Ailao Shan-Red River Fault Research Articles

Discovery of the ∼35 Ma granite porphyry dikes in the giant Dachang Sn ore field: Tectonic and metallogenic implications

The world’s second-largest Dachang Sn ore field in the southwest of China exhibits genetic connections to the Late Cretaceous highly evolved S-type granitoids (93–91 Ma). In this study, we report the newly identified Eocene granite porphyry dikes with zircon U-Pb ages of 35–33 Ma at the Gaofeng Sn deposit from the Dachang ore field. The petrogenesis and mineralization potential are also constrained by employing whole-rock and zircon geochemical data. These Eocene dikes coincide with the emplacements of granitoids (35–30 Ma) along the Ailaoshan–Red River fault and its neighboring regions, indicating that they could be related to a similar lithospheric extension setting after the Indo-Asian collision. However, unlike those of the latter granitoids (35–30 Ma), which are grouped into I-type granites, the high-silica granite porphyry dikes show elevated ASI values (1.32–1.38) with high P2O5 (0.41–0.44 wt%) and normative-CIPW corundum (4.15–4.71 wt%) contents and could be classified as S-type granites. The strongly negative whole-rock Sr, Ba, and Eu anomalies signify separation of plagioclase and K-feldspar during magma crystallization, which is also confirmed by the zircon trace elements compositions. The Gaofeng Eocene granite porphyry dikes have a high initial zircon Tti-zr of 855 ℃, high Ba/Pb ratios (11.5–16.8), and low Pb contents (7.56–9.98 ppm) with low zircon εHf(t) values (−7.4 to −0.6), hinting at the dehydration melting of biotite from the metasedimentary basement. Additionally, the dikes are characterized by low oxygen fugacity (zircon Ce/Ce* <70; ΔFMQ from −6.0 to +0.6 with an average of −2.0) and show evidence of magmatic-hydrothermal interaction (whole-rock Nb/Ta < 5), in line with those of the Dachang Late Cretaceous fertile granites, suggestive of Sn mineralization potential. Therefore, it is recommended that attention should be paid to the Eocene granite porphyry dikes during future prospecting.

Application of SVD combined with PCA in delineation and evaluation of ore-prospecting targets in the Gejiu tin polymetallic cluster region, SW China

The Gejiu tin polymetallic cluster region, situated at the northeastern side of the Ailaoshan-Red River Fault and the junction area between the Cathaysia terrane and the Yangtze terrane, is associated with the late Cretaceous magmatic-hydrothermal metallogenetic event. In this study, a singular value decomposition (SVD) is effectively used to extract the anomaly components from the metallogenetic element groups established by the principal component analysis (PCA) for the identification of the ore-prospecting targets. Three steps are shown: (1) the metallogenetic element groups are obtained using PCA; (2) the element anomaly components formed by magma-hydrothermal metallogenetic events are extracted using SVD from the metallogenetic element groups; and (3) the local and regional element concentration anomaly components are identified by applying the element anomaly components corresponding to different eigenspaces. Finally, ten ore-prospecting targets for the Bi-Pb-Sn-Th-W-Cu mineralization, two ore-prospecting targets for the Ag-Cd-Hg-Zn-Mn mineralization, and two ore-prospecting targets for the La-Nb-Th-Zr-U-Y mineralization are delineated, respectively. The results certify that SVD can discriminate not only the regional element concentration anomaly components formed by the magmatism but also the local element concentration anomaly components triggered by the metallogenetic processes from the metallogenetic element groups established by the PCA.

Metallogenic Mechanism and Geodynamic Background of the Chang’an Chong Cu-Mo Deposit in Southern Ailaoshan Tectonic Belt: New Evidence from Garnet U-Pb Dating and In-Situ S Isotope

The Chang’an Chong Cu-Mo deposit is located in the Chang’an Cu-Mo-Au ore cluster in the southern Ailaoshan tectonic belt in southwestern China. There are six intrusive bodies in the mining area, among which the No.Ⅱ intrusive body is the largest and most closely related to Cu-Mo mineralization of skarn. The No. 1 main orebody is composed of the No. 1 copper orebody and No. 1 molybdenum orebody, which are distributed in parallel with similar shapes. In this paper, the age of skarn is determined by the LA-SF-ICP-MS U-Pb dating of garnet, and it is nearly consistent with the age of alkaline porphyry in this region (41–32 Ma). Compared with the U-Pb age of zircon from the ore-bearing porphyry and the Re-Os age of molybdenite, the U-Pb age of garnet was consistent with them within error, indicating that they were the same mineralization event, which further proves that the porphyry-skarn Cu-Mo-Au mineralization event along the Ailaoshan-Red River fault zone mainly occurred at 38~32 Ma. In-situ S isotope results show that the δ34S mean values of disseminated pyrite (PyI), pyrite of sulfide veins (PyⅡ) and chalcopyrite (Ccp) in the main mineralization period are 2.35‰, 3.60‰ and 0.55‰, respectively. These δ34S values are similar to those of magma and slightly enriched in δ34S, and the δ34S value of chalcopyrite is mainly concentrated near 0‰, so it can be considered that the S of the ore-forming fluid came from magmatic-hydrothermal fluids. Based on the comprehensive analysis of the regional metallogenic background, deposit chronology and isotope geochemistry, it is concluded that the Chang’an Chong Cu-Mo deposit was formed in an intra-plate post-collision strike-slip environment.

Genesis and mechanisms of metal enrichment in the Baimazhai Ni-Cu-(PGE) deposit, Ailaoshan Orogenic Belt, SW China

ABSTRACT The ∼259 Ma Baimazhai Ni-Cu-(platinum-group element) deposit is located in the Ailaoshan-Red River fault zone on the southwest margin of the Yangtze Plate in the Jinping area of southeastern Yunnan Province. The intrusion is lenticular (∼530 m long × 190 m wide × 24–64 m thick) and concentrically zoned (margin to core) from gabbro through pyroxenite to peridotite. It contains ∼50 kt of Ni-Cu-(platinum-group element) mineralization, concentrically zoned (margin to core) from disseminated through net-textured to massive sulfides with an average grade of 1.03 wt.% Ni, 0.81 wt.% Cu, and 0.02∼0.69 ppm Pd+Pt. The sulfide assemblage comprises pyrrhotite, chalcopyrite, and pentlandite, with lesser magnetite, violarite, galena, and cobaltite. The mineralization is enriched in Ni-Cu-Co relative to the platinum-group elements and the host rocks are enriched in highly incompatible lithophile elements relative to moderately incompatible lithophile elements with high Th/Yb and intermediate Nb/Yb ratios. These host rocks, and those at most other Ni-Cu-platinum-group element deposits in the Emeishan Large Igneous Province, have high γOs and intermediate εNd values, indicating that they crystallized from a magma derived from a subduction-modified pyroxenite mantle source and modified by crustal contamination. The initial concentrations of metals in the primary magma are estimated to have been on the order of 200 ppm Ni and 100 ppm Cu, but only 0.4 ppb Pd, 0.2 ppb Pt, 0.005 ppb Rh, 0.02 ppb Ru, and 0.01 ppb Ir. The δ34S values of ores and separated sulfides range from 5.8‰ to 8.6‰, between the ∼10‰ value of sulfides in the metasedimentary country rocks and the 0 ± 0.5‰ value expected for magmas derived from MORB-type mantle, or the –2.5 ± 0.3‰ value expected for subduction-modified mantle, consistent with equilibration at magma:sulfide mass ratios (R factors) of 100–1000. Variations in Ir100 and Pd100 (metals in 100% sulfide) are consistent with 40–60% fractional crystallization of monosulfide solid solution to form Ni-Co-intermediate platinum-group element (Ru, Os, Ir)-rich massive ores and Cu-palladium/platinum-group elements (Pt, Pd, Rh)-Au-rich residual sulfide liquids. This process is also recorded by magnetite: Type I (early magmatic), type II (late magmatic), and type III (secondary) magnetites exhibit progressively lower Cr-Ti-V concentrations. The platinum-group element contents in base-metal minerals are low, and only pentlandite, violarite, and cobaltite contain detectable concentrations of Pd, Rh, and Ru. There is abundant textural evidence for metamorphic-hydrothermal alteration of sulfides in the Baimazhai intrusion, with secondary violarite, chalcopyrite, and pentlandite being enriched (Ag, Sb, Au, Pb) or depleted (Sn) in more mobile chalcophile elements. The different tectonic and petrogenetic settings of the Baimazhai and other deposits in China highlight the potential of Ni-Cu-platinum-group element deposits to occur in subduction or post-subduction settings and demonstrate that the key controls are magma flux and access to crustal S. Exploration potential remains for the Ailaoshan orogenic belt to host additional magmatic Ni-Cu deposits.

Massive lithospheric delamination in southeastern Tibet facilitating continental extrusion.

Significant left-lateral movement along the Ailao Shan-Red River fault accommodated a substantial amount of the late Eocene to early Miocene India-Asia convergence. However, the activation of this critical strike-slip fault remains poorly understood. Here, we show key seismic evidence for the occurrence of massive lithospheric delamination in southeastern Tibet. Our novel observation of reflected body waves (e.g. P410P and P660P) retrieved from ambient noise interferometry sheds new light on the massive foundered lithosphere currently near the bottom of the mantle transition zone beneath southeastern Tibet. By integrating the novel seismic and pre-existing geochemical observations, we highlight a linkage between massive lithospheric delamination shortly after the onset of hard collision and activation of continental extrusion along the Ailao Shan-Red River fault. This information provides critical insight into the early-stage evolution of the India-Asia collision in southeastern Tibet, which has significant implications for continental collision and its intracontinental response.

Geometric and kinematic analysis of normal faults bordering continental shelves: A 3D seismic case study from the northwest South China Sea

Normal faults border many a continental shelf, but have only been considered to play a subordinate role to basin-controlling faults in previous studies. Their detailed geometries and kinematic histories are still poorly known. In this study, high-quality three dimensional (3D) seismic data are used to investigate distinct families of normal faults bordering the continental shelf of the Qiongdongnan Basin, northwest South China Sea. Sixty-six (66) normal faults are interpreted and found to mostly tip out in the Upper Miocene. Three large-scale faults offsetting Horizon T30 (the base of Pliocene strata), and three other faults terminating beneath this same horizon, were selected and studied in detail. We discriminate between growth and blind faults by analysing upper tip folding geometry and fault displacement distribution using throw-depth (T-z) plots. The dips of the upper parts of faults are almost twice that of their lower parts. A new three-stage model for fault reactivation in Qiongdongnan Basin is therefore proposed based on our results. In the northwest South China Sea, the propagation of normal faults on the continental shelf can be attributed to: (1) the rotation of the South China block and a reversal in the movement of the Ailao Shan-Red River Fault Zone from left-to right-lateral slip, and (2) large-scale slope instability in the Qiongdongnan Basin at ~5.5 Ma. The results presented here outline the structural evolution of the continental shelf of the northwest South China Sea after 5.5 Ma, and how regional-scale structures such as the Ailao Shan-Red River Fault Zone controlled the evolution of normal faults on continental shelves.

Petrogenesis of Eocene to early Oligocene granitic rocks in Phan Si Pan uplift area, northwestern Vietnam: Geochemical implications for the Cenozoic crustal evolution of the South China Block

The Phan Si Pan uplift area of NW Vietnam is a part of the Archean to Paleoproterozoic Yangtze Block (SW China) that was displaced along the Ailaoshan-Red River Fault shear zone and adjacent structures (i.e., Song Da zone). The region is of interest as it is at the forefront of the debate regarding the extrusion of SE Asia during the Early Paleogene. Two Cenozoic silicic plutonic complexes of contrasting composition intruded the basement rocks of the Phan Si Pan uplift area. The Ban Xeo granite is ferroan (A-type), alkalic and yielded a zircon weighted-mean 206Pb/238U age of 49 ± 0.9 Ma. The major and trace element compositions of the Ban Xeo granite are similar to the spatially associated Late Permian Muong Hum granites. The εNd(t) values (−2.5 to −1.4), 87Sr/86Sr initial ratios (0.70793, 0.70989, and 0.75341), and zircon εHf(t) values (−1.1 to +3.0) indicate that the Ban Xeo granite is likely derived by differentiation of a mafic parental magma that had EMII-like mantle source characteristics. The younger Ye Yen Sun (YYS) granite is magnesian, alkali-calcic to calc-alkalic, and yielded zircon U–Pb weighted-mean ages of 35.3 ± 0.5 to 34.1 ± 0.4 Ma. The YYS granites have 87Sr/86Sr initial ratios of 0.70636 to 0.70860, εNd(t) values from −8.7 to −2.6, and zircons εHf(t) values from −9.3 to +0.6 which indicate a crustal origin. The YYS dioritic rocks range from syeno-diorite to granodiorite, and have similar Sr (87Sr/86Sr = 0.70627) and εNd(t) (−3.8) values as the YYS granites. However, the zircon Hf isotopes (εHf(t) = +0.3 to +3.2) of the syeno-diorite are slightly more chondritic than the granites and indicates that the parental magma was likely derived from a mantle source. Rhyolite–MELTS modeling results show that the YYS granites were likely generated by fractionation of a parental magma similar in composition to the granodiorite. Furthermore, the YYS syeno-diorites may be representative of a cumulate rock that developed during the early stages of emplacement or crystallization. The parental magma of YYS igneous rocks likely formed by partial melting of mafic Mesoproterozoic juvenile rocks (lower crust/uppermost mantle) by heat from the upwelling lithospheric mantle during a period of crustal relaxation. The formation of the Ban Xeo granites and Ye Yen Sun igneous rocks are likely related to lateral melt migration along the eastern edge of the India-Asia collision zone. It is possible that repeated episodes of Cenozoic magmatism was pivotal in the development and initiation of fault movement of the Ailaoshan-Red River shear zone.

Reappraisal of the Jianchuan Cenozoic basin stratigraphy and its implications on the SE Tibetan plateau evolution

We present a new stratigraphy of the Jianchuan basin, one of the largest Cenozoic sedimentary basins in southeastern Tibet. This basin was regarded as recording sedimentation from the Eocene up to the Pliocene, and as such has been the focus of several studies aiming at constraining the environmental, tectonic and topographic evolution of the area. Within the Shuanghe and Jianchuan formations thirteen new zircon U/Pb ages and one biotite 40Ar/39Ar age of interbedded and cross-cutting ultrapotassic magmatic rocks show that a brief magmatic event occurred from ~35.7 to ~34.5Ma (35.2±0.4Ma on average). The uppermost formation (Jianchuan Fm), supposedly Pliocene in age, is related to this magmatic event and is 35.4±0.8Ma old. All sedimentary formations are thus Eocene in age, with neither Oligocene nor Miocene sediments. The coal-bearing Shuanghe Formation yields a fossil of a large amynodontid typical of the Upper Eocene Ergilian interval (37.2 to 33.9Ma). Sedimentation of the Shuanghe Formation took place in a short time interval at ~35.9±0.9Ma, after a large-scale drainage reorganization that induced the abandonment of a large braided-river system. This reorganization was possibly linked with the initiation of the left-lateral Ailao-Shan Red River fault and/or to widespread magmatism in the Jianchuan basin. Previous high paleoaltitude estimates for the Jianchuan basin are thoroughly re-evaluated and yield a value of 1200±1200m.a.s.l. for the Upper Eocene.

Curie Point Depth from Spectral Analysis of Magnetic Data in the Southeast Tibet

The satellite magnetic anomalies are used to calculate the Curie point depth of the Southeast Tibet by spectral analysis method in the study. The relationship between the Curie point depth and the regional faults or heat flow will be discussed. The results show that the Curie point depth of the study area ranges from 15 km to 36 km and the average depth is 26.3 km. The Curie point depth is cluster-like on the north of the Ailaoshan-Red River Fault, while it is strip-like distribution on the north side. The Curie point depth in the Xiaojiang Fault zone, the Xiaojinhe Fault zone, the Dien Bien Phu Fault zone and the Gaoligong Fault zone are shallow. It could be related to their strong frictional heat induced by these faults. The Curie point depth in the middle Sukhothai Block is shallow, which it is not only related to the Phayao Fault, the Mae Chan Fault and the Nam Ma Fault, but also to the subduction of the Palaeo-Tethys into the Indochina Block. There is a negative but nonlinearly correlation between the heat flow and the Curie point depth in this study area. The areas of low heat flow value correspond to the areas of deep Curie point depth. However, both the high and low heat flow values can be found in the areas of shallow Curie point depth. The possible reason is thought to be related to the low thermal conductivity of the rock.

Rheological weakening of high-grade mylonites during low-temperature retrogression: The exhumed continental Ailao Shan-Red River fault zone, SE Asia

We present a detailed case study of an exhumed continental strike-slip fault zone, the Ailao Shan-Red River (ASRR) strike-slip fault zone, to investigate how deformation promotes strain localization, and how the weak second phases and fluids trigger rheological weakening during retrogression near the ductile to brittle transition during exhumation. Along the ASRR strike-slip fault zone, in the Diancang Shan (DCS) metamorphic massif, high-temperature ductile deformation (D1) pervasively occurred during shearing and exhumation since late Oligocene. The high-temperature microstructures and textures are in part or entirely altered by subsequent low-temperature shearing (D2) since late Miocene, which is under the conditions of frictional-viscous transition of K-feldspar (ca. 450°C) during further exhumation to the upper crustal levels. The formation of D2 microstructures and shear bands overprinted high-temperature intracrystalline plasticity phases (D1) in mylonitic rocks. Depending on the main rock-forming minerals, the results also demonstrate that the brittle–ductile transition involves a combination of different deformation mechanisms and possible rheological paths. In quartz-rich rocks, quartz was deformed in the dislocation creep regime and records transition of microfabrics and slip systems during decreasing temperature, which lasted until retrogression related to exhumation. As a result, grain-size reduction associated with fluids circulating within the ASRR strike-slip fault zone at brittle–ductile transition leads to reaction and texture weakening. Rheological weakening is the consequence of the syntectonic deformation, fluid flow, reaction softening, reaction creep and textural softening. The hydrous fluids resulted in hydration of silicates. Decompression occurred during shearing and as a result of tectonic exhumation. All these results demonstrate that the exhumation through the ductile to ductile-brittle transition involves a combination of different deformation mechanisms, rheological transition features and feedbacks between deformation, decreasing temperature and fluids in the ASRR fault zone.

Late Cenozoic tectonic evolution of the Ailao Shan-Red River fault (SE Tibet): Implications for kinematic change during plateau growth

Surface uplift, river incision, shear zone exhumation, and displacement along active faults have all interacted to shape the modern landscape in the southeastern margin of the Tibetan Plateau. The Ailao Shan-Red River fault, a major structure in the tectonic evolution of southeastern Asia, is an excellent recorder of these processes. We present new stratigraphic, structural, and low-temperature thermochronologic data to explore its late Cenozoic tectonic and geomorphic evolution. The stratigraphic and structural observations indicate that the major bend in the fault was a releasing bend with significant Miocene sedimentation in the early–middle Miocene but became a restraining bend with abundant shortening structures developed after the late Miocene reversal of displacement. We also document exhumation of the shear zone from two low-temperature thermochronologic transects. New apatite (U-Th)/He(AHe) data and published thermochronologic results reveal two accelerated cooling episodes, backed by stratigraphic and geomorphic observations. The first rapid cooling phase occurred from ca. 27 to 17 Ma with removal of cover rocks and exhumation of the shear zone. The second accelerated cooling episode revealed by our AHe data commenced at 14–13 Ma, lasting 2–3 Myr. The Ailao Shan range may have risen to its modern elevation with high-relief topography developing due to river incision. We interpret the onset of this rapid exhumation to reflect renewed plateau growth associated with lower crustal flow.

Petrology, geochemistry, and metamorphic evolution of meta-sedimentary rocks in the Diancang Shan–Ailao Shan metamorphic complex, Southeastern Tibetan Plateau

Meta-sedimentary rocks are widely distributed within the Diancang Shan–Ailao Shan metamorphic complex in the Southeastern Tibetan Plateau. Detailed geochemical analyses show that all of them have similar geochemical features. They are enriched in light rare-earth elements (LREEs) and depleted in heavy rare-earth elements (HREEs), with moderately negative Eu anomalies (Eu/Eu∗=0.55–0.75). Major and trace element compositions for the meta-sedimentary rocks suggest that the protoliths were probably claystone, siltstone, and greywacke and deposited in an active continental margin. Garnet porphyroblasts in meta-sedimentary rocks have distinct compositional zonation from core to rim. The zonation of garnet in St–Ky–Grt–Bt–Ms schist indicates an increasing P–T trend during garnet growth. In contrast, garnets from (Sil)–Grt–Bt paragneiss show diffusion zoning, implying a decreasing P–T trend. Based on mineral transformations and P–T estimates using conventional geothermobarometers and pseudosection calculations, four metamorphic stages have been determined, including an early prograde metamorphic stage (M1), a peak amphibolite–granulite facies metamorphic stage (M2), a near-isothermal decompression stage (M3), and a late amphibolites-facies retrograde stage (M4). The relic assemblage of Ms+St±Ky±Bt±Kfs+Qz preserved as inclusions in garnet porphyroblasts of the meta-sedimentary rocks belongs to prograde (M1) stage and records P–T conditions of 560–590°C and 5.5–6.3kb. Matrix mineral assemblages of Grt+Bt+Ky/Sil+Pl+Qz and Grt+Bt±Sil+Pl±Kfs+Qz formed at peak (M2) stage yield P–T conditions of 720–760°C and 8.0–9.3kb. M3 is characterized by decompression reactions, dehydration melting of assemblages that include hydrous minerals (e.g., biotite), and partial melting of felsic minerals. The retrograde assemblages is Grt+Bt+Sil+Pl+Qz formed at 650–760°C and 5.0–7.3kb. At the amphibolites-facies retrograde (M4) stage, fine-grained biotite–muscovite-bearing symplectites were formed, which occur at the grain boundaries of garnet and felsic minerals; such assemblage as Bt+Ms+Pl±Kfs±Grt+Qz were formed at 553–613°C and 4.0–5.0kb. The metamorphic history of the Diancang Shan–Ailao Shan metamorphic complex is thus defined by a clockwise P–T trajectory which includes a nearly isothermal decompression recording the subduction, collision and exhumation events between the Yangtze and Indochina blocks at Oligocene to Early Miocene, prior to the regional strong left-lateral shearing along the Ailao Shan-Red River Fault.

Expansion of the South China Sea basin: Constraints from magnetic anomaly stripes, sea floor topography, satellite gravity and submarine geothermics

The widely distributed E–W-trending magnetic anomaly stripes in the central basin and the N–E-trending magnetic anomaly stripes in the southwest sub-basin provide the most important evidence for Neogene expansion of the South China Sea. The expansion mechanism remains, however, controversial because of the lack of direct drilling data, non-systematic marine magnetic survey data, and irregular magnetic anomaly stripes with two obvious directions. For example, researchers have inferred different ages and episodes of expansion for the central basin and southwest sub-basin. Major controversy centers on the order of basinal expansion and the mechanism of expansion for the entire South China Sea basin. This study attempts to constrain these problems from a comprehensive analysis of the seafloor topography, magnetic anomaly stripes, regional aeromagnetic data, satellite gravity, and submarine geothermics. The mapped seafloor terrain shows that the central basin is a north-south rectangle that is relatively shallow with many seamounts, whereas the southwest sub-basin is wide in northeast, gradually narrows to the southwest, and is relatively deeper with fewer seamounts. Many magnetic anomaly stripes are present in the central basin with variable dimensions and directions that are dominantly EW-trending, followed by the NE-, NW- and NS-trending. Conversely such stripes are few in the southwest sub-basin and mainly NE-trending. Regional magnetic data suggest that the NW-trending Ailaoshan-Red River fault extends into the South China Sea, links with the central fault zone in the South China Sea, which extends further southward to Reed Tablemount. Satellite gravity data show that both the central basin and southwest sub-basin are composed of oceanic crust. The Changlong seamount is particularly visible in the southwest sub-basin and extends eastward to the Zhenbei seamount. Also a low gravity anomaly zone coincides with the central fault zone in the sub-basin. The submarine geothermic distribution demonstrates that the southwest sub-basin has a higher geothermal value than the central basin, and that the central fault zone is defined by a low thermal anomaly. This study suggests that NW–SE expansion of the southwest subbasin is later than the N–S expansion of the central basin with the sub-basin extending into the central basin and with both expansions ending at the same time. The expansion of southwestern sub-basin, similar to the Japanese Sea, is likely caused by left-lateral strike slip on the central fault zone in the South China Sea, which may have significance for finding oil and gas in this region.

Late Permian to Early Oligocene granitic magmatism of the Phan Si Pan uplift area, NW Vietnam: their relationship to Phanerozoic crustal evolution of Southwest China

The Phan Si Pan uplift area of NW Vietnam is a part of the Archean to Paleoproterozoic Yangtze Block (SW China) that was displaced along the Ailaoshan-Red River fault shear zone and adjacent structures (i.e., Song Da zone). This area is of particular interest because it experienced a number of Phanerozoic crustal building events, including the Emeishan large igneous province, the India-Eurasia collision, and Ailaoshan – Red River Fault displacement. In the Phan Si Pan uplift area, there are at least three different geochronological complexes, including (1) Late Permian, (2) Eocene, and (3) Early Oligocene. (1) The Late Permian silicic plutonic and volcanic rocks are correlated with the Emeishan large igneous province (ELIP). The silicic rocks of the Phan Si Pan uplift area, and Tu Le basin are ferroan alkalic and have zircon CA-ID-TIMS weighted-mean ages of 256.3 ± 0.4 Ma to 257.3 ± 0.1 Ma for granite and 257.1 ± 0.6 Ma to 257.9 ± 0.3 Ma for rhyolite. The high precision CA-ID-TIMS of Phan Si Pan – Tu Le silicic rocks suggest that Emeishan lavas erupted over a period of ~6 million years with plutonism ending shortly thereafter. Granitic rocks from Phan Si Pan uplift and Tu Le basin have variable ISr values ranging between 0.70214 and 0.70357 that could be the result of significant fractional crystallization (i.e., highly depleted Sr) or mobility of Rb, and eNd(t) values are broadly chondritic (eNd(t) = 0 ± 1). Zircon eHf(t) values range from +3.3 to +8.8 suggesting the granitic rocks of Phan Si Pan uplift and Tu Le basin are derived from a mantle source. Reported fractional crystallization modeling suggests the Phan Si Pan – Tu Le silicic rocks can be produce from Emeishan high-Ti basaltic rocks. (2) The Ban Xeo granite is ferroan (A-type), alkalic, and yielded a zircon weighted-mean 206Pb/238U age of 49 ± 0.9 Ma. The major and trace element compositions of the Ban Xeo granite are similar to the spatially associated Late Permian Muong Hum granites. The eNd(t) values (-2.5 to -1.4), 87Sr/86Sr initial ratios (0.70793, 0.70989, and 0.75341), and zircon eHf(t) values (-1.1 to +3.0) indicate that the Ban Xeo granite is likely derived by differentiation of a mafic parental magma that had EMII-like mantle source characteristics. (3) The younger Ye Yen Sun (YYS) granite is magnesian, alkali-calcic to calc-alkalic, and yielded zircon U-Pb weighted-mean ages of 35.3 ± 0.5 to 34.1 ± 0.4 Ma. The YYS granites have 87Sr/86Sr initial ratios of 0.70636 to 0.70860, eNd(t) values from -8.7 to -2.6, and zircon eHf(t) values from -9.3 to +0.6 which indicate a crustal origin. The YYS dioritic rocks range from syeno-diorite to granodiorite, and have similar Sr (87Sr/86Sr = 0.70627) and eNd(t) (-3.8) values as the YYS granites. However, the zircon Hf isotopes (eHf(t) = +0.3 to +3.2) of the syeno-diorite are slightly more chondritic than the granites and indicates that the parental magma was likely derived from a mantle source. Rhyolite–MELTS modeling results show that the YYS granites were likely generated by fractionation of a parental magma similar in composition to the granodiorite. Furthermore, the YYS syeno-diorites may be representative of a cumulate rock that developed during the early stages of emplacement or crystallization. The parental magma of YYS igneous rocks likely formed by partial melting of mafic Mesoproterozoic juvenile rocks (lower crust/uppermost mantle) by heat from the upwelling lithospheric mantle during a period of crustal relaxation. The formation of the Ban Xeo granites and Ye Yen Sun igneous rocks are likely related to lateral melt migration along the eastern edge of the India-Asia collision zone. It is possible that repeated episodes of Cenozoic magmatism was pivotal in the development and initiation of fault movement of the Ailaoshan-Red River shear zone.

Structure and tectonic geomorphology of the Qujiang fault at the intersection of the Ailao Shan–Red River fault and the Xianshuihe–Xiaojiang fault system, China

The northwest-striking Qujiang fault is located at the southeastern tip of the Sichuan–Yunnan block, which is bounded by the Ailao Shan–Red River fault and the Xianshuihe–Xiaojiang fault system. The Qujiang fault is one of the most seismically active faults in western Yunnan, China and is considered to be the seismogenic fault of the 1970 Ms 7.7 Tonghai earthquake. In this study, we examine the geometry, kinematics, and geomorphology of this fault through field observations, satellite images, and DEMs. It is a Holocene active fault characterized by dextral strike–slip movements with dip–slip components. It can be divided into northwest and southeast segments depending on different kinematics. The northwest segment from Baiyizhai to Wujie shows right-lateral strike slip with normal components, whereas it is characterized by dextral movements with the northeast wall thrusting over the opposite in the southeast segment from Wujie to Miaobeishan. The geophysical data combined with our investigations indicate that the fault dips steeply to the NE at depth, while secondary faults with different dip directions developed at shallow surface owing to fault bifurcation. During the Tonghai earthquake, a blind fault may have developed on the southwest wall, resulting in aftershocks concentrated to the southwest of the surface raptures. Multiple tectonic landforms are developed along the fault. The right-lateral Holocene slip rate was determined to be 2.84–3.27mm/year based on radiometrically dated offset gullies. Our analyses show that the present deformation at the intersection of the Ailao Shan–Red River fault and the Xianshuihe–Xiaojiang fault system is partitioned into a shortening distributed over a region ~150km wide and a right-lateral strike slip localized on the Qujiang and Jianshui faults. Since the late Pleistocene, the Qujiang fault has been playing an important role in accommodating the ongoing SE-directed movements of the Sichuan–Yunnan block, creating strong earthquakes to release strain energy.

Partitioning of the Cretaceous Pan-Yangtze Basin in the central South China Block by exhumation of the Xuefeng Mountains during a transition from extensional to compressional tectonics?

The formation of a series of intermountain basins is likely to indicate a geodynamic transition, especially in the case of such basins within the central South China Block (CSCB). Determining whether or not these numerous intermountain basins represent a division of the Cretaceous Pan-Yangtze Basin by exhumation of Xuefeng Mountains, is key to understanding the late Mesozoic to early Cenozoic tectonics of the South China Block (SCB). Here we present apatite fission track (AFT) data and time–temperature modeling in order to reconstruct the evolution history of the Pan-Yangtze Basin. Fourteen rock samples were taken from a NE–SW-trending mountain–basin system within the CSCB, including, from west to east, the Wuling Mountains (Wuling Shan), the south and north Mayang basins, the Xuefeng Mountains (Xuefeng Shan) and the Hengyang Basin. Cretaceous lacustrine sequences are well preserved in the south and north Mayang and Hengyang basins, and sporadically crop out in the Xuefeng Mountains, whereas Paleogene piedmont proluvial–lacustrine sequences are only found in the south Mayang and Hengyang basins. AFT results indicate that the Wuling and Xuefeng mountains underwent rapid denudation post-84Ma, whereas the south and north Mayang basins were more slowly uplifted from 67 and 84Ma, respectively. Following a quiescent period from 32 to 19Ma, both the mountains and basins have been rapidly denuded since 19Ma. Both the AFT data and sedimentary facies changes suggest that the Cretaceous deposits that cover the south–north Mayang and Hengyang basins through to the Xuefeng Mountains define the Cretaceous Pan-Yangtze Basin. Integrating our results with tectonic background for the SCB, we propose that rollback subduction of the paleo-Pacific Plate produced the Pan-Yangtze Basin, which was divided into the south–north Mayang and Hengyang basins by the abrupt uplift and exhumation of the Xuefeng Mountains from 84Ma to present, apart from a period of tectonic inactivity from 32 to 19Ma. This late Late Cretaceous to Paleogene denudation resulted from movement on the Ziluo strike–slip fault, which formed due to intra-continental compression most likely associated with the Eurasia–Indian plate subduction and collision. Sinistral transpression along the Ailao Shan–Red River Fault at 34–17Ma probably transformed this compression to the extrusion of the Indochina Block, and produced the quiescent window period from 32 to 19Ma for the mountain–basin system in the CSCB. Therefore, the initiation of exhumation of the Xuefeng Mountains at 84Ma indicates a switch in tectonic regime from Cretaceous extension to late Late Cretaceous and Cenozoic compression.

Discussion of tectonic models for Cenozoic strike-slip fault-affected continental margins of mainland SE Asia

Understanding the roles of Cenozoic strike-slip faults in SE Asia observed in outcrop onshore, with their offshore continuation has produced a variety of structural models (particularly pull-apart vs. oblique extension, escape tectonics vs. slab-pull-driven extension) to explain their relationships to sedimentary basins. Key problems with interpreting the offshore significance of major strike-slip faults are: (1) reconciling conflicting palaeomagnetic data, (2) discriminating extensional, and oblique-extensional fault geometries from strike-slip geometries on 2D seismic reflection data, and (3) estimating strike-slip displacements from seismic reflection data.Focus on basic strike-slip fault geometries such as restraining vs. releasing bends, and strongly splaying geometries approach the gulfs of Thailand and Tonkin, suggest major strike-slip faults probably do not extend far offshore Splays covering areas 10,000’skm2 in extent are characteristic of the southern portions of the Sagaing, Mae Ping, Three Pagodas and Ailao Shan-Red River faults, and are indicative of major faults dying out. The areas of the fault tips associated with faults of potentially 100km+ displacement, scale appropriately with global examples of strike-slip faults on log–log displacement vs. tip area plots. The fault geometries in the Song Hong-Yinggehai Basin are inappropriate for a sinistral pull-apart geometry, and instead the southern fault strands of the Ailao Shan-Red River fault are interpreted to die out within the NW part of the Song Hong-Yinggehai Basin. Hence the fault zone does not transfer displacement onto the South China Seas spreading centre. The strike-slip faults are replaced by more extensional, oblique-extensional fault systems offshore to the south. The Sagaing Fault is also superimposed on an older Paleogene–Early Miocene oblique-extensional rift system. The Sagaing Fault geometry is complex, and one branch of the offshore fault zone transfers displacement onto the Pliocene-Recent Andaman spreading centre, and links with the West Andaman and related faults to form a very large pull-apart basin.

Potassic magma genesis and the Ailao Shan-Red River fault

Two types of K-rich magma of Eocene to Early Oligocene (ca. 40–30) and Plio-Pleistocene (ca. 5–0.1Ma) age were emplaced prior to and following left-lateral slip on the Ailao Shan-Red River (ASRR) fault, a regional shear zone extending between southwest China and the Tonkin Gulf (South China Sea) that accommodated ‘escape’ of the Indochina block. The first type is exposed in the Dali-Lijiang and adjacent regions of western Yunnan and Sichuan and comprises ultramafic potassic to ultrapotassic ‘absarokites’ and their shoshonite, banakite, and SiO2-rich derivatives which were emplaced immediately prior to activation of the ASRR fault. They are characterized by high Mg.-nos, and low contents of fusible oxides (FeO*, CaO, Al2O3), for equivalent MgO content, and pronounced primitive mantle-normalized high-field strength element (HFSE) depletions. In contrast, ‘post-escape’ K-rich magmas were erupted in the Puer, Maguan-Pingbian regions of south and southeast Yunnan. Apart from their relative enrichments in potassium they show typical HFSE-rich intra-plate compositional affinity. Geological and geomorphic evidence, and thermochronologic age dating of metamorphisc events, suggest that left-lateral shearing occurred between ca. 30 and 17Ma; thereby accommodating the southeastward ‘escape’ of Indochina and (possibly) two episodes of spreading in the South China Sea. The southwestern part of Dali-Lijiang magmatic products was detached and offset by ca. 600km and are now located in Phan Xi Pang in northern Viet Nam. The same is true for the Permo-Triassic Emeishan flood basalts, whose western exposures were likewise displaced by the same amount and are now represented by the Song Da complex, also in northern Viet Nam. Here, we report geochemical, isotopic, and 40Ar/39Ar age data for samples from both the ‘pre-escape’ Dali-Lijiang magmas and the ‘post-escape’ K-rich Puer, Maguan-Pingbian basalts and basanites, with a view to comparing and contrasting their interpolated source compositions, estimated conditions of upper mantle melt segregation and, by inference, their mantle dynamic and contamination histories insofar as these were conditioned by the India-Asia collision. Our interpretations yielded two complementary conclusions. The first contends that the pre-escape magmas result from adiabatic melting of crust-contaminated asthenosphere comprising a ‘mélange’ of continental lithospheric mantle (CLM) (hydrated by sab-derived hysdrous fluids released at 0.2–0.5GPa) and lower crust, delaminated from the overriding plate during mantle wedge corner flow and further enriched by metasomatic melts of subducted continental crust. We postulate that incipient H2O-saturated melting of the ‘mélange’ occurs at depths of between ca. 100 and 200km after being ‘dragged’ down by relict oceanic slab fragments, in response to the dehydration of supra-subduction amphibole- and phlogopite. The ensuing viscosity ‘crisis’ and buoyancy relative to ambient ‘fertile’ convecting mantle of such asthenospheric ‘pockets’, and the collision-related change from lithospheric compression to extension, almost certainly predisposes such a refractory yet crust-contaminated ‘pockets’ to rapid adiabatic melting. The second conclusion concerns the post-escape K-rich basalts and basanites and is based on the contention that decompression melting of thermally anomalous K-rich asthenospheric occurred in response to regional post-escape transtension, concomitant with the cessation Indochina escape and contiguous seafloor spreading. However, although these magmas share the HFSE-rich fertile source character of other, widely dispersed, post-escape Cenozoic basalts they more specifically resemble relatively rare examples of intra-plate, K-rich activity observed in northeast China, central Spain, and elsewhere in Asia and Europe, arguably (indirectly) reflecting mantle perturbations caused by major continental collisions.

Relationships between porphyry Cu–Mo mineralization in the Jinshajiang–Red River metallogenic belt and tectonic activity: Constraints from zircon U–Pb and molybdenite Re–Os geochronology

The Jinshajiang–Red River porphyry Cu–Mo metallogenic belt is an important Cenozoic porphyry Cu–Mo mineralization concentrating zone in the eastern Indo‐Asian collision zone. New zircon U–Pb and molybdenite Re–Os ages and compilation of previously published ages indicate that porphyry Cu–Mo deposits in the belt did not form at the same time, i.e., the porphyry emplacement and relevant Cu–Mo mineralization ages of the Ailaoshan–Red River ore belt in south range from 36.3Ma to 34.6Ma, and from 36.0Ma to 33.9Ma, respectively, which are obviously younger than the porphyry emplacement ages of 43.8–36.9Ma and the relevant Cu–Mo mineralization ages of 41.6–35.8Ma of the Yulong ore belt in north. Tectonic studies indicated that the Jinshajiang fault system in north and Ailaoshan–Red River fault system in south of the Jinsjiang–Red river belt had different strike-slip patterns and ages. The right-lateral strike-slip motion of the Jinshajiang fault system initiated at ca. 43Ma with corresponding formation of the Yulong porphyry Cu–Mo system, whereas the left-lateral strike-slip motion of the Ailaoshan–Red River fault system initiated at ca. 36Ma with corresponding formation of the Ailaoshan–Red River porphyry Cu–Mo system. Therefore, the different ages of porphyry Cu–Mo systems, between in north and south of the Jinshajiang–Red River belt, indicate that the porphyry Cu–Mo mineralization is closely related to the divergent strike-slip movements between the Jinshajiang and Ailaoshan–Red River strike-slip faulting resulted from the Indo‐Asian collision. The tanslithospheric Jinshajiang–Red River faulting caused partial melting of the enriched mantle sources of alkali-rich porphyries by depressurization or/and asthenospheric heating, and facilitated the migration of alkali-rich magmas and the corresponding formation of alkali-rich porphyries and relevant Cu–Mo deposits in the belt.

Oroclinal origin of the Simao Arc in the Shan-Thai Block inferred from the Cretaceous palaeomagnetic data

SUMMARY An active oroclinal bending is discovered in the Shan-Thai Block to the south of the eastern Himalayan syntaxis. To investigate the evolution of the Simao Arc using palaeomagnetic techniques, the Middle Cretaceous red beds of the Nanxin Formation were sampled at the Zhengwan (22.8°N, 100.9°E) and Dadugang (22.4°N, 101.0°E) localities in the southern Simao Basin. Most of the studied samples revealed the presence of characteristic remnant magnetization with unblocking temperatures around 680 °C. A primary nature for this magnetization is interpreted on the basis of a positive fold and reversal test. Tilt-corrected mean directions calculated for Zhengwan and Dadugang localities are characterized by large easterly deflected declination; D= 51.8°, I= 47.9°, ks= 45.0, α95= 6.9°, N= 11 and D= 64.1°, I= 48.1°, ks= 36.0, α95= 7.3°, N= 12, respectively. Steep inclination values at both these localities with respect to those expected are in the range previously reported from the Shan-Thai Block, confirming their southward displacement by 6.2°± 1.7° as part of the Shan-Thai Block. Combination of the present data (two localities) with those previously reported from Simao Basin (seven localities) reveals a positive palaeomagnetic oroclinal test, indicating that the present-day arc-like geometry of the Simao Basin was formed by oroclinal bending. Comparison with recent GPS and structural data suggest that formation of the Simao curvature started after the early Pliocene (after 4 Ma) and continuing until the present. Origin of the Simao Arc is ascribed to southwestward movement of the crustal material across the Ailao Shan-Red River Fault (around the eastern Himalaya syntaxis), which was formed by westward movement of the decollement with progressive eastward deepening of the Lanping-Simao Basin. Decoupling between the upper and the middle–lower crusts is a requisite condition for the arc formation on the continent.