Ailao Shan Shear Zone Research Articles

Clockwise extrusion of the Sichuan‒Yunnan block toward the Red River Fault in the southeastern Tibetan Plateau

The Xiaojiang Fault and the Red River Fault, which are located on the southeastern margin of the Tibetan Plateau, are the eastern and southern boundaries of the Sichuan–Yunnan Block, respectively. The relationship between these two faults is highly important for the study on the tectonic evolution and seismic risk of the southeastern margin of the Tibetan Plateau. Using the digital elevation model (DEM) data, we extracted and analyzed the maximum elevation, peak steepness index, maximum slope, and relatively flat surface of the Ailaoshan Shear Zone on the southwest side of the Red River Fault. The results revealed that the geomorphic indices result around Yuanjiang were significantly lower than those on the northern and southern sides of Yuanjiang. On the basis of lithology, climate and tectonics, it is inferred that tectonic activity is the main factor controlling landform development. On the basis of existing geophysical and geomorphic survey results in this area, a geodynamic model of this area was constructed: in the mid-Miocene, tectonic inversion of the Red River Fault occurred, and material from the Tibetan Plateau flowed into the Sichuan‒Yunnan Block around the East Himalayan Syntax. The resulting extrusion caused the Red River Fault to bend of ∼50 km; in the early Pliocene, the middle and lower crust broke through the barrier and entered the interior of the Ailaoshan Shear Zone. Because of the extrusion of the middle and lower crust, the role of the Red River Fault as the boundary has weakened.

A magnetotelluric study of 3D electrical resistivity structure underneath the southern segment of the Red River fault zone, South China

• Imaged 3D resistivity structure beneath the southern Red River (RRF) and Xiaojiang Faults (XJF). • The southern RRF was shown electrically resistive and proposed mechanically strong. • The XJF shows a subvertical fault conductor zone under its eastern branch fault. • The southern RRF may have no large earthquakes in the future based on our results. The Red River Fault (RRF) is a continental-scale strike-slip fault forming the southwestern boundary of the Chuan-Dian block in the southeastern Tibetan Plateau. The kinematics and seismicity along the RRF are under debate over the years. Here, based on broadband magnetotelluric array data, we present a fine 3D electrical resistivity model in the southern segment of the RRF and its adjacent Xiaojiang fault zone (XJFZ). Our MT model shows a prominent electrical resistor extending to a depth of ∼20 km beneath the RRF and Ailao Shan shear zone. The southern RRF is imaged as a resistive fault zone, indicating no crustal fluids within this fault. The XJFZ represents a sub-vertical fault conductor zone bounded by bilateral crustal resistive blocks but a sharp contrast of resistivity across its western branch. The fault conductor zone along the eastern branch of the XJFZ ends at the resistive RRF, indicating the XJFZ may not develop beyond the RRF southward. Our MT model indicates that the resistive southern RRF is possibly not capable of occurring large seismicity in the future, but the southern end of the XJFZ is with seismic risk potential.

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.

Contrast between metamorphic and ore-forming fluids in the Ailaoshan belt, southeastern Tibet: New constraints on ore-fluids source for its orogenic gold deposits

The source of ore-forming fluids for orogenic gold deposits is still disputed with models ranging from derivation from metamorphism of continental crust to derivation from metasomatized mantle lithosphere. This dispute is addressed by this study of the Ailaoshan orogenic gold belt, southeastern Tibet, whose evolution involved intense metamorphism on the eastern side of the Ailaoshan shear zone. Both the regional metamorphic barren quartz veins and gold-related quartz veins in this belt are genetically related to movement on the Ailaoshan shear zone and thus formed over the same limited age span. Microthermometry, fluid compositions and stable isotope analyses conducted on both metamorphic fluids extracted from regional metamorphic quartz veins and ore-forming fluids collected from auriferous quartz veins at Zhenyuan and Jinchang gold deposits reveal significant differences. These include: (1) regional metamorphic quartz veins contain three types of fluid inclusions, including CO2-rich, aqueous-CO2 and aqueous types. The CO2-bearing inclusion types are more abundant than those in auriferous quartz veins; (2) those fluids from barren metamorphic veins with high CO2 content (up to 45 mol%) are typified by low-medium temperature (217–418 °C), low salinity (2–8 wt% NaCl) and variable pressure (28–440 MPa), and belong to the NaCl-CO2-H2O system. In contrast, the ore-forming fluids, which exhibit lower temperature (mainly 170–270 °C), higher salinity (mainly 5–13 wt% NaCl), and lower pressure (20–282 MPa and 14–44 MPa for Zhenyuan and Jinchang, respectively), belong to the NaCl-H2O-CO2 system; (3) the metamorphic fluids contain high Ca2+ and Mg2+ in liquids and CO2, N2 and CO in gases, whereas the ore-forming fluids are enriched in K+ in liquid and H2O, CO2, CH4 in gases; (4) the pyrites from regional metamorphic quartz veins have δ34S values ranging from 2.08 to 2.86‰ (mean = 2.50‰), indicating a single sulfur source of metamorphic fluids, thus contrasting with the broad range of δ34S values of ore sulfides. These distinctions negate a genetic relationship between crustal metamorphism and gold mineralization and demonstrate that ore-fluids that deposit orogenic gold deposits, at least in the Ailaoshan belt cannot be derived or evolved from crustally-derived regional metamorphic fluids. This provides a new line of evidence for the derivation of ore-forming fluids from a sub-crustal, metasomatized mantle lithosphere source for at least some orogenic gold deposits.

Source and evolution of ore fluids in the Zhenyuan orogenic gold deposit, SE Tibet: Constraints from the S-C-O isotopes

Orogenic gold deposits commonly have large variation of stable isotopic values of ore minerals induced by complicated ore fluid evolution, which was considered to provide equivocal evidence for ore sources. In contrast, this study has revealed relatively narrow range of S and C-O isotopes of pyrites and carbonate minerals in the Zhenyuan gold deposit (>100 t, at 3.5 g/t) on Ailaoshan shear zone, southeastern margin of Tibet. The Zhenyuan deposit is a disseminated orogenic gold deposit with mineralization and alteration occurring in various types of wall rocks including carbonaceous slate, meta-quartz sandstone, lamprophyre, quartz porphyry and meta-basalt. The pre-ore pyrites include the diagenetic pyrites in carbonaceous slate and meta-quartz sandstone and the barren hydrothermal pyrites in quartz porphyry. Diagenetic pyrites have much varied δ34S values from −54.58 to 38.31‰, and barren hydrothermal pyrites have elevated positive δ34S values (8.76–43.24‰). The ore-related pyrites, disseminated in all types of host rocks and commonly with repetitive gold-rich zones, are the most important gold-bearing minerals. They contain high concentration of Au along with other trace elements, including Cu, As, Sb, Tl, Pb and Bi. Little difference of δ34S values is observed for the pyrites in different types of host rocks and for the low-Au and high-Au zones in single pyrite grain. All ore-related pyrites display a relatively narrow range of δ34S values (−5.22 to 4.16‰) with a peak at ~0‰. This narrow range implies that the deposition of gold from ore fluids at Zhenyuan deposit was mainly controlled by sulfidation of iron-bearing minerals in the wall rocks. The δ34S values of ore-related pyrites are consistent with the explanation that the ore fluids were from the subcrustal source, instead of the crustal source with much varied sulfide δ34S values. Carbonate minerals from different types of host rocks also have consistent and relatively narrow range of δ13CPDB (−3.77 to 0.68‰) and δ18OSMOW (14.85–20.77‰) values, indicating that the composition of ore fluids were little affected by water-rock reaction at deposit scale. The δ13CPDB and δ18OSMOW values of carbonates minerals are between those of the mantle peridotite and marine carbonates, which can be explained by that the ore fluids have experienced crustal carbonate contamination in the subcrustal level or/and along the migration conduit of the ore fluids.

Petrogenesis and tectonic significance of the Fenshuiling and Mengha granitic plutons, SW China: insights from bulk elements, zircon U-Pb ages and Sr-Nd-Hf isotopes

ABSTRACT The Ailaoshan shear zone developed plenty of Cenozoic granitic plutons, located between the Yangtze block in the east and the Simao block in the west. Although a mass of previous studies have enriched the understanding of petrogenesis of these plutons, there are few reports on the plutons within the Mojiang-Lvchun terrane, a suspect domain between the Yangtze and Simao blocks. Among two plutons investigated in this paper, the Fenshuiling pluton is characterized by strongly shoshonitic (K20 = 5.8–6.4 wt.%) nature, high Y (6.5–30.0 ppm) and Yb (0.88–2.97 ppm) contents, and has limited variations in La/Yb (8.6–20.5) and Dy/Yb (1.6–1.9) with increasing SiO2. In Mengha, the pluton is also strongly shoshonitic (K20 = 5.7–6.1 wt.%), high Y (18.7–32.3 ppm) and Yb (2.03–3.50 ppm) contents, and shares limited variations in La/Yb (17.0–17.6) and Dy/Yb (1.5–1.7) with increasing SiO2. These tendencies correspond with the differentiation from a shoshonitic mafic melt by olivine, clinopyroxene and plagioclase (gabbroic) dominated fractional crystallization. These plutons mainly composed of quartz syenites are characterized by island-arc basalt (IAB) patterns, which are consistent with those evolved by shoshonitic mafic melt derived from metasomatic lithospheric mantle within the Yangtze block. Zircon εHf(t) values of Fenshuiling and Mengha plutons range from −4.1 to −0.7 and −2.8 to −1.1, respectively, and their whole-rock εNd(t) values are −5.0 to −4.6 and −4.7 to −4.6, respectively. These Hf isotopes are similar to those from Neoproterozoic granitoids caused by a persistent Neoproterozoic subduction around the southwestern margin of the Yangtze block. This similarity supports a scenario that the enriched lithospheric mantle source area of these plutons caused by the Neoproterozoic subduction, also means that the suspect terrane belongs to the Yangtze block. The location of the Fenshuiling and Mengha plutons separated from those within the western margin of the Yangtze block is attributed to Cenozoic sinistral shearing events.

Evolution of the Miocene Ailaoshan orogenic gold deposits, southeastern Tibet, during a complex tectonic history of lithosphere-crust interaction

The orogenic gold deposits along the NNW-trending Ailaoshan shear zone are formed during Miocene continental collision, after a complex history of Neoproterozoic subduction, Late Triassic amalgamation between the Indochina and South China blocks after closure of the Paleo-Tethyan Ocean, and Oligocene-Eocene continental-scale shearing related to the Indian-Eurasian continental collision. As a result, the eastern Neoproterozoic basement with ductile shear zones was thrust over the western metasedimentary rocks and Paleo-Tethyan ophiolites affected by brittle faulting. These tectonically juxtaposed successions were underlain by lithosphere metasomatized in the Neoproterozoic. The gold-hosting western succession is characterized by tightly folded Silurian to Permian metasedimentary rocks, overlain by gently folded Late Triassic metasedimentary rocks, and cut by secondary NW-trending shear zones. This crustal architecture controlled migration of ore fluid towards the cessation of regional ductile deformation at ca. 22 Ma, with major ductile shear zones serving as the main conduit, tight folds, and jogs or intersections on subsidiary brittle shear zones as the fluid trap, and Late Triassic rocks as the seal. Gold-rich rims on disseminated gold-poor sedimentary pyrite with a limited δ34S range from − 3 to 3‰, pyrite He-Ar isotope ratios, and the timing of mineralization within retrograde metamorphism suggest an important metal contribution from mantle lithosphere. The auriferous fluid, derived from previously metasomatized mantle lithosphere, is interpreted to have advected up the Ailaoshan shear zone into the metasedimentary sequence during mantle upwelling, clearly contrasting with widely accepted crustal metamorphic models for Phanerozoic orogenic gold deposits.

Alteration of Eocene lamprophyres in the Zhenyuan orogenic gold deposit, Yunnan Province, China: Composition and evolution of ore fluids

The Zhenyuan orogenic gold deposit (>100 t Au), located in the northern part of the Ailaoshan shear zone (ASSZ) in South China, is characterized by extensive alteration and mineralization in a variety of host rocks. In the deposit, abundant Eocene lamprophyres have been infiltrated by ore fluids to produce carbonate veins with zones of disseminated fine-grained gold-bearing pyrite and arsenopyrite on both margins of the veins. Primary phlogopite, K-feldspar and magnetite have been altered to carbonate, sericite and fuchsite. Carbonate minerals and sericite in altered lamprophyre have higher Mg contents than in other host rocks, reflecting the primitive geochemistry of the lamprophyres. Since carbonatization is the primary alteration type in the lamprophyres, whose main primary components are low-Ca phlogopite and K-feldspar, the CaO content of the lamprophyres reflects their degree of alteration. As alteration increases, incompatible element ratios remain essentially constant whereas Sr is enriched with Ca. In addition, Au with Co, Ni, As, Ag, Sb, and Bi are enriched, reflecting the addition of these metal elements during alteration of the lamprophyres. Altered lamprophyres have slightly lower 143Nd/144Nd (t) (0.51240–0.51253) and substantially higher 87Sr/86Sr (t) (0.70716–0.71234) than unaltered or weakly-altered lamprophyres, indicative of crustal contamination. The auriferous ore-forming fluids are thus interpreted to have migrated upwards and interacted with the sedimentary host rocks before altering the lamprophyres. Lamprophyres commonly show a close spatial and commonly temporal association with orogenic gold deposits worldwide. At Zhenyuan, the lamprophyres shared a common deep source to the auriferous ore fluids but did not themselves contribute to this fluid or its gold content.

Alteration and mineralization styles of the orogenic disseminated Zhenyuan gold deposit, southeastern Tibet: Contrast with carlin gold deposit

Orogenic disseminated and Carlin gold deposits share much similarity in alteration and mineralization. The disseminated orogenic Zhenyuan Au deposit along the Ailaoshan shear zone, southeastern Tibet, was selected to clarify their difference. The alteration and mineralization from the different lithologies, including meta-quartz sandstone, carbonaceous slate, meta-(ultra)mafic rock, quartz porphyry and lamprophyre were researched. According to the mineral assemblage and replacement relationship in all types of host rocks, two reactions show general control on gold deposition: (1) replacement of earlier magnetite by pyrite and carbonaceous material; (2) alteration of biotite and phlogopite phenocrysts in quartz porphyry and lamprophyre into dolomite/ankerite and sericite. Despite the lamprophyre is volumetrically minor and much less fractured than other host rocks, it contains a large portion of Au reserve, indicating that the chemically active lithology has played a more important role in gold precipitation compared to structure. LA-ICP-MS analysis shows that Au mainly occurs as invisible gold in fine-grained pyrite disseminated in the host rocks, with Au content reaching to 258.95 ppm. The diagenetic core of pyrite in meta-quartz sandstone enriched in Co, Ni, Mo, Ag and Hg is wrapped by hydrothermal pyrite enriched in Cu, As, Sb, Au, Tl, Pb and Bi.Different host rock lithology has much impact on the alteration and mineralization features. Carbonate and sericite in altered lamprophyre show they have higher Mg than those developed in other of host rocks denoting that the carbonate and sericite incorporated Mg from phlogopite phenocrysts in the primary lamprophyre during alteration. The ore fluid activated the diagenetic pyrite in meta-quartz sandstone leading the hydrothermal pyrite enriched in Cu, Mo, Ag, Sb, Te, Hg, Tl, Pb and Bi, but the hydrothermal pyrite in meta-(ultra)mafic rock is enriched in Co and Ni as the meta-(ultra)mafic rock host rock contain high content of Co and Ni. However, Au and As shear similar range in both types of host rocks indicating that these two elements most likely come from the deep source fluid rather than the host rocks. It was shown in the disseminated orogenic gold deposit that similar hydrothermal alteration with mineral assemblage of carbonate (mainly dolomite and ankerite), sericite, pyrite and arsenopyrite develops in all types of host rocks. This is different from the Nevada Carlin type, in which alteration is mainly dissolution and silicification of carbonate host rock. On the other hand, Au mainly occur as invisible gold in both disseminated orogenic and Carlin gold deposits.

Late Cenozoic landscape evolution along the Ailao Shan Shear Zone, SE Tibetan Plateau: Evidence from fluvial longitudinal profiles and cosmogenic erosion rates

In tectonically active regions, geomorphic features, such as catchment slopes, terraces, and river profiles can be interpreted in the context of tectonic and climatic forcing; however, distinguishing tectonic impacts from other factors such as pre-existing geologic complexities and climate changes is challenging. We use fluvial longitudinal profiles, catchment slopes, and catchment mean erosion rates derived from in-situ cosmogenic 10Be and 26Al to examine the late Cenozoic landscape evolution of the Ailao Shan Shear Zone (ASSZ) in the southeastern Tibetan Plateau. The trunk stream of the Red River, flowing along the eastern side of the shear zone, consists of three sections with distinct channel parameters, separated by knickzones (the Midu, Ejia, and Nansha sections from NW to SE). Tributaries to the Red River within the Ailao Shan Shear Zone in the Ejia and Nansha sections consistently display two channel segments (upper low-gradient and middle steep channel segments); a third set of lower, less steep channel segments are identified only along the tributaries in the Nansha section. Catchment mean erosion rates contrast sharply along strike: ca. 300 m/Myr in the Ejia section and ca. 100 m/Myr in the Nansha section. Collectively, our results provide strong evidence that: 1) two waves of incision induced by pulsed and declining regional uplift are propagating up the Red River shaping the background pattern of river incision; 2) vertical fault displacements, river reorganization and additional factors locally affect river profile morphology. Normalized steepness indices (ksn), catchment slopes, and knickzone distribution vary systematically along the Ailao Shan Shear Zone, indicating long-wavelength regional surface uplift during plateau growth in the middle-late Miocene, which points to a tectonic model involving crustal thickening and diffuse or continuous deformation in the southeastern margin of the Tibetan Plateau.

Tracing escaping structure in the Northern Indo-China Peninsula by Openness and remote sensing

The Openness based on DEM emphasizes the terrain convexity and concavity. It facilitates the interpretation of detailed landforms on the Earth’s surface. Compared with the layer stacking of ETM+ with less three-dimensionality and visualizability and with indefinite details of linear images in the deep cutting or deep covered region, the Openness is used for accurate interpretation of tectonic geomorphic units and linear structures. In this paper, the ETM+ images (741 RGB) and RRIM based on Openness combined with the field geological investigation are used to trace the escaping structure in SE Asia. The east boundary is Ailaoshan shear zone and the west boundary is Uttaradit-Dien Bien Phu fault, which together form the southwards extruding wedge block. The arc boundary surface of the southern Khorat Plateau is jutted to the north. The NW and NE sides of Khorat Plateau are traversed by Uttaradit-Dien Bien Phu fault and Thakhek-Da Nang fault, respectively, resulting in a blocked escaping structure. The SE margins of Truong Son structure belt and Song Ma structure belt are both arcs jutting to SE. These arc structures clamped by faults or related to the fault on one side indicating the material flow direction obviously, are the most specific manifestation of escaping structures. Moreover, these push units are extruded from south to north successively.

Structural geometry and kinematics of the Ailao Shan shear zone: insights from integrated structural, microstructural, and fabric studies of the Yao Shan complex, Yunnan, Southwest China

ABSTRACTThe Yao Shan complex, a massif near the southern segment of the Ailao Shan–Red River (ASRR) shear zone, bears important information on the structural framework of the massif and the kinematics of ductile shearing along the ASRR shear zone. In this contribution, structural, microstructural, quartz c-axis fabric, magnetic fabric, and geochronologic data are used to determine the structural framework of the Yao Shan massif and its tectonic implications for the ASRR shear zone. The Yao Shan complex is characterized by an overall linear A-type antiform that contains a core of high-grade metamorphic rocks with Palaeoproterozoic to Mesozoic protoliths and a mantle of Permo-Triassic low-grade rocks. Both the high-grade metamorphic core and low-grade Permo-Triassic rocks have experienced progressive ductile shearing. Anisotropy of magnetic susceptibility (AMS) results from 17 samples collected along the Xinjie–Pingbian section across the complex show that magnetic lineation (Kmax) and foliation (Kmax–Kint) are generally subparallel to the corresponding structural elements in the sheared rocks. The shape parameter E values of the magnetic ellipsoids are indicative of dominantly oblate and plane strain, but vary with protolith type and degree of strain among the various rock types. In agreement with the field and microstructural observations, the corrected degree of anisotropy (Pj) values reflect high shear strain in the core rocks and relatively low shear strain in the low-grade strata. A kinematic analysis based on structural and magnetic fabric data shows that both left- and right-lateral shear occurred during the deformation of the Yao Shan complex. Therefore, instead of being an element of the ASRR shear zone, the Yao Shan complex constitutes a crustal-scale inharmonic A-type fold with a fold axis parallel to the stretching lineation. Geochronologic data reveal that the folding occurred coevally with ductile shearing of the middle to lower crust between ca. 30 and 21 Ma.

The boundary between the Simao and Yangtze blocks and their locations in Gondwana and Rodinia: Constraints from detrital and inherited zircons

The Simao block, which is the northern extension of the Indochina block, and the Yangtze block were amalgamated by the closure of a Paleo-Tethys branch ocean commonly referred to as the Ailaoshan ocean. The exact location of the suture between these two continental blocks has been contested. Some researchers believed that it is located within the Cenozoic Ailaoshan shear zone whereas other researchers thought that it is present >50km farther to the west. The detrital and inherited-xenocrystic zircons from Paleozoic sedimentary rocks and a Triassic granite dyke respectively in the Laowangzhai–Mojiang suspect terrane, which occurs between the two suggested locations for the same suture, show that neither of these previously suggested locations is accurate. The age distribution pattern of detrital and inherited-xenocrystic zircons from the Laowangzhai–Mojiang suspect terrane is similar to that of detrital zircons from the Simao–Indochina block but different from that of those from the Jianshui area in the western margin of the Yangtze block. This indicates that the Laowangzhai–Mojiang suspect terrane belongs to the Simao–Indochina block. The actual location of the suture between the Simao–Indochina and Yangtze blocks is suggested to superimpose the Ailaoshan late-Devonian to early-Carboniferous ophiolite belt within the suspect terrane. The new interpretation is also supported by Hf isotopes of detrital zircons from the related continental blocks. The age distribution pattern of Precambrian detrital zircons from the western part of the Yangtze block is different from that of those from Australia or Tethyan Himalaya (the northern part of the Indian continent), indicating that the Yangtze block was not an integral part of Australia or India in Rodinia. The age distribution pattern of pre-Carboniferous detrital zircons from the Simao–Indochina block is remarkably similar to that of those from the Tethyan Himalaya, indicating that the Simao–Indochina block was derived from Indian Gondwana.

Paleogene sedimentation, volcanism, and deformation in eastern Tibet: Evidence from structures, geochemistry, and zircon U–Pb dating in the Jianchuan Basin, SW China

The Cenozoic tectonics of southeastern Tibet feature several large-scale strike-slip ductile shear zones along block boundaries and complex rotation and internal deformation of the blocks that are widely thought to be the result of India–Eurasia collision. The way in which we can match the rotation and internal deformation with the strike-slip movement remains an open issue. The easternmost of these shear zones, the 1000-km-long Ailao Shan/Red River sinistral shear zone, consists of three segments, and the gaps between the segments are occupied by the Chuxiong and Jianchuan Basins. Geologic data from these basins may provide positive constraints on the nature of Cenozoic deformation of continental crust of southeastern Tibet, but few data are available. This paper documents structural, geochronological, and geochemical results to constrain the tectonics of the Cenozoic Jianchuan Basin, eastern Tibet. Detailed field observations along a transect across the Jianchuan Basin revealed a horizon of volcanic rocks that overlies thick massive Eocene mudstones and siltstones, and which in turn is overlapped by coarse-grained Oligocene sandstones and conglomerates. The Late Eocene volcanics are shoshonitic and high in magnesium, and they consist of trachybasalts, trachyandesites, and trachydacites. In situ zircon U–Pb dating demonstrates that the volcanism was short-lived from 35 to 36Ma. Structural data reveal that some of the basement of the Jianchuan Basin has been uplifted, and then thrust over the Cenozoic sediments. Mudstone dikes developed along the main fault plane, and diapirs developed along other minor fractures of the hanging wall. Except for a few gentle folds, no solid-state deformation-induced structures are found in the Cenozoic strata, whereas syndepositional structures are common. These observations suggest that the basement uplift, thrusting, and sedimentation were coeval. Widespread pervasive NW–SE striking vertical fracture-cleavages in the basement suggest that the syndepositional uplift and thrusting were caused by NE–SW directed compression. These structures are distinct from ductile deformational features associated with the sinistral Ailao Shan shear zone. Available data revealed that the ductile shearing lagged ca. 10Ma behind the NE–SW compression. Our data thus suggest that deformation was widely distributed within continental block(s) at an early stage of the India–Eurasia collision, and then localized into some ductile shear zones likely due to block rotation. New zircon U–Pb geochronology and geochemistry of the igneous rocks in the basement show that these igneous rocks formed during the Early Triassic and the earliest Middle Triassic, and not as previously thought during the Late Triassic; they are therefore probably part of the Jomda–Weixi arc rather than the Yidun arc.

Cenozoic high-K alkaline magmatism and associated Cu–Mo–Au mineralization in the Jinping–Fan Si Pan region, southeastern Ailao Shan–Red River shear zone, southwestern China–northwestern Vietnam

The Jinping–Fan Si Pan (JFP) Cenozoic magmatic and Cu–Mo–Au metallogenic belt in the southeastern part of the Ailao Shan shear zone host the Tongchang, Chang′an, Habo, and Chinh Sang Cu–Mo–Au deposits. These deposits form an integrated epithermal-porphyry regional mineralization system associated with 40–32Ma high-K alkaline magmatism. The magmatic rocks in the belt have relatively low TiO2 (<0.73wt%), P2O5 (<0.29wt%), and FeO* (<4.99wt%), and high Na2O (2.86–4.75wt%) and K2O (4.01–7.98wt%). They also have high contents of incompatible trace elements, and are enriched in LILE (Rb, Ba, K, Sr) and LREE. They have marked Nb, Ta, Ti and P depletion in primitive mantle-normalized spidergrams, and plot close to the EMII mantle field in the Sr–Nd isotopic diagram. These characteristics are similar to those of the Eocene high-K alkaline rocks along the northern Ailao Shan belt, eastern Tibet plateau. The sulfur and lead isotope analyses of sulfide minerals from both the ores and related magmatic rocks confirm the involvement of a magmatic ore fluid. The Cenozoic alkaline intrusions and Cu–Mo–Au mineralization in the JFP were formed prior to the initiation of left-lateral shearing along the Ailao Shan shear zone. The magmas appear to have been derived from enriched mantle, possibly with mixing of materials from the buried Tethyan oceanic lithosphere, and/or crust.

Timing of left-lateral shearing along the Ailao Shan-Red River shear zone: constraints from zircon U–Pb ages from granitic rocks in the shear zone along the Ailao Shan Range, Western Yunnan, China

As the boundary between the Indochina and the South China blocks, the Ailao Shan-Red River (ASRR) shear zone underwent a sinistral strike-slip shearing which is characterized by ductile deformation structures along the Ailao Shan range. The timing issue of left-lateral shearing along the ASRR shear zone is of first-order importance in constraining the nature and regional significance of the shear zone. It has been, therefore, focused on by many previous studies, but debates still exist on the age of initiation and termination of shearing along the shear zone. In this paper, we dated 5 samples of granitic plutons (dykes) along the Ailao Shan shear zone. Zircon U–Pb ages of four sheared or partly sheared granitic rocks give ages of 30.9 ± 0.7, 36.6 ± 0.1, 25.9 ± 1.0 and 27.2 ± 0.2 Ma, respectively. An undeformed granitic dyke intruding mylonitic foliation gives crystallization age of 21.8 ± 1 Ma. The Th/U ratios of zircon grains from these rocks fall into two populations (0.17–1.01 and 0.07–0.08), reflecting magmatic and metamorphic origins of the zircons. Detailed structural and microstructural analysis reveals that the granitic intrusions are ascribed to pre-, syn- and post-shearing magmatisms. The zircon U–Pb ages of these granites provide constraints on timing of the initiation (later than 31 Ma from pre-shearing granitic plutons, but earlier than 27 Ma from syn-shearing granitic dykes) and termination (ca. 21 Ma from the post-shearing granitic dykes) of strong ductile left-lateral shearing, which is consistent with previous results on the Diancang Shan and Day Nui Con Voi massifs in the literature. We also conclude that the left-lateral shearing along the ASRR shear zone is the result of southeastward extrusion of the Indochina block during the Indian–Eurasian plate collision. Furthermore, the left-lateral shearing was accompanied by the ridge jump, postdating the opening, of the South China Sea.

Young, active conjugate strike–slip deformation in West Sichuan: evidence for the stress–strain pattern of the southeastern Tibetan Plateau

The active kinematics of the eastern Tibetan Plateau are characterized by the southeastward movement of a major tectonic unit, the Chuan-Dian crustal fragment, bounded by the left-lateral Xianshuihe–Xiaojiang fault in the northeast and the right-lateral Red River–Ailao Shan shear zone in the southwest. Our field structural and geomorphic observations define two sets of young, active strike–slip faults within the northern part of the fragment that lie within the SE Tibetan Plateau. One set trends NE–SW with right-lateral displacement and includes the Jiulong, Batang, and Derong faults. The second set trends NW–SE with left-lateral displacement and includes the Xianshuihe, Litang, Xiangcheng, Zhongdian, and Xuebo faults. Strike–slip displacements along these faults were established by the deflection and offset of streams and various lithologic units; these offsets yield an average magnitude of right- and left-lateral displacements of ∼15–35 km. Using 5.7–3.5 Ma as the time of onset of the late-stage evolution of the Xianshuihe fault and the regional stream incision within this part of the plateau as a proxy for the initiation age of conjugate strike–slip faulting, we have determined an average slip rate of ∼2.6–9.4 mm/year. These two sets of strike–slip faults intersect at an obtuse angle that ranges from 100° to 140° facing east and west; the fault sets define a conjugate strike–slip pattern that expresses internal E–W shortening in the northern part of the Chuan-Dian crustal fragment. These conjugate faults are interpreted to have experienced clockwise and counterclockwise rotations of up to 20°. The presence of this conjugate fault system demonstrates that this part of the Tibetan Plateau is undergoing not only southward movement, but also E–W shortening and N–S lengthening due to convergence between the Sichuan Basin and the eastern Himalayan syntaxis.

Geometry, kinematics, and regional significance of the Chong Shan shear zone, Eastern Himalayan Syntaxis, Yunnan, China

The geology east of the Eastern Himalayan Syntaxis is poorly known, although it figures prominently in many models for the Cenozoic tectonics of the India-Eurasia collision and subsequent intracontinental deformation. The little known Chong Shan shear zone a ∼250-km–long and ∼10-km–wide metamorphic belt composed mainly of mylonitic augen gneisses and migmatites, forms a major shear zone developed during early Cenozoic extrusion of the Indochina crustal (lithospheric?) fragment. Foliation within the shear zone is moderately to steeply west dipping, and stretching lineations are subhorizontal, consistent with dominantly strike-slip transport. Kinematic indicators including rotated porphyroclasts, S-C fabrics, and asymmetric folds provide evidence for both dextral and sinistral movements. Our preliminary geo-chronological studies indicate that the Chong Shan shear zone has been active since at least ca. 34 Ma, and perhaps as early as 41 Ma. Strike-slip shearing continued at least until ca. 29 Ma, perhaps as late as ca. 24 Ma, and terminated by ca. 17 Ma. The Chong Shan shear zone, therefore, is not a belt of Precambrian metamorphic rocks as previously interpreted, but a Cenozoic shear zone of great significance, which was contemporaneous with movement on the left-lateral Ailao Shan shear zone and the right-lateral Gaoligong Shan shear zone two shear zones that bound the Indochina fragment on the east and west, respectively. Our data from the Chong Shan shear zone along with the data presented elsewhere from the Gaoligong Shan shear zone indicate that while the region between the Gaoligong Shan shear zone and Ailao Shan shear zone extruded to the southeast, it did not extrude as a single rigid block, but rather it was dismembered into at least two major fragments, the Baoshan to the west and the Lanping-Simao to the east separated by the Chong Shan shear zone. Our study of the Chong Shan shear zone suggests it and the other major early Cenozoic shear zones formed part of a broad major shear zone that passes into eastern Tibet between the Qiang-tang and Lhasa tectonic units.

Miocene to present activity along the Red River fault, China, in the context of continental extrusion, upper-crustal rotation, and lower-crustal flow

Detailed mapping of field relationships along the Red River fault reveals information about the distribution and magnitude of slip along the fault, about its interactions with other regional fault systems, and the relationship between river incision and growth of the eastern margin of the Tibetan Plateau. The Red River fault is complex, consisting of up to four strands, and is dominated by right-lateral strike-slip displacement. Evidence for an extensional component of displacement is strongest along the northern part of the fault, and decreases to the southeast, to zero southeast of a major bend in the fault. Results of this study indicate dextral displacement on the Red River fault is probably at least ∼40 km, 15–16 km of which predates incision of the Red River in Pliocene time or later, and probably also predates plateau growth and development of other regional fault systems. Long-term average slip rate on the Red River fault is a minimum of ∼5 mm/yr. However, regional active tectonics are characterized by rotation of the upper crust around the eastern Himalayan syntaxis, bounded to the east by the Xianshuihe-Xiaojiang fault system, which deflects but does not cut the Red River fault. The distributed nature of the Xianshuihe-Xiaojiang fault system as it approaches the Red River fault is important for accommodating shearing across the strong crustal anisotropy formed by the Red River fault and Ailao Shan shear zone. The Red River fault appears to terminate in the extending Dali fault system in northwest Yunnan. Pliocene surface uplift, river incision, and rotation around the eastern Himalayan syntaxis are inconsistent with the pre-Pliocene displacement on the Red River fault and lateral extrusion along the Ailao Shan shear zone advocated by the “extrusion model.” This suggests a significant change in crustal conditions on the southeast margin of the Tibetan Plateau in Pliocene time, possibly the result of lower-crustal flow.

Exhumation of the Ailao Shan shear zone recorded by Cenozoic sedimentary rocks, Yunnan Province, China

The role of strike‐slip faults, such as the Ailao Shan shear zone in southwestern China, in accommodating the India‐Eurasia collision remains controversial. Cenozoic sedimentary rocks preserved along the northeast side of the Ailao Shan shear zone, however, record the nature and timing of exhumation of shear zone rocks. Field mapping shows that the lower strata consist of early Oligocene lacustrine red mudstone and gypsum and the upper of late Oligocene to Miocene fluvial conglomerate with rare, fine‐grained, fossil leaf‐bearing intervals. Sediment derived from shear zone gneiss is present only in the upper part of the section. Our observations indicate deformation began in Oligocene time, with the onset of rapid exhumation by early Miocene time, consistent with existing geochronologic results. Exhumation must have ended before the formation of a regional low‐relief erosion surface and the onset of brittle deformation along the Red River fault, probably in early Pliocene time. Rapid exhumation of the Ailao Shan shear zone has been interpreted to have occurred in transtension, accommodated by a normal fault on the northeast margin of the shear zone. However, we observe consistent and pervasive shortening structures within the Cenozoic strata, including overthrusting and folding of the basin along faults defined by thick gouge zones within weak Cenozoic lithologies. Deformation is likely syndepositional. These structures are distinct from subvertical deformational features associated with the active Red River fault. Our field data thus support mid‐Cenozoic transpressional exhumation of the Ailao Shan shear zone. This is consistent with regional evidence for widespread left‐lateral and compressional deformation in the region to the east of the eastern Himalayan syntaxis during this time. Focused erosion along the Red River, established by late Oligocene time, may have contributed to deeper exhumation of the northeast part of the shear zone.