depths.
Consequently, we sampled metapelitic BMS units from the ARSZ and aimed to: (a) determine their metamorphic and geophysical characteristics by applying petrological modeling to examine the effects of mineral phase transition reactions, specifically focusing on their pressure-temperature (P-T ) evolution, density (ρ), P-wave velocity (Vp), and P-wave/S-wave velocity ratio (Vp/Vs) according to metamorphic grade; (b) perform in situ monazite and zircon U-Pb geochronology in order to determine the age of various metamorphic and deformation events across the massif; and (c) integrate calculated results with previously published geophysical survey data to determine the possible causes of exhumation across the ALS massif.These observations and model results will improve our understanding of how middle-lower flow of continental crust can be accommodated by deep-seated fault systems.

Geological Setting and Sample Description
Several crustal-scale strike-slip ductile shear zones have developed in the SETP along previous zones of weakness, such as margins between ancient crustal blocks and/or in response to concentrations of stresses associated with orogeny (Figure 1a).These shear zones impart fundamental controls on the style of crustal deformation, magma emplacement, and metamorphic complex exhumation in these regions (Chung et al., 1998;Harrison et al., 1996;Liu et al., 2012).Among these mega shear zones, the NW-SE trending

Journal of Geophysical Research: Solid Earth
ARSZ is a critical discontinuity that represents the easternmost extrusion boundary of the Sunda terrane (Leloup et al., 1995;Tapponnier et al., 1982).The northeastern margin of the ARSZ is defined by a set of north-south striking faults, including the Xiaojiang Fault, the Nanen River Fault, the Yimen Fault, and the Lvzhijiang Fault, which was historically referred to as the Xiaojiang Fault system (Schoenbohm et al., 2006;Wang & Burchfiel, 1997).Importantly, middle-lower crustal channel flow away from the southeastern margin of the Tibetan Plateau was reportedly accommodated by the Xiaojiang Fault system (Bai et al., 2010;Bao et al., 2015).
The ARSZ is over 1,000 km long and exposes four NW-SE trending, elongated, and high-grade metamorphic complexes: the Xuelong Shan, Diancang Shan, and Ailao Shan (ALS) in southwest China, and the Day Nui Con Voi (DNCV) complex in northeast Vietnam.Among them, the ALS complex is the largest, extending for over 500 km and ranging in width between 10 and 30 km (Figure 1b).The major rock types exposed within the ALS massif comprise micaschist, garnet-biotite-sillimanite gneiss, marble, dolomite, garnet-amphibolite, and felsic orthogneiss (Figure 1c).Zones recording high shear strain expose extensively mylonitized equivalents of these units, such as augen gneiss, which often contain prominent quartz ribbons.Intensely sheared granitic gneisses in the ALS massif core contain concordant, locally boudinaged layers of mafic granulite with fold axial planes striking between 040°and 060°, and the gneissic schistosity and dominant foliations dip steeply (ca.50-85°) to the northeast.In situ leucosomes that formed by partial melting of preexisting mafic, sedimentary, and granitic rocks are present as both discontinuous and concordant layers and lenses within the migmatites and adjacent rocks (Liu, Wang, et al., 2015;Searle, 2006).Previous work within the ARSZ has mainly focused on the timing and nature of strike-slip displacement, as constrained from field structures, microstructures, and petrofabric studies (Cao et al., 2017;Liu et al., 2012;Wu et al., 2017;Zhang et al., 2012Zhang et al., , 2017aZhang et al., , 2021)).It is generally accepted that significant left-lateral strike-slip shear initiated after the Eocene-Oligocene (Cao et al., 2009;Liu et al., 2007Liu et al., , 2012;;Tang et al., 2013;Yang et al., 2019) and accompanied clockwise rotation of the Indochina Block (Briais et al., 1993;Li et al., 2017).Metamorphic rocks exposed within the ALS massif record two major thermal events during the Triassic and Cenozoic (Ji et al., 2020;Liu et al., 2013).Medium-pressure ultramafic granulite of Late Eocene (∼35 Ma; biotite Ar-Ar) age has been documented in the south end of the massif (Liu et al., 2017).Kyanite-bearing, high-pressure (>14 kbar) pelitic granulite has also recently been reported from the middle segment of the ARSZ (Ji et al., 2020).
Here, we studied a set of BMS rocks and associated cross-cutting granitic rocks from the ALS massif.Additionally, these samples are plotted on geological profiles (Figure 1d) with multiple independent sets of geochronological constraints for the timing of metamorphism and shear.The samples included a garnetbearing micaschist (15G79-2) collected from the Ejia area, which is thrust above a Triassic biotite granite (15G79-3) (Figure 2a).Similar brittle reverse and thrust faults also occur within the granite itself (Figure 2b).Two staurolite-zone samples were collected by drilling (Figure 2c) adjacent to the ALS fault.One staurolite-bearing garnet micaschist (18J61-1) was sampled from the Yaoshan massif (Figure 1b) and a garnet-and kyanite-bearing two-mica schist (18J44-1) was collected from Yuanyang, adjacent to the ALS fault (Figure 1b).Asymmetric folds and sheared lenses in outcrop indicated sinistral shear stresses (Figures 2d and 2e).Finally, relict kyanite-bearing garnet-sillimanite-K-feldspar gneiss (15G72-5, 17HA14 and 17HA57-1) was collected from the Yuanjiang and the Yuanyang areas (Figure 2f).In the northeastern part of the shear zone, both leucosomes and high-grade metamorphic rocks show clear sinistral shearing, and typical δ-type and σ-type porphyroblasts may observed in melanosomes (Figures 2g and 2h).Further, some samples that record sub-horizontal and near-vertical foliations also preserve two generations of lineations (Figure 2i).Multiple geochronological dates and lower hemisphere equal-area stereoplots of foliation and mineral stretching lineation are plotted in these two profiles.The geochronological dates from number 1 to 13 are cited from Chen (2018), Liu et al. (2013), Wang et al. (2013aWang et al. ( , 2013b)), Li et al. (2009), Sassier et al. (2009)

Analysis Methods of Zircon and Monazite U-Pb Dating
Monazite and zircon U-Pb dating were performed using 193-nm laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) at the Wuhan Sample Solution Analytical Technology Co., Ltd and Beijing Createch Testing Technology Co., Ltd, respectively.The monazites were analyzed using a 16 μm spot size and a frequency of 2 Hz.A T-connector was utilized for the carrier gas (helium) and the make-up gas (argon) to mix before entering the inductively coupled plasma.Additionally, a "wire" signal-smoothing device was included during monazite dating.In contrast, zircon analyses were conducted with a beam diameter of 25 μm, a 10-Hz repetition rate, and energy of 2.5 J/cm 2 .Harvard 91,500 and GJ-1 (zircons), and 44,096 (monazite) were used as external standards for all U-Pb geochronology corrections.The concentration of 29 Si in NIST610 glass was used for concentration correction and trace element calibration (Liu et al., 2008).All analyzed data were handled by the ICPMSDataCal 10.4 program, which we used to perform off-line selection and integration of background and analyzed signals, time-drift correction and quantitative calibration for correction and age determination (Liu et al., 2010).Concordia diagrams, weighted average age calculations, and probability density plots were performed using Isoplot/Ex 3.0 (Ludwig, 2003).
Electron backscatter diffraction (EBSD) analysis was performed on select thin sections using a FEI Quanta 450 FEG SEM chamber with a 70°tilt angle.EBSD data were acquired by an Oxford Nordlys F+ high-speed detector with 20 kV acceleration voltage, 18-20 mm working distance, and 8-μm step size.EBSD data were collected using Aztec Synergy software and were analyzed using the HKL Channel 5 software on a FEI Quanta 450 scanning electron microscope.

Garnet Zone
Garnet-bearing micaschist sample 15G79-2 contains quartz, plagioclase, biotite, muscovite, and garnet, alongside accessory rutile, ilmenite, tourmaline, apatite, allanite, zircon and monazite (Figure 3a).Garnets range in diameter from 0.1 to ∼1.5 cm and show variable morphologies: grains with euhedral shapes contain rare inclusions, whereas those with anhedral rims often have biotite and quartz inclusions.In BSE images, allanite exhibits bright rims and dull cores, indicating a strong zonation in LREE content.Matrix rutile grains are rimmed by ilmenite, with or without spatially associated tourmaline.Microstructural relationships imply that the following reaction occurred during prograde metamorphism of sample 15G79-2 to grow garnet during burial:

Staurolite Zone
Staurolite-zone samples contain fine-grained crystals of staurolite within clots of muscovite, and a matrix dominated by chlorite, biotite, and quartz.These microstructures likely represent secondary muscovite pseudomorphs after primary (prograde) staurolite, which indicates hydration of these samples during retrogression/ exhumation (Figure 3b).Units situated towards the high-grade end of the staurolite zone contain larger staurolite porphyroblasts (Figure 3c).Sample 18J61-1 exhibits multiple foliations, with a pre-tectonic fabric defined by oriented quartz inclusions in staurolite (S n ) that show oblique relations with the muscovite-dominated matrix foliation (S n+1 ).Inclusions in garnet are mainly quartz, ilmenite, epidote/allanite and monazite.Fine-grained monazite surrounding allanite in garnet suggests that monazite was grown at the expense of allanite.Quartz inclusions in staurolite cores are coarser than those in mantle and rim domains.Muscovite flakes show evidence of deformation-driven kinking.EBSD analysis of quartz shows distinctly different fabric characteristics between inclusions in staurolite and those in the matrix (Figure 4a), which indicates that a recrystallization event occurred between both sets of fabric development.Quartz in the matrix shows evidence for sinistral shear at temperatures of 550-600°C, as determined by the opening angle of their pole figure (Figure 4b).Staurolite orientation within the rock fabrics suggests that their long axes are near-parallel with shear-induced lineations, and misorientations are mainly located in crystal rims (Figure 4c).Thus, we interpret that the following metamorphic reaction occurred during metamorphism of these samples:

Kyanite Zone
By contrast with the staurolite zone samples, staurolite in the kyanite zone is more euhedral and has fewer inclusions.Both biotite and muscovite in kyanite-zone samples show sinistral shear textures, including sigmoidal fish.Rutile inclusions are mainly located in garnet mantles and rims (Figure 3d), although grains in the matrix are partially replaced by ilmenite.Kyanite-bearing two-mica schist (18J44-1) has a distinct S/C fabric and garnet porphyroblasts show rotational internal microstructures (Figure 3e).Biotite, muscovite, quartz and plagioclase form the S-foliations and the C-foliations are dominantly comprised of muscovite.Kyanite grains are pre-tectonic and are wrapped by the S-foliation, and contain reddish-brown biotite inclusions.Garnet porphyroblasts have a helicitic texture and the orientation of quartz inclusions show a large angle with the matrix foliation (Figure 3e).This inclusion pattern indicates that garnet cores crystallized prior to the initiation of sinistral shear, but the garnet rims grew during this deformation.Rare staurolite grains occur as inclusions in garnet rim domains.Relevant prograde metamorphic reactions for this zone include:

Sillimanite Zone
Based on their mineral assemblages, sillimanite-zone units of the ALRS can be divided into two zones: a lowgrade zone containing fibrous/felty sillimanite, alongside biotite, garnet, plagioclase, quartz, and/or muscovite and kyanite, and a high-grade zone containing prismatic sillimanite, garnet, K-feldspar, and biotite, and lacks muscovite.In the former (17HA54), sillimanite is closely associated with reddish-brown biotite, and Fe-Ti oxides are common around biotite or in garnet fractures (Figure 3f).Sample 15G72-5, a garnet-sillimanite-K-feldspar gneiss, contains abundant K-feldspar (∼60%-70%), and lesser plagioclase (5%) and quartz (<1%).Biotite and sillimanite in the matrix define the dominant foliation and penetrating lineation.Relict kyanites occur as inclusions in garnet and form partially pseudomorphed grains in the matrix, where they are mantled by plagioclase (Figures 3g and 3f).K-feldspar exhibits a large dihedral angle when adjacent to garnet (Figure 3h).Garnet porphyroblasts range in diameter from 0.5 to 3 mm, and always show anhedral shapes.Compared with the relatively clear rim, garnet cores contain abundant plagioclase and quartz inclusions.Some garnets contain rings of inclusions in mantle domains.Relevant reactions for rocks within this zone include:

Compositional Mapping of Garnet
Compositional maps of garnet from the garnet and staurolite zones were acquired using X-ray EDS, as described previously, whereas garnet in the kyanite zone was analyzed via EPMA scanning.Large porphyroblasts in micaschist (15G79-2) revealed that the garnet core is strongly enriched in Mn and depleted in Mg (Figure 5a), and Ca and Fe contents are relatively consistent across the grains.Many grains exhibit an extremely narrow Mn-rich reabsorption rim.Garnet in staurolite-bearing micaschist sample 18J34-4 is euhedral and mainly contains quartz, apatite, and ilmenite inclusions.The high-Mn core correlates with the inclusion-rich region, while the inclusionfree rim has a very low Mn content (Figure 5b).Garnet in kyanite-bearing micaschist contains fine-grained quartz, staurolite and ilmenite inclusions, and shows patchiness in compositional element maps that indicates dissolutionreprecipitation (Figure 5c).Garnet cores with higher Ca and Mn contents shows embayed boundaries, although rims are relatively homogeneous in composition.

EPMA Results of Garnet and Staurolite in BMS Rocks
Garnet compositions from the garnet, staurolite, kyanite, and sillimanite-K-feldspar zone were acquired via EPMA (Figure 6).All data are provided in Table S1 in Supporting Information S1.Garnet megacrysts in garnetbearing micaschist (15G79-2) exhibit an asymmetric composition profile across the grains (Figure 6a).From core to rim, the composition ranges from Alm 64 Prp 8 Grs 18 Sps 10 to Alm 72 Prp 15 Grs 13 Sps 0 .The garnet core has a relatively homogeneous spessartine content (10 mol.%) and falls to zero at the rim domain that contains quartz and biotite inclusions.The Fe # value (Fe 2+ /(Fe 2+ + Mg)) also shows a pronounced decrease.
Garnet in staurolite-bearing micaschist (18J61-1) has a composition of Alm 64-73 Prp 7-13 Grs 9-13 Sps 5-18 .From the garnet core toward the rim, the spessartine proportion and Fe # values gradually decrease.Garnet rims in contact with biotite show elevated Fe # values in the outmost 200 μm, indicating modification via cation exchange (Figure 6b).Fe # values of matrix staurolite first decrease from the core to the mantle, and then increases at the outer rim.
Garnet grains in staurolite-kyanite zone sample (18J15) show multiple stages of growth, evidenced by dramatic two-step increases in pyrope contents and a decrease in the spessartine content.These changes align with textural differences between cores containing abundant quartz inclusions to mantles containing staurolite inclusions, to inclusion-free rims (Figure 6c).The grossular content is low in the core, higher in the mantle region, and finally decreases again in the rim.Overall, the garnet composition lies between Alm 66 Prp 6 Grs 18 Sps 11 and Alm 76 Prp 17 Grs 6 Sps 2 .Corresponding Fe # values range from 79 to 93.
Akin to the lower-grade metamorphic zones, garnet in the kyanite zone (18J44-1) displays a gradual decrease in grossular content from core to rim and has a compositional range from Alm 65 Prp 9 Grs 16 Sps 12 and Alm 71 Prp 13 Grs 11 Sps 4 (Figure 6d).The Fe # values are relatively consistent, lying between 85 and 81.
Garnet in sillimanite-K-feldspar zone sample (15G72-5) have extremely low spessartine contents (<2%) and their grossular proportions drop dramatically from core to rim (Figures 6e and 6f).The pyrope content reaches 30 mol. % and is the highest content of all BMS units.Overall, garnets have compositions of Alm 65 Prp 30 Grs 10 Sps 2 to

P-T Evolution
All P-T conditions of metamorphism and density variations within BMS rocks were calculated using the phase equilibrium modeling software GeoPS (http://www.geops.org;Xiang & Connolly, 2021), which is a highperformance visualization program that uses Gibbs free energy minimization to calculate stable assemblages at defined metamorphic conditions.We used the internally consistent thermodynamic data set HP62 (Holland & Powell, 2011, created   Powell, 2003), and spinel (White et al., 2002).Pure phases included quartz, aqueous fluid (H 2 O), rutile, sphene, and the polymorphs of Al 2 SiO 5 .The P-T evolution of the BMS rocks during prograde, peak, and retrograde metamorphism were mainly constrained by isopleths for pyrope [(X Mg = Mg/(Mg + Fe + Ca + Mn)] and grossular [(X Ca = Ca/(Mg + Fe + Ca + Mn)] in garnet, which are highly sensitive to changes in P and T conditions during Barrovian metamorphism (Palin et al., 2012).For low-grade schist samples, isopleths for spessartine [(X Mn = Mn/(Mg + Fe + Ca + Mn)] were also used.Due to the abundance of hydrous minerals in all units except for upper sillimanite zone samples, aqueous fluid (H 2 O) was set as a saturated phase.Uncertainties related to the absolute positions of assemblage field boundaries on calculated phase diagrams may be up to ±1 kbar and ±50°C at the 2σ level (Palin, Weller, et al., 2016).Finally, calculated P-T paths (Figure 7) were used to determine changes in mineral proportion (wt.%) and bulk-rock density variation during metamorphism of the ARSZ (Figure 8).
The prograde metamorphic evolution of the study region was constrained by garnet-bearing micaschist sample 15G79-2, staurolite-mica schist sample 18J61-1, and kyanite-bearing micaschist sample 18J44-1.Bulk-rock pseudosections were constructed for samples 15G79-2 and 18J61-1 over a P-T range of 3.4-10.0kbar and 500-750°C (Figures 7a and 7b).The observed metamorphic assemblage in sample 15G79-2 is matched by the field Chl-Ms-Bt-Qz-Grt-Pl-Rt, which is stable up to ∼7.6 kbar and 590°C.At higher grade, chlorite would dehydrate to form additional biotite, garnet, and new staurolite.Measured garnet compositions, evidence for retrograde reactions, and decomposition of garnet indicate that this sample likely equilibrated at slightly highergrade metamorphic conditions of ∼7.8 kbar, ∼610°C (Figure 7a).Garnet in staurolite-micaschist sample 18J61-1 records a relatively simple P-T evolution that initiates in the Chl-Ms-Bt-Grt-Ilm-Qz-Pl-St field at 3.5 kbar, 530°C (Figure 7b).Increases in P and T due to burial and heating led to the complete breakdown of chlorite, plagioclase, and ilmenite, and biotite partial decomposition at 7.5 kbar, ∼600°C (Figure 7b).This result is consistent with an ∼80°open angle of the EBSD pole figure for matrix quartz (Figure 4b).The destabilization of chlorite in both schists leads to bulk-rock density being mainly controlled by biotite breakdown and the production of garnet.
Isopleths for high-Mn and Ca and low-Mg garnet cores for sample 18J44-1 intersect at ca. 4.8 kbar, 565°C in a calculated mineral assemblage of Ms-Bt-Grt-Rt-Qz-Pl-St.Kyanite first appears at ∼7.2 kbar and ∼660°C during burial due to the breakdown of staurolite, and partial melting would begin in a fluid-saturated environment at >670-690°C (Figure 7c).The observed peak metamorphic assemblage (Ms + Bt + Ky + Grt + Pl + Qz + Rt) is constrained in our modeling to ca. 7.4 kbar and 680°C, which also matches intersections of garnet isopleth compositions.
Modeling of suprasolidus rocks is complicated by issues related to bulk composition modification by open-system processes related to melt migration in and out of source regions (Palin, White, et al., 2016).A pseudosection for sillimanite-zone sample 15G72-5 was constructed over the P-T range 5.4-16.0kbar and 630-880°C (Figure 7d).The bulk-rock oxidation state was considered highly reducing and set to a value of XFe 3+ /Fe total = 0.01 due to the absence of magnetite or other Fe 3+ -rich oxides in the sample (e.g., Guilmette et al., 2011;Indares et al., 2008;Zhang et al., 2017b).As the kyanite-bearing garnet-sillimanite gneiss experienced notable melt loss, the phase equilibria modeling allows exploration of the metamorphic evolution history of near-to-the peak and the retrograde path, but may not be valid for the prograde process (Groppo et al., 2012;Indares et al., 2008;White et al., 2005;Zhang et al., 2017b).This is consistent with our petrographic observations, which support an isothermal decompression P-T path (Figure 7d).Muscovite dehydration occurred during exhumation from ∼16.0 kbar to ∼800°C to ∼8.8 kbar and ∼750°C, as constrained by garnet rim compositions.This steep isothermal decompression path also induces melt generation, with molar proportions reaching 8%, allowing for melt extraction into the overlying crust.Melt extraction ceased following decompression through the kyanitesillimanite polymorphic transition.We note that some kyanite remains in the sample as metastable relics preserved in garnet porphyroblasts (Figures 3g-3i).

Density Variations During Metamorphism
The density of rocks within the continental crust is controlled by their mineral assemblages, the P-T conditions of equilibration, and their water content (Tassara et al., 2006).The density of each mineral in the metamorphic units studied here was computed from the pressure derivative of the molar Gibbs energy and their respective molar mass (Almqvist et al., 2013;Connolly & Kerrick, 2002): where G is the molar Gibbs energy and N is the molar mass.Among the micaschists, two abrupt density variation domains were controlled by chlorite dehydration reactions (520-570°C, 3.0-8.0kbar) and partial melting when crossing the wet solidus at 670-700°C (Figures 8a-8c).The bulk-rock densities increase commensurately with increases in T and P during the dehydration of chlorite, while the density of the system decreases during partial melting.Above biotite-zone conditions, density increases with metamorphic grade when at the same pressure (Figures 8a-8c).The kyanite-bearing garnet two-mica schist shows a density range from 2.89 to 2.93 g/cm 3 between 5.0 and 7.6 kbar (Figure 8c).During prograde burial, the staurolite zone and garnet zone micaschists show corresponding density increases from 2.80 to 2.85 g/cm 3 and from 2.75 to 2.82 g/cm 3 , respectively (Figures 8a and 8b).In contrast, high-P granulite has a peak density of 2.87 g/cm 3 at peak pressure conditions (Figure 8d), which reflects the loss of partial melt during exhumation.

Seismic Compressional Wave Velocity and Wave Speed Ratio of BMS Rocks
Seismic compressional (Vp) and shear wave (Vs) velocities in rocks are determined by the constituent minerals, their proportions, and overall rock texture (Christensen & Mooney, 1995;Connolly & Kerrick, 2002).The compressional wave velocities and Vp/Vs variation of the BMS rocks from the ARSZ were simulated (Figure 8) by Perple_X v. 6.9.1 (Connolly, 1990(Connolly, , updated 2022) ) using the same ds62 thermodynamic data set (Holland & Powell, 2011) and a-x models used to calculate P-T pseudosections using GeoPS.The bulk modulus was determined using the Voigt-Reuss-Hill method, where the upper bound was defined by X i× Z i (Voigt, 1928) and the lower bound was defined by (Reuss, 1929).Here, X i is the volume fraction of phase i, and Z i is the elastic modulus of phase i. Adiabatic bulk modulus (Ks) and shear modulus (μ) were expressed as the following formulas (Connolly & Kerrick, 2002): where G is the molar free Gibbs energy and σ is the Poisson's ratio.After determining the modal mineral composition, mineral densities and the individual mineral elastic modulus, isotropic Vp and Vs were calculated as Almqvist et al., 2013;Connolly & Kerrick, 2002).
Among the upper part of the BMS, chlorite and staurolite decomposition reactions cause large increases in the seismic compressional wave velocity (Figures 8a-8c), whereas Vp/Vs values decrease following staurolite breakdown.Along the prograde P-T path, the Vp of the garnet-bearing two-mica schist increased from 5.97 km/s to 6.14 km/s, and the Vp/Vs values ranged from 1.586 to 1.594.The staurolite micaschist showed a similar Vp variation (5.95-6.14km/s) and a wider range of Vp/Vs values (1.582-1.606).For garnet-and staurolite-zone samples, the calculated Vp of kyanite-bearing micaschist ranged from 6.03 km/s to 6.15 km/s and corresponds to Vp/Vs values from 1.651 to 1.669.In contrast, at near-solidus temperatures, the high-P granulite showed an abrupt Vp decrease from 6.36 to 5.86 km/s and an increase in Vp/Vs values from 1.70 to 2.16 (Figure 8d).During isothermal decompression, Vp decreases from 6.71 km/s to 6.35 km/s and exhibits higher Vp/Vs values (2.01-2.16)than micaschists.

Thermal Activity Recorded by Monazite and Zircon in the BMS
Eleven samples from 10 locations were subject to monazite and/or zircon geochronology to determine the timing of metamorphism and partial melting.The majority of geochronological dates were acquired from monazite due to its abundance in metapelites and its compositional sensitivity to low-temperature activity and fluid-bearing metamorphic reactions.All U-Pb geochronological data and trace element measurements of monazite/zircon are listed in Table S2 in Supporting Information S1.Corresponding 207 Pb/ 235 U versus 206 Pb/ 238 U diagrams and chondrite-normalized REE diagrams of samples are illustrated in Figure 9 and Figure S1 in Supporting Information S1, respectively.

Garnet Zone
Zircons from garnet-bearing two-mica schist sample 15G79-2 are euhedral to subhedral in shape, 100-200 μm in diameter, and show a distinct core-rim texture (Figure 9a).Magmatic core domains exhibit high-contrast oscillatory zoning, whereas metamorphic rim domains have less variation in CL and are duller than cores.An irregular CL-dark zone (∼5 μm) is preserved between both domains.Fractured regions of cores also appear dark in CL.A total of 25 U-Pb spot analyses were made on gray rims, which yielded 21 concordant ages.Except for three ages belonging to the Indosinian orogeny, 18 analyzed spots exhibit 206 Pb/ 238 U ages ranging from 47.1 to 77.8 Ma.Two pre-Himalayan ages and two ages with large errors were excluded from consideration (Figure 9a), such that the remaining 14 analyses yielded a weighted mean age of 52.1 ± 2.4 Ma.The REE pattern exhibits a scattered HREE (Ho-Lu) pattern (Figure S1a in Supporting Information S1).In addition, the rims show distinctly lower Y, Ti, Ca, Nb, REE, Pb, and Th content than magmatic cores, but have a higher U content Table S3 in Supporting Information S1.Monazite from the same sample (15G79-2) shows a clear core and mantle structure, although our spot size did not allow for each domain to be analyzed separately (Figure 9b).A total of 23 out of 30 spots yielded ages that ranged from 29.4 to 31.8 Ma, and gave a weighted mean age of 30.8 ± 0.4 Ma.Weakly deformed biotite granite (15G79-3) in the vicinity of the schist shows Cenozoic and Indosinian ages that range from 29.1 to 34.1 Ma and 228 to 233 Ma, respectively (Figure 9c).In a REE diagram (Figure S1b in Supporting Information S1) the latter group has lower Eu and Ce contents than the former.

Kyanite Zone
In contrast to monazite in garnet and staurolite zone samples, monazite in kyanite zone samples has duller cores than rims (Figure 9h).The dark cores of monazite in kyanite-garnet micaschist (18J44-1) were less abundant than those in sample 18J43-3 (Figure 9h), and all record high Y and LREE contents (Table S4 in Supporting Information S1).A total of 32 monazite spots including 27 bright rims and five dark cores were analyzed.A total of 28 spots yielded a weighted mean 238 U/ 206 Pb age of 31.87 ± 0.37 Ma (MSWD = 3.4).The REE pattern of 18J44-1 has similar features to sample 18J61-1 which show distinct depletion of high REE (Figure S1f in Supporting Information S1).

Sillimanite-K-Feldspar Zone
Monazite grains in kyanite-bearing sillimanite-garnet-K-feldspar gneiss (15G72-5) are anhedral in shape and some grains display greatly embayed rims (Figure 9i).In comparison with monazite in other metapelites from the ALS massif, the monazite in high-P granulite shows significantly lower REE and Y contents (Table S4

BMS Spatial Variations, Metamorphic Evolution and Tectonic Implication of the ARSZ
The Himalayan orogenic belt is the largest and best-preserved collisional orogen on Earth, and exposes an almost continuous series of BMS units along the 2,500 km-long Main Central Thrust (Caddick et al., 2007;Chakraborty et al., 2016;Mottram et al., 2014;Yakymchuk & Godin, 2012).The spatial distribution and zonation of isograds, and quantitative P-T-t paths for individual BMS units provide key evidence that can be used to constrain the geodynamics of intracontinental crustal deformation and thermal evolution during post-collisional convergence (Khanal et al., 2021 and references therein).However, in the SETP, spatial juxtaposition, differential uplift, and heterogeneous thermal overprinting have led to widely variable P, T, ρ, Vp conditions and ages within the middlelower crustal BMS rocks.Four discrete metamorphic massifs are exposed within the 1000-km-long ARSZ, which share a common geodynamic history, but unique tectonic characteristic (Leloup et al., 2001).For example, regional antiforms with gneissose cores and micaschist limbs occur in the Xuelong Shan (Zhang et al., 2017a), DCS (Zhang et al., 2021) and DNCV massif (Anczkiewicz et al., 2007;Leloup et al., 2001).Nevertheless, the spatial distribution and structural features of metamorphic supracrustal rocks in the largest massif-the ALSshow significant differences.First, the metasedimentary rocks display an inverted BMS profile, with metamorphic grades spanning from the chlorite zone to the second-sillimanite zone (Harrison et al., 1992(Harrison et al., , 1996;;Ji et al., 2020;Liu et al., 2013).Second, adjacent to the southwest boundary fault, the micaschists are tectonically emplaced above older granites or Paleozoic phyllites/slates via a high-angle thrust (Leloup et al., 1995;Fan et al., 2021;Zhang et al., 2017a; Figure 2a).Third, as first identified in this study, a high-P granulite belt outcrops adjacent to the Red River Fault.Detailed study of the causes of these discrepant characteristics along the ARSZ can shed new light on the ridge block extrusion model and crustal channel flow hypothesis.Here, we focused on the ALS massif.

The Cenozoic High-P Granulite Belt in Southeastern Tibet
A progressive increase in metamorphic grade toward the northeast, and exhumation of rocks from significantly greater depths along the northeast edge of the ALS massif have been documented by many previous workers (Harrison et al., 1992(Harrison et al., , 1996;;Schoenbohm et al., 2005).Nevertheless, pronounced retrograde metamorphism and deformation have obscured the history of these lower-crustal high-P granulites.Peak metamorphic pressures in Journal of Geophysical Research: Solid Earth 10.1029/2023JB027253 the study area were previously assumed to be recorded in the sillimanite zone rocks (Anczkiewicz et al., 2007;Gilley et al., 2003;Harrison et al., 1996;Leloup & Kienast, 1993;Leloup et al., 2001;Liu FL et al., 2013;Liu PP et al., 2017;Nam et al., 1998;Palin et al., 2013) on account of the lack of kyanite in high-grade paragneiss.In previous works, kyanite has only been documented in micaschists on the southwest side of the shear zone (Liu et al., 2013;Wang et al., 2016a;Wang HB et al., 2019).Ji et al. (2020) first reported Cenozoic high-P granulite (peak pressure ∼14 kbar) in the sillimanite-K-feldspar zone near the Mosha area, situated in the middle segment of the ALS massif.These rocks were characterized by relict kyanite in the matrix or included in garnet, and record metamorphic reactions ( 7), ( 8), and ( 9) documented in Section 4.4.Here, we determined a maximum metamorphic pressure of ∼16 kbar (Figure 8d), equivalent to a crustal thickness of ∼55 km, indicating greater uplift and exhumation on the northeast side.Moreover, we emphasize that the classical decompression reaction Ky = Sill and/or 2Ky + Grt + Qz = 3An is observed in many locations (south Ejia, Mosha, Yuanjiang, Honghe, and Yuanyang area), with one example of peak pressure preserved in the >250 km long high-P granulite belt in the eastern edge of the ALS massif.Rocks in this high-P granulite belt experienced extensive melt production and loss during exhumation, and are likely source rocks for the widespread felsic plutons, sills, and dykes that occur throughout the BMS.Such a phenomenon has also been reported in the type-locality Barrovian metasediments of northeast Scotland (Palin et al., 2018) and in the Greater Himalayan Sequence in the Himalaya (Khanal et al., 2021).Thus, the discovery of high-P granulite belt is an important extension of the traditional BMS conditions reported in metapelites of the ARSZ into the suprasolidus field.Indeed, the melt-absent/melt-poor high-P granulites and low grade/upper level BMS (sub-solidus micaschists) were thus vertically separated by an intermediate melt-bearing layer.
The discovery of high-P granulite also sheds new light on degree of differential exhumation and variations in thermal evolution processes in crustal-scale shear zones.When compared with micaschists that record a peak pressure up to 7-8 kbar (Figures 8a-8c), the ∼16.0 kbar peak metamorphic pressure of kyanite-bearing garnetsillimanite gneiss demonstrate that the northeastern margin was exhumed from ∼25 to 30 km deeper than the southwest side.As metamorphic grade progressively increases from southwest toward to the northeast across the massif, rocks in the sillimanite zone exhibit pronounced partial melting (Liu, Wang, et al., 2015;Liu JL et al., 2019).It has been suggested that there may have be a temperature difference of up to 200-300°C in the middle crust across the massif at the time of sinistral shearing (Ji et al., 2020(Ji et al., , 2021; Figure 8d).Cooling age discrepancies across the shear zone are closely associated with higher grade metamorphic rocks that have been differentially exhumed, thus forming an inverted BMS (Harrison et al., 1996).Many mechanisms have been proposed to explain this inverted metamorphic sequence, including shear heating (Leloup et al., 1999), emplacement of a hot overlying thrust sheet (Gilley et al., 2003), folding of pre-existing isograds followed by juxtaposition by normal faulting (Chen et al., 2018), and differential displacement along shear zones (Fan et al., 2021).In other crustal-scale shear zones on worldwide, the differential unroofing of high-P granulite-facies compared to greenschist-facies rocks has been documented, such as the Coimbra-Cordoba shear zone in Spain (Pereira et al., 2010), the Tauá and Senador Pompeu shear zone in northeast Brazil (Ávila et al., 2020, 2023), the Legs Lake shear zone in Canadian shield (Mahan et al., 2003), and in Fiordland, western New Zealand (Klepeis et al., 2007).The exhumation mechanism discrepancy between partial melted high-P granulites from the lower crust and the brittle micaschist from middle crust beyond the scope of this study.

P-T-t-d Path of the BMS
The P-T-t-d trajectory of BMS micaschists and high-P granulites profoundly reflects the spatial variations in the thermal structure, rheology, and dynamic processes that occurred during ARSZ evolution.Combined with previous geochronological data (Figure 10a), we built a P-T-t-d path for the upper and lower units of the BMS in the ALS massif (Figure 10b).Our metamorphic zircon age (52-41 Ma) for garnet-bearing micaschist sample 15G79-2, the thrust structures (Figure 2a), high-angle lineation (Figure 2i), and the tight fold transect by syn-shearing veins (Figure 10b) indicate that prograde metamorphism was contemporaneous with development of a foldand-thrust system during the early Eocene (Figure 12a).On the southwest side of the ALS massif, prograde P-T paths for the high-grade sections of the BMS (garnet to kyanite zones) record extensive dehydration of chlorite, muscovite and biotite in the vicinity of the ALS fault (Figures 8a-8c).The peak P-T conditions of pelitic rocks increased from 3.5-5.2kbar to 7.2-7.8kbar, which corresponds to ∼10 km of crustal thickening and an increase in temperature of ∼100°C from ∼550 ± 20°C.Lower Y, Ca and REE contents in zircon rims relative to magmatic cores demonstrate that these regions are concurrent with garnet growth, and so date prograde burial.
Garnet-bearing leucogranitic veins might also represent important reservoirs for yttrium (Y), which explain the notable depletion in Y observed in kyanite and sillimanite zone rocks (Figure S1h in Supporting Information S1).
A regional NE-SW compression and crustal thickening event (D1) prior to sinistral shear is also evidenced by: (a) granitic mylonite adjacent to both the ALS and the Red River fault shows early Eocene mica or amphibole Ar/Ar cooling ages (Zhang et al., 2006, Figure 2d); (b) the Ludian-Zhonghejiang fold-and-thrust system, which lies parallel and is situated close to the ARSZ, was active during the period 50-39 Ma (Cao et al., 2020); (c) a thinskinned fold-thrust stratigraphic succession and the unconformity exists between Paleocene and Lower Eocene strata (45 ± 5 Ma) in Indochina (Liang et al., 2022;Wang & Burchfiel, 1997); (d) Mesozoic granitic and sedimentary rocks situated on the interior of the ARSZ and its flanks record Eocene zircon/apatite fission track and (U-Th)/He ages (Liu-Zeng et al., 2018;Wang et al., 2020); (e) gneissose folding with sillimanite-and garnetbearing fabrics that formed due to pure shear deformation in the ARSZ, and which were pervasively developed prior to left-lateral shearing initiation (Jolivet et al., 2001;Liu et al., 2012;Zhang et al., 2014Zhang et al., , 2017aZhang et al., , 2017b)); (f) the chronostratigraphic age of formation of the Jianchuan and Lühe syncontractional basins (Eocene: 37-35 Ma), which are interpreted to document synchronous E-W shortening of the ARSZ (Gourbet et al., 2017;Li et al., 2020); and (g) Eocene-Oligocene potassic granitoids are interpreted as having formed in response to crustal thickening and collapse in the southeastern Tibetan Plateau (Gou et al., 2021).These lines of evidence lead us to propose that micaschists on the southwest side of the ALS massif recorded an episode of crustal stacking above the brittle-ductile transition zone (∼25 km) during the soft collision stage between the Indian plate and the Eurasian plate (Lee & Lawver, 1995).Nonetheless, more detailed field investigation is required to determine the significance and evolution of the Eocene structures present in the ARSZ.
Extensive anataxis occurred during decompression stage D2, and has obscured microstructural evidence of middle Eocene regional extension.As such, only microstructural evidence of high-pressure mineral breakdown reactions is recorded in thin section (Figure 10b).However, most ultra-potassic magma and lamprophyres in the ALS massif were emplaced during the Middle to Late Eocene, which indicate a period of mantle perturbation and partial melting (Huang et al., 2010;Liang et al., 2007).The middle-late Eocene (41.0 ± 0.5 Ma) anatectic age in the eastern ALS massif (Guo, 2017;Liu, Wang, et al., 2015) indicated that the middle-lower crust of the northeast margin was notably weak immediately prior to the initiation of left-lateral slip.SHRIMP analysis of monazite inclusions within garnet porphyroblasts show U-Th-Pb ages older than 43 Ma, which may suggest that prekinematic garnet growth stopped at ∼43 Ma (Gilley et al., 2003).Furthermore, zircon overgrowth rims that contain sillimanite inclusions also display middle Eocene ages, which demonstrates that high-P granulite was exhumed to middle crustal depths during isothermal decompression (Ji et al., 2020).A middle Eocene monazite age (41.3 ± 0.2 Ma) obtained from staurolite zone sample 18J61-1 in this study is interpreted to record exhumation of the amphibolite facies rocks in the ALS massif.
Left-lateral strike-slip shear (D3) is pervasive documented by multiple rock types at different scales (Figure 10b), and the P-T-t path has been well constrained by many previous workers (Ji et al., 2020(Ji et al., , 2021;;Leloup et al., 1995;Liu et al., 2013Liu et al., , 2017)).Differences between these paths are mainly due to conflicting interpretations of the heat source for high-temperature metamorphism.In particular, Leloup et al. (1995) proposed that shear heating associated with sinistral deformation caused a temperature increase of >200°C, although others interpret that upwelling magmas supplied thermal energy to drive localized granulite-facies metamorphism (Gilley et al., 2003;Liu et al., 2017).Nonetheless, we note that most samples in the sillimanite zone recorded both the early stages of ductile shear and later fluid-driven recrystallization along shear planes (Figure 10b).The pervasiveness of Early Oligocene ages (34-29 Ma) in ALS massif rocks can be explained by the onset of shearing and rapid uplift at that time (Chen et al., 2016;Harrison et al., 1996).The age of prograde metamorphism decreases with increasing structural depth, as expected in an inverted metamorphic sequence (Gibson et al., 1999).This indicates that previous estimations of peak pressure (<8 kbar) in the ALS (Gilley et al., 2003;Harrison et al., 1996;Leloup et al., 2001;Leloup & Kienast, 1993) may only record exhumation from the middle crust.In addition, a higher thermal flux on the northeast side may also account for higher rates of erosion during and following sinistral shearing (Harrison et al., 1996).We therefore support the proposition that melt-enhanced large-scale, rapid upward of deeper, hotter portions of the crust over cooler portions of the crust was responsible for forming an inverted BMS, as has been documented in other major shear zones worldwide (Burg & Moulas, 2022;Hollister & Crawford, 1986;Mottram et al., 2014;Searle, 2015).

Phase Transformation Reactions Control Melt Migration
During prograde metamorphism, micaschists should gradually strengthen due to breakdown of hydrous minerals, such as phyllosilicates, to form porphyroblasts (e.g., garnet, staurolite, and kyanite) (Groome et al., 2006).This process occurs in tandem with dehydration stiffening, whereby expulsion of free aqueous fluid promotes tighter coupling between grains.However, with increasing P-T conditions, partial melting occurs, causing strain localization along fluid-rich domains and promoting the initiation of shear.The studied BMS rocks are expected to produce <5% melt via fluid-saturated solidus reactions (Wei and Zhu, 2016), although continued heating or decompression during exhumation from high-pressure conditions (>10.0 kbar) associated with muscovite dehydration can produce an additional 7% melt (Wei and Zhu, 2016).Together, this generated melt can form a connected melt network and significantly reduce rock strength and viscosity (Rushmer, 1991;Sawyer, 1994).As reactants are consumed, the muscovite dehydration reaction (muscovite + quartz + fluid → melt + Sill/Ky + Kfs) ceases at higher temperatures (Hollister & Grujic, 2006;Rosenberg & Handy, 2005).Biotite dehydration melting then occurs and can produce an additional 30%-40% melt (Wei and Zhu, 2016), as it is a major melt-forming reaction in metapelites.The accumulation of such large melt volumes leads to significant expansion of the rock and breakdown of the solid matrix, which enhances crustal permeability in ductile shear zones (Etheridge et al., 2020).Fractures at all scales cause negative pressure gradients and allow melt to escape from its source and forms veins, sills, dikes, or plutons at higher levels in the crust (Demartis et al., 2011).Thus, compared with muscovite dehydration, biotite dehydration will greatly increase the proportion of melt and will destroy the structure of protolith.
Impermeable rocks that overlie middle or lower crustal melt-bearing layers may serve as a sort of "top seal" that promotes horizontal melt migration (Cavalcante et al., 2016;Weinberg and Podladchikov, 1994).Here we suggest that this roughly coincides with the kyanite to sillimanite transformation that occurs during exhumation of high-P granulite, and so this change in mineralogy may inhibit melt escape from deeper crustal levels (Figure 12b).This transformation is important due to: (a) sillimanite (3.23-3.27g/cm 3 ) having a lower density than kyanite (3.53-3.65 g/cm 3 ), which promotes a large positive change in volume across the decompression reaction kyanite → sillimanite, forcing melt to migrate away.The expulsion of melt acts to further stiffen the residue, with melt-depleted metapelites being commonly stronger than adjacent rocks in the same terrane (Diener & Fagereng, 2014).(b) The polymorphic phase transition between kyanite and sillimanite is known to be sluggish (Carmichael, 1969) and may not go to completion in some rocks that are fluid undersaturated; thus, within the crust, there will likely be a transition with depth between kyanite-dominated rocks through to sillimanite-dominated rocks, with some units in between containing both polymorphs.(c) The P-T evolution of high-P granulites crosses the solidus during retrograde metamorphism at around 7-8 kbar (Figure 7d; Ji et al., 2020;Wang et al., 2022), immediately after passing through the kyanite → sillimanite transformation.Density contrasts and the magnitude of deviatoric stress are two important factors that control the extent of melt migration in both the vertical and horizontal directions within a shear zone (Rabinowicz & Vigneresse, 2004).Thus, intergranular melt is likely to have been gradually squeezed out from the cooling and contracting granulite, with upward migration of melt occurring while in the kyanite stability field, but lateral motion would be preferred while at lower pressures and temperature in the suprasolidus sillimanite-bearing zone due to a horizontal gradient in lithostatic pressure.Together, we interpret that the P-T region constrained between muscovite dehydration melting and the polymorphic kyanite → sillimanite transition reaction causes a rheological change that promotes horizontal "flow" of melt in the deepest levels of the ALS shear zone.

Physical Properties of the Exhumed BMS Rocks and Evidence for Channel Flow in the Middle-Lower Crust
Lateral ductile flow of the middle-lower crust over long distances was likely inhibited by large-scale strike-slip faults adjacent to the SETP.These features also coincide with pronounced geophysical anomalies, including lower P-wave velocity, lower gravity anomaly, higher Poisson's ratio, and higher Vp/Vs values than ambient crust (Bao et al., 2015;Zhao et al., 2020).Most earthquakes along giant fault systems in the SETP occur in the upper crust, and are common in the northern part of ARSZ and the Xiao Jiang Fault system (Wang et al., 2020).Based on the calculated density, Vp and Vp/Vs, we discuss seismic imaging of present-day geophysical information of the crustal density structure (Shi et al., 2015), the velocity structure obtained from deep seismic wide-angle reflection/refraction (Wang et al., 2014;Wen et al., 2022), and Vp/Vs velocity ratio (Hou et al., 2023) across the ARSZ.

Density Variation and Earthquake Distribution
The density of rocks is a function of pressure, temperature, and chemical composition (Duesterhoeft et al., 2014;Semprich et al., 2010).Key metamorphic mineral reactions and phase transitions can therefore produce significant jumps in density within the crust, causing instability.To investigate the relationship between mineral phase transformations and density variations, we simulated a one-dimensional isobaric heating path for ALS rocks at 7 kbar (Figure 11).Rocks at this pressure (i.e., a depth of 20-25 km) may have been involved with crustal channel flow (Bai et al., 2010;Liu et al., 2014) and would have experienced several phase transformations due to variations in geothermal gradient (Figure 11a).For micaschists, density is positively correlated with metamorphic grade up to the point of partial melting, with chlorite dehydration at ∼580-600°C being significant (Figure 11a).Other increases in density are associated with garnet and staurolite formation.The modeling result of density is well consistent with systematic density simulation of metapelite in a large ranges of whole rock compositions (Semprich et al., 2010).The generation of partial melts due to reaction between plagioclase, sillimanite and quartz will dramatically decrease bulk-rock density.We plotted relocated earthquakes in the last 10 years (cate2019, http://www.cses.ac.cn/wp-content/uploads/2020/01/cata2019.txt)across the Yuanjiang-Mojiang section against density variations for four representative BMS samples (Shi et al., 2015).The peak metamorphic pressure of micaschists indicate exhumation from the middle crust (∼25-30 km), which coincides with the depth of greatest density variation in our models (Figure 12d).Most earthquakes along the Yuanjiang-Mojiang section are located at 10-15 km (Figure 12d), which coincides with the conditions of chlorite dehydration (i.e., at 3-4 kbar).The dehydration of chlorite across a narrow temperature interval is believed to be a major driving force for earthquakes in the upper crust (10-15 km) and for generating low-velocity/high conductivity layers in the middle crust (Gao et al., 1994).When compared against the steeply dipping Red River Fault, most earthquakes develop along the middle parts of the ALS fault and extend into the lower crust (Figure 12d).This suggests that the crust on the southwest side of the ARSZ is more brittle and the outline of the ARSZ may be a wedge shape.This matches observations that most large earthquakes occur within the boundaries of two low-velocity channels in southeast Tibet (Bao et al., 2015).

Seismic Wave Velocity and Vp/Vs Ratios
Petrophysical modeling along prograde P-T paths for ALS micaschists depicts a change in slope for bulk-rock Vp at 7.0-8.0kbar (∼25 km).All three micaschist metamorphic samples have a Vp of 6.1-6.2 km/s (Figures 7a-7c), which can be used to divide the upper and lower crust in deep seismic wide-angle reflection studies (Wang et al., 2014).This result implies that the upper part of the BMS was exhumed from a critical discontinuity zone within the crust.As high-P granulite was exhumed from ∼16.0 to 8.8 kbar, its bulk-rock Vp value decreased from 6.71 km/s to 6.35 km/s, which corresponds to the C3 boundary (∼30 km) in wide-angle reflection (Wang et al., 2014, Figure 12e).We interpret this result as showing that high-P granulite was first uplifted into a lower crust low-velocity zone and then was further exhumed during sinistral shearing in a second stage of exhumation.Indeed, our calculated P-T path for the granulite shows an isothermal decompression stage that transitions to decompression cooling at 8.0-9.0 kbar (Ji et al., 2020).Our observation of subvertical foliation in the high-P granulite also demonstrates that there was a component of perpendicular kinematic deformation during exhumation.
Based on our modeling, the Vp and Vs values of micaschists shows three distinct jumps at staurolite formation, sillimanite formation, and during first melting at the solidus (Figures 10b and 10c).In all micaschists, more gradual increases in seismic velocity are associated with the breakdown of hydrous minerals.The Vp displays a positive relationship with metamorphic grade (Figure 10b), although the kyanite-bearing sample exhibited the lowest Vs, and the other two micaschists have similar Vs value (Figure 10c).Moreover, different from the staurolite-and kyanite-bearing samples, the garnet-bearing micaschist shows an interruption at 490-560°C, which was related to garnet formation and zeolite decomposition.After zeolite decomposition, the rate of Vp increase in garnet-bearing micaschist also reduced.
Laboratory and geophysical studies demonstrate that the Vp/Vs ratio is more sensitive to the bulk-rock silica content than Vp or Vs alone (Sammon et al., 2020).In all three profiles across the ALSZ along latitude 23°N, 24°N and 25°N, distinct-scale high Vp/Vs anomalies at lower crust (30-40 km) are observed (Hou et al., 2023).For felsic granulite, higher feldspar contents produce a higher Vp/Vs value, while increased quartz contents result in a much lower Vp/Vs value (Kern, 1982).Traditionally, partial melting or an increase in mafic components (lower silica contents) may markedly elevate a rock's Vp/Vs or Poisson's ratio (Christensen, 1996;Kern, 1982;Kern et al., 2001).The Vp/Vs variation of progressive metamorphism of pelitic rocks correlates reasonably well with mineral reactions (Christensen, 1996).Specific to our simulated result, chlorite dehydration, kyanite transformation to sillimanite, and partial melting all significantly affected the Vp/Vs ratios (Figures 8 and 10d).Along the isobaric heating path, the Vp/Vs value initially increased during progressive metamorphism, it increased dramatically when staurolite stabilized at ∼580°C, then gradually decreases until crossing the solidus (Figure 10b).For the garnet-bearing micaschist, the Vp/Vs ratio initial increase began at plagioclase formation and terminated when zeolite disappeared.After chlorite was consumed at ∼600°C, the Vp/Vs ratio exhibits a slow increase which was distinct from the staurolite-and kyanite-bearing micaschist.
The micaschists in the BSM have a high quartz-feldspar ratio, which will induce a low Vp/Vs value in the upper crust, coinciding with geophysical observation (Hou et al., 2023).In contrast, after melt was extracted from the high-P granulite, quartz was consumed and the higher feldspar-quartz ratio increased the Vp/Vs value.In the presence of ∼8% melt, the lower crust may show high (>2.0)Vp/Vs values (Figure 8d), which is consistent with the lower limit value of melt accumulation in a migmatite before it can connect to form a free-flowing network (Jamieson et al., 2011;Watanabe, 1993).Here we conclude that conditions constrained by muscovite decomposition reactions and the Ky-Sil transition is a vital region that facilitates crustal flow development.At these P-T conditions, a rock's temperature, melt content, and physical properties are appropriate to form a low viscous sheet, but also difficult to maintain.In general, granulites are too viscous to flow, and so suitable lowdensity (or melt-bearing/buoyant) rocks are needed instead.

New Tectonic Evolution Model of the ARSZ
Our new data for the metamorphic evolution of crustal rock in the ARSZ (Ji et al., 2020(Ji et al., , 2021;;Liu, Wang, et al., 2015;Liu, Chen, et al., 2015;Liu et al., 2013;Wang et al., 2016a) has revealed P-T-ρ-Vp-d-t paths for garnet, staurolite, kyanite and sillimanite grade BMS rocks in the ALS massifs.The overall petrological, structural, simulation and geochronological results presented and discussed in this article enable us to divide the tectonic evolution processes of the ARSZ into pre-and syn-kinematic stages (Figures 12a-12c).
During the Early to Middle Eocene, the soft collision between India and Eurasia caused widespread thin-skinned thrust imbrication in the SE Tibetan Plateau (Figure 12a).At the boundaries of the blocks, such as the ARSZ, which separates the Sunda block and the SCB, crustal thickening may be more apparent due to weaker basement strength and apparent viscous discrepancy.Zircon and monazite growth in BMS units in the ARSZ that formed thrust faults preserve prograde metamorphic age.After ∼10-15 Myr of conductive thermal relaxations, mica experienced dehydration reactions and partly converted to sillimanite (Figure 12b), partial melting occurred in the eastern part of the ARSZ.Significant rheological weakening and density decreases led to uplift of lower crustal rocks to the weak zone (∼30 km), which is characterized by a high Vp/Vs ratio.Rapid isothermal decompression preserved kyanite and garnet in a metastable state and induced phase transform reactions ( 7), (8), and (9).Sinistral shearing along the ARSZ caused widespread overprinting of previous tectonic processes (Figure 12c) and decompression cooling is documented by monazite in high-P granulite recorded the exhumation of rocks through the solidus (∼750°C) at the onset of ARSZ sinistral shearing.

Conclusions
A >250 km long, high-pressure granulite belt characterized by kyanite decompression reactions has been identified for the first time on the northeast side of the ARSZ.Micaschists in the southwest side of the ALS massif were exhumed from the middle crust by thrusting during the Eocene, while high-pressure granulites experienced incipient decompressional partial melting and exhumation accompanied by melt ascent and sinistral shear.We also find that the northeast edge of the ALS massif was uplifted from at least ∼25 km deeper crustal level than the southwest side.Thermobarometry shows that the middle crust recorded a ∼200-300°C difference in peak metamorphic temperatures between the northeast and southwest edge of the ARSZ when sinistral shearing initiated.Therefore we conclude that the Red River Fault generated a thermal perturbation and allowed exhumation to be focused in the ALS segment since the early Eocene.Moreover, we suggest that the region of P-T space delimited by muscovite dehydration and the kyanite to sillimanite polymorphic transition may have promoted lateral melt migration in lower crust due to formation of a less permeable "cap" in the middle crust.The inverted nature of the BMS in the ARSZ provides valuable clues to elucidate its tectonic evolution and the dynamic processes that operate in the middle-lower crust during crustal-scale shearing.

Figure 2 .
Figure 2. (a) Field photographs showing garnet-bearing micaschist thrust onto weakly deformed granite.(b) Reverse fault in granite near the ALS Fault.(c) Steeply dipping staurolite-bearing micaschist and the locations of fresh samples collected by drilling.(d) Field photographs showing the boudinaged structure of competent layers in micaschist, which indicate sinistral shearing.The pink dotted box marks the area of coarse garnet porphyroblasts.(e) Enlarged view of the yellow box in figure (c) showing the asymmetric folds and σ-style boudinage in kyanite-bearing micaschist.(f) Intense ductile deformation and vertical extrusion of melt that has developed near to the Red River Fault.(g) Subhorizontal leucosome layers with strong mylonitic lineation interlayered with melanosome gneiss which show typical "δ" and "σ" Kfeldspar porphyroclasts, indicate left-lateral shearing.(h) Enlarged area of the red box in figure (g) shows two types of shear criteria in granitic rock.(i) Two episodes of lineations in garnet-bearing micaschist.

Figure 3 .
Figure 3. Microphotographs showing petrographic characteristics of the Barrovian metamorphic sequence rocks.(a) Deformed garnet megacrysts in micaschist containing inclusions of epidote, rutile, and ilmenite.The dashed line A-B shows the location of the zoning profile obtained via EPMA.(b) Fine-grained staurolite that overgrew muscovite in the chlorite-garnet-staurolite micaschist.(c) Diablastic texture of staurolite in the two-mica schist and the large angle of intersection between the external foliation and the orientation of inclusions.(d) Kyanite with inclusions of biotite, which coexists with staurolite in micaschist.(e) The rim of prismatic kyanite transform to sillimanite and accompany with biotite and K-feldspar.(f) Kyanite-garnet micaschist shows S-C fabric which indicated sinistral movement and the rotated garnet shows inverse direction.(g) Piliform sillimanite accompany with biotite.(h) Kyanite relicts surrounded by plagioclase in sillimanite gneiss.(i) Metastable kyanite included in garnet in garnet-sillimanite-K-feldspar gneiss.(j) Embayed kyanite included in garnet and matrix are dominated by sillimanite and K-feldspar. Figure parts (b), (e) and (f) are plane-polarized light, (c), (g), and inset figures in (h) and (j) are cross-polarized light, and other parts are BSE images.

Figure 4 .
Figure 4. Quartz c-axis fabrics measured by EBSD from the staurolite-bearing micaschist.(a) Pole figure of quartz inclusions in staurolite and (b) pole figure of quartz in the matrix.(c) Misorientation figure of staurolite in the micaschist.

Figure 5 .
Figure 5. X-ray compositional maps of representative garnet in the (a) garnet zone, (b) staurolite zone, and (c) kyanite zone.

Figure 7 .
Figure 7. Petrological modeling results for BMS rocks from the ALS massif.(a) P-T pseudosection and metamorphic evolution paths of the (a) garnet zone sample, (b) staurolite zone sample, (c) kyanite zone sample and (d) sillimanite K-feldspar zone sample.

Figure 8 .
Figure 8.(a) Petrological modeling results for the BMS rocks from the ALS massif.(a) Variation of mineral proportions along the P-T paths of BMS rocks.Changes in bulk-rock (b) density, (c) Vp, and (d) Vp/Vs ratio during phase transformations with depth in BMS rocks.

Figure 9 .
Figure 9. Representative CL images and U-Pb concordia diagram of zircon and monazite from BMS rocks and granitic intrusion in the ALS massif.

Figure 10 .
Figure 10.(a) Compilation of age data and (b) P-T-t-d paths relevant to BMS units of the Ailao Shan metamorphic complex.Thin dashed lines show data for samples that were analyzed with multiple analysis methods.The solidus in (b) is based on that for a high Al 2 O 3 metapelite from White et al. (2014), the Al 2 SiO 5 phase transformation lines (solid) are fromHoldaway and Mukhopadhyay (1993), whereas the dashed gray line for sillimanite-andalusite is fromPattison (1992).

Figure 12 .
Figure 12.Schematic showing the (a) crustal thickening, (b) regional extension and Ky = Sill transition zone formation, (c) and left-lateral strike-slip evolution model of the Ailao Shan metamorphic complex.Present-day geophysical observations along the Yuanjiang profile are shown for (d) density structure, and (e) velocity structure.